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I digit - type of coating
1. high cellulose content 2. high titanium givingslag 3. appreciable titanium giving fluid slag
4. high content of oxides and silicates
5. high content of iron oxides or iron silicates or both giving heavy solid slag
6. high content of calcium carbonate (or) calcium floride
II digit - The welding position in which the electrode may be used
0 - F, H, V, D, O 1 - F, H, V, O 2 - F, H
3-F 4 - F, Hf 9 - any other combination of positions
F - flat, H - Horizontal - Vertical, V - Vertical up, D - Vertical down, O - over head,
III digit - current conditions
0 - D+ 1 - D+, A90 2 - D-,A70 3 - D-, A50 4-D+,A 70
5 - D, A90 6-D, A70 7-D,A50 9- Any other current conditions.
D - D.C electrode positive D- -D.C electrode negative
- A.C 50,70,90 - open circuit voltage
Second 3 digits :
4th and 5th digit combine - The two ranges of tensile strength that is 410-510 Mpa and 510-610 Mpa with
appropriate yield strength indicated by the two digits that is 4th and 5th by 41 and 51 respectively. 6th digit
indicated the % elongation in combination with impact values of the deposited
Suffix letter :
H - low H2 electroodes
J - Iron powder coating giving a metal recovery of 110 to 130%
K- Iron powder coating giving a metal recovery of 130 to 150%
L- Iron powder coating giving a metal recovery of greater than 150%
P - Deep penetration electrodes.
Comparison between D.C andA.C supply for arc welding
Properties D.C A.C
1. Adaptability in factories Not good Good
2. Initial cost 3 to 4 times of aAC transformer 1/4 to 1/3 of DC generator
3. Maintenance cost High Low
4. Efficiency Less More
5. Heat distribution between work Can be controlled by changing Can not be controlled
piece and electrode polarity
6. Power factor problem No problem Acute problem
7. No load voltage Low High
8. Magnetic arc blow Problem No problem
9. Electrodes Both bare and coated electrodes Only coated electrodes
10. Non-ferrous metals Can be welded by using DCRP Cannot be welded.
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Preheating and post heating in welding
The heat treatment of welds is sometimes necessary for making efficient weld joints and carrying out suc-
cessful repair work.
Heating carried out before the commencement of welding and during the period of welding is called
preheating and heating carried out after the completion of weld is called post heating.
Preheating may be necessary for the following reasons.
a) To slow down the cooling rate of the deposited weld metal and the heat affected zone of parent metal.
This preheating prevents undue hardening and cracking of weld metal and HAZ for ex:- preheating is done
on heavy M.S. sections, before making the root pass in a heavy T-joint. Similarly preheating is done in the
case of high tensile steels.
b) Preheating of entire base metal may be necessary to distribute the expansion and contraction stresses
caused by welding as in the case of C.I which has pool ductility.
c) Preheating may be necessary to maintain sufficient heat at the joint to ensure proper fusion. For ex:-
metals of high which thermal conductivity such as copper andAl.
d) Local preheating which cause initial expansion in one direction so that it will later balance the expansion
and contraction caused by welding for ex:- in repairing fractures in wheels and pulleys
Post heating is done for the following reason.
a) To slow down the cooling rate of the weld there by minimizing the cracking tendencies
b) To reduce the residual stresses in the component for ex:- stress relieving
Carbon arc welding : It is a process in which the fusion of metal is accomplished by the heat of an arc is
struct between the carbon electrode and work piece.
Generally no shielding atmosphere and no pressure is used but filler rod is used only when it is neces-
sary.
• Temperature of weld pool is controlled by changing the arc length, transfer of carbon into the weld is
also minimized bylarge arc length.
• DCSP is preferred to restrict electric disintegration and the amount of carbon going into the weld
metal.
• Electrode point - is ground to the shape shown in fig. If the point is too small it will burn off too quickly
leaving a broad face that will make the arc unstable.
• Applications
1) It is commonly used for welding of Al before inversion of TIG and MIG
2) It may be used for welding copper, Ni, Nonel and other non-ferrous as well as ferrous
metals
3) It can also be used as excellent heating source for brazing, braze welding, soldering and
general purpose heating
4) The process can also be used for repairing castings of CI and steels.
• Advantages
1) Heat input to the WP can be easily controlled by changing arc length
2) Negligible work piece distortion
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3) Process can be easily mechanized
4) Less cost of welding and high speed
5) Process is very much suitable for butt welding fo thinner sheets.
• Disadvantages
1) There is a possibility of transferring carbon to weld metal harder weld deposits in case of
ferrous metals
2) Separate filler rod must be used
3) Arc blow exists because of D.C supply
Twin electrode carbon arc welding : Here power supply will be given to two electrodes only. The heat
transfer in this type of carbon arc welding is basically by radiation from the arc to the work piece and hence
the amount of heat available for the work-piece is very much lower than in the case of ordinary carbon arc
welding.
Tungsten inert gas welding (TIG) : Also called as
TAGS - Tungsten arc gas shielded welding GTAW - Gas tunsten arc weldingAAW -Argon arc welding
The equipment required for TIG are
a) Power supply (DCSP,DCRP and ACHF) b) TIG torch c) Tungsten electrode
d) Gas supply e) Gas regulator along with flow meter
Power supply :
DCRP - The electrode tends to melt off at the end of it and contaminate the weld. Hence for any given
current DCRP required larger dia electrode than DCSP for example a 1.5mm dia tungsten electrode can
handle about 125 amps in DCSP circuit. However if reverse polarity is used with this amount of current the
tip of the electrode will melt off quickly, consequently a 6mm dia electrode will be required to handle 125
amps of welding current is DCRP. The polarity also affects the shape of the weld. The DCRP with its larger
dia electrode and lower current density forms a wide and shallow weld. For this reason DCRP is rarely used
in TIG welding expect for welding Al and Mg. These metals have a heavy oxide coating which is more
readily removed by the cleaning action of DCRP.
DCSP - This is used for the welding of most of metals because better welds are achieved with the heat
concentrated at the plate. The welding process is more rapid, there is less distortion of the base metal and
the the weld puddle is deeper and narrower than with DCRP.
ACHF - withACHF we have better arc starting capability, longer arc lengths and better arc stability than
with normal frequencyAC. In the standardAC cycle 1/2 of the heat is absorbed into the tungsten electrode.
Tungsten electrode : available in 0.5 to 6.4 mm dia 75 to 610 mm long
1. Pure tungsten - M.P 3410oC and provides
a) Desired property of high M.P and less cost
b) Low electrical resistance and good heat conductivity
c) It has low current carrying capacity
e) Used in less important jobs (because it is more prone to contaminate)
2. Thoriated tungsten - M.P 3366oC (Thorium 0.8 to 2.2%) and provides
a) Provides high current carrying capacity
b) Better electron emissivity
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c) Longer lifeand greater resistence to contamination
d) Easy arc initiation and better arc stability.
3. Zirconiated tungsten - M.P 3366oC (Zirconium 0.15 to 0.4%) and provides
a)All the propeties fall between pure tungsten electrode and Thoriated tungsten electrode
Shielding gases for TIG welding : In general there are 3 types of shielding gases are used as Argon,
Helium and mixture ofArgon and He
Argon - 1. More extensively used because it is expensive than He.
2. 1.4 times as heavy as air and 10 times as heavy as He. Hence provide a
blanket over the weld and there is less cloudy during welding and
consequently it permits better control of weld puddle and arc.
3.Argon has less electrical resistance and therefore lower voltages can be
used at any current setting. Therefore there is a smoother and quitter arc action.
It is also advantages in welding thin material.
4. The arc is easier to start than with He. For a given welding speed the
weld produced is narrower with smaller heat affected zone.
Helium -
1) Helium is operated at higher voltages therefore a lower current is possible to get the same arc power,
hence welds can be made at higher speeds. (30 to 40% = faster than argon).
2) He is used for machine welding or in welding heavy metals or metals having high heat conductivity.
Mixture ofAr and He - The required penetration pattern at the given welding speed can be obtained by this
mixture. (20%Ar 80% He)
Material Electrodes Power Supply Shielding Gas
SS Thoriated Tungsten DCSP Argon
Al All Types AC - do -
Mg Pure Tungsten AC - do -
Deoxidized
Copper Monel TT DCSP - do -
H.C steel and Cl TT AC/DCSP - do -
Applications :
1) Originally developed for weldingAl and Mg
2) The other metals are SS, H.C Steels, Cu, Monel, Inconel (Cu +Cr + Fe), Brass, Bronze, Silver,
Molybdenum etc
3) For joining various combinations of dissimilar metals like brazing and braze welding
4) Pipe work required for H.P steam lines, chemical and petroleum industries
5) Welding of air craft frame, jet engine casting, rocket motor casting
6) Precision welding of parts in atomic energy
7) Expansion bellows, transistor cases, instrument diaphrragms etc
Advantage of TIG :
1. TIG welds are stronger and more ductile
2. No danger corrosion due to no flux is used
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3. No past weld cleaning because of no slag
4. Wide variety of joints can be made because no flux is used
5. There is very little or no smoke, fumes or sparks all of which will contribute to making a meter and
sounder weld.
6.As the shielding gas is transparent, operator can clearly observe the weld
7. Fusion welds can be made in merely all commerical metals.
Disadvantage of TIG :
1. Higher equipment cost
2. Under similar conditions MIG is faster because of no separate filler rod
3. Tungsten if transfer to molten weld pool can contaminate weld, and tungsten inclusion is hard and
brittle
MIG welding : Also called GMAW (Gas Metal Arc Welding). It was consumable electrode.
Metal Transfer in MIG
1. Short arc method (or) short circuit transfer method - a standard short circuit metal transfer in MIG
welding m/c occurs at a rate of approximately 100 to 200 short circuiting cycles/sec. The voltage used in this
is usually below 30V. here the penetration can be controlled precisely. So this type of arc is readily useful for
welding light gage metals. It can be used in all the welding positions without affecting weld quality. The heat
affected zone (HAZ) is much smaller than that with spray arc transfer
2. Spray arc transfer - In this both voltage and current densities are much higher and so more heat energy
is injected into the base metal. This process uses an electrode small dia about 1.5 mm at an amperage of 275
amps and voltage of 35 to 40V. the spray arc operates on DCRP. The higher the voltge, the finer the size of
the particles. The distance of filler metal from the base metal determines the effectiveness of the spray arc.
Because of the metal ions and their large mass the penetration qualities of the spray arc are confined to
heavier metals and cannot be used for welding thin sheets. The distance of the electrode from the base metal
determines the success of spray arc, an optimum distance is around 12.5 mm. If the distance is more than
12.5 mm, the arc column becames erratic and the weld bead will difficult to control.
3. Pulsed spray arc mechanism : - It is achieved by pulsing the current back and forth between the spray
transfer and short circiut transfer
The above figure illustrates on the left, the current time relationship for two power sourcesAand B. “A”
puts current in the short circuit transfer range and ‘B’ gives out current in spray transfer range. On the right
side two outputs contain, which produce simple pulsed output by electrically switching back and forth
between them. The metal transfer is restricted to spray type. The short circuit transfer is supported by not
allowing sufficient time for a transfer by this method to occur. For a given electrode by this pulsed spray
method all the advantages of spray transfer process are availble at average current levels.
Power system used in MIG (Arc control in MIG)
1. constant arc voltage - CAV (self adjucting arc)
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2. drooping arc voltage (constant current) - DAV VL ...................................DAV
.....................
VL VN
VN V
V
VS
S
............................................
.....................
V CAV IL IN IS
IL IN IS >
>I I
Shielding gases for MIG :
Argon -
1) Has less ionization potential, arc is more stable and hence no spatter. Because of low ionization poten-
tial voltage is reduced and results in lower power in arc and so lower penetration.
2) Reduced spatter and lower penetration make use of argon desirable for welding sheet metal
3) Straight argon is rarely used for arc shielding except in welding ofAl, Cu, Ni, and Ti.
4) If we weld steel with straight argon gas leads to under cutting and poor bead contour penetration. At
lead edges is shallow and deep at the center. This can lead to lack of fusion at the root of the weld if the arc
is not directed exactly over the center of the weld.
Argon + O2 In order to reduce pool bead contour and improve the penetration pattern with argon no steel
add O2 1 - 2% or 5% most preferably 5% oxygen improves the penetration pattern by broadening the deep
penetration. It also eliminates under cutting used for welding all steels. But the problem with O2 is it leads to
oxide formation.
Helium - It has high ionization potential, it is lighter than air and has high thermal coductivity. It results in0
broader and shallow weld bred. Used for shielding during welding of non-ferrous metals likeAl Mg and Cu.
CO2 -
1) Unlike argon and helium CO2 is made up of molecules, each molecule of CO2 consists of one
carbon and two O2 atoms.
2) At normal temperature CO2 is an inert gas but at high temperature it decomposes into CO and O2.
The result is 20 to 30% of shielding will be O2 , so the electrode wire in MIG should have deoxidizing
elements such as Si, Mn,Al, Ti etc.
3) The oxygen instead of combining with carbon or iron, it combines with deoxidizers because the
deoxidizers have greater affinity to O2
4) CO2 eliminates many undesirable characteristic which are obtained while using argon
5) Broad and deep penetration is obtained. The welding defects like lack of penetration and lack of
fusion are eliminated.
6) The bead contour is good and there is no under cutting.
7) Cost is less when comparedArgon and Helium.
8) The main problem while using CO is that the arc is violent leading to weld spatter.
2
Applications :
1) It is equally effective for welding thin gauge metals as well as heavy structural plates.
2) Most of the metals and alloy can be welded
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3) Used in welding of structures, ship building, pressure vessels, tanks, pipes, domestic equipments,
heavy electrical equipment etc.
Advantages of MIG :
a) It has an all position welding capability
b) It can be easily applied to an extremely wide range of metals both ferrous and non-ferrous metals.
c) It can be easily mechanized.
d) It can deposit large quantity of weld metal in a relatively short period of time.
e) It can be manual or semi automatic or automatic also
f) In semi automatic MIG welding the operators skill required is less than TIG
g) Modern MIG welding is available in the spray arc, short arc and pulsed spray arc
h) Just bychanging electrode wire and shielding gas, many varieties of metals can be welded with same
MIG welding equipment.
Note : The melting rate in terms of various parameters is
Mr = al + bL2
Where Mr = melting rate kg/HR I = welding current, amps L= electrode extension, mm
a, b are constants depends upon wire size and type of power source.
Submerged arc welding (SAW)
It is a consumable electrode type welding, the arc is produced while the consumble electrode is con-
tinuously fed into the weld zone as in MIG, but the welding zone is completely covered by means of large
amount of granulated flux which delivered ahead of the welding electrode by means of welding flux feed
tube. The arc is completely submerged in the flux and not visible from outside. Here some unused flux will
float on the top and this is collected and re used again.
• Saw is used for doing faster welding jobs and it is possible to use larger welding electrodes say 12mm
and very high currents say 4000amps, so that very high metal deposition rates of the order of 20 kg/hr or
more, also with very high welding speeds say 5m/min are possible.
• Some SAW mechines are able to weld plates of thickness as high as 75 mm in butt joints in single pass
even though SAW is used for welding thin sections also, it is economical only when we use if for thicker
sections.
•AC and DC power source are used in SAW, with both constant voltage and constant current type m/
c’s can be effectively used. But for lager electrodes a constant current power supply is preferred. The
current ratings of SAW are 2 to 3 times the MIG m/c’s
• Since the arc is completely sub-merged in the flux, there is no spatter of the molten metal. Since this
process uses loose granulated flux to cover the joint, it is not possible to carry the welding any position
except flat or down hand position.
• With single electrode, no arc blow occur withAC supply but if we go for two electrodes, arc blow is
likely to occur because of the interference of the two magnetic fields surrounding the two electrodes, and if
the two currents in phase. To overcome this problee, the two power supplies are so adjusted that when one
supplies in peak, the other would be set to zero current so that no opposing magnetic fields will be set.
• With two electrodes SAW can be done by
a) Tandem arc welding
Creates a deep narrow weld bead. The welding if faster, used for heavier and thicker plates
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b) Parallel arc welding
The penetration is deep but the weld bead is wide. So used where wide gaps are welded.
c) Series arc welding
Least amount of penetration but permits welding of thin gauge metals at high speed (t<12mm)
• It is a large volume process, it produces large amount of molten weld pool which takes some time to
solidify. So it is required to provide backup in SAW.Different backing methods used in SAW are
a. Flux backing
b. Copper backup plates - When work pieces are in perfect fit up without offset we use smooth
copper backup plate, but the wattage of arc should be increased by 10 to 15% to compensate for the heat
loss through copper plates.
c. Flux - copper backing - Whenever the work pieces cannot be held tightly to the copper backing
plate, this type of arrangement is used.
d. Integrated backing shell - Due to difficulty in edge preparation this method is rarely used. However
on thick walled vessels, pipes, tank bottoms etc, This method is employed.
e. Gas backing
• Application of SAW - The welds made by SAW have high strength and ductility with low H2 and N2
content. It is suitable for welding low alloy steel, high tensile steel, LC and MC steels high resisting steel,
corrosion resistant steel, highs strength steel and many of non-ferrous alloys.
• Advantages of SAW are
a) Smooth welds of highs strength and ductility with low H2 and N2 contents.
b) Because of high current, high metal deposition high welding speeds and good penetration are achieved.
c) Due to high speeds less distortion
d) Elimination of fumes and spatter
e)Absence of visible arc and ease of penetration
• Disadvantages of SAW are
a) Arc is not visible, judging the weld progress is difficult and so accessories like jigs, fixtures and
guides are required
b) Pre-placing of flux may not always possible
c) Process is limited to flat position
d) Flux is subjected to contamination that may cause weld porosity.
e) CI, Al, Mg, Lead and Zn can not be welded
“Resistance welding”
Resistance welding is a group of welding processes where in coalescence is produced by the heat
obtained from resistance of the work to the flow of current in circuit of which the work is a part and by the
application of pressure. No filler metal is needed.
The two important factors to be considered in welding are
a) Generation of heat at the place where the two pieces are to be joined.
b) The application of pressure at the place where the weld should be formed.
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Heat : The heat is obtained by large electrical current of the order of 3000A to 100000A with voltages of
1 to 225 volts.
So H I2 R t
Where H = heat generated I = current passing in the circuit
R = resistance at the interface t = time of current passing
a) Current (I) :
• With other parameters constant the temperature in resistance welding is regulated by controlling the
magnitude of current and time
• Enough current is needed to heat the metal pieces being welded to their plastic state
• Low welding current leads to insufficient fusion
• High welding current results in the entire thickness between the electrodes being heated to plastic
state by the time the weld is reaches to fusion temperature and the electrodes are indented into work
•As the current or current density is increased, the weld time can be decreased sufficiently to produce
a weld without overheating the contact surfaces.
• As the current increases, the nugget dia, breaking load of welded joint, electrode indentation are
increased.
• 3 types of current supplies are used
a) AC - Single phase AC transformer
b) DC - Energy deliver directly from a power line
c) Stored energy system - Such as batteries, electromagnetic system, capacitor system, homo-
polar generator.
b) Resistance (R) :
The total resistance of the system between electrodes consists of
i) Resistance of electrodes “R1”
ii) Contact resistance between eletrode and work “R2””
iii) Resistance of work “R3”
iv) Resistance between laying surfaces of the two work pieces to be welded “R ”
4
In order to acquire a sound weld and to avoid over heating of electrodes R , R and R should by kept as
12 3
low as possible.
R1 - To keep R1 as low possible electrodes are made by high conductivity and low Resistivity materials
such as Cu and its alloys
R2 - It can low minimized by keeping the electrodes clean and smooth, controlling the shape and size of
electrodes etc.
R3 - It depends on the nature of the metal and its thickness, it cannot be changed
R4 - It varies with quality of surface, surfaces that are not clean and poses scale, dust and other contami-
nates on them offered more resistance. Smooth surfaces and high electrode force reduce the value or R4
c) Time : Four definite segments of times are set up on the resistance welding m/c during one cycle.
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• Squeeze time is the time between the initial application of electrode force and the initial application of
current. During this period upper electrode comes in contact with the work-piece and develops full elec-
trode force.At the end of the squeeze time the welding current is applied.
• Weld time is the during which the welding current flows through the circuit, it enters from on electrode,
passes through work pieces and goes out from the second electrode.
• Hold time is the time during which the force acts at the point of welding after the last impulse of
welding current stops. The electrode pressure is maintained until the metal has some what cooled.
• Off time is the time gap between end of hold time and beginning of squeeze for next cycle.
Variations f Resistance welding cycle :
a. Just before weld time the electrode pressure is reduced a little in order to internationally increase the
resistance between the plate surface on the work pieces. This will hepl rise the temperature quickly at the
place of welding.
b. During the hold time, the electrode pressure is increased again in order to forge the weld metal as it
cools, this will improve the weld strenghth
c. Post heat current is applied along with the forging pressure in this cycle. The welding force is usually
maintained until the post heat current is applied, after which it is increased to forging force. Such a cycle is
used mainly for grain refinement in hardenable carbon steels and alloy steels and are not used on low carbon
steels.
Electrode Force : It is the force applied to the work pieces by the electrodes during the weling cycle. The
pressure exerted on the work pieces by the welding electrodes does following.
• It bring the various interfaces to intimate contact and thus affects the contact resistance between work
pieces.
• It ensures the completion of electrical circuit between the electrodes and through the work pieces.
• By its forging action it reduces weld porosity.
If smaller electrode force is used contact resistances between the two work pieces is high and the
surface burning and pitting of the electrode may result.
When the electrode forec is increased very much, the contact resistance of the work metal decreases
at the saying surfaces and therefore reduces the total heat generated between the faying surfaces. Very high
electrode force may also squeeze the softened hot meal between the faying surfaces or the work pieces may
be intended by the electrodes.
Applications of RW :
• Joining sheets, bars and tubes
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• Making tubes for metal furniture and cycles
• Welding aircraft and automobile parts
• Welding cutting tools
• Welding of fuel tanks for cars etc.
Advantages of RW :
• Fast rate of production
• No filler rod is needed
• Process is semi automatic and less skilled worker can do the job
• Similar and dissimilar metals can be welded
• Reliabilityand reproducibilityare high
Disadvantages of RW :
• Initial cost of equipment is high
• Skilled persons are needed for the maintenance of equipement
• Special surface preparation is required in certain cases
• Larger job thickness cannot be welded.
Heat balance in RW :
Heat balance may be defined as a condition in which the fusion zones in the pieces of material to be
joined undergo the same degree of heating.
The problem arises when two diffirent thickness of the same material, or same or different thicknesses
of different materials having different thermal and electrical conductivities are may be spot welded. The heat
balance also affected by
• The relative electrical and thermal conductivities of the materials to be joined
• Relative geometry of the parts t be joined
• Thermal and electrical conductivities of the electrodes
• Geometry of electrodes
Weld spot a pacing : Ideally spot spacing should be such that there is no shunting of welding current
through the previous welding otherwise the nugget size will be reduced for the same nominal current.
With the second and subsequent welds the short circuit provided by the first or previous spot weld
offers an alternative low resistance spot and the welding current is partly diverted or shunted through the
route as shown in fig.
Shunting
>>>
<
The total current is thus shared between the new site and already existing nugget. The relative propor-
tions mainly depend upon the distance between the two points along the interface. General rule is to allow a
distance 16t between the consecutive spot welds where “t” is the thickness of weld material. If distortion is
more important than weld strength, then weld spacing can be increased to 48K.
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When spot welds are to be placed more closely current shunting is bound to take place, for obtaining
the same sized weld nugget, the current must be increased for second and subsequent welds to compensate
for shunting loss.
Metals spot welded :
Ferrous metals - L.C steels, H.S.S bits, S.S coated steels
Non ferrous metals -Al,Al- Mg alloys,Al-Mn alloys
In seam welding the heat at the interface is generated by contact resistance, which can be increased by
decreasing the electrode force or by weld time. The amount of heat generated is controlled by speed of
rotation of electrode disc, The heat generated decreases with increase in speed of rotation.
In early days attempts were made to cool rotating electrode disc byt injections external spary of water.
This external sprayis necessary and difficult to control, therefore modern machines are cooled by refrigerant
fluids that flow inside the copper discs.
If cooling is not sufficient
1. Distortion takes place in work piece
2. The copper alloy used in electrode breaks down because of excessive heat in welding.
Seam welding is applied in welding of metal coatings, gasoline takns, automobile mufflers, refrigerator
cabinets etc., seam welding gives neat design, saving in material, produce leak proof joints and low cost of
construction.
Resistance butt welding : Butt welds in resistance welding can be produced by 3 ways
a) upset butt welding :
The current and pressure are maintained throughout the operation, the pressure being gradually in-
creased upto the value which will forge the two pieces together, Final pressure is 15 to 150Mpa. The joint
is somewhat upset as shown, but this defect can be eliminated by subsequent rolling or grinding.
Areas of application are
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a) For welding small ferrous and non ferrous strips or rods, such as bond saw blades
b) For welding of longitudinal butt joint in tubing and pipe and transverse butt joints in heavysteel rings.
b) Flash butt welding : It is the most popular among the resistance butt welding process.
The work piece held in a movable clamp is brought towards the other stationary work piece held in a
statinoary clamp until the two come in light contact.As the welding current with higher voltage tuned on,
flashing occurs, while in condescend metal particles are being expelled byflashing the movable clamp keeps
on moving constantly towards the stationary one at a careful controlled rate in order to maintain the light
contact. As the flashing continues, the ends of the two work pieces reach higher and higher temp, at then
stage when the welding temp is reached, the pressure of moving clamp is greatly increased to forge the two
parts together and expel molten metal and slag out of joint, there by making a good solid weld. The metal
expelled forms a ragged fin around the joint which is later removed by cutting or grinding.
Note : All metals can be welded by using method but alloys with high percentage of Zn, tin, Cu, Pb are to
be avoided. The flash butt welding consumes much less current but the time allowed for welding is however
is more.
c) Percussion butt welding :
It is a welding process where in a coalescence is produced over the entire area of abusing surfaces by
heat obtained from an arc produced by a rapid discharge of electrical energy with the pressure percussive
(rapidly) applied during or immediately following electrical discharge.
The arcing is by sudden discharge of energy by electro-static or electromagnetic means. The process is
very rapid taking about 0.1 of a second.
Metals welded :
Cu-alloys, Ni-alloys,Al-alloys, L.C steels, M.C steels, SS, dissimilar metals like Cu to Mo, Cu toAl
or SS CI to steel.
NOTE : Since the control of path of an arc is difficult the joint is limited to small areas that is less than 3.5
cm2 area
High frequency resistance welding :
The phenomena of H.F current which carries it to flow at or near the surface of the conductor and not
through the entire thickness makes it possible to resistance weld extremelythin pieces of material, as thin as
10.1 mm.Another phenomena of H.F current is the proximityeffect that is the current flow in the part of low
inductance rather than low resistance which means that the effective resistance of the metal being welding is
much higher with H.F current than the normal 50Hz current. Therefore the amperage requirement for a given
amount of heat release is only a fraction of that needed for conventional RW.
H.F current (450000Hz) is supplied to the two contacts on the base metal as shown in the fig. Because
of the fast cycling the conductor of the current assumes the shape of “V” between the conductors. This
“V”path acts as return conductor for low inductance which causes surfaces of the two pieces of base metal
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to be heated.At the point of “V” there are two rollers which force the metal together which slightly upset the
base metal causing the weld to take place.
Metals welded are Cu and alloys, Ni and alloys,Al and its alloys, many types of steel, all are for 0.1mm
to 0.35mm thick.
Advantages :
a) Joining can be done at high speed about 300 m/min in light gauge metals
b) Power required is only a fraction of that of conventional RW
c) The HAZ is very shallow resulting in welds of high tensile strength, good ductility.
d) Most of the process variables that affect conventional resistance welding such as contact resistance,
contact pressure, surface condition, the amount of squeeze applied to joint etc do not apply in H.F RW.
Black smiths forge welding :
• This is the first form of welding, the process consists of heating the metal in a forge (in black smiths
fire) to white heat and then uniting them by pressure (byhand hammering or power hammering or hydraulic
press)
• The heating is usually done in a coal fired or coke fire forge, although modern installations frequently
employed oil or gas furnaces. The manual process is generally limited light work, because the welding is
accomplished by hand sledge.
• Before the weld is made the pieces are formed to correct shape so that when they are welded. They
united at the center first, and then progresses to outside edges, any oxide or foregign particles forced out.
• Forge welding is rather slow and there is considerable danger of an oxide scale forming on the
surface. Oxidation can be encountered byusing a thick fuel bed and by covering the surfaces with the fluxing
material, which dissolved the oxide borax in combination with salt ammonia is commonly used
• Butt welds lapped welds can be obtained. In the case of material of larger thickness the edges of parts
to be welded are chamfered at wedge shaped filling piece.
• LC steels and wrought iron are recommended for welding this because they have broad temp range
for forge welding. The welding temp range decreases as the carbon content increases.
Cold pressure welding :
This is a process which can be used to join certain similar and dissimilar metals, its feature is complete
absence of heat during welding operation, which eliminates any need for electrical, gas, or chemical heating
equipment. The two parts are to be welded are subjected to pressures, the specific pressure per unit area
being above the flow point of the material. This pressure brings the molecules into intimate contact and
produces surface molecular fusion resulting in a homogeneous weld.
• In making a weld the pressure is applied over the narrow strip so that the metal can flow away from
the weld on both sides. It may be applied either by impact or with slow squeezing action, both being equally
effective.
• Pressure required to weld Al using this method is 200 to 300 MPa.
• Impurities such as layers of oxides or foreign matter on the surface to be welded would prevent
welding and it is there fore essential to have the material carefully cleaned before welding pressure is applied.
Wire brushing (scratch brushing) is superior to chemical cleaning, in case ofAl.
• It is claimed that most of the material can be cold pressure welded, but till not the process has been
successfully applied toAl and Cu. However it can also be used for lead, Ni, Zn Monel etc.
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• There exists for each material a definite value for the amount of deformation under pressure which
results in satisfactorywelds and the optimum ratio between the remaining thickness of material after welding
and the original thickness of material befor welding expressed as a percentage called the figure of merit. The
harder and less ductile, the material to be welded, the higher is the required welding pressure and the lower
the figure of merit.
• Areas of applications are
a) For wlding metals in explosive areas
b) For assembling small transistors where welding heat may cause damage
c) For joining wires where they break during drawing operations.
“Defects in Weld”
Inclusion of Slay
• Inclusion of slag in welding take place due to flux employed and electrode coating. In multlilayer
welding proces. It is impossible to remove slag between layers results inclusion in these zone other causes
are slow speed less electrode angle.
Inadequate Penetration
• It exists in groove weld when the deposited metal and base metal are not fused at root of the weld.
This condition occurs when the root fence has not reached fusion temperature along it’s entire depth. Inad-
equate penetration may lead to cracking.
Incomplete Fusion
• This is defined as the failure of to weld metal layer to fuse together. It is caused by failure to reach the
base metal at it’s melting point.
Under Cut
• It is the melting or burning away of base metal at the toe of the weld, in multilayer weld it may occur
at the puncture of weld layer and the wall of the groove.
Undercut due to excessive weaving speed. big electrode, incorrect electrode angle.
Porosity
• Porosity is caused by entrapment of gas bubbles by the freezing dendrities during the cooling of
molten pad. This may be caused by excessive welding heat. Porosity occurred along the entire weld length
may be due to impurities, rust, grrease or dust, surface of metal excessive moisture in covering of electrode.
Spatter
• Spatter refers to particles of metal scattered around the vicinity of the weld along it’s length. It occurs
(i) excessive current (ii) too high arc voltage (iii) arc blow making arc uncontrollable.
Cracking in Weld Metal
• It mayoccur due to incorrect welding technique, or using a filler material having different rate contrac-
tion compared to parent metal.
Hot Cracking
• Hot cracking is influenced by sulpher and carbon content of mild steel weld metals. Sulpher form with
iron FeS as it remains liquid after the metal has frozen therefore not cohension between the grains and the
weld metal, it is prevented bypresence3 of magenase which form magenes sulphide solidifies slightlyabove
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that of steel.
S - 0.40% wire.
- 0.06% parent metal.
Cold Cracking
• This occurs at a relatively low temperature (considerably below at which steel solidfies).
* Volume changes accompanying transformation from austenite to ferrite/bainite.
* Cooling rate, dependent upon size and temperature of weldment and heat input of welding process a
sudden drop in temperature is sufficient to produce stresses of significant valume.
* Hydrogen : Introduced by the electrode or it’s coating as chemically combined water, damp coating.
• Cracking in HeatAffected Zone . The above remarks have been concerned with weld metal itself but
apply equally to the heat affected zone. This is zone of parent metal which has it’s structure altered but not
completely melted by the heat of the welding process, and into which hydrogen may diffuse from the weld
metal.
• Pinch Effect is the result of electromagnetic forces.
Welding of Different Metals Alloys
Welding of Cast Irons
• Carbon in cast iron present in either free or in combined form, both free on combined carbon undergo
into solution in molten metal. If it is suddenly cooled, large amount of carbon is retained, this results in hard
metal in the weld. Thus to reduce this heating effect preheating of cast iron is done proper attention is
focussed to prevent breakage due to uneven or expansion or contraction of base metal.
• GreyCast Iron: Generally Gas welding is used for it due to lack of ductilily in the material. Preheating
is done about 250oC. filler metal of low melting point; and welding is done by neutral flame and backhand
technique. To prevent chilling effect entire casting is covered bysuitable high temperature insulating material,
Other welding methods are :
* shielded metal arc welding
* thermit welding, brazing.
Welding for Stainless Steel
• Welding of stainless steel can be done by all techniques but when it is heated in the range 470oC to
880oC or cooled slowly through this range. In this range carbon is precipitated from solid solution at grain
boundaries and to form chromium rich carbide. This condition is very much susceptible to corrosion. To
overcome (1) Proper heat treatment (2) Other methods consist in adding the suitable amount of either
titanium or colubiums.
• In welding of stainless steel, problem of warpage distrotion, canbon precipitation and matching up of
the chemical elements may be arise.
• The matching up to chemical elements is essential to ensure that both weld and base metal have same
properties and act on union under load. This can be done by using same filler metal grade as the base metal
to be welded.
• Stainless steel have lower thermal conductivity results in an uneven, distribution of heat and distortion.
This can be taken care of by lowering the heat input by using small weld beads, intermittent or back steping
beds, stringers.
Welding of Aluminium
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• pureAluminium can be easily welded by resistance, atomic hydrogen, welding methods.Aluminium
and it’s alloy have melting point range from 600oC to 700oC. At elevated temperature aluminium forms
aluminium oxide. It is very refractory meterial. It’s melting point is 2750oC. Another difficulty is that it’s
specific gravityis higher than aluminium. If it is not removed. Proper flux should be used to remove it. In arc
welding, DC welding preferred for aluminium, spot welding can be used for welding aluminium due to it’s
heat and electrical conductivity.
• Aluminium alloys are generally available in two forms heat treatable and non heat treatable, Strength in
non heat treatable alloy is developed by alloying elements and strain hardening in annealed alloy is devel-
oped by alloying elements and strain hardning in annealed or soft tempered type, the composition of alloy
wholly controls the strength while in strain hardened type the degree of strain hardening is the factor.
• Oxcetyline welding method, edge preparation, selection of proper welding rod and the flux depends
upon whether the aluminium is in the form of sheets alloy tubind or cast alloy.
Welding of Carbond Steel
• Low Carbon Steel. (Carbon upto 0.30%). These are easily welded by all welding processes and the
resultant weld joints are of extremelyhigh quality.
• Medium Carbon Steel. (from 0.3 to 07%). These can be done by various fusion process. The
techniques and materials used are dictated by the characteristics of the base metal in some instance preheat-
ing and heat treatment may be required to produce desired weld quality especially in steels containing more
than 4% carbon.
• High carbon Steel: (more than 0.7%). These are more difficultly welded. These are welded to very
few applications much as repairing chacked or worn parts of H.C.S.
With proper care gas welding, shielded metal arc welding; submerged arc welding, resistance welding,
pressure gas welding is used.
• Cast Steel. The weld ability of cast steel is comparable to that of wrought iron steel with variations
according to the chemical composition of the steel.Arc welding, gas welding, brazzing and pressure welding
is used.
• Tool steel. (0.80 to 1.50%) it is more difficult to weld.
* Gas welding and pressure gas welding isused.
* Preferably carburising flame shuld be used for strong weld.
* Proper flux is used.
* It is also necessary to preheat the steel before welding and anneal the steel after welding.
* Proper welding rods should be used (contain carbon percentage at least equal to base metal).
Welding of Plastics
• Welding of plastics of thermoplastic type can be welded successfully by using hot gas torch or elec-
trically heated tool. In the hot gas torch a gas, usually air is heated by electrical coil as air passed through it.
Electrical tools are similar to an electric soldering iron and moved in contact with material until desired
temperature is obtained.
• V grooves butt or fillet welds usually are employed. Because the plactic can not be made to flow as
in fusion welding of metals, some filler matiral has to be added.
• The plastic filler rod are used and heated simultaneously with work piece and then work piece are
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composed to form joint.
• But welds are sometimes made by friction welding.
• Heated tool sometimes made by friction welding.
• Heated tool welding is employed for making lap - seem welds in flexible plastic sheets.
Note
• Hard Facing. (Surfacing)- Surfacing is operation of deposition of a layer of metal of one composition
upon the surface or edge of base metal of a different composition.
The objective of hard facing is to improve resistance to wear abrasion, chemical reaction.
• Surface Metals. (1) high alloy steals and irons (2) Copper base alloy (3) carbides (4) cobalt base
alloy (5) Nickel base alloy (6) low alloy steel (7) stainless steel.
• Surfacing Methods. -All surfacing metals can be easily deposited by gas flame and arc welding, arc
welding is used for surfacing of high melting point alloys. Submerged arc welding is used for surfacing large
area.
• Metallizing. - In metallizing surfacing metals are melted and atomized in a torch or gun and then
sprayed over base metal process sometimes called metallizing or metal spraying acetyline flame usually
applied sometimes other gas flame maybe used. Wire of surfacing automaticallyfed in to centre of flame and
simultaneously atomized by compressed air and blown on the base material.
“SPECIAL WELDING PROCESSESS”
Electro slag welding :
During welding
Molten metal - 50mm deep
Molten slag - 30 to 40mm deep
Temp of slag bath - 2000oC
This process which was developed in Russia used a continuously fed current carrying wire which dips
into a pool of molten slag. The resistance of the slag carrying current to the earthed work piece generates
heat which melts the wire and the related parts of the work piece.
All welds are done in a vertical position. The plates or slabs to be welded are aligned together with a
gap between them. Run on plates of equivalent. The plates or slabs to be welded are aligned together with
a gap between them. Run on plates of equivalent thickness to the work pieces are fixed at the beginning of
welding. Movable water cooled copper shoes were clamped on either side of weld gap overlapping the
plate edges, to form an open box which will contain the molten slag and the molten metal. The height of the
shoes is just in excess of the combined depths of the molten salg and molten metal.
A moving electrode is introduced into the box as it strikes an arc in the base of the box, some granu-
lated flux is added, the intial heat of the arc is used to melt the flux and as it becomes molten, it put off the arc.
Now the electro-slag melting of the electrode commences that is the melting of the electrode wire is due to
the heat developed by the resistance offered by the molten slag. To the flow of current.As the wire continu-
ous to melt and further quantities of flux are added. A provision is made for the copper shoes to slide up
slowly as the metal solidifies the weld thus continues upwards until it is finished off on run off plates which
allow for shrinkage continues in the weld metal at the ends.
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The electrode is made to oscillate back and fourth across the weld so that weld penetration into the
palate is equal. For larger thickness multiple electrodes can be used.
• Power source - about 1000A with 100% duty cycle
DC - constant potential for feeding one or two electrodes
AC - 3 phase for feeding one electrode per phase
• Electrode wire - It is solid wire 3mm copper coated, each electrode deposits at the rate of 15 to 20
kg/hr. some times tubular wire with flux and alloying elements inside tube.
• Flux -Additional flux at the rate of 5kg/every 100kg of metal deposited. The type of slag depends on
the parent metal for example in welding of MS the following compositions of flux is suitable.
SiO2 (33 - 36), Al2 O3 (11 - 13), MnO (21-26), CaO (4 - 7), MgO (5 - 7), FeO (1.5 max), CaF2
• Applications : Used for welding heavy pates forgings, castings etc of metal like
a) L.C and M.C steels
b) Medium alloy steels
c) High strength carbon steels
d) Alloy steels such as SS
e) CI, and titanium etc.
• Advantages :
a) Minimum of flux consumption that is roughly 5% of the metal deposited
b) Less electrical energy per kg of deposited metal, than submerged arc process or shield arc process.
c) Welding in single pass irrespective of the thickness up to 75mm thick single eletrode, For unlimited
thickness multiple eletrodes are used
d) Uniform heating of the weld area resulting in minimum of distortion and residual stresses
e) Good welding speed say 40 to 75 mm/min, though the speed apparently looks low, it is to be
remembered that the no. of passes is only one. In effect the welding spped is high
f ) The loss of by spatter and vulcanization is low
g) The porosity is low
h) The min edge preparation
i) The flux consumption results in the chemical composition of the welding metal to be more consistent
than in arc welding.
• Disadvantages :
a) The SAW is more economical than eletro slag weling for joints below 60mm
b) In eletro slag welding there is some tendency towards hot cracking in the HAZ
c) It is difficult to close cylindrical shells
d) The process tends to produce large grains.
Electro gas welding : It is similar to electro slag welding but the heat required for welding is obtained
maintaining a continuous arc between the eletrode and molten weld pool during welding, but in electro slag
welding this heat is obtained through the electrical resistance of molten slag. Also in eletro gas welding a
shielding gas such as CO2. Shields the molten weld pool from oxidation. Only DC (DCRP) can be used in
this process. The process is most suitable for joining paltes in thickness range of 12.5 to 75mm. For t>75mm,
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electro slag welding is usually more practical because in eletro gas welding it may be difficult to obtain
adequate shielding gas coverage. This can lead to defects in the weld deposit.
Electron beam welding :
filament - W or Ta, which is heated to 2600oK by electrical means. Discharges
about 6 x 108 electrons per second per cm2 of filament area.
Vacuum - 10-4 to 10-2 torr.
Work - joint in a straight line rectangle or a curve. The work table is actuated on x
and y axes by NC
High voltage equipment - 70 KV to 150KV
Low voltage equipment - 15 KV to 30 KV
The system can be self accelerated or work accelerated. In self accelerated system the gun has a separate
anode. Where as in work accelerated system the work piece becomes the anode.
When the beam strikes the welding point K.E of the electrons is converted into heat energy. (The
velocity of electrons is as high as half the speed of light). The process variables are
a)Accelerating voltage
b) Beam current (current is in order of milli-amps)
c) Welding speed
d) Distance from gun to work
The penetration increases when accelerating voltage or beam current is increased or when the welding
speed is decreased. The product of accelerating votalge and beam current determine the amount of heat
input which decides the spot size. The gun to work distance determines the beam spot size on the work.
Spot size = 0.025 to 6.25 mm
In the beginning the electron beam penetrates a layer of metal 0.0025 mm thick as the beam travels
further down the eletrodes are acttered slowed down and stopped by collision of stoms of the crystalline
structure. The result is heating in pear shaped pattern.
This process can be applied to nearly every area where TIG welding is used.
Advantages of EBW :
1. The welds are nearly straight sided, narrow and deep
2. Welds can be done with a rapid but low energy input. This reduces work piece distortion and combines
the HAZ t norrow limits
3. The weld size and location can be controlled producing welds with better metallurgical properties
4. There is no contamination of welds.
Disadvantages of EBW :
1. The work must be done in vacuum chamber. Therefore work piece must be small enough to fit into the
chamber
2. As the work piece is inside vacuum chamber it is in accessible and hence manipulation should be done
by special devices like NC.
3. High initial cost
4. Some metals like Zinc and lead cannot be welded.
5. metals likeAl and Mg means pulsing procedure
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Plasma welding and cutting :
1. Arc plasma is the temporary state of a gas. The gas gets ionized after passage of electric current
through it and it becomes a conductor of eletricity. In ionized state the gas atoms brake into negatively
charged eletrons and +vely charged ions and the system contains a mixture of ions, eletrons and highly
excited atoms. The degree of ionization may be between 1% and greater than 100%. That is double and
triple degrees of ionization may be there such stages exists as more and more no. of eletrons are pulled from
their orbits.
The energy of the plasma jet and thus the temp depends on the electrical power employed to create the
arc plasma.
Plasma jets - temp capability of - 8000 to 25000oC
Oxyacetylene flame - temp capability of - 3200oC
Eletric arc - temp capability of - 6000oC
Practicallyall welding arc are partially ionized plasmas, but one in plasma arc welding is a restricted plasma.
Definition of PAW : It is an arc welding process where in “coalescence” is produced by the heat obtained
from a restricted arc setup between a tungsten or alloy tungsten eletrode and the job (transferred arc.) The
process employs two inert gases, one forms the arc plasma and the second does the work of shielding. Filler
metal may or may not be used. Pressure is not employed. Non transferred arc is used for welding.
Plasma cutting :
Used for cutting of carbon steels,Al (up to 125 mm thick), SS (up to 50mm thick without any loss of
corrosion resistance), Monel, Inconel, and many other metals which are hard to cut.
Areas of applications are risers, man holes in tank shells, heavy bulk items not possible by shears or
saws for cutting intricate contours.
Key hole concept :
The actual process of welding with plasma jet is done with what is called the key whole concept
method. The jet columm melts a small hole through the material that is to be welded.As the torch progresses
along the material the hole also progresses, however it is filled up bythe molten metal as the torch progresses.
This method ensures 100% penetration.
Correct orifice diameter and right selection of current and travel speed create a forceful plasma jet to
penetrate completely through the work piece, resulting in the key hole effect. The key hole effect results in
1. ability to penetrate through relatively thick pieces
2. uniform under bead
3. ratio of depth of penetration to width of weld is high.
Thermit welding :
It is two types : 1) fusion Thermit welding and 2) pressure Thermit welding.
Fusion Thermit welding :
The thermo chemical reaction based on the ignition of Thermit mixture is
metal oxide + Al (powder) Al2O3 + metal + heat ‘
Though the metal oxide used in Themit welding is usuallyiron oxide, however oxides of copper, Nickel,
Chromium, Mn can also be employed the chemical reactions and the corresponding theoretical temps at-
tained are given below :
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i) with iron 9Fe + 4Al2O3 + 3010 KJ/mole (3090oC)
3Fe3O4 + 8Al 2Fe + Al2O3 + 3010 KJ/mole (2960oC)
Fe3O4 + 2Al 3Fe + 4Al2O3 + 783 KJ/mole (2500oC)
3FeO + 2Al
ii) with copper 6Cu + Al2O3 + 1089 KJ/mole (3140oC)
3Cu2O + 2Al 3Cu + Al2O3 + 1152 KJ/mole (4865oC)
3CuO + 2Al
iii) with Nickel 3Ni + Al2O3 + 864 KJ/mole (3170oC)
3NiO + 2Al
iv) with chromium 2Cr + Al2O3 + 2287 KJ/mole (2975oC)
Cr2O + 2Al
v) with Mn
2MnO + 2Al 3Mn + Al2O3 + 1685 KJ/mole (2425oC)
3MnO2 + 4Al 3Mn + 2Al2O3 + 4356 KJ/mole (4990oC)
From first equations of Thermit welding of steel, contains 3 parts by wt of iron oxide and I part ofAl
power, when ignited it produces a theoretical temp of 3090oC and is unsuitable as Al vaporizes at 2500oC.
It is therefore necessary to reduce the temp to a working limit of 2480oC by adding non reacting constituents
like steel scrap. However the molten metal temp must not be lower below 2100oC as Al2O3 solidifies at
about 2040oC.Alloying elements in the form of Ferro alloys may be added to the Thermit mixture to get the
desired composition of molten metal.
However in calculating the weight of Thermit miture only the weight of iron oxide andAl powder are
considered.Additions made for adjustment of composition and temp are not included in the calulation of the
mixture. Thus a 50 kg of metal oxide andAl powder mixture to which 12 kgs of metals and alloys are added
is termed as 50 kg of Thermit.
The Thermit reaction is non-explosive and is completed less than 2 min irrespective of quantity in-
volved.
Apart from the high purity of the Thermit metal the presence ofAl strongly promotes rapid nucleation
and small grain size. However the max amount ofAl is steel is restricted to 0.7%
Pressure Thermit welding :
The process uses Thermit reaction only as a heat source. The Thermit liquid is poured from the top of
the crucible therefore slag enter the mould first, putting a protective coating of slag around the joint to be
welded which prevents the iron in the Thermit solution from fusing with the work piece. When the work
pieces are thus heated sufficiently the required force is applied to effect a butt weld.
Stud welding :
Stud welding is an interesting application of metal arc welding for welding studs to the plate. It
incorporatesa method of drawing an arc between a stud or a rod and the surface of the metal thus the molten
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surface are brought into contact with each other under pressure. The equipment required for stud welding
consists of stud welding gun, a device to control the time of current flow, a source of DC welding current (up
to 600A), studs and ferrules. The time interval will depend upon the size of the stud to be welded and is
usually less the one second.
The ferrule made of force Len perform the following functions.
1) Concentrate the heat of the arc in the weld zone
2)Act as dam confining molten metal to an area
3) Protect the operator from harmful effects of the arc
4) Prevent over heating of the base metal
5) Protect the weld puddle from contamination.
Stud welding is two types :
1) Drawn arc type - explained above 2) Capacitor discharge type
In the capacitor discharge stud welding the stud usually less than 6 mm dia has a small pip on the weld
end and with this in contact with the late a group of condensers is discharged through it. The pip is vaporized
and on arc is established which heats the base metal and end of stud. The spring loaded stud is pushed into
the weld pool completing the weld. The weld pool is extremely shallow and the solidification almost instan-
taneous ferrous and non ferrous steel can be attached by this process. For Al usually to have an argon gas
shielding.
The current requirement is high that is on the order of 5000A but the duration of the arc is usually less
than 5 seconds which results in low heat input giving veryshallow penetration. The base metal is in effect not
over heated and hence the process is frequently used to sttach studs to thin gauge metal (As thin as 0.45
mm) backed PVC or paint, without disturbing backing.
******************
“You make a living by what you get,
you make a life by what you give”
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4 METAL FORMING PROCESSES
Metal forming is a process in which the desired and shape are obtained through the plastic deformation
of a material. The stresses induced during the process are greater than yield and less than fracture. The type
of loading may be tensile, compressive, bending, shearing or combination.
- This process is very economical of no loss of material
- Some time strength also can be improved through strain hardening
The forming process can be divided into
i) cold forming ii) hot forming
- If the working temperature is above recrystallization temperature, it is termed as hot working and those
below are termed as cold forming or cold working.
- Under the action of heat and force, when the stons reach a certain higher energy level, the new crystals
start forming which is termed as recrystallisation.
- Recrystallisation temperatureis the approximateminimum temperature at which complete recrystallisation
of a cold worked metal occur with in a specified time.
- R. C. T will be generally 1/3 to ½ of M.P. of most of metals.
- R. C. T also depends on the amount of cold work a metal has already received. Higher the cold work,
lower will be RCT.
- For some of the metals like lead and Tin RCT is below room temperature, Cadmium and Zinc RCT is
room temperature.
- In hot working, the temperature at which the working is completed is important since any extra heat left
after working will aid in grain grouth thus giving poor mechanical properties.
Advantage of Hot working are :
1. Any amount of work can be imparted because no strain hardening.
2. No limit on working because at high temperature material is more ductile and also brittle material also
can be hot worked.
3. Due to reduction in shear stress at higher temperature, hot working requires much less force for
achieving necessary deformation.
4. Very favorable grain size can be achieved.
5. Coefficient of friction for cold working is 0.1 , but for hot working it is 0.6.
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Disadvantages :
1. Some metals can be hot worked due to their brittleness at high temperature.
2. Surface finish is poor due to formation of scales.
3. No dimensional stability
4. Handling and maintaining of hot metal is difficult
The Typical forming process are :
i) Rolling ii) Forging iii) Drawing
iv) deep drawing v) Bending vi) Extrusion
vii) Punching and blanking - actually it is not forming processbut due to similarity it also comes under this.
Rolling :-
It is a process where the metal is compressed between two rotating rolls for reducing its CS area.
It is most widely section etc.
- Rolling is a hot working process unless specifically mentioned as cold rolling.
- In rolling metal is taken into rolls by friction.
- Rougher rollers will achieve greater reduction than smoother rolls, but rolled metal also be rough
The reduction that would be achieved with a given set of rolls is designated as the angle of bite.
- Volume flow of metal remains scene before rolling and so velocity of billet after rolling is higher than
before rolling
- When the metal enters the rolls the furface speed fo rolls is higher than incoming metal, but metal
velocity at exist is higher than that of surfacespeed of rolls
- Between the entrance and exit is velocity of metal is continuously changing, but roll velocity remains
constant. Somewhere in the contact length the velocities of metal and rolls are same which is called neutral
plane.
- Typical value of angle of bite are
Cod rolling with lubrication 30 to 40
Hot rolling of tubes 12 to 14
Hot rolling of sheets 15 to 22
Hot rollingwith rough rolls 27 to 34
- Pressure on rolls increases form inlet to neutral point, maximum at neutral point and then dereases till it
reaches exit
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- Average roll pressure is a function of contact lengh, so roll separating force can be reduced by reducing
contact length.
- Contact length can be reduced by roll diameter, because smaller rollers will have less contact length
than larger rolls for the same reduction.
- So smaller rollers are used for large reductions and cold rolling
- The smaller rollers are less rigid to withstand for roll separating force, so back up rolls are attached to
provide necessary rigidity.
Roll stand arrangements :
- Most commonly used (2 - high non-reversible)
- 2 high reversible - more expensive for reversing direction
- 3 - high - used for two continuous process
- 4-high- similar to2-high, but alter two roller acts back up rollers
- Cluster - similar to 4-high, but better back up is achieved
- Planetary rolling mill - used for achieving large reduction.
ANALYSIS OF ROLLING PROCESS
a) Anlge of bite : - The metal contacts each of the rolls along the arc AE which is known as the arc of
contact.
The angle “” between the entrance plane and the centre line of rolls is called angle of bite.
- Materials of rolls, and the bar being rolled.
- Roughness of their surfaces
- Rolling temperature and speed
- = 24 to 30o in rolling heavy billets
- = 15 to 20o hot rolling sheets and strip stock
- = 2 to 10o in cold rolling oilet and strip.
Form the figure
CD = Neutral plane ACDB = Lagging zone (V strip < V )rolls
CDFE = Leading zone (V strip < V )rolls
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H = Draft = H0 - H1 = 2 . FL
= D (1 - Cos )
Where R = Roll redius D = Roll diameter
Cos = 1 - (H / D )
b) Length of deformation zone
From the OAE,
l = R Sin = length of deformation Zone
= R (1 - Cos2 ) 1/2
= R [1 - 1 - (H / D )2 ] 1/2
_____________
= RH - (H/2)2
but <<<< RH, and so neglected
1 = RH
Also Tan 1 H)
=R-----(---H--/-2) = -R-----(--H--/-2-)--
Since R >>>> H/2
Tan = RH H H
---------- = -------- = ------
R
R R
c) Velocities
Let V0 = Entrance speed of strip ;
V1 = Speed at which strip is delivered
V0 < V < V1
Forward slip = (V1 - V) / V
Forward slip can be increases with
- Increase in radius of roll
- decrease in thickness of bar
- Increases of coefficient of friction
Backward slip = (V - V0) / V
The point on the path of metal deformation where velocity of strip is equal to velocity of Roll, the
corresponding point is called no slip point (or) neutral plane
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d) Forces acting during Rolling :
Two forces will act at the entry of the metal as
N = normal (or) radial forces T = Tangential force
=Angle of friction
T = . N
= Coefficient of friction
Rx = T Cos - N Sin 0
For the biting (strip is pushed into roll opening) to take place when Rx 0
Rx = T Cos N Sin 0
Tan Tan i.e,
Therefore rolls bite a strip only when the angle of friction is greater than the angle of biting.
When
, it gives the maximum possible angle of bite, i.e is maximum
H = R = max. Possible reduction
Note :- Max. draft condition is called Coulomb friction condition.
e) Neutral plane
Let n = constant = 2 1 / H
Pressure on the surfaece of lagging zone is
Px = (Y /n) [(n-1) (H0 / Hx) n + 1 ] - (H0 / Hx)n .
Pressure on the surface of leading zone
Px = (Y /n) [(n+1) (Hx / H1)n - 1] - (Hx/H1)n
2
Where =back tension
1
= front tension
1
Hx = thickness at neutral section.
Backward slip = [(V - V0 ) / V] = 1 - (V0 / V) 1 - (Hx / H0)
Forward slip = [(V1 - V ) / V] = (V1 / V) - 1= (Hx / H1 - 1
Max. pressure = (Px) max = (Y /n) [(n - 1) (H0 / Hx)n + 1 ]
Roll separating force = P = (2 / 3) Y . w . 1 . [1 + ( 1/4 H)]
Where
Y = yield stress (or) flow stress of work
W = width of the strip being rolled
L = length of deformation zone = R H = R
b
H = average thickness of sheet = (H0 + H1) / 2
Specific roll pressure P = (P / Contact area) = (P / W.1)
Roll separating force can be reduced by
- using small roll diameter
- Lower friction
- Higher work piece temperature
- Small bites
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- By applying front tension and back tension
Angle of nip = 2 x max. angle of bite
Rolling torque on each roll = T = P.a
Where P = Roll separating force
a = moment arm = 1
= arm factor = 0.3 to 0.4
l = length of defomation zone = R H
Power required to drive a roll = (T / 1000) ........... KW
Total power required = 2 x (T / 1000) .............. KW
Power loss in bearing = PL = P db .
b
Where b = Coefficient friction in bearing
db = diameter of bearing
angular speed roller
P = roll separating force.
Drawing (Wire and tube)
In this process rod or tube is pulled through a dia. resulting in reduction on cross sectional area.
Drawing is usually in the cold state and is applied to
i) non ferrous metals and alloys ii) Steels and alloys steels
In drawing process the dia. of hole is divided into four zones as
a) deforming zone (I)
b) Lubrication zone (II)
c) Sizing zone (III)
d) Out let zone (IV)
The sizing zone is usually of a cylindrical form and other zones are conical. The taper angle “” of the
deforming zone varies between 100 to 240 depending on type of work, and metal being drawn
The commonly used lubricants in drawing are
- Mineral and vegetable oils and their emulsions
- Graphite
- Soap solution.
Let D0 = Intial diameter of rod = 2 R0 D1 = final diameter of rod = 1 R1
A0 = (/4) (D0)2 = cross section of rod before drawing
A1 = (/4) (D1)2 = cross section of rod after drawing
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K = coeffcient of elongation = (A0 /A1) = 1.25 to 1.3
J = % reduction in area = (A0 /A1) A0 = 30 to 35%
= half diameter angle = 6 to 240 , = half the angle of entry = 36 to 400
The variables which will affect drawing process are :
- Work material propertied
- die angle
- lubrication
- die material
- final size of rod
* Die materials can be alloy steels, carbides and diamonds
* Drawing speed ranges :
- 9 mpm for largest diameter rod
- 90 mpm for small rods and coils
- 1500 mpm for very fine wires
Drawing stress = ( 1+B ) / B [1 - (R / R )2B ] + (R / R0)
Y 1 0 1
Where B =Cot
= back tension
For the case of strip drawing, the expression is
= (1 + B) / B [1 - h1 h0)B ] + (h1 / h0)B
y
h1 = intial depth of strip ; h2 = final depth of strip
(y
Total drawing stress = y - - 2 ) e -2uL/R 1 = t
Where = coefficient of friction L = length of die land
Total Drawing load = . R12
Die pressure = Px = y - x
= ( 1 + B ) / B [1 - (Rx / R0)2B ] + (Rx / R0)2B
2 y 1
Max. draft : It is limited by the strength of the deformed product. The exit end of the drawn rod will
fracture at the die exit when / = 1 (if there is no strain hardening)
2y
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With zero back stress
= (1 + B) / B [1 - (R1 / R0)2B ] + 0
2 y
2 = y = 1 = ( 1 + B ) / B [1 - (R1 / R0)2B ]
1 = 1 + B / B [1 - (1 - J)B ] and B =Cot
From the above equation the value of “J” can be calulated which gives max. reduction
For example
= 0.1 , and = 60
B = 0.1 Cot 60 = 0.9515
On substitution, J = 0.505, i.e with = 0.1, = 60 the max possible reduction is 50.5%
Note : harder the metal to be drawn, smaller the die angle, for ex :
= 240 = for AL
= 120 = for Cu
= 60 = for steel
Tube drawing : In tube drawing in general the reduction in internal diameter is less, but only thickness of
pipe is reduced
Let h0 =initial thickness h1 = final thickness
drawing stress = 2 = y (1 + B) / B [1 - (h1 / h0)B ]
where B = 1+ 2/ (tan - tan
For cylindrical plug = 0
B = 1+ 2Cot
1= coefficient of friction between die and pipe
2= coefficient of friction between pipe and plug
In plug drawing, the friction drag acts in the backward direction (i.e towards the die entrance)
both at die tube interface and tube plug interface. But in tube drawing with moving mandrel interface get
reversed because while the tube is getting elongated, the mandrel remains un-deformed.
In tube drawing with moving mandrel
B =1+ 2/ (tan - tan )
If the mandrel is cylindrical, = 0
B = 1+ tan
2
1=
if 2 then B = 0, by taking equilibrium condition is basic equation
drawing stress = = log10 [1 / (h0 / h1) - 1] .
2 y
For max. draft, similar to wire drawing 2 / y = 1 (non work hardening)
(1 + B) / B [1 - h1 / h0)B] = 1
Let 12
12tan = 0.1 / 0.268 = 0.373
On substitution
(h / h ) = 0.4275 = 42.75%
1 0 max
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Extrusion = It is the process of forcing metal enclosed in a container through the opening of a die. In its
CS the extruded metal acquires the contour and dimensions of the die opening.
Extrusion is more widely used in fabrication non-ferrous metals and their alloys. However steel and other
ferrous alloys can also be successfully processed.
The main advantage of extrusion is that high compressive stresses are set up in the billet due to its
reaction with the container and the die. These stresses are effective in reducing the cracking of materials
during primary breakdown form the billet. Due this large reductions are possible and also the difficult
metals like, SS, Nickel based alloys, high temperature materials can be extruded.
Extrusion is generally considered as a closed die forging, the difference is that the main body of
the metal is the product and flash is cut awayand almost no slug remaining but in extrusion the flash itself
is the product and there is a slug remaining in the die.
There are two types of extrusion process:
1. Direct extrusion
2. Indrect extrusion
In the direct extrusion, the heated metal billet comes out from the die opening as the ram moves.
In this billet has to move w.r.t container, and so lot of frictional forces will comes into picture, which are to
be over come by the ram force, But in the indirect extrusion there is no it is going back in the hallow ram.
Also in the indirect extrusion the force is independent of length of billet.
* Force in indirect extrusion is 25 to 30% less than direct extrusion.
* A long hallow ram is required in indirect extrusion, which limits the loads can applied.
* To reduce frictonal force, a hydraulic press is used, so that it can also act as lubricant.
* The pressures used in hydrostatic extrusion are 1100 to 3150 Mpa.
* The commonly used fluids in hydrostatic extrusion are Glycerne, Ethyleglycol, SAE 30, Castor oil
etc.
* Applications are-making wires of less ductile materials, and extrusion of nuclear reactor fuel rods.
* The cold impact extrusion is used to produce short length of hallow shapes such as collaspsible
tooth paste tubes, however it is limited to soft and ductile materials likeAl, tin, lead etc.
* Hollow work pieces can be produced by attaching a mandrel to the end of the ram in forward
extrusion and also known as Hooker extrusion.
* Extrusion ratio is the ratio of billet area to product area which is 40 in case of steel and 400in
case of Al.
Let AC = cross sectional area of container, A = cross section area of die opening Coefficient of
elongation or Extrusion Ratio = K =A0 A1= 8 to 50 (in general)
Reduction of metal = ( A0 A1) / A0
The process is similar to wire drawing without any back tension.
Extrusion pressure = x0 = y (1 + B) [1- (R0 / R1)2B]
B
= y (1 + B) [1- (A0 / A1)2B]
B
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= y (1 + B) [1- KB]
B
For a flat strip K=h /h
01
Where h0 = initial thikcness of strip
h1 = final thickness of strip
To account for the container friction.
Let Pf = ream pressure required by container friction.
= uniform interface shear stress between billet and container wall
i
Pf D2 D L, Pf = 4i L / D
i
where L= length of billet in container, D = inernal diameter of container
Total extrusion pressure = pt = 4 + pf
x0
Extrusion load = pt x R02
Forging: Forging is the process of shaping heated metal by the application sudden blows or steady
pressure and by making use of charaterisitc of plasticity of the material.
Forgion process can be classified as
1. Smith forging
a) Hand forging
b) Power forging
i) power forging
ii) press forging
2. Impression die or closed die forging
a) drop forging
b) machine forging
c) press forging
Bar foging: This process is used to reduce an ingot heated at around 13000 c into a bar. Two flat dies
placed opposite to each other squeeze the metal to reduce its thickness. In this ta / t2 should be less than
1.3.
Forging force = 2 b w [1 + (b / 4 t22)]
s
where b = width of die;
w = width of work
= shear flow stress
s
t2 = final thickness
t1 = initial thickness
2s [1 + (b / 4t22)] = average pressure under the dies
* In order to avoid distortion w / t 1.5
1
* In order that the billet is properly squeezed t1 b 3.
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MECHANICAL ENGINEERING
Extrusion : It is the process of forcing metal enclosed in a container through the opening of a die. In its
CS the extruced metal acquires the contour and dimensions of the die opening. Extrusion is more widely
used in fabricating non-ferrous metals and their alloys. However steel and other ferrous alloys can also be
successfully processed.
The main advantage of extrusion is that high compressive stresses are set up in the billet due to its
reaction with the container and the die. These stresses are effective in redcuing the cracking of materials
during primary breakdown from the billet. Due to this large reductions are possible and also the difficult
metals like, SS, Nickel based alloys, high temperature materials can be extruded.
Extrusion is generally considered as a closed die forging, the difference is that the main body of
the metal is the product and flash is cut awayand almost no slug remaining but in extrusion the flash itself
if the product and there is a slug remaining in the die.
There are two types of extrusion process :
1. Direct extrsion
2. Indirect extrusion
In the direct extrusion, the heated metal billet comes out from the die opening as the ram moves.
In this billet has to move w.r.t. container, and so lot of frictional forces will comes into picture, which are
to be over come by the ram force. But in the indirect extrusion there is no frictional force between con-
tainer and billet because extruded metal is not moving in the die, but it is going back in the hallow ram.
Also in the indirect extrusion the force is independent of lengh of billet.
* Force in indirect extrusion is 25 to 30% less than direct extrusion.
*Along hallow ram is required in indirect extrusion, which limits the loadsd can applied.
* To reduce frictional forces, a hydraulic press is used, so that it can also act as lubricant.
* The pressures used in hydrostatic extrusion are 1100 to 3150 MPa.
* The commonly used fluids in hydrostatic extrusion are Glycerine, Ethyleglycol, SAE 30, Castor
oil etc.
* Hollow work pieces can be produced by attaching a mandrel to the end of the ram in forward
extrusion and also known as Hooker extrusion.
* Extrusion ratio is the ratio of billet area to product area which is 40 in case of steel and 400 in
case of A1.
Let AC = cross sectional area of container, A = cross sectional area of die opening Coefficient of
elongation or Extrusion Ratio = K = A0/A1 = 8 to 50 (in general)
Reduction of metal = = (A0 - A1) / A0
The process is similar to wire drawing without any back tension
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Extrusion pressure = x0 = y (1+B) [1 - R0/R1)2B]
B
= y (1+B) [1 - (A0/R1)2B]
B
= y (1+B) [1 - KB]
B
For a flat strip K = h0/h1
Where h0 = intial thickness of strip
h1 = final thickness of strip
To account for the container friction
Let pf = ream pressure required by container friction
i = uniform interface shear stress between billet and container wall
pf /4D2 = D L, pf = L/D
i i
where L= length of billet in container, D = internal diameter of container
Total extrusion pressure = pt = x0 + pf
Extrusion load = pt x R02
**************************
“Success is not measured by what one brings,
but rather by what one leaves”
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MECHANICAL ENGINEERING
5 METAL CUTTING THEORY
ANALYSIS OF ORTHOGONAL MACHINING
(USING SINGLE POINT CUTTING TOOL-)- - <
Tool
Let shear - - -<> --
= b (ASA)
= 0 (ORS) plane t 1 (--- < t2
t1 = Uncut chip thickness - - - ------ - >
t = Chip thickness
- - - 0 B
2
90 - ---
--------
A
work piece
= Shear angle
(Angle made by shear plane w.r.t. cutting velocity vector
Basic assumptions are :
1. Material of machining operation is always incompressible
Volume before cut = volume after cut
t1.b1.L1 = t2.L2b2,
where L1, L2 = length of chip before and after machining
b1, b2 = width of chip before & after machining
2. Width of chip remains constant
i.e. b1 = b2 (plane strain condition)
From AOP sin = t1/OP (1)
From POB sin(90 - + ) t /OP (2)
2
Divide equation (2) by equation (1), we get
t2 = sin(90 - + ) , t2 = cos( - )
t1 sin t1 sin
t2 = cos. cos +sin. sin , t2 = cot. cos +sin
t sin t1
1
t2 - sincot. cos , t2 - sin cos
t1 t1 tan
tan cos , tan cos (3)
- sin
t2 - sin
t1
where = chip reduction coefficient
= t2 / t1
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from above equation, it is clear that with increase in , shear angle decreases and vice-versa.
Larger value of means more thickening that is, more effort in terms of forces / energy is required
to accomplish the machining work. Therefore it is always desirable to reduce the value of t2 or without
sacrificing productivity.
The chip thickness (t2) becomes larger than the uncut chip thickness (t1). The reason can be attrib-
uted to the following :
1. Compression of the chip ahead of the tool.
2. Lamellar sliding of chip segments.
t2
= t1 > 1 (because t2 > t1 )
therefore
chip thicke1nin=g is also often expressed by the reciprocal /ofcutatisng ratio
r = < 1 , where r = chip thickness ratio
can be expressed aslo by the ratio of
1. Total length of the chip before cut (L1) and after cut (L2)
2. Cutting velocity Vc and chip velocity Vf
we have
t1.b1.L1 = t2.b2.L2
t1.L1 = t2.L2 [since b1 = b2]
= t2 = L1 t2 > 1 , thus L1 >1
t1 L2 t1 L2
L1 > L2
Again considering unchanged material flow rate
(t1.b1)Vc = (t2.b2)Vf
t1.Vc = t2.Vf Vc
t2 Vc Vf
= t1 = Vf > 1 , Vc > Vf
chip velocity Vf will be lesser than the cutting velocity Vc and the ratio would be equal to the cutting
ratio Vf t1 L1
Vc t2 L2
r = = = =
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Equation (3) can be written as
tan r cos
1 - r sin
from above expression it is clear that shear angle is a function of cutting ratio and rake angle i.e.
= tan-1 r cos
1 - r sin
, as r
, r cos , ( 1 - r sin )
As , cos less and sin more, therefore
Higher value of indicates higher machinability.
Chip reduction coefficient () b1 >
t2 > >
t1 >
= , t1 = f cos
s
m
t2 ( Vf
(
An elementary mass m is assumed to be t1 R (
travelling with the chip in a circular path. VR
The velocity of that mass gradually decreases t (
from Vc to Vf due to friction Force F. 2
Therefore
Vm
F = -m.a c
F = - m dv F
N
dt
(negative indicates retardation)
and N = mw2r , where N = centrifugal force
w =Angular velocity
N = m.w.wr
N = m.v. d , d
dt v = wr and w = dt
Now = F/N = - m . dv m.v. d
dt dt
= - dv . 1
v d
dv = . d
v
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Intergrating above equation
V
Fdv
v= . d
Vc 0
- [ ln v ] VF = ( / 2 - a )
V
c
- [ln Vf - ln Vc ] = ( / 2 - a )
ln Vc - ln Vf = ( / 2 - a )
ln Vc = ( / 2 - a )
Vf
Vc = e ( / 2 - a )
Vf
= e ( / 2 - a ) A
The simple but very significant expression clearly depicts that the value of can be desirably reduced by
1. Using tool having large positive rake
2. Reducing friction byusing lubricant
Optimum value of
We know that
= cos ( - ) (1)
sin
For optimum value of
d
d = 0
d cos ( - )
d sin = 0
- sin - . sin ( - ) + cos ( - ) cos = 0
cos ( - + ) = 0
cos (2 - ) = 0
cos (2 - ) = cos /2
= +
4 2
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from equation (1) , we get
cos
4 +2
opt. =
sin +
42
cos cos . cos . + sin . . sin .
opt. = 4-2 4 2 4 2
=
sin 4 +2 cos . cos .
sin . cos 2 . + 42
4
opt. = 1 i.e. t2 =1, t1 = t2
t1
From equation (A) , , ln = ( / 2 - a )
= e ( / 2 - a )
= e ( / 2 - ) ln
( / 2 - ) (/2-a)
ln 1 = ln e classical friction theroy
0 = ( / 2 - )
, / 2 = (impossible)
The value of = 0 indicates that the machining takes place under zero friction force (Ideal case).
velocity relation : Vf < - -
( --- - -
-----------
- -
- <
--- --- V6 Tool
--
-- - --- B
-
- - - - - -
V
c
------
- < ( Vf 90 - V
s
<
90 -
Vc
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Chip velocity (V ) - The velocity with which the chip moves over rake face is known as the velocity of
F
chip.
Shear velocity (Vs) - The velocity with which shearing of metal along the shear plane takes place is known
as shear velocity
Applying sine rule, we get
Vc = Vs = Vf
sin (90 - + ) sin (90 - ) sin
Vc = Vs = Vf
cos ( - ) cos sin
FORCES INDUCED IN MACHINING (MERCHANT’S CIRCLE DIAGRAM)
This display and inter - relationship of different force components in continuous chip formation
under orthogonal cutting is very easilyvisualized and established bya simple diagram called the merchant’s
circle diagram (MCD).
In orthogonal cutting , chip flows along the orthogonal plane and cutting force and their major
components remain in that plane. Consider chip as a rigid body moving at constant speed and is in equilib-
rium state. The forces acting on chip segments are -
1. From the work piece side - (a) FS - Shear force parallel to shear plane
(b) FN - Normal Force on shear plane
Here FS + F =R
N
2. From the tool side (a) F - Frictional Force at chip tool interface
(b) N - Normal force on rake face
Here F + N = R1 and R = R1
The resultant force R and RL can be resolved further as
R = RL = FC + FT
Where Fc= Cutting force taken along the direction of Vc
FT= Thrust force taken perpendicular Fc and Vc
Thus it clearly appear that
FS + FN = R = R1 = F + N = FC + FT
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It is noted that the circle drawn with R and R1 as diameter are identical and contains the pair of the resolved
component FS & FN , F & N and FC & FT which are perpedicular to each other.
Those two circles with their contents when combined into a single circle produce an MCD.
The significance and important roles of the forces involved in chip formation are :
1. F - It is the shear force which is mainly responsible for chip separation from parent body by shearing
s
and is used to determine the dynanic yield shear strength of the work material.
2. F - It is the force to be induced into workpiece at the cutting point to provide the forces especially
FS which is req. for shearing the chip out.
3. RI - It is the resultant force to be provided through the cutting edge to induce the force R in the w/p.
4. FC - Main cutting force component used an index of machinability and for the evalution of cutting power
consumption.
5. FT - Thrust force, responsible for maintaining the continuous contant of cutting tool with w/p
Among all the forces i.e. F, N, FS , FN , FC , FT First four forces are dependent (Actual forces) and
last two forces are independent or measurable force. FC & FT can be measured with tool dynamometer or
force transducer.
Advantage of MCD :-
1. Easy, quick and resonably accurate determination of magnitude and direction of the forces.
2. Derivation of mathematical expression for different force component.
3. Easy & quick evaluation of frition force, apparent COF occurring at chip tool interface, Shear
strength of work material under any cutting condition.
Disadvantage of MCD
1. MCD is Valid onlyfor orthogonal machining
2. The ratio F/N gives only the apparent COF.
3. MCD is valid only for single shear plane.
4. MCD is valid only for smooth, Continuous chip formation in stable condition.
From MCD, we have FT
<
FC = R cos (-) R= FC
FC FT = R sin (- ) cos ()
R -
R= FT
sin ()
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Again F cos
N = C
R N = R cos (- )
N F cos
F = R sin F= FT sin
sin (- )
Again
FN FS = R cos FS cos
FS = cos (- )
R + - FS FT sin
sin (- )
FN= R sin FN =
Coefficient of friction between rake face & chip underside
tan = F/N = FT sin sin(
FC cos cos ()
tan FT tan
FC tan ()
tan ( = FT
FC
1 tan tan = FT
+ tan tan FC
FC (tan tanFT + FT tan tan
tan FC - FTtanFT + FCtan
tan FT + FCtan
FC - FTtan
Note :- FT < FC General case
FT > FC Special case like face turning, Broaching grinding etc. (FT/FC 2.5)
when FT/FC > 1 , > 450
= tan > 1 (not possible)
whenever the coefficient of friction becoming >1, classical friction theory can be used.
ln 1
= 2 r rchip thickness ratio
- = Back rake angle
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Shear stress induced in material
FS
AS
AS = b x t1
sin
= F sin
S
b.t1
Let = stresses induced normal to shear plane
n
= FN sin
n b.t1
For machining to be possible
=
u
Shear Strain
The magnitude of average strain, that develops along the shear plane the to machining action, is called shear
strain.
PN + NM PN + NM
ON ON ON
= cot + tan ( - )
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Relation among the various forces
Ernst and Merchant’s theory
This theory, first proposed by Ernst - Merchant in 1941, is based on the principle of minimun energy
consumption. It implies that during cutting, the metal shear should occurs in that direction in which the energy
requirement for shearing is minimum,Assumptions made are an follows -
1. The tool is perfectly sharp
2. The cutting edge is perfectly straight
3. Cutting should be orthogonal
4. Depth of cut is constant
5. The width of tool is greater than that of the w/p.
6. Work moves relative to the tool with uniform velocity.
7. Continuous chip is produced with no built up edge
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8- Plane strain condition exist.
9- Material is homogeneous
10- Shear zone extends in a very narrow region which can be approximated by a straight
Proof :- we know that
FS sin
b.t1
Ernst and Merchant theory is based on the principle of minimum power consumption during machining.
Minimum power (P) = FC.VC
Shear force as calculated earlier
FS = .b.t1
sin
< FS
R cos ( + ) = FS <FC
R cos ( ) = FC
F <R
N
<
FT <
FC = R cos (
FS R cos (
FC = FS cos (
cos (
P = FC . VC = .b.t1cos (
sin .cos (
To minimize power consumption , denominator is to be maximized.
Let x = sin .cos ()
dx
d = 0
d = sin .cos () = - sin .sin () + cos.cos () = 0
d
cos (20
cos (2 cos
These formula should be used only when sufficient data is not given in the problem to find out in a
normal way.
90 +
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Note :- For minimum shear strain to be induced
90 +
Lee and shaffer tried to validate this theoritical eqn but they failed because some amount of work done was
neglected.
Lee and shaffer
Stabler
Heat generation in machining
Machining operation is also taking place according to the 1st law of thermodynamics i.e. whatever the
energy supplied for machining operation is simply converted into heat energy only.
work done = FC x VC = Heat generated
Zone I :- It is the shear zone. In this zone the energy supplied for machining operation is used for breaking
the atomic bonds between the atoms of material. During breaking of the atomic bonds, the atom will release
the heat energy which is equal to energy supplied for breaking the bonds
work done = Heat generated [Primary Heat zone + secondary heat zone + Tertiary heat zone]
I0 H00 = FS x VS (Energy required for producing 60 to 65% shear action)
II H00 = F x Vf (30 to 35%)
= frictional force x Flow velocity of chip
Zone II & III :- The heat is generated due to friction between tool rake face and chip underside, tool Flank
and machined surface.
During machining without the use of cutting fluid, whatever the heat generated in machining opera-
tion is simply carried away by the three elements i.e. chip w/p and tool.
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Chip > w/p > tool
75-80% 50-20% 4-6%
of H.G. of H.G. of H.G.
work done = Heat carried away by the (chip + w/p + tool)
out of these, heat carried away by the tool is very small therefore it may be neglected.
W.D. = Heat carried by (chip + w/p)
Distribution of heat
A percentage share of heat by tool and workpiece gradually decreases, when increasing cutting velocity.
The increase in cutting velocity means improvement in metal removalrate or productivity. This diagram
indicates that work material should be machine at speed as high as possible from heat sharing and produc-
tivity point of view.
Specific cutting energy
Energy required for removing unit volume of material is called as specific cutting energy.
Specific cutting energy = Work done
Material removal rate
FC x VC
= t1 . b1 x VC
FN
Specific cutting energy = t1 . b1 mm2
It is also called specific cutting pressure (by comparing units)
This is used for defining the machinability of work material Higher specific cutting energy indicates lower
machinabilityof work material.
Factors influencing forces in machining operation
1). Machanical properties of work material
. Hardness - Forces
. Ductility - Forces
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2) process parameters (VC. f. d) (speed, feed & depth of cut )
< FC
< FT
< d >
FC FC
Forces FT FT
VC > f>
If feed rate increases, the section of sheared chip increases because the metal resists the rupture
more and requires larges effort for chip removal.
VC , reduces the cutting force because the to rise in temperature in the cutting zone which makes
the metal machined more plastic and consequently the effort necessary for machining decrease.
with increase in depth of cut chip thickness becomes significant that causes the growth of volume
deformed metal and that requires enormous cutting force to cut the chip.
3) , Force (cutting)
b
for every I0 increase in b , Force are reduced by 10%
4) Use of coolants reduces the force when compared to without cutting fluid. 25% to 40% force is
reduced with the use of cutting fluid as compared to without cutting fluid.
In orthogonal machining operation :-
FC = 67% i.e. FC = 2 : 1
FT = 33% FT
In oblique machining
FC = 67% Fa : Fr : FC = 1 : 4 . 5 : 11
Fa = 6%
Fr = 27%
Effect of the high cutting temperature
a) On job :- 1) Thermal distortion and dimensional in accuracy - when temp. rises the work becomes
relatively soft because of being soft, under the action of force it undergoes deformation and at top of the
surface there will be a surface roughness.
2) Deterioration of surface integrity (Surface roughness, macrocracks)
b) On tool :- 1) rapid dulling - It loses its geometry. sharpness of cutting edge and due to plastic deforma-
tion the tool is unable to cut
2) rapid wearing - The high temperature accelerates the wear growth
c) on the environment :- Inconvenience to the worker/operator
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Determination of temperature
work done = Heat generated = Heat carried by (chip + workpiece)
w = Heat carried by chip + aw
where a = 0.15 to 0.2
-aw + w = Heat carried by chip
w(1-a) = Heat carried by chip
M.C. w (1-a)
w (1-a)
m-c
FC x VC x (1-a) [volume = MRR = t1 . b . VC]
p x volume x C
FC x Vc x (1-a)
p x t1 x b x Vc x c
FC (1-a)
p x t1 x b x c
Also TC - Tr
where TC = Cutting Temperature
Tr = Room Temperature
C = Speific heat
TC = Tr + F (1-a)
C
p. t1. b. c
TC < Melting point of the workpiece material
if TC Melting point, use cutting fluid/reduce work done orpower consumption by changing machining
parameter
Temperature Measurament techinques colorimetric method
It gives approximate and average temperature produced during machining. Here drill tool, work material are
taken inside the container which is partiallyfilled with a liqnid of known mass and known temperature along
with a thermometer.
when drill tool starts drilling operation, metal removal takes place in the form of chip. This chip goes
to the liquid bath and raise the temperatire of liquid which is measured with the help of thermometer.
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