| Summary: | Al/Mg dissimilar welds were successfully fabricated by refill friction stir spot welding using a grooved sleeve tool. Influences of sleeve penetration depth and rotational speed on the weld formation and mechanical performance were systematically evaluated in terms of welding parameter optimization, interfacial bonding mechanism, hardness distribution and welded joint strength. The results indicated that the success of joining Al alloy to Mg alloy significantly depends on tool sleeve penetration depth. The interfacial bonding mechanism compromised both metallurgical bonding and mechanical inter-locking. Intermetallic compound layers of Al<sub>3</sub>Mg<sub>2</sub> and Al<sub>12</sub>Mg<sub>17</sub> were formed at the Al/Mg interface. The thickness of the intermetallic compound (IMC) layer at the weld center increased from 20–30 μm to 40 μm when the rotational speed increased from 1000 to 2000 rpm. The minimum hardness was 80 HV in Al 7075 and 52 HV in ZEK 100; both were measured in the heat affected zone. The welded joint lap shear strength decreased, and the scatter increased with the increasing of rotation speed, whose maximum was 3.6 kN when the rotational speed was 1000 rpm. In addition, the failure mechanism was determined by tool rotational speed, and found to be interfacial failure under a rotational speed of 1000 rpm and nugget pullout under a rotational speed of 2000 rpm.
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