Page 95 - Welding of Aluminium and its Alloys

84 The welding of aluminium and its alloys

5.17 Plug and spot welds using TIG or MIG welding.

electron beam welding are capable of spot and ‘stake’ welding through a
substantial plate thickness, in the case of electron beam up to 200mm.
TIG welding tends to be confined to thin sheet, less than 2mm thick,
and finds only a very limited application in production. The bulk of spot
welding is performed using MIG welding and is covered in greater detail
in Chapter 7. The designer must be aware, however, of the variable quality
of the spot weld which results in low strength and poor fatigue performance.
The high restraint inherent in this weld form almost always results in
distortion, particularly when the welds are close pitched, and may pro-
duce hot cracks in the HAZ. These features limit the applications of spot
welding.
Plug welding is similar to spot welding except that the upper plate is cut
to provide a hole which is either completely filled with weld metal or is fillet
welded around its rim.This type of weld suffers from the same problems of
variable quality and inadequate strength as do the spot welds. They both
tend to be avoided when structural integrity is required.
The slot weld is rather more useful in a structural application than the
spot or plug weld since there is a reasonable length of weld to be deposited.
This permits a stable weld pool to be established and a sound joint to be
made.The weld may be a single pass completely filling the slot or it may be
fillet welded both sides. For best quality the start and stop positions should
be on the upper plate, clear of the slot. Fillet welded slots are preferred
when the plate thickness exceeds 5mm.The strength of a slot weld is deter-
mined by the shear strength of the weld deposit.

5.6 Distortion
Residual stress due to heating and cooling of the HAZs and the contrac-
tion of the weld metal as it cools from a molten state to ambient tempera-
ture is an unavoidable feature of welded joints.The stress deriving from this
shrinkage results in distortion. This distortion may be localised, evenly dis-
tributed and acceptable or may render the entire structure unfit for its
purpose. In a ship’s hull, for instance, buckling of the hull plates can induce
turbulence and increase drag; in piping it can restrict fluid flow; and in archi-
tectural applications it can be aesthetically unacceptable.