Perform Gas Metal Arc Welding - Department of Training and

Perform Gas Metal Arc Welding - Department of Training and
Perform Gas Metal
Arc Welding
Workbook
(AUM8057A)
AUT032
AUM8057A
Perform Gas Metal Arc Welding
Workbook
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First published 2008
ISBN 978-0-7307-9919-1
© Department of Education and Training 2008
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AUM8057A
Perform Gas Metal Arc Welding
Contents
Introduction.............................................................................................................. 1
Gas metal arc welding process (GMAW) ................................................................. 2
Advantages of the GMAW process .......................................................................... 3
Limitations of the GMAW process ........................................................................... 3
Safety in gas metal arc welding ............................................................................... 3
Darker welding filters............................................................................................ 3
Body protection .................................................................................................... 4
Ventilation............................................................................................................. 5
Protecting others .................................................................................................. 6
Equipment .................................................................................................................. 6
Power source ....................................................................................................... 7
Wire feed unit ....................................................................................................... 7
Gun cable assembly............................................................................................. 8
Gas supply system ............................................................................................... 8
Interconnecting cables ......................................................................................... 9
Wire feed systems................................................................................................ 9
Drive rollers ........................................................................................................ 10
Wire conduit (liner) ..............................................................................................11
Contact tip .......................................................................................................... 12
Metal transfer models ............................................................................................. 12
Dip transfer......................................................................................................... 12
The dip transfer cycle ......................................................................................... 13
Spray transfer..................................................................................................... 14
Globular transfer ................................................................................................ 15
Comparison between GMAW and MMAW ............................................................. 19
Weld positions ......................................................................................................... 20
Weld symbols .......................................................................................................... 22
Joint symbols ..................................................................................................... 23
Length and pitch of fillet welds ........................................................................... 24
Classification of consumables ............................................................................... 30
Solid wire electrodes .......................................................................................... 30
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Shielding gases ....................................................................................................... 32
Carbon dioxide ................................................................................................... 32
Argon ..................................................................................................................32
Gas mixtures ...................................................................................................... 33
Flow rates........................................................................................................... 34
GMAW variables ...................................................................................................... 34
Wire speed/amperage ........................................................................................ 34
Arc voltage ......................................................................................................... 35
Electrical stickout ............................................................................................... 36
Travel speed ...................................................................................................... 38
Torch angle......................................................................................................... 38
Angle of travel .................................................................................................... 39
Gas flow rate ...................................................................................................... 40
Other machine controls .......................................................................................... 41
Spot timer ........................................................................................................... 41
Burnback control ................................................................................................ 41
Gas purge switch ............................................................................................... 41
Inch wire control ................................................................................................. 41
Spool brake ........................................................................................................ 41
Fillet and butt weld structures ............................................................................... 43
Joint design for GMAW........................................................................................44
Control of distortion ................................................................................................ 48
Before welding ................................................................................................... 49
During welding ................................................................................................... 52
After welding ...................................................................................................... 53
Where not to weld ................................................................................................... 54
Procedure sheets .................................................................................................... 58
GMAW defects ......................................................................................................... 59
Porosity .............................................................................................................. 59
Cold lap/lack of fusion ........................................................................................ 60
Lack of root penetration ..................................................................................... 61
Excessive penetration ........................................................................................ 61
Contour defects .................................................................................................. 62
Undercut ............................................................................................................ 62
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Weld cracking..................................................................................................... 63
Stray arcing ........................................................................................................ 64
Excessive spatter ............................................................................................... 64
Trouble shooting/equipment malfunction ............................................................. 64
Tanks and containers .............................................................................................. 68
Practical exercises .................................................................................................. 70
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Perform Gas Metal Arc Welding
Introduction
Since its introduction in the 1940s, gas metal arc welding (GMAW) has become a
very popular welding process for the vehicle body building industry, as it has several
advantages over other welding systems. It is particularly suited to a wide range of
light and heavy applications ranging from one-tonne tray bodies and tradesman vans
to semi-tippers and road trains. The versatility, ease of operation and relative low
distortion rate allow this process to be used on a wide range of materials.
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Gas metal arc welding process (GMAW)
Gas metal arc welding (GMAW) is an arc welding process where the necessary heat
for fusion is produced by an electric arc maintained between a continuously fed wire
electrode and the part to be welded. The heated weld zone, the molten weld metal, and
the consumable electrode are shielded from the atmosphere by a shroud of gas which
is delivered through the welding torch to the weld pool.
Gas nozzle
Direction of travel
Wire guide and contact tip
Shroud of gas
Solidified weld metal
Parent metal
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Advantages of the GMAW process
The major advantage of the GMAW process is its high deposition rate compared with
the Manual Metal Arc, and Gas Tungsten Arc welding processes. This is brought about
by the high ratio of current to wire diameter, and the removal of the need to change
electrodes, chip slag etc. These advantages are facilitated by the use of a continuously
fed, gas-shielded electrode. The advantages of this process compared to manual metal
arc welding (MMAW) are summarised below:
•
high deposition rates when compared to manual metal arc welding
•
no wastage from electrode stubs
•
elimination of slag removal
•
less operator skill required
•
has a wide range of applications
•
low hydrogen deposit
•
reduced distortion on thin materials when compared to manual metal arc welding.
Limitations of the GMAW process
Although GMAW is a popular and versatile welding process offering the advantages
listed above, it is also limited by the following:
•
high initial equipment cost
•
high maintenance requirements and low mechanical reliability of the welding
equipment
•
cannot be used in windy conditions (AS 1554-1 limits the use of gas-shielded
processes where the wind velocity exceeds 10 km/hr). This makes the process
generally unsuitable for work outside the factory
•
lack of fusion defects can be a major problem under some circumstances
•
a degree of expertise required for setting weld parameters
•
requires a knowledge of equipment trouble shooting.
Safety in gas metal arc welding
Darker welding filters
The primary concern in this regard is arc intensity, which is much greater than that
associated with MMAW electrodes. A darker welding filter will be required for the
GMAW process when compared with MMAW. A filter one shade darker than that used
for welding at the same amperage with the MMAW process will be required.
For example:
•
up to 200 amps: a shade 11 is required
•
200–300 amps: a shade 12 is required.
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Safety glasses worn at all times are essential, as the higher emission of ultraviolet (UV)
radiation may result in increased and more severe arc flashes.
Dark filter protected
by clear lens
Body protection
This same arc intensity also requires operators to ensure their body is completely
covered with protective clothing. Even extraneous light from the arc (UV radiation
bouncing from a reflecting wall) can result in a rather uncomfortable ‘ray burn’.
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You must wear safety boots, gloves, long sleeves, and a suitable face shield. For more
intense work, wearing a leather apron and a cap are also necessary.
Experience has shown that cotton materials have less resistance to ultraviolet rays
than woollen materials. Cotton, and particularly synthetics, will quickly break down and
eventually disintegrate. It is therefore preferable to wear leather or woollen materials.
Ventilation
During arc welding a toxic gas called ozone (O3) is given off from the arc, with higher
current densities producing higher ozone levels. Although ozone is not dangerous
under most conditions, it is advisable to use exhaust extraction when working in
confined spaces where ventilation is restricted. Natural ventilation and exhaust fans
can also be advantageous. Any ventilation system used must not interfere with the gas
shielding of the weld zone.
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Protecting others
To protect other workers, you must shield your working area with suitable screens to
prevent stray arc rays escaping the work area as well as any sparks from welding or
grinding.
Equipment
The major equipment items which make up a GMAW plant are:
•
the power source
•
the wire feeder
•
the welding gun cable assembly
•
the gas supply system
•
the interconnecting cables.
Power source
Wire feeder
Transformer
rectifier
Flowmeter
Heater
(if required)
Inductor
Regulator
Power
cable
Cooling water
(if required)
Welding wire
Shielding
gas supply
Contact
tip
Shielding
gas
Nozzle
Work
GMAW equipment
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Gas diffuser
Nozzle
Contact tip
Shielding gas
Electrode wire
Contact
tip to
work
Electrode stickout
Arc length
Power source
A constant voltage (constant potential) power source is required for GMAW. This is
commonly a transformer/rectifier or, increasingly, an inverter. The output requirement
is for direct current. All solid wires for GMAW run on direct current electrode positive
(DC+). The GMAW process is intolerant to variations in arc voltage, and the constant
voltage output provided by the constant voltage (CV) power source ensures that the
arc length is self-adjusting and remains constant despite uneven torch movement.
OCV (Open Circuit Voltage)
Operating point
V
Volts
A
Current
Wire feed unit
The primary function of the wire feed unit is to feed wire to the arc pool. This unit
houses a reel of electrode wire and a DC motor to which feed rollers are attached.
Feed rollers push the electrode wire to the arc pool. The speed of the drive motor is
governed by a potentiometer (the wire feed control) and is influenced by variations in
arc voltage. Increasing the wire speed also increases the amperage. Incorporated into
the unit are the shielding gas connections, gas solenoid and, in the case of a watercooled torch, water connections.
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Gun cable assembly
•
The gun cable assembly consists of a large outer cable which covers and protects
several smaller conduits by which electrode wire, current and shielding gas are
conveyed to the welding arc pool. It connects to the wire feeder and terminates at
the ‘gun’ or ‘handpiece’.
•
The electrode wire travels through the wire conduit or ‘liner’ which runs through
the centre of the gun cable. The welding current is carried through the cable by a
heavy copper lead within the cable.
•
Shielding gas is also carried through the cable, and is distributed at the weld pool
via the gas diffuser and gas nozzle.
•
There are two trigger control wire cables, a positive and a negative, which
send back a signal to the power unit when the torch trigger on the handpiece is
depressed. This starts the whole welding operation.
Shielding gas
Power cable
Outer cable casing
Wire liner
Electrode wire
Trigger control cables
Welding is started by depressing the torch trigger. This initiates three separate
functions:
1.
The welding current contactor solenoid is ‘pulled in’ (closed) and welding current
becomes available. Welding current is transferred to the wire as it passes through
the contact tip.
2.
The gas solenoid valve opens and allows shielding gas to flow.
3.
The wire feed motor starts up and feeds wire at the preset, constant speed
through the wire conduit.
Because of the heat generated in the weld pool and the heat generated through
electrical resistance at the contact tip, torches have to be efficiently cooled. The
majority of torches are air-cooled; however, water-cooled torches may be required
when high amperages are used on a continuous basis.
Gas supply system
Shielding gases for gas metal arc welding (GMAW) are usually supplied from a single
cylinder; however, large consumers may use manifolded systems. The components of
the gas supply system are:
•
a cylinder of gas containing either carbon dioxide (CO2), argon (Ar) or an
argon/CO2 mixture which may include oxygen (O2)
•
a regulator to reduce cylinder pressure
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•
a flowmeter to control shielding gas flow rate
•
a heater – when CO2 is used as a shielding gas, a heater is fitted between the
cylinder and the regulator to prevent freezing at the regulator.
Interconnecting cables
These consist of:
•
the work return lead
•
the electrode lead – from the power source to the gun cable adaptor of the wire
feeder
•
the control cable from the power source to the wire feeder.
Wire feed systems
There are three basic types of GMAW wire feeding systems, each requiring different
torches.
1.
The push system
The push system is by far the most popular wire feed system. The wire feed unit
pushes the electrode wire along the wire conduit (liner), through the gun and contact
tip and to the weld pool. Push systems are generally robust, lightweight and very
functional, as well as being the least expensive of the three systems. The system works
very well with hard wires such as steel and stainless steel up to 4.5 metres in length.
Wire in spools of 15 kg or larger are usually used with this system. This keeps costs
down and increases efficiency.
The major disadvantage of the push system is unreliability of wire feeding caused by
friction in dirty liners or kinked gun cables. This is a particular problem when feeding
soft wires such as aluminium.
Push system
2.
The pull system
The pull system is ideally suited to feeding soft wires such as aluminium or where
welding is to be carried out at a location remote from the power source. The drive
motor and drive rollers are built into the handle of the welding torch. This offers a short,
direct wire travel, with little friction through the conduit.
The drawbacks of this system are the high initial cost of equipment, the cost of
consumable wire on small spools and the weight of wire carried on the gun. Although
this system is mainly used for aluminium work, mild steel and stainless steel wires can
also be used.
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Pull system
3.
The push/pull system
As the name implies, both push and pull motors are employed. One motor is in the
welding torch handle and pulls the wire through the torch. The other motor is in the wire
feeder and pushes the wire through the wire conduit. The motors are synchronised to
feed the wire at the same speed. This enables the feeding of both hard and soft wires
up to 10 metres from the welding machine and still offers the economy of 15 kg (or
larger) spools of wire. The push/pull system is a versatile system; it is particularly suited
to aluminium, but may also be used for hard wires as well.
Push/pull system
Drive rollers
Friction created by the pressure applied to the electrode wire as it passes through
the rotating drive rollers enables the electrode to be pushed along the wire conduit.
Resistance in the gun cable may cause the wire to slip as it passes through the drive
rollers. Increasing the pressure of the top roller increases friction and prevents this
slippage. However, excessive pressure can deform the wire, making it more difficult to
feed.
Upper roller
Grooved roller
Deformation caused by excessive roll pressure
Wire feeders use either a two or four roll drive system. Two roll systems are cheaper
to buy and are best suited to feeding hard wires such as carbon steels and stainless
steels through short gun cables.
Four roll feeders create greater friction between the rollers and the wire with less roller
pressure, resulting in a smoother wire feed with less slippage and less distortion of the
wire.
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The four roll system offers advantages for:
•
feeding soft wires such as aluminium
•
feeding wires through long gun cables
•
use with cored wires.
Four roll feeder
Two roll feeder
Cross-sectional shapes of the rollers vary according to the manufacturer and the type
of wire being used.
Common configurations/sections of drive rolls and their uses:
Flat top roller with a ‘V’
bottom roller.
Used for general purpose
feeding of hard wires such
as steel and stainless steel
Flat top roller with a ‘U’
bottom roller.
Used mainly for aluminium
wires. The ‘U’ profile
reduces deformation of the
soft wire
Serrated ‘V’ top roller
with a serrated ‘V’
bottom roller.
Used for cored wires and
large diameter solid wires
Note: This list is not exhaustive, but these are the most commonly used.
Wire conduit (liner)
The liner is used to guide the wire through the gun cable to the handpiece, and through
to the contact tip. When welding with carbon and stainless steels the liner is made of
spiral wound wire. Teflon® is used when feeding aluminium wire. To ensure reliable
wire feeding, it is imperative that the liner is cut to the correct length and properly fitted
in the gun cable. It is also important that the gun cable is kept as straight as possible
when welding.
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Contact tip
The contact tip serves two functions:
•
to guide the wire to the arc
•
to transfer welding current to the wire.
The contact tip is a most important component of the welding torch. It is here that the
filler-wire is energised or ‘picks-up’ the welding current. It is usually made from copper
and is directly attached to the power lead via the gas diffuser and torch body. Contact
tips are matched to each wire size. It is important that the contact tip is maintained in
a clean condition free from spatter on the end, and with a smooth internal bore. Worn
contact tips reduce the efficiency with which the welding current is transferred to the
electrode wire and contribute to uneven wire feeding. They should be replaced when
worn.
Metal transfer modes
With most of the commonly used welding processes the operator has little control over
the way metal is transferred across the arc. With GMAW the operator can select and
control the type of metal transfer. This is done primarily by selection of arc voltage,
although wire diameter and shielding gas also influence metal transfer.
The metal transfer mode determines the characteristics of the GMAW process. The
operator must select the most appropriate mode of transfer and set the machine
accordingly before starting the weld.
Apart from the pulsed transfer mode, which requires sophisticated power sources, the
welding operator can select from three transfer modes:
•
dip (or short arc) transfer
•
globular transfer
•
spray transfer.
Dip transfer
Dip transfer is also known as ‘short arc’ transfer (short for ‘short circuiting arc’). In the
dip transfer mode, low current and low voltage settings are used. The low voltage
employed cannot maintain a continuous current flow across the gap between the
electrode wire and the work piece. As the electrode nears the work piece the electrical
resistance across the arc gap is overcome and an arc is established.
During welding, the tip of the electrode wire contacts the work piece and a short circuit
occurs. This results in a rapid temperature rise in the wire (caused by the short circuit
current flowing through to the work piece) and the end of the electrode wire is melted
off. An arc is immediately formed between the tip of the wire and the weld pool. This arc
maintains the electrical circuit for a short time until the electrical resistance across the
increasing arc gap causes the arc to be extinguished.
The electrode wire continues to feed, and the tip once again dips into the pool and
the cycle is repeated. This sequence of events is repeated at a frequency of up to 200
times per second, and produces sufficient heat for fusion and to keep the weld pool
fluid.
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This method of transfer is suitable for all positional welding due to rapid freezing of
the weld pool, and has the advantage that the heat input to the work piece is kept to a
minimum. This limits distortion and enables thin sheet material to be welded. However, on
thicker material, the low heat input tends to give rise to lack of fusion defects if care is not
taken with machine adjustment and welding technique.
Typical weld conditions:
Volts
13–23
Amps
60–200
Stickout
6 mm–15 mm
Electrode
Work
Schematic diagram of short arc transfer
The dip transfer cycle
1.
Trigger is depressed – wire starts to feed.
2.
Wire contacts the work piece and heats up due to electrical resistance and begins
to melt.
3.
Wire melts off and an arc is established.
4.
Arc length increases as the end of the wire melts slightly.
5.
Arcing ceases due to the low arc voltage being unable to overcome the electrical
resistance across the arc gap.
6.
Wire is fed into the weld pool which has been created and the cycle begins again.
Features of dip transfer:
•
low currents are used
•
low heat input
•
low penetration
•
moderate spatter
•
low deposition rate compared with other transfer modes
•
relatively cold weld pool
•
ideal for thin materials
•
can be used for out-of-position welding such as vertical ups and overhead welds
•
‘lack of fusion’ faults are a problem, particularly when plate thickness exceeds 5 mm.
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Spray transfer
Unlike dip transfer, spray transfer employs an arc which burns continuously. To achieve
this, the arc voltage is relatively high and must be above approximately 23 volts when
welding steel (depending on wire size and shielding gas composition).
Additionally, the amperage used must be above the ‘threshold current’. The threshold
current is the current above which tiny droplets are pinched off and projected axially
across the arc gap. Below the threshold current, droplet detachment is brought about
by the molten droplet of wire growing in size, until it is heavy enough to be detached by
gravitational forces.
Typical weld conditions:
Volts
24–40
Amps
200–Upwards
Stickout
15 mm–30 mm
Electrode
A
A A
A
B
Arc
B
Spray transfer
Spray transfer offers greatly increased deposition rates compared with dip transfer,
minimal spatter, and is not accompanied by the lack of fusion faults sometimes
associated with dip transfer. Because of the hot, fluid weld pool associated with spray
transfer, it is only suitable for use on plates above approximately 5 mm thick, and in the
downhand (flat) position.
Features of spray transfer:
•
high currents are used
•
high heat input
•
moderate/deep penetration
•
high deposition rates
•
low spatter
•
good appearance
•
fluid weld pool
•
unsuitable for out-of-positional welding such as overhead welding
•
requires a shielding gas with high argon content.
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Globular transfer
Globular transfer occurs at current levels between those used for dip and spray
transfer. Voltages are high enough to ensure a constant arc, but amperage is set below
the threshold current that produces spray transfer. The result is that the wire melts in
the arc, and a molten globule forms on the end of the wire. As melting continues, the
size of the globule grows until its own weight causes detachment of the droplet due to
gravitational forces. This droplet detachment is erratic because of the influence of arc
forces repelling the droplet away from the wire; high spatter levels result. Droplet size is
considerably larger than the wire diameter.
Typical weld conditions:
Volts
20–26
Amps
200–280
Stickout
12 mm–22 mm
Electrode
Arc
Globular transfer
Features of globular transfer:
•
moderate amperages are used
•
low to moderate penetration
•
moderate to high spatter levels
•
coarse appearance
•
metal droplets are detached by gravitational forces
•
largely unsuitable for out-of-position welding
•
occurs even at high amperages when the shielding gas contains in excess of
approximately 23% CO2.
The tables on the next page show the amperage and voltage ranges for the transfer
modes described above.
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Volt and current ranges for transfer modes
Working range for the different types of transfer modes.
Median range
Voltage
(V)
40
Spray arc range
30
Globular arc range
20
10
Short arc range
Current (amps)
100
200
300
400
By carefully selecting the amperage (wire speed) and the voltage, it is possible to
set the parameters to weld effectively in the three modes of transfer shown above.
However, if the welding parameters are set outside the three circles, your welding
conditions will become erratic and uncontrollable.
The GMAW transfer modes
Transfer
mode
Welding
positions
Volts
Amps
Wire
diameter
Uses
Short arc
(dip)
All
13 to 23
60 to 200
0.6 mm to
1.2 mm
Light
material.
Out-ofposition
welding
Globular
Flat or
horizontal
fillets
20 to 26
200 to 280
0.6 mm to
1.6 mm
Between
dip and
spray
Spray
Flat or
horizontal
fillets
24 to 40
210 to 410
0.8 mm to
1.6 mm
Material
over 5 mm
thick
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Questions
1.
What is the primary function of a shielding gas used in the GMAW process?
_________________________________________________________________
2.
List four advantages offered by the GMAW process over MMAW.
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
3.
State the shade of lens which would be required when welding with a setting of
190 amps.
_________________________________________________________________
4.
In what type of situation would ozone gas present a problem?
_________________________________________________________________
5.
List the five major equipment items which make up a GMAW plant.
_________________________________________________________________
6.
In a GMAW plant, what type of current is used for welding with solid wire?
_________________________________________________________________
7.
When the torch trigger is depressed, what three separate functions occur?
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
8.
State the difference between the wire electrode push system and the pull system.
_________________________________________________________________
9.
Where would a four roller system be used in preference to a two roller system?
_________________________________________________________________
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10. State the two major functions of a contact tip.
_________________________________________________________________
_________________________________________________________________
11. Of the three modes of metal transfer, which has the highest deposition rate?
_________________________________________________________________
12. In what weld positions can short arc (dip) welding be used?
_________________________________________________________________
13. How many amps are required before spray welding can begin?
_________________________________________________________________
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Comparison between GMAW and MMAW
The following table shows a comparison of gas metal arc welding (solid wire) with
manual metal arc welding.
Gas metal arc
welding
Item
Capital cost
Deposition rates
Manual metal arc
welding
Higher
Lower
More complicated plant
Relatively simple plant
Higher
Lower
Higher current densities
result in higher deposition
rates
Length of run that can be
deposited is dependent on
the electrode length
Increased arcing times
(longer weld lengths)
Consumable costs
Operator appeal
Higher
Lower
Electrodes by weight are
classed as similar but
additional costs of gases
must be considered
Lower, although there
is a high wastage of
consumables as electrode
ends are not used
Good
Good
Less cleaning
More portable
Control of variables
Less maintenance
Less stopping
Ease of machine control
Thin material can be
welded
Greater control
Deposited metal
Good
Good
Standard wires considered
to be hydrogen-controlled,
resulting in higher
mechanical properties
Greater selection of
electrodes available
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AUM8057A
Perform Gas Metal Arc Welding
Weld positions
Vehicle body builders are required to weld in many different positions. The common
weld positions are:
•
•
•
•
flat
vertical
horizontal
overhead.
Overhead
Vertical
Horizontal
Flat
When deciding which mode to set the welding machine, the position of the weld must
be considered. In positions between flat and horizontal vertical, all modes can be
employed. Below horizontal vertical, spray and globular modes are not recommended.
Flat
Horizontal
vertical
Horizontal
vertical
Horizontal
Horizontal
overhead
Horizontal
Horizontal
overhead
Overhead
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AUM8057A
Perform Gas Metal Arc Welding
For the following weld positions the ISO (International Organization for Standardization)
have codes rather than the entire word. The codes for these positions run from
‘A’ to ‘G’.
‘P’ stands for position and a letter indicates the area it is situated in.
PA
PB
PB
PC
PC
PD
PD
PE
PF stands for Vertical Up.
PG stands for Vertical Down.
On plate the welds would look like this:
Weld
Flat
Horizontal
Vertical
Overhead
1G/PA
2G/PC
PF
3G/PG
4G/PE
1F/PA
2F/PB
PF
3F/ PG
4F/PE
Butt
Fillet
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AUM8057A
Perform Gas Metal Arc Welding
Weld symbols
Welding symbols are found on drawings to inform the vehicle body builder what the
designer requires in regard to the following:
•
what type of weld is required
•
how the parent material must be prepared
•
which side of the joint to weld
•
how high the weld must be
•
how long the weld must be
•
how large the spacing in between the welds
•
whether the weld is to be done in the factory or out on site.
This information would take too much time to write out and take up too much space on
a drawing so ISO symbols are used. The welds themselves are not actually drawn on
the drawing; only the symbols are used.
The two most common types of weld used in the vehicle body building industry are the
butt weld and the fillet weld.
Butt weld
Fillet weld
So that the weld symbols can convey as much information as possible, several
elements are included.
Welding symbol elements
These are the welding symbol elements that would be typically shown on a drawing
within the vehicle body building industry.
Size of weld
Basic symbol
(type of weld)
Length of weld
Pitch of weld
(from centre to centre)
Specifications,
process and
procedures
T
Tail
L-P
S
Weld all around
Reference line
Arrow connecting
reference line to
arrow side of joint
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Perform Gas Metal Arc Welding
T
Other information can be added here. If there is none the tail will be left off.
S
Size of the weld: throat thickness in mm.
L
Length of the weld: end to end in mm.
P
Pitch: distance between welds measured from centre to centre.
O
Weld all around: fully weld total joint.
>
Tail: Omitted when reference information is not used.
___
Reference line: basis of the symbol.
Arrow: This indicates where the weld will be and which side of the joint will be
welded.
Joint symbols
This is how the weld symbols would be shown on a drawing. Note: if the joint symbol is
on the top of the reference line, the weld will be on the opposite side to the arrow. If the
joint symbol is on the bottom of the reference line, the weld will be on the same side as
the arrow.
Square butt
Weld on arrow side
Weld on other side
Weld on both sides
Square butt symbols
V butt
Weld on arrow side
Weld on other side
Weld on both sides
V butt
Fillet
Weld on arrow side
Weld on other side
Fillet symbols
23
Weld on both sides
AUM8057A
Perform Gas Metal Arc Welding
Length and pitch of fillet welds
In addition to the symbols the size, length and spacing may be included. These will
appear as symbols on a drawing but the weld joint will look like the interpretation.
See note
50–100
100 100
Drawing
50
50
50
Interpretation
Length and pitch of increments
of intermittent welds
See note
50
50–125
50–125
50
50
125
Drawing
125
Length and pitch of increments
of chain intermittent welding
75
75
75
See note
75-250
75-250
75
125
Drawing
125
250
Length and pitch of increments of
staggered intermittent welding
Note: In the last drawing the fillet symbol is slightly offset to indicate that the top and
bottom fillet welds are staggered.
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AUM8057A
Perform Gas Metal Arc Welding
Basic symbols for welding
Type of weld
Sketch of weld
Symbol
General butt
Full penetration butt
weld by a welding
procedure to be agreed.
Square butt
Single V butt
Single bevel butt
Single U butt
Single J butt
Fillet
Bead
Surfacing
Stud
Plug or Slot
25
Indication of drawing
AUM8057A
Perform Gas Metal Arc Welding
Supplementary symbols
Backing strip
or bar
Weld all round
Field or site weld
Flush contour
Convex contour
26
Single V butt
Fillet
Type of weld
Sketch of weld
Indication of
drawing
Type of weld
Ground flush
Sketch of weld
Indication of drawing
AUM8057A
Perform Gas Metal Arc Welding
Fill in the missing sections of this exercise
27
AUM8057A
Perform Gas Metal Arc Welding
Questions
1.
On the chart below indicate which is higher or lower when comparing GMAW with
MMAW.
Item
Manual metal arc
welding
Gas metal arc welding
Consumable costs
Deposition rates
Capital cost
2.
Correctly label the weld positions in the diagram below.
a)
d)
˚
˚
˚
˚
b)
c)
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AUM8057A
Perform Gas Metal Arc Welding
3.
On the diagram below indicate the weld positions between flat and horizontal
vertical.
4.
Using the ISO codes for weld positions, what do the following letters indicate?
PE______________PA____________PC_____________PF__________
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AUM8057A
Perform Gas Metal Arc Welding
Classification of consumables
There are many different types of solid and flux-cored electrode wires commercially
available. They are classified to a particular standard, which makes it possible to
identify and select the most suitable type of wire for a job. It is important to understand
classification systems and the information they represent.
Solid wire electrodes are always connected to the positive (+) terminal. They contain
a number of de-oxidising agents to promote a cleaning or scavenging action in the
weld pool. The wires are also copper-coated for two main reasons: to prevent them
from rusting and to allow good electrical conductivity when the wire is passed through
the contact tip. The wire is wound onto a spool or coil; this ensures the wire is passed
through the feed rolls and flexible conduits as continuously as possible.
Classification systems list a number of essential features about the wire; for example,
its chemical composition, gas shielding requirements, mechanical strength of the weld
deposit and whether it is hydrogen controlled or not.
Solid wire electrodes
Australian Standards AS2717 Part 1 classifies solid wire electrodes under three groups
of elements separated by hyphens. Each group consists of a number of letters or
letters and numerals.
For example, the classification ES2-GM-W503H on a reel of wire can be broken down
to decipher the chemical composition of the wire and its strength.
Group 1
ES2, the first group of digits, indicates that it is an electrode, solid (ES) with the
numeral 2, which denotes the chemical composition of the wire by putting it in the
chemical classification 2.
From the chart below you can see that a wire ES2 contains 0.07% carbon, 0.9% to
1.4% manganese and 0.4% to 0.7% silicon.
Chemical composition chart
Classification
ES2
ES3
ES4
ES5
ES6
ES7
Carbon %
0.07
0.06 to 0.15
0.07 to 0.15
0.07 to 0.19
0.07 to 0.15
0.07 to 0.15
Manganese %
0.9 to 1.4
0.9 to 1.4
1.0 to 1.5
0.9 to 1.4
1.4 to 1.85
1.5 to 2.0
Silicon %
0.4 to 0.7
0.45 to 0.7
0.6 to 0.85
0.3 to 0.6
0.8 to 1.15
0.5 to 0.8
Note: Electrodes may also contain very small additions of copper, titanium, zirconium
and aluminium.
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AUM8057A
Perform Gas Metal Arc Welding
Group 2
The second group consists of two letters that indicate the type of shielding gas used
during qualification tests. In our example ES2-GM-W503H, the digits GM stands for
Gas which is a Mixture.
Gas chart
Classification
Type of gas
C
Gas shielding which is followed by the
type
Carbon dioxide (CO2)
M
Mixture of gases
I
Inert gas
G
The classification ES2-GM-W503H indicates that the wire is to be shielded by use of a
mixture of gases.
Group 3
In our example of ES2-GM-W503H the third group of digits involves a letter W followed
by a three-digit number. W stands for weld metal. The first two digits refer to the
minimum strength of the deposited weld, which is measured in Mega Pascals
(500 MPa), and presented as 10% of that value, in this case 50 MPa. The third digit
3 refers to the minimum impact value set at different temperatures. The letter H
completes the classification which indicates that the process is hydrogen-controlled.
Classification
W
Weld metal
The tensile strength of the weld metal
expressed as 10% of the total MPa, ie
50 = 500 MPa
Impact strength (J value) at a particular
temperature. 3 equals a J value of 47 at
–30° C as specified in AS 2717
Hydrogen-controlled
50
3
H
Here is an example of a plain carbon steel wire electrode, ES4-GC-W503H.
The chemical composition will be 0.07% to 0.15% carbon, 1.00% to 1.50% manganese,
and 0.60% to 0.85% of silicon.
When deposited with CO2 shielding gas, the weld metal will have a minimum tensile
strength of 500 MPa and an impact value of 47 J at –30 °C. The weld is hydrogencontrolled.
Filler wires for welding of steels are de-oxidised with manganese and silicon, and are
generally copper-coated (nickel is sometimes used). The copper coating of the wire
serves three purposes:
•
prevents corrosion of the wire
•
improves current pick-up
•
improves feeding characteristics.
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AUM8057A
Perform Gas Metal Arc Welding
Common wire sizes for GMAW of steels:
0.6 mm
0.8 mm
0.9 mm
Generally used for sheet
metal and other light
applications
General purpose GMAW
1.0 mm
1.2 mm
1.6 mm
Welding of heavy plates
Shielding gases
In Australia GMAW is also commonly known as ‘MIG Welding’ (Metal Inert Gas). This
is misleading, as it suggests that all shielding gases are inert. All GMAW of carbon
and low-alloy steels employs the use of an active shielding gas. This means there is a
reaction between the shielding gas and the metal droplets as they travel across the arc.
Inert shielding gasses are used for welding stainless steels and non-ferrous metals.
To achieve the desired arc stability when welding carbon and low-alloy steels, some
oxidising action is required in the arc. This can be achieved in one of two ways:
•
using carbon dioxide (CO2) as a shielding gas
•
using argon (Ar) as the base with the addition of CO2 and/or O2 (oxygen).
Carbon dioxide
Carbon dioxide, when used as a shielding gas, produces a highly reactive arc. CO2
promotes the following characteristics to the welding arc:
•
deep penetration
•
high spatter levels
•
high deposition rates
•
high heat input
•
true spray transfer cannot be achieved when using CO2 as a shielding gas.
Carbon dioxide is best suited to dip transfer. The additional heat of CO2 helps to
overcome the problem of ‘lack of fusion’ and increases deposition rates. Carbon
dioxide tends to produce convex bead shapes and high spatter levels.
Argon
Argon is a true inert gas, which by itself cannot be used to weld carbon and low-alloy
steels. When used by itself to weld non-ferrous metals, it produces an arc which, when
compared with CO2, has the following characteristics:
•
smooth arc
•
lower penetration
•
lower heat input
•
lower spatter
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AUM8057A
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•
improved bead shape
•
promotes spray transfer.
Gas mixtures
Gas mixtures for welding steel use argon as a base, with the addition of differing levels
of CO2 and/or O2 to achieve desirable arc characteristics.
The greater the O2 and CO2 levels, the more the arc characteristics align to the
characteristics of CO2. The reverse is true: the lower the addition of CO2 and O2, the
more the arc aligns toward characteristics produced by argon shielding gas.
Shielding gas
Argon
Chemical behaviour
Inert
CO2
Active
Argon/CO2
Active
Argon/CO2/O2
Active
Effect/Uses
For welding all metals
except carbon and lowalloy steels
Produces high spatter
and deep penetration.
Used with de-oxidised
wire on carbon steels
For welding carbon
and low-alloy steels.
Produces low spatter
and moderate
penetration
Additional oxygen
increases penetration.
Used with de-oxidised
wire to weld carbon and
low-alloy steels
Each gas company will supply mixtures of their own formulation. However, as a rough
guide for welding carbon and low-alloy steels, uses for mixtures approximating the
following compositions are as follows:
Gas
Purpose
CO2
Dip transfer, particularly on thicker plates
Ar + 25% CO2
General use in dip transfer
Ar + 15% CO2
Multi-purpose for dip and spray transfer
Ar + 5% CO2
For spray transfer
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AUM8057A
Perform Gas Metal Arc Welding
The ionising effect of the shielding gas influences bead shape as well as the amount of
penetration obtained. The effect of shielding gas upon bead shape can be seen below.
Parent metal
Argon
Argon/CO2
CO2
Argon/CO2 /O2
Effects of shielding gas
Flow rates
Flow rates for CO2 should be set at 16–18 litres per minute (L/min). Flow rates for
Ar/CO2 mixtures should be set at 14 L/min.
GMAW variables
The variables affecting the GMAW process are:
•
wire speed/amperage
•
arc voltage
•
electrode stickout
•
travel speed
•
torch angle
•
shielding gas flow rate.
Wire speed/amperage
Wire speed and amperage are controlled by the same potentiometer on a GMAW plant.
Consequently these variables cannot be adjusted independently of each other.
As amperage is increased, the current density in the wire increases, and the melt-off
rate of the wire increases. Amperage is the most important factor when determining
heat input into the metal being welded. Turning up the wire speed/amperage control
will:
•
increase the wire feed speed
•
increase amperage
•
increase deposition rate
•
increase penetration
•
increase heat input
•
for a given travel speed, increase the size of the weld bead.
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AUM8057A
Perform Gas Metal Arc Welding
Decreasing wire speed will have the opposite effect.
Increased amperage
Arc voltage
Arc voltage determines the mode of metal transfer during GMAW welding. At low arc
voltages, resistance across the arc causes extinguishment of the arc, which results
in dip transfer. Higher arc voltages are enough to maintain the arc by overcoming the
electrical resistance. As the arc voltage is increased, arc length is increased. This
enables more wire to be melted off without ‘stubbing’, as sometimes occurs when high
wire feed speeds and low arc voltages are used. Increased arc length also increases
the width of the weld bead. To fully understand why, it is necessary to consider
shielding gas for a moment.
The ionising potential of the shielding gas determines the width of the arc column.
Helium promotes a wider arc
column with increased side
wall fusion, and shallower
penetration.
Argon promotes a narrow arc
column with deep and narrow
penetration.
Bead shape due to shielding gas composition
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AUM8057A
Perform Gas Metal Arc Welding
If the arc column is lengthened, but the angle the arc column burns at does not change,
the weld bead is widened.
Long arc length
Short arc length
Bead shape due to arc length
It can be seen therefore that, if arc voltage is increased without changing the wire
speed or travel speed, a wider, flatter bead will result.
Increased voltage
Electrical stickout
When discussing GMAW, two types of ‘stickout’ are referred to:
1.
Visible stickout – the distance that the electrode protrudes beyond the gas nozzle.
2.
Electrical stickout – the distance that the electrode protrudes from the contact tip.
Nozzle
Contact tip
Electrode wire
Electrical
stickout
Visible stickout
Stickout length
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AUM8057A
Perform Gas Metal Arc Welding
Visible stickout has little effect upon welding conditions except that, if excessive,
gas shielding efficiency will be reduced. However, electrical stickout is an important
consideration. Welding current is transferred to the wire via the contact tip. The wire
between the end of the contact tip and the arc offers electrical resistance. If the
electrical stickout is halved, so is the electrical resistance.
Contact tip
Electrode
Stickout
distance
Electrical
resistance
Stickout
distance halved
Electrical
resistance halved
Parent material
Reduced resistance due to reduced electrical stickout
The effect of this increased resistance is:
•
reduced amperage
•
reduced penetration
•
reduced heat input
•
higher deposition rate.
The increased deposition rate is brought about by:
•
preheating of the wire
•
the wire feed motor.
There is an increase in electrical resistance due to the increase in electrical stickout;
this in turn preheats the wire, which tends to melt off sooner. This has the effect of
increasing the arc length, which in turn tends to increase arc voltage. The drive motor
senses the increase in arc voltage and speeds up as a means of reducing the arc
voltage to the preset level. The drive motor will speed up to compensate for increased
arc voltage and slow down when the arc voltage is reduced. Increased deposition rates
are obtained with a longer electrical stickout because the arc voltage is self-adjusting.
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AUM8057A
Perform Gas Metal Arc Welding
Travel speed
As travel speed is reduced, the weld bead becomes more convex due to greater
deposition of filler wire. Heat input is increased due to the fact that the arc remains
above any particular point for a greater period of time.
The opposite is achieved when travel speed is increased.
Increased speed
Torch angle
As with any welding process, the angle of approach must be adjusted to distribute the
weld metal evenly in the joint.
90°
45°
Angle of approach
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AUM8057A
Perform Gas Metal Arc Welding
Angle of travel
The angle of the gun is maintained such that it is ‘pushed’ in the direction of travel.
Angle of travel
Direction of travel
Angle of travel
The exception to this is when making heavy welds in spray transfer where the gun is
‘dragged’. This is done to direct shielding gas over the solidifying and cooling weld
metal, which will remain hot for an extended period of time.
The operator determines the actual angle of travel used, by seeking the best
compromise between good visibility and efficient shielding.
As the torch angle is lowered, shielding efficiency is reduced due to the Venturi effect,
which draws air into the gas shield.
80º–90°
Too steep – poor visibility
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AUM8057A
Perform Gas Metal Arc Welding
60º–70°
A good compromise
30º– 40°
Too low – air induced
Nozzle angle
Gas flow rate
Gas flow rates should be set so as to provide adequate shielding.
Recommended rate of flow for argon mixtures
=
14 L/min.
Recommended rate of flow for CO2
=
16–18 L/min.
It should be kept in mind that excessively high flow rates cause turbulence and
increase the Venturi effect when torch angles are too low.
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AUM8057A
Perform Gas Metal Arc Welding
Other machine controls
Spot timer
The spot timer allows the weld time to be preset as a means of making consistent weld
sizes for spot welding. The timer is activated when the gun trigger is depressed.
Burnback control
This control enables wire to feed for a small amount of time after current flow is
terminated when the torch trigger is released. This can be adjusted to prevent the wire
fusing to the contact tip, or stop it sticking to the weld pool when welding is terminated.
Gas purge switch
The gas purge switch enables shielding gas to flow without feeding wire. It is used to
set gas flow rates or to purge lines of contaminated gas before starting welding.
Inch wire control
This enables wire to be fed without gas or current flow.
Spool brake
The wire spool carrier employs a braking device to prevent overrun of the wire due
to the inertia of the spool of wire. It should be adjusted to provide enough braking to
prevent overrun, but with no unnecessary drag that would cause slippage of the wire at
the drive rollers.
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AUM8057A
Perform Gas Metal Arc Welding
Questions
1.
The Australian Standards classifies solid wire electrodes under three groups of
elements. Name the three elements.
a) ____________________ b) ___________________ c) ___________________
2.
Name the three gases commonly used in a tri gas mix.
a) ____________________ b) ___________________ c) ___________________
3.
Which gas produces the deepest penetration?
_________________________________________________________________
4.
The flow rate for an Ar/CO2/O2 gas mixture should be set at __________ litres per
minute.
5.
List five controllable GMAW variables which affect the outcome of a weld.
a) _______________________________________________________________
b) _______________________________________________________________
c) _______________________________________________________________
d) _______________________________________________________________
e) _______________________________________________________________
6.
Will moving the electrode from a short arc length to a long arc length increase or
decrease the weld bead width?
_________________________________________________________________
7.
Give a definition of an electrical stickout.
_________________________________________________________________
8.
What may occur when the nozzle angle used for welding is too low?
_________________________________________________________________
9.
What does a wire inch control do?
_________________________________________________________________
10. State the purpose of a gas purge switch.
_________________________________________________________________
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AUM8057A
Perform Gas Metal Arc Welding
Fillet and butt weld structures
Fillet weld
1
6
4
5
7
2
1
1.
2.
3.
4.
5.
6.
7.
8.
Parent metal
Reinforcement
Fusion zone
Weld face
Weld metal
Toe
Heat-affected zone
Root fusion
1.
2.
3.
4.
5.
6.
7.
8.
9.
Parent metal
Reinforcement
Fusion zone
Weld face
Weld metal
Toe
Heat-affected zone
Root fusion
Penetration
3
8
Butt weld
1
2
4
5
6
7
1
9
3
8
1.
2.
3.
4.
Fillet weld measurements
2
2
1
4
3
43
Reinforcement
Leg length
Nominal throat thickness
1. throat
Reinforcement
Actual
thickness
2.
3.
4.
Leg length
Nominal throat thickness
Actual throat thickness
AUM8057A
Perform Gas Metal Arc Welding
Joint design for GMAW
Pre-qualified joint preparation for GMAW of steel structures can be found in
AS 1554.1. It can be seen that joint design is similar to that used for MMAW butt welds
in steels but with the following variations:
•
Included angles of butt welds are reduced by 10 degrees. This is because the
thinner electrode and lack of flux provides easier access to the root of the joint.
30º
Root gap 1.5–2 mm
Root face 1.5–2 mm
The root face for butt welds is decreased when dip transfer is used because
penetration is limited, and increased when spray transfer is used as a means of
preventing burn-through.
Weld sizes
Fillet and butt welded joints are designed to carry certain loads. These loads are
calculated from tests carried out on similar joints. An allowance is made for safety.
The welding operator must deposit welds to the dimensions specified by the designer.
The designer knows how the welds will behave in service and asks for weld deposits of
a particular size to meet the conditions. If the welder then deposits an undersized weld,
the weld may fail in service. If the weld is over-reinforced, the joint will be less flexible
and the vehicle body may fail. The rigidity of an oversized weld can cause excessive
stress loading on other sections of the weld.
Example:
For a 6 mm fillet weld there should be a 6 mm leg length and a 4.2 mm
throat thickness.
For a butt weld there should be an even curved reinforcement slightly
above the alignment of the parent metals.
Where the size of the weld is not specified, the deposit should be in proportion to the
plate thickness; for example, on a 10 mm plate there should be a 10 mm fillet weld.
Butt welds should be built up so that the weld section is at least equal to the thickness
of the parent metal.
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AUM8057A
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Fillet weld dimensions
The size of a fillet weld is determined by the following dimensions: leg length and throat
thickness. These may be checked with a weld gauge.
throat thickness
convex
leg length
Leg length
Throat thickness
There is a vast difference between throat thickness and effective throat thickness, as
the following figures indicate.
The strength of a weld is determined by the effective throat thickness, which for a mitre
weld should be 70% of the leg length.
effective throat
effective throat
mitre
convex
Mitre weld
Convex weld
effective throat
concave
Concave weld
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AUM8057A
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The assembly of parts for a fillet weld also influences the weld size. The parts should
be close-fitting so that there is fusion over the entire area of the joint surfaces. In
the diagram below there is a gap between the parts. The weld size is correct but the
effective throat thickness is reduced.
effective
throat thickness
actual
throat thickness
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AUM8057A
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Questions
1.
Using weld definitions, identify the features of the fillet weld in the diagram below.
1
6
4
5
7
2
1
3
8
1.
2.
3.
4.
5.
6.
7.
8.
2.
In butt weld preparation the included angles vary between GMAW and MMAW.
What is the difference?
_________________________________________________________________
3.
As a ‘rule of thumb’, what should be the size of a fillet weld when joining 12 mm
plate?
_________________________________________________________________
4.
On the diagrams below, indicate where the effective throat thickness would be
measured.
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Perform Gas Metal Arc Welding
Control of distortion
When metal is heated it expands and on cooling it contracts. Laying down a bead of
expanded metal (the weld bead) onto a comparatively unheated and unexpanded
piece of metal (the parent metal) results in the weld bead contracting or shrinking to
a far greater degree compared with the parent metal. It is this uneven expansion and
contraction between the two metals that causes distortion when welding.
The effects of distortion cannot be totally eliminated but they can be controlled. There
are three general types of distortion:
•
longitudinal shrinkage
•
transverse shrinkage
•
angular shrinkage.
Longitudinal shrinkage
As the weld contracts in the direction of its length (longitudinal shrinkage) it will pull
the fabrication into a distorted shape. The weld is in a state of high tension in its long
direction.
Contraction
Plate distorts when
weld cools
Transverse shrinkage
If a butt weld is made between two plates which are free to move, the plates will be
drawn toward the weld. In extreme cases they may overlap.
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Angular distortion
This is the result of rotation of the welded parts around the axis of the weld due to
transverse contraction.
Angular distortion
The control of distortion can be broken into three areas:
•
before welding
•
during welding
•
after welding.
Before welding
The control of distortion before welding can be facilitated by:
•
good design
•
tack welding
•
jigs, clamps and fixtures
•
uniform preheating
•
presetting.
Good design
Well-designed joints use a minimum weld length and an appropriate joint preparation to
prevent over-welding, which results in minimal distortion.
Good design would include:
•
reducing the size of the welds to the minimum allowed
•
reducing the number of runs to achieve the weld size
•
reducing gaps to a minimum.
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AUM8057A
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Double ‘V’ welds can help control distortion
Double ‘V’ preparation
preferred as it can be
welded from both sides
30°
Reduced bevel angles
with larger root gap
Tack welding
Tack welds are small additional welds that act like clamps.
The number and size of tack welds needed depends on the type and thickness of the
material. For example, stainless steel requires more tack welds than mild steel and
thicker material requires fewer tacks than thinner material.
1
4
5
3
6
7
2
Tack sequence
Jigs, clamps and fixtures
Jigs, clamps and fixtures are used to hold the parts being welded in place during the
welding process.
Commence weld
and travel outwards
Joint to
be welded
Correct
angle set
Parts restrained by clamps
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AUM8057A
Perform Gas Metal Arc Welding
Uniform preheating
Suitable preheating of parts to be welded can help reduce distortion. Preheating can be
carried out using oxyacetylene equipment. If the part to be welded can be preheated
and evenly expanded, welded and then evenly cooled, the effects of distortion will be
minimised. This technique is often employed when welding king pins onto semitrailers,
as it also minimises the stresses applied to the surrounding material.
Presetting
This technique consists of predicting how much distortion can be expected and
misaligning the material by that amount before welding, so that on completion of the
weld the distortion is minimal.
Presetting
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Perform Gas Metal Arc Welding
During welding
The control of distortion during welding can be facilitated by:
•
backstep welding
•
intermediate chain welding
•
intermediate staggered welding
•
balanced sequence welding.
1
2
4
3
Intermittent chain welding
Backstep welding
Intermittent staggered welding
4
8
17
1 5
12
8
4
10
15
12
13
6
9
2
1
3
10
5
7
11
14
11
9
6 2
16
Balanced sequence welding
18
7
52
3
AUM8057A
Perform Gas Metal Arc Welding
After welding
Correction of distortion after welding is often difficult and sometimes impossible. For
this reason it is best to control distortion before and during welding. Correction of
distortion after welding can be done by:
•
hammering or peening the weld
•
pressing
•
heating.
Hammering or peening the weld
Peening is used to stretch the weld and nearby parent metal by hitting the hot metal
with hammer blows.
Peening the weld
A sledgehammer applied to the correct area can be used to straighten out lighter
materials such as square tubing on the side rails of a bus.
Pressing
This is a more subtle form of straightening distorted material. Pressing can be a more
controlled process but often takes time to set up. Pressing can be performed by
machines such as a ‘hydrabends’, ‘H’ frame press, ‘porta powers’ or hydraulic bottle
jacks or other forms of rams.
Heating
The contraction of the weld bends the material towards the weld. By heating the
opposite side of the material from the weld and then cooling it, the material will shorten
slightly and will tend to straighten.
Spot heat here
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AUM8057A
Perform Gas Metal Arc Welding
Where not to weld
A vehicle body is not a static item; it moves and flexes in service. The body must be
able to twist and flex in relation to uneven road surfaces. A torsionally flexible body
has the advantage of decreasing suspension loading when the vehicle is on uneven
surfaces as all wheels can make contact with the road surface, thus sharing the
payload over all the wheels. With this in mind, vehicle body builders must weld the
vehicle body in a way that allows the vehicle to remain flexible but at the same time
have the strength to remain in service without failing.
There are several general rules to apply to achieve a flexible but strong body. These
rules may not always be possible to apply, but where practicable they should always be
followed.
Always weld longitudinally along the body, not across the body. For example, weld in
the direction of a main runner, not across it.
Do not weld across the main runner
Weld underneath the cross bearer in the
same direction as the main runners
Cross bearer
Weld here in the direction
of the main runner
Main runner
Section on centre line
This is extremely important where stresses are concentrated, such as in a gooseneck
or where a drawbar meets the front of the body.
Do not weld
across here
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AUM8057A
Perform Gas Metal Arc Welding
Weld along the drawbar
not across the drawbar
Do not fully weld gussets if there is to be a lot of flexing. Some manufacturers prefer to
not weld to the extreme ends of gussets.
Weld at the extreme outer edges and the inner section. Depending on the size of the
gusset, intermediate welds may be necessary.
It is not generally recommended to weld on a chassis frame except when joining
after a wheelbase alteration. If it is necessary to weld on a chassis, check with the
manufacturer and your workshop policy before beginning the work. Generally it is not
accepted to weld on the flange or in an area within 40 mm from the top and bottom
edge of the web.
40 mm
40 mm
No welding in
these areas
40 mm
40 mm
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AUM8057A
Perform Gas Metal Arc Welding
Where cross bearers pass through the main runners on a semitrailer body, only weld
on the web of the cross bearers. This allows for maximum flexibility of the body.
Main runner
Holes cut in
main runner
Only weld web of
cross bearers
Cross bearers
•
To weld spring retainers to the axle, do not weld across the axle – weld
longitudinally along the axle.
•
Never attach a work lead to components such as axles, springs, engines or drive
lines. Arcing on these components may cause serious damage to bearings and
springs. Parabolic leaf springs are particularly sensitive to surface damage.
•
Pipes and conduits made of synthetic material such as those used for brakes and
electrical systems must be protected from weld spatter and temperatures
exceeding 80°C.
•
Fuel tanks and fuel pipes in the vicinity of welding should be removed.
•
Disconnect the battery to protect electronic components such as ABS, onboard
computers and alternators.
•
Unplug all onboard computers.
•
Protect airbag suspensions and parabolic leaf springs which may fracture by even
momentary exposure to weld spatter.
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AUM8057A
Perform Gas Metal Arc Welding
Questions
1.
There are three general types of distortion, name them.
a) ____________________ b) ___________________ c) ___________________
2.
The control of distortion can be broken into three areas; give three methods which
can be employed in each area to reduce distortion.
a)
Before welding
1. __________________
2. __________________
3. __________________
b)
During welding
1. __________________
2. __________________
3. __________________
c)
After welding
1. __________________
2. __________________
3. __________________
3.
What is the advantage of having a torsionally flexible body when travelling over
uneven road surfaces?
________________________________________________________________
4.
It is not recommended to weld across a drawbar. Why?
_________________________________________________________________
5.
List four places or items where the work lead (earth) should not be attached on a
truck.
a)
_____________________________________
b)
_____________________________________
c)
_____________________________________
d)
_____________________________________
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Perform Gas Metal Arc Welding
Procedure sheets
Procedure sheets vary from firm to firm and are used as a means of keeping track of
how a weld was done and the type of material used. It is also useful to record who did
the welding. These sheets are not used for all welds but often for selected welds such
as king pin attachment.
PROCEDURE SHEET
NAME
DATE
Type of welding machine
Control data
Run
Wire speed
setting
Amperage
reading
Voltage
control setting
Transfer mode
1
2
3
4
Electrode wire
Material data
Size Ø mm
Type
Type
Thickness
Classification
Shielding gas
Remarks:
Type
Flow rate litres/min
Signature
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AUM8057A
Perform Gas Metal Arc Welding
GMAW defects
Apart from slag inclusions, all the common weld defects that occur with other
processes may occur with GMAW. Defects such as porosity and lack of fusion can be a
particular problem with GMAW.
The defects commonly encountered in GMAW are:
•
porosity
•
cold lap/lack of fusion
•
lack of root penetration
•
excessive penetration
•
contour defects
•
undercut
•
weld cracking
•
stray arcing
•
excessive spatter.
Undercut
Porosity
Overroll
Spatter
Parent metal
Incomplete penetration
Lack of fusion
Weld faults
Porosity
Definition: a pore or group of gas pores in the weld metal. Porosity may be
conveniently differentiated according to size and distribution. A number of different
terms are used related to size. These are:
•
Gas pore – a cavity (usually spherical) formed by entrapped gas during the
solidification of molten metal.
•
Wormhole – an elongated or tubular cavity in the weld metal caused by entrapped
gas being forced away from the solidifying weld metal.
•
Cluster – a group of pores in close proximity to each other.
As is the case with other welding processes, porosity may be caused by moisture, or
surface contaminants on the plate.
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AUM8057A
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With GMAW, by far the greatest cause of porosity is due to inadequate gas shielding.
This may be due to:
•
flow rate set too low
•
flow rate set too high
•
no gas flow at all
•
excessive wind or air movement at the
gun
•
contaminated shielding gas
•
stickout length too long
•
gun angle too low.
Porosity
Cold lap/lack of fusion
Definition: portions of the weld run that do not fuse to the surface of the metal or edge
of the joint. With GMAW lack of fusion is commonly referred to as ‘cold lapping’ as it
usually takes the form of lack of sidewall fusion over an extensive part of the joint.
Cold lapping is common when welding in the dip transfer mode, particularly when the
plate thickness exceeds 5 mm. Welding downhand, or with high wire speed and low arc
voltage settings, further increases the risk of this occurrence. Plates that are dirty or
heavily scaled further exacerbate the problem.
Cold lapping does not generally occur when welding in the spray transfer mode.
Therefore to minimise the likelihood of cold lapping, one or more of the following should
be employed:
•
Weld in the spray transfer mode.
•
Clean plates.
•
If in doubt, set the arc voltage slightly high.
•
Set enough amperage to ensure sufficient heat for fusion.
•
Keep the electrical stickout short.
•
Use CO2 shielding gas or a mixed gas high in CO2.
Lack of fusion
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AUM8057A
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Lack of root penetration
Definition: the failure of the weld metal to completely fill the root of the joint.
Root runs in butt welds are normally made in the dip transfer mode except for those
in heavy plate, in which case spray transfer would be used. The dip transfer mode
is inherently ‘cold’, employing low amperages and voltages. This means that root
penetration is limited in this mode.
The solution to overcoming lack of root fusion is to use thinner root faces on butt welds
than would be the case with other processes, ie typically in the range of ½ mm to 1 mm.
In fillet welds, the solution is to use comparatively high amperage settings when in the
dip transfer mode. Additionally, CO2 or a gas mixture high in CO2 will help.
Incomplete penetration
Excessive penetration
Definition: excess weld metal protruding through the root of a butt weld. This defect
normally only occurs on thin (sheet) materials or when the spray mode of transfer is
used. Adjustment of wire speed and arc voltage will usually overcome this problem with
relative ease.
Another form of this defect is electrode wire protruding through the root of the butt in
the form of ‘spikes’ or ‘icicles’. This is caused when arcing to the root face of the butt
weld momentarily ceases, a small amount of wire penetrates the butt, and the arc is
re-established when the wire contacts the parent metal.
The solution to this problem is to limit the width of the root gap and/or to increase the
arc voltage, which results in a wider spread of the arc so that arcing to one or both
sides of the weld is always present.
Parent metal
Excessive penetration
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Contour defects
Contour defects may be in the form of overroll or overlap, excessive convexity or
excessive concavity of the bead, or simply rough, uneven appearance.
Travel speed and torch angle adjustments may fix many of these problems, but the
GMAW operator has an advantage in that he/she can control weld profile by adjusting
the arc voltage.
Excessive convexity may be remedied by increasing arc voltage, and beads which are
too wide or too concave may be remedied by decreasing arc voltage.
Overall
Undercut
Definition: a groove or channel in the parent metal occurring continuously or
intermittently along the toes or edge of a weld.
Undercut is not a common problem in GMAW. However, it is likely to be encountered in
two situations:
1.
When fillet welding in spray transfer: This is normally caused by setting the arc
voltage too high, causing a long arc length which results in undercutting of the
toe of the weld of the vertical plate. The solution is quite simple and good practice
for all welds in spray transfer. To facilitate this, set a smooth spray transfer mode
using the lowest arc voltage.
2. Vertical up welds: Solid wires are largely unsuitable for making stringer beads in
the vertical-up position. Convex beads with some undercut generally result. When
a weave technique is used, a bead that is convex in the middle, with undercut
toes, may result. The solutions are:
•
reduce the arc voltage, or
•
reduce the overall heat of the welding, or
•
pause longer at the toes.
Undercut
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Weld cracking
Definition: discontinuities produced either by tearing of the metal in the plastic
condition (hot cracks) or by fracturing when cold (cold cracks). Hot cracks are common
in materials with high coefficients of expansion and/or which suffer from hot shortness.
Hot cracking occurs at elevated temperatures soon after solidification. This mode of
cracking is common in aluminium and stainless steel. Cold cracking is most common in
hardenable materials, particularly when cooling rates are rapid. Cracking is considered
to be a serious defect and rarely tolerated.
Cracks may also be described depending on how, when and where they occur, eg
longitudinal, transverse, crater, centre line, hot, cold, toe and underbead. Cracks may
occur in either the parent metal, usually as fusion or heat-affected zone cracks or in the
weld metal.
Hot cracking – Usually occurs in metals that are hot short and/or have high rates
of thermal expansion. Hot cracking most commonly occurs in the weld metal with
longitudinal cracks and crater cracks being the most common examples.
Cold cracking – Most commonly occurs in the base metal adjacent to the fusion zone.
The most common example of this is underbead cracking in hardened steels.
Crater cracks –These come from hot shrinkage. The crater solidifies from all sides
toward the centre, leading to a high concentration of stress at the centre of the crater.
If the metal lacks ductility, or the hollow crater cannot accommodate the shrinkage,
cracking may result. Crater cracks may, under stress, propagate from the crater and
lead to failure of the weld.
Cracking in GMAW welds is not generally a major problem due to the following factors:
•
GMAW is a ‘low-hydrogen’ process.
•
Hollow craters are not usually a characteristic of GMAW welds.
•
The inherent low heat input is ideal for stainless steels and other metals which are
prone to hot cracking.
Heat affected zone (HAZ) cracking
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Stray arcing
Definition: damage on the parent metal resulting from the accidental striking of an arc
away from the weld. Stray arcing is not a major problem associated with GMAW as the
electrode is usually only live when the gun trigger is depressed. Care should be taken
that the gun is not put down with the weight resting on the trigger, and also that arcing
does not occur between the job and the work return lead connection.
Excessive spatter
Definition: the metal particles expelled onto the surface of the parent metal or weld,
during welding, and not forming part of the weld.
This usually occurs due to one of the following factors:
•
shielding gas or plate contaminated with moisture
•
high levels of CO2 or O2 in the shielding gas
•
excessive arc voltage in the dip transfer mode
•
welding in the globular transfer mode.
Note: Spatter is not usually present in the spray transfer mode.
Trouble shooting/equipment malfunction
Compared with the manual welding processes, GMAW requires higher levels of care
and maintenance. Major sources of frustration are the problems associated with
feeding of the electrode wire. This is a particular problem when welding with aluminium
wire, feeding wire through long gun cables, or when using a gun cable that has been
poorly maintained.
Equipment malfunctions with GMAW fall into two main categories:
1.
electrical
2.
mechanical.
The main problems with regard to electrical malfunctions and their likely causes are:
Problem
No power at machine
Likely cause
Mains switch off
Machine switched off
Mains power on but no
welding power
Wire feeds, but no arc
Blown fuse
Trigger switch not
working
Wire feeder not
connected
Work return not
connected
Blown fuse
64
Rectification
Check switches and
fuses – If intact call
electricians
Check – If trigger is
working wire feeder will
operate, wire will feed
Check work return
Check fuses
AUM8057A
Perform Gas Metal Arc Welding
Mechanical problems manifest themselves in the form of wire feeding problems.
Common wire feeding problems and their likely causes are:
Problem
No wire feed at all
Uneven wire feed
Spool overrun
Wire fused to contact tip
Likely cause
Spool brake excessively
tight
Rectification
Check tension on spool
brake
No friction at drive rolls
Check drive rolls and
adjust as necessary
Wire jammed at drive
rolls or in gun cable
Check guide tubes
Dirty or damaged liner
Check wire conduit
Clean or replace
Slippage at drive rolls
Increase pressure
Liner cut too short
Replace
Kinks in gun cable
Keep as straight as
possible
Insufficient roll pressure
Tighten drive rolls
Wire distorted due to
excessive roll pressure
Reduce roll pressure
Wire is kinked or twisted
Look for misalignment of
drive rolls or damaged
liner
Contact tip worn or dirty
Inspect and replace
Spool brake excessively
tight
Spool brake too loose
Excessive arc voltage
Check tension on spool
brake
Tighten
Reduce arc voltage
Excessive burnback time
Reduce burnback time
Intermittent wire feed
See above
GMAW equipment requires a regular inspection and maintenance schedule:
•
•
•
contact tips should be inspected at least daily
liners, drive rolls and spool brake should be inspected weekly
gas and electrical connections should be inspected monthly.
Feeding aluminium wire presents additional problems. It is essential that all sources of
friction upon the wire be minimised. Recommendations are as follows:
•
•
•
•
•
•
reduce spool braking
use a Teflon® liner
ensure the correct liner is used
keep the gun cable as straight as possible
avoid small diameter wire if possible
fit a straighter gooseneck to the gun
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AUM8057A
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•
•
pay particular attention to drive roll pressure
use good quality wire
•
additionally, a welding machine with the following features is highly recommended:
◦
a push/pull gun
◦
a four roll wire feeder
◦
a soft-start feature.
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Questions
1.
List three types of useful information which can be taken off a procedure sheet.
a)
____________________________________________
b)
____________________________________________
c)
____________________________________________
2.
On the diagram below, label the defects on a butt weld that the arrows are
pointing to.
3.
List three ways in which cold lapping could be minimised.
4.
5.
6.
a)
____________________________________________
b)
____________________________________________
c)
____________________________________________
Undercut in GMAW is more likely to be encountered in two situations.
What are they?
a)
____________________________________________
b)
____________________________________________
Give three likely causes of wire being fused to the contact tip.
a)
____________________________________________
b)
____________________________________________
c)
____________________________________________
What type of liner is recommended for use with aluminium welding?
_________________________________________________________________
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Tanks and containers
Whenever possible always avoid welding tanks or containers which have held volatile
substances. If there is another way around the problem, such as replacement of the
tank, then make welding the very last resort.
Water-soluble substances
Tanks and containers that have held substances which dissolve in water (watersoluble) may be welded or cut with relative safety, provided a few simple precautions
are taken. It is very important to be certain about the water solubility of the contents.
Before welding do the following:
•
Rinse the container with water several times.
•
Fill the container with as much water as possible.
•
Be sure there is a vent hole to let out the fumes created during the welding or
cutting operation before starting to weld or cut.
Important:
Never weld or cut containers until you know what has been stored in
them.
Small free space with opening
Welding
point
Welding
point
12 mm
12 mm
Open
pipe
Open
pipe
Container full of water
Arrangements for
water filling
Non-water-soluble substances
Welding and/or cutting tanks and containers that have held flammable or combustible
substances present dangers if cleaning, purging and other procedures are not carried
out carefully.
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AUM8057A
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Perform Gas Metal Arc Welding
Never trust your eye or sense of smell to determine if it is safe to
weld or cut the container. Even a small amount of flammable
substance is extremely dangerous and will cause an explosion.
Cleansing procedures are outlined below:
•
Thoroughly wash out the container with a hot caustic soda solution, steam or
some other cleansing agent. Do not use carbon tetrachloride because of its toxic
fumes.
•
Fill the container with an inert gas such as nitrogen, carbon dioxide, argon or
helium to cleanse the remaining fumes.
•
Fill the container with as much water as possible.
•
Vent the container before starting welding or cutting, to let out the fumes during the
welding or cutting operation.
Pressure
regulator
Container
Air exit
Entry of
carbon
dioxide
Carbon
dioxide
cylinder
Protection by filling with
carbon dioxide
Pressure
regulator
Container
Nitrogen
cylinder
Air
exit
Inlet tube
Protection by filling with
nitrogen
Important:
•
When using steam or caustic soda solution, wear safety glasses and clothing
which gives your body full protection.
•
While cleansing and washing the tanks/containers, make sure they are well
aired (ventilated), as some cleansing agents give off toxic fumes.
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AUM8057A
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Practical exercises
Type of weld
10 mm plate
Sign off
Pad weld
Flat position
Single run fillet
Three run fillet
Single vee butt weld
Flat position
Type of weld
1.6 mm plate
Single run fillet
Flat position
Closed butt
Flat position
Single run fillet
Vertical down position
Closed butt
Vertical down position
70
Sign off
AUM8057A
Type of weld
Perform Gas Metal Arc Welding
3 mm plate
Single run flat position
Open butt
Flat position
Single run fillet
Vertical down position
Open butt
Vertical down position
71
Sign off
AUM8057A
Type of weld
Perform Gas Metal Arc Welding
10 mm plate
Sign off
3 mm plate
Sign off
Pad weld
Horizontal position
Out side corner
Horizontal position
Single V butt
Horizontal position
Multi-run fillet
Vertical up
Multi-run fillet
Overhead position
Type of weld
Open butt weld
Horizontal position
72
AUM8057A
Type of weld
Perform Gas Metal Arc Welding
1.6 mm plate
Open butt weld
Horizontal position
73
Sign off
AUT032
Perform Gas Metal Arc
Welding
Workbook
(AUM8057A)
DESCRIPTION
This workbook and guide is intended as an introduction to GMAW for trades which
fabricate metals such as vehicle body building.
It contains the basic operation of a GMAW machine, modes of metal transfer, codes
and symbols on through to practical exercises.
Incorporated into the workbook is a section of how to approach the welding of a
dynamic unit such as a vehicle body.
EDITION
First edition
CATEGORY
Automotive Manufacture
COURSES AND QUALIFICATIONS
• Certificate III Automotive Manufacture (Bus, truck and trailer)
RELATED PRODUCTS
AUT031
Fabricate Parts for Sub-Assemblies Workbook
AUT033
Prepare and Operate Equipment, Tools and Machinery
- Hand Tools Workbook
AUT034
Prepare and Operate Equipment, Tools and Machinery
- Power Tools Workbook
AUT035
Modify or Repair Chassis/Frame and Associated Components
Workbook
Produced by WestOne Services
AUT032 PERFORM GAS
METAL ARC WELDING
WORKBOOK (AUM8057A)
ISBN 978-0-7307-9919-1
ORDERING INFORMATION:
Contact WestOne Services on Tel: (08) 9229 5200 Fax: (08) 9227 8393 Email: sales@westone.wa.gov.au
Orders can also be placed through the website: www.westone.wa.gov.au
9 780730 799191
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