Modeling the Stress-Strain Behavior and Hot Tearing during Direct Chill Casting of an AZ31 Magnesium Billet

Quantitative knowledge of the thermal mechanical history experienced during direct chill (DC) casting aids the scientific understanding of the process especially in terms of defect formation such as hot tearing. In this work, a thermomechanical finite element (FE) model has been developed to simulate the DC casting of magnesium alloy AZ31 billets. The mathematical model simulates the evolution of temperature, stress, and strain within the billet during an industrial DC casting process. These quantities were then used to calculate the evolution in pressure, and hence hot tearing tendency, within the semisolid regime via the Rappaz–Drezet–Gremaud (RDG) criterion. The temperature predictions were validated against experimental thermocouple data measured during a plant trial at an industrial magnesium DC casting facility. In addition, the residual elastic strains predicted by the model were compared to residual strain measurements made at the Canadian Neutron Beam Centre (CNBC) using a magnesium billet produced during the industrial casting trial. The validated model was then used to quantitatively assess the impact of casting speed on the hot tearing tendency in AZ31 billets.     

Cold Cracking Development in AA7050 Direct Chill–Cast Billets under Various Casting Conditions

Cold cracking is a potentially catastrophic phenomenon in direct chill (DC) casting of 7xxx series aluminum alloys that leads to safety hazards and loss of production. The relatively low thermal conductivity and wide solidification temperature range in these alloys results in accumulation of residual thermal stress under nonuniform cooling conditions of the billets. In addition, such alloys show a severe loss in ductility below a critical temperature of 573 K (300 °C). This brittleness along with high stress concentration at the tips of voids and microcracks can lead to catastrophic failure. Casting process parameters affect the magnitude and distribution of stresses in the billet and increase the susceptibility of the material to cold cracking. In order to investigate the effect of casting process parameters such as casting speed, billet size, and water flow rate, thermomechanical simulations were applied using ALSIM5 casting simulation software. Among the studied casting process parameters, the increased billet size and high casting speed resulted in the most dramatic increase in residual stress level. Critical crack sizes that led to catastrophic failure were also calculated and are reported against process parameters.     

Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.     

小方坯齿轮钢连铸过程中的宏观偏析模拟

基于国内某厂齿轮钢小方坯连铸生产过程,利用ProCAST软件建立移动切片模型,能够高效模拟连铸过程中的宏观偏析,模型分别模拟研究了不同过热度、二冷水量和拉坯速度等对宏观偏析的影响。模拟结果与碳偏析检测结果吻合良好,验证了移动切片模型模拟连铸坯宏观偏析的准确性。由于溶质浮力的影响,内弧侧的宏观偏析强于外弧侧。随着过热度的增加,铸坯中心碳偏析度从1.06增加至1.15。过热度控制在25 ℃范围内,可以保证铸坯的宏观碳偏析度控制在1.10范围内。随着连铸二冷水量的增加,铸坯中心偏析改善程度较小,铸坯中心碳偏析度从1.16降低至1.13。随着拉坯速度的增加,铸坯中心偏析呈现加重的趋势,铸坯中心碳偏析度由1.14增加至1.21,拉坯速度控制在1.4 m·min–1范围内,可保证铸坯中心碳偏析度低于1.15。      

Development and validation of a thermal model of the direct chill casting of AZ31 magnesium billets

A mathematical model of the direct chill (DC) casting process for magnesium billets has been developed to predict the temperature in the billet, dummy block, and center pin using the commercial finite-element (FE) package, ABAQUS. Boundary conditions used for primary and secondary cooling and interfacial cooling between the billet and dummy block were defined based on the literature and knowledge of the physical phenomena occurring during the process. Temperatures in the billet, mold, and dummy block, as well as sump depth, were measured for a series of casting conditions during a plant trial conducted at Timminco Metals. The data obtained during the plant trial were used to “fine-tune” and validate the model for a variety of industrial casting conditions.     

Three-dimensional analysis and design of petal-like mould for continuous casting of steels

Abstract

Understanding the shrinkage behaviour of a steel billet is very important for designing the continuous casting mould, and conversely, a well designed mould is beneficial when matching the shrinkage behaviour of the billet. The shrinkage behaviour of a billet is hard to measure in situ during continuous casting but can be calculated by numerical simulation. A three-dimensional finite element model has been built to simulate the thermal and stress fields of the billet in the mould. The dynamic thermal boundary condition, the effect of ferrostatic pressure and the temperature dependent thermophysical parameters have been considered in the model. The shrinkage of billet when considering ferrostatic pressure is on average 0·08 mm smaller than when not considering ferrostatic pressure. The temperature and stress distributions are analysed in the present paper, and based on this analysis, a novel petal-like mould was designed and its taper determined. The designed mould has been tested in industrial practice showing better lifetime and billet quality.     

Residual stresses after orthogonal machining of AlSl 304: numerical calculation of the thermal component and comparison with experimental results

Residual stresses due to the thermal influence of orthogonal machining have been calculated with a finite element model using stationary workpiece temperatures during cutting calculated with the finite difference method. Calculated results are compared with experimental data obtained with the X-ray diffraction method. In this way, the thermal and mechanical/frictional influences of the machining operation on the workpiece residual stress state can be separated. The influence of cutting speed and cutting depth on machining residual stresses is discussed. It is shown that the thermal as well as the mechanical impact of the orthogonal cutting process causes tensile residual stresses. The mechanical impact of the machining operation causing tensile residual stresses is due to (a) compressive plastic deformation in the surface layer ahead of the advancing tool and (b) greater elastic relaxation upon unloading with respect to the underlying material of a thin, strongly work-hardened surface layer. CHRISTOPH WIESNER, formerly Research Assistant with the Laboratoire de Métallurgie Mécanique, Ecole Polytechnique Fédérale de Lausanne, MX-D Ecublens, 1015 Lausanne, Switzerland.     

Deformation,residual stress,and constitutive relations for quenched W319 aluminum

A study of the development of deformation and transient and residual stresses during quenching in aluminum alloy W319 is presented. Rapid tension tests were performed on W319 in the super-saturated solution state at several temperatures and strain rates. A material model following the mechanical threshold stress (MTS)-Voce formulation is developed and implemented in both a simple one-dimensional code and a fully three-dimensional form as a user material subroutine in ABAQUS. The results of the tension tests are used to determine the parameters in the thermomechanical constitutive model. Unidirectional beam quenching experiments are performed to test the applicability of the constitutive model. Residual stresses in the beams are measured using a groove removal technique upon completion of the quenching process. Residual stress and deformation results from beam quenching experiments compare well to the analytical results computed using the constitutive model.     

逆相变退火对连铸坯组织和力学性能的影响

 研究了逆相变退火温度对0.1C5Mn钢连铸坯的组织结构和力学性能的影响规律,采用SEM进行组织结构的表征,利用XRD技术分析连铸坯退火后奥氏体含量,并测试了退火试样的力学拉伸性能。试验结果表明,连铸坯退火过程中发生奥氏体逆转变且在较低退火温度下有少量碳化物析出,随着退火温度升高,奥氏体含量先增加后减少,析出物逐渐溶解消失。提高退火温度可以显著提高试验钢的抗拉强度但却降低它的屈服强度,另外随退火温度升高,断后伸长率和强塑积先增高后降低。在625~650℃退火,可以获得20%~25%的伸长率。研究结果说明利用逆转变退火可以大幅度提高中锰钢铸坯的力学性能。     

Finite element method simulation of mushy zone behavior during direct-chill casting of an Al-4.5 pct Cu alloy

In this article, the stresses, strains, sump depth, mushy zone length, and temperature fields are calculated through the simulation of the direct-chill (DC) casting process for a round billet by using a finite-element method (FEM). Focus is put on the mushy zone and solid region close to it. In the center of the billet, circumferential stresses and strains (which play a main role in hot cracking) are tensile close to the solidus temperature, whereas they are compressive near the surface of the billet. The stresses, strains, depth of sump, and length of mushy zone increase with increasing casting speed. They are maximum in the start-up phase and are reduced by applying a ramping procedure in the start-up phase. Stresses, strains, depth of sump, and length of mushy zone are highest in the center of the billet for all casting conditions considered.     

Deformation Characteristics of Steel Billets with Liquid Core

To investigate the deformation characteristics of billets with liquid core during soft reduction and to clarify the correlation between internal cracks and deformation of the billet in the mushy zone, a fully coupled thermo‐mechanical Finite Element Model was developed in ABAQUS, furthermore, casting and soft reduction tests were carried out in a laboratory strand casting machine. During soft reduction the temperature distribution, the stress and strain states in the billet were calculated, the deformation characteristics of the billet during soft reduction were determined and the relation between internal cracks and equivalent plastic strain as well as maximal principal stress was analysed. The results show that tensile stresses can develop in the mushy zone during soft reduction and the equivalent strain nearby the Zero Ductility Temperature (ZDT) increases with a decreasing solid fraction. Internal cracks can be initiated when the accumulated strain exceeds the critical strain and /or the applied tensile stress exceeds the critical fracture stress during solidification. In addition, the factors (reduction efficiency and internal cracks) that should be considered to determine the optimal parameter for the soft reduction were established.     

Three-dimensional modelling of thermal residual stresses and mechanical behavior of cast SiC/A1 particulate composites

Thermal residual stresses developed during casting of SiC/aluminum particulate-reinforced composites were investigated as a function of cooling rate and volume fraction of particles using thermo-elastoplastic finite element analysis. The phase change of the matrix during solidification and the temperature-dependent material properties as the composite is cooled from the liquidus temperature to room temperature were taken into account in the model. Further, the effect of thermal residual stresses on the mechanical behavior of the composites was also studied. Based on the study, it was found that the matrix undergoes significant plastic deformation during cool down and has higher residual stress distribution as the cooling rate increases. The model which does not include the solidification of the matrix tends to overestimate the residual stresses in the matrix and underestimate the tensile modulus of elasticity of the composites. In addition, the presence of thermally induced residual stresses tends to decrease the apparent modulus of elasticity and increase the yield strength of the composites compared to those without residual stresses.     

Analysis of bulging for high speed continuous casting

In order to research the temperature distribution and mechanical deformation of slab bulging during high speed continuous casting,mathematical models have been developed to analyze the thermal and mechanical behavior of the slab.The thermal history of the slab has been predicted by a two-dimensional transient finite element heat transfer model,whose results serve as the input to the stress model.The stress model has been formulated for a two-dimensional longitudinal plane.In this case,the maximum tensile strain during the bulging process is located at the solidification front just past the top of the upstream roll,which may contribute to crack formation.The maximum tensile stresses are located at the cold surface in the middle of the two back-up rolls,just at the point of the maximum bulging.Stresses near the solidification front are small because of the high temperatures which produce lower elastic modulus values.Finally,the effect of the casting speed on the bulging deformation is discussed.     

冷却强度对超高硫中碳钢方坯凝固过程MnS生长的影响

 中碳超高硫易切削钢SAE144是兼具力学性能与切削性能的结构钢,用于制造汽车发动机密封阀件等,产品多采用转炉/电炉→LF精炼→连铸小方坯→线棒材热轧→冷拉及机加工成型流程生产,近年来市场热度稳步提升。若钢中MnS尺寸过大,零件加工使用过程易发生探伤不合、切削性能差、带状组织严重、力学性能各相异性显著,甚至拉拔加工断裂等问题。MnS夹杂物多在铸坯凝固后期形成,随着轧制与钢基体同步变形,控制该类钢种铸坯内MnS原始尺寸成为控制热轧材中MnS夹杂物形态及尺寸的最关键环节。为控制热轧超高硫中碳钢盘条中MnS夹杂物,利用钢坯凝固数值模拟、第二相析出理论、Ostwald熟化理论计算分析了160 mm2钢坯中硫元素偏析及MnS的生成、长大和熟化过程。计算结果表明,当固相分数f_s为0.446、硫微观偏析比达到2.19时,铸坯在凝固末期生成MnS。凝固过程中MnS的生长过程决定了钢坯中MnS颗粒的直径。理论计算表明,当连铸二次冷却水量固定为0.6L/kg时,拉速为1.6、2.1和2.6 m/min时,160 mm2方坯中心的MnS分别增长到30.6、32.2和34.6 μm,与实际测试结果一致。控制该类钢种线材中MnS尺寸的关键是提高二冷区的冷却强度,降低连铸拉速。基于该系列计算方法,提出了160 mm2钢坯中与MnS直径控制目标相匹配的连铸工艺参数控制范围。     

SWRH82B铸坯凝固质量控制

针对安钢SWRH82B钢生产的工艺现状,研究了拉速、过热度、二冷配水等工艺参数对于铸坯中心缩孔、疏松和偏析等内部质量的影响。在实验室进行了铸坯凝固组织的研究,测量了二次枝晶间距,进一步了解连铸工艺参数对铸坯凝固组织的影响。     

A Benchmark Study on Casting Residual Stress

A benchmark study was undertaken for casting residual stress measurements through neutron diffraction, which was subsequently used to validate the accuracy of simulation prediction. The “stress lattice” specimen geometry was designed such that subsequent castings would generate adequate residual stresses during solidification and cooling of ductile cast iron, without any cracks. The residual stresses in the cast specimen were measured using neutron diffraction. Considering the difficulty in accessing the neutron diffraction facility, these measurements can be considered as a benchmark for casting simulation validations. Simulations were performed using the identical specimen geometry and casting conditions for predictions of residual stresses. The simulation predictions were found to agree well with the experimentally measured residual stresses. The experimentally validated model can be subsequently used to predict residual stresses in different cast components. This enables incorporation of the residual stresses at the design phase along with external loads for accurate predictions of fatigue and fracture performance of the cast components.     

Spectral factors in the perception of vowel quantity in Icelandic

The aim of this study was to clarify the effect of heating rate on the development of both transient and residual stresses in investment molds. Solid, cylindrical, gypsum-based molds were modeled and theoretical calculations made of temperature distributions with a constant heating rate. These calculations used experimental thermal diffusivity data obtained with a laser flash method. The simulations calculated transient thermal stresses during heating and at the end of heating for two surface conditions: an unrestricted surface, and a surface restricted by a casting ring. The simulation model developed nonuniform strains and stresses at casting temperature; tangential stress was compressive at the surface and tensile at the mid-point for the unrestricted surface model. The surface restricted model developed compressive tangential stresses throughout the mold at the casting temperature. This resulted in significant thermal strain differences compared to the magnitudes of expansion of the mold due to heating.     

Study on Solidification Structure of Wheel Steel Round Billet Using FE‐CA Coupling Model

Solidification structure of wheel steel round billet during the continuous casting process was simulated using FE (Finite Element)–CA (Cellular Automaton) coupling model. Variation of thermo‐physical parameters during solidification was considered based on a thermodynamic database. Meanwhile effect of electromagnetic stirring (EMS) was reflected by increasing both the thermal conductivity and crystal formation rate in liquid phase. It was found that the cooling curves and solidification structure calculated by this model agreed well with that in experiments. Optimum casting temperature range was discussed based on the simulation results and actual conditions in plant. An optimized casting superheat will be no more than 25 °C in order to obtain at least 50% equiaxed crystal ratio, while the degree of segregation in the billet is less than 1.05 correspondingly.