Heat treatment strengthening of friction stir welded AZ61 magnesium alloy joints

Friction stir welding was applied to join 5 mm thick AZ61 magnesium alloy, which exhibited a significant microstructural gradient across the joint region. The nugget zone (NZ) and the advancing side thermo-mechanically affected zone (AMAZ) were primarily characterized by micro-tearing and mechanical bonding, leading to a joint tensile strength of only 79.2% relative to the matrix material. The fracture morphology exhibited distinct features of tear platforms and tear steps. After solution heat treatment at 325 °C, the microstructural gradient at the NZ–AMAZ interface was reduced, and the bonding mechanism shifted from mechanical bonding to metallurgical bonding. As a result, elongation increased from 14.4% to 36.1%. TEM analysis indicated that the main strengthening phases in the AZ61 magnesium alloy belong to the AlMn, Mg16Al12Zn, and Mg(ZnAl) systems. The solution-precipitation phenomenon of the AlMn strengthening phase during heat treatment is not significant, and its strengthening effect on the joint is limited. The morphology of the AlMn phase is mostly granular or lath-like, among which the lath-like phase is identified as Al11Mn4. In the FSWed sample, Mg16Al12Zn and β’(MgZn’) phases were nearly absent; however, significant precipitation occurred following complex heat treatment. Dislocations interacted strongly with these fine precipitates through pile-up and Orowan mechanisms, substantially enhancing the alloy’s strength. This study demonstrates that complex heat treatment improves the overall performance of friction stir welds. The FSWed samples contain mainly dislocations, but a certain proportion of <c> dislocations reduce elongation. After heat treatment, the <c> dislocations nearly vanish and dislocations increase, improving elongation.