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Study on Extrusion-Shearing Composite Deformation Process of Magnesium Alloys

Author: ZhangJunPing
Tutor: ZhangDingFei
School: Chongqing University
Course: Materials Science and Engineering
Keywords: magnesium alloy ES process finite element simulation (FEM) microstructure
CLC: TG379
Type: Master's thesis
Year: 2011
Downloads: 50
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Abstract

Equal channel angular extrusion (ECAE or ECAP) is a severe plastic deformation. The alloys should be pre-extruded before ECAP. In order to obtain tiny grains, the alloys are extruded in ECAP die for several passes. This paper has developed a novel extrusion process through connecting extrusion and ECAP. The process includes extrusion and two passes ECAP with route C, so it is called extrusion-shear process (ES). The evolution of microstructure and the mechanical properties of magnesium alloys were investigated during ES process in the paper. ES process was investigated by Deform-3D finite element simulation software.The AZ series alloys were used in the experiments. AZ31 alloys were thermally compressed by gleeble-1500D thermal simulation instrument to obtain the material properties and provide material model for finite element simulation. The alloys were extruded in one-shear ES die through thermal simulation instrument. The extrusion results were compared with finite element simulation. According to industrial extrusion process, finite element models were established to analyze the effects of ES process parameter on extrusion force, effective strain, velocity field and temperature field. The industrial ES experiments were carried out with ES dies of different extrusion ratio, different channel angulars and different temperatures. The ESed bars were cut for metallography experiment and tension experiment. The results were showed as follow:The extrusion force and the changes of die temperature were obtained through thermal simulation of one-shear ES process. The results of thermal simulation agree well with the results of finite element simulation. It means ES process can be well simulated by finite element simulation in the paper. The microstructure at two edges of cross section is inhomogeneous. The microstructure at the outer corner is finer.The results of simulation show that the extrusion forces of ES process typically change. Two platforms of extrusion force are observed. As the rise of temperature, reduce of extrusion ratio and friction factor, increase of channel angular, extrusion force decreases. The effective strain increases with the increase of extrusion ratio and friction factor or reduce of channel angular. Besides, the friction factor is the main factor that affects the homogeneity of effective strain distribution. The velocity of metal flow in the ES dies continuously increases. However, because of the back-pressure like effect of second channel, the velocity at the first channel decreases. Increase channel angular and decrease friction increase the velocity of metal flow but have few effects on the homogeneity of velocity distribution. As increase of extrusion ratio, friction factor and extrusion speed, the die temperature rises. The higher the die temperature, the more temperature rises.Compared with extrusion, ES process obtains finer grains and higher strength. But when temperature rises to some extent, the mechanical properties of ESed bars and extrusion bars are the same. As the extrusion temperature rises, the grain size of ESed bars becomes coarse, the recrystallization fraction as well as the homogenization and elongation increases. As increase of the channel angular, the effective strain at the channel areas decreases, the grain size of ESed bars becomes coarse and dynamic recrystallization volume fraction decreases. As the extrusion ratio increases, the grain size of extrusion zone decreases. Ultimately, the grain size of ESed bar also reduces.ES process can not only refine the microstructure at the edge of the bar, the central microstructure has also been refined thanks to the shearing effects of the equal channel. In ES process, the microstructures of alloys are preliminary refined at the direct extrusion zone. After the first shearing, the central microstructure has been significantly refined. And after the second shearing, all of the microstructure has been significantly refined. The dynamic recrystallization of entire cross section of the alloy completely occurs, which results the same grain size of the edges and the centre zone. Ultimately, the bars with uniform microstructure are obtained.

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