Although friction stir welding (FSW) was developed primarily for making butt joints in wrought aluminium alloys which are difficult or virtually impossible to fusion weld, the process can also be used to weld light alloy castings.
Almost all aluminium alloys - both in the solution treated and, where appropriate, hardened conditions - can be friction stir welded. Additionally, the process can be used to weld dissimilar alloys of the same family of alloys and sometimes from different families. If fusion welding were used, the process could create intermetallics from the mix of alloying elements, which in turn could lead to a reduction in the mechanical properties of the welded joint.
FSW is a solid phase process so there is no molten pool into which gases can be dissolved, and which will subsequently be rejected during solidification. This means that FSW welds do not exhibit the porosity that can occur with fusion welding and by the same token hot cracking problems can almost always be avoided.
Early FSW trials on pressure die cast aluminium alloys did not display the turbulent welding conditions typically experienced with fusion techniques (when a pocket of trapped gas is released into the weld pool and expels some of it) and it was found that fully consolidated welds were achieved. The FSW process uses a fairly high down force and it was shown that pre-existing gas pores immediately around the weld nugget region were eliminated, resulting in a soundweld and HAZ interface. Cast components can be FSW welded to wrought plate and extrusions as dissimilar metal combinations when required, and there have been several applications where cast end caps and closure lids have been welded into extruded and cast containers in order to seal the components for low pressure applications.
In other families of alloys, high pressure die-cast magnesium plates have been successfully friction stir welded to the same, and different, magnesium alloys. The cast plates contained residual porosity from the manufacturing process, and this was not found to affect the resultant weld quality. In fact in some welded samples, tensile test failures occurred in the parent plate because of its inherent porosity, the weld nugget being overmatched with respect to the parent material strength. The weld surface produced by FSW can be very smooth, and the weld nuggets exhibit a very fine grain size as seen in aluminium alloys. Magnesium alloys investigated to date include AM50, AM60, AZ91 andZK60, although the processing parameters need to be optimised for each specific alloy. Also, as with aluminium, wrought magnesium material such as AZ31 can be welded to cast magnesium plates without undue problems, although indissimilar material welding there may be a preferred orientation of the components being welded.
It should be noted that friction stir welding has also been applied to other cast materials. Welding trials have been performed on both nickel aluminium bronzes and on cast steels. In the bronzes, one advantage of FSW is that a finegrain size is conferred onto the welded material, while in some cast steels the same high weld quality has been achieved as with wrought steel plates. The beneficial effects seen in the FSW in cast bronzes has led to another aspect of the friction stir - that of processing surfaces of components to improve the characteristics of the material in this region. The FSP can be used not only to refine the grain size of the alloy, but also to eliminate porosity and even fine micro-cracking on component surfaces that might otherwise lead to premature failure in service.
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