Friction stir spot welding of AA6061-T6 and Cu with preheating: Strength and failure behavior at different test temperatures

In the current study, the influence of test temperatures was investigated on lap shear strength and fracture behavior of dissimilar and similar friction stir spot-welded (FSSW) joints between Cu and AA6061-T6 sheets prepared without and with employing additional heating. Formation and distribution of intermetallic compounds (IMCs) in Cu-AA6061 FSSW joints was further appraised. An increase in maximum lap shear force was observed in Cu-AA6061 and Cu-Cu joints when preheating was employed. Lap shear force decreased with increase in test temperature for Cu-AA6061 joints prepared without preheating; however, for joints prepared employing preheating, the lap strength increased from 25 to 50 degrees C following a slight drop at 100 degrees C test temperature. For Cu-Cu joints, lap shear force also increased with test temperature for the range studied in the present work. X-ray diffraction, and energy dispersive spectroscopy analysis illustrates the formation of IMCs in the form of Al2Cu and Al(4)Cu(9)in Cu-AA6061 and oxides in Cu-Cu joints. The interface failure from IMC rich regions in dissimilar joints was observed whereas similar joints failed under plug failure. Intermittently, thick IMC layer was observed in dissimilar Cu-AA6061 joint prepared with preheating. Presence of tearing/rupture from oxide dominated intermetallic region observed in Cu-AA6061 FSSW joints whereas dimples and micro-voids were noted in Cu-Cu joints.