Spot friction welding of thin AZ31 magnesium alloy

Spot friction welding (SFW) is a novel variant of the linear friction stir welding process with the potential to create strong joints between similar, as well as dissimilar sheet metals. It is particularly suitable for soft, low melting point metals such as aluminum, magnesium, and their alloys where resistance spot welding can cause defects such as voids, trapped gas and microcracks due to the intense heat requirement for joint formation. Until now, spot friction welding has focused primarily on aluminum alloys. This paper presents a feasibility study on spot friction welding of AZ31, a wrought magnesium alloy available in sheet form. Lap joints of 1.58-mm-thick magnesium alloy AZ31B-0 sheet were produced by spot friction welding. The spot welds were made in 2 sec with a 15-mm-diameter pin tool rotating at 500-2,000 rpm. The tool was inserted into 2-sheet stack-ups to depths of either 2.4 or 2.8 mm relative to the top sheet surface. Tensile-shear testing showed that joint strengths up to 4.75 kN could be obtained. The removal of surface oxides from the sheets prior to welding increased lap shear strengths by about 50% at the 2.4-mm insertion depth and it promoted failure by nugget pull-out rather than by interface separation.