Abstract
Local melting and cracking are investigated during friction stir spot welding of binary Mg-Al alloys containing from 3% to 23% Al by using a combination of stir zone temperature, rotational torque and detailed metallographic. The stir zone temperatures during Mg-15%Al and Mg-23%Al alloys friction stir spot welding correspond with the (α-Mg + Mg17Al12) eutectic temperature in the binary Mg-Al equilibrium phase diagram. The stir zone microstructures of friction stir spot welds in Mg-15%Al and Mg-23%Al alloys comprise fine equiaxed α phases and Mg17Al12. Narrow stir zones and lower rotational torques are observed when Mg-15%Al and Mg-23%Al alloys are spot welded since energy generation is diminished by tool slippage due to existing eutectic liquid at contact interface. These results support that local melting of the (α-Mg + Mg17Al12) eutectic microstructure occurred at the contact interface between the tool surface and material when the stir zone temperature attained to the eutectic temperature in the binary Mg-Al equilibrium phase diagram.Liquation cracking is observed in the location beneath the tool shoulder in friction stir spot welds of Mg-9%Al alloy. The crack surface morphology is formed of dendrite shape. It is confirmed that the cracking occurred during Mg-9%Al alloy friction stir spot welding is liquation cracking.
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