Mg - Gd - Y - Nd - Zr Alloy Friction Stir Welding Research

Mg-Gd-Y alloy has become one of the hot topics in the materials filed of magnesium alloy in recent years, because of its superior age-hardenable and up to300℃heat-resistant characteristics. Welding is one of the most important link in engineering fields for metal material, but the quality of the joints are bad or well which has a directly influence on material’s applications. Friction Stir Welding is a high-quality, efficient, economical, and environmental friendly techniques, however, reported about it is less at present, especially the Mg-Gd-Y alloy with high strength and heat-resistant ability.Therefore, the research of this magnesium alloy is discussed in this paper, and advanced friction stir welding technology was used on it. The research results followed in this paper.Friction stir welding (FSW) can be used for Mg-Gd-Y-Nd-Zr magnesium alloy. The joints can be clearly divided into three different zones by the microstructure and hardness evolutioned. The weld nugget zone (WNZ) where the microstructures were consisted of refined and equiaxed grains had the highest hardness. The thermo-mechanically affected zone (TMAZ) where the matrix grains had the characteristic of hot-deformation had the higher hardness. The heat affected zone (HAZ) where the grain size was equal to the base metal had the lower hardness than that of the base metal. During the present experimental condition, the ultimate-tensile-strength of the sample perpendicularly cut to the welding line reached as much as0.91of the one of the base metal, and the elongation has been greatly improved. The fracture analysis by SEM showed that the fracture mode of the joint was ductile.The friction stir processing (FSP) experimental results showed that there were a wider range parameters in this magnesium alloy processing. We could obtianed refined and equiaxed grains on the stirring zone which had a higher hardness than base metal under the tool rotational speed at 475-1500r/min and the traverse speed at30-118mm/min. In addition, under the present experimental condition, the tool rotational speed had a more significant effect on the microstructure than the traverse speed, and the appropriate rotational speed is750r/min.Through the Closely-passes FSP used to Mg-Gd-Y-Nd-Zr magnesium alloy processing, we could get a refined and uniformed grains microstructure in the processing area, even the junction areas were also well. The coarse eutectic phase in as-cast alloys would be broken, scattered and melted into the magnesium matrix under the stirring tool of high rotational speeding, and the second-phases with rich rare earth would greatly be refined and distributed. The tensile experiment showed that the casting alloy’s mechanical properties can be improved greatly by the Closely-passes FSP and subsequently aging treatment, especially the Cast-T4alloy’s tensile strength would reach390MPa. So for this kind of rare earth magnesium alloy, we can make a new technology for obtaining high performance magnesium alloy plate——"Casting and Solid solution and Closely-passes FSP and Aging", which can significantly improve the cast magnesium alloy’s microstructures and properties.