【作者】 宋毅；

【导师】 李元元；

【作者基本信息】 华南理工大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 粉末冶金技术作为一项绿色制造技术,相比于传统的制造工艺,具有低成本、高效率以及近净成形等优势。粉末冶金工艺可以更为精确地控制微观组织、密度选择及合金成分等材料性能。粉末冶金零件的大量需求也进一步促进了粉末冶金技术的推陈出新,涌现出了包括热等静压(HIP)、粉末喷射成形、放电等离子烧结(SPS)等一批先进技术。因此,采用数值模拟的方法来研究粉末成形的过程并改进粉末成形的工艺成为了该领域的一个研究热点。本文主要针对金属粉末的压制成形过程以及放电等离子烧结过程进行了数值模拟和有限元建模,对相关实验研究进行了讨论和分析,并验证了模拟中所采用的理论模型的准确性。基于粉末材料的椭球形屈服准则和塑性流动理论,建立了增量形式的热弹塑性本构模型;采用返回映射的算法得到了粉末材料在加载、卸载条件下的弹塑性应力、应变关系;采用岩土塑性力学的研究方法,对比了不同屈服曲面在静水应力和Mises等效应力平面上的表现形式;采用Mises等效应力和等效静水应力的比值定性地分析了粉末的剪切屈服对椭球形屈服准则所造成的误差。运用商业有限元软件Marc开发了粉末材料本构模型的子程序,适用于复杂三维零件的粉末成形过程的模拟。通过与铁粉材料的压制实验的对比,新模型能够更好地描述铁粉在冷压工艺下的致密化过程。相比于其他模型,新模型可以更精确地计算粉末材料流动应力的变化。由于模型中采用了更为合理的粉末材料参数,所以能够很好地权衡偏应力和静水应力的影响,从而在压制的后期阶段,更准确地反映了载荷与位移的实际变化关系。研究并分析了金属粉末材料在放电等离子烧结工艺下出现的孔洞击穿的实验现象,通过试样的微观组织分析得出结论,在SPS烧结过程中可能出现的局部烧结致密化的行为将显著改变后续烧结过程中的电能、应力及温度的分布,从而为研究烧结过程中的不均匀压坯密度分布及电流分布所导致的粉末局部致密化对SPS有限元模拟的重要性提供了有力的证据。基于SPS实验现象的研究,提出了用于描述SPS烧结金属粉末材料一步成形工艺的多场耦合方案,实现了温度场、电场、应力场、位移场以及密度场的耦合模拟。该方案由热电耦合求解模块和热机耦合子程序求解模块组成。模拟结果表明,外部压力的提升有助于烧结温度的降低。在较大压力的作用下,压坯和模具间随位移场变化的接触热阻有效地抑制了粉末压坯的内部温差。而且,应力的综合作用极大地促进了压坯边缘位置低温区域的致密化进程。与实验结果的对比验证了SPS多场耦合模型的可靠性。更多还原

【Abstract】 As an important green manufacturing technique, powder metallurgy (P/M) has many compelling advantages over the traditional manufacturing processes, such as the cost and material saving, the high production rates and the near net-shape capacity. P/M technique can control the material properties of microstructure, density and alloy compositions et al. The large demand of P/M parts promotes the innovation of P/M techniques and gives birth to a lot of advanced technologies including hot isostatic pressing (HIP), spray forming, spark plasma sintering, and so on. The finite element method (FEM) is generally applied to study and optimize the forming process of powders, which has becomed a hot spot in P/M research field.The numerical modeling and FEM simulation of the compaction forming and SPS processes are presented in this paper for the metal powder material. The corresponding experimental discussions and analyses are conducted to validate the accuracy of the adopted theoretical model.The thermal elastoplastic constitutive model in the incremental form is deduced on the basis of the ellipsoidal yield criterion and the plasticity flowing rule. The relationship of the elastoplastic stress and strain in the complex loading and unloading situations is calculated by the returning mapping algorithm. Different yield models are compared in the plane of hydrostatic stress and Mises equivalent stress. It is considered that the ratio of Mises equivalent stress and hydrostatic stress can be used to analyze the error of the ellipsoidal yield criterion qualitatively which is derived from the shear yielding.The user subroutines of the constitutive model are integrated with the commercial FEM software Marc, which is applicable to the complex 3D simulation of the forming processes for powder materials. The simulative results show that this new model gives a better description on the densification behaviors of powders in the cold compression processes. The increase of the flowing stress can also be calculated more precisely. Because of the more rational material parameters, the influences of the deviator of the stress tensor and hydrostatic stress are weighed more veritably. As a result, the present model fits better with the experimental load-displacement curves in the later stage of the compaction process.A perforation phenomenon which occurs in the SPS process of the metal powder material is discussed. It can be concluded from the analyses of microstructure that the unexpected local densification behavior could affect the distributions of electrical potential, stress and temperature significantly, which demonstrates the importance of the density gradient to the SPS FEM study.Based on the discussions of these SPS experimental phenomena, this paper proposed a simulation scheme of the multi-physical fields, which realizes the coupling of temperature field, electric field, stress field and density field. The simulation shows that the increased outer pressure helps to decrease the sintering temperature and restrain the intrinsic radial temperature difference effectively through affecting the variational compact/die contact thermal resistance within the displacement field. Moreover, the comprehensive actions of stress promote the densification process of the colder regions in the interior of the powder compact. And the reliability of the coupled multi-physical-field FEM model is confirmed by the corresponding experiments.更多还原