ASSEMBLY PROCESSES Welding FUNDAMENTALS OF WELDING

ME477 Fall 2004

PART VII JOINING & 1. Overview
ASSEMBLY PROCESSES
FUNDAMENTALS OF WELDING • Welding – A joining process of two materials that
coalesced at their contacting (faying) surfaces by the
Joining - welding, brazing, soldering, and adhesive bonding application of pressure and/or heat.
to form a permanent joint between parts
– Weldment – The assemblage
Assembly - mechanical methods (usually) of fastening parts together – Sometime a filler material to facilitate coalescence.
Some of these methods allow for easy disassembly.
• Advantage: portable, permanent, stronger than the
1. Overview of Welding Technology parent materials with a filler metal, the most
2. The Weld Joints economical method to join in terms of material usage
and fabrication costs .
3. Physics of Welding
4. Features of a Fusion Welded Joint • Disadvantage: Expensive manual Labor, high
energy and dangerous, does not allow disassemble
1 and defects

2

Two Types of Welding Welding Operation

• Fusion Welding – melting base metals • 50 types processes (American Welding Society)
• Applications: Constructions, Piping, pressure vessels,
– Arc Welding (AW) – heating with electric arc
– Resistance welding (RW) – heating with resistance to boilers and storage tanks, Shipbuilding, Aerospace,
Automobile and Railroad
an electrical current • Welder - manually controls placement of welding gun
– Oxyfuel Welding (OFW) – heating with a mixture of • Fitter assists by arranging the parts prior to welding
• Welding is inherently dangerous to human workers
oxygen and acetylene (oxyfuel gas)
– Other fusion welding – electron beam welding and – High temperatures of molten metals,
– Fire hazard fuels in gas welding,
laser beam welding – Electrical shock in electric welding
– Ultraviolet radiation emitted in arc welding (a special helmet with a
• Solid State Welding – No melting, No fillers
dark viewing window) and
– Diffusion welding (DFW) – solid-state fusion at an – Sparks, spatters of molten metal, smoke, and fumes (good
elevated temperature
ventilation).
– Friction welding (FRW) – heating by friction
– Ultrasonic welding (USW) – moderate pressure with • Automation - Machine, Automatic and Robotic welding

ultrasonic oscillating motion 4

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2. The Weld Joint 3. Physics of Welding

• Types of Joints • Coalescing Mechanism: Fusion via high-density energy

– Butt joint • Process plan to determine the rate at which welding can
– Corner joint
– Lap joint be performed, the size of the region and power density
– Tee joint
– Edge joint • for fusion welding PD = P
Powder density (PD):
• Types of Welds A

– Fillet weld where P = power entering the surface, W (Btu/sec); and
– Groove weld
– Plug and slot welds A = the surface area, mm2 (in2)
– Spot and Seam welds – With too low power density, no melting due to the heat conducted into
– Flange and Surfacing welds
work
– With too high power density, metal vaporizes in affected regions
– Must find a practical range of values for heat density.

• In reality, pre & post-heating and nonuniform

• For metallurgical reason, less energy and high heat
density are desired.

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Physics of Welding II Approximate Power Densities and
Efficiency
• The estimated quantity of heat:
U m = KTm2 where K=3.33x10-6 Welding process W/mm2 (Btu/sec-in2)
Oxyfuel 10 (6)
• Heat waste: Arc 50 (30)
Resistance
– Heat transfer efficiency, f1, between heat source and surface Laser beam 1,000 (600)
• Heat problem: Oxyfuel gas welding is inefficient while Arc welding Electron beam 9,000 (5,000)
is relatively efficient. 10,000 (6,000)

– Melting efficiency, f2 , due to the conduction of a work material Arc Welding Process f1 8
• Conduction problem: Al and Cu have low f2 Shield Metal Arc Welding 0.9
Gas Metal Arc Welding 0.9
• Net Heat Available for Welding: Hw = f1 f2H Flux-cored Arc Welding 0.9
• Balance between energy input and energy for welding: Submerged Arc Welding 0.95
Gas Tungsten Arc Welding 0.7
H w = UmV
• Rate Balance: HRw = U mWVR

= f1 f2 HR = U m AwV

where WVR=volume rate of metal welded
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4. Features of Fusion Welded Joint
Weld
Interface Fusion zone

UBMZ HAZ

• A typical fusion weld joint consists of fusion zone, weld WELDING PROCESSES
interface, heat affected zone and unaffected base metal
zone. Fusion welding – Heat & melting
Arc Welding
• Fusion zone: a mixture of filler metal and base metal
melted together homogeneously due to convection as in Resistance Welding
casting. Epitaxial grain growth (casting) Oxyfuel Welding

• Weld interface – a narrow boundary immediately Other Fusion Welding
solidified after melting. Solid-state welding – Heat and pressure, but

• Heat Affected Zone (HAZ) – below melting but no melting & no filler
substantial microstructural change even though the Weld Quality
same chemical composition as base metal (heat Weldability
treating) – usually degradation in mechanical properties
Design Consideration
• Unaffected base metal zone (UBMZ) – high residual
stress 10

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1. Arc Welding (AW) AW with Consumable Electrodes

• A fusion welding where the • Shielded Metal Arc Welding (SMAW)
coalescence of the metals – A consumable electrode – a filler metal rod
(base metals and filler) is coated with chemicals for flux and shielding
achieved by the heat from (230-460mm long and 2.5-9.4mm in
electric arc. diameter)
– The filler metal must be comparable with
• Productivity: Arc time base metals.
– Current: 30-300A and Voltage: 15-45V
• Technical issues Arc Welding Process f1 – Cheaper and portable than oxyfuel welding
– Less efficient and variation in current due to
– Electrodes – consumable and non- Shield Metal Arc Welding 0.9 the change in length of consumable
0.9 electrodes during the process.
consumable electrodes Gas Metal Arc Welding 0.9
0.95 • Gas Metal Arc Welding (GMAW)
– Arc Shielding – To shield the arc Flux-cored Arc Welding 0.7 – Use a bare consumable electrode
– Flooding the arc with a gas which depends
from the surrounding gas. Helium on the metal
– No slag build-up and higher deposition rate
and argon are typically used. Flux Submerged Arc Welding than SMAW
– Metal Inert Gas (MIG) or CO2 welding
does a similar function. Gas Tungsten Arc Welding
12
– Power source – dc for all metals or
H w = f1 f2H = U mV
ac for typically steels

• Heat loss due to convection, where f1 is the heat efficiency
conduction and radiation f2 is the melting efficiency

H is the total heat generated 11

V is the metal volume melted

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AW with Consumable Electrodes AW with non-consumable Electrodes

• Flux-cored Arc Welding (FCAW) • Gas Tungsten Arc Welding (GTAW)
– Tungsten (Wolfram) Inert Gas (TIG) Welding
– Use a continuous consumable tube – With or without a filler metal
with flux and others such as deoxidizer – Tungsten melts at 3410°C
and alloying elements – Shielding gas: argon, helium or a mixture
– All metals (commonly Al and Stainless steels)
– Two types in a wide range of thickness
– Slow and costly but high quality weld for thin
• Self-shielded – flux has an ingredient for sections
shielding
• Plasma Arc Welding (PAW) – a special form
• Gas-shielded – external gas of GTAW but with a constricted plasma gas to
attain a higher temperature
– Produce high quality weld joint
• Carbon Arc Welding – Graphite is used as
• Electrogas Welding (EGW) electrode

– Flux-cored or bare electrode with • Stud Welding – for cookware, heat radiation
external shield gas and water-cooled fin.
molding shoes.
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– Used in shipbuilding

• Submerged Arc welding (SAW)

– Shielding is provided by the granular
flux

– Large structures

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2. Resistance Welding Force Spot Welding Cycle

• RW – heat and pressure to + electrode (1) (2) (3) (4) (5)Force, Current
accomplish coalescence. Weld nugget
Force
• Power source: heat generated: H = I 2Rt - electrode Current
Force
• Resistance Welding Processes (1) (2) (3) (4) (5) time
15
– Resistance spot welding (RSW) 16

• Electrodes – Cu-based or
refractory(Cu+W)

• Rocker-arm spot welders

– Resistance seam welding (RSEW)

– Resistance projection welding (RPW)

– Flash welding (FW) – Heating by
resistance

• Upset welding – similar to FW but pressed
during heating and upsetting.

• Percussion welding – similar to FW but
shorter duration

– High-frequency (induction and resistance)
welding

3. Oxyfueld gas Welding 4. Other Fusion Welding

• Oxyfuel gas weldings (OFW) – Use • Electroslag Welding – similar to electrogas welding, no arc is
various fuels mixed with oxygen used

• Oxyacetylene welding – A mixture of • Thermit (from Thermite™) Welding, dated 1900, is a fusion –
acetylene and oxygen welding process that uses a mixture of Al powder and iron
oxide in 1:3 ratio for exothermic reaction (reaching 2500°C)
– Total heat: 55x106J/m3 – Used in railroad, repair cracks in ingot and large frame and shaft.

– Acetylene: odorless but commercial 18
acetylene has a garlic order.

– Unstable at 1atm thus dissolved in
acetone.

• Other gases

– MAPP (Dow), Hydrogen,

Propylene, Propane and Natural gas Outer Envelope

(1260°C)

Acetylene feather

Inner cone (2090°C)
(3480°C)
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High Energy beam Welding 5. Solid-State Welding

• Electron Beam Welding • No filler metals but w/o local melting with either
pressure-alone or heat and pressure.
– A high-velocity, narrow-beam electron converting into heat to
produce a fusion weld in a vacuum (Multiple degrees of vacuum) • Intimate contact is necessary by a through cleaning
or other means.
– From foil to plate as thick as 150mm
• Solid-state Welding Processes
– Very small heat effected zone
– Forge welding – Samurai sword
– Power density PD = f1EI – Cold welding – high pressure
A – Roll welding
– Hot-pressure welding
• Laser Beam welding – Diffusion welding at 0.5Tm
– Explosive welding – mechanical locking commonly used to
– A high-power laser beam as the source of heat to produce a fusion
weld without a filler material bond two dissimilar metals, in particular to clad one metal
on top of a base metal over large areas
– Due to the high density energy on a small focused area, narrow and – Friction welding – friction to heat
deep penetration capability – Ultrasonic welding – oscillatory shear stresses of ultrasonic

– Pulsed beam for spot-weld thin samples 20

– Continuous beam for deep weld and thick sample

– e.g.: Gillette Sensor razor

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Explosive, Friction & Ultrasonic Comparison
Welding

21 22

6. Weld Quality 24

• Residual Stress and Distortion 4
– Welding fixtures, Heat sink,Tack welding, control weld condition, Preheating,
Stress-relief heat treatment, Proper design

• Welding Defects
– Cracks, Cavities, Solid Inclusions, Incomplete Fusion
– Imperfect shape, Miscellaneous Defects such as arc strike and excessive spatter.

• Visual Inspection – Most widely used welding inspection,
– dimensional, warpage, crack

• Limitations:
– Only surface defects are detectable
– Internal defects cannot be discovered
– Welding inspector must also determine if additional tests are warranted

• Nondestructive
– dye- and fluorescent-penetrant - detecting small defects open to surface
– Magnetic particle testing - iron filings sprinkled on surface reveal subsurface
defects by distorting magnetic field
– Ultrasonic - high frequency sound waves directed through specimen, so
discontinuities detected by losses in sound transmission
– Radiograph - x-rays or gamma radiation to provide photographic film record of any
internal flaws

• Destructive – mechanical & metallurgical tests

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ME477 Fall 2004

Mechanical Tests for Welding 7. Weldability

25 • Similar to Machinability, it defines the capacity of a metal
to be welded into a suitable design and the resulting
weld joint to perform satisfactorily in the intended
service.

• The factors affecting weldability, welding process, base
metal, filler metal and surface condition.

• Base metal – melting point, thermal conductivity and
CTE

• Dissimilar or filler materials, Strength, CTE mismatch
and compatibility must be considered.

• Moisture and oxide film affects porosity and fusion
respectively.

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8. Design Considerations BRAZING SOLDERING AND
ADHESIVE BONDING
• Design for welding
• Minimum parts 1. Brazing
• Arc Welding 2. Soldering
3. Adhesive Bonding
– Good fit-up of parts
– Access room for welding 28
– Flat welding is advised

• Spot welding

– Low carbon steel up to 3.2mm
– For large components: reinforcing part or flanges
– Access room for welding
– Overlap is required

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Introduction 1. Brazing

• Brazing and soldering – A filler metal is • If properly designed and performed, solidified joint will
melted and distributed by capillary action but be stronger than filler metal.
no melting of parent metals occurs.
• Why?
• Brazing & soldering instead of fusion welding
– Small part clearances used in brazing
– Join the metals with poor weldability. – Metallurgical bonding that occurs between base and filler

– Join dissimilar metals. metals
– Geometric constrictions imposed on joint by base parts
– No heat damage on the surfaces.
• Applications
– Geometry requirement is more relaxed than
welding. – Automotive (e.g., joining tubes and pipes)
– Electrical equipment (e.g., joining wires and cables)
– No high strength requirement – Cutting tools (e.g., brazing cemented carbide inserts to shanks)
– Jewelry-making
• Adhesive Bonding – similar to brazing and 29 – Chemical process industry, plumbing and heating contractors
soldering but adhesives instead of filler
metals. join metal pipes and tubes by brazing
– Repair and maintenance work

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Advantages and Disadvantage Brazed Joints

• Advantages • Butt
• Lap – a wider area for brazing metal
– Any metals can be joined
– Certain methods are quickly and consistently or • Lap joints take more load than butt joints.

automatically done 32
– Multiple brazing at the same time
– Very thin parts can be joined
– No heat affected zone
– Joints inaccessible by welding can be brazed

• Disadvantage

– Strength,
– Low service temperature,
– Color mismatch with the color of base metal parts

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Brazed JointsJoint strength Common Filler Metals

• Clearance between mating surface for capillary clearance Filler Metal Typical Brazing Base metals
action (0.025 and 0.25mm) Composition Temp.(°C)
Al
• Cleanliness of the joint – chemical (solvent cleaning Al & Si 90Al, 10Si 600 Ni and CU
& vapor degreasing) and mechanical (wire brushing 1120
& sand blasting) treatments Cu 99.9Cu 850 Cu
925 Steels, Cast Iron
• Fluxes are used during brazing to clean surfaces and Cu & P 95Cu, 5P
to promote wetting 950 and Ni
Cu & Zn 60Cu, 40Zn Stainless steel
• Common filler metals 1120 and Ni alloys
Au & Ag 80Au, 20Ag Stainless steel
– Compatible melting temperature compatible with base metal 730 and Ni alloys
– Low surface tension for wetting Ni alloys Ni, Cr, others Ti, Monel, Inconel,
– High fluidity, Strength and no chemical and physical Tool steel and Ni
Silver alloys Ag, Cu, Zn, Cd
interactions with base materials 34

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Brazing method 2. Soldering

• Several techniques for applying filler metal • Similar to Brazing but the filler material melts below
450°C
• Brazing fluxes
• A filler material is solder and sometimes tinning (coating
– Avoids oxide layers or unwanted by-product the faying surfaces) is needed.
– Low melting, low viscosity, wetting, protection until brazing
• Typical clearance ranges from 0.076 to 0.127mm.
metals solidify • After the process, the flux residue must be removed.
– Borax, borates, fluorides and chlorides in a form of powder, • Advantage

paste or slurries – Low energy, variety of heating methods, good electrical and
thermal conductivity, air-tight & liquid-tight seams and reparable
• Brazing methods depending on heat source
• Disadvantage
– Torch, Furnace, Induction, Resistance, Dip (either molten salt
bath or molten metal bath), Infrared and brazing welding – Low strength, weak in high temperature applications

35 • For mechanical joints, the sheets are bent and the wires
are twisted to increase joint strength.

– Electronic applications: electrical connection.
– Automotive application: vibration.

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ME477 Fall 2004

Materials and Methods 3. Adhesive bonding

• Solders – mainly alloys of tin and lead (low melting point) but in soldering • The filler material is called adhesive (usually polymer)
copper, intermetallic compounds of copper and tin and in soldering alloys requiring curing sometime with heat.
silver and antimony.
• Strength depends on chemical bonding, physical
• Fluxes: Melt at soldering temperature, Remove oxide films, Prevent oxide interaction (secondary bonds) and mechanical locking.
formation, Promote wetting, Displaced by the molten solder
• Surface preparation
– Types: Organic and inorganic
– clean and rough surfaces
• Methods
– Hand soldering – soldering gun • Application methods
– Wave soldering
– Brushing, rollers, silk screen, flowing, splaying, roll coating
• Multiple lead wires on a printed circuit board(PCB)
• Advantage
– Reflow soldering – A solder paste consists of solder powders in a flux
binder, which is heated either using vapor phase reflow or infrared – a wide variety of materials, different sizes, bonding over an entire
reflow. surface and flexible adhesives, low temp. curing, sealing, simple
joint design
37
• Disadvantage

– weaker bonding, compatible, limited service temperature, curing

times and no inspection method 38

Adhesive types Joint Design

• Natural adhesives - derived from natural sources, • Adhesive joints are not as strong as welded, brazed, or soldered joints
including gums, starch, dextrin, soy flour, collagen • Joint contact area should be maximized
• Adhesive joints are strongest in shear and tension
– Low-stress applications: cardboard cartons, furniture, • Joints should be designed so applied stresses are of these types
bookbinding; or large areas: plywood • Adhesive bonded joints are weakest in cleavage or peeling
• Joints should be designed to avoid these types of stresses
• Inorganic - based principally on sodium silicate and
magnesium oxychloride Tension Shear cleavage peeling

– Low cost, low strength 40

• Synthetic adhesives - various thermoplastic and
thermosetting polymers

– Most important category in manufacturing

– Synthetic adhesives cured by various mechanisms, such as
Mixing catalyst or reactive ingredient with polymer prior to
applying, Heating to initiate chemical reaction, Radiation curing,
such as ultraviolet light, evaporation of water from liquid or paste,
Application as films or pressure-sensitive coatings on surface of
one of adherents

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