Finite-element thermal modeling of casting microstructures and defects

Finite-element thermal modeling offers the capability of simulating casting solidification conditions. By combining thermal modeling and inspection, a defect map can be established to enable prediction of casting microstructures and defects. Gamma-prime size, dendrite-arm spacing, equiaxed grain size, shrink, hot tearing, misrun, and other features can all be predicted.     

Estimation of local fatigue behaviour in A356–T6 gravity die cast engine head based on solidification defects content

Al–Si–Mg cast alloys have widespread applications, especially in the aerospace and automotive industries, due to their excellent combination of castability and high specific strength. Among Al–Si alloys, hypoeutectic A356 (Al–7Si–0·3Mg) ranks as one of the most widely used for the production of a variety of components, including engine blocks and engine heads, due to its excellent castability and good mechanical properties. The microstructure of this alloy greatly depends on chemical composition, solidification conditions, metal soundness and heat treatment. Furthermore, its mechanical properties are strongly affected by solidification microstructure and defects, which can vary greatly in complex shaped castings. Among the different microstructural features, only secondary dendrite arm spacing and percentage defect content can currently be predicted with sufficient accuracy by casting simulation software. This makes the prediction of the fatigue life of complex shaped Al–Si castings very difficult, since it is widely accepted that fatigue behaviour mainly depends on the size of solidification defects (gas pores and cavity shrinkages). In this study, the experimental work was carried out on an industrial A356–T6 gravity die cast engine head, with the aim of finding relationships among the main microstructural features and solidification defect parameters. The goal of this analysis was to correlate the defect size, which is the most important variable affecting the fatigue behaviour, to the other microstructural parameters that can be predicted by casting simulation software. Moreover, by applying literature models for fatigue behaviour prediction, based on maximum defect size, the local expected fatigue life/fatigue limit on a section of the casting will be evaluated and compared with those obtained by rotating bending fatigue tests. This study would demonstrate the effectiveness of a new approach of coengineering design, with a strong synergy between the structural finite element method and the casting simulation process, able to estimate the local fatigue strength in complex shaped A356 castings.     

Modeling of alloy casting solidification

Alloy casting solidification processes involve many physical phenomena such as chemistry variation, phase transformation, heat transfer, fluid flow, microstructure evolution, and mechanical stress.1 Simulation technologies are applied extensively in casting industries to understand the effects of these phenomena on the formation of defects and on the final mechanical properties of the castings. As of today, defect prediction is still one of the main purposes for casting solidification simulation. In this paper, we will first present the commonly used microstructure simulation methods, then discuss the predictions of the major defects of a casting, such as porosity, hot tearing, and macrosegregation. The modeling of casting solidification can be chained with later stages of heat treatment such that the resultant microstructure, defects, and mechanical state will be used as the initial conditions of the subsequent processes, ensuring the tracking of the component history and maintaining a high level of accuracy across metallurgical stages.     

Using solidification parameters to predict porosity distributions in alloy castings

Quality criteria used in the computer-aided design and analysis of casting processes typically relate geometric, thermal, or solidification parameters to structural features such as centerline shrinkage and microporosity. Quality criteria for the prediction of porosity in castings have been used successfully in steel, but the application of criteria functions to nonferrous alloys has been less successful. Recent work suggests that the dominating mechanism that determines the amount and distribution of porosity in castings is strongly dependent on the solidification mode of the alloy and the solidification conditions. Accordingly, casting processes and alloy types are divided into four groups, and a different set of criteria functions are obtained for each group.     

The application of defect maps in the process modeling of single-crystal investment casting

Because single-crystal components are Jmrd to cast, accurate models of the casting parameters can help to reduce the scrap rate and lower costs. A defect map, which plots the occurrence of defects based on the temperature gradient and the solidification rate, can be used as a performance criterion in process modeling. This article describes the use of defect maps in the investment casting of a single-crystal turbine blade.     

Modeling and simulation of 3D thermal stresses of large-sized castings in solidification processes

When heavy machines and large scaled receiver system of communication equipment are manufactured, it always needs to produce large- sized steel castings, aluminum castings and etc. Some defects of hot cracking by thermal stress often appear during solidification process as these castings are produced, which results in failure of castings.Therefore predicting the effects of technological parameters for production of castings on the thermal stress during solidification process becomes an important means. In this paper, the mathematical models have been established and numerical calculation of temperature fields by using finite difference method (FDM) and then thermal stress fields by using finite element method (FEM) during solidification process of castings have been carried out. The technological parameters of production have been optimized by the results of calculation and the defects of hot cracking have been eliminated. Modeling and simulation of 3D thermal stress during solidification processes of large-sized castings provided a scientific basis, which promoted further development of advanced manufacturing technique.     

Porosity distribution in directionally solidified test bars sand cast from a controlled A356 melt

Heat transfer modelling of directionally cast A356 bars has been used to obtain values for all relevant thermal and solidification parameters throughout the freezing process. Porosity distributions of unmodified and Sr-modified bars were compared to previously proposed criteria functions for microporosity distributions. It was shown that porosity increases continuously from the chill to the feeder ends of the bars. Notwithstanding the obvious influence of other factors such as solidification shrinkage or mass feeding, this behaviour could be explained by the hydrogen build-up occurring in the remaining liquid as solidification proceeds.

Experimental results confirm the higher microporosity usually associated with modified castings and the fundamentally different distribution between micro- and macro-shrinkage for the unmodified and modified conditions. From a practical standpoint, the average rate of solidification between solidus and liquidus temperatures, or the solidification time at a given point, were found to be adequate and simple criteria to which microporosity distribution may be related.     

The effect of grain refinement on the formation of casting defects in alloy 356 castings

One of the main reasons for rejection of wheel castings is the formation of shrinkage defects at the spoke-rim junction. Industrial trials show that the severity of defects is accentuated by the addition of grain refiner beyond an optimum level. Subsequent laboratory castings also found that the addition of grain refiners beyond this optimum level increased the size of the defect at hot spots in a casting. This is explained by a decrease in the size of interdendritic channels during feeding when the grain size is reduced. Therefore feeding becomes restricted earlier in grain refined castings and thus a larger amount of shrinkage needs to be accommodated in the hot spot. It was also observed that when plentiful fresh oxide films are present then the nucleation of porosity is favoured. However, when fresh oxide films have not been generated, then external shrinkage (slumping of the casting surface) is more likely to occur.     

A METHOD OF AS-CAST CRACK PREDICTION BASED UPON NUMERICAL SIMULATION OF SOLIDIFICATION

Based on the numerical simulation of solidification of castings, a thermal stress formula and a thermal crack initiation criterion are established, which suggest that the cracks initiate at the two-thirds thickness of solidified layer from outside of castings. Cast steel wheels whose diameters are 800mm are employed to testify the positions of cracks through thermal elastic-plastic analyses and low magnifying structure observations conventionally. The results show that the numerical prediction of cracks coincides with the measured result, and the cracks do not necessarily follow where the defects such as shrinkage holes and porosities occur. It is also found that surface temperature control is an effective factor to avoid the crack formation.     

金属型重力铸造铝合金油底壳裂纹成因分析及对策

以油底壳为例,分析了壁薄、深型腔、大平面铝合金金属型重力铸件,产生裂纹缺陷的原因,确定是合金凝固时期的热应力和收缩应力所致。通过控制化学成分、强化精炼、分散引注、提高模具温度、调整涂料厚度强化凝固顺序等工艺措施,成功解决了该类缺陷,获得了良好的经济效益。这些工艺措施对同类产品的同类缺陷非常有效,同时对拓宽金属型在有色铸造领域的应用有着重要的意义。     

A study of thermal parameters and interdendritic feeding in lost foam casting

Thermal parameter-based criterion functions are of great practical importance to predict dispersed microporosity in castings. Using an experimental approach, the present study is a pioneering application of the thermal parameter-based criterion functions to predict the microporosity formation in lost foam casting (LFC) of A356 alloy. A series of plate castings with controlled hydrogen content were made with various thermal conditions by varying the dimensions of feeders and in gate design. Extensive analysis of the thermal parameters shows that changing the gate and feeder design modifies the distribution of the thermal parameters; furthermore, it influences the distribution of microporosity along the central line of the plate castings. The thermal parameters and criterion functions based on Darcy’s law show consistency, thus supporting the interdendritic feeding mechanism of microporosity formation. The criterion functions such as Niyama, LCC, and KCL can be used to predict microporosity formation during solidification of LFC, depending on the hydrogen content in the casting. The limitation of the interdendritic feeding mechanism and the effect of hydrogen on the gas pore formation are also discussed.     

预热对Al2O3陶瓷激光熔覆层温度梯度影响的模拟

针对铝合金微熔滴沉积成形过程中存在的缺陷问题,沉积制备了7075高强铝合金试样,采用金相显微镜(OM)、扫描电镜(SEM)等分析测试手段对其内部缺陷形貌进行观察,进而研究不同类型缺陷的形成机理.结果表明:铝合金微熔滴沉积成形件内部缺陷主要有孔洞和裂纹.其中孔洞缺陷包括间隙孔洞和凝固收缩孔洞;裂纹包括熔合线裂纹和细微热裂纹,其形成与延展和成形件内部残余热应力集中效应、熔滴重熔效果以及晶粒生长状况等因素有关.在此基础上提出减少或消除各类缺陷的方法,从而促进了该技术在高熔点合金零件制备中的应用.     

铝合金铸件中的凝固缺陷形成机理及预测

孔洞与热裂是铸件在凝固过程中最常遇到的典型缺陷。两者均形成于合金凝固末期,与液相补缩不足相关,但又有各自的形成机理。铝合金熔体对氢气有很大的溶解度,凝固过程中又因为溶解度的剧降而析出。孔洞是液相中过饱和的气体压力与凝固收缩引起的压力降共同作用的结果。热裂则是在凝固末期由于铸件收缩受阻而产生的应力以及液相补缩不足而导致的,其不仅与合金性质、铸造条件有关,并且受铸件形状的影响。文章基于近年来该方面的理论研究成果,总结了关于孔洞与热裂的形成机理以及几种目前所应用的经典预测模型,并对这几种模型的理论基础以及所考虑的关键参数进行了分析与讨论,在此基础上提出了未来新模型的研究方向和亟需解决的关键问题。     

SOLIDCast在轮辐铸造工艺设计中的应用

运用SOLIDCast进行轮辐的铸造工艺设计,通过SOLIDCast对工艺的充型及凝固过程进行模拟,分析了缺陷形成的原因.对工艺进行了优化:增加冷铁、选用适当的浇注时间.用优化后的工艺投入生产,实际生产出的铸件外表光洁、加工后没有铸造缺陷,证实了优化后的工艺是合理的.     

Influence of platform position on stray grain nucleation in Ni-based single-crystal dummy blade clusters

Stray grains are the most severe of the solidification defects that occur in the industrial single-crystal blade preparation process. In this study, a single-crystal dummy blade cluster with different crystal orientations controlled by the seeding method was prepared, and the influence of the position of the circular platform (relative to the sample and furnace body) on stray grain nucleation was investigated. Results show that the microstructure of the circular platforms could be divided into the center, expansion, and stray grain regions. The inside of the circular platform facing the center of the cluster is more prone to stray grain formation than the outside of the circular platform facing the furnace body. With an increase in the distance between the circular platform and the bottom of the dummy blade cluster, the stray grain region expands, whereas the expansion region narrows. The stray grain is slightly aggravated with increase of the misorientation. Finally, the mechanism underlying the influence of platform position on the formation of stray grains in single-crystal dummy blade clusters is discussed based on the temperature evolution during directional solidification.

     

Uncertainty quantification in modelling equiaxed alloy solidification

Numerical simulations can provide insight into physical phenomena during alloy solidification processes that cannot be observed experimentally. These model predictions depend on the material, process and numerical parameters which contain inherit uncertainties due to their origin in experimental measurements or model assumptions. As a step towards understanding the effect of uncertain inputs on solidification process modelling, uncertainty quantification and sensitivity analysis are performed on a transient model of transport phenomena during the solidification of grain refined Al-4.5 wt.% Cu in a rectangular cavity. The effect of microstructural model parameters, thermal boundary conditions and material property input uncertainties are examined for their effect on macrosegregation levels and solidification time. Predictions of the macrosegregation level are most sensitive to the dendrite arm spacing of the rigid mushy zone.     

调整温度场消除球铁件缩松缺陷

介绍球铁件收缩、补缩行为，并根据温度场特点，分析球铁件凝固过程形成的热场不良元的原因。通过改变浇注系统，安放冷铁等工艺手段，建立良性温度场和理想的凝固条件，达到获得致密铸件的目的。     

凝固模拟在球铁飞轮工艺优化中的应用

应用MAGMAsoft对四种不同结构球铁飞轮的不同工艺方案进行凝固模拟，根据凝固模拟结果和实际试生产铸件外观缺陷及内部缩松情况，提出不同改进工艺方案并逐个进行凝固模拟，选择最佳方案用于生产。优化后的工艺方案投入批量生产后，球铁飞轮的缩孔、缩松缺陷达到用户要求。     

Effect of Grain Refinement and Cooling Rate on the Microstructure and Mechanical Properties of Secondary Al-Si-Cu Alloys

The effect of AlTi5B1 grain refinement and different solidification rates on metallurgical and mechanical properties of a secondary AlSi7Cu3Mg alloy is reported. While the Ti content ranges from 0.04 up to 0.225 wt.%, the cooling rate varies between 0.1 and 5.5 °C/s. Metallographic and thermal analysis techniques have been used to quantitatively examine the macro- and microstructural changes occurring with grain refiner addition at various cooling rates. The results indicate that a small AlTi5B1 addition produces the greatest refinement, while no significant reduction of grain size is obtained with a great amount of grain refiner. On increasing the cooling rate, a lower amount of AlTi5B1 master alloy is necessary to produce a uniform grain size throughout the casting. The combined addition of AlTi5B1 and Sr does not produce any reciprocal interaction or effect on primary α-Al and eutectic solidification. The grain refinement improves the plastic behavior of the alloy and increases the reliability of castings, as evidenced by the Weibull statistics.     

铸件充型凝固过程数值模拟研究

概述了铸件充型及凝固过程数值模拟研究的最新进展。采用并行计算技术，对压铸充型模拟过程数值计算方法进行了改进。提出了用动态膨胀收缩累积法来预测球墨铸铁缩孔缺陷，在球铁件上进行了缩孔形成的模拟和工艺改进。采用的有限差分与有限元相结合的方法模拟铸件应力场。实现了有限差分温度载荷到有限元模型的自动转换与传递，并采用有限元法计算出铸件应力场。