Abstract: AZ31 Mg alloy and 5A06 Al alloy were welded successfully with refill friction spot welding process.The shear stress of the spot joint with different tool rotation frequencies was tested.The effect of rotation frequency on the cross-section,interface layer and element distribution was analyzed.With the increase of the tool rotation speed,the shear stress of the joints increases firstly and then decreases.In particular,when the rotation speed is 2400 r/min,the average shear stress can reach a maximum of 1.9 kN.When the rotation frequency is lower,the interface layer of Mg/Al is thiner.Due to the insufficient interface reaction,the phenomenon of incomplete fusion appears.If the rotation frequency is higher,the interface layer of Mg/Al is thicker (about 5 μm).The interface bonding is very good.