Workshop Technology 2, Unit 4 shielded gas arc welding

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UNIT 7
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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.
.
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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
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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
Work piece
Water inlet
Figure 7.1. TIG welding equipment
Welding
machine
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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.
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Good joints make it easier to obtain a good weld. In production
work, carefully fitted joints can help save money and can help the
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).
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7.1.2. Joint Fit.
welding operator develop standardized welding techniques.
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Welding
machine
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Electrode
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
machine
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Electrode
Positive surface
particles travel
Direction of electron
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
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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
machine
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Electrode
Surface
particles lifted
Electron flow
Work piece
Medium penetration
Figure 7.5 Alternating current power supply
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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
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
Size
WELDING CURRENT IN AMPERES
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.
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7.1.4. Welding Torch.
torches
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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
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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
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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.
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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.
Argon
Better control of penetration (16 gauge
and thinner).
Argon-helium
Higher welding speeds.
mixtures
Copper
and Argon
nickel
Easy to control penetration and weld
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.
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Table 7.2 Selection of gases for manual application of tungsten-arc welding.
Stainless steel
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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).
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Table 7.3
Material
Operating data for TIG
Aluminium
Stainless Steel
Magnesium
Deoxidized
ACHF
DCSP
ACHF
DCSP
Current:
60-80
80-100
60
110-140
Argon:
15 cfh
11 cfh
13 cfh
15 cfh
Passes:
1
1
1
1
125-145
120-140
115
175-225
Argon:
17 cfh
11 cfh
19 cfh
15 cfh
Passes:
1
1
1
1
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*
1.6mm electrode
3.2mm electrode
Current:
4.7mm electrode
Current:
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Copper
Type of Current
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*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
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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
o
30
Direction of
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.
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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
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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.
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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).
Gas flow
meter
Arc welding
power supply
Welding
power
cable
Spool of
electrode
wire
Inert gas
cylinder
Electrode
feed
rools
Contacto
r cable
Ground
cable
Control head
forelectrode feed
and gas supply
Figure 7.9 . MIG welding equipment
Contactor lead,welding
current,electrode, and
inert gasto welding
gun
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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
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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
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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
Aluminium and copper
Shielding Gas
Remarks
Argon + helium
High heat input
20-80% mixture
Copper
Minimum of porosity
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
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selecting a wire feed rate. Slope adjustment to limit current is not a
Metal
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Mixture of argon, Increases toughness of weld
helium and carbon deposit
dioxide
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7.3.2. Selection of Gas.
The primary purpose of the inert gas is to shield the weld crater
from contamination. Shielding gas may also affect (1) the transfer of
metal across the arc, (2) fusion and penetration, (3) the shape of weld
deposit, (4) the speed of completing the weld, (5) the ability of filler
metal to flow over the surface without undercutting, and (6) the cost of
the finished weld.
No single inert gas is satisfactory for all welding conditions.
Some specific jobs are more efficiently welded with a mixture of gases.
For example, low alloy steels are welded with a mixture of argon,
helium, and carbon dioxide (Table 7.4).
7.3.3. Filler Metal.
Electrodes used for filler metal with the MIG process are much
smaller in diameter than those used with the metal-arc process. Sizes
may range from 0.4 mm to 5.5 mm in diameter. Small diameter
electrodes require high feed rates, from 100 to 1,400 inches per minute.
The composition of the electrode usually matches that of the base
metal, but for welding high-strength alloys, the composition of the
electrode may vary widely from that of the base metal.
For example, an aluminium-zinc-magnesium alloy (7039) is
welded with an aluminium-magnesium alloy (5356).
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7.4. JOB PROCEDURES
High-quality welds are obtained by controlling process variables which
include current, voltage, travel speed, electrode extension, cleanliness, and
type of joint.
7.4.1. Current.
Welding current varies with the melting rate of the electrode.
Extreme values of current tend to promote defects, but a high current
(1.1 mm. electrode at 220 amp) reduces the drop size of the transfer,
improves arc stability, and improves penetration.
7.4.2. Voltage.
With the MIG welding process, the voltage control determines
the arc length. The higher the voltage setting, the longer the arc. A
desirable voltage range to establish a short arc is 19-22 volts; defects
are more likely to occur outside this range (Fig. 7.14).
Curve representing
undercutting
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Severity of defect (Increase)
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Voltage
Fig. 7.13. Defects related to voltage settings.
Curve representing
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Voltage
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higher voltage is more desirable for flat-position welding than for vertical or
overhead welding. Table 7-5 indicates typical voltage values.
Table 7-5 Typical arc voltage for MIG using drop transfer and 1/16 inch
diameter electrode.
Argon
Helium
Ar-O2 Mixture
CO2
1-5%O2
Aluminium
25
30
*
*
Carbon Steel
*
*
28
30
Low-alloy Steel
*
*
28
30
Stainless Steel
24
*
26
*
Nickel
26
30
*
*
Copper
30
36
*
*
*Not recommended.
7.4.3. Travel Speed.
After selecting a current and voltage setting, select the rate of
travel. A typical example is 0.6m – 0.76m per minute (in./min). If the
rate is changed more than a few mm per minute, weld quality will be
greatly affected (Fig. 7.15).
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Position of welding will determine voltage needed. For example, a
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No undercut.
Travel speed
26 in/min
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Undercutting.
Travel speed
32 in/min
Fig. 7.15. Undercutting of horizontal fillet on 6.3mm thick aluminium as
affected by travel speed. Gas metal arc process was used.
Position of welding will affect the travel speed. For example, if
the weld direction is dropped 15 degrees from flat so that the position
is slightly downhill, travel speed can be increased.
7.4.4. Electrode Extension.
Electrode extension is important. The further the electrode
extends from the gun to the arc, the greater the electrical resistance
between the output terminals. Higher resistance increases the
temperature of the electrode, and the resistance-heated electrode uses
less current in the weld puddle. In the spray-arc process, the electrode
extension should be about 12 mm to 25 mm, for short-circuiting
transfer; it should be approximately half this distance.
7.5.
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MIG WELDING TECHNIQUES
There are three methods of initiating the arc.
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i.
The gun switch operates the gas and water solenoids and
when released the wire drive is switched on together with
the welding current.
ii.
The gun switch operates the gas and water solenoids and
strikes the wire end on the plate operates the wire drives
and welding current (known as ‘scratch start’).
iii.
The gun switch operates the gas and water solenoids and
wire feed with welding current known as ‘scratch start’.
As a general rule dip transfer is used for thinner sections up to 6.4 mm
and for positional welding, whilst spray transfer is used for thicker sections.
The gun is held at an angle of 80o or slight less to the line of the weld to
obtain a good view of the weld pool, and welding proceeds from right to left
with nozzle held 6 – 12 mm from the work.
The further the nozzle is held from the work less the efficiency of the
gas shield, leading to porosity. If the nozzle is held too close to the work
spatter may build up, necessitating frequent cleaning of the nozzle, while
acting between nozzle and work can be caused by a bent wire guide tube
allowing the wire to touch the nozzle, or by spatter build-up short-circuiting
wire and nozzle. If the wire burns back to the guide tube it may be caused by
a late start of the wire feed, fouling of the wire in the feed conduit or the feed
rolls being too tight. Intermittent wire feed is generally due to insufficient
feed rolls pressure or looseness wire due to wear in the rolls. Excessively
sharp bends in the flexible guide tubes can also lead to this trouble.
Root run is performed with no weave and filler runs with as little
weave as possible consistent with good fusion since excessive weaving tends
to promote porosity. The amount of wire projecting beyond the contact tube
is important because the greater the projection, the greater the I2R effect and
the greater the voltage drop which may reduce the welding current and affect
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penetration. The least projection commensurate with accessibility to the
joint being welded should be aimed at.
Backing the strips which are welded permanently on to the reverse
side of the plate by the root run are often used to ensure sound root fusion.
Backing bars of copper or ceramics with grooves of the required penetration
bead profile can be used and are removed after welding. It is not necessary to
back-chip the root run of the light alloys but with stainless steel this is often
done and a sealing run put down. The importance of fit-up in securing
continuity and evenness of the penetration bead cannot be over-emphasized.
Flat welds may be slightly tilted to allow the molten metal to flow
against the deposited metal and thus give a better profile. If the first run has
a very convex profile poor manipulation of the gun may cause cold laps in the
subsequent run.
7.6. DIRECT CURRENT STRAIGHT POLARITY
The welding circuit shown in figure 7.16, is known as a straight
polarity circuit. It is understood that the electrons are flowing from the
negative terminal (cathode) of the machine to the electrode. The electrons
continue to travel across the arc into the base metal and to the positive
terminal (anode) of the machine.
Approximately two-thirds of the total heat produced with DCSP is
released at the base metal while one-third is released at the electrode. The
choice of direct current straight polarity depends on many variables such as
material of the base metal, position of the weld, as well as the electrode
material and covering.
Reactor
Arc gap
Electrode
Work piece
Cathode
Field
Holder
Anode
Figure 7.16. Wiring diagram of a direct current, straight polarity (DCSP)
arc circuit
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7.7.
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DIRECT CURRENT REVERSE POLARITY ARC WELDING
It is possible, and sometimes desirable, to reverse the direction of
electron flow in the arc welding circuit. When electron flow from the negative
terminal (cathode) of the arc welder to the base metal, this circuit is known
as direct current reverse polarity (DCRP). In this case, the electron returns
to the positive terminal (anode) of the machine from the electrode side of the
arc, as shown in Figure 7.17.
Reactor
Arc gap
Electrode
Work piece
Anode
Field
Holder
Cathode
Figure 7.17. Wiring diagram of a direct current, reverse polarity (DCRP)
arc circuit
When using DCRP, one-third of the heat generated in the arc is
released at the base-metal and two-thirds is liberated at the electrode. With
two-thirds of the heat released at the electrode in DCRP, the electrode metal
and the shielding gas are super-heated. This superheating causes the molten
metal in the electrode to travel across the arc at a very high rate of speed.
Deep penetration results due to the force of the high velocity arc. There is
theory that, with a covered electrode, a jet action and/or expansion of gases in
the metal at the electrode tip causes the molten metal to be propelled with
great impact across the arc.
The choice of direct current reverse polarity depends on many
variables such as material of the base metal, position of the weld, as well as
the electrode material and covering.
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ACTIVITY 7
7.1.
Explain the term nonconsumable electrode.
7.2.
What does the term inert signify?
7.3.
List the gases used for shielding a welding arc.
7.4.
Explain how TIG welding electrodes are shaped.
7.5.
How far should the electrode extend beyond the nozzle of the TIG
torch?
7.6.
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Explain why MIG welding is classified as a semiautomatic process.
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FEEDBACK ON ACTIVITY 7
7.1.
The electrode does not melt into the weld.
7.2.
The gas does not combine with the base metal or filler.
7.3.
Argon, helium and carbon dioxide.
7.4. The electrode diameter should match the current value. If the current is
too high for the diameter of the electrode the life of the electrode will
be short. When the current is too low for a given electrode diameter,
the arc will not be stable. The end of the electrode should remain
bright, as if it was polished.
7.5.
The electrode should extend beyond the nozzle a distance equal to the
electrode diameter.
7.5.
MIG welding is classified as semi-automatic because 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.
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SELF-ASSESSMENT 7
1.
From the standpoint of operation, how are TIG and MIG processes
different? How are they similar?
2.
What polarity does anode signify?
3.
In what direction do the electrons travel when using straight polarity?
4.
How much of the heat used for arc welding is liberated at the electrode
when using straight polarity?
5.
Why is it recommended that a tungsten electrode arc be started on a
scrap tungsten surface?
6.
What would happen if the tungsten electrode were bent off centre?
7.
Name two defects that could occur with gas shielded-arc welding
processes and explain how each could be avoided.
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FEEDBACK OF SELF-ASSESSMENT 7
1.
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TIG uses a tungsten electrode that does not melt into the weld;
because the electrode is shielded and cooled by inert gas flow. A
separate filler rod is used as needed
MIG uses a continuous electrode which feeds into the weld
automatically as an arc is maintained. . They both use inert gas.
Electrode
(tungsten)
Inert/noble
gas
Filler rode
Shielded gas
arc
Direction of travel
80 – 90o
20 – 30o
Melting pool
Work piece
TIG in progress. The tungsten does not melt into the puddle for
filler. This is a nonconsumable electrode.
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Inert/noble gas
Shielded gas
Arc
Melting pool
Work piece
MIG in progress
2.
Positive (+)
3.
Across the arc into the base metal and to the positive terminal.
4.
One-third (1/3)
5.
To keep the tungsten electrode clean.
6.
Uses more current and electrode will be jagged or contaminated.
7.
(a) Eyes and skin – arc is more intense. Wear leather and specially
treated cloth.
(b) Breathing – provide adequate ventilation.
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