安定化熱處理對摩擦攪拌Mg-9Li-2Al-1Zn鎂合金拉伸性質及沖剪表面之影響

Abstract

Magnesium alloys have advantages such as low density, high specific strength and good mechanic damping properties. They are often applied for 3C and transportation industries as structural materials. Magnesium alloys with adding lithium element can make the density lower and the workability better than pure magnesium. A recent report suggested that the second phase α with plank-like shape in extruded dual-phase Mg-Li alloy might be the initial points of fracture under tensile test. In order to redistribute the second phase α, friction stir processing (FSP) followed by stabilization heat treatment is applied for trying to improve the mechanical properties of extruded Mg-9Li-2Al-1Zn alloy (LAZ921-F). This study focus on the effects of distribution of the second phase α on the tensile fracture characteristics and the effects of the ductility change on the roughness of blanking surfaces of Mg-9Li-2Al-1Zn alloy after FSP (LAZ921-FSP) and stabilization heat treatment (LAZ921-FSP-S). According to the shifts of X-ray diffraction peaks, supersaturated solid solution induced by FSP may be eliminated after stabilization heat treatment. Stabilization heat treatment also makes aging time less. The β phase greatly affected by stabilization heat treatment may dominate the changes of tensile mechanical properties of LAZ921-FSP. In the microstructure of FSPed LAZ921 magnesium alloys, there are two kinds of microstructures in SZ. The one is composed of equal-axed α particles. The other is composed of α phase on the grain boundaries of β phase and the α phase is dispersed in the matrix with needle-like shapes. On tensile mechanical properties, the experimental results indicate that LAZ921-FSP has better strength and worse elongation than LAZ921-F. The initial points of fracture in LAZ921-FSP were located on β phase. LAZ921-FSP-S and LAZ921-F have similar tensile mechanical properties and similar initial points of fracture occurred within α phase and α/β interfaces. The AlLi phase and θ phase in heat affected zone (HAZ) may be dissolved into matrix during FSP. Because that the θ phase re-precipitates after stabilization heat treatment and induces greater precipitation hardening effect than that induced by AlLi phase, the hardness of HAZ is higher than other zones affected by FSP. The fractures caused by tensile stress perpendicular to processing direction are on the zones with higher hardness and where hardness difference ocurr. After blanking, there are cracks parallel to extrusion direction found in LAZ921-F. Also, there are cracks induced by blanking found in LAZ921-FSP due to the brittleness of β phase. The quality of blanking surfaces of LAZ921-FSP-S is better than LAZ921-F and LAZ921-FSP because of finer second phase α fined by FSP and better ductility improved by stabilization heat treatment.