The welding of aluminium and its alloys

MIG spot welding

MIG spot welding may be used to lap weld sheets together by melting through the top sheet and fusing into the bottom sheet without moving the torch. The equipment used for spot welding is essentially the same as that used for conventional MIG, using the same power source, wire feeder and welding torch. The torch, however, is equipped with a modified gas shroud that enables the shroud to be positioned directly on the surface to be welded (Fig. 7.20). The shroud is designed to hold the torch at the correct arc length and is castellated such that the shield gas may escape. The power source is provided with a timer so that when the torch trigger is pulled a pre-weld purge gas flow is established, the arc burns for a pre-set time and there is a timed and controlled weld termination. The pressure applied by positioning the torch assists in bringing the two plate surfaces together. Because of this degree of control the process may be used by semi-skilled personnel with an appropriate amount of training.

The process may be operated in two modes: (a) by spot welding with the weld pool penetrating through the top plate and fusing into the lower one or (b) by plug welding where a hole is drilled in the upper plate to enable

the arc to operate directly on the lower plate so that full fusion can be achieved. Plug welding is generally required when the top sheet thickness exceeds 3 mm. The size of the drilled hole is important in that this deter­mines the size of the weld nugget and the diameter should be typically between 1.5 and 2 times the top sheet thickness. Typical welding parame­ters are given in Table 7.7.

Of the shield gases argon is the preferred choice as it produces a deep, narrow penetration. Argon also provides better arc cleaning than helium, important in maintaining low levels of oxide entrapment. Arc stability is also superior. Surface preparation is important, cleanliness being crucial to defect-free welds. As with butt welds, degreasing and stainless steel wire brushing, supplemented by scraping if a hole is drilled, are most important.

Welding can be carried out with equal ease with the plate in the horizontal, vertical or overhead position although in other than the flat position the welding time needs to be reduced from that listed in Table 7.7. This may result, however, in an increased level of porosity. Other defects include cracking, lack of fusion and burn-through. To prevent and control burn-through a temporary backing bar may be used. Fit up is important and for the highest strength the gap between the plates should be as small as possible.