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