两种不同约束条件下发动机缸体铸件热应力场的数值模拟

采用FDM/FEM集成热应力分析系统(其中采用铸造之星FT-STAR进行温度场计算,ANSYS进行热应力场计算,采用FT-STRESS进行有限差分网格向有限元网格转换和有限差分/有限元温度载荷转换),对某厂柴油机发动机缸体铸件进行从浇注到冷却至室温全过程的温度场、热应力场数值模拟,得到冷却变形情况及残余应力分布,并研究比较了将气缸处砂芯考虑为部分刚性,和将砂芯考虑为完全退让性的两种不同约束模拟方案对计算结果的影响。前者应力值和变形值远远大于后者的结果。     

基于有限差分网格的铸件热应力分析

提出了有限差分网格向有限元网格转换的方法,进而在该有限元模型基础上采用FDM/FEM集成铸件热应力分析系统实现了铸件的热应力分析。采用典型的应力框试件,将该方法计算结果和直接剖分得到的有限元网格基础上的计算结果进行了对比,结果表明该方法是可行的。     

THERMOMECHANICAL MODELING OF SOLIDIFICATION PROCESS OF SQUEEZE CASTING I. Mathematic Model and Solution Methodology

开发了模拟挤压铸造凝固过程中铸件温度、应力及形状变化的有限元模型. 该模型包括了凝固过程中潜热的释放和体积收缩效应、界面传热和变形的相互作用以及凝固壳在冲头压力下的变形等. 应力场模拟中采用热弹粘塑性本构模型描述凝固壳的变形, 并对液相和糊状区进行了特殊处理. 利用接触算法处理铸件与模具界面, 并且采用一种特殊的迭代法来模拟冲头的运动. 该模型可以用来研究模具设计和工艺参数(如模具温度及冲头压力等)对铸件质量的影响.     

基于有限单元法的铸件热应力场数值模拟

重点论述了基于有限单元法(FEM)对铸件热应力场进行数值模拟的理论基础和实际方法,开发了基于有限单元法的铸造热应力场模拟程序.采用典型试件进行了算例验证,并与ANSYS的模拟结果及实际铸件浇注结果进行了对比,结果吻合.研究表明,开发的基于有限单元法的铸件热应力场数值模拟程序,模拟结果精确,运行可靠.     

Development of efficient numerical heat transfer model coupled with genetic algorithm based optimisation for prediction of process variables in GTA spot welding

Abstract

Although numerical heat transfer models based on conduction mode of heat transfer have become a strong basis for the quantitative analysis of fusion welding, they still find limited use in actual design for three primary reasons. First, these traditional models consider a volumetric heat source term, which ironically requires a-priori knowledge of the final weld pool dimensions. Second, the numerical models need confident values of a few parameters, e.g. arc efficiency and arc radius, which are usually uncertain and requires many trial and error simulations to realise their suitable values. Third, these models are rarely attempted for the prediction of possible weld conditions for a requisite or target weld dimensions, which is of paramount interest in design for welding. The present work attempts to circumvent these issues by linking a genetic algorithm (GA) based global optimisation scheme with a finite element based three-dimensional numerical heat transfer model. The numerical model includes a volumetric heat source that adapts itself to the computed weld pool geometry at any instant. The GA module identifies the optimum values of a set of uncertain parameters needed for the reliable modelling calculations and next, identifies the suitable values of the process variables, e.g. weld current, for a target weld dimension. In each case, the GA module guides the numerical model to compute weld dimensions for a given set of inputs, traces the sensitivity of the error in prediction on the inputs being optimised, updates them accordingly and reuses the numerical model to finally obtain their optimised values. The complete integrated model is validated with a number of experimental results in gas tungsten arc spot welding processes.     

基于有限差分网格的发动机缸体铸件热应力模拟

对发动机气缸体铸件从浇注完到开箱落砂后的铸造过程应力场进行计算机数值模拟.为了实现形状复杂铸件的热应力分析,应用了有限差分网格向有限元网格的转换方法,得到缸体的有限元模型,并将该模型与经过大量简化后直接剖分得到的有限元模型进行了比较.在此有限元模型的基础上,采用FDM/FEM集成铸件热应力分析系统实现了对发动机缸体铸件热应力分析,模拟结果和实际生产情况相吻合.     

呋喃树脂自硬砂铸造大型平台变形问题探讨

由于呋喃树脂自硬砂高温强度低，在铸件保温过程中，铸件温度处于塑性变形区，铸件刚性比较差，铸件可能由于本身自重及上部砂型重力作用产生向下弯曲变形，而随后由于热应力引起的弹性变形可以与之全部或大部抵消。因此在生产大型平台类铸件时，可以不放置反变形量，通过控制吊压铁时间和保温时间获得合格铸件。     

二维结晶器铜板传热及力学行为研究

结晶器是连铸的核心部件,其传热状态及力学行为直接影响到铸坯的质量以及结晶器的寿命。基于有限元方法,建立了全尺寸结晶器模型,模型考虑了宽面铜板、窄面铜板、弧形水槽、镍镀层等结构,基于热电偶获得的实测数据,利用反问题算法求解热流,将数值模拟与实际数据相结合。通过此模型,全面分析了结晶器的温度、应力及变形情况,为连铸过程优化及结晶器设计提供理论依据及指导。     

A three-dimensional steady-state finite element analysis of square die extrusion by using automatic mesh generation

Steady-state finite element analysis is made for three-dimensional hot extrusion of sections through square dies by using an automatic mesh generator which can generate three-dimensional meshes by shifting two-dimensional meshes. In industrial practice, the design of extrusion dies is still an art rather than science, especially for complicated profiles, because the die design for a new extrusion is developed from previous experience and in-plant trials. The objective of this study is to develop a steady-state finite element method for hot extrusion through square dies, and to provide a theoretical basis for an optimal die design and process control for the extrusion technology. In the present investigation, steady-state assumption is used for both the analyses of deformation and temperature. The analysis of temperature distribution includes heat transfer, and is carried out by decoupling from the analysis of deformation. Convection link element is adopted for the heat transfer analysis between the billet and the container, and also between the billet and the die. Computations are carried out for solid and hollow extrusion of several sections. The present method of analysis has been shown to provide good results comparable with the non-steady-state method with reduced computation time. Distributions of temperature, effective strain rate, velocity and mean stress are discussed for effective design of an extrusion die.     

Distortion and Residual Stress in High-Pressure Die Castings: Simulation and Measurements

Two individual high-pressure die casting geometries were developed to study the influence of process parameters and alloy composition on the distortion behavior of aluminum alloy castings. These geometries, a stress lattice and a V-shaped lid, tend to form residual stress due to a difference in wall thickness and a deliberate massive gating system. Castings were produced from two alloys: AlSi12(Fe) and AlSi10MnMg. In the experimental castings, the influence of important process parameters such as die temperature, ejection time, and cooling regime was examined. The time evolution of process temperatures was measured using thermal imaging. Subsequent to casting, distortion was measured by means of a tactile measuring device at ambient temperatures. The measured results were compared against a numerical process and stress simulations of the casting, ejection, and cooling process using the commercial finite element method software ANSYS Workbench. The heat transfer coefficients were adapted to the temperature distributions of the die, and the castings were observed by thermal imaging. A survey of the results of the comparison between simulation and experiment is given for both alloys.     

电磁离心凝固过程熔体流动和传热的有限元数值模拟

利用电磁流体动力学有关理论,建立了电磁离心凝固过程熔体流动与热量传输的耦合分析模型,采用有限元法对电磁离心凝固过程进行了分析,得出了瞬态速度场和温度场。分析结果表明,电磁离心凝固过程中,正是由于电磁力引起的受迫对流运动,对金属熔体起电磁搅拌作用并使晶粒得到细化,从而提高铸件质量。     

基于MATLAB平台有限单元法的铸造热应力数值模拟

采用有限单元法(FEM)对铸件凝固过程热应力场进行了三维数值模拟研究,开发了基于MATLAB平台的FEM铸造热应力场模拟程序,采用典型应力框试件对所开发的算法和程序进行了算例验证,并与采用商品化软件ANSYS进行模拟的结果进行了对比,二者吻合良好.结果表明,所采用的算法及开发的程序能较好地预测铸件变形及热应力的集中位置.     

Determination of pattern allowances for steel castings using the finite element inverse deformation analysis

In recent years, the development of computational models to predict casting distortions has led to improvements in efficiency and accuracy over traditional pattern design that relies on shrink rules. Unfortunately, the determination of pattern dimensions using computer simulation remains a trial-and-error process that requires several design iterations. In this study, the finite element inverse deformation analysis is utilized to calculate the pattern geometry in a single iteration for a plastically deformed (i.e., distorted) body. This method is evaluated through a loop test, for which an inverse simulation is performed first to calculate the pattern shape. This configuration is then used as the input geometry for a forward simulation, which is shown to successfully recover the original as-cast shape used for the inverse analysis. Through this sequence, the inverse deformation method is shown to be a viable technique for the determination of pattern allowances in production castings.     

Numerical simulation of deformation in large scale hydroturbine blade casting

Abstract

Deformation is one of the most common defects during the casting of large scale hydroturbine blades because of their curved three-dimentional (3D) surface geometry. Inverse deformation is usually applied to the pattern for sand moulds to finally obtain proper shape. However, the value of inverse deformation is hard to be determined. In this paper, a method is presented to determine the inverse deformation values by cycling numerical simulation. The inverse deformation is determined by the criteria of the achievement of even and appropriate machining allowance of the whole casting. This method is applied to a large scale blade casting used in the Three Gorges Power Plant. The calculated inverse deformation is obtained after three cycles. By using the electronic coordinate determination system (ECDS), its surface is measured and the machining allowance values of some points are acquired. The measured and calculated results are in agreement.     

Numerical simulation of welding-induced distortion in thin-walled structures

Abstract

A sequentially coupled thermal stress analysis approach is presented for modelling temperature and distortion profiles resulting from welding thin-walled structures. The material is modelled as thermo-elastic–plastic with isotropic strain hardening. The heat source is modelled as a three-dimensional (3-D) double ellipsoid, and 3-D finite element (FE) models are employed for predicting ensuing distortions. Comparisons between the simulation results and experiments performed for eight weld configurations are presented. The weld configurations include bead-on-plate, butt weld and tee joint welds with varying plate thicknesses. Temperature measurements using thermocouples and an infrared (IR) imaging radiometer are directly compared to the thermal simulations. Likewise, distortions measured directly on the experimental set-ups are compared to the FE distortion predictions. Very good correlation is obtained for temperature as well as distortion predictions between experimental and proposed numerical approaches. Lastly, details of a weld simulation for the rear section of a motorcycle frame are presented.     

Up-pulling force analysis for ESC hollow cylindrical casting

Electroslag casting(ESC)is an important method to produce high quality castings.In this study,the ESC up-pulling inner mold method(EUPIM)was used to produce hollow cylindrical castings with the multiple consumable electrodes.The radial deformation,the axial and radial internal stress of the inner mold,and the axial internal stress of the slag shell were analyzed using the finite element method(FEM)with the aid of ANSYS software.The ProCAST software was used to calculate the specific heat,heat conductivity and density curve of Cu.Simulation results show that the radial deformation,the axial and radial internal stress of the inner mold,and the axial internal stress of the slag shell near the slag-metal interface of hollow cylndrical casting gradually increase from 0 s to 360 s after the ESC starting(slagging)process but before applying the up-pulling force.The suitable initial up-pulling moment of the inner mold is at around 180-198 s after the starting process.     

A Free Thermal Contraction Method for Modelling the Heat-transfer Coefficient at the Casting/Mould Interface

Abstract

This study is intended to explore a simple and inexpensive method that is able to model the variation and distribution of the heat transfer coefficient at the casting/mould interface. It has been assumed that, in a rigid mould, the magnitude of interface gap size primarily depends on the thermal contraction of cast metal as it solidifies. Consequently, a free thermal contraction method has been developed to describe the thermal contact phenomena at the casting/mould interface, based on the above assumptions. This method has been used in the solidification simulations for three sand castings of different shapes. The numerical solutions of the simulations agree closely with the experimental results of the thermocouple measurements in the castings and their moulds. For the casting and mould materials involved, the empirical parameters in the equation of the heat transfer coefficient are the same for each of these cases, i.e. independent of the geometric shape of the castings.     

Numerical Simulation and Experimental Validation of Residual Stresses in Water-Quenched Aluminum Alloy Castings

Aluminum alloy castings are normally water quenched after solution treatment to improve mechanical properties. Rapid water quenching can result in high-residual stress and severe distortion which significantly affect functionality and performance of the products. To optimize product design and durability, one needs to model and predict residual stress and distortion produced in the water-quenched components. In this article, a finite element-based approach was developed to simulate the transient heat transfer and residual stress development during water quenching. In this approach, an iterative zone-based heat transfer algorithm was coupled with material constitutive model called mechanical threshold stress (MTS). With the integrated models, a good agreement was achieved between the numerically predicted and the experimentally measured residual stresses in the aluminum alloy frame-shape casting. The integrated FEA-based heat transfer and residual stress models were also applied to a water-quenched cast aluminum cylinder head with a great success.     

Thermal crown analysis of the roll in the strip casting process

In the twin roll strip casting process, coupled analyses of heat transfer and deformation for the casting roll are carried out by using the finite element program MARC to examine the thermal crown. The shrink fit effect and plastic deformation are considered. The result shows that the thermal crown is greatly influenced by the shrink fit and that the thermal crown for POSCO Pilot Caster 2 Copper Roll has an “M” shape. The effects of several factors on thermal crown are also investigated. The amount of thermal crown increases as the heat flux, casting speed, sleeve thickness and casting roll width increase. On the other hand, it decreases as the casting roll diameter increases.     

Cooling control for castings by adopting skeletal sand mold design

The cooling control of the melt during the casting process is of great significance. A comprehensive closed-loop cooling control of castings by adopting a skeletal sand mold design was proposed. The skeletal sand mold consisting of an adaptive shell, functional cavities and a support was designed and created based on the finite difference meshes of a casting. It was applied to a round wall test casting. Two kinds of skeletal sand molds, one with lattice support and the other with enforcing ribs for this casting were designed and printed out by the 3 D printing(3 DP) method. Aluminum alloy A356 was cast by using these two sand molds. The first mold was cooled by natural convection, the other one by water spray cooling. Two sound castings were obtained. The sand mold temperature, cooling curves, microstructures, mechanical properties, residual stress and deformation were measured, compared and discussed. Water spray cooling hastened the cooling rate by 62%, increased the content of Mg and Cu in the α-Al matrix, improved the mechanical properties, and altered the surface residual stress state.