Strengthening and toughening mechanism of Mg–RE–Al alloy repair welded joints by suppressing abnormal grain growth during heat treatment

Mg–RE alloys refined with Al demonstrate excellent grain thermal stability, enhancing the mechanical properties of repair welded joints after heat treatment. This research examines the impact of heat treatment on the microstructure and mechanical properties of repair welded joints composed of Mg–4.02Y–3.06Nd–1.56Al alloy. The findings indicate that the microstructure of the repaired joint comprises primarily of an equiaxed α–Mg matrix, an Mg–RE eutectic structure, particle and needle–like Al–RE phases. The particle and needle–like Al–RE phases have a high–melting–point and do not readily dissolve during the heat treatment process. Eliminating the Mg–RE eutectic structure is crucial for improving the mechanical properties of the repair welded joint. If the solution temperature is too low (500°C), an excessive amount of the residual Mg–RE eutectic structure will hinder the improvement of mechanical properties. At a solution temperature of 550°C, although the pinning effect of Al₂RE on the grain boundary inhibits abnormal grain growth (AGG), localized AGG still occurs at the FZ edge. At 525°C, the eutectic structure fully dissolved, and no AGG occurred, achieving an optimal solid solution influence. Subsequently, after aging at 200°C for 16 h, a high density β' strengthening phase uniformly precipitated in the matrix, markedly enhancing the alloy's strength. As a result, the joint's UTS (292 MPa), YS (215 MPa), and EL (5.8%) were comparable to those of the original, unrepaired alloy.