【作者】 周家勇；

【导师】 彭向和；

【作者基本信息】 重庆大学 ， 固体力学， 2009， 博士

【摘要】 随着科学技术的进步,大厚径比微型件正大量应用于工业和民用产品之中,批量生产要求采用效率较高的冲切技术加工这类零件。在室温下冲切加工这类零件时的高冲切力常使模具破坏或加速其失效,而通常采用的蚀刻和电化学等非冲切加工技术则存在效率相对较低、制造成本较高或加工过程可能污染环境等缺点。作为大厚径比微型件批量冲切加工的富有前景的方法之一,热辅助微型件冲切技术近年来引起了人们强烈的关注。本文在分析国内外有关研究现状基础上,对热辅助微型件冲切的若干基本问题进行了实验、理论与数值模拟研究。主要工作和结论如下:(1)以LY12铝合金为研究对象进行了温升率对金属材料力学性能影响的实验研究。试验结果表明:中等温升率产生的局部热失配及局部残余应力场可引起材料强化,而高温升率引起的严重局部热失配及局部残余应力与外加热—力载荷的耦合作用可加速材料微缺陷的形核与扩展,加剧材料的损伤与性能劣化。对试件断面的SEM图分析表明:不同温升率下试件断口的金相组织具有明显的差异,从而表现出不同的宏观力学性能。低温升率下材料再结晶比例较高,呈现出韧性断裂特征,而高温升率时试件断面平整,缺陷较多,具有脆性断裂特征。(2)根据实验结论,基于热力学相容的简单机械模型,建立了一个计及温度、温升率、应变率和损伤的大变形弹塑性本构模型,该模型充分考虑了温度对材料参数的影响,并在材料的损伤和强化函数中计及应变、应变率、温升率和再结晶的影响。鉴于由微分型本构方程直接改为增量形式,会导致较大的计算误差,因此发展了相应的数值算法,并据此算法推导了该本构方程的增量型表达式及其有限元列式。再结晶可消除材料微结构的畸变和相应的应变强化,将基于包含温升率的变温Johnso-Mehl方程计算的再结晶转变体积分数引入强化函数,以此考虑温升率及再结晶对材料强化的影响。(3)根据有限元软件ABAQUS的子程序接口原理,利用Fortran语言基于本文模型编制了用户自定义材料子程序。并利用UMAT材料子程序,首先模拟了一种低碳钢试件在室温下的瞬态热力耦合墩粗过程,与实验结果的比较表明,模型能较好地描述该试件的热力耦合大变形过程。然后又分别模拟了以三种温升率加热LY12铝合金试件至450?C后的拉伸试验过程,建立的本构模型较好地描述了该材料经历不同温升率历史后的力学性能差异。(4)以304不锈钢、铜和LY12铝合金冲件为研究对象,选取碳化钨硬质合金和普通碳素工具钢两种模具材料,进行了六组冲件—模具材料组合下电辅助加热微型件冲切的热电耦合模拟。结果表明,电加热可在较短时间内在工件的冲切区形成合适的冲切温度,显著降低材料的破坏强度,从而可有效降低冲切力。对比同一种冲件分别和以上两种模具材料组合的模拟结果可以看出,如果模具材料的导电性能和传热性能均优于冲件材料,可避免模具中过多的热量沉积。(5)利用ABAQUS和基于本文模型的用户自定义材料子程序,模拟了激光辅助加热FeNi42、电辅助加热304不锈钢和LY12铝合金微型件的冲切过程。模拟结果表明,对冲件进行的辅助加热显著降低了其材料强度,在较高温度下冲切时,冲切力的有效降低可提高模具寿命并实现大厚径比微型件的冲切加工。更多还原

【Abstract】 With the requirement of the development of science and technology, the parts with small size and large aspect ratio have been more and more extensively used in various industries and consumers. It is known that, although blanking is an efficient and environmental friendly method for mass production of micro-parts compared with etching and electrochemical micromachining, blanking of the micro-parts with larger aspect ratio remains unsolved, due to that it results in excessive tool loads, with consequent tool failure or decrease in tool life. Two parameters dominate the attempt to use large aspect ratio - the strength of the currently available tool-materials and the shear strength of the work-material. Of the two, the availability of blanking would naturally opt to reduce the latter by making use of heating-assisted technology than developing high-strength tool-materials. On the basis of the progress in the research of the heating-assisted micro-parts blanking, some fundamental problems related to micro-parts blanking are investigated in this dissertation. The main contribution of the dissertation is listed as follows:(1) Since a heating-assisted micro-part blanking process may involve both high temperature and large heating-rate, the effect of heating rate on the mechanical properties of work-materials should also be investigated. For LY12 aluminum alloy, experiment shows that, the local thermal inconsistency and residual stress field at moderate heating rate may result in hardening, while the severe local thermal inconsistency and residual stress field at high heating-rate may accelerate the initiation and growth of microdefects, resulting in damage and property degradation of the material. SEM observation and analysis reveal distinct difference between the metallographs of the materials undergoing different heating-rate histories. The materials undergoing higher heating-rate history appears more brittle, attributed to more damage and less recrystallization.(2) Based on a simple thermomechanically consistent mechanical model, a constitutive model is proposed for finite elastoplastic strain and deformation of materials. It takes into account the effects of temperature on the properties, and the effects of strain, heating-rate, strain-rate and recrystallization on the hardening and damage of materials. The incremental form of the constitutive model, the corresponding numerical algorithm and the finite element formalism are also developed. Based on variable-temperature Johnson-Mehl equation, the volume fraction transformed is calculated and it is involved in hardening function.(3) The user defined material subroutines of the proposed constitutive model are developed based on the subroutine interface of commercially available finite element code ABAQUS. The coupled transient thermal-mechanical processes of the upsetting of a mild steel specimen are simulated. The responses of the aluminum alloy LY12 specimens heated at different heating-rate to a prescribed temperature followed by tension until fracture are also simulated. The computed results are in satisfactory agreement with the experimental results, and the effects of the main influencing factors can be well described, demonstrating the validity of the proposed constitutive model in the analysis of heating-assisted finite elastoplastic deformation and damage processes.(4) The coupled thermal-electric simulations, with copper, LY12 aluminum alloy and 304 stainless steel as workpiece materials, and tungsten carbide and plain carbon tool steel as tool materials, respectively, are numerically simulated with the proposed constitutive model and the corresponding approach. The results show that the shear zone of workpiece can be heated by electrical current to an appropriate blanking temperature in a short time interval, which enables a sufficient reduction of the shear strength of the work material as well as the blanking force. Analysis also shows that, in order to avoid improper deposition of energy in a workpiece-tool system, the electrical and thermal conductivities of the tool material should be superior to that of the work-material as much as possible.(5) The laser-assisted and electricity-assisted heating micro-part blanking processes, with FeNi42 alloy, 304 stainless steel and LY12 aluminum alloy as workpiece materials, respectively, are numerically simulated with the proposed constitutive model and the corresponding approach. The results show that assisted heating can provide an appropriate blanking temperature in a short time interval, and sufficiently reduce the shear strength of the work material and the corresponding blanking force, to meet the requirement of the blanking with large aspect ratio and mass production.更多还原