金属热成形过程的综合数值模拟

金属在热成形过程中的微观组织演变是影响产品力学性能的关键因素,该演变过程取决于温度、应变和应变速率。本文基于有限变形理论和微观组织演变的数学模型,建立了能够模拟变形过程、温度变化过程和微观组织演变过程的有限元法,研制了通用的三维有限元计算软件,并在H型钢三维热轧模拟方面进行了深入开发,给出了原材料为C-Mn钢的H型钢热轧过程综合模拟结果。综合对比了8组不同工艺下的热轧实验结果和计算机模拟结果,二者均吻合良好,表明本文方法能够较好地预报金属热成形过程。     

Al-4Cu-Mg合金半固态压缩时微观组织的计算模型

研究了Al-4Cu-Mg合金半固态压缩变形过程中微观组织参数的变化.结合模糊数学和人工神经网络理论,以Al-4Cu-Mg合金半固态压缩变形时的变形温度、变形程度和应变速率为输入参数,以变形后的晶粒等效直径或分形维数为输出参数,建立了Al-4Cu-Mg合金半固态压缩时的微观组织计算模型.研究结果表明,计算结果与实验结果吻合较好.因此,应用本模型可对Al-4Cu-Mg合金半固态成形过程中的微观组织演变进行预测,为优化变形工艺参数提供了简便、适用的方法.     

Monte Carlo Simulation of Three-Dimensional Nonisothermal Grain-Microstructure Evolution: Application to LENS Rapid Fabrication

A stochastic three-dimensional (3-D) model for grain-microstructure evolution during transient nonisothermal annealing of metallic materials is developed, validated, and applied to the LENS (Laser-Engineered Net Shaping) advanced rapid fabrication process. The model is based on the assumption that the main driving force for microstructure evolution is the reduction in energy contribution arising from the grain boundaries. A temperature-dependent grain-boundary mobility factor is introduced into the expression for the transition probability in order to account for nonisothermal effects, such as those induced by the rastering laser during LENS-based manufacturing. The grain-boundary mobility factor and its temperature dependence are determined using the available experimental isothermal-annealing data. The simulation of grain growth (under nonisothermal annealing conditions encountered in the LENS process) is carried out by coupling a Monte Carlo method for microstructure evolution with a finite difference-based solution to the three-dimensional (3-D) transient energy equation. In response to the computational challenges of the simulations, a highly efficient interprocessor communications methodology is developed, which greatly reduces the simulation time on parallel computers. The results obtained show that under isothermal annealing conditions, the kinetics of grain growth is governed by a temporal power-law behavior and that, after an initial transition period, the grain-size distribution (normalized with respect to the average grain size) becomes time invariant. Furthermore, the application of the model to the LENS process is found to enable establishment of the relationships between process parameters (the laser power, beam rastering velocity, etc.) and the microstructure (grain size distribution, depth of the heat-affected region, etc.) of the deposited material.     

Microstructural modelling of dynamic recrystallisation using an extended cellular automaton approach

Dynamic recrystallisation (DRX) governs the plastic flow behaviour and the final microstructure of many crystalline materials during thermomechanical processing. Understanding the recrystallisation process is the key to linking dislocation activities at the mesoscopic scale to mechanical properties at the macroscopic scale. A modelling methodology coupling fundamental metallurgical principles with the cellular automaton (CA) technique is here derived to simulate the dynamic recrystallisation process. Experimental findings of a titanium alloy are considered for comparison with theory. The model takes into account practical experimental parameters and predicts the nucleation and the growth kinetics of dynamically recrystallised grains. Hence it can simulate different stages of microstructural evolution during thermomechanical processing. The effects of hot working temperature and strain rate on microstructure were studied, and the results compared with experimental findings.     

Investigation of the effect of temper rolling on the texture evolution and mechanical behavior of IF steels using multiscale simulation

The main objective of this study is to simulate texture and deformation during the temper-rolling process. To this end, a rate-independent crystal plasticity model, based on the self-consistent scale-transition scheme, is adopted to predict texture evolution and deformation heterogeneity during temper-rolling process. For computational efficiency, a decoupled analysis is considered between the polycrystalline plasticity model and the finite element analysis for the temper rolling. The elasto-plastic finite element analysis is first carried out to determine the history of velocity gradient during the numerical simulation of temper rolling. The thus calculated velocity gradient history is subsequently applied to the polycrystalline plasticity model. By following some appropriately selected strain paths (i.e., streamlines) along the rolling process, one can predict the texture evolution of the material at the half thickness of the sheet metal as well as other parameters related to its microstructure. The numerical results obtained by the proposed strategy are compared with experimental data in the case of IF steels.     

Three-dimensional numerical analysis and experimental investigation of grain refinement in multi-pass equal channel angular pressing for round-workpieces

Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass ECAP process show that the different processing routes result in the different deformation distributions. The grain in the workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model) and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent with FE analysis results.     

Numerical Simulation of Microporosity Evolution of Aluminum Alloy Castings

A mathematical model to calculate the size and distribution of microporosities was studied and coupled with a stochastic microstructure evolution model. The model incorporates various solidification phenomena such as grain structure evolution, solidification shrinkage, interdendritic fluid flow and formation and growth of pores during solidification processes. The nucleation and growth of grains were modeled with a cellular automaton method that utilizes the results from a macro scale modeling of the solidification process. Experiments were made to validate the proposed models. The calculated results of aluminum alloy castings agreed with the experimental measurements.     

Effect of process parameters on microstructural variables of a commercial TC6 titanium alloy disc

Abstract

The interaction between the deformation behaviour and the microstructure evolution is the main characteristic in the forging process of titanium alloy and this interaction is researched using finite element (FE) simulation. Coupled simulation of deformation behaviour with microstructure evolution has been carried out by means of a new constitutive equation presented by Li et al. (Mater. Sci. Technol., 2004, 20, 1256–1260). The effect of deformation temperature, hammer velocity,height reduction and shear factor on the microstructure variables, including grain size and volume fraction, has been studied in the forging process of the TC6 titanium alloy disc with deformation temperatures of 880–940°C, hammer velocities of 1·2–12 000 mm min^?1 and shear factor (m) of the friction of 0·1–0·4. The simulated results show that deformation temperature, hammer velocity and height reduction have a significant effect on themicrostructure evolution and this effect is more significant on the microstructure evolution in hot forging than that in isothermal forging. The simulated results are in good agreement with the experimental results.     

Three dimensional simulation of microstructure evolution for ceramic tool materials

The observation and scientific quantitative characterization of three dimensional microstructure evolution during sintering process of ceramic tool materials is important to investigate the influence of nano-particles on mechanical properties. The relationship between microstructure and mechanical properties of ceramic tool materials can be established to direct the development of nano-composite ceramic tool materials by the research of the grain growth, grain boundary migration, distribution of nano-particles and microstructure densification at the different sintering temperature and pressure. In this paper, a 3D Monte Carlo model of three-phase nano-composite ceramic tool material is built and applied to simulate the microstructure evolution during sintering process. In this model, the grain boundary energy of each phase and interfacial energy between two phases are taken into consideration as the driving forces for grain growth. The sintering temperature and pressure are successfully coupled into the Monte Carlo simulation model. The microstructure evolution of defect free three-phase nano-composite ceramic tool materials is successfully simulated at different sintering temperature and pressure. The simulation results show that the higher the sintering temperature is, the faster the grain growth. However, the sintering pressure has little effect on the grain growth.     

Grain growth in sintering of clustered powder compacts

A theoretical model of grain growth in sintering of clustered powder compacts is outlined, showing that the microstructure evolution is stepwise continuous in time and its general trends can be predicted independently of the particular system and process being considered. The dependence of coarsening on densification can be accounted for by introducing a densification-scaled time variable (intrinsic time). The theory is successively applied to systems where particular local mechanisms of matter transport are supposed to operate, respectively in the initial/intermediate and the intermediate/final stage of sintering. The obtained mathematical models are solved numerically to follow the evolution of three regularly packed clusters. The model predictions are in good agreement with experimental data obtained by other researchers.     

Study of the dynamic recrystallization of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process via Finite Element Method modeling and microstructure characterization

By integrating the thermomechanically coupled simulation with the mathematically modeling of microstructure evolution using Finite Element Method (FEM), the study of the dynamic recrystallization (DRX) of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process is conducted. Through physical experiment, microstructure characterization and FEM-based microstructure modeling, the DRX behavior of the Ti-alloy in β-forging process is extensively explored. The effects of plastic deformation strain, strain rate and deformation temperature on the DRX of the Ti-alloy in terms of DRX volume fraction, DRX grain size and the average grain size are systematically investigated. The simulation results show that the increase of plastic deformation strain, deformation temperature, and strain rate contributes to the DRX of the alloy. The simulation and experimental results further reveal that the FEM-based microstructure evolution modeling is able to predict the DRX behavior and the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process.     

Mathematical model of deformation and microstructural evolution during hot rolling of aluminium alloy 5083

Abstract

A mathematical model to predict the through thickness temperature, strain and strain rate distributions during hot rolling and the subsequent microstructure evolution was developed using the commercial finite element package ABAQUS. Microstructure evolution predictions included the amount of recrystallisation through the thickness of the sheet based on its thermomechanical history during rolling and thermal history after rolling. The equations used to predict the microstructure evolution were based on semiempirical relationships found in the literature for a 5083 aluminium alloy. Validation of the model predictions was done using comprehensive experimental measurements which were conducted using the Corus research multimill, a pilot scale experimental rolling facility, in Ijmuiden, The Netherlands. The results indicate that the through thickness temperature and strain distribution predictions for the rolling operation are reasonable. Hence, the boundary conditions used in the finite element model adequately represent the interface heat transfer and friction conditions. Microstructure predictions using the literature based equations significantly underestimate the amount of recrystallisation occurring in the sheet. A sensitivity analysis indicates that the recrystallisation kinetics are extremely sensitive to the fitting parameters used in the microstructure equation, and that the gradient in the recrystallisation kinetics is the result of the temperature gradient experienced by the specimen during deformation.     

SDC99钢淬火过程中应力和组织演变的有限元模拟

为了研究钢铁材料在淬火过程中内部组织和应力的变化,以自主研发的SDC99钢为研究对象,考虑相变潜热的影响,采用有限元方法对偏心圆环的淬火过程进行模拟仿真,并对淬火过程中模型的温度场、应力场和组织场的变化进行分析和研究.结果表明:经实验测定淬火过程中温度场及残余应力的分布与模拟结果吻合较好,偏心圆环上最大残余应力出现在45°及315°位置;模型硬度的分布与其马氏体含量分布趋势一致,模拟的硬度值略小于实测值.     

热锻过程中动态再结晶的数值模拟

在热锻过程中,由动态再结晶引起的微观组织演化是决定锻件最终性能的重要因素之一。基于更新的拉格朗日格式刚塑性有限元和考虑动态再结晶的粘塑性模型,建立了能够模拟变形过程、温度变化过程和动态再结晶过程的有限元方法,研制了轴对称有限元软件,并用该软件模拟26Cr2Ni4MoV小圆柱试样镦粗过程中由动态再结晶引起的微观组织演化。在软件中应用网格重新划分技术降低由于网格畸变引起的计算误差。模拟结果和实验结果符合良好。     

Integrated modeling of friction stir welding of 6xxx series Al alloys: Process, microstructure and properties

Compared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The “model chain” is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a “material by design approach” from process to performances.     

Microstructure Evolution in a Rapidly Solidified Cu85Fe15 Alloy Undercooled into the Metastable Miscibility Gap

1.IntroductionThe Cu-Fe alloy system is well known as a peritecticsystem[1].It also exhibits a metastable miscibility gap inthe undercooled liquid state,as shown in Fig.1.Whena single-phase liquid is cooled into the miscibility gap,itseparates into two liquids:one is Cu-rich(L1),and theother is Fe-rich(L2).Although many researches on theCu-Fe system have been carried out[2~6],most of themfocused on the thermodynamic aspect.The effect of thecooling rate and undercooling on the liquid phase …     

Implementation of a phase field model for simulating evolution of two powder particles representing microstructural changes during sintering

In this work, we present a multiphysics phase field model for capturing microstructural evolution during solid-state sintering processes. The model incorporates modifications of phase field equations to include rigid-body motion, elastic deformation, and heat conduction. The model correctly predicts consolidation of powder particles during sintering because of two competing mechanisms—neck formation and grain growth. The simulations show that the material undergoes three distinctive stages during the sintering process—stage I where neck or grain boundary between two particles is formed, stage II in which neck length stabilizes and growth or shrinkage of individual particles initiates, and finally stage III with rapid grain growth leading to disappearance of one of the grains. The driving forces corresponding to different mechanisms are found to be dependent on the radius of the particles, curvature at the neck location, surface energy, and grain boundary energy. In addition, variation in temperature is found to significantly influence the microstructure evolution by affecting the diffusivity and grain boundary mobility of the sintered material. The model is also used to compare sintering simulation results in 2D and 3D. It is observed that due to higher curvature in 3D, model predicts faster microstructural evolution in 3D when compared to 2D simulations under identical boundary conditions.     

In situ characterization of interface-microstructure dynamics in 3D-Directional Solidification of model transparent alloys

Microstructure plays a central role in determining properties of materials so that the fundamental understanding of the physics of microstructure selection is critical in the design of materials. Under terrestrial conditions fluid flow effects are dominant in bulk samples which preclude precise characterization of fundamental physics of microstructure selection. Experiments in thin samples, carried out to obtain diffusive growth, give microstructures that are neither 2D nor 3D. Rigorous theoretical models, using the phase-field method, have shown that the fundamental physics of pattern selection in 2D and 3D is significantly different. A benchmark experimental study is required in bulk samples under low gravity conditions. Also, the selection of microstructure occurs during the dynamical growth process so that in situ observations of spatio-temporal evolution of the interface shapes are required. Microgravity experiments on ISS are thus planned in a model transparent system by using a new Directional Solidification Insert (DSI), designed for use in the DECLIC facility of CNES and to be adapted to also fit ESA experiments. The critical aspects of hardware design, the key fundamental issues identified through 1g-experiments, the proposed experimental study on ISS, and the results of rigorous theoretical modeling are presented.     

Microstructure and texture evolution during the drawing of tungsten wires

Tungsten wires develop during their forming process a pronounced fibre texture that causes anisotropic deformation of single grains. The aim of this work is to simulate the crystallographic texture and microstructure evolution that arises during wire drawing using two different texture models. A visco-plastic self-consistent model that allows simulations using a large number of grains is compared with a crystal plasticity finite element model that provides a more detailed insight into the wire’s microstructure. Texture predictions of both models are discussed and quantitatively compared with experimental texture measurements obtained by neutron diffraction. The developed fibre texture causes plane strain deformation of single grains, which induces grain curling. The prediction of grain curling is of importance because it allows studying the residual stresses that trigger splits, at the grain level.