Improved thermal stress analysis for castings

Abstract

Three algorithms are presented to address three problems in stress analysis of castings: difficulties for enmeshment of complicated casting into finite element meshes, coupling of thermal and stress analysis, and determination of inverse deformation, respectively. An algorithm of conversion of finite difference meshes into finite element meshes is achieved by reformatting the data of meshes. An algorithm of the heat transfer coefficient at the casting/mould interface is presented and the coupling of thermal and stress analysis is realised with the feedback of stress and deformation to heat transfer. Machining allowance is applied as a criterion for deformation evaluation of castings. And insufficient machining allowance is transformed to inverse deformation which is fed back to the original casting design for recalculation of stress and deformation. Therefore, the design of casting with appropriate inverse deformation is obtained. Case studies about a cylinder block and a hydro turbine blade casting are illustrated.     

Determination of the metal/die interfacial heat transfer coefficient and its application in evaluating the pressure distribution inside the casting during the high pressure die casting process

Abstract

High pressure die casting (HPDC) experiments were conducted on a 650 t cold chamber die casting machine to study the interfacial heat transfer behaviour between casting and die. A 'step shape' casting and two commercial alloys namely ADC12 and AM50 were used during the experiments. Temperature and pressure measurements were made inside the die and at the die surface. The metal/die interfacial heat transfer coefficient (IHTC) was successfully determined based on the measured temperature inside the die by solving the inverse heat transfer problem. The IHTC was then used as the boundary condition to determine the 3-D temperature field inside the casting. Based on the predicted temperature distribution, the pressure distribution inside the casting was evaluated by assuming that the transferred pressure from the plunger tip of the injection side to the casting is primarily influenced by the solid fraction of the casting. Reasonable agreement was found between the determined pressure values and the measured pressures at the die surface of the casting.     

大型水轮机叶片的变形预测与控制研究

将数值模拟应用于大型水轮机叶片变形的预测与控制。综合考虑凝固、冷却、打箱、切除浇注系统和热处理影响建立数值模型,准确预测铸件在整个热加工工序中的变形。根据铸件结构特征和不同区域拘束度的不同,设置不同的反变形系数,对原始模型进行反变形处理。针对添加了反变形的模型,进行整个热加工过程的数值模拟计算,最终确定合理的铸件模样尺寸,生产出加工余量合适均匀的铸件。     

铸件曲面及其模型曲面逆向工程并行设计技术

研究目的是通过逆向工程方法实现铸件曲面及其模型曲面的并行设计。已知与铸件安装配合的曲面数据及铸件各截面厚度,确定铸件的加工变形和加工余量后,分别求出铸件曲面和模型曲面的离散有序数据作为神经网络的样本数据。建立RBF神经网络拓扑结构,利用神经网络拟合铸件曲面和模型曲面,实现两种曲面的并行设计,曲面精度高,计算速度快。通过实例证明该方法可以实现曲面的并行设计,缩短研制和设计周期,提高效率。     

基于曲面重构方法的大型叶片加工定位问题研究

大型水轮机叶片曲面翘曲,双面加工,笨重不易翻动,给其找正定位带来极大不便。长期以来,加工现场通常采用人工找正定位,其缺点是找正困难、效率低、精度差。本文通过反求工程方法将测量得到的大量离散毛坯表面数据,依据CAD模型的拓扑特征,重构毛坯表面,然后利用重构曲面与CAD模型曲面对应节点距离平方和构造目标函数,采用最小二乘法（LSQ）求解欧氏变换矩阵,从而达到毛坯加工余量均匀,实现计算机辅助定位的目的。实算证明：该方法能快速辅助找正;且对于表面质量高、边界规范的毛坯,定位比较准确。最后给出了一辅助定位实例。     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.     

Finite-element analysis on thermomechanical behavior of a marine propeller casting in the sand-casting process

Thermal deformation of a large marine propeller casting during solidification and cooling was analyzed by a coupled thermomechanical finite-element analysis (FEA). The calculated displacements on the blade in the z-direction were compared with the measured values that confirmed the effectiveness of FEA for the prediction of thermal deformation. The effect of the shake-out time and method on the deformation of the propeller casting was also investigated. The longer the shake-out time, the smaller the deformation. For controlling the deformation of the propeller casting, the sequential shake-out was a more efficient method than the simultaneous shake-out.     

航发钛合金风扇叶片自适应砂带磨削实验研究

航空发动机叶片的材料多为钛合金,该类材料强度和机械性能良好,但加工性能差,而模锻、精铣后的叶片型面精度和表面质量需要加工来满足航空发动机的要求。通过三坐标测量仪最佳拟合法检测模锻叶片,确定叶片型面加工余量,再采用自适应砂带磨削方式对航发模锻叶片材料定量去除。结果表明:磨削加工能精准去除模锻叶片表面的加工余量,保证叶片进排气边的圆弧过渡,且叶片表面粗糙度R_a值均在0.4 μm以下。      

大直径铸钢套筒的离心铸造研究

主要介绍了卧式离心铸造大直径铸钢套筒的方法。讨论了降低套筒气孔和铸造裂纹的工艺措施，离心铸造铸钢套筒具有组织致密、均匀、工艺出口率高和铸件加工量少等特点，生产成本比普通静态铸造降低20%以上，用作铝箔轧机的出口端卷筒，获得了较好的效果。     

实型铸造冲模毛坯的主要问题及解决对策

针对实型铸造冲模毛坯存在的模具表面粗糙、加工余量大、浇注时黑烟等问题，提出了3点建议：(1)推广适合数控加工的泡沫板材；(2)开发加工泡沫模样的专用机床和刀具；(3)治理浇注黑烟，实施烟气净化。     

熔模铸件加工余量分析及铸件尺寸的确定

具体分析了实际生产中熔模铸件的最小加工余量,详细阐述了熔模铸件加工余量设计值及铸件加工面尺寸的确定方法,并进行了实例计算。     

A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper

High pressure die casting copper is used to produce rotors for induction motors to improve efficiency. Experiments were carried out for a special "step-shape" casting with different step thicknesses. Based on the measured temperature inside the die, the interfacial heat transfer coefficient(IHTC) at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem. The IHTC peak value was 4.5×10~3-11×10~3 W·m~(-2)·K~(-1) under the basic operation condition. The influences of casting pressure, fast shot speed, pouring temperature and initial die surface temperature on the IHTC peak values were investigated. Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate. An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.     

A method for the identification of the specific force coefficients for mechanistic milling simulation

This paper presents a new method to obtain the specific cutting coefficients needed to predict the milling forces using a mechanistic model of the process. The specific coefficients depend on the tool–material couple, the cutting conditions and the geometry of the tool, being usually calculated applying the force model in an inverse way. The most used inverse method is based on the calculation of the average cutting force per revolution values measured in a series of slot machining tests at different feed rates. In this research work, the inverse method is applied using the instantaneous cutting force values, solving the equations system by a constrained least squares fitting method. Furthermore, the cutting force and specific cutting coefficients relation with rake angle and chip thickness is analysed. The results are validated by the comparison of the simulations and experiments in orthogonal cutting test, showing the advantages of using the new method.     

Studying elastic deformation behaviour of cast irons by acoustic emission

Abstract

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.     

Cracking and Healing of Oxide Scales on Ti-Al Alloys at 900°C

The behavior of oxide scales on Ti-50Al andTi-50Al-2Nb (at.%) was investigated in constantstrain-rate tensile tests at 900°C in air. Thestrain rates ranged between 1 × 10^-9sec^-1 and 3 × 10^-4sec^-1. The tests were accompanied byacoustic-emission measurements in order to detectscale-cracking processes during deformation. Thecritical strains to scale cracking amounted to 0.12-0.5%for the scales on TiAl and 0.17-0.58% for Ti-50Al-2Nb. Thesevalues were found to depend strongly on the size of thepores in the scales and, by using a fracturemechanics-based model, the results for the criticalstrains could be condensed into a narrow scatterband,which is independent of the applied strain rate. Healingof scale cracks was found for strain rates below 1.5× 10^-6 sec^-1 (Ti-Al) and 1.9× 10^-4 sec^-1 (Ti-Al-Nb), respectively. It turned out that the healingprocess is dominated by TiO₂ growth. In alater healing stage, the originalAl₂O₃ barrier is, however,restored in the scale on Ti-50Al. For Ti-50Al-2Nb, an Al₂O₃ layer is found onthe former scale-crack contours. The healing process isalso described by a quantitative model. As a generalconclusion from the investigations, it turned out thatcritical strains to oxide-scale cracking can be estimated fromjust simple oxidation experiments without sophisticatedmechanical testing if the microstructural parameters ofthe scale are determined quantitatively as a function of oxidation time by metallographicmeans.     

20Cr2Ni4A钢制内环工艺改进

针对液力变矩器的20Cr2Ni4A钢制内环经渗碳淬火后圆盖板翘曲度过大导致无法满足机加工要求这一问题,采用有限元方法模拟了装炉方式和在圆盖板上下端增加机加余量对内环渗碳淬火过程的应力场和变形场的影响,并进行了试验验证。结果表明,装炉方式和增加机加工余量对内环渗碳淬火过程应力场和变形场的影响很大,采用4种装炉方式进行淬火后内环的最大变形量按从大到小依次为长轴竖放、短轴竖放、吊放、横放。采用吊放装炉能使?215 mm薄壁处平面翘曲变形有效减小至0.29 mm,横放装炉翘曲度能控制到0.21 mm。当圆盖板上端机加工余量增厚至11 mm时,翘曲度能控制到0.19 mm。综合生产效率和成本,宜选用横放装炉生产。     

Numerical Simulation Of Molten Pool And Control Strategy Of Kiss Point In A Twin-Roll Strip Casting Process

Coupled turbulent flow, temperature fields of the twin-roll casting strip process were simulated by three-dimensional finite element method. Based on the heat balance calculation and using inverse methods between the simulations and real experiments, the relational models among casting speed, location, and coefficient of heat transfer between molten metal and rolls in different regions are given. In the simulation, the calculated surface temperatures are in good agreement with the measured values. An on-line model of kiss point is derived by simulations and the geometry of molten pool, corresponding control strategy is also proposed.     

Determination of heat transfer coefficients by extrapolation and numerical inverse methods in squeeze casting of magnesium alloy AM60

In this work, a different wall-thickness 5-step (with thicknesses as 3, 5, 8, 12, 20 mm) casting mold was designed, and squeeze casting of magnesium alloy AM60 was performed under an applied pressure 30, 60 and 90 MPa in a hydraulic press. The casting-die interfacial heat transfer coefficients (IHTC) in the 5-step casting were determined based on thermal histories throughout the die and inside the casting which were recorded by fine type-K thermocouples. With measured temperatures, heat flux and IHTCs were evaluated using the polynomial curve fitting method and numerical inverse method. For numerical inverse method, a solution algorithm was developed based on the function specification method to solve the inverse heat conduction equations. The IHTCs curves for five steps versus time were displayed. As the applied pressures increased, the IHTC peak value of each step was increased accordingly. It can be observed that the peak IHTC value decreased as the step became thinner. Furthermore, the accuracy of these curves was analyzed by the direct modeling calculation. The results indicated that heat flux and IHTCs determined by the inverse method were more accurately than those from the extrapolated fitting method.     

细长梁数控侧铣加工变形的预测、补偿与验证

针对铸造钛合金ZTC4材质的弱刚度细长梁结构件加工变形问题,借助有限元仿真方法对该加工过程进行分析,并根据仿真预测出来的零件变形量数据,结合镜像补偿加工的方法完成了试验验证。通过建立细长梁数控侧铣加工的三维有限元仿真模型,预测出零件本体在不同位置处的变形量范围,并以此为基础利用数控铣床进行试验验证。结果表明,无补偿的加工方式零件的表面质量差,而补偿加工后的零件表面,其平面度和直线度误差都有较大幅度的降低,下降幅度分别为17%和33%,从而确定了补偿加工的方式能够有效降低该弱刚度结构件的形状误差,提升其加工精度。     

凸模铸造工艺

通过凸模铸件生产摸索,将原工艺4个角1/4补块改为1/2补块,以解决长方形变形问题;采用冷热冒口联合使用及加大大平面加工量措施,以解决大平面夹渣、气孔问题;操作上增加测量4个角垂直度和表面尺寸.经过上述工艺措施生产,彻底解决了铸件变形及夹渣、气孔问题.