Formation mechanism and evolution of surface coarse grains on a ZK60 Mg profile extruded by a porthole die
来源期刊：JOURNAL OF MATERIALS SCIENCE TECHNOLOG2020年第12期
论文作者：Jianwei Tang Liang Chen Guoqun Zhao Cunsheng Zhang Xingrong Chu
文章页码：88 - 102
摘 要：Porthole die extrusion of Mg alloys was studied by means of experimental and numerical studies. Results indicated that an inhomogeneous microstructure formed on the cross-section of the extruded profile. On the profile surface, abnormal coarse grains with an orientation of <11-20> in parallel to ED（extrusion direction） appeared. In the profile center, the welding zone was composed of fine grains with an average size of 4.19 um and an orientation of <10-10> in parallel to ED, while the matrix zone exhibited a bimodal grain structure. Disk-like, near-spherical and rod-like precipitates were observed, and the number density of those features was lower on the profile surface than that in the profile center. Then, the formation and evolution of coarse grains on the profile surface were investigated, which were found to depend on the competition between static recrystallization and grain growth. The stored deformation energy was the factor dominating the surface structure through effective regulation over nucleation of the precipitates and recrystallization. A profile with a low stored deformation energy suppressed formation of precipitates and consequently facilitated grain growth rather than recrystallization, resulting in the formation of abnormal coarse grains. Finally, the surface coarse grains contributed detrimentally to hardness, tensile properties, and wear performance of the bulk structure.
Jianwei Tang1，Liang Chen1，Guoqun Zhao1，Cunsheng Zhang1，Xingrong Chu2
1. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University2. Associated Engineering Research Center of Mechanics and Mechatronic Equipment, Shandong University
摘 要：Porthole die extrusion of Mg alloys was studied by means of experimental and numerical studies. Results indicated that an inhomogeneous microstructure formed on the cross-section of the extruded profile. On the profile surface, abnormal coarse grains with an orientation of <11-20> in parallel to ED（extrusion direction） appeared. In the profile center, the welding zone was composed of fine grains with an average size of 4.19 um and an orientation of <10-10> in parallel to ED, while the matrix zone exhibited a bimodal grain structure. Disk-like, near-spherical and rod-like precipitates were observed, and the number density of those features was lower on the profile surface than that in the profile center. Then, the formation and evolution of coarse grains on the profile surface were investigated, which were found to depend on the competition between static recrystallization and grain growth. The stored deformation energy was the factor dominating the surface structure through effective regulation over nucleation of the precipitates and recrystallization. A profile with a low stored deformation energy suppressed formation of precipitates and consequently facilitated grain growth rather than recrystallization, resulting in the formation of abnormal coarse grains. Finally, the surface coarse grains contributed detrimentally to hardness, tensile properties, and wear performance of the bulk structure.
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