【作者】 李妍；

【导师】 崔振山；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 硕士

【摘要】 大锻件作为大型成套设备的核心零件,在国民经济建设、国防装备发展中发挥着举足轻重的作用。大锻件的锻造过程研究不仅仅着眼于如何生产出满足要求、具有一定形状尺寸的锻件,更重要的是研究如何修复缺陷、改善微观组织和创造有利的力学条件使锻造过程中不出现新的裂纹或夹杂性裂纹。镦粗成形作为最基本的锻造工艺,可以锻合锻件的先天性空洞缺陷,改善材料的力学性能,从而提高锻件的内在质量及承载能力。本文通过深入分析大锻件内部空洞型缺陷在镦粗过程中的变化,得到了空洞的演化规律、影响空洞闭合的因素和提高空洞闭合效果的条件。本文的主要工作和研究成果如下:1)基于细观损伤力学的Gurson模型,对空洞的演化规律和影响空洞闭合的因素进行分析,得到材料内部空洞闭合的力学条件。2)通过对Gurson模型的本构关系及更新计算的研究,对Marc进行二次开发,将包含空洞的本构模型嵌入到Hypela子程序中,实现了空洞演化的数值模拟运算。3)通过比较包含Gurson本构模型和人工空洞模型的数值模拟结果,发现Gurson模型实现空洞闭合的条件是材料的静水应力或等效应变无穷大。由于这一条件并不符合实际情况,本文又基于正交试验设计,模拟了不同试验参数下的空洞体积变化,从而获得了修正的Gurson模型参数。修正后的Gurson模型给出的空洞体积比接近直接设置人工空洞的模拟结果。研究表明,空洞位于锻件心部时闭合最容易,其位置越接近端面越难于闭合;空洞的闭合程度随基体材料硬化程度的增高而变差。4)以铅作为模拟大锻件成形的材料并在试件上设置微小空洞,进行镦粗成形物理试验模拟,验证了空洞的闭合准则。对比发现,空洞体积变化规律与数值模拟结果基本一致。更多还原

【Abstract】 As key parts of large complete sets of equipment, heavy forgings have played important roles in national economic constructions and defense development. In addition to producing dimensional satisfactory forgings, the research on upsetting process pays greater attention on modification of inner defects, improvement of material tissue as well as creating favorable mechanical conditions which is able to prevent new fractures or inclusion-induced cracks. As basic forging techniques, upsetting can eliminate void defects so that the mechanical performance, inner quality and carrying ability of materials will be greatly improved. The law of void evolution, criterion of void closure and factors which affect void closure are revealed in this paper. The main work is as follows:1) The law of void evolution and factors which affect void closure were investigated based on Gurson model which comes from Meso-mechanics. The criterion of void closure was also put forward. 2) The constitutive relationship of Gurson model and the update algorithm were studied and implemented through subroutine Hypela which defines the material property of porous material and facilitates the numerical simulation.3) The simulation results of artificial model and Gurson model were compared. It is found that infinite hydraulic stress or equivalent strain is required when Gurson Model is employed to realize void closure of materials. However, such condition does not conform to reality. This paper obtained modified parameters of Gurson Model by Orthogonal experimental design and simulation under different sampling parameters through embedded subroutine. The modified Gurson model can be applied to simulate void volume variation in heavy forgings. The results demonstrate that the void volume change ratios are quite close to those obtained by artificial models, which implies that the predictions of void closure are greatly improved. It is also found that void closure in the center is the easiest one to achieve while the difficulty increases as it approaches the surface. The harder the matrix material, the worse the void closure degree will be.4) By setting tiny artificial voids inside lead samples whose recrystallization temperature equates to room temperature, the criterion of void closure is verified by upsetting experiment. The void volume change ratios obtained from physical experiments are in agreement with those from numerical simulations.更多还原