| Magnesium and magnesium alloy materials have the advantages of high specific strength/stiffness,low density,good forging and cutting processability,abundant resources and high recyclability,known as the "21st century green materials".In recent years,it has a wide range of application prospects in the aviation,aerospace,digital products,automotive industry and many other fields.But the structure characteristic of magnesium alloy dense hexagonal crystal decided that its independent slip system under room temperature is less,resulting in poor room temperature plasticity,high difficulty of deformation processing and easy cracking of deformation.Therefore,improve the deformational capacity and the strength of room temperature plasticity of magnesium alloy is conducive to the popularization and promotion of magnesium alloy engineering applications.Research shows that grain refinement is an effective method that can augment the strength of metal components and improve plasticity and toughness.In recent years,Severe plastic deformation(SPD)has been recognized as the most promising advanced method for the preparation of ultrafine crystals.Among them,Equal Channel Angular Pressing(ECAP)is one of the most widely researched and rapidly developing SPD techniques.However,the ECAP process still suffers from cumbersome and discontinuous processes and low efficiency.In addition to ECAP process,semi-solid forming technology has the advantages of short process flow and low energy consumption,and its emergence also brings new opportunities for the development of metal material forming technology.In response to the above situation,this paper combines the traditional ECAP process with the semi-solid forming process,based on the idea of "shortening the process,process coupling",and takes magnesium alloy as the research object to design and develop a new Semi-Solid Extrusion Shear(SSES)composite forming process to achieve high efficiency the goal of effectively and economically preparing magnesium alloys with fine and uniform structures.This paper integrates the theoretical knowledge of metal solidification principle,plastic deformation principle,finite element numerical simulation technology,etc.,uses a combination of theoretical analysis,numerical simulation and experimental research to formulate proper parameters for SSES process,explores the casting solidification process and plastic rheological characteristics of SSES forming process,discusses the influence law of different material parameters on microstructure and mechanical properties and clarifies the mechanism of structure evolution and grain refinement in the SSES process.Thus,the intrinsic connection between SSES process and microstructure and mechanical properties is revealed and the main research contents and conclusions are as follows:Firstly,the numerical models of the filling process and solidification processfor the numerical simulation of the alloy casting process were researched.Then,the assumptions of the rigid-plasticity finite element theory applicable to the plastic deformation simulation were analyzed,and the theoretical formulas were derived.Next,Pro CAST casting simulation software and Deform-3D deformation simulation software were adopted to perform segmented simulation analysis for the whole composite process,so as to simulate the whole semi-solid extrusion-shear composite process simply and accurately,and reveal the distribution of extrusion load,stress,strain and other field quantities of the material during the deformation process and the changing law of casting process.Meanwhile,the key influencing factors in the deformation process of the SSES process were clarified,and the proper process parameters and forming scheme were constituted.The simulation results show that 680 ??700 ? of the casting temperature and 300 ??350 ? of the mold temperature are the optimal setting for SSES process.(2)Combining the results of simulation and experiment,appropriate process parameters can ensure that the smooth forming of magnesium alloys in the SSES process the and also effectively avoid defects such as porosity and shrinkage caused by solidification.In this paper,the casting temperature is 700 ?,and the mold temperature is 350 ?.Then the effect of different shear angles on the structure,macro-texture and mechanical properties of extruded bars in SSES process is explored.The results show that the shear angle can not only conduce a large amount of strain,promote recrystallization,but also help refine the grains.However,as the shear angle decreases,the average grain size does not continue to decrease either.When the angle is up to 135,the size of recrystallized grains has shown signs of growth,and the comprehensive mechanical properties are basically the same as when the shear angle is at 150°.Therefore,the shear angle of the SSES process should not be too small,but be better between 150° and 180°.(3)It's found that compared with the 380? Direct Extrusion(380?-DE)extruding process,extruding magnesium alloy with Semi-Solid Direct Extrusion(SSDE)has a homogeneous structure,small grain size,and weaker weave structure.The macroscopic weave extreme strength of the sample extrusion with SSDE is only 44% of that of 380?-DE,meanwhile,its elongation is 1.6 times as much as that of 380?-DE when with relatively high strength.After analyzing the similarities and differences of the structural evolution of the two processes,the structure evolution law and grain refinement mechanism of AZ31 magnesium alloy SSES forming process were deduced.That is at the beginning of the SSES process,the liquid phase formed necklace-like fine grain clusters through crystalline nucleation;as extrusion proceeded,dislocation slip started,and the sub-grain boundaries in the large solid crystal grains are prone to occur dynamic recrystallization through slip and climbing,and form clustering-like small crystal groups.After entering the shear corner,more shear strain can be introduced,and two mechanisms of continuous dynamic recrystallization and discontinuous dynamic recrystallization occur simultaneously,which further refines the grain size.(4)Inspired by the large deformation rate of SSES,a study of Fast Extrusion Shear(FES)was carried out.The same set of dies was used to study the FES process for AZ31 magnesium alloy at different temperatures and shear angles.Comparing the simulation results with the experimental results,it was found that properly increasing the extrusion speed of the Extrusion Shear(ES)process is conducive to accumulate equivalent strain,reduce stress concentration and excessive stress,and further improve the processing efficiency and practicability of the ES process.Compared with the ES process,the FES process can get the same grain refinement effect with a smaller extrusion ratio,a larger shear angle,and a simpler process flow,and the comprehensive mechanical properties are also better.During the FES process,the shear angle part can effectively introduce the strain difference along the transverse direction of the bar,which promotes the dynamic recrystallization and grain rotation,thus changing the microstructure and structure of the AZ31 magnesium alloy bar.As the shear angle decreases,the recrystallization process is more complete,but it causes inhomogeneity in the cross-sectional structure of the bar.Besides,with the increase of extrusion temperature,dynamic recrystallization process could be promoted and structure could be refined further.However,the migration rate of grain boundaries is fast when the temperature(450 ?)is too high,which leads to the increase of the average grain size up to 55.7 ?m.Therefore,considering the grain refinement effect,structural uniformity and comprehensive mechanical properties,the range of shear angle should be between 135° and 180°,and the extrusion temperature should be between 350 ? and 400?.By contrast of the three processes of ES,FES and SSES,it can be seen that SSES has the best effect on grain refinement of AZ31 magnesium alloy,high structural uniformity and excellent comprehensive mechanical properties. |
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