Influence of the mechanical grooving surface pretreatment on metal–polymer joining by friction stir spot welding

Abstract This study investigates the effect of mechanical grooving texturing parameters on the friction stir spot welding (FSSW) of 1 mm thick aluminium AA6082-T6 and 6 mm thick polyamide 6 (PA6). Two levels of texturing depth (0.1 mm and 0.2 mm) were analysed, both performing equally well in tensile–shear testing. The equivalent mechanical performance was attributed to the fact that the joints tend to fail through the thermally affected polymer, due to the rupture of pores, rather than through the metal–polymer interface. As the level of porosity is equivalent in both conditions, the mechanical performance is not affected by varying the plunge depth. In turn, doubling the plunge depth parameter reduced the pretreatment time by less than half, making the mechanical grooving process much more efficient. Then, the use of lubricant on the texturing procedure was evaluated, but the difficulties in eliminating the lubricant residues negatively affected the interaction of materials at the interface, significantly reducing the mechanical performance of the welded joints. Finally, the size and length/width ratio of the textured area was studied. Overall, increasing the textured area led to improved mechanical performance, but a greater gain was obtained by increasing the number of grooves instead of making them longer. The maximum tensile–shear force of about 7.9 kN was obtained when performing a 30 × 30 mm2 textured area and using an optimised texturing depth of 0.2 mm, which is the greatest value already reported in the open literature to aluminium–polymer joining by FSSW.