Modelling the pressure die casting process with the boundary element method: Steady state approximation

This paper describes a model which can predict the temperatures on the cavity surfaces of a die. The time varying boundary conditions are averaged so that the process can be modelled as a steady state problem. Since the model considers only thin components, it is reasonable to assume that the melt has totally solidified before ejection, and therefore the quantity of heat energy entering the die over the casting cycle can be estimated. This and other assumptions relating to the boundary conditions also enable the value of thermal resistance between the melt and the die to be estimated. Under certain conditions, subcooled nucleate boiling takes place in the cooling channels of the die. An iterative procedure is used to take account of this, which involves the repeated calculation of global heat transfer coefficients for the cooling channels, with the criteria that the total energy transferred through the channels is equal to that transferred due to boiling and convection. The boundary element method is used to predict the cavity temperatures. In die casting, only the temperatures on the cavity surfaces are of interest since the surface quality of a component is related significantly to the temperature distribution over the cavity. Since only thin components are considered herein, it is not necessary to model the solidifcation process and discretize the cast. These factors make the BEM ideally suited for the work described in this paper. To verify the model, the predicted temperatures for two components are compared with experimental values measured using thermocouples and a thermal imaging camera. It was found that there is fairly good agreement between the two sets of results.     

Modelling the pressure die casting process using a hybrid Finite‐Boundary Element model

In this paper an efficient three‐dimensional hybrid thermal model for the pressure die casting process is described. The Finite Element Method (FEM) is used for modelling heat transfer in the casting, and the Boundary Element Method (BEM) for the die. The FEM can efficiently account for the non‐linearity introduced by the release of latent heat on solidification, whereas the BEM is ideally suited for modelling linear heat conduction in the die, as surface temperatures are of principal importance. The FE formulation for the casting is based on the modified effective capacitance method, which provides high accuracy and unconditional stability. This is essential for accurate modelling of the pressure die casting process and efficient coupling to the BEM. The BE model caters for surface phenomena such as boiling in the cooling channels, which is important, as this effectively controls the manner in which energy is extracted. The die temperature is decomposed into two components, one a steady‐state part and the other a time‐dependent perturbation. This approach enables the transient die temperatures to be calculated in an efficient way, since only die surfaces close to the die cavity are considered in the perturbation analysis. A multiplicative Schwarz method for non‐overlapping domains is used to couple the individual die blocks and casting. The method adopted makes use of the weak coupling between the domains, which is a result of the relatively high interfacial thermal resistance that is present. Numerical experiments are performed to demonstrate the computational effectiveness of the approach. Predicted die and casting temperatures are compared with thermocouple measurements and good agreement is indicated. Copyright © 1999 John Wiley & Sons, Ltd.     

Computer aided foundry die-design

In the design of die casting dies, the object is to produce sound casting as cheaply and rapidly as possible. At the same time consideration must be given to suitable die size, locations of gating system, and selection of an appropriate die casting machine. Foundries in many cases encounter difficulties to ensure shortened lead-time in designing a new die for new product. A lot of estimations have to be done which use fundamentally based on previous experience and application of various mathematical and empirical equations. In this work, main die design procedures and related equations are presented in a logical way. A computer program is developed to estimate main die elements based on the geometry input of casting shape. After initial inputs have been given, the system does full calculations, optimizes selections, and lists main die element sizes. The program can present die characteristics and casting machine characteristics. From both characteristics the optimum die elements were optimized. Optimum filling time and gating dimensions among other elements of die are estimated. Cooling time, cooling channel locations, and flow rates relations are estimated and presented.     

低压铸铝件缩孔缺陷数值模拟与工艺改进

目的 为了避免铝合金盖板低压铸造过程产生的缩孔缺陷,对盖板结构进行分析。方法 采用数值分析技术,对盖板低压铸造过程进行数值模拟,将模拟结果与实际铸件缺陷进行对比,分析缩孔缺陷的产生原因。为消除铸件缩孔缺陷,在模具内增设冷却水道,并对该工艺进行模拟分析。结果 铸件凝固过程中,当合金固相率达到75%时,铸件特征位置处的补缩通道开始陆续关闭,形成了孤立液相区,导致凝固结束时形成缩孔缺陷。当模具内增设冷却水道后,铸件的缩孔体积分数由3.5%下降到0.8%。结论 通过在模具内增设合理的冷却水道,能加快铸件厚大部位的凝固速度,有利于实现顺序凝固,从而避免缩孔缺陷,提高铸件质量。     

基于遗传算法的低压铸造铝合金车轮工艺优化

为解决低压铸造铝合金车轮质量控制难度大的问题,采用遗传算法对工艺参数进行优化.基于铸造数值模拟结果,利用BP人工神经网络建立了铸造工艺参数与质量控制目标缩松缺陷和凝固时间的非线性关系,采用遗传算法实现了铸造工艺参数的优化.以某型低压铸造A356铝合金车轮为例,对浇注温度、上模温度、下模温度、侧模温度、模芯温度5个参数进行优化,得到的最佳工艺组合,可有效控制缩松缺陷和凝固时间.利用数值模拟结果、建立神经网络模型,采用遗传算法优化的方法,获得近似最优解,有助于优化低压铸造工艺.     

Prediction of die casting process parameters by using an artificial neural network model for zinc alloys

Pressure die casting is an important production process. In pressure die casting, the first setting of process parameters is established through guess work. Experts use their previous experience and knowledge to develop a solution for a new application. Due to rapid expansion in the die casting process to produce better quality products in a short period of time, there is ever increasing demand to replace the time-consuming and expert-reliant traditional trial and error methods of establishing process parameters. A neural network system is developed to generate the process parameters for the pressure die casting process. The system aims to replace the existing high-cost, time-consuming and expertdependent trial and error approach for determining the process parameters. The scope of this work includes analysing a physical model of the pressure die casting filling stage based on governing equations of die cavity filling and the collection of feasible casting data for the training of the network. The training data were generated by using ZN-DA3 material on a hot chamber die casting machine with a plunger diameter of 60 mm. The present network was developed using the MATLAB application toolbox. In this work, the neural network was developed by comparing three different training algorithms: i.e. error backpropagation algorithm; momentum and adaptive learning algorithm; and Levenberg-Marquardt approximation algorithm. It was found that the Levenberg-Marquardt approximation algorithm was the preferred method for this application as it reduced the sum-squared error to a small value. The accuracy of the developed network was tested by comparing the data generated from the network with those of an expert from a local die casting industry. It was established that by using this network the selection of process parameters becomes much easier, so that it can be used by a novice user without prior knowledge of the die casting process or optimization techniques.     

Einfluss der Porosität auf die Schwingfestigkeit von Proben und Bauteilen aus Aluminiumdruckguss

Influence of Porosity on Fatigue Strength of Aluminium Die‐Cast Specimens and Components Aluminium die‐casting is widely used in the automotive industry. Brackets, engine blocks, wheels and suspension arms etc. are typical applications. Due to the gases contained in the melt, more or less spherical pores arise in the cast components during the cooling procedure, usually with a higher concentration in the middle. The distribution of the pores depends both on the casting condition and on the form of the components and is stochastic. In the past, the simple way is often chosen for the fatigue life estimation of aluminium die‐casting: The fatigue strengths of the material, e.g. in the form of S‐N curves, are determined by an unnotched specimen which are then used in the fatigue calculation. However, the application of these data to real components with notches leads in many cases to fatigue life estimations which are by far too conservative. In the present paper a simple geometrical model of pores is presented. Using this model, the notch stress caused by a pore can be estimated. The behaviour of pores in a real component with notches will be treated. It will be shown that the mentioned conservative nature of the fatigue life prediction and the apparent notch insensitivity of aluminium die‐casting can be explained by the facts that the stress distribution in a real component, contrary to an unnotched specimen, is inhomogeneous and that the porosity density on the surface is often low.     

Rapid Tooling Die Cast Inserts Using Shape Deposition Manufacturing

Solid Freeform Fabrication (SFF) platforms such as stereolithography have successfully created rapid tooling injection molding inserts to produce plastic parts. However, the rapid tooling of viable aluminum die casting inserts presents more challenges because of the high pressures and temperatures associated with aluminum die casting. The objective of this research was to optimize the Shape Deposition Manufacturing (SDM) process to produce die cast inserts. SDM is a SFF process which can produce fully dense metal structures with Computer Numerical Control (CNC) accuracy. SDM laser-based metal deposition of tool steel powder was optimized for material properties by a Taguchi based design of experiment utilizing the process parameters of laser power, laser scanning rate, and powder feed rate. An insert was created using the optimized settings and run in a 600 ton die cast machine. The insert was used in the “as deposited” condition without any heat treatments. Over 150 aluminum pieces wsre produced without any signs of insert wear or fatigue.     

Thermal stresses in aluminium alloy die casting dies

The aim of this research is to analyze the influence of Aluminium Alloy die casting parameters, die material, and die geometry on in-service tool life. An innovative immersion testing apparatus is developed, at which Aluminium Alloy die casting is simulated. It enables controlled thermal fatigue cycling. Special specimens with different edge geometry and specimens with maraging steel welds deposited by Gas Tungsten Arc (GTA) welding are prepared. They are subjected to cyclic heating in bath of molten Aluminium Alloy 226 and cooling in bath of water-based lubricant. The specimens are continuously internally cooled with cold water. The microstructure, hardness profile, and the surface cracks developed are periodically analyzed after completion of a particular number of cycles. Temperature transients at different locations of the specimens are measured and used in calibration of finite element model (FEM). The computation of transient stresses is performed by developed FEM. The influence of immersion test parameters, material, specimen edge geometry, and thickness of maraging steel surfacing welds on thermal stresses is studied. To improve thermal fatigue testing efficiency, a specimen of particular geometry and immersion test parameters are developed based on finite element analysis. The results showed significant differences in produced thermal stresses for analyzed materials, test parameters, and edge geometries. Maraging steel is found to be superior material for die casting dies, due to generation of lower stresses.     

Development of an A356 die casting setup for determining the heat transfer coefficient depending on cooling conditions,gap size,and contact pressure

In this paper, the development of an experimental die casting setup to perform simultaneous in‐situ measurements of temperatures, air‐gap formation and contact pressures is presented. The derivation of the resulting heat transfer coefficients between mold and melt is also included. To take the influence of different cooling rates into account, an active die tempering is applied. For the implementation of this experimental setup, a special mold for a rotationally symmetrical test specimen is developed which incorporates the necessary measuring technique and can be adapted to different cooling conditions by means of exchangeable die inserts. Concluding, first results and the corresponding heat transfer coefficients are presented.     

Investigation on the transferability of algorithms for the numerical optimization of cooling channel design in injection molding on metal gravity die casting

The thermal mold design and the identification of a proper cooling channel design are primordial steps in the development of complex molds for injection molding. In order to find a suitable cooling channel system, a lot of effort is needed to avoid part warpage after solidification. In current research, a simulative procedure to optimize the cooling channel layout iteratively is being developed at the Institute of Plastics Processing. These algorithms are transferred to the metal gravity die casting process, which has several similar requirements to the mold. Effectively, the simulation is simplified to a heat conduction problem. Instead of water, high temperature resistant oil is deployed and the casted material is a A356 aluminum alloy instead of semi‐crystalline plastics. The algorithm is adapted to these changed boundary conditions and the calculation of the optimized heat distribution is performed. Aim of this procedure is the construction of a mold producing parts with less warpage than a conventional mold.     

薄壁陶瓷型铸造工艺的研究与应用

薄壁陶瓷型铸造工艺是将陶瓷型厚度控毛制在2～3mm,并采用金属国外衬套,辅以喷雾冷却等措施.大大提高了铸件凝固时的冷却速度.有效地减少脱碳层,同时细化晶粒,提高了铸件的使用寿食.由于陶瓷型厚度的减薄.大大减少了铸型的脱液收缩及由此而引起的铸型变形和型裂.提高了铸件的精度.应用此工艺成功地试制了汽车万向节热锻模及自行车曲柄辊锻模,工艺简单,成本低,创作周期短.模具使用寿命长.     

Zr基块体非晶合金玻璃形成能力与压铸成型性能研究

本文分析了熔体温度、铸造压力和化学成分对非晶合金玻璃形成能力(GFA)与压铸成型性能的影响,并探究两种性能的工艺关联。研究发现:随熔体温度升高,Zr基非晶合金GFA先提高后减小,且合金成分不同,熔体化学成分,局域原子团簇特征和合金实际冷却速率就不同,非晶合金GFA与玻璃稳定性也就存在差异。非晶合金的压铸成型能力随熔体温度和铸造压力升高不断提高,但与GFA存在相互限制的作用:当合金GFA较强时,熔体内原子堆垛密实,粘滞系数较高严重阻碍过冷液流动成形,且玻璃稳定性越好压铸成型性能越差;而当熔体温度过高,非晶合金GFA减弱,过冷液粘度降低时,才能快速提高非晶合金的压铸成型性能。因此,选择最佳的合金成分、优化工艺参数有利于非晶精密结构件成型。     

Development of an interactive simulation system for the determination of the pressure-time relationship during the filling in a low pressure casting process

An interactive computer simulation system has been developed in this study to aid the determination of the pressure–time relationship during the filling of a low pressure casting to eliminate filling-related defects while maintaining its productivity. The pressure required to fill a casting in a low pressure casting process can be separated into two stages. The first stage is to exer pressure to force the molten metal to rise in the riser tube up to the gate of the casting die, whichvaries from casting to casting due to the drop of the level of the molten metal in the furnace, whilst the second stage is to add an additional pressure to push the molten metal into the die cavity in away that will not cause much turbulence and have the proper illing pattern to avoid the entrapment ofgas while maintaining productivity.

One of the major efforts in this study is to modify the filling simulation system with the capability to directly predict the occurrence of gas porosity developed earlier to interactively determine the proper gate velocity for each and every part of the casting. The pressure required to ill the die cavity can then be obtained from the simulations.

The operation principles and the interactive analysis system developed are then tested on an automotive wheel made by the low pressure casting process to demonstrate how the system can aid in determining the proper pressure–time relations, the p–t curve, required to produce a sound casting without sacrificing productivity.     

Development of an interactive simulation system for the determination of the pressure–time relationship during the filling in a low pressure casting process

An interactive computer simulation system has been developed in this study to aid the determination of the pressure–time relationship during the filling of a low pressure casting to eliminate filling-related defects while maintaining its productivity. The pressure required to fill a casting in a low pressure casting process can be separated into two stages. The first stage is to exert pressure to force the molten metal to rise in the riser tube up to the gate of the casting die, which varies from casting to casting due to the drop of the level of the molten metal in the furnace, whilst the second stage is to add an additional pressure to push the molten metal into the die cavity in a way that will not cause much turbulence and have the proper filling pattern to avoid the entrapment of gas while maintaining productivity.One of the major efforts in this study is to modify the filling simulation system with the capability to directly predict the occurrence of gas porosity developed earlier to interactively determine the proper gate velocity for each and every part of the casting. The pressure required to fill the die cavity can then be obtained from the simulations.The operation principles and the interactive analysis system developed are then tested on an automotive wheel made by the low pressure casting process to demonstrate how the system can aid in determining the proper pressure–time relations, the p–t curve, required to produce a sound casting without sacrificing productivity.     

Two-phase flow boiling in small channels: A brief review

Boiling flows are encountered in a wide range of industrial applications such as boilers, core and steam generators in nuclear reactors, petroleum transportation, electronic cooling and various types of chemical reactors. Many of these applications involve boiling flows in conventional channels (channel size ≥ 3 mm). The key design issues in two phase flow boiling are variation in flow regimes, occurrence of dry out condition, flow instabilities, and understanding of heat transfer coefficient and vapor quality. This paper briefly reviews published experimental and modeling work in these areas. An attempt is made to provide a perspective and to present available information on boiling in small channels in terms of channel size, flow regimes, heat transfer correlations, pressure drop, critical heat flux and film thickness. An attempt is also made to identify strengths and weaknesses of published approaches and computational models of boiling in small channels. The presented discussion and results will provide an update on the state-of-the-art and will be useful to identify and plan further research in this important area.     

热流道辅助长筒薄壁形锌合金件压铸成型研究

ZA8锌合金以其卓越的耐冲击性能和尺寸稳定性,在精密电子类产品中得到广泛的应用.但这种合金材料的热流动性能较差,在其压铸成型中会造成很多的压铸缺陷.为减少薄壁ZA8锌合金铸件压铸生产中出现的流纹、缩松、气孔、起泡等缺陷,本文在采用传统的锌合金压铸流道实验的基础上,调节各种工艺参数,对其产生的压铸缺陷进行了统计分析.分析结果显示,流道结构是该铸件缺陷产生的主要原因,改用热流道辅助压铸成型.重新统计后发现,热流道能有效地减少铸件出现的各种缺陷,且材料利用率可以从30.6%提高到41.9%.在对比压铸传统流道和热流道特点的基础上,给出了热流道的设计和安装结构,详细阐述了压铸热流道技术的原理,分析了采用压铸热流道技术所带来的经济效益.研究得出,锌合金压铸热流道相比于传统的流道,在生产中具有加温时间快、产品成型周期短、温度压力损失小、材料利用率高、铸件缺陷低等优点.     

Nanofluids for power engineering: Emergency cooling of overheated heat transfer surfaces

The possibility of emergency cooling of an overheated heat transfer surface using nanofluids in the case of a boiling crisis is explored by means of synchronous recording of changes of main heat transfer parameters of boiling water over time. Two nanofluids are tested, which are derived from a mixture of natural aluminosilicates (AlSi-7) and titanium dioxide (NF-8). It is found that the introduction of a small portions of nanofluid into a boiling coolant (distilled water) in a state of film boiling (t _heater > 500°C) can dramatically decrease the heat transfer surface temperature to 130–150°C, which corresponds to a transition to a safe nucleate boiling regime without affecting the specific heat flux. The fact that this regime is kept for a long time at a specific heat load exceeding the critical heat flux for water and t _heater = 125–130°C is particularly important. This makes it possible to prevent a potential accident emergency (heater burnout and failure of the heat exchanger) and to ensure the smooth operation of the equipment.     

Process monitoring of partially reinforced metal-matrix composite components by acoustic emission

Partial reinforcement using a fibre bundle embedded in a part of a component has been investigated in the development of a composite piston for use in an internal combustion engine. A trial piston was fabricated by a casting operation in which molten aluminium was poured into a die containing an annular continuous fibre bundle. The most probable defects introduced during the manufacturing operation are (i) microcracks generated in the fibre bundle due to residual thermal stresses and (ii) imperfect impregnation of the molten aluminium into the fibre bundle. Acoustic emission measurements have been used as a technique to detect the presence of defects in the trial pistons. The acoustic emission was measured during cooling of the trial piston after casting. Microcracking in the fibre bundle during cooling could be detected. Imperfect impregnation of the aluminium into the fibre bundle could also be detected. The acoustic emission due to microcracking was found to be strongly dependent on the mechanical properties of the fibres, while the acoustic emission from incomplete impregnation was found to depend on process conditions. It is believed that acoustic emission measurements can not only be used for the detection of microcracks but can also be of value in the selection of fibre materials and in the adjustment of the process conditions.     

The effect of die geometry on the microstructure of indirect squeeze cast and gravity die cast 7050 (Al-6.2Zn-2.3Cu-2.3Mg) wrought Al alloy

The indirect squeeze casting process has been used to cast a 7050 (Al-6.2Zn-2.3Cu-2.3Mg) wrought Al alloy to near-net shape with excellent die replication. Defects which occur with gravity casting, in particular (1) shrinkage pipe, (2) macro-porosity and (3) hot-tearing, are largely removed by squeeze casting, although regions of macro-porosity can re-appear when thick sections are fed through substantially thinner sections. Squeeze casting results in a considerable refinement of microstructure compared to gravity casting due to a marked decrease in solidification time. The decrease in solidification time is caused by intimate contact between the pressurised melt and the die, which leads to an increase in the heat transfer coefficient. Decreasing the section thickness also results in a refinement of the microstructure due to a reduction in solidification time. This revised version was published online in September 2006 with corrections to the Cover Date.