Welding aluminium and its alloys
The equipment used is standard DC welding
equipment. The operating voltage is about 25 volts,
depending upon arc length and type of electrode, and
over a current range of 40–450 amperes. The electrode
coating on the rods usually consists of a mixture
of chloride and fluorides though other salts may
be present. For manual metal arc welding, mixtures
generally similar to those used for gas welding are
supplied as the coating, but in this case the flux
may include binders which prevent the coating from
chipping and also help to stabilize the arc. The composition
of the core wire is important and in general
falls into three types: pure aluminium, aluminium
silicon or aluminium manganese. If a wrought alloy
is to be welded containing less than 2 per cent alloying
element, then silicon-type electrodes are used.
Pure aluminium should be welded with a pure
aluminium electrode.
Vertical welding is possible with materials
thicker than 5 mm, although downhand welding is
preferable with the electrode moving in a straight
line along the seam without weaving. The speed of
welding depends on the current and the operator’s
skill, but is usually about three times that of mild
steel. It increases as the electrode size is increased
and it should also be increased as the weld progresses
in order to allow for the rise in temperature
of the parent metal. Too rapid welding on low currents
does not give the required penetration, while
welding that is slow, or a current that is too high,
gives an excessive bead which may result in burning
through the parent metal. When the arc is broken
the coating tends to cover the tip of the
electrode, thus obstructing the re-establishing of
the arc. It is usually sufficient to tap the end of the
electrode on the work to crack off this coating,
although it may be necessary to cut the tip off. The
slag on MMAW should flake off readily, especially
if the weld is allowed to cool before the slag is
removed. With all electrodes all traces of slag
should finally be removed by scrubbing with a
wire brush and washing.
11.8 Safety precautions for the welder
1 Never look at a welding arc without a shield.
2 Always replace the clear cover glasses when
they become pitted and encrusted due to metal
spatter.
3 Examine the closed lenses in the helmet. If they
are cracked, replace them immediately.
4 Wear goggles when chipping slag off a weld.
5 Always wear gloves and an apron when welding.
6 Use a holder that is completely insulated. Never
lay it on the bench while the machine is running.
7 Welding processes should be carried out only
in the areas where there is adequate ventilation.
308Repair of Vehicle Bodies
8 When welding outside a permanent welding
booth, be sure to have screens around so the arc
will not harm persons nearby.
9 Prevent welding cables from coming in contact
with hot metal, water, oil or grease.
10 Make sure that you have a good earth connection.
11 Keep the cables in an orderly manner to prevent
them from becoming a stumbling hazard. Avoid
dragging the cables over or around sharp corners.
Whenever possible, fasten the cables overhead
to permit free passage.
12 Do not weld near inflammable materials.
13 Be sure tanks, drums or pipelines are completely
cleaned of inflammable liquids before welding.
14 Always turn off the welding machine when not
in use.
Questions
1 What is the difference between alternating and
direct currents?
2 Describe, with the aid of a line diagram, the
principle of manual metal arc welding.
3 Name four factors which control the quality of
manual metal arc welding.
4 Describe the process of manual metal arc
welding and explain how the necessary heat is
produced.
5 What is the function of the coating of a manual
metal arc welding electrode?
6 What is meant by the term ‘arc length’?
7 Give four essential factors in making a good arc
weld.
8 In what way does a current affect a weld?
9 State the current in amperes and the size of
electrode needed, for welding over 5 mm thick
steel plate.
10 Explain the function of an AC metal arc welding
transformer.
11 What would be the resulting effect if the
amperage is set too high in manual metal arc
welding?
12 Why is it important not to look at an electric arc
without proper eye protection?
13 Compare the use of a head shield to that
of a hand shield for continuous welding
operations.
14 What causes the defect known as undercutting,
and how can this be avoided?
15 How is the weld affected when the arc is too
short?
16 What happens when the welding arc is too long?
17 What determines the number of runs that should
be made on a weld?
18 Why must a welder take into account the
expansion and contraction of metal?
19 How are welding electrodes identified?
20 Why is it important to have adequate air
extraction when welding in confined situations?
Gas shielded arc
welding
12.1 Development of gas shielded arc
welding
Originally the process was evolved in America in
1940 for welding in the aircraft industry. It developed
into the tungsten inert-gas shielded arc
process which in turn led to shielded inert-gas
metal arc welding. The latter became established in
this country in 1952.
In the gas shielded arc process, heat is produced
by the fusion of an electric arc maintained between
the end of a metal electrode, either consumable or
non-consumable, and the part to be welded, with a
shield of protective gas surrounding the arc and the
weld region. There are at present in use three different
types of gas shielded arc welding:
Tungsten inert gas (TIG) The arc is struck by a
non-consumable tungsten electrode and the metal
to be welded, and filler metal is added by feeding a
rod by hand into the molten pool (Figure 12.1).
Metal inert gas (MIG) This process employs a
continuous feed electrode which is melted in the
intense arc heat and deposited as weld metal:
hence the term consumable electrode. This process
uses only inert gases, such as argon and helium, to
create the shielding around the arc (Figure 12.2).
Metal active gas (MAG) This is the same as MIG
except that the gases have an active effect upon the
arc and are not simply an inert envelope. The gases
used are carbon dioxide or argon/carbon-dioxide
mixtures.
The following should also be noted:
Gas tungsten arc welding (GTAW) This is the
terminology used in America and many parts of
Europe for the TIG welding process, and it is
becoming increasingly accepted as the standard
terminology for this process.
Gas metal arc welding (GMAW) This is the
terminology used in America and many parts of
Europe for the MIG/MAG welding processes, and
it is becoming increasingly accepted as the standard
terminology for these processes.
Figure 12.1TIG welding equipment AC/DC (Murex
Welding Products Ltd )
310Repair of Vehicle Bodies
12.2 Gases used for shielded arc
processes
The shielding gases used in the MIG/MAG and
TIG welding processes perform several important
functions:
1 Protection from atmospheric contamination
2 Arc support and stabilization
3 Control of weld bead geometry
4 Control of weld metal properties.
It is necessary to prevent contamination of the weld
pool by atmospheric gases which cause deterioration
of the weld bead quality, by surface oxidation, porosity
or embrittlement. In the consumable electrode
process MIG/MAG, it is also necessary to consider
the potential loss of alloying elements in the filler
wire owing to preliminary oxidation in the arc
atmosphere. In TIG welding, oxidation of the nonconsumable
tungsten electrode must be prevented.
For these reasons most welding shielding gases are
based on the inert gases argon (Ar) and helium (He).
Active gases such as carbon dioxide (CO2), oxygen
(O2) and hydrogen (H2) may be added to the shielding
gas to control one or other of the functions
stated, but the gas chosen must be compatible with
the material being welded (Table 12.1).
The successful exclusion of atmospheric contamination
depends on the ability to provide a
physical barrier to prevent entrainment in the arc
area, and in the case of some reactive metals such
as titanium it may be necessary to extend this
cover to protect the solidified weld metal whilst it
is cooling. The gas cover depends on the shielding
efficiency of the torch and the physical properties
of the gas. The higher the density of the gas the
more resistant it will be to disturbance, and gases
which are heavier than air may offer advantages in
the downhand welding position (Table 12.2).
Figure 12.2MIG welding equipment (Migatronic
Welding Equipment Ltd )
Table 12.1Chemical and physical compatibility of
welding shielding gases and materials
Material Gas Compatibility
Plain carbon Argon, helium No reaction
steel and CO2, oxygen Slight oxidation of
low alloy alloying elements
steel Hydrogen Porosity and HICC
risk
Nitrogen Porosity and loss of
toughness
Austenitic Argon, helium No reaction
stainless H2 Reduces oxide
steel O2 Oxidizing
CO2 May cause carbon
Aluminium Argon, helium pick-up
and alloys H2 No reaction
O2 Gross porosity
Oxidizing
Copper Argon, helium, N2 No reaction
H2 Porosity
Nickel Argon, helium No reaction
N2 Porosity
Titanium Argon, helium No reaction
O2, N2, H2 Embrittlement
Table 12.2Density of common welding shielding
gases
Gas Density (kg/m3)
Argon 1.784
Helium 0.178
Hydrogen 0.083
Nitrogen 1.161
Oxygen 1.326
Carbon dioxide 1.977
Gas shielded arc welding 311
The shielding gas used in MIG/MAG processes
displaces the air in the arc area. The arc is struck
within this blanket of shielding gas, producing a
weld pool free from atmospheric contamination.
The type of gas used determines the heat input, arc
stability and mode of transfer, as well as providing
protection for the solidifying weld metal. With any
gas shielded arc process the type of shielding gas
used greatly influences the quality of the weld
deposit, weld penetration and weld appearance.
The heat affected zone can also be influenced by
the composition of the gas.
One of the important functions of the shielding
gas is to protect the weld zone from the surrounding
atmosphere and from the deleterious effects of
oxygen, nitrogen and hydrogen upon the chemical
composition and properties of the resulting weld.
In this capacity the gas fulfils the major function
of the fluxes used as electrode coverings or
deposited as an enveloping layer during welding
with other processes. The obvious advantages
derived from the use of gas shielding are that the
weld area is fully visible; that little, if any, slag is
produced; and that the absence of abrasive flux
increases the life of jigs and machine tools. In the
MIG welding process, gas shielding enables a
high degree of mechanism of welding to be
achieved. Few gases possess the required shielding
properties, however, and those that do with
certainty – the inert gases, notably argon – are relatively
expensive.
Argon
Argon is one of the rare gases occurring in the
atmosphere and is obtained from liquefied air in
the course of the manufacture of oxygen. Argon is
an inert gas. It does not burn, support combustion,
or does not take part in any ordinary chemical
reaction. On account of its strongly inactive properties
it can prevent oxidation or any other chemical
reaction from taking place in the molten metal
during the welding operation.
The argon is supplied in steel cylinders coloured
blue, a full cylinder having approximately 200 bar
pressure which is reduced by a regulator to approximately
15 litres per min when drawing for welding.
Normally the cylinder should not be allowed to
empty below 2 bar. This prevents air from entering
the cylinder and helps to preserve the purity of the
contents. Gas pressures are shown as bar: 1 bar _
14.505 lbf/in2; 10 lbf/in2 _ 0.689 bar. Gas consumptions
are in litres per hour (litre/h): 1 ft3/h _
28.316 litre/h.
Argon has been more extensively used than
helium because of a number of advantages:
1 Smoother, quieter arc action
2 Lower arc voltage at any given current value
and arc length
3 Greater, cleaner action in the welding of such
materials as aluminium with AC
4 Lower cost and greater availability
5 Lower flow rates for good shielding
6 Easier arc starting.
The lower arc voltage characteristics of argon are
used to advantage in the welding of thin metals,
because the tendency towards burn-through is lessened.
Pure argon can be used for welding aluminium
and its alloys, copper, nickel, stainless
steel and also for MIG brazing.
Helium
Helium is a colourless, odourless inert gas almost
as light as hydrogen. It is found in the United
States in a natural gas and is therefore more
widely used in America than anywhere else.
Helium has a high specific heat, so that a given
quantity requires much more heat to raise its temperature
by 1 °C than does air; therefore the weld
temperature is reduced and so distortion is minimized.
A disadvantage of helium is that, because
of its lightness, two and a half times the gas flow
is required for a given performance than would be
needed with argon. Helium is more favourable
than argon where high arc voltage and power are
desirable for welding thick material and metal
with high heat conductivity such as copper.
Helium is more often used in welding heavy than
light materials.
Carbon dioxide
This gas is used as a shielding gas in the MAG
welding process. It is not an inert gas: when it
passes through the welding arc there is some
breakdown into carbon monoxide and oxygen. To
ensure that the oxygen is not added to the weld,
deoxidants such as silicon, aluminium and titanium
312Repair of Vehicle Bodies
are included in the welding wire, which is specially
made for carbon dioxide MAG welding. The
deoxidant combines with the released oxygen to
form a sparse slag on the surface of the completed
weld. A gas heater with an electrically heated element
is used to prevent freezing of the gas regulator
after prolonged use with carbon dioxide gas.
Since the development of the carbon dioxide
process it has become widely used for the welding
of plain carbon steels.
Argon mixtures
The gas mixtures that are suitable for vehicle body
repair work consist of 95 per cent argon and 5 per
cent carbon dioxide, or 80 per cent argon and 20 per
cent carbon dioxide. These mixtures give smoother
results with a better, cleaner and more attractive
spatter-free weld. They also improve metal transfer
and weld finish. These gases can be used to weld
low-carbon mild steel, high-strength steel (HSS) and
very low-carbon rephosphorized steels.
Helium mixes
These are specially formulated for MIG welding
of stainless steels. They are a mixture of highpurity
helium and argon, with small controlled
additions of carbon dioxide. They contain no
hydrogen, and are suitable for welding all grades
of stainless steel including weldable martensitic
grades, extra-low-carbon grades and duplex stainless
steel.
Choice of shielding gases
Shielding gases must be carefully chosen to suit
their application (Table 12.3). The selection will
depend on:
1 The compatibility of the gas with the material
being welded
2 Physical properties of the material
3 The welding process and mode of operation
4 Joint type and thickness.
12.3 TIG welding
Principles of operation
The necessary heat for this process (Figures 12.3
and 12.4) is produced by an electric arc maintained
between a non-consumable electrode and the surface
of the metal to be welded. The electrode used
for carrying the current is usually a tungsten or
tungsten alloy rod. The heated weld zone, the
molten metal and the electrode are shielded from the
atmosphere by a blanket of inert gas (argon or
helium), fed through the electrode holder which is in
the tip of the welding torch. A weld is made by
applying the arc heat so that the edges of the metal
are melted and joined together as the weld metal
solidifies. In some cases a filler rod may be used to
reinforce the weld metal.
Before commencing to weld it is essential to
clean the surfaces that are to be welded. All
oil, grease, paint, rust and dirt should be
removed by either mechanical cleaning or chemical
cleaners.
Striking the arc may be accomplished in one of
the following ways:
1 Using an apparatus which will cause a spark to
jump from the electrode to the work (arc stabilizer
AC equipment); or
2 By means of an apparatus that starts and maintains
a small pilot arc which provides a path for
the main arc.
Once the arc is struck, the welding torch is held
with the electrode positioned at an angle of about
75° to the surface of the metal to be welded. To
start welding, the arc is usually moved in a small
circle until a pool of molten metal of suitable
size is obtained. Once adequate fusion is
achieved, a weld is made by gradually moving
the electrode along the parts to be welded so as
progressively to melt the adjoining edges. To
stop welding, the welding torch is quickly withdrawn
from the work and the current is automatically
shut off.
The material thickness and joint design will
determine whether or not filler metal in the form
of welding rod needs to be added to the joint.
When filler metal is added it is applied by feeding
the filler rod from the side into the pool of molten
metal in the arc region in the same manner
as used in oxy-acetylene welding. The filler rod
is usually held at an angle of about 15° to the
surface of the work and slowly fed into the
weld pool.
The joints which may be welded by this process
include all the standard types such as square
Gas shielded arc welding 313
groove, V groove, as well as T and lap joints
(Figure 12.5). It is not necessary to bevel the
edges of material that is 3.2 mm or less thickness.
Modes of operation
The TIG process may be operated in one of the following
modes:
DC electrode negative In this mode the electrode
remains relatively cool whilst the workpiece is
effectively heated. This is the most common mode
of operation for ferrous materials, copper, nickel
and titanium alloys.
DC electrode positive With DC electrode positive
there is a tendency for the electrode to overheat
and fusion of the workpiece is poor. The advantage
of this mode of operation is the cathodic cleaning
effect which removes the tenacious oxide film
from the surface of aluminium alloys.
AC alternating current This offers a good compromise
between plate heating and the cathodic
cleaning effect and is used with aluminium and
with manganese alloys.
Table 12.3Gas mixtures available and their applications
Gas Applications Features
Argon TIG: all metals. MIG: spray pulse, Stable arc performance. Poor wetting
aluminium, nickel, copper alloys characteristics in MIG
Efficient shielding. Low cost
Helium TIG: all metals, especially copper High heat input. Increased arc voltage
and aluminium. MIG: high-current
spray, aluminium
Argon _ 25 to 80% He TIG: Aluminium, copper, stainless Compromise between pure Ar and pure He.
steel. MIG: aluminium and copper Lower He contents normally used for TIG
Argon _ 0.5 to 15% H2 TIG: austenitic stainless steel, some Improved heat input, edge wetting and weld
copper nickel alloys bead profile
CO2 MAG: plain carbon and low-alloy Low-cost gas. Good fusion characteristics and
steels shielding efficiency, but stability and spatter
levels poor. Normally used for dip transfer only
Argon _ 1 to 7% CO2 MIG/MAG: plain carbon and Low heat input, stable arc. Finger penetration.
_ up to 3% CO2 low-alloy steels. Spray transfer Spray transfer and dip on thin sections. Low CO2
levels may be used on stainless steels but carbon
pick-up may be a problem
Argon _ 8 to 15% CO2 MIG/MAG: plain carbon and Good arc stability for dip and spray pulse
_ up to 3% CO2 low-alloy steels. General purpose Satisfactory fusion and bead profile
Argon _ 16 to 25% CO2 MIG/MAG: plain carbon and Improved fusion characteristics for dip
low-alloy steels. Dip transfer
Argon _ 1 to 8% O2 MIG/MAG: dip, spray and pulse, Low O2 mixtures suitable for spray and pulse, but
plain carbon and stainless steel surface oxidation and poor weld profile often
occur with stainless steel
No carbon pick-up
Helium _ 10 to 20% argon MIG: dip transfer, stainless steel Good fusion characteristics, high short-circuit
_ oxygen _ CO2 frequency
Not suitable for spray pulse transfer
Argon _ 30 to 40% He MIG: dip, spray and pulse welding Improved performance in spray and pulse transfer.
_ CO2 _ O2 of stainless steels Good bead profile. Restrict CO2 level for
minimum pick-up
Argon _ 30 to 40% He MIG: dip, spray and pulse welding General-purpose mixture with low surface oxidation
_ up to 1% O2 of stainless steels and carbon pick-up. (It has been reported that
these low-oxygen mixtures may promote
improved fusion and excellent weld integrity for
thick-section aluminium alloys)
314Repair of Vehicle Bodies
12.4 TIG spot welding
TIG spot welding is an adaptation of the main
process. This method utilizes the heat from the
tungsten arc to fuse the base material in much the
same way as ordinary TIG welding. The tungsten
electrode is set inside the argon shield to ensure
that fusion takes place in a completely shrouded
atmosphere. The sheets or parts to be joined are
held in close contact by the manual pressure of the
gun, and fusion is made from one side of the joint
only (Figure 12.6). The equipment comprises a
water-cooled torch with associated cables for
argon, water and power, a standard AC/DC welding
set and a timer and contact unit. The arc is
struck by pressing a switch on the gun activating
the timer which carries the welding current to the
tungsten electrode. The arc is struck automatically
and fusion takes place between the top and bottom
components of the joint to be made. The depth of
penetration through the component part is controlled
by the current and time cycle.
The process is used for joining mild steel and
stainless steel not exceeding 1.6 mm, but is not suitable
for aluminium and magnesium base alloys. The
results of the spot welds differ according to the type
and quality of the materials used, but providing the
Figure 12.3Basic principles of the TIG welding
process (BOC Ltd )
Figure 12.4Principles of the TIG welding process
Gas shielded arc welding 315
surfaces of the joint are clean and free from rust,
scale, greases or dirt, satisfactory spot welds can
generally be made.
12.5 Equipment used in TIG welding
The equipment required for TIG welding (Figure
12.1) consists of a welding torch equipped with a
gas passage and nozzle for directing the shielding
gas around the arc, a non-consumable electrode, a
supply of shielding gas (argon), a pressure reducing
regulator and flow meter, a power unit and a supply
of cooling water (Figure 12.7) if the welding
Figure 12.5Recommended edge preparation
T should not exceed 3 mm for manual welding or 5 mm for machine welding without filler wire.
With filler wire addition, machine welds up to approximately 6 mm thick material are possible
with this edge preparation
Used on material thickness up to 2 mm
This form of preparation is only used when filler wire is to be added. Gap should not normally
exceed 3 mm and then only used when plate is to be welded from both sides.
T should not exceed 10 mm
This method of preparation avoids the use of a separate filler wire and is suitable mainly for
machine welding applications. Thickness should not normally exceed 8 mm and the thickness
of the filler strips should not exceed approximately 3 mm. The edges of the plate must butt
closely to the filler strip throughout the whole length of the weld seam. H, the height of the
strip above the plate, should not exceed 3 mm for the whole range of metal thickness involved
Single-pass welds up to 10 mm T are possible with both hand and machine applications, but
multipass welds are recommended where T exceeds 8 mm. Single-pass machine welds with
filler wire addition and heavy currents are possible on thicknesses up to 3 mm but the
preparation shown in 6 is preferred where T exceeds 10 mm
This preparation is recommended for T in excess of 10 mm, assuming both sides of the joints
are accessible
Figure 12.6Principles of the TIG spot-welding
process
316Repair of Vehicle Bodies
torch is water cooled. The individual components
may differ considerably, depending upon power
requirements and the type of work to be carried out.
Power unit
Standard AC and DC welding equipment may be
used, but in most cases special welding units are
used which are capable of producing AC or DC,
have automatic control of argon and water flow, and
have fine current control switches for the stopping
and starting of welding. The choice of welding current
is determined by the material to be welded.
Metals having a refractor surface oxide film, like
magnesium, aluminium and its alloys, are normally
welded with AC, while DC is used for carbon
steels, stainless steels, copper and titanium. When
direct current is used the electrode may be connected
either to the positive or to the negative side
of the power source, depending on the material to
be welded. Usually the majority of general welding
requires direct current with negative polarity, as the
heat distribution and current loading are used to the
best advantage and the tungsten arc electrode can
carry at least four times as much current, without
overheating the electrode, as an equivalent positive
arc. Practically all metals other than magnesium
and aluminium are suitable for this method, which
gives deep penetration with a very narrow weld
bead, whereas DC positive gives a shallow penetration
with a wide bead (Figure 12.8).
Welding torch
Various types of torches are available to suit the different
applications and current requirements.
Torches may be water or air cooled: for currents
below 150 A air-cooled torches are used, while from
150 to 400 A water-cooled torches are used (Figure
12.9). The air-cooled torches are used for welding
light materials and the water-cooled torches for
welding heavier materials, as the water cooling then
prevents cracking of the ceramic shield at the tip of
the torch. The torch is fully insulated electrically
and has a quick release collet arrangement to facilitate
convenient adjustment or changing of the tungsten
electrodes. Tungsten having a melting point of
3400 °C, is used as the electrode material owing to
its refractory nature. It is almost non-consumable
when used under ideal welding conditions. A
Figure 12.7Gas, water and power supply for TIG welding
Gas shielded arc welding 317
ceramic shield, which is interchangeable, directs the
gas so as to form a shroud around the arc and weld
metal. The argon and the electric current are supplied
to the torch through a combined cable and gas
hose. In the water-cooled models a third cable is
added to carry the water to and from the torch. A
water flow switch can be provided to give complete
protection to equipment and operator by shutting off
the welding current if the water supply should fail.
To avoid contamination of either the electrode tip or
the work, which would occur if the normal method
of arc striking were employed, a high-frequency
spark unit or an arc stabilizing device is used to stop
the operator from having to touch the electrode on
the surface of the work.
Gas supply
The inert gas, argon or helium, is supplied to the
welding torch from the storage cylinder via a gas
pressure regulator and a gas economizer valve
(Figure 12.7), which may be a dual-purpose valve
when cooling water is used, and a special flow
meter calibrated in cubic feet per hour or litres per
minute of gas flow. The gas flow required varies
with current setting, shroud size, material being
welded, and type of weld joint.
Filler metals
Filler materials for joining a wide variety of metals
and alloys are available for use with the gas tungsten
arc welding process. Among them are filler rods for
welding various grades of carbon and alloy steels,
stainless steels, nickel and nickel alloys, copper and
copper alloys, aluminium and most of its alloys,
magnesium, titanium, and high-temperature alloys
of various types. There are also filler materials for
hard surfacing. Wherever a joint is to be reinforced,
a filler rod is added to the molten puddle. In general,
the diameter of the filler rod should be about the
same as the thickness of the metal to be welded. For
sound welds, it is important that the physical properties
of the rod be similar to the base metal.
TIG electrodes
Normally pure tungsten electrodes are used, but to
improve are striking and stability an addition of
either thorium oxide or zirconium oxide is added
to the tungsten. For alternating current welding
zirconiated electrodes are used, while for direct
current welding thoriated electrodes are used. The
chosen electrode must be of a diameter which suits
the current.
Improved performance can be obtained by alloying
the electrodes lanthanum, yttrium and cerium,
particularly in automatic TIG welding where consistency
of operation is important (Table 12.4). The
electrode diameter is determined by the current and
Figure 12.9A water-cooled TIG welding torch
Figure 12.8Alternative methods of DC connections
(a) Theoretical distribution of heat in the argon
shielded arc with the alternative methods (b) Average
differences in arc voltages with equal arc lengths,
using (left) negative polarity at the electrode and (right)
positive (c) Relative depths of penetration obtainable
with (left to right) DC positive, DC negative and AC
318Repair of Vehicle Bodies
polarity: recommended diameters are given in Table
12.5. The angle to which the electrode is ground
depends on the application. The included angle or
vertex angle (Figure 12.10) is usually smaller for
low-current DC applications. In order to obtain
consistent performance on a particular joint it is
important that the same vertex angle is used.
12.6 TIG welding techniques
The normal angles of the torch are 80–90° and of
the filler rod 10–20° from the surface of the horizontal
plate respectively (Figure 12.11). The arc
length, defined as the distance between the tip of the
electrode and the surface of the weld crater, varies
between 3 mm and 6 mm, depending on the type of
material and the current used. The filler rod is fed
into the leading edge of the molten pool and not
directly in the arc core, and should be added with a
Table 12.5Recommended diameters and current
ratings (BS 3019: Part 1) for TIG electrodes
Maximum current (A)
Diameter DC Electrode (_) AC electrode (_)
(mm) thoriated zirconiated
0.8 45 –
1.2 70 40
1.6 145 55
2.4 240 90
3.2 380 150
4.0 440 210
4.8 500 275
5.6 – 320
6.4 – 370
Figure 12.10Appropriate vertex angles of
electrodes (BOC Ltd)
Figure 12.11Recommended angles for torch and
filler rod in TIG welding
Table 12.4Electrodes for TIG welding
Electrode type Use
1–2% thoriated tungsten DC electrode negative
Ferrous metals, copper, nickel,
titanium
Ceriated tungsten As above
Improved restriking and shape
retention
Zirconiated AC
Aluminium and magnesium
alloys
slightly transverse scraping motion, with the tip of
the filler rod actually making contact with the weld
metal. This ensures that the rod is at the same electrical
potential as the plate during transfer of metal
to the weld and avoids any tendency of the rod to
spatter. The heated end of the filler rod should
always be kept within the influence of the shrouding
argon gas in order to prevent its oxidation.
Butt welds in thin-gauge materials are carried
out with a progressive forward motion without
weaving, but a slightly different technique is
required when tackling medium- and heavy-section
plate. As the filler rod diameter increases with
increasing thickness of plate, there is a tendency
for the end of the filler rod to foul the tungsten
electrode. Contamination of the hot tungsten by
particles of molten metal causes immediate spattering
of the electrode and particles of tungsten may,
according to the degree of contamination, become
embedded in the weld pool. Loss of tungsten in
this manner will cause the arc to become erratic
Gas shielded arc welding 319
and unstable, and the electrode will certainly
require to be replaced before further welding is
attempted. To avoid repetition of this occurrence,
the arc length must be increased slightly to accommodate
the insertion of the larger filler rod. This
procedure cannot be taken too far, however,
because there is a maximum arc length beyond
which good welding becomes impossible. The
upper limit is usually about 6 mm thick for aluminium,
which does not allow for the free access
of a 6 mm diameter filler rod. For heavier sections,
therefore, a forward and backward swinging
motion of the torch is employed. The weld area is
melted under the arc; the torch is withdrawn backwards
for a short distance from 6 mm to 3 mm
along the line of the seam and the filler rod is
inserted in the molten pool (Figure 12.12). The
torch is moved forward and the filler rod is withdrawn
from the pool simultaneously. A rhythmical
motion of both torch and filler rod backwards and
forwards in a progressive forward motion melts
down filler rod and plate without the filler rod
entering the core of the arc, and is recommended
when welding plate in excess of 6 mm thick.
steel. The advantage of the method is that materials
which would normally require flux can be welded
without it; therefore cleaning of the weld is minimized
and the effects of distortion are greatly
reduced. This process is especially adapted for
welding light-gauge work requiring the utmost in
quality or finish because of the precise heat control
possible and the ability to weld with or without filler
metal. It is one of the few processes which permit
the rapid, satisfactory welding of tiny or lightwalled
objects.
Among the materials which are readily weldable
by this process are most grades of carbon, alloy
or stainless steels, aluminium and most of its
alloys, magnesium and its alloys, copper, coppernickel,
phosphor-bronze, tin bronzes of various
types, brasses, nickel, nickel-copper (Monel
alloy), nickel-chromium-iron (Inconel alloy), hightemperature
alloys of various types, virtually all of
the hard surfacing alloys, titanium, gold, silver and
many others.
12.8 MIG/MAG welding
The development of the MIG/MAG processes is in
some ways a logical progression from the manual
metal arc and TIG processes. The effort throughout
has been to obtain and maintain maximum versatility,
weld quality, speed of deposition, simplification
of the welding operation and lower operating costs.
MIG/MAG welding has been adapted for many
industrial applications, and over the past years has
become widely used for car body repairs. This
method of welding is an electric arc process using
DC current and a continuous consumable wire
electrode without the addition of flux. On account
of the absence of flux, gas is used to shield the arc
and weld pool from atmospheric contamination.
The process can utilize argon, argon/carbon dioxide
mixture or carbon dioxide as the shielding gas,
the choice being dependent upon the type of material
being welded and the economics associated
with the selected gas. If non-ferrous metals or
stainless steels are to be welded, argon is the usual
choice for shielding gas on grounds of compatibility.
However when mild steel, low-alloy steels or
high-strength steels are to be welded, argon/carbon
dioxide mixture or carbon dioxide is generally
used for reasons of overall efficiency, weld quality
and economy.
Figure 12.12Motion of torch and filler rod for TIG
welding heavy sections
12.7 Application of TIG welding
This process has found an application in the body
building side of the industry, where it is used to
ideal advantage for fabricating components by welding
in materials such as aluminium and stainless
320Repair of Vehicle Bodies
Many types and grades of metal can be welded
using this method: aluminium, aluminium alloys,
carbon steel, low-carbon and alloy steels, microalloy
steel, nickel, copper and magnesium. The success
of this welding method is due to its capability
of giving a consistently high-quality weld while
also being very easy to learn. In addition it has the
advantage of spreading very little heat beyond the
actual welding point, and this helps to avoid distortion
and shrinking stresses which are a disadvantage
in the oxy-acetylene process.
Principles of operation
Metal inert-gas or active-gas shielded arc welding
(consumable) is accomplished by means of a gas
shielded arc (Figures 12.13 and 12.14) maintained
between the workpiece and a consumable (bare
wire) electrode from which metal is transferred to
the workpiece. The transfer of metal through the
protected arc column provides greater efficiency of
heat input than that obtained in the TIG welding
process. The resultant high-intensity heat source
permits very rapid welding. In this process a continuously
fed electrode passes through a gun, during
which it passes through a contact area which
impresses the preselected welding current upon the
wire. The current causes the wire to melt at the set
rate at which it is fed. The shielding gas issuing
from the nozzle protects the weld metal deposit
and the electrode from contamination by atmospheric
conditions which might affect the welding
process. The arc may be started by depressing the
trigger of the welding gun and scratching the electrode
wire end on the work.
The equipment is designed so that the wire automatically
feeds into the weld area as soon as the
arc is established. Most MIG/MAG welding sets
that are manufactured for the automobile repair
trade are semi-automatic, the operator only being
concerned with the torch-to-work distance, torch
manipulation and welding speed. Wire feed speeds,
power settings and gas flow are all preset prior to
commencement of welding.
12.9 MIG/MAG spot/plug welding
An advantage of MIG/MAG welding is the ability
of the process to be adopted for single-side spot
welding applications, either semi or fully automatically.
By extending the welding gun nozzle
to contact the workpiece, one-sided spot welds
may be performed using dip transfer conditions.
Predetermined weld duration times may be
employed, the gun trigger being coupled to a suitable
timer and, if desired, fully mechanized.
Unlike resistance spot welding, no pressure is
Figure 12.13Basic principles of operation of
MIG/MAG welding (BOC Ltd)
Figure 12.14Principles of the MIG/MAG welding
process: argon, argon/CO2 or CO2
Gas shielded arc welding 321
required on the workpiece with MIG/MAG spot
welding, and neither is a backing block. Mismatch
of the sheets is permissible with a maximum gap
equivalent to half the sheet thickness, the extra
metal being provided by the electrode wire. Up to
30 spots per minute may be welded, which compares
reasonably well with the 100 spots per
minute from resistance welding techniques. The
deep penetration characteristics of this welding
process enable spot welding of widely differing
metal thicknesses to be performed successfully,
together with multisheet thicknesses.
MIG/MAG plug welding differs from MIG/MAG
spot welding in that the outer metal panel has a
predrilled or punched hole which is filled up with
the weld metal to form the ‘plug’. The hole sizes
used are 5–6 mm with currents of 50–60 A for
panel thicknesses of 0.75–1.2 mm. Care should be
taken when MIG/MAG plug welding to avoid an
excess build-up of weld metal, to reduce the necessity
of dressing the finished weld. This method is
used to advantage in the fitting of new panel sections,
where the original was resistance spot welded
and access is now difficult to both sides.
Higher-strength steels are used for selected
panel sections, but as they are vulnerable to heat
they are not as easily welded as mild steel. MIG
seam and butt welding produce hard weld joints;
they will tear from the sheet metal on impact.
However, MIG spot welds and particularly puddle
(or plug) welded MIG spots can be made ductile.
The weld time has to be short to reduce the heataffected
zone around the weld.
Modern car body designs are constructed with
deformation zones both front and rear, and the
shear impact properties of the original welding
have been carefully calculated to ensure that the
energy of an impact is fully absorbed and contained
within the zone. Changing the manufacturers’
original welding specification could impair the
safety of the vehicle. The crumplability (impact
energy absorbing) design of car bodies makes new
demands on welds. For its success the body design
relies on the sheet metal to crumple or fold rather
than tear in a collision. This protective design
depends on the ductility of the metal, and the
welds too have to be ductile. If they are soft they
will break without forcing the assembly to crumple,
but if they are hard the welds will unbutton
and the assembly will fly apart instead of folding
up slowly. Welds therefore must be ductile as well
as large and strong.
12.10 Equipment used in MIG/MAG
welding
The basic equipment required comprises a power
source, a wire feed unit and a torch. The power
sources commonly used have constant-voltage characteristics,
and controls are provided for voltage and
inductance adjustment. This type of power source is
used in conjunction with a wire feed unit which
takes the wire from a spool and feeds it through a
torch to the arc. A control on the wire feeder enables
the speed of the wire to be set to a constant level
which will in turn determine the arc current.
The welding torch should be reasonably light
and easy to handle, with provision for gas shield
shrouding, control switch, easy wire changes and
good insulation. The torch is connected to a wire
feed and control unit by means of:
1 A flexible armoured tube carrying the welding
wire
2 A plastic tube carrying the shielding gas
3 A pair of plastic tubes carrying cooling water,
the return tube often carrying the welding supply
to cool the welding cable (light-duty torches are
air cooled)
4 The control wires for the switchgear.
The whole feed unit is bound together by a plastic
sleeve, and is 1.5–3 m long. The wire feed unit
houses the drive unit for the wire feed, which can be
varied in speed to suit current/voltage conditions.
The wire reel is also carried in the unit and is loaded
with some 15 kg of welding wire.
Power unit
DC power units are used as either rotary generators
or rectifying units which are specially designed to
give full versatility of arc control. The equipment
is either single phase (130–240 A) or three phase
(250–500 A). The principal components of most
machines are welding transformer, rectifier, choke
coil, wire feed unit, gas solenoid valve and electronic
control box.
The welding transformer is dimensioned so as to
achieve optimal welding properties. The transformer
is manufactured from materials able to withstand a
322Repair of Vehicle Bodies
working temperature of up to 180 °C. By way of a
further safeguard against overloading, there is a
built-in thermal fuse which cuts out the machine at
120 °C. The thermal fuse is automatically reconnected
once the transformer has cooled down.
The rectifier is constructed from a fan, thyristors,
diodes and a capacitor battery. During welding,
or when the machine is hotter than 70 °C, the
fan cools the rectifier and the transformer. The rectifier
is electronically protected against overloading
in the event of any short-circuit between
welding positive and negative, with the machine
cutting out approximately 1 second after the onset
of short-circuiting.
The transformer converts the high mains voltage
into a low alternating voltage, which is rectified
and equalized into DC voltage in the rectifier module.
A choke coil reduces the peaks in the welding
current and thus eliminates the cause of welding
spatter. When the switch on the torch is depressed,
the thyristors come on. The rectifier emits a DC
voltage, which is determined by the remote control
in the welding trigger and the gas/wire matching
switch on the box. Simultaneously, shielding gas is
turned on and the wire feed motor is started up at a
speed also determined by remote regulation from
the torch.
When the trigger is released the motor decelerates,
and after a short time lag the gas flow and
welding voltage are interrupted. This time delay is
called burn-back and causes the welding wire to
burn a little way back from the molten pool, thus
preventing it from sticking to the workpiece.
Depending on the type of equipment selected,
the following functions are available: seam, spot,
stitch and latching (four-cycle).
Seam Welding starts when the switch on the
welding trigger is actuated and stops when the
switch is released. For use in short-duration welding
and tacking.
Spot Welding starts when the switch is actuated
and is subsequently controlled by the welding
timer for a time between 0.2 and 2.5 s. It makes no
difference when the switch is released. This function
ensures uniform spot welds, providing the correct
setting has been found.
Stitch The wire feed motor starts and stops at
intervals which are set on the welding timer and
pause timer. When welding is interspersed with
pauses in this way, the average amount of added
heat is reduced, which prevents any melting
through on difficult welding jobs.
Latching Welding starts when the switch is actuated;
the switch can then be released and welding
continues. By reactuating the switch, welding stops
when the switch is released. For use on long
seams.
A typical welding control panel (Figure 12.15) has
the following features:
1 Selection switch This selects between the
functions seam, spot, stitch and latch as
described above.
2 Power light This lights up when the machine
has been turned on.
Figure 12.15MIG welding control panel (Migatronic Welding Equipment Ltd )
Gas shielded arc welding 323
3 Overheating warning light If this light comes
on, the welding equipment is automatically
switched off owing to overheating of the transformer.
When the temperature is back to normal,
the welding can be continued.
4 Welding timer switch With this switch the
welding time is chosen, when the selection
switch is in the stitch or spot position.
5 Adjustable pause time button With this button
the pause time is chosen, when the selection
switch is in the stitch position.
6 Burn-back button Pre-adjustment of the burnback
delay button indicates the time for stopping
the wire feed until the arc is switched off.
This varies between 0.05 and 0.5 s.
7 Adjustment of wire speed switch This gives
the adjustment of the wire feed from 0.5 to
14 m/min.
8 Welding voltage switch This sets the welding
voltage of the transformer. When set at gas test,
the gas flows by pressing the switch on the
torch handle.
The characteristics built into the welding power supply
are such as to provide automatic self-adjustment
of arc conditions as the weld proceeds. Depending
on the relevant current and voltage used, metal
transfer between electrode and work takes place in
the following distinct forms each of which has certain
operational advantages (Figure 12.16).
Dip transfer (short arc)
This condition requires comparatively low current
and voltage values. Metal is transferred by a rapidly
repeated series of short circuits when the electrode
is fed forward into the weld pool.
Metal dip transfer is the most suitable mode of
metal transfer for welding on car repairs, as it
offers good bead control and low heat input, thus
cutting down distortion when welding in panel sections.
This type of transfer will also permit the
welding of thinner gauges of sheet steel, and is
practical for welding in all positions.
The principle of the method (short-circuiting
transfer) is briefly as follows. The molten wire is
transferred to the weld in droplets, and as each
drop touches the weld the circuit is shorted and the
arc is momentarily extinguished. The wire is fed at
a rate which is just greater than the burn-off rate
for the particular arc voltage being used; as a result
the wire touches the weld pool and short-circuits
the power supply. The filler wire then acts as a
fuse, and when it ruptures a free burning arc is created.
After the occurrence of this short-circuit, the
arc re-ignites. This making and breaking, or arc
interruption, takes place from 20 to 200 times per
second according to the setting of the controls. The
result is a relatively small and cool weld pool, limiting
burn-through.
Free flight transfer
In this metal transfer a continuous arc is maintained
between the electrode and the workpiece
and the metal is transferred to the weld pool
as droplets. There are three subdivisions of the
system:
Spray transfer In this type the mode of transfer
consists of a spray of very small molten metal
droplets formed along the arc column, which are
projected towards the workpiece by electrical
forces within the arc and collected in the weld
pool. No short-circuiting takes place; the welds are
hotter, and the depositing of weld metal is faster. It
is ideally suited for the rapid welding of thick sections
in the downhand position and the positional
welding of aluminium and its alloys.
Globular transfer This occurs at currents above
those which produce dip transfer, but below the
current level required for spray transfer. The
droplet size is large relative to the wire diameter
and transfer is irregular. This mode of transfer
occurs with steel wires at high currents in carbon
dioxide, but is generally regarded as unusable
unless high spatter levels can be tolerated. The use
of corded wires gives a controlled form of globular
transfer which is acceptable.
Pulse arc transfer In this mode the droplets are
transferred by a high current which is periodically
applied to the arc. Ideally, one drop is transferred
with each pulse and is fired across the arc by the
pulse. Typical operating frequencies are 50–100
droplets per second. A background current is maintained
between pulses to sustain the arc but avoid
metal transfer. Maximum control is obtained with
this type of metal transfer, which utilizes a power
supply to provide a pulsed welding condition in
which the metal transfer takes place at pulse peaks.
This leads to extreme control over the weld penetration
and weld appearance.
324Repair of Vehicle Bodies
Figure 12.16Metal transfer forms
Gas shielded arc welding 325
Welding torch
Air cooled torches are available for the various
welding applications ranging from 180 A to 400 A.
The design of the torch is in the form of a pistol or
is curved similar to the shape of an oxy-acetylene
torch (Figure 12.17), and has wire fed through the
barrel or handle. In some versions where the most
efficient cooling is needed, water is directed
through passages in the torch to cool the wire contact
means as well as the normally cooled metal
shielding gas nozzle (Figure 12.18). The curved
torch carries the current contact tip at the front end
through which the feed wire, shielding gas and
cooling water are also brought. This type of torch is
designed for small diameter feed wires, is
extremely flexible and manoeuvrable and is particularly
suitable for welding in confined areas. The
service lines consisting of the power cable, water
hose and gas hose on most equipment enter at the
handle or rear barrel section of the torch. The torch
is also equipped with a switch for energizing the
power supply and controls associated with the
process. Some welding torches also have a current
control knob located in the torch so that the welding
current can be altered during welding. This
ensures that the welding voltage is altered at the
same time as any current alteration, so allowing the
welder to respond immediately to variations in
weld gaps and misaligned joints. Welding characteristics
are excellent. Fingertip control increases
both productivity and weld quality (Figure 12.19).
Feed unit
The consumable electrode for welding ferrous and
non-ferrous materials is supplied as a continuous
length of wire on a spool or reel, and the wire varies
from 0.6 to 0.8 mm diameter. The feeding of the
wire is achieved by the unit feed mechanism housing
the necessary drive motor, gear box and feed
rolls which draws wire from the adjacent reel, or by
an integrally built motor drive connected to the torch
which pulls the wire in the desired direction. The
feed wire unit also houses the controls which govern
the feeding of the wire at the required constant
Figure 12.17Air-cooled MIG welding torch (Murex Welding Processes Ltd )
326Repair of Vehicle Bodies
speed, plus the automatic control of current and gas
flow. The wire from the feed rolls is fed along a
carefully designed conduit system which also carries
the welding current and shielding gas. The outer
end of this conduit system is connected directly to
the welding torch (Figure 12.20).
Figure 12.18Range of MIG welding torches, air and
water cooled, 180–600 A (Murex Welding Products Ltd )
Figure 12.19Migatronic Dialog torch with current
control in the torch handle (Migatronic Welding
Equipment Ltd )
Figure 12.20Wire feed unit (Migatronic Welding
Equipment Ltd )
Gas supply
The primary purpose of the shielding gas used in
MIG/MAG welding is the protection of the molten
weld metal from contamination and damage by the