As the welding gun trigger is pressed, the power, wire feed and gas flow are engaged simultaneously. The intense heat of the arc melts the wire and the parent metal in a molten pool. As the MIG wire is consumed, more is fed into the molten pool and in so doing a weld bead is deposited. Figures 5.7 and 5.8 show the difference between metal-arc welding and MIG welding.
Figure 5.7: Metal-arc welding
Did you know?
Consumable components are components which are discarded (thrown away) after they have become worn out.
Gooseneck
Trigger
Gas shroud
Nozzle
Wire
Flux coating
Electrode
Arc
Shielding gas
Parent metal Weld pool
Figure 5.8: MIG welding
Without shielding gas, it is difficult to control the welding arc. Atmospheric gases also react with the weld, causing a lot of spatter and a poor quality weld.
Figure 5.9: MIG welding without shielding gas
Assessment 1
1. What type of shielding gas is used when MIG welding? Why is shielding gas used?
2. Sketch the MIG welding set-up and label the four important components.
3. State four important safety precautions to be taken when MIG welding.
Molten weld pool
Nozzle
Continuous feed electrode wire
Arc
Inert shielding gas
Parent metal Gas shroud
Did you know?
TIG welding stands for tungsten inert gas welding.
TIG welding is similar to MIG welding but does not include the continuous wire feed.
The electrode is made from tungsten (an element with a very high melting point) which is used to establish and maintain a welding arc. The filler rod is fed into the molten pool manually. The process resembles gas welding but has an electrical arc as its source of heat.
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Defects
The following section deals with defects which can occur during the welding process. Commercially, defects are categorised according to severity.
Welding runs are either accepted or rejected, depending on the criteria set by engineers and designers. There is a specific career in weld testing which we will look at later in the chapter.
Numerous errors can occur during welding.
We will only look at six main defects:
1. Porosity 2. Slag inclusion
3. Weld craters/faulty restart 4. Incomplete penetration 5. Lack of fusion
6. Undercutting Porosity
Porosity refers to gas pores (tiny bubbles) found in the solidified weld bead. As you can see in Figure 5.10, these pores may vary in size and are randomly distributed.
Pores can occur either under or on the weld surface (the latter being called surface porosity).
The most common causes of porosity are:
• atmospheric contamination
• surface contamination
• dirty or wet electrodes when arc welding
• rusted MIG wire.
Atmospheric contamination
Atmospheric contamination during MIG welding can be caused by:
• inadequate shielding-gas flow
• excessive shielding-gas flow (this can cause aspiration of air into the gas stream)
• a severely clogged gas nozzle or a damaged gas supply system (leaking hoses, fittings, etc.)
• excessive wind in the welding area (this can blow away the gas shield: see Figure 5.9).
The atmospheric gases primarily responsible for porosity in steel are nitrogen and excessive oxygen. Inspect the gas supply regularly to ensure there are
no leaks, thus ensuring continuity of the shielding gas and minimising atmospheric gases from coming into contact with the weld.
Surface contamination
Surface contamination can be caused by dirty, oxidised (rusted), oily, wet or painted surfaces. In all these cases, the gases formed by the melted surface
impurities become entrapped in the weld surface, usually resulting in a brittle weld.
Brittle
Hard but easily broken
Dirty or wet electrodes when arc welding
Even if the welding electrodes have not come into contact with water, they are hygroscopic, which means that they tend to absorb moisture from the atmosphere if not stored correctly.
Rusted MIG wire
MIG wire which is not often used may start rusting around the outer surface of the reel. When the wire has rusted, welding should not be carried out as the rust can damage the wire liner of the feed mechanism (see Figure 5.4).
Figure 5.10: Porosity in a fillet weld
Slag inclusion
Slag inclusions are non-metallic solids entrapped in the weld metal or
between the weld metal and the base metal. Slag inclusions are solid regions within the weld cross-section or at the weld surface where the once-molten flux used to protect the molten metal is mechanically trapped within the solidified metal. This solidified slag represents a portion of the weld’s cross-section where the metal is not fused to itself. This can result in a weakened condition which reduces the serviceability of the component.
Inclusions may also appear at the weld surface. Like incomplete fusion, slag inclusions can occur between the weld and base metal or between individual weld passes. In fact, slag inclusions are often associated with incomplete fusion. Slag inclusions can be avoided by thoroughly chipping off the slag from previous weld runs and brushing the weld bead with a wire brush before doing any further welding.
Slag inclusion can also result from incorrect current settings. To remove slag inclusion, grind out the offending part of the weld and re-weld the section.
Figure 5.11: Slag inclusion in a butt weld SlagThe layer on top a weld,
resulting from the melted flux
Fusion Joining
Porosity
Slag inclusion