高压扭转成形过程数值模拟及试验
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摘要
大塑性变形法(severe plastic deformation, SPD)是一种制备块体超细晶(ultra-fine grained, UFG)材料的新型塑性加工方法。作为大塑性变形法的典型代表,高压扭转法(high-pressure torsion, HPT)能显著细化晶粒,提高材料的力学性能,已成为材料科学与工程领域内的研究热点。
高压扭转过程中,试样内部的微观组织结构与变形量、温度等宏观场量参数存在密切的联系,因此获得试样在变形过程中材料的流动信息和相关场量参数的大小及分布状况,对于选择合理的工艺参数、优化模具结构以及实现对变形过程的主动控制都是十分重要。本文针对无约束高压扭转的特点,使用三维造型软件UG建立了三维几何模型,并将模型导入3D有限元模拟软件DEFORM-3D中,从而建立了用于HPT工艺分析的有限元模型。通过对HPT过程的有限元分析,获得了材料在HPT过程中流动信息、变形行为、温度分布等场量信息。数值模拟结果表明,高压扭转可以使材料获得大的塑性变形量,试样的温度分布特征与等效应变分布特征较为一致,坯料与模具之间存在着较大的相对滑动,因此坯料的实际扭转角度与下模的旋转角度差别很大。
在上述研究的基础上,对不同工艺参数条件下的HPT过程进行了全面的数值模拟,对坯料的应变、相对扭转角度、下模的扭矩等信息进行了获取和分析。模拟结果表明,摩擦因子影响着坯料径向的等效应变分布,其中当摩擦系数为1时,坯料径向的等效应变呈“M”型分布;摩擦因子、扭转圈数、压力和温度的增加有助于增大坯料的等效应变及相对扭转角度,但变形温度应避开钢的脆性转变区;高径比对坯料相对扭转角度的影响不大,但高径比的减小可以提高坯料轴向的变形均匀性。
本文在有限元模拟分析的基础上,设计了试验模具,利用实验室现有2000KN压扭液压机,对退火态的20CrMnTi坯料进行了高压扭转试验,研究了不同扭转圈数和压力对材料显微组织和显微硬度的影响。试验结果表明,高压扭转后,试样子午面上距中心5mm处的显微组织最为细小、显微硬度值最高,显微硬度沿试样径向也呈“M”型分布,这与有限元模拟中坯料径向的等效应变分布的趋势较为一致,可以认为本文所建立的有限元模型是可靠的;随着压力的升高和扭转圈数的增加,试样子午面和端面的组织趋于细小均匀,珠光体沿试样径向呈针状或细带状分布在铁素体基体上,显微硬度显著升高;高压扭转过程中在较低压力下扭转较多圈数与在较高压力下扭转较少圈数所获得的组织十分相似,说明增大压力和增加扭转圈数对增大试样的变形程度、促使晶粒细化都是十分重要的。
目前,高压扭转法较多的应用在制备超细晶材料方面,也有望应用于航空航天及医学领域。本文以纯铝粉末材料为研究对象,将高压扭转法应用于粉末烧结体锥形件的制备。有限元模拟表明HPT工艺可以有效地提高锥形件的致密度。在模拟的基础上进行了试验,成功制备出铝粉烧结体锥形件,并对锥形件进行了金相观察、显微硬度测量及相对密度测量。模拟和试验结果表明,锥形件整体的致密度达到0.98以上。HPT过程中试样顶部和边缘处的变形量不同,顶部的变形量小,该处致密度在变形后有所提高,加工硬化效果不明显,显微硬度仅为41.6kg/mm2;边缘处的变形量大,组织方向性更强,内部孔隙的平均尺寸仅2μm,致密度显著提高,加工硬化效果显著,显微硬度达到61.12kg/mm2。
Severe plastic deformation (SPD) is a new method of plastic working to obtain ultra-fine grained (UFG) materials. As a typical representative of severe plastic deformation method, high-pressure torsion (HPT) could significantly refine the grain and improve the mechanical properties of materials, it has become an attractive research field of materials science and engineering.
In the process of HPT, the microstructure of the specimen has intimate relationship with field parameters, such as strain and temperature etc. Therefore, it is important to get the flowage information of the metal and the distribution of relative field parameters, that we can choose reasonable technical parameter, optimize die structure and realize initiative control the whole deformation processing. Based on the characters of nonrestrictive HPT, this paper used three-dimensional modeling software UG to establish three-dimensional geometric models and imported the models into DEFORM-3D which is a 3D finite element simulation software, then established a process finite element model to analyze the HPT process. Through analyzing the HPT process, the information of material flowage, deformation behavior and temperature distribution in the HPT process was obtained. The simulation result shows that HPT can severely deform the material. Temperature distribution characteristics and equivalent strain distribution characteristics in the specimen are consistent. Because of the existence of large relative sliding between the specimen and die, the actual torsion angle of specimen is distinctly different from the torsion angle of the bottom die.
Based on the research, comprehensive numerical simulation of HPT was done under different conditions. Strain, relative torsion angle and torque was obtained and analyzed. The simulation result shows that the friction factor affects the equivalent strain distribution along radius of specimen. When the friction factor is 1, the equivalent strain along radius of specimen shows the distribution of "M"-type; equivalent strain and relative torsion angle increase significantly with the increasing of friction factor, torsion turns, pressure and temperature; aspect ratio has little effect on the relative torsion angle while deformation uniformity along axial direction of specimen is improved if aspect ratio is decreased.
Based on FEM analysis, this paper designed die for experiment. 20CrMnTi specimen which had been annealed was processed by HPT utilizing 2000KN hydraulic press in laboratory. Microstructure and microhardness of specimen which were processed in different press and turns were researched. The experimental result shows that microstructure was severely refined in the place which is 5mm from the center along radius of specimen, while microhardness reach to peak value in the same place and shows the distribution of "M"-type too. The distribution of microhardness and equivalent strain has the same trend, so the FEM model is credible in the paper. As pressure and torsion turns are increased, the microstructure of meridian plane and end face turn to fine gained, pearlite turns to acerate or striate in the basal body of ferrite and microhardness is elevated evidently; Both low pressure many torsion turns and high pressure few torsion turns can obtain the similar microstructure after HPT. It shows that both increasing pressure and increasing torsion turns are very important for increasing the deformation of specimen and refining gain.
Presently HPT was used in making ultra-fine grained and it was also expected be used in aerospace and medicine. With pure aluminum powder as the research object, this paper made use of HPT for making cone-shaped part prepared by pure Al powder sintered material. FEM simulation shows that HPT can effectively improve relative density of cone-shaped part; Based on simulation,cone-shaped part prepared by pure Al powder sintered material was made by experiment. metallographic observation, microhardness measurement and relative density measurement were carried on subsequently. Simulation and experiment show that the relative density of cone-shaped part reachs more than 0.98. The deformation of top part in cone-shaped part is different from that of edge in cone-shaped part. In top part, the deformation of top part is not severe, the relative density is improved somewhat after HPT, microhardness reach only to 41.6kg/mm2 because work hardening is inapparent. In edge, the deformation is severe, so microstructure has strong directional, the size inner pore is about 2μm, relative density is improved markedly, and microhardness reach to 61.12kg/mm2 because work hardening is distinct.
高压扭转过程中,试样内部的微观组织结构与变形量、温度等宏观场量参数存在密切的联系,因此获得试样在变形过程中材料的流动信息和相关场量参数的大小及分布状况,对于选择合理的工艺参数、优化模具结构以及实现对变形过程的主动控制都是十分重要。本文针对无约束高压扭转的特点,使用三维造型软件UG建立了三维几何模型,并将模型导入3D有限元模拟软件DEFORM-3D中,从而建立了用于HPT工艺分析的有限元模型。通过对HPT过程的有限元分析,获得了材料在HPT过程中流动信息、变形行为、温度分布等场量信息。数值模拟结果表明,高压扭转可以使材料获得大的塑性变形量,试样的温度分布特征与等效应变分布特征较为一致,坯料与模具之间存在着较大的相对滑动,因此坯料的实际扭转角度与下模的旋转角度差别很大。
在上述研究的基础上,对不同工艺参数条件下的HPT过程进行了全面的数值模拟,对坯料的应变、相对扭转角度、下模的扭矩等信息进行了获取和分析。模拟结果表明,摩擦因子影响着坯料径向的等效应变分布,其中当摩擦系数为1时,坯料径向的等效应变呈“M”型分布;摩擦因子、扭转圈数、压力和温度的增加有助于增大坯料的等效应变及相对扭转角度,但变形温度应避开钢的脆性转变区;高径比对坯料相对扭转角度的影响不大,但高径比的减小可以提高坯料轴向的变形均匀性。
本文在有限元模拟分析的基础上,设计了试验模具,利用实验室现有2000KN压扭液压机,对退火态的20CrMnTi坯料进行了高压扭转试验,研究了不同扭转圈数和压力对材料显微组织和显微硬度的影响。试验结果表明,高压扭转后,试样子午面上距中心5mm处的显微组织最为细小、显微硬度值最高,显微硬度沿试样径向也呈“M”型分布,这与有限元模拟中坯料径向的等效应变分布的趋势较为一致,可以认为本文所建立的有限元模型是可靠的;随着压力的升高和扭转圈数的增加,试样子午面和端面的组织趋于细小均匀,珠光体沿试样径向呈针状或细带状分布在铁素体基体上,显微硬度显著升高;高压扭转过程中在较低压力下扭转较多圈数与在较高压力下扭转较少圈数所获得的组织十分相似,说明增大压力和增加扭转圈数对增大试样的变形程度、促使晶粒细化都是十分重要的。
目前,高压扭转法较多的应用在制备超细晶材料方面,也有望应用于航空航天及医学领域。本文以纯铝粉末材料为研究对象,将高压扭转法应用于粉末烧结体锥形件的制备。有限元模拟表明HPT工艺可以有效地提高锥形件的致密度。在模拟的基础上进行了试验,成功制备出铝粉烧结体锥形件,并对锥形件进行了金相观察、显微硬度测量及相对密度测量。模拟和试验结果表明,锥形件整体的致密度达到0.98以上。HPT过程中试样顶部和边缘处的变形量不同,顶部的变形量小,该处致密度在变形后有所提高,加工硬化效果不明显,显微硬度仅为41.6kg/mm2;边缘处的变形量大,组织方向性更强,内部孔隙的平均尺寸仅2μm,致密度显著提高,加工硬化效果显著,显微硬度达到61.12kg/mm2。
Severe plastic deformation (SPD) is a new method of plastic working to obtain ultra-fine grained (UFG) materials. As a typical representative of severe plastic deformation method, high-pressure torsion (HPT) could significantly refine the grain and improve the mechanical properties of materials, it has become an attractive research field of materials science and engineering.
In the process of HPT, the microstructure of the specimen has intimate relationship with field parameters, such as strain and temperature etc. Therefore, it is important to get the flowage information of the metal and the distribution of relative field parameters, that we can choose reasonable technical parameter, optimize die structure and realize initiative control the whole deformation processing. Based on the characters of nonrestrictive HPT, this paper used three-dimensional modeling software UG to establish three-dimensional geometric models and imported the models into DEFORM-3D which is a 3D finite element simulation software, then established a process finite element model to analyze the HPT process. Through analyzing the HPT process, the information of material flowage, deformation behavior and temperature distribution in the HPT process was obtained. The simulation result shows that HPT can severely deform the material. Temperature distribution characteristics and equivalent strain distribution characteristics in the specimen are consistent. Because of the existence of large relative sliding between the specimen and die, the actual torsion angle of specimen is distinctly different from the torsion angle of the bottom die.
Based on the research, comprehensive numerical simulation of HPT was done under different conditions. Strain, relative torsion angle and torque was obtained and analyzed. The simulation result shows that the friction factor affects the equivalent strain distribution along radius of specimen. When the friction factor is 1, the equivalent strain along radius of specimen shows the distribution of "M"-type; equivalent strain and relative torsion angle increase significantly with the increasing of friction factor, torsion turns, pressure and temperature; aspect ratio has little effect on the relative torsion angle while deformation uniformity along axial direction of specimen is improved if aspect ratio is decreased.
Based on FEM analysis, this paper designed die for experiment. 20CrMnTi specimen which had been annealed was processed by HPT utilizing 2000KN hydraulic press in laboratory. Microstructure and microhardness of specimen which were processed in different press and turns were researched. The experimental result shows that microstructure was severely refined in the place which is 5mm from the center along radius of specimen, while microhardness reach to peak value in the same place and shows the distribution of "M"-type too. The distribution of microhardness and equivalent strain has the same trend, so the FEM model is credible in the paper. As pressure and torsion turns are increased, the microstructure of meridian plane and end face turn to fine gained, pearlite turns to acerate or striate in the basal body of ferrite and microhardness is elevated evidently; Both low pressure many torsion turns and high pressure few torsion turns can obtain the similar microstructure after HPT. It shows that both increasing pressure and increasing torsion turns are very important for increasing the deformation of specimen and refining gain.
Presently HPT was used in making ultra-fine grained and it was also expected be used in aerospace and medicine. With pure aluminum powder as the research object, this paper made use of HPT for making cone-shaped part prepared by pure Al powder sintered material. FEM simulation shows that HPT can effectively improve relative density of cone-shaped part; Based on simulation,cone-shaped part prepared by pure Al powder sintered material was made by experiment. metallographic observation, microhardness measurement and relative density measurement were carried on subsequently. Simulation and experiment show that the relative density of cone-shaped part reachs more than 0.98. The deformation of top part in cone-shaped part is different from that of edge in cone-shaped part. In top part, the deformation of top part is not severe, the relative density is improved somewhat after HPT, microhardness reach only to 41.6kg/mm2 because work hardening is inapparent. In edge, the deformation is severe, so microstructure has strong directional, the size inner pore is about 2μm, relative density is improved markedly, and microhardness reach to 61.12kg/mm2 because work hardening is distinct.
引文
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