Formation mechanism of abnormal coarse grains on weld seam of extruded ZK60 alloy and the effects on mechanical properties

Abstract

The abnormal grain growth of ZK60 alloy during porthole die extrusion was studied. The results indicated that the abnormal grains (AGs) discontinuously formed along the weld seam, due to the growth priority of some grains and the restriction on normal grain growth. The grain boundaries with large misorientation angles owned high energy and mobility, and the different sub-grain structures between AGs and normal grains (NGs) provided the driving force for AGs to sweep out NGs. Moreover, the pining effects of precipitates restricted the growth of NGs, while rare precipitates formed on AGs boundaries. In case of NGs, the grain size was firstly refined due to dynamic recrystallization, and then it increased due to the grain growth. <0001>//TD basal plane and <11–20>//ED fiber textures formed, and then the fiber texture rotated 30° around c-axis by {10-10} <1–210> prismatic slip, resulting in the transformation into {0001} <10-10>. The formation of AGs deteriorated the tensile properties. Moreover, high extrusion temperature or speed increased the sizes of AGs and NGs, and decreased the number of precipitates, which caused the decreasing hardness, yield/tensile strength, and elongation.