【作者】 邓小虎；

【导师】 张立文；

【作者基本信息】 大连理工大学 ， 材料学， 2009， 博士

【摘要】 金属材料性能很大程度上取决于其微观组织,因此控制材料微观组织演变对热加工过程的理论发展和技术优化有重要的意义。长期以来,对金属成型过程微观组织演变的研究主要建立在实验和半经验公式基础上,这种方法既增加了产品成本、延长了生产周期,又无法直接观察材料的微观组织演变过程。近年来,随着高性能计算设备和新的模拟技术的出现,计算机模拟成为研究热加工过程微观组织演变的重要手段。元胞自动机(CA)法是一种广泛应用的材料介观组织模拟方法,与传统的数值计算和模拟方法相比,它在物理系统的计算和模拟中的应用更为灵活。CA法的转变规则兼具确定性和随机性,可以很好的和材料加工理论模型结合在一起,因此适合于模拟晶粒形核与长大过程。本文采用CA方法分别对金属热变形和焊接凝固过程的微观组织演变进行了模拟,主要内容如下:(1)基于动态再结晶组织转变原理,建立了纯金属动态再结晶过程的二维CA模型。模型综合考虑了位错密度演变、动态再结晶形核及再结晶晶粒长大等一系列过程。在纯金属模型基础上,建立了包含粒子的合金动态再结晶CA模型。模型认为第二相粒子存在使粒子附近发生粒子激发形核,并且会增强位错累积和阻碍晶界迁移。模型分别被应用到纯铜和GCr15轴承钢单道次热压缩过程。模拟结果表明初始晶粒尺寸及热加工参数都对动态再结晶动力学有很大影响。将模拟结果和实验结果进行了对比,吻合较好。(2)建立了预测金属材料双道次热变形过程的二维CA模型。模型综合考虑了热变形涉及的动态再结晶、静态回复、静态再结晶、亚动态再结晶和晶粒长大等单个的物理现象。利用模型分别对纯铜和GCr15轴承钢双道次热压缩过程进行模拟来验证模型。探讨了道次间隔时间和第一道次变形量对微观组织演变、应力-应变曲线和再结晶动力学的影响。模拟结果和实验数据及理论模型基本一致。(3)在二维CA模型基础上,建立了动态再结晶过程的三维CA模型。三维模型采用了立方元胞单元和改进的Moore型邻居。将动态再结晶三维CA模型应用到纯铜和GCr15轴承钢单道次热压缩过程中,探讨了不同初始晶粒尺寸和热变形参数对动态再结晶过程的应力-应变曲线和再结晶动力学的影响。(4)将晶粒变形技术引入到动态再结晶过程的CA模型中,建立了准同步的耦合模型。基于元胞尺寸变化的均匀拓扑变形技术能有效的模拟晶粒形状的改变。模型被用来模拟纯铜和GCr15轴承钢单道次热压缩过程,讨论了热压缩过程晶粒变形对动态再结晶微观组织演变和再结晶动力学的影响。(5)通过耦合CA和蒙特卡罗(MC)方法,建立了熔焊焊缝凝固过程的二维模型。模型通过有限差分(FD)法求解热传导及溶质扩散过程,采用CA/MC法构建固相晶粒的形核和长大规则,通过凝固潜热处理及固/液界面前沿溶质再分配将CA/MC和FD方法结合在一起。将模型应用到17Cr-Ti-N不锈钢的焊缝凝固过程,预测了TiN作为形核孕育剂条件下焊缝凝固组织的柱状晶向等轴晶转变(CET)。模拟结果和实验结果基本一致。更多还原

【Abstract】 The microstructure exerts a strong influence on the properties of metallic materials. Therefore,microstructural control during thermo-mechanical and subsequent process is an important and support factor in theoretical development and technology optimization.In the past decades,the studies of microstructural evolution are mainly based on a number of experiments and semi-empirical formulations.It can cause great expense of time and costs, and can not provide a direct view of microstructural development.Recently,with the advent of powerful computers and new simulated techniques,computer simulation has been playing an increasing important role in the studies on microstructural evolution during thermo-mechanical process.Cellular automaton(CA) is one of meso-scale simulated methods widely used in materials science.Compared with the traditional numerical methods,CA is relatively flexible in simulating different physical systems.It combines the utilities of both deterministic and stochastic transitional rules.Theoretical models of materials process can be readily incorporated into the CA simulation through appropriate transformation function to model nucleation and grain growth.In this paper,microstructural evolution of metal hot deformation and weld solidification are simulated by using CA method.The main contents are as follows:(1) Based on dynamic recrystallization(DRX) metallurgical principles,a dynamic recrystallization in pure metal two-dimensional(2-D) CA model is developed.The evolution of dislocation density,DRX nucleation and grain growth are under consideration.Then,a modified DRX 2-D CA model for particle-containing alloy is constructed.Particle stimulated nucleation(PSN) is incorporated into the CA model to determine the influence of second-phase particles on the nucleation of DRX.It is considered that dispersed second-phase particles enhance the accumulation of dislocation density and resist the movement of grain boundary.The models are applied to model single hit compression tests of pure copper and GCr15 bearing steel.It is observed that DRX kinetics depends on both thermo-mechanical parameters and initial grain sizes.The simulated results are compared with the experimental results and are found to be in good agreement.(2) A 2-D CA model is developed to predict the microstructural evolution during two-stage hot deformation of metallic materials.The physically-based model has integrated the effects of the individual metallurgical phenomena related with the hot deformation, including DRX,static recovery,static recrystallization(SRX),metadyanmic recrystallization (MDRX) and grain growth,etc.The model is validated by simulating double hit compression tests of pure copper and GCr15 beating steel.The effects of the interval and pre-strain on microstructural evoutions,stress-strain curves and recrystallization kinetics are investigated. The simulated results agree well with the experimental results and theoretical models.(3) The 2-D DRX CA model is extended to a three-dimensional(3=D) one.The simulation domain is subdivided into a regular lattice of cubic cells and the modified Moore’s neighbour rule is introduced.The 3=D model is applied to simulate the microstructural evolution of pure copper and GCr15 bearing steel single hit compression tests.The effects of intial grain size and thermo-mechanical parameters on stress-strain curves and kinetics of DRX are discussed.(4) A quasi=synchronous integration model is constructed by coupling a CA model with a grain deformation model.The uniform topology deformation technique is introduced by changing the size of CA grid so as to keep the volume constant.The model is applied to simulate single hit compression process for pure copper and GCr15 bearing steel.The influences of grain deformation on microstructural evolution and the kinetics of recrystallization are investigated.(5) A 2-D welding solidification model is developed by coupling CA with monte carlo (MC) method.The finite difference(FD) method is introduced to calculate heat transfer and solute diffusion.The CA/MC model is used to construct the evolution rule of nucleation and growth of solid.The key problems of combination between CA/MC and FD method are solidification latent heat and solid/liquid interface solute partitioning.The model is introduced to simulate welding solidification process in 17Cr-Ti-N stainless steel.Based on the experimental results that titanium nitride(TiN) works as an inoculant of equiaxed grain formation,given number of TiN is allocated to CA cells randomly.The columnar-to-equiaxed transition of welding solidification microstructure is predicted.The simulated results agree well with experimental results.更多还原