42CrMo钢塑性成形中的损伤开裂研究

【作者】 刘革；

【导师】 蔺永诚；

【作者基本信息】 中南大学 ， 机械工程， 2011， 硕士

【摘要】 在金属材料的塑性加工中,损伤开裂是影响工件成形性能的重要因素之一,也逐渐成为塑性加工领域的研究热点。但是,如何准确描述塑性成形中的损伤开裂仍是一个难题。本文主要是以典型大锻件材料42CrMo钢为研究对象,建立了一种适用于该材料的韧性开裂损伤模型,并分析其锻造过程中的损伤演化规律。研究内容与主要成果包括以下四方面：通过热模拟试验,获得了典型大锻件材料42CrMo钢高温拉伸时的应力应变数据,结合微观组织与塑性理论深入分析了该材料的高温流变特性,建立了基于Zeller-Hollomon参数的高温流变本构方程。通过试探法确定了有限元模拟所用的材料属性；在连续介质力学框架下,引入变形温度与应变速率对材料损伤的影响,改进了Freudentha、Cockroft-Latham、Normalized C-L三种常用的韧性断裂准则,并进一步修正了Normalized C-L准则的改进模型,最终建立了适用于42CrMo钢的韧性开裂损伤模型,其预测结果与试验吻合很好,有效地证明了在建立大锻件材料的损伤开裂模型时,必须考虑变形温度和应变速率对损伤的影响。利用有限元计算的方法,研究了42CrMo (?)冈在锻造过程中的损伤程度及分布情况。结果表明：当温度为1200℃时,材料出现损伤的极限压下率为44%；当温度为950℃时,材料出现损伤的极限压下率为53%。此结果与试验结果相吻合,充分说明了所建立损伤开裂模型适用于该研究材料的锻造过程。通过有限元模拟了典型尺寸锻件的锻造过程,讨论了料宽比、砧宽比、变形温度、应变速率对材料损伤演化的影响规律,获得了不同变形工艺下的成形极限图。结果表明：变形温度与变形速率对材料锻造过程中的损伤影响较大,而料宽比与砧宽比对其影响较小,可以忽略；当速率为0.3s-1,温度为1150℃时,大锻件在锻造过程中的塑性最好,适于高温锻造。更多还原

【Abstract】 In the hot forming process of metals or alloys, damage and fracture is one of the important factors affecting the final product quality, and also gradually becomes a research focus in plastic processing field. However, to describe the damage and fracture in hot forming process accurately is still a big problem. A ductile damage model is put forward for 42CrMo alloy, which is typical material for large forgings. The damage evolution behavior during the forging process is analyzed. The main contents of the dissertation include the following four parts:The high temperature tensile true stress-true strain curves of 42CrMo are obtained through thermal simulation experiments. The rheological properties of high temperature deformation for the studied material are discussed through microstructure and plasticity theory. The constitutive equation of the material at high temperature is established by introducing Zeller-Hollomon parameter.Based on the DEFORM-3D computing platform, the material properties for the finite element simulation are amended by the method of trial tests. Under the framework of continuum mechanics, three commonly used ductile damage models, including Freudenthal model, Cockroft-Latham model and Normalized C-L model, are improved, considering the effects of the deformation temperature and strain rate on the ductile damage. Further, the improved Normalized C-L model is modified. Finally, the ductile damage model is established for the studied material. A good agreement between the predicted and experimental results is obtained, which indicates that the effcts of the deformation temperature and strain rate on damage evolution should be considered while establishing the damage model for the studied material.By the method of FEM, the degree and distribution of ductile damage for 42CrMo steel in forging process are studied. The results show that the limit reduction ratio of 42CrMo steel is 44% when the deformation temperature is 1200℃, and the limit reduction ratio is 53% for the cases of 950℃. The results are consistent with the experimental results, and it shows that the established damage model is suitable for the forging process of the studied material.The forging processes are simulated for 42CrMo steel. The effects of blank width ratios, tool width ratios, deformation temperatures and strain rates on the damage evolution are investigated. The forming limit diagram is obtained under different deformation temperatures and strain rates. The results verify that the effect of width ratios and anvil width ratios on the damage evolution is small, and can be neglected; the optimized forming processes are:the deformation temperature is 1150℃and the strain rate is 0.3s-1.更多还原