Work

Heat treatment strengthening of friction stir welded AZ61 magnesium alloy joints

Year: 2025

Type: article

Abstract: In order to significantly enhance the mechanical properties of friction stir welded (FSWed) AZ61 magnesium alloy and elucidate underlying strengthening mechanisms, FSW 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 AMAZ-NZ interface was reduced, and the bonding mechanism shifted from mechanical bonding to metallurgical bonding. As a result, the elongation rate has increased by 14.4% to 36.1%. Transmission electron microscopy 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 friction stir welded (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 dislocations reduce elongation. After heat treatment, the dislocations nearly vanish and dislocations increase, improving elongation. This offers theoretical basis and technical reference for optimizing high-reliability joining processes for AZ61 magnesium alloys effectively.

Source: Composite Design and Manufacturing

DOI: 10.1093/cdm/wqag003