Material flow behavior, microstructure evolution and mechanical properties in FSWed AZ31B Mg alloy with varied cooling conditions

Cooling medium-assisted friction stir welding (FSW) has proven effective in improving the joint quality of magnesium (Mg) alloys, but its underlying mechanisms remain unclear. In this study, we combined numerical simulations and experiments to investigate the multi-scale effects of three cooling conditions—air cooling (AC), water cooling (WC), and alcohol–nitrogen extreme cooling (ANEC)—on the FSW of AZ31B Mg alloy. The results show that the peak welding temperature decreases from 460 °C (AC) to 235 °C (ANEC), and the thermal cycle duration shortens from 120 s to 60 s. Stronger cooling weakens material flow and narrows the heat-affected zone by about 45 %. These changes significantly affect the dynamic recrystallization (DRX) behavior in the stir zone. With increasing cooling intensity, continuous DRX (CDRX) is suppressed while discontinuous DRX (DDRX) is promoted, resulting in grain refinement from 18.17 μm to 3.67 μm and a higher dislocation density (from 1.13 × 10 14 m −2 to 1.42 × 10 14 m −2 ). Consequently, the yield strength increases from 115.3 MPa to 141.3 MPa, while elongation decreases overall from 8.69 % to 5.72 %. However, the ANEC joint exhibits better ductility than the WC joint, owing to its finer and more uniformly recrystallized microstructure, which improves the strength–ductility synergy. This study establishes a quantitative coupling between cooling intensity, thermal–mechanical behavior, and DRX mechanisms, revealing that controlled cooling can tailor the balance between strength and ductility. The findings provide new insights into the thermo-mechanical coupling in FSW and a theoretical basis for optimizing Mg alloy joint performance through adaptive cooling design.