Microstructure and mechanical behavior of similar butt-joints of ZK60 and ZK60–1.5RE magnesium alloys produced by linear friction stir welding

Abstract

The microstructure and mechanical properties of butt-joints produced by linear friction stir welding of similar plates of as-cast ZK60 and modified ZK60 with 1.5 wt.% RE (ZK60–1.5RE) are investigated. The thermomechanical affected zone is investigated in both advancing and retrieving sides, and the microstructure is compared to the base metal and the stirred zone. Electron backscattered diffraction measurements provide the average microstructural features of the transformed microstructure. The mechanical properties are assessed using hardness, tensile testing, and surface residual stress measured using X-ray diffraction. Higher torque and heat input are obtained for the ZK60–1.5RE compared to the ZK60. The thermomechanical affected zone is notably larger for the ZK60 compared to the ZK60–1.5RE. A gradient microstructure is formed in thermomechanical affected zones where deformed grains are progressively more recrystallised towards the stirred zone. There is no visible interface between the thermomechanical affected zone and the stirred zone. A fine and partially recrystallised microstructure is formed in the stirred zone. The retrieving side of the ZK60–1.5RE has a slightly more refined microstructure compared to the other investigated zones. Anisotropy measured with increment in the maximum intensity of the (0001) increases towards to stirred zone. The formed microstructure is correlated with the role of twinning, recovery, static, and dynamic recrystallisation that can occur during friction stir welding. Twinning and grains with large misorientation spread are more pronounced in the thermomechanical affected zone and regions towards the base metal. Sharp low angle grain boundaries are observed towards the stirred zone. From refined recrystallised grains decorating the grain in the thermomechanical affected zone, a mosaic-like of low and high angle grain boundaries are observed in the stirred zone. The joints have comparable surface residual stresses. The friction stir welding improved the ductility and strength of the as-cast ZK60–1.5RE alloy since the tensile samples fractured in the BM.

Keywords

• ZK60
• Mischmetal
• FSW
• Recrystallization
• Microstructure
• Thermomechanical