【作者】 门正兴；

【导师】 周杰；

【作者基本信息】 重庆大学 ， 材料加工工程， 2010， 博士

【摘要】 传统的热成形工艺由于高耗能、低材料利用率和较长的生产周期已难以满足当今社会对材料成形领域提出的节能、低耗、绿色制造的要求;另一方面,随着时代的发展,新型材料的不断产生,其成形窗口窄,对温度敏感,成形过程中相变复杂等特点也为材料成形提出了更高的要求。为了适应产业发展的需求,许多新兴的加工技术手段应运而生,不仅扩展了材料成形的领域,也改变了锻造行业落后的面貌。连续加热成形技术是将加热过程贯彻到整个成形过程的先进成形技术,它将连续加热技术和传统的锻造成形技术相结合,以保证材料始终在锻造温度内实现塑性变形,在发挥传统成形方法优势的同时,大大的降低了成形力,减少了零件的整个生产周期,减少了能量的损耗,避免了由于工件多次加热对成本和产品质量的影响,越来越受到人们的关注,其中电阻连续加热成形技术尤为引人瞩目。电阻连续加热成形技术是采用电阻加热方式在成形工件的工作位置对工件实行的局部或整体的加热,并在成形过程中不断的为工件加热使之始终处于高温状态。电阻加热具有热效率高、设备简单、控制方便、受模具结构影响较小等特点,是连续加热成形技术的理想热源。更为重要的是,在电阻连续加热成形过程中,工件中由电流通过而引起的电场将对金属的变形行为进行改善。但是,电阻连续加热成形技术是一个复杂的成形过程,涉及到电学、传热学、塑性成形力学、材料科学、计算机控制技术等多门学科,影响因素众多且相互之间的关系复杂,阻碍了电阻连续加热技术的进一步发展。本文在对以往连续加热成形技术的研究基础上,对电阻连续加热成形技术进行的分类,并通过自主设计的电阻连续加热成形装置对42CrMo4棒料进行了热成形试验并成功的开发了能够模拟电阻连续加热成形过程的电-热-力耦合有限元模型,为今后电阻连续加热成形的发展提供了坚实的基础。本文首先根据电极与成形工件的关系将电阻连续加热成形技术的模具结构分为三类,即电极不与工件直接接触、电极直接与工件接触及电极与模具、工件部分接触三类,并将电阻连续加热成形系统分为加热系统、温度控制及检测系统、成形系统、绝缘系统四个子系统。采用电极不与工件直接接触的模具结构设计了42CrMo4棒料电阻连续加热成形试验装置。将电阻连续加热成形过程分为成形前模具内加热和电阻加热情况下的成形两个部分,通过试验的方法分别研究了工艺参数、材料性能、模具结构及铝合金垫层等对加热温度、工件温度场分布、成形力和成形过程的影响,并对成形过程中产生的缺陷进行了研究,分析了缺陷产生的机理并提出了解决的方法。由于电阻连续加热成形过程包括了导电、导热和塑性变形的复杂非线性过程,采用试验的方法难以对其进行全面深入的研究。而目前真正的电-热-力耦合模拟还无法实现,因此本文采用将电-热耦合和热-力耦合分别计算后顺序耦合的方法,建立了电阻连续加热成形技术的电-热-力耦合有限元模型。通过模拟与试验结果的对比,在电-热耦合中建立的电阻率与接触面温度成反比关系的接触电阻模型能有效的反映加热过程中接触电阻对加热温度的影响,验证了数学模型的准确性,得到了电阻连续加热过程中温度、电流密度、应力应变及等分布,为复杂零件的电阻连续加热成形和进一步的优化参数提供了条件。为了使工件在电阻连续加热成形过程中温度保持恒定,在成形过程中对加热电流强度的适当控制是该方法成功与否的关键。为了得到合理的加热电流强度曲线,采用拉丁超立方抽样方法及二次多项式相应法建立了加热电流与与坯料温度之间的十元二次近似模型,并采用遗传算法对加热电流强度进行参数优化,从而得到等够在成形过程中保持坯料温度变化在12 oC左右的加热电流曲线。更多还原

【Abstract】 The traditional hot forming technology is difficult to satisfy the real time demand because of high energy consumption, low material utilization and long production cycle. On the other hand, new material get a still higher demand on the matal forming technology because of the special properties of temperature sensitive, narrow forming range and complex phase transition during forming. The hot forming technology by continuous resistance heating is one of the advanced and filled with hopes technologies in the furture, which heats the billet in the dies during the whole forming process in order to keep the billet as the higher temperature. The troditional forming technology and the continouos heating technology has been combined. It have advantages of low forming force, low energy consumption, and can avoid the increase of produation cost due to repeated heating.The hot forming technology by continuous resistance heating can heating the billet to the forming temperature in the die prior forming by the resistance heating and continue heating it during forming. The resistance heating is the ideal continuous heat source for it has characters such as high heat efficiency, simple device, convenient control and so on. The more imporatant thing is which can improve the metal platstic deformation by the exist of current field. Howerer, the forming process by resistance heating is compex, which include heat transfer theory, material science, computer control technology and so on. Many influencing factors and the complex relations each other serevely obstructed the futher development of the hot forming technology by continuous resistance heating. In the paper, based on the previous studies, the die structure has been classify and the experiments of the 42CrMo4 upsetting have been contacted by the independent designed experimental system. Finally, an axis-symmetric electro-thermo-mechanical model has been developed to analyze a hot-forging process by direct resistance heating.The mold structure of continuous resistance heating has been dedived into four kinds: 1) Non-contact of the electrode and billet; 2) Directly contact of the electrode and billet; 3) Part-contact of the electrode and mold and billet. The mold system has four subsystems: heating, temperature control, forming and isolation system. The upsetting experiment device for 42CrMo4 bar has been designed with the first mold structure.The forming process is consist of two parts: heating inside mold before forming and forming during resistance heating.The parameter,material property,mold structure and aluminium alloy buffer,which influence temperature and temperature field distribution of billet and forming force and forming process,have been studied. And the reasons of defect have been researched and solution has been put forward.It is hard to further study with traditional experiments because the continuous resistance heating forming process is a nonlinear complex of electric conduction, thermal conduction and plastic deforming. The coupling simulation of electric, thermal and force can not be inplemented at present. This article separately calculate the electric-thermal coupling and thermal-force coupling and then calculate sequential coupling of them and finally build the finite element model of electric-thermal-force coupling for the continuous resistance heating forming. Comparing the results of simulation and experiments, contacting resistance model built in electric-thermal coupling effectively reflect influence of contacting resistance to temperature, in which the resistivity has an inverse relation with temperature of contact surface. The veracity of math model has been validated and the distribution of temperature, current density and stress and strain has been obtained. The results have provided contion to complicated part’s continuous resistance heating forming and further optimization. In order to keep the billet temperature constant during the continuous resistance heating process, the control of current strength is the key point which decides the results of this method. In this article, Latin hypercube sampling method and quadratic polynomial has been used to build a decimal quadric form approximate model for heating current and billet temperature, and then genetic algorithm has been used to optimize the current intensity, and finally obtained the heating current curve which could maintain the billet temperature varying within 12oC.更多还原