等通道转角挤压模具探讨与展望

等通道转角挤压是能够获得块状超细晶组织的一种新的材料加工方法。其工艺和模具结构有他自身的特点。模具结构的合理设计对改善材料变形均匀性，增强晶粒细化效果，节省加工成本，优化工艺等有着较为重要的作用。本文对各种不同结构的等通道转角挤压模具进行了较为详细地评述，着重探讨了单转角模具的结构特点，尤其是方形通道模具各结构参数之间的关系。并总结了目前模具发展中存在的不足和未来的发展趋势，为工艺优化和模具设计提供了重要的参考依据。     

等通道转角挤压变形工艺综述

通过对不同变形路径、道次、温度、模具转角等工艺参数对等通道转角挤压变形的影响机理进行分析。总结了各工艺参数对成型工艺的影响规律。     

新型等通道转角模具挤压AZ91D镁合金的数值模拟

设计了一种新型的等通道转角挤压模具外形并推导出了在该模具中变形工件上分布的等效应变的理论公式.基于该公式,采用3D有限元分析技术,分析了平值的变化对工件变形的应变分布和挤压所需的最大载荷的影响.模拟的平均等效应变值和理论值非常吻合,ψ=40°时,工件上的等效应变分布最为均匀;ψ的增大将导致最大载荷的逐渐减小,且下降的幅度越来越小.     

新型等通道转角挤压模具优化设计

设计出一种新型的等通道转角挤压模具,推导出该模具下工件变形等效应变值的理论表达式,并应用3D有限元分析技术,分析了模具转角区的外倒角半径R和内倒角半径r的变化对工件变形的变形行为、应变分布和挤压所需的最大栽荷的影响.研究结果表明,R和r的增大都可以减小变形死区面积,但r影响更显著;R的增大将导致最大载荷的迅速减小,而r对其影响甚微.     

圆形挤压件等通道弯角挤压过程三维数值模拟与参数分析

等通道弯角挤压(ECAP)工艺可以将材料晶粒有效细化至1μm以下，是目前获得块状超细晶粒材料的重要方法之一。利用DEFOM-3D对圆形纯铝挤压件ECAP工艺进行大量三维有限元模拟，获得了挤压件变形过程晶粒细化机理，得出了挤压过程中模具拐角、模具圆心角和摩擦条件对挤压件变形均匀程度的影响规律，从而为ECAP模具设计、工艺参数拟定以及挤压工艺路线规划提供有效的理论指导。     

挤压工艺参数对4032铝合金组织与性能的影响

采用YK28-800型双动数控液压机研究了挤压温度及变形程度等工艺参数对4032铝合金挤压后显微组织和力学性能的影响。结果表明:挤压温度不变而变形程度增大时,初晶硅在基体上分布更细小均匀;当挤压温度高于350℃变形程度在25%～70%时,挤压变形后合金的抗拉强度变化不明显,而伸长率随变形程度的增加而提高,但在变形程度超过70%后伸长率明显下降;挤压变形温度为400℃时,变形后的合金可获得最大的伸长率和一定的抗拉强度。     

基于ANSYS的无人机机身模具三维热分析

某无人机复合材料机身结构是由模压固化成型的。模具在固化过程中的温度分布不均匀和模具变形会影响到机身结构的变形,为控制机身成型质量,就要了解模具在成型过程中的温度分布和变形情况;基于ANSYS软件,采用1/2对称模型,模拟了机身下模具在加热、保温阶段的温度分布和瞬态温差,直观显示了模具在各时刻的瞬态应力分布和变形情况,为优化固化工艺和模具设计提供依据。     

等通道弯角多道次挤压工艺累积变形均匀性研究

通过等通道弯角挤压(Equal channel angular pressing,ECAP)工艺累积足够的变形制备块状超细晶粒材料(亚微米或纳米微观结构材料)。采用节点映射法将前一次有限元分析终了结果准确映射到下一次挤压初始阶段的相应节点,实现等通道弯角多道次挤压过程有限元分析。通过有限元模拟不同模具拐角的等通道弯角多道次挤压工艺,获得相应挤压件累积等效应变分布及其规律,为ECAP挤压件晶粒均匀细化提供合理的模具设计与工艺路线。     

等通道转角挤压超细晶ZL203合金的晶间腐蚀行为

采用浸泡腐蚀和电化学腐蚀试验研究了等通道转角挤压制备的超细晶ZL203合金的晶间腐蚀行为。结果表明:等通道转角挤压细化了合金的显微组织,同时也显著降低了合金晶间腐蚀敏感性;合金晶间腐蚀敏感性降低的原因一是θ相网状结构的破碎打断了晶间腐蚀扩展通道,二是变形-αAl基体氧化膜耐蚀性能的提高。     

挤压及热处理对6063铝合金组织及性能的影响

对6063铝合金采用了等通道转角挤压工艺及随后的热处理,利用光学显微镜、扫描电子显微镜及拉伸实验分析了挤压及热处理对6063铝合金的微观组织及力学性能的影响。研究表明:等通道转角挤压显著细化了铝合金的晶粒尺寸并改善了晶粒分布的均匀性。挤压道次越多,晶粒细化作用越明显且晶粒分布的均匀性也越好。经4道次挤压及时效处理后,铝合金的平均晶粒尺寸减小到1.3μm,材料的强度和硬度得到显著提高而延伸率仍保持较高。增强的强度及硬度归功于位错强化、晶界强化和析出的Mg2Si相。     

A computational study of die geometry and processing conditions effects on equal channel angular extrusion of a polymer

Equal channel angular extrusion (ECAE) is an efficient process to obtain enhanced microstructures via super-plastic deformation. In view of its optimisation, it is of prime importance to assess the relationships between processing conditions and material flow. More precisely, detailed knowledge of the plastic strain distribution in the extruded material in relation to the ECAE processing variables is required. The key parameters of the ECAE process are primarily die geometry, ram speed, extrusion temperature, use of back-pressure, number of extrusion sequences and processing route (e.g. rotation of the sample between successive passes). A numerical investigation was achieved to check out the influence of these parameters on the homogeneity of plastic strain distribution in the case of a conventional thermoplastic polymer. Material parameters of a phenomenological elastic viscoplastic model were deduced from compressive deformation tests at different temperatures and strain rates on high-density polyethylene (HDPE). Recommendations on tool geometry and processing conditions can then be provided, according to the numerical results.It was found that optimum ECAE die geometry is strongly material dependent. The application of a back-pressure significantly contributes to reduce the corner gap and consequently promotes the homogeneity of the plastic strain field. A slight sensitivity of plastic strain to ram speed and friction conditions was pointed out. The extrusion temperature strongly influences the magnitude of the plastic strain and has a slight effect on its homogeneity. The number of passes has a significant effect on the magnitude of the plastic strain but has a negligible influence beyond a certain temperature. The extruded material reaches a stationary strain state after few passes. The homogeneity of the plastic strain field is strongly affected by the processing route.     

Rate-dependent transient extrusion

Plane strain slip line field solutions are developed for deformation at the edge of a cylindrical billet when this is backward extruded to form a thin-walled cup. The extrusion pressure for a perfectly plastic material is then derived assuming homogeneous compression in the centre of the billet. A geometric factor, derived from the slip line field, is identified which permits the extrusion pressure to be determined for a nonlinear viscous material.Theoretical results agree well with recent experimental data on back extrusion of highly rate-sensitive superplastic alloys. It is shown that an apparent steady state exists in the process and that extrusion pressure increases with ram speed with a rate index approximately equal to that of the material. The punch profile shape has a greater effect on extrusion pressure for viscous materials than for perfectly plastic ones. The method of analysis appears to have a generality beyond the particular process considered and may be used to optimize extrusion equipment designs.     

GCr15切削过程的有限元仿真

应用MSC.SuperForm软件的单元去除和网格重划分技术,考虑到GCr15机械物理性能随温度的变化和流动应力受应变率和温度影响的特性,建立了二维完全热—力耦合有限元模型,模拟出切削过程各变形区的温度、应力以及切削力的分布。通过仿真得到了切削温度、切削力和已加工表面残余应力的影响规律。仿真结果可为机床、工装设计以及切削参数优化提供帮助。     

Control of surface temperature of an aluminum alloy billet by air flow during a heating process at elevated temperature

The procedure of semi-solid forming is composed of heating a billet, forming, compression holding and ejecting step. There are several methods to heat a billet during semi-solid forming process such as electric heating and induction heating. Usually in semi-solid forming process, induction heating has been adopted to achieve more uniform temperature of semi-solid material. Although induction heating is better method than any others, however, there is still difference of temperature between internal part and surface part of semi-solid material. Worse yet, in case of high liquid fraction of semi-solid material, liquid of the billet will flow down though solid of the billet still remains, which is very difficult to handle. In the present study, induction heating of the billet during thixoforging process with forced surface cooling has been performed to obtain more uniform distribution of temperature, microstructure and shape of the billet. Distribution of temperature of the billets was measured and compared with that of conventional distribution of temperature. Microscopic and macroscopic aspects of the billets were discussed according to location of the measuring points. By this new induction heating method, not only temperature distributions over the whole billet become uniform, but also control of temperature distribution between inside and outside part of the billet is possible as user’s experimental intentions.

     

Evaluation of a pyramid die extrusion for a hollow aluminum profile using FE simulation

The pyramid die extrusion for a hollow aluminum profile was analyzed to investigate the potential of such innovative dies. For this purpose, the pyramid and conventional porthole dies were respectively designed for a given hollow aluminum profile. And the extrusion process was comprehensively studied by performing different types of finite element simulation, such as the analysis of steady state, transient state and billet skin tracking. The effects of pyramid angle on the evaluation parameters of extrusion, such as extrusion load, material flow, exit temperature, length of transverse weld, quality of longitudinal weld, back end defect and die stress were overall analyzed and compared with the conventional porthole die. Through this study, the advantages and shortcomings of pyramid die were well concluded, which should be important information for die designers and makers.

     

Hot backward extrusion comparative analyses by a combined finite element method

Deformation and temperature of hot backward extrusion are complex owing to interaction between deformation and temperature. In this paper, two- and one-way axisymmetric hot backward extrusion problems are analyzed by a combined finite element method, which consists of the volumetrically elastic and deviatorically rigid–plastic finite element method and the heat transfer finite element method. The volumetrically elastic and deviatorically rigid–plastic finite element method is different from the conventional rigid–plastic finite element methods, and has some merits in comparison with the conventional methods. Because contact surfaces between workpiece and tools of the one-way extrusion are different from those of the two-way one, the deformation and temperature of the one-way extrusion are different from those of the two-way one. Contours of effective strain rate, effective strain, temperature, effective stress and hydrostatic stress, as well as plots at different reductions for the two extrusions are obtained successfully. Differences of calculated results for the two extrusions can be clearly seen through comparative analyses.Because the bulk modulus is introduced into the volumetrically elastic and deviatorically rigid–plastic finite element method, influence of temperature on hydrostatic stress can be considered in this paper.     

Multi-field coupling finite element analysis for determining the influence of temperature field on die service life during precision-forming process of steel synchronizer ring

Failure analysis shows that increased die temperature caused by severe plastic deformation of material and heat conduction between hot billet and cavity significantly affects the distortion of gear cavity in steel synchronizer ring forging process. The forging process of steel synchronizer ring and die temperature distribution under different forging conditions are analyzed through finite element method. Simulation results show that severe plastic deformation occurs in the gear cavity. The improvement of lubrication condition results in decreased die temperature. When the initial billet temperature is high, the die temperature is also high. Increasing forging speed in a certain range facilitates the die temperature decrease. The distribution of die temperature in synthetic forming technology is more reasonable than that of one step forging. The synthetic forming technology is adopted in production to reduce the effects of severe plastic deformation caused by die temperature. The ejection mechanism and control system of the double disc friction press are improved to reduce the contact time between the hot billet and cavity. Experimental results show that synthetic forming technology is reasonable, and that the die service life is prolonged.     

大钢锭内部空洞锻合过程的数值模拟

本文采用实验和塑性有限元法模拟相结合的方法研究了大钢锭在锻造过程中内部空洞的锻合机制。结果表明,空洞的闭合度λ和空洞周围的最大压应变ε 1成线性关系。工具与坯料间的磨擦影响坯料内部的应力应变分布,但不改变λ和ε 1的线性关系。当用上下平砧拔长大钢锭时,使ε 1峰值到达坯料中心的最小砧宽比为0.51,最佳砧宽比为0.7。对于FM法拔长,最小砧宽比为0.4,最佳砧宽比为0.6。     

基于热-结构耦合的干气压缩机出口输气管道应力分析

结合高温高压管道在生产中的重要性,利用大型有限元分析软件ANSYS对输气管道的温度场和应力应变场的变化进行分析研究。采用具备符合高温高压管道受力特点的ANSYS单元,对管道进行热结构耦合的应力分析,掌握了管道在内压恒定、不同温度载荷下的应力分布规律。计算结果表明,当管道温差过大时,外壁所受应力较大,应进行必要的保温措施,从而防止管道泄漏或破裂等恶性事故的发生。     

Matrix-presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting

The main purpose of this paper is to develop the matrix presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting to obtain the friction coefficients during the upsetting process. This inverse model assumed the linear material and based on the modified experimental loading increments using the linear modified experimental upsetting loading standard proposed in this paper. Then the friction coefficients of contact boundary between the workpiece and the die at specific finite element analysis stages can be derived. Finally, using the cubic spline fitting, the history of friction coefficient during the upsetting process can be obtained. It is demonstrated that the workpiece profile of upsetting experiment is quite identical to the workpiece profile of simulation using the result obtained in this paper as the history of friction coefficient of contact boundary, and furthermore the distribution of stress and strain of the workpiece during upsetting process can be understood.