Homogeneity improvement of friction stir welded ZK61 alloy sheets in microstructure and mechanical properties by multi-pass lowered-temperature rolling

Multi-pass lowered-temperature rolling was applied on the friction stir welded ZK61 magnesium alloy sheets not only for fabricating wide high-performance wrought magnesium alloy sheets, but also for reducing the sheet inhomogeneity including the microstructure and mechanical properties induced by friction stir welding. Results showed that obvious microstructure inhomogeneity including plannar texture anisotropy between the base metal and the stir zone were introduced in the friction stir welded ZK61 alloy sheets, but can be strikingly weakened by hot rolling deformation via multiple dynamic recrystallization. These led to a uniform fine-grained structure of about 8 mu m in average grain size and similar (0002) basal texture in the final 1.4 mm thick sheets, and brought a continuous improvement in the associated property differences, where the relatively lower yield strength in base metal zone was strongly correlated with spreading (0002) texture while higher values in stir zone with stable intense (0002) texture. Moreover, the experimental and calculated Hall-Petch relationships were investigated to explain the different evolution of yield strength between the base metal and the stir zone. Meanwhile, enhanced fracture elongations larger than 20% in both the base metal zone and the stir zone were primarily attributed to grain refinement.