往复挤压镁合金再结晶组织表征

Characterization of recrystallized microstructure of reciprocating extruded magnesium alloy

利用往复挤压在300～360℃细化铸态Mg-6Zn-1Y-1Ce合金组织，研究其组织演变和挤压参数对再结晶组织的影响。结果表明：往复挤压合金横截面边缘存在不均匀环，由靠近筒壁的细晶环和粗晶环组成，其宽度随着挤压温度提高而减小；细晶环是由边缘区域与挤压筒壁摩擦而发生第二轮再结晶所致，粗晶环是由再结晶晶粒长大所致；合金晶粒度由变形速率和温度决定，经340℃挤压合金晶粒最细，平均粒径8．2um。除边缘外，往复挤压过程中合金在挤压阶段发生一次再结晶，墩粗过程和后续多道次挤压变形都是通过晶界滑移实现。因此，随着挤压道次的增加，保温时间随之延长，晶粒随之被粗化。

The microstructure of as-cast Mg-6Zn-IY-1Ce alloy was refined by reciprocating extrusion （REX） at the temperature range of 300-360 ℃, the microstructural evolution and the effect of extrusion parameters on recrystallized microstructure were investigated. The results show that the as-REXed alloys have an inhomogeneous ring on cross section, which consists of a fine-grain ring and a coarse-grain ring on the periphery. The width of the ring decreases with increasing the extrusion temperature. The fine-grain ring is attributed to the secondary recrystallization occurring because of friction between the periphery of sample and container wall, the coarse-grain ring is attributed to grain growth followed recrystallization. The grain size of REXed alloy is determined by deformation rate and temperature, a minimum grain size of 8.2 pm is obtained at 340℃. During REX process, recrystallization occurs only once during extrusion except for the periphery, the deformation processes during upsetting and subsequent multi-pass REX are achieved by grain boundary sliding. Therefore, with increasing REX passes, the holding-time is prolonged and grains coarsen.