Researches On Friction Stir Spot Welding Of AZ Magnesium Alloys

In the paper, hot-extruded magnesium alloy AZ31was selected and the effects of pindiameter, penetration length, the types of overlap and post weld heat treatment on themacro-morphologies, microstructures and mechanical properties of friction stir spotwelding (FSSW) welded AZ31magnesium alloy joints were systematically studied bymicrostructural observations, tensile tests and microhardness tests. The microstructuralevolutions and the relationships between microstructures and mechanical properties ofthe welded joints were revealed. Based on the experimental facts and combined withtheoretical analysis, main conclusions may be summarized as follows:The effects of pin diameter on the microstructure and the mechanical properties ofFSSW welded AZ31B magnesium alloy joints were investigated. The results show thatwith an increase of the pin diameter, the height and the width of the curved interface inthe FSSW welded AZ31B magnesium alloy joints increased because of the strong effectof the stirring and the high temperature when large sized pins were adopted. An increaseof the pin diameter led to the coarsening of α-Mg grains in the stir zone (SZ),thermo-mechanically affected zone (TMAZ) and heat affected (HAZ) due to the heatgeneration, resulted in the decrease of the microhardness of the SZ, TMAZ and HAZ.The tensile shear force of the FSSW welded AZ31B magnesium alloy joints increasedwith the increase of the pin diameter because the height and the width of the curvedinterface dominated the failure of the specimens.The effects of penetration length on the microstructure and the mechanical propertiesof FSSW welded AZ31B magnesium alloy joints were studied. The results show thatthe input, the width of the curved interface and the size of α-Mg all increased with theincrease of the penetration length. The equiaxed structure was more uniform as a resultof higher penetration length. When the penetration length was lower, α-Mg in thethermo-mechanically affected zone was more uniform because of the effect of thestirring. When the penetration length was higher, α-Mg in the thermo-mechanicallyaffected zone coarsened and was irregular because of the effect of the thermal cycle.The microhardness of the FSSW welded joints decreased with the increase of thepenetration length. The shear tensile force of the FSSW welded joints increased initiallyand then decreased. When the penetration length was0.8mm, the value of shear tensileforce was highest. When the penetration length was1.0mm, liquid penetration induced cracking (LPI) cracking occurred in the FSSW welded joint, leading to the decrease ofthe shear tensile force.The effects of the types of overlap on the mechanical properties of the FSSWwelded AZ series magnesium alloy joints were investigated. The results show that themicrostructure of the stir zone adjacent to the periphery of the rotating pin was mainlycomposed of the upper sheet. The value of the average width of the curved interface, thevalue of the tensile shear force and the microhardness of the stir zone of the FSSWwelded AZ61alloy joint were the highest in all samples. In the FSSW welded AZ31andAZ61dissimilar magnesium alloy joints, the irregular deformation of the longitudinalsegment of the curved interface appeared. While, the microhardness of the stir zone washigher when AZ61alloy was the upper sheet. In addition, the microhardness of the stirzone increased initially and then decreased sharply in the longitudinal test positionduring FSSW process.The effects of post weld heat treatment on the microstructure and the mechanicalproperties of FSSW welded AZ31B magnesium alloy joints were studied. The resultsshow that when the heat treatment temperature was200℃, α-Mg in the SZ adjacent tothe periphery of the rotating pin began to coarsen and the grains adjacent to theperiphery of the rotating pin coarsened obviously, and the recrystallization grainsoccurred in the thermo-mechanically affected zone. When the heat treatmenttemperature was250℃, more coarsening grains occurred in the SZ adjacent to theperiphery of the rotating pin, some grains in the SZ which was far away from theperiphery of the rotating pin and the recrystallization grains in the thermo-mechanicallyaffected zone coarsened. When the heat treatment temperature was300℃or above,grains in the SZ coarsened dramatically. The shear tensile force of the FSSW weldedjoints increased initially and then decreased with the increase of the heat treatingtemperature.