Characterization of microstructure evolution and weld geometry in micro-friction stir spot welding of lightweight AZ31B magnesium alloy

• µFSSW technique was successfully applied to join thin AZ31B magnesium alloy sheets (0.3 mm & 0.5 mm). • Weld geometry, shear strength, and cross-tensile strength were strongly influenced by plunge depth and dwell time. • The highest shear load (326 ± 37 N) and cross load (88 ± 2 N) were achieved at 500 ms dwell time and 600 µm plunge depth. • Microstructural analysis revealed grain refinement in the Stir Zone (SZ) and partial recrystallization in the HAZ. • Results suggest µFSSW is a promising green and precise joining technique for lightweight magnesium components. The micro-friction stir spot welding (µFSSW) procedure is investigated in this study. The process is used for magnesium alloy AZ31B with different thicknesses of 0.3 mm and 0.5 mm. This study examines the impact of varying plunge depths and dwell times on the geometry of welds and their mechanical performance and microstructural features. The results of the welding experiments showed that the ideal mechanical properties were achieved at a dwell duration of 500 ms and a plunge depth of 600 µm. This resulted in a maximum shear load of 326 ± 37 N and a cross-tensile load of 88 ± 2 N. The weld geometry study revealed that increasing the plunge depth and the dwell duration contributed to an increase in the weld nugget size and the thermo-mechanically affected zone (TMAZ). The results of the microhardness tests showed that the stir zone (SZ) had a higher level of hardness, generating an inverted “V” distribution with a peak of 118 HV. Metallographic investigations demonstrated that SZ included smaller grains due to dynamic recrystallization. On the other hand, the TMAZ contained coarser grains, and the heat affected zone (HAZ) contained granules that had partially recrystallized. Based on the study’s findings, it can be concluded that µFSSW is a viable approach for joining thin sheets of AZ31B. The study also highlights that dwell duration and plunge depth are crucial parameters that influence the integrity of the weld.