Effect of sand dilation on distortions and pattern allowances during steel sand casting

Mechanical interactions between the sand mold and casting have a great impact on pattern allowances. In this study, the effect of core expansion on distortions during steel casting is investigated. A hollow steel cylinder is cast using silica and zircon sand cores. The evolution of the cylinder’s inner diameter is measured in situ using LVDTs. During solidification, core expansion is found to distort the inner diameter into a barrel-shaped profile, generating the largest expansion at the mid-height. The experiments are simulated using a sequential thermo-mechanical coupling. In the stress analysis, the steel and sand are modeled using an elasto-visco-plastic constitutive law and the Drucker–Prager Cap model, respectively. The simulations reveal that sand dilation due to shear stresses, as opposed to thermal expansion, accounts for the majority of the increase in the inner diameter. The measured and predicted pattern allowances are found to be in excellent agreement.     

Prediction of distortions and pattern allowances during sand casting of a steel bracket

Mechanical interactions between the casting and mould generate unwanted distortions and lead to dimensional inaccuracies. In this study, the effects of mould expansion and mould restraint are investigated through sand casting experiments involving a U-shaped steel bracket. Distortions are quantified by in situ measurements of the evolution of the gap opening between the bracket legs. Mould expansion is observed immediately after filling. Outer mould restraint prevents distortions in the bracket legs until the time of mould fracture, after which the legs are pushed outward. The experiments are simulated using a sequential thermo-mechanical coupling. The steel and bonded sand are modelled using previously calibrated elasto-visco-plastic and Drucker–Prager Cap constitutive laws, respectively. Excellent agreement between measured and predicted pattern allowances (PA) is obtained. Distortions are greatly under-predicted unless mould fracture is considered. Variations in the packing density of the moulds are also shown to have an impact on PA.     

新型马氏体耐热钢G115大型铸件热处理过程有限元分析

为了优化G115钢大型铸件正火及回火热处理制度,通过有限元方法,分析了正火和回火热处理工艺对铸件温度场分布的影响。结果表明,在正火过程中,铸件下端边缘处温度最高且升温速率最快,距上端约1/4处内表面温度最低,且升温速率最慢。在回火过程中,铸件的温度变化规律与正火过程相似。铸件下端边缘处在目标温度下的保温时间最长,上端约1/4截面厚度中心处的保温时间最短。结合加热速率、温差、加热效率及生产成本,正火优选工艺②,回火优选工艺③。采用优化后的热处理工艺所生产G115钢铸件的显微组织及力学性能均匀且明显高于CB2钢。     

采用热弹塑性有限元方法预测低碳钢钢管焊接变形

用有限元分析软件ABAQUS Code开发了用于模拟焊接温度场、焊接残余应力和焊接变形的热弹塑性有限元计算方法.通过建立三维有限元模型,预测了低碳钢钢管多层焊接时的温度场和焊接变形,并通过试验验证了所提出计算方法的精度和有效性.结果表明,在多层焊接条件下,采用有限元方法计算得到的焊接变形与试验结果十分吻合.     

Review on finite element analysis of welding deformation and residual stress

Welding deformation and residual stress have negative influence on assembly accuracy and service performance. Thermal elastic plastic (TEP) and inherent strain finite element analysis (FEA) methods were used to study this challenge. Basic principle of these two methods was first introduced. The influence of welding process, constraints, solid phase transformation and multi-pass welding on deformation and residual stress was discussed, and computation accuracy and efficiency were summarised. Loading method of inherent strain in inherent strain FEA was analysed, interface element was introduced to simulate effects of the gap on deformation in assembly welding especially for large structures. The future work, including accurately multiscale TEP model, efficiently transient prediction method of large structures, and flexible evaluation software, was planned.     

剪切机机架应力变形的有限元分析

采用 ANSYS有限元软件的结构分析模块对剪切机机架进行了三维应力变形分析 ,找出了其薄弱环节 ;并对机架应力进行实际测试 ,经比较 ,有限元分析与测试结果相吻合     

Crack prediction using nonlinear finite element analysis during pattern removal in investment casting process

A three-dimensional transient thermo-mechanical coupled nonlinear finite element model was developed to predict the possible crack formation of ceramic shell during rigid polymer pattern removal in the investment casting process. A smeared crack model was used to describe the response of the ceramic shell when crack initiates. A foam degradation model was implemented to account for the loss of mechanical properties of the foam during firing process. The effects of firing method, pattern type and complex geometry were investigated. The simulation results were validated with experimental findings. The developed model not only serves as a useful tool for designing foam patterns but also can be used for optimizing firing process parameters in investment casting process.     

Effect of cooling condition on deformation of continuously cast stainless steel 304

This work investigated the effect of the cooling condition on the deformation that was found for continuously cast stainless steel 304 slabs. The transient heat-transfer finite element analysis under various cooling conditions yielded temperature distributions within the slab, which were then used for subsequent calculations of the deformed shape. The asymmetric cooling condition in a cooling pit induced non-uniform and asymmetric thermal gradients with elapsed time. The non-zero final deflection and the reversal of curvature for the slab during cooling were attributed to the combined effect of asymmetric temperature profile and inelastic material property. Parametric studies revealed that a slab material of a low thermal conductivity or yield strength resulted in an increase for final deflection. The present analysis explained the physical cause of the deformation reasonably well.     

Experimental and finite element analysis for fracture of coating layer of galvannealed steel sheet

Mechanical properties of galvannealed (GA) steel sheet used for automotive exposed panel and predicted failure phenomenon of its coating layer were evaluated using finite element method. V-bending test was performed to understand better the fracture of coating layer of GA steel sheet during plastic deformation. Yield strength of the coating layer was calculated by using a relative difference between hardness of coating layer measured from the nano-indentation test and that of substrate. To measure shearing strength at the interface between substrate and coating layer, shearing test with two specimens attached by an adhesive was carried out. Using the mechanical properties measured, a series of finite element analyses coupled with a failure model was performed. Results reveal that the fracture of coating layer occurs in an irregular manner at the region where compressive deformation is dominant. Meanwhile, a series of vertical cracks perpendicular to material surface are observed at the tensile stressed-region. It is found that 0.26-0.28 of local equivalent plastic strain exists at the coating and substrate at the beginning of failure. The fracture of coating layer depends on ductility of the coating layer considerably as well.     

An inverse analysis using a finite element model for identification of rheological parameters

In order to reduce the discrepency between experimental and numerical development, a parameter automatic identification procedure from rheological test is formulated as an inverse problem. The direct model which permits to simulate the large strain behaviour during the rheological test is a Finite Element Code. The inverse problem is formulated as finding a set of rheological parameters starting from a known constitutive equation. The goal is to compute the parameter vector which minimizes an objective function representing, in the least square sense, the difference between experimental and numerical data. The high nonlinearity of the problem to be solved, requires the use of an accurate evaluation of the sensitivity matrix by analytical differentiation of governing equations with respect to the parameters. Thus the optimisation algorithm is strongly coupled with the finite element simulation. This method, namely a Computer Aided Rheology (CAR) methodology is possible in principle for all tests able to be simulated. This paper concerns the thermoviscoplastic deformation during torsion and tension tests.     

Analysis of T-shaped equal channel angular pressing using the finite element method

Plastic deformation behavior in T-shaped equal channel pressing, a modified equal channel angular pressing (ECAP) using T-shaped channel instead of conventional L-shaped channel, is analyzed by using the commercial finite element code DEFORM. Simulations were carried out under realistic conditions by considering the strain hardening of material and friction. The deformation behavior is more complicated and the strain induced is highly localized. Severe plastic strain is localized in the bottom region of the workpiece and very small strain is developed in the other region, which is in good agreement with the experimental results reported in the literature showing the nonuniformity in microstructure and hardness distribution. In addition, the load requirements of the T-ECAP are much higher compared to conventional ECAP.     

高阶单元在焊接热弹塑性有限元分析中的应用

为研究高阶单元对热弹塑性有限元分析中变形的影响,建立了不同网格尺寸、单元阶次的三维有限元模型,利用通用有限元软件ABAQUS对CO₂气体保护堆焊过程进行热弹塑性有限元分析,比较线性单元和二阶单元在温度和变形的计算精度及时间.结果表明,二阶单元能够更好地反映焊接区域的变形特点,焊接变形的计算结果与试验结果更加接近;单元阶次仅对应力应变场分析有较大影响,对于温度场计算结果影响不大.     

高阶单元在焊接热弹塑性有限元分析中的应用

为研究高阶单元对热弹塑性有限元分析中变形的影响,建立了不同网格尺寸、单元阶次的三维有限元模型,利用通用有限元软件ABAQUS对CO₂气体保护堆焊过程进行热弹塑性有限元分析,比较线性单元和二阶单元在温度和变形的计算精度及时间.结果表明,二阶单元能够更好地反映焊接区域的变形特点,焊接变形的计算结果与试验结果更加接近;单元阶次仅对应力应变场分析有较大影响,对于温度场计算结果影响不大.     

T形焊接接头固有变形逆解析方法的开发

基于固有应变方法的弹性有限元方法是适用于大型复杂焊接结构件变形预测的有效方法,但这一方法得以运用的必要条件是必须已知存在于焊缝附近的固有变形.文中利用物理试验和数值模拟技术相结合的研究手段开发了典型焊接接头固有变形的逆解析数值计算方法.利用逆解析得到的固有变形值,采用固有应变弹性有限元方法对T形焊接接头的焊接变形进行了预测.通过比较数值模拟结果和试验测量值,验证了基于数值模拟技术的逆解析方法的求解精度.结果表明,T形焊接接头固有变形的逆解析算法是正确有效的.     

不锈钢板翅结构钎焊残余应力及其高温蠕变松弛行为三维有限元分析

利用有限元软件ABAQUS,对不锈钢板翅结构在钎焊过程中产生的残余应力及其高温下的蠕变松弛行为进行三维有限元分析,得到了残余应力的变化规律.结果表明,由于镍基钎料BNi-2和不锈钢母材304力学性能的差异以及夹具的约束作用,钎焊后在接头处产生了较大的残余应力.在高温环境下,由于蠕变松弛,残余应力大幅度下降.在蠕变稳态阶段,钎缝处仍然存在一定的残余应力;钎角处蠕变应力和应变集中,易萌生裂纹,成为最薄弱环节.研究结果为板翅结构的高温强度设计提供参考.     

Sensitivity analysis of a finite element model for the simulation of stainless steel tube extrusion

In this work, a sensitivity analysis has been performed on a finite element model of glass-lubricated extrusion of stainless steel tubes. Fifteen model parameters, including ram speed, billet and tool temperatures, friction coefficients and heat transfer coefficients, were considered. The aim of the study was to determine the parameters that are most important for the response of the extrusion force. The relationship between the model parameters and the responses was analyzed by a calculation of two different regression models: one linear polynomial model and one model that includes interaction terms. Additional simulations were then carried out to validate the regression models. The results show that the initial billet temperature is the factor that has the strongest impact on the extrusion force within the parameter ranges studied in this work. The goodness of prediction and goodness of fit are very good for both regression models.     

滚动工况下钢质车轮和铝合金车轮的有限元分析对比

根据车轮的结构和实际工作条件,采用ANSYS有限元分析技术,建立1/16车轮的模型。计算了滚动工况下钢质车轮、铝合金车轮在不同转速下的应力值和变形值。通过对比分析计算结果,得出的结论可为铝合金车轮的设计提供依据,最后作试验验证了所建模型的合理性。     

变形参数对半固态轧制影响规律的有限元模拟

建立了半固态材料刚 -粘塑性分析模型 ,以高温钢铁材料为研究对象 ,用MARC软件模拟分析了半固态轧制时 ,轧辊辊缝、轧辊转速等变形参数和浆料初始温度对应力场、速度场的影响 ;结合有限元模拟结果 ,进行了半固态实际轧制试验。实际轧制试验表明 ,轧辊辊缝、轧辊转速和浆料初始温度对半固态轧制试验的影响规律基本与有限元模拟一致 ,其中浆料初始流入速度和温度控制是保证半固态轧制过程顺利进行的关键     

基于有限元分析的摩擦系数测定

在塑性体积成形有限元模拟中,摩擦模型和摩擦系数的确定是一个关键问题.本文采用圆环镦粗法研究体积成形中摩擦系数的确定.利用有限元分析确定一组摩擦系数标定曲线,用圆环镦粗试验确定圆环内径变化率百分比与圆环高度压缩百分比关系曲线.通过曲线对比得到模具材料和成形材料之间的摩擦系数.针对6A02CZ材料在钢模具中体积成形时6A02CZ材料与钢之间的摩擦系数进行确定,6A02CS材料在无润滑剂和润滑油做润滑剂两种情况下的摩擦系数分别为0.325和0.3.研究表明,采用有限元方法和圆环镦粗试验相结合的方法更适合塑性体积成形中的有限元分析.     

Sensitivity analysis based preform die shape design using the finite element method

This paper uses a finite element-based sensitivity analysis method to design the preform die shape for metal forming processes. The sensitivity analysis was developed using the rigid visco-plastic finite element method. The preform die shapes are represented by cubic B-spline curves. The control points or coefficients of the B-spline are used as the design variables. The optimization problem is to minimize the difference between the realized and the desired final forging shapes. The sensitivity analysis includes the sensitivities of the objective function, nodal coordinates, and nodal velocities with respect to the design variables. The remeshing procedure and the interpolation/transfer of the history/dependent parameters are considered. An adjustment of the volume loss resulting from the finite element analysis is used to make the workpiece volume consistent in each optimization iteration and improve the optimization convergence. In addition, a technique for dealing with fold-over defects during the forming simulation is employed in order to continue the optimization procedures of the preform die shape design. The method developed in this paper is used to design the preform die shape for both plane strain and axisymmetric deformations with shaped cavities. The analysis shows that satisfactory final forging shapes are obtained using the optimized preform die shapes.