板坯连铸结晶器内铸坯凝固及变形的热力耦合有限元分析

建立板坯连铸结晶器内凝固坯壳形成及其应力和变形的热力耦合有限元分析模型,采用二维瞬态热传导有限元方法分析温度场,采用二维热弹塑性接触有限元方法计算应力分布。编制了有限元程序,计算分析了三种结晶器锥度、三种拉坯速度和三种保护渣层情况下,结晶器内Q235铸坯凝固坯壳的形成过程。得到凝固坯壳的温度场、厚度、变形、与结晶器壁间的气隙以及应力分布等,得到该三种工艺参数对结果的影响规律。     

REVIEW OF NEW PROCESS TECHNOLOGIES IN THE ALUMINUM DIE-CASTING INDUSTRY

There have been many challenges in aluminum die casting to establish casting processes to produce high-integrity components from aluminum alloys. Advances in new casting technology mainly have been in pressure die casting; in particular, to obtain mold filling at low speed. This can be achieved by using innovative filling processes with aluminum alloys in the liquid or semisolid state. Different techniques such as high-pressure die casting (HPDC), Cosworth process, low-pressure systems, squeeze casting, indirect squeeze casting, metal compression forming (MCF), and semisolid metal (SSM) processing have been developed. Semisolid forming includes thixoforming and rheoforming. During the semisolid casting process, preheating by induction is needed to obtain the same temperature and the same liquid fraction through the billet in a short time. Thixocasting in the semisolid state helps to avoid turbulence during mold filling.     

压铸模温度场与铸件质量关系的研究与应用

运用有限元分析软件ANSYS对薄壁铝合金压铸件的压铸过程进行温度场数值模拟分析,研究了浇注温度、模具预热温度对温度场分布的影响,分析了热应力集中的位置,从而有效控制了铸件因热应力引起的体积收缩,保证了铸件质量及其精度,优化了压铸工艺参数,为压铸模具的热应力分析奠定了基础。     

Residual stresses due to gas arc welding of aluminum alloy joints by numerical simulations

This study deals with the numerical simulation of the gas arc welding process of Aluminum tee joints using finite element analysis and evaluation of the effect of welding parameters on residual stress build up. The 3D simulations are performed using ABAQUS code for thermo-mechanical analyses with moving heat source, material deposition, solid-liquid phase transition, temperature dependent material properties, metal elasticity and plasticity, and transient heat transfer. Quasi Newton method is used for the analysis routine and thermo-mechanical coupling is assumed; i.e. the thermal analysis is completed before performing a separate mechanical analysis based on the thermal history. The residual stress build up and temperature history state in a three-dimensional analysis of the tee joint is then compared to experimental results. Hole drilling method is used for measuring the residual stress, while temperature history is measured by thermocouples. After carrying out numerical simulations, the effects of voltage/current, welding speed, material thickness and size of electrode on residual stress build-up and resulting distortions are evaluated.     

CF／Al复合材料的热膨胀行为

测试和分析了用热压和挤压铸造工艺制备的ＣＦ／Ａｌ复合材料热膨胀行为。指出ＣＦ／Ａｌ的热膨胀规律与其内部残余应力的存在方式密切相关，其制备方式对热膨胀过程也有影响。从热膨胀变化规律能够间接了解材料内部残余应力的生成、消亡及存在方式，为材料的尺寸稳定化处理及选择热处理制度提供理论和实验依据。     

Virtual Mold Technique in Thermal Stress Analysis during Casting Process

It is important to analyse the casting product and the mold at the same time considering thermal contraction of the casting and thermal expansion of the mold. The analysis considering contact of the casting and the mold induces the precise prediction of stress distribution and the defect such as hot tearing. But it is difficult to generate FEM mesh for the interface of the casting and the mold. Moreover the mesh for the mold domain spends lots of computational time and memory for the analysis due to a number of meshes. Consequently we proposed the virtual mold technique which only uses mesh of the casting part for thermal stress analysis in casting process. The spring bar element in virtual mold technique is used to consider the contact of the casting and the mold. In general, a volume of the mold is much bigger than that of casting part, so the proposed technique decreases the number of mesh and saves the computational memory and time greatly. In this study, the proposed technique was verified by the comparison with the traditional contact technique on a specimen. And the proposed technique gave satisfactory results.     

Squeeze-casting conditions of Al/Al2O3 metal matrix composites with variations of the preform drying process

The characteristics of the preform play a role in determining the final properties of MMCs. Effects of organic binder and microwave drying on preform microstructure have been examined by SEM. In the preform with organic binder, flocking processes are observed during drying. The preform has a uniform distribution of binder and dries quickly with microwave drying owing to its internal and volumetric heating patterns. The fundamental manufacturing process and controlling parameters of squeeze casting, including preform temperature, mould temperature, applied pressure and molten metal temperature, have been studied in Al/Al₂O₃ composites. MMCs have poor mechanical properties with too high temperatures of preform and molten metal due to thermal shocking of the preform, oxidation of the matrix and thermal damage to the fibers. Mould temperature barely affects the tensile strength of MMCs. High applied pressure reduces voids and solidifies the matrix faster. Conditions for squeeze casting to achieve optimal processing, are suggested. The tensile strength of MMCs can be improved by up to about 20% compared with the unreinforced matrix alloy.     

Finite element simulation of the temperature and stress fields in single layers built without-support in selective laser melting

Overhanging and floating layers which are introduced during the build in selective laser melting (SLM) process are usually associated with high temperature gradients and thermal stresses. As there is no underlying solid material, less heat is dissipated to the powder bed and the melted layer is free to deform resulting undesired effects such as shrinkage and crack. This study uses three-dimensional finite element simulation to investigate the temperature and stress fields in single 316L stainless steel layers built on the powder bed without support in SLM. A non-linear transient model based on sequentially coupled thermo-mechanical field analysis code was developed in ANSYS parametric design language (APDL). It is found that the predicted length of the melt pool increases at higher scan speed while both width and depth of the melt pool decreases. The cyclic melting and cooling rates in the scanned tracks result high VonMises stresses in the consolidated tracks of the layer.     

Three-dimensional finite element analysis of temperatures and stresses in wide-band laser surface melting processing

During laser surface melting of steel components, obtaining the desired distribution of microstructure and residual stresses with minimum distortion is essential for production goals and reliable service performance. In this study, a three-dimensional finite element based model, which is integrated into commercial finite element analysis (FEA) software SYSWELD by means of user subroutines, has been developed to simulate the wide-band laser surface melting (LSM) processing and predict temperature history and stress field with different laser scanning speed. In the proposed computational procedure, thermal, metallurgical transformation and mechanical aspects were taken into account, and the heat transfer analysis, the temperature dependent on material properties and a coupled transient thermo-mechanical analysis were used. Effects of laser scanning speed on melting temperature field and residual stress were investigated. The simulation results show that laser scanning speed changes have significant effects on melting residual stress. For experimental verification, laser surface melting of thin plate 42CrMo4 steel was achieved by a 5 kW continuous wave CO₂ laser with laser scanning speed from 10 m/s to 30 m/s. The computational results are in good agreement with experimental measurements.     

挤压铸造6082铝合金的高温流变行为和变形激活能分析

目的 研究不同挤压铸造压力下所制备的6082铝合金的高温应力应变关系。方法 对100 MPa和50 MPa挤压铸造压力下制备的6082铝合金进行高温压缩实验,建立了相应的本构模型,并分析了挤压铸造压力和变形参数对流变行为和变形激活能的影响。结果 相同变形条件下,挤压铸造压力为100 MPa时,6082铝合金流变应力更高。当温度较高和应变速率较低时,两种不同的6082铝合金流变应力值差距明显缩小。挤压铸造6082铝合金的激活能随着变形温度和应变速率的增加而降低。结论 高挤压铸造压力下制备的6082铝合金变形激活能更大,变形更困难,但高温中低应变速率时,挤压铸造6082铝合金的变形难易程度相近。     

The processing and characterization of sintered metal-reinforced aluminium matrix composites

The current investigation involves the fabrication and characterization of an aluminium metal matrix composite reinforced with sintered metal preforms. Two types of metallic preforms were used (steel and stainless steel) and a variety of squeeze casting conditions were investigated using systematic design-of-experiments techniques to determine the effect of casting conditions on the composite microstructure and mechanical properties. It was observed that a detrimental reaction phase containing iron, aluminium and silicon formed around the metallic preform particles, with a lower volume fraction of reaction phase forming at the lower melt casting temperature. This reaction phase appears to promote premature fracture by facilitating crack initiation and propagation. The stainless steel-reinforced composites had a smaller volume fraction of reaction phase and exhibited superior properties compared to the steel-reinforced composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Application of contact element method in the numerical simulation of thermal stress

The interaction relationship between casting and mold (core) decides the stress level of casting and mold (core). In this article, the stress field during the casting process of stress frame casting was simulated by contact element method provided of the professional casting simulation software, ProCAST, compared its results with the simulation results of sand mold with full rigidity. Meanwhile, the influence of shake-out temperature on residual stress was also in study. It showed that the stress result of full rigidity mechanical model is bigger than that of the contact element method. The casting residual stress first increases and then decreases along with the elevation of the shake-out temperature, and the residual stress reaches the maximum when the shake-out temperature is 600°C. The lower is the shake-out temperature, the smaller is the casting deformation.     

Simulation analysis of thermal stress during casting process of large-sized alloy steel ingot

To quantitatively analyze the main reasons of common crack in the surface of alloy steel ingot with 5% Cr during production and to propose the direction of improvement, a physical model system, which consisted of steel ingot mold, casting, riser of heat insulation, slag layer, sprue pipe, and runner, was primarily established by three-dimensional CAD software. The joint simulation method concerned with pouring, solidification, temperature field, and cast stress was determined by using ADSTEFAN cast simulation software. The stress distribution of casting and the quantitative effect of shake-out timing were analyzed in detail. An effective plan of decreasing stress was proposed in cooling mode.     

Application of contact element method in the numerical simulation of thermal stress

The interaction relationship between casting and mold (core) decides the stress level of casting and mold (core). In this article, the stress field during the casting process of stress frame casting was simulated by contact element method provided of the professional casting simulation software, ProCAST, compared its results with the simulation results of sand mold with full rigidity. Meanwhile, the influence of shake-out temperature on residual stress was also in study. It showed that the stress result of full rigidity mechanical model is bigger than that of the contact element method. The casting residual stress first increases and then decreases along with the elevation of the shake-out temperature, and the residual stress reaches the maximum when the shake-out temperature is 600°C. The lower is the shake-out temperature, the smaller is the casting deformation.     

铝合金活塞铸造过程中模具易失效区域预测

采用ProCAST软件对铝合金活塞浇铸过程中模具的温度场、应力场进行了有限元仿真。根据热、应力场仿真结果,预测活塞模具的易失效区域,通过与实际失效结果对比,发现仿真结果中易失效区域位置与实际模具失效区域位置相吻合;并通过研究不同模具预热温度对浇铸过程中模具热应力场的影响,发现提高模具的初始预热温度,可以有效降低浇铸过程中模具型腔表面所受的热应力,从而有助于模具寿命的提高。     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

离心铸管金属型的热弹塑性分析

本文针对离心铸管用金属型的实际工作条件,假定材料的弹性模量为常数并具有线性强化性能,而屈服极限和塑性强化系数及热膨胀系数均随温度变化,采用塑性力学中的全量理论,在解析的基础上利用数值解法进行离心铸管金属型的热弹塑性分析。最后,给出其应力、应变变化过程和各阶段的应力场及循环应力-应变关系。     

FTSC薄板坯连铸结晶器内的热/力行为研究

基于SPHC钢在H2结晶器内的凝固过程,利用有限元软件ANSYS中的单元生死技术,通过改变薄板坯单元的状态建立其在结晶器内随时间变化的二维非稳态模型。模拟出薄板坯在结晶器不同部位处的温度场及应力场分布情况,其结果可为优化结晶器锥度及改善薄板坯质量提供理论依据和技术基础。     

单晶铸件凝固过程工艺优化的数值模拟

采用ProCAST软件系统研究了LMC(Liquid Metal Cooling)以及HRS(High Rate Solidification)工艺下,不同工艺参数对单晶铸件凝固过程中纵向温度梯度、温度梯度角、凝固界面位置的影响。结果表明:HRS工艺受型壳厚度影响很小,型壳表面的辐射散热是HRS工艺的主要影响因素,型壳的导热或者型壳和合金之间的换热是LMC工艺的主要影响因素;提高保温炉温度有利于提高纵向温度梯度;拉速是影响定向凝固最重要的参数,随拉速的增加,单晶铸件的纵向温度梯度先增大后减小,因此,制备不同合金铸件时应当采用不同的拉速;不同浇注温度时,经过10min的静置时间后,单晶铸件的初始温度分布趋于一致,对后续凝固过程影响很小。提出了以纵向温度梯度G∥、温度梯度角θ以及凝固界面位置Rp考察定向凝固工艺参数优劣的标准,纵向温度梯度、温度梯度角、凝固界面位置是评价定向凝固参数优劣的有效手段。     

金属材料挤压铸造成形技术的研究进展

挤压铸造技术是一种结合了铸造和塑性加工特点的短流程、高效、精确成形技术,广泛应用于机械、汽车、家电、航空、航天、国防等领域生产高性能和高精度的零件。首先简要介绍了金属材料挤压铸造成形技术的特点和历史。重点分析了液态金属压力下结晶的物理冶金行为和力学过程,此外,还总结了挤压铸造技术涉及的材料体系、挤压铸造过程的工艺参数优化、挤压铸造过程的零件成形、挤压铸造过程的数值模拟、挤压铸造成形装备研究方面的研究进展。最后展望了金属材料挤压铸造成形技术的发展重点。