Microstructure, interface characterization and tensile shear strength of dissimilar friction stir welded AZ31/LZ91 alloys lap joints

The microstructure, interface characterization and tensile shear strength of AZ31/LZ91 alloys friction stir lap welding were investigated. The results show that the grain size of the upper AZ31 stirred zone gradually decreases from top to bottom. Compared with the thermo-mechanically affected zone on the retreating side, the grain size of the thermo-mechanically affected zone on the advancing side is smaller, and the distribution is more uniform. The grain size of the alpha-Mg phase and the beta- Li phase in the thermo-mechanically affected zone on the advancing side and the retreating side of the lower LZ91 are different. The grain size of the alpha- Mg phase on the retreating side is smaller, while the grain size of the beta-Li phase is larger, compared to the thermo-mechanically affected zone on the advancing side. The grain size and misorientation angle of the alpha-Mg phase are different on both sides of the thermo-mechanically affected zone/stirred zone- side interface on the advancing side of the AZ31/LZ91 joint. The average grain size of the alpha-Mg phase in the thermo-mechanically affected zone is similar to 4.5 mu m, while the alpha-Mg phase at the stirred zone- side is similar to 3.3 mu m. However, the proportion of the misorientation angle at the stirred zone-side ranging from 10 degrees to 45 degrees reaches the maximum value of 68.8%. The AZ31/LZ91 interface is well connected, and the line scan from the AZ31 side to the LZ91 side shows that the contents of Mg and Al elements are reduced, and the content of Zn elements is basically unchanged. When the welding speed increases in the range of 30 mm/min-120 mm/min, the tensile shear strength first increases and then decreases. When the welding speed is 100 mm/min, the tensile shear strength reaches the highest value of 145.3 MPa.