WELDING METALLUGY

DPP C2 (b)

Institut Kemahiran MARA
Kuching, Sarawak

INFORMATION SHEET

COURSE : STK SEMESTER : 3
SESSION : TMW 3032 METALLURGY SHEET NO : 5.2
CODE/SUBJECT : WEEK : 15
LECTURER :

TOPIC : MELTING AND SOLIDIFICATION OF WELD METAL
SUB-TOPIC : 1. HEAT AFFECTED ZONE (HAZ)

LEARNING At the end of the lesson, students will be able to :
OUTCOME : 1. Identify Heat Affected Zone

TAJUK: HEAT AFFECTED ZONE

PENERANGAN:

The heat-affected zone (HAZ) is the area of base material, either a metal or a
thermoplastic, which has had its microstructure and properties altered by welding or heat
intensive cutting operations. The heat from the welding process and subsequent re-cooling
causes this change in the area surrounding the weld. The extent and magnitude of property
change depends primarily on the base material, the weld filler metal, and the amount and
concentration of heat input by the welding process.

1

DPP C2 (b)

The thermal diffusivity of the base material plays a large role—if the diffusivity is high,
the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity
leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process
plays an important role as well, as processes like ox fuel welding use high heat input and
increase the size of the HAZ. Processes like laser beam welding and electron beam welding
give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls
between these two extremes, with the individual processes varying somewhat in heat input. To
calculate the heat input for arc welding procedures, the following formula is used:

v x I x 60
Q=

S x 1000

Where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed
(mm/min). The efficiency is dependent on the welding process used, with shielded metal arc
welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas
tungsten arc welding, 0.8.

One disadvantage of the laser ablation process is that a heat affected zone is left behind
where molten material re-solidified in situ or where material was sufficiently heated and cooled
rapidly enough to result in embitterment. This change in material properties can alter
subsequent laser ablation and material performance. The advantage of ablating with a short
duration excimer laser is that the pulse width is short enough to greatly reduce the transfer of
heat out of the ablation zone. This tends to localize the heat more and reduces the extent of the
heat affected zone.

2

DPP C2 (b)

The size of the heat affected zone is a function of the laser pulse duration and the
material parameters such as thermal conductivity and specific heat. The heat affected zone will
depend on the distance the heat is conducted within the material and varies with material and
laser wavelength. A plot of the heat conduction distance as a function of wavelength is shown
below.

The better the material conduction (thermal diffusivity) the greater is the extent of the
heat affected zone. The effect of the HAZ on the material (embitterment, for example) is more a
function of the material thermo mechanical properties.

Associated with the heat affected zone is recast and formation of burrs. During the
ablation process, the expulsion of material in the plasma jet creates a compressive on the
molten pool of material under the laser spot. This will cause a portion of the liquefied material to
be forced out of the ablation zone and it will deposit onto the

surrounding region. The amount of recast material can be minimized by a small absorption
depth which will reduce the melted volume and a high laser power which will convert more of
the melted material into vapor faster. Coupled with a higher power is a shorter pulse duration
which will have less dynamic impact on the relatively massive liquid. The duration of the forcing
function is small compared to the dynamic response time of the liquid.

Debris is also a related problem of the ablation process. Particulate will redeposit onto
the surface as solid fallout of higher boiling point second phases from the plume and as
condensation of the primary material. The debris is commonly particulates of the substrate
material if the ablation is sufficiently deep, oxides and nitrides of the work material or other
contaminants, and products created by interactions within the plume. Taken together, all of the
preceding effects reduce the quality of the ablated region.

The following examples of laser ablation of micro- and mille-structures are intended to
show typical process results. The results are not necessarily the best attainable but are for
illustrative purposes only.

3

DPP C2 (b)

• Diamond Nozzle

4

DPP C2 (b)

• Test Structure

5

DPP C2 (b)

• Close-up of Diamond Tip

How does welding affect the HAZ of the weld?
Q - I work with aluminum-welded structures of different alloy types and designs. I
also work with both heat treatable and non-heat treatable alloys. I would like to know
how the arc welding of these alloys affects the strength of the heat-affected zone of the
weld.
A - To appreciate the affect of arc welding on the heat-affected zone of various
aluminum alloys, evaluation of how these alloys obtain their strength and the potential
for changes in strength after welding is necessary.

There are seven series of aluminum alloys. Each one is designated by the
primary alloying element used to produce that particular type of material. Additionally,
we can divide the seven aluminum alloy series into the heat treatable and non-heat
treatable categories:

6

DPP C2 (b)

Series Primary Alloying Element
1xxx
2xxx Aluminum - 99.00% or Greater (Non-Heat Treatable)
3xxx
4xxx Copper (Heat Treatable)
5xxx
6xxx Manganese (Non-Heat Treatable)
7xxx
Silicon (Non-Heat Treatable)

Magnesium (Non-Heat Treatable)

Magnesium and Silicon (Heat Treatable)

Zinc (Heat Treatable)

The addition of alloying elements to aluminum is the principal method used to produce a variety
of different materials that are useable in a wide assortment of applications. The principle reason
for adding the primary alloying elements is to facilitate an improvement in the alloys physical
and/or mechanical characteristics. Typically, the addition of the primary alloying elements to
aluminum is to provide improvement in work hardening and or precipitation hardening
characteristics.

How aluminum alloys obtain there strength:

4.2.1 Work Hardening

Work hardening is an important process used to increase the strength of materials that are
unable to be strengthened by heat treatment. It can be applied to both heat treatable and non-
heat treatable series material but it is used extensively to produce the strain-hardened tempers
in the non-heat treatable aluminum alloys. This process involves a change of shape brought
about by the input of mechanical energy. As deformation proceeds, the material becomes
stronger but harder and less ductile. The strain hardened temper of full-hard material –H18 for
example is usually obtained with a cold work equal to about a 75% reduction in area. The -H16,
-H14, and -H12 tempers are obtained with lesser amounts of cold working, and they represent
three-quarter-hard, half-hard, and quarter-hard conditions, respectively. It is important to
understand that the full affect of strain-hardening on the non-heat treatable alloy can be
removed by heating the base material to its annealing temperature for a very short time period.

7

DPP C2 (b)

4.2.2 Precipitation Hardening
A second method of strengthening aluminum is precipitation heat treatment, which is

preceded by solution heat-treating. Solution heat-treating is achieved by heating a material to a
suitable temperature, holding at that temperature for a long enough time to allow constituents to
enter into solid solution, and then cooling rapidly to hold the constituents in solution. This
process is usually followed by precipitation hardening or what is also termed artificial aging,
which is achieved by re-heating the alloy to a lower temperature and holding it at this
temperature for a specified time. The result of these heat treatments is to produce a
metallurgical structure within the material that provides superior tensile strength. If, during the
artificial aging procedure, the material is held at a temperature for too long or the temperature
used is too high, the material can become over-aged, resulting in a decrease in tensile
strength. It is important to recognize that the precipitation hardening process is both time and
temperature dependent. This method of strengthening is only used on the heat treatable
aluminum alloys.

4.2.3 The Affect of Arc Welding on the Heat Affected Zone
In order to make a welded joint in an aluminum structure using the arc welding process,

the base material must be melted. During the melting process, heat is transferred through
conduction into the base material adjacent to the weld. Typically the completed element can be
divided into three distinct areas: the weld metal, the heat affected zone adjacent to the weld and
the base material beyond the HAZ which has been unaffected by the welding operation. If arc
welding on materials which have been strengthened by work hardening or precipitation
hardening, because the HAZ will experience cycles of heating and cooling during the welding
operation, its properties will change and may be extremely different than that of the original
base alloy and the unaffected area of the base material (see fig 1)

8

DPP C2 (b)

4.2.4 Non-Heat Treatable Alloys
What is important from a HAZ perspective is that the non-heat treatable aluminum alloys

that are strengthened by strain hardening can be restored to a full soft, ductile condition by
annealing. Annealing eliminates the strain hardening, as well as the microstructure that is
developed as a result of cold working. The heating of the HAZ, which takes place during the arc
welding operation, is sufficient to anneal the base material within the HAZ area. For this reason
the minimum tensile strength requirements for as welded non-heat treatable alloys, regardless
of the original work hardened temper, is based on the annealed strength of the base alloy.
Typical tensile strengths of non-heat treatable alloys in their tempered condition and as-welded
are shown in table 1.

4.2.5 Heat Treatable Alloys
In the case of the heat treatable alloys, the HAZ will normally not be fully annealed. The

HAZ will experience a wide variety of temperatures as seen in Fig 1. The time at these
temperatures and the rate of cooling back to room temperature will determine the amount of
annealing that actually takes place in the HAZ. This is not to suggest that there will not be a
reduction in strength in the HAZ. The affect on the HAZ of a heat treatable alloy that is welded
in the solution heat treated and artificially aged condition (T-6) is typically one of partially
annealed and over-aged. This condition can be significantly affected by the heat input during
the welding operation. The general rule is, the higher the heat input, the lower the as welded
strength. Typical tensile strengths of some of the heat treatable alloys in their temper condition
and as welded are shown in table 2.
Summary
Dependant on the particular aluminum alloy type and its temper, there can be significant
difference between the tensile strength of the HAZ and the tensile strength of the unaffected
area of the welded component. The reduction in tensile strength of the HAZ under controlled
conditions, particularly with the non-heat treatable alloys, can be somewhat predictable. The
reduction in tensile strength of the HAZ in the heat treatable alloys is more susceptible to
welding conditions and can be reduced below the required minimum requirement if excessive
heating occurs during the welding operation.

9

DPP C2 (b)

Table 1
Typical Tensile Strength Properties of Groove Welds

Non-Heat Treatable Alloys

Base Alloy & Temper Base Alloy Tensile Strength - ski As welded Tensile Strength - ski

1060-H18 19 10
5052-H32 33 27
5052-H39 42 27
5086-H34 47 38
5086-H38 53 38
5083-H116 46 43
3003-H34 35 16
3004-H38 41 24

Table 2
Typical Tensile Strength Properties of Groove Welds

Heat Treatable Alloys

Base Alloy & Temper Base Alloy Tensile Strength - ski As welded Tensile Strength - ski

6063-T6 31 19
6061-T6 45 27
6061-T4 35 27
2219-T81 66 35
2014-T6 70 34
7005-T53 57 43

10

DPP C2 (b)
11

DPP C2 (b)
4.3 QUESTION.
1. State the three distinct regions in the element?
2. Give two types of region of heat affected zone?
3. How to calculate the heat input for arc welding procedures?
4. State 4 examples of types of microstructure of Heat Affected Zone (H.A.Z)?
5. Discuss about H.A.Z as u known?
4.4 REFERENCE.
1. Ellis D J : Mechanized narrow gap welding ‘joining and material 1988 Vol 1.
2. Handbook of structure welding, Abington publishing 1992.
3. Yeo R B G :’ joining and Material 1989 vol 2

12

DPP C2 (b)
13

DPP C2 (b)
14

DPP C2 (b)
15