1. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
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he
he
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SHIELDED GAS ARC WELDING
C
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A B B Y Y.c A B B Y Y.c
UNIT 7 SHIELDED GAS ARC WELDING
OBJECTIVES
General Objective: To understand the principles of shielded gas arc
welding i.e. TIG and MIG welding.
Specific Objectives : At the end of the unit you will be able to :
Ø Identify the principles of shielded gas arc
welding i.e. TIG and MIG welding.
Ø Elaborate on the TIG and MIG welding
principles, welding procedures, welding
machines, gas, etc.
Ø State the advantages and disadvantages of TIG
and MIG compared to manual arc welding.
Ø State the weaknesses of TIG and MIG welding
and how to prevent them.
.
2. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
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y
bu
bu
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to
to
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he
he
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SHIELDED GAS ARC WELDING
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INPUT
7.0. INTRODUCTION
The objective of welding is to produce a welding joint that contains the
same mechanical properties as the base metal. The objective can be achieved
if the molten metal is free from atmospheric air. If not, nitrogen and oxygen
gases in the atmosphere will be absorbed by the melting pool. The welding
produced will have small pore that will weaken the weld.
To prevent the welding, molten metal and the end of the filler rode and
electrodes from atmospheric air pollution before the molten metal become
solid inert gas is blown out from the welding point. These gases will cover
the welding pools, the filler rod points and electrode tips to avoid oxidation.
7.1. TUNGSTEN INERT GAS (TIG)
The welding of aluminium and magnesium alloys by the oxy-acetylene
and manual metal arc processes is limited by the necessity to use a corrosive
flux. The gas shielded, tungsten arc process enables these metals and a wide
range of ferrous alloys to be welded without the use of a flux. The choice of
the either a.c. or d.c. depends upon the metal to be welded. For metals
having refractory surface oxides such as aluminium and its alloys,
magnesium alloys and aluminium bronze, a.c. is used whilst d.c. is used for
3. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
ABB
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y
bu
bu
2.0
2.0
to
to
re
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he
he
k
k
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SHIELDED GAS ARC WELDING
C
C
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A B B Y Y.c A B B Y Y.c
carbon and alloy steels, heat-resistant and stainless steels, cooper and its
alloys, nickel and its alloys, titanium, zirconium and silver.
The arc burns between a tungsten electrode and the work piece within
a shield of the inert gas argon, which excludes the atmosphere and prevents
contamination of electrode and molten metal. The hot tungsten arc ionizes
argon atoms within the shield to form a gas plasma consisting of almost
equal numbers of free electrons and positive ions. Unlike the electrode in the
manual metal arc process, the tungsten is not transferred to the work and
evaporates very slowly, being classed as ‘non-consumable’. Small amount of
other elements are added to the tungsten to improve electron emission.
Gas flow
Torch
Water outlet
Welding
machine
Work piece
Water inlet
Figure 7.1. TIG welding equipment
4. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
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y
bu
bu
2.0
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he
he
k
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SHIELDED GAS ARC WELDING
C
C
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A B B Y Y.c A B B Y Y.c
Electrode
(tungsten)
Inert/noble
gas
Filler rode
Shielded gas
arc
Direction of travel
80 – 90o
20 – 30o
Melting pool
Work piece
Figure 7.2. TIG in progress. The tungsten does not melt into the
puddle for filler. This is a nonconsumable electrode.
7.1.1. Preparation of Metal.
Gas tungsten-arc processes must start with clean metal which
has the proper joint design i.e., V, U, or J. Mechanical and chemical
cleaning are often necessary to prepare the base metal. The edges of
the joint should be shaped to permit adequate fusion and penetration.
It is common practice to reduce or bevel the adjoining edges to 1.6 mm
thickness.
A strip (backup bar) to support the back side of the base metal
should be used when needed. This is especially helpful on aluminium
since it aids in shielding. The backup bar may be removed after
welding.
5. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/5
he
he
k
k
lic
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SHIELDED GAS ARC WELDING
C
C
w om w om
w
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A B B Y Y.c A B B Y Y.c
7.1.2. Joint Fit.
Good joints make it easier to obtain a good weld. In production
work, carefully fitted joints can help save money and can help the
welding operator develop standardized welding techniques. Root
opening (distance apart) and angle of bevel are two major factors
requiring close tolerance when fitting joints.
7.1.3. Welding Machine.
Gas tungsten-arc welding requires a conventional welding
machine, with the following accessories:
1. Torch, lead cable, and hoses.
2. Inert gas supply and flow meter for measuring amount
of shielding gas.
3. Water cooling system for water-cooled torches.
Air-cooled torches are limited to 150 ampere capacity.
4. High-frequency spark unit attached to the output leads
of the power supply (to start and stabilize arc).
The finished weld will be greatly affected by type of current and
polarity. For example, aluminium is welded with alternating current
plus superimposed high-frequency current (ACHF). Stainless steel is
welded with direct current straight polarity (DCSP). Improper
electrical connections will cause (a) the electrode to overheat, (b) poor
penetration, or (c) insufficient cleaning effect upon the base metal.
Current selection must be made with care. When an electrode is
connected to the negative terminal (DCSP), electrons pass through the
arc to bombard the base plate (Fig. 7.3).
6. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
ABB
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y
bu
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2.0
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SHIELDED GAS ARC WELDING
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Welding Electrode
machine
Direction of electron
travel
Positive surface
particles travel
Work piece
Deep penetration
Figure 7.3 Power supply with direct current straight polarity
This causes nearly 70% of the arc heat to accumulate in the
base metal to assist fusion and penetration. When the electrode is
made positive (DCRP), a cleaning effect is created on the surface of the
base plate (Fig. 7.4).
Welding
Electrode
machine
Direction of electron
Positive surface travel
particles travel
Work piece
Shallow penetration
Figure 7.4 Power supply with direct current reverse polarity
In welding aluminium this method is used to remove surface
oxidation. While an electrode positive connection furnishes a cleaning
effect, it also heats the tungsten electrode. The electrode may get hot
7. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
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ABB
ABB
y
y
bu
bu
2.0
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SHIELDED GAS ARC WELDING
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A B B Y Y.c A B B Y Y.c
enough to melt, transfer to the weld pool, and contaminate the base
metal. When this happens, the electrode must be removed, its end
broken off, and it must be ground to shape.
Alternating current offers the advantages of both direct current
straight polarity (DCSP) and direct current reverse polarity (DCRP).
Gas tungsten-arc welding of aluminium and magnesium requires an
AC power supply (Fig. 7.5).
Gas tungsten-arc welding is not recommended for metal more
than 20 mm thick. Welds have been completed on 25 mm thick plate
but require a great deal of time and, consequently, are expensive.
Most applications are less than 12 mm thick, and require less than 500
amperes of current.
Welding Electrode
machine
Surface
particles lifted Electron flow
Work piece
Medium penetration
Figure 7.5 Alternating current power supply
8. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
ABB
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y
bu
bu
2.0
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re
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J3103/7/8
he
he
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SHIELDED GAS ARC WELDING
C
C
w om w om
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A B B Y Y.c A B B Y Y.c
7.1.4. Welding Torch.
The welding torch has a round collet which compresses to hold
the electrode and a nozzle to control the gas (Fig. 7.2). Water-cooled
torches are used when current values exceed 150 amperes.
Maintenance of either torch is more time consuming than with the
metal-arc process. Careful selection of nozzle size, proper shaping of
the working end of the electrode and correct extension of electrode
beyond nozzle are important. Nozzle size influences the flow of gas.
End shape of electrode and extension of electrode beyond nozzle control
the stability of the arc. Further, it is important that electrode diameter
match current value (Table 7.1). If the current is too high for the
diameter of an electrode, the life of the electrode will be reduced. When
the current is too low for a given electrode diameter, the arc will not be
stable.
Table 7.1. Selection of nozzle size and electrode size for gas tungsten-arc
welding
Electrode Nozzle or WELDING CURRENT IN AMPERES
Size Cup Sizes ACHF DCSP DCRP
(Diameter, Pure Thoriated Pure or Pure or
Inches) Tungsten Tungsten Thoriated Thoriated
0.020 4,5 5-15 5-20 5-20 *
0.040 4,5 10-60 15-80 15-80 *
1/16 4-6 50-100 70-150 70-150 10-20
3/32 5-7 100-160 140-235 150-250 15-30
1/8 6-8 150-210 225-325 250-400 25-40
*Not applicable.
9. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
ABB
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y
bu
bu
2.0
2.0
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J3103/7/9
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SHIELDED GAS ARC WELDING
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A B B Y Y.c A B B Y Y.c
The end of the electrode should remain bright, as if it was
polished. On some metals, such as aluminium and magnesium, the end
is contaminated when starting or by touching the base plate.
Contamination can be burned off by welding on a scrap plate of metal,
or it can be removed by grinding (Fig. 7.6). The electrode should be
adjusted to extend beyond the nozzle a distance equal to the electrode
diameter (Fig. 7.7)
15o 30o
45o
Grind here
DCSP DCRP AC
Figure 7.6 Electrode shapes for gas shielded tungsten-arc welding
3/8” max
Electrode diameter
Figure 7.7. Adjustment of electrode from nozzle
10. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
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ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/10
he
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k
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SHIELDED GAS ARC WELDING
C
C
w om w om
w
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A B B Y Y.c A B B Y Y.c
7.1.5. Shielding Gas.
Gas used with this process produces an atmosphere free from
contamination and also provides a path for arc transfer. The path
creates an environment that helps stabilize the arc. The gas and arc
activity also perform a cleansing action on the base metal. Both argon
and helium are generally used for this process but argon is preferred
because it is cheaper and provides a smoother arc. Helium, however,
helps produce deeper penetration (Table 7-2).
7.1.6. Filler Metal.
Filler metals are selected to meet or exceed the tensile strength,
ductility, and corrosion resistance of the base metal. The usual practice
is to select a filler metal having a composition similar to that of the
base metal. For most efficient application, select clean filler metals of
proper diameter; the larger the diameter of the filler metal, the more
heat is lost from the weld pool.
11. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/11
he
he
k
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SHIELDED GAS ARC WELDING
C
C
w om w om
w
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A B B Y Y.c A B B Y Y.c
Table 7.2 Selection of gases for manual application of tungsten-arc welding.
Metal Shielding Gas Remarks
Aluminium Argon Easy starting
Good cleaning action.
Helium Faster and more penetration.
Argon-10% helium Increase in penetration over pure argon.
Stainless steel Argon Better control of penetration (16 gauge
and thinner).
Argon-helium Higher welding speeds.
mixtures
Copper and Argon Easy to control penetration and weld
nickel contour on sheet metal.
Argon-helium Increases heat into base metal.
Helium Highest welding speed.
7.2. TIG WELDING TECHNIQUES
After the base metal has been properly cleaned and clamped or tacked
together, welding can be started. On aluminium, the arc is usually started by
bringing the electrode near the base metal at a distance of about one
electrode diameter so that a high-frequency spark jumps across the gap and
starts the flow of welding current. Steel, copper alloys, nickel alloys, and
stainless steel may be touched with the electrode without contamination to
start the arc. Once started, the arc is held stationary until a liquid pool
appears. Filler rod can be added to the weld pool as required (Fig. 7.8).
Highest current values and minimum gas flow should be used to produce
clean, sound welds of desired penetration (Table 7-3).
12. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/12
he
he
k
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SHIELDED GAS ARC WELDING
C
C
w om w om
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A B B Y Y.c A B B Y Y.c
Table 7.3 Operating data for TIG
Material Aluminium Stainless Steel Magnesium Deoxidized
Copper
Type of Current ACHF DCSP ACHF DCSP
1.6mm electrode
Current: 60-80 80-100 60 110-140
Argon: 15 cfh 11 cfh 13 cfh 15 cfh
Passes: 1 1 1 1
3.2mm electrode
Current: 125-145 120-140 115 175-225
Argon: 17 cfh 11 cfh 19 cfh 15 cfh
Passes: 1 1 1 1
4.7mm electrode
Current: 190-220 200-250 120-175 250-300
Argon: 21 cfh 13 cfh 19 cfh 15 cfh
Passes: 1 1 1,2 1 at 257.4*
*Preheat to temperature indicated.
The shielded gas is pure argon and pre-heating is required for drying
only to produce welds of the highest quality. All surfaces and welding wire
should be degreased and the area near the joint and the welding wire should
be stainless steel wire brushed or scrape to remove oxide and each run
brushed before the next is laid.
The angles of torch and filler rod are shown in Fig. 7.8. After
switching on the gas, water, welding current and HF unit, the arc is struck
by bringing the tungsten electrode near the work (without touching down).
The HF sparks jump the gap and the welding current flows. Arc length
should be about 3 mm. Practice starting by laying the holder on its side and
bringing it to the vertical position, but using the ceramic shield as a fulcrum
can lead to damage to the holder and ceramic shield. The arc is held in one
13. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/13
he
he
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SHIELDED GAS ARC WELDING
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position on the plate until a molten pool is obtained and welding is
commenced, proceeding from right to left, the rod being fed into the forward
edge of the molten pool and always kept within the gas shield. It must not be
allowed to touch the electrode or contamination occurs. A black appearance
on the weld metal indicates insufficient argon supply.
15o
Direction of
30o travel
Figure 7.8. Example of TIG
The flow rate should be checked and the line inspected for leaks. A
brown film on the weld metal indicates presence of oxygen in the argon while
a chalky white appearance of the weld metal accompanied by difficulty in
controlling the weld indicates excessive current and overheating. The weld
continues with the edge of the portion sinking through, clearly visible, and
the amount of the sinking which determines the size of the penetration bead
is controlled by the welding rate.
7.3. METAL INERT GAS (MIG)
It is convenient to consider, under this heading, those applications
which involve shielding the arc with argon, carbon dioxide (CO2) and
mixtures of argon with oxygen and/or CO2, since the power source and
14. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
to
to
re
re
J3103/7/14
he
he
k
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SHIELDED GAS ARC WELDING
C
C
w om w om
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A B B Y Y.c A B B Y Y.c
equipment is essentially similar except for gas supply. With the tungsten
inert gas shielded arc welding process, inclusions of tungsten become
troublesome with currents above 300 A. The MIG process does not suffer
from these advantages and larger welding current giving greater deposition
rates can be achieved. The process is suitable for welding aluminium,
magnesium alloys, plain and low-alloy steels, stainless and heat-resistant
steel, copper and bronze, the variation being filler wire type of gas shielding
the arc.
The consumable electrode of bare wire is carried on the spool and is fed
to a maually operated or fully automatic gun through an outer flexible cable
by motor-driven rollers of adjustable speed, and rate of burn-off of the
electrode wire must be balance by rate of wire feed. Wire feed rate
determines the current used.
In addition, a shielding gas or gas mixture is fed to the gun together
with welding current supply, cooling water flow and return (if the gun is
water cooled) and a control cable from gun switch to control contractors.
A d.c. power supply is required with the wire electrode connected to the
positive pole ( Fig. 7.9).
Arc welding Gas flow
power supply meter
Spool of Inert gas
Welding
electrode cylinder
power
cable wire
Electrode
feed Contactor lead,welding
rools current,electrode, and
inert gasto welding
gun
Contacto Control head
r cable forelectrode feed
Ground and gas supply
cable
Figure 7.9 . MIG welding equipment
15. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
er
ABB
ABB
y
y
bu
bu
2.0
2.0
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to
re
re
J3103/7/15
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SHIELDED GAS ARC WELDING
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A B B Y Y.c A B B Y Y.c
During this process an electric arc is used to heat the weld zone. The
electrode is fed into the weld pool at a controlled rate and the arc is shielded
by a protective gas such as argon, helium, or carbon dioxide (Fig. 7.9). Gas
metal-arc welding can be either the short-circuiting process or the spray-arc
process (Fig. 7.10).
Inert/noble gas
Shielded gas
Arc
Melting pool
Work piece
Figure 7.10. MIG in progress
The short-circuiting arc process (short arc) operates at low currents
and voltages. For example, 18-gauge sheet metal can be welded at 45 amps
and 12 volts.
Work piece
Figure 7.11. Mechanics of the short circuiting transfer process as
shown between the electrode and work piece. Electrode dips into pool
an average of 90 times a second
In contrast, the spray-arc process uses high currents and voltages, e.g.,
Arc action is illustrated in Fig. 7.12. This results in high heat input to the
weld area, making possible deposition rates of more than 0.4 lb per minute.
(The deposition rate is the weight of filler metal melted into the weld zone
16. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
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ABB
ABB
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bu
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2.0
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J3103/7/16
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SHIELDED GAS ARC WELDING
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per unit of time.) Most applications of the spray-arc process are in thick
metal fabrications, e.g., in heavy road-building machinery, ship construction,
and beams for bridges.
Electrode maintains steady arc length
Work piece
Figure 7.12. Mechanics of the spray-arc transfer
process as shown between the electrode and work
All metal inert-gas (MIG) welding is classified as semi-automatic, since
the electrode feeds into the weld according to a preset adjustment. After
making an initial adjustment, the welding operator merely moves the gun
along the joint. For effective applications, the welding operator needs
information concerning power requirements, welding gun, selection of
shielding gas, type of filler metal, and job procedures.
7.3.1. Power Requirements.
Conventional power supplies used for shielded metal-arc
welding are not satisfactory. A welding machine designed for the MIG
process is called a constant potential power source; it produces a
constant voltage and also permits the operator to adjust electrode feed
rates. The adjustments on the power supply are voltage, slope (limits
current), and wire feed rate. Welding current is established by
17. F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y
Y
er
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ABB
ABB
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y
bu
bu
2.0
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to
re
re
J3103/7/17
he
he
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SHIELDED GAS ARC WELDING
C
C
w om w om
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A B B Y Y.c A B B Y Y.c
selecting a wire feed rate. Slope adjustment to limit current is not a
problem with spray-arc type transfer. However, in short-circuiting arc
processes, limitations on short-circuit current are essential to prevent
excessive spatter.
The electrode feed mechanism, an important part of the welding
machine, consists of a storage reel for electrode wire and a power drive
which feeds the electrode into the weld at a controlled rate.
Table 7.4 Shielding mixtures for MIG
Metal Shielding Gas Remarks
Aluminium and copper Argon + helium High heat input
20-80% mixture Minimum of porosity
Copper Argon + nitrogen Good heat input on copper
25-30% mixture
Carbon steels Argon + oxygen Stabilizes arc
Low alloy steels 3-5% mixture Reduces spatter
Causes weld metal to flow
Eliminates undercut
May require electrode to
contain deoxidizers
Low alloy steels Mixture of argon, Increases toughness of weld
helium and carbon deposit
dioxide