Parameter-mechanical properties regression model and fine grain strengthening phenomenon of Mg-9Al-Zn magnesium alloy weldments prepared by FSW

Abstract

The solid-state joining of Mg-9Al-Zn magnesium alloy under multi-parameter interaction was carried out by friction stir welding (FSW). A mathematical relationship between welding parameters and mechanical properties was established. The macro-micro detection was carried out by using laser scanning confocal microscope (LSCM) and scanning electron microscope energy dispersive spectrometer (SEM-EDS), and the mechanism of different parameters affecting the mechanical properties and microstructure evolution was revealed. The results indicate that the influence of welding parameters on the mechanical properties of the joint follows the order: "rotational speed > press-in volume > welding speed”. LSCM shows that there are obvious high-frequency vibration folds on the weld surface, which affect the fatigue resistance and corrosion resistance of the joint. Fine grain reinforcement is the main mechanism affecting weld mechanical properties, with weld hardness closely correlating with its size distribution trend. The morphology of the tensile fracture exhibited characteristics of quasi-cleavage fracture, with precipitated phases such as Al₆Mn and AlMnSi observed in the fracture area. A high welding speed accelerates the migration of silicon (Si) and promotes its accumulation at the edge of the stirring zone, which increases the brittleness of the alloy. At a rotational speed of 400 rpm, a welding speed of 70 mm/min, and a reduction of 0.15 mm, the precipitated phases of Al₆Mn and AlMnSi in the weld are evenly distributed, showing no Si accumulation. The fracture surface displays weak dimple characteristics, and the mechanical properties of the weld are optimized, reaching 146.15 % of the strength of the base metal.