Numerical Simulation of the Liquid-Gas Interface Shape in the Shot Sleeve of Cold Chamber Die Casting Machine

A mathematical model has been developed to simulate the flow pattern of molten metal and to predict the liquid-gas interface shape in the shot sleeve of a cold chamber die casting machine during the injection stage. The flow pattern in the shot sleeve is known to be closely related to the extent of gas entrapment of molten metal in the sleeve during the injection operation. In this study, a Variable Spacing Even Mesh (VSEM) method is proposed to incorporate with a computational fluid dynamics technique, named SOLA-MAC, to simulate the flow pattern in the shot sleeve. SOLA-MAC can deal with free surface flow problems while the VSEM method is used to handle the problem where the space in the shot sleeve keeps decreasing as the plunger moves to push the molten metal. The model is then tested on the shot sleeve of a cold chamber die casting machine. Four plunger speeds are tested to demonstrate the effects on the flow pattern of molten metal in the shot sleeve. The critical speed found in this study is 38 cm/s and it is close to the reported critical speed under the conditions that the space between the plunger and the sleeve end is 5 cm in diameter and 30 cm in length, and the fill ratio is 50%. As the plunger speed is slower than the critical speed, the wave front propagates along the sleeve faster than the plunger and reflects against the end wall of the sleeve. The remaining air in the shot sleeve is entrained as the wave front enters to the gate. As the plunger speed is higher than the suggested critical speed, the melt is immediately pushed higher in front of the plunger and forms a surge. The surge traps air in the early stage of the injection process.     

基于数值模拟的压铸过程低速工艺优化

低速压射过程中冲头的运动分为封闭浇料口、匀加速和匀速低速三个阶段,各个阶段的工艺参数对压室内金属液的流动形态都有重要作用,进而影响铸件中气孔缺陷的产生。采用数值模拟技术来研究不同工艺参数下压室内金属液的流动形态,对低速工艺进行优化。针对压射过程冲头的运动特征,引入了移动速度、压力边界条件,开发了压室压射过程三维数值模拟程序。结合慢压射临界速度理论,模拟了不同料口封闭速度、加速度和慢压射速度下压室内金属液波形的发展。模拟结果表明采用优化的工艺参数可以降低压室内的卷气。     

压室慢压射过程流场模拟

气孔是压铸生产中的常见问题,导致这一缺陷的原因之一就是在慢压射过程中压室内的金属液流动卷入了气体.文中在压铸流场模拟的基础上,增加了移动边界条件处理冲头的移动过程,模拟了慢压射过程中压室内流体的流动形态.模拟结果与理论计算值及国外商品化软件的计算结果进行了比较,均比较吻合.     

On the optimum plunger acceleration law in the slow shot phase of pressure die casting machines

The objective of this paper is to analyse a plunger acceleration law that is expected to minimize air entrapment in the slow shot phase of pressure die casting in horizontal cold chambers, and thus to reduce porosity in manufactured parts. The study is carried out using results from an analytical model of the flow of molten metal in the shot sleeve, which is based on the shallow-water approximation, and whose predicted optimum acceleration parameters are in good agreement with available experimental results. The results for the surface profiles of the wave formed during plunger movement using plunger acceleration laws which are typically used in pressure die casting are compared with those corresponding to the proposed law. Some analytical predictions for the wave profiles and for the mass of trapped air are compared with numerical results obtained from a finite-element code, which solves momentum and mass conservation equations. The limiting values of the initial filling fraction required for appropriate operating conditions are determined for wide ranges of acceleration parameters and pouring hole locations.     

压室液态金属流动耦合温度场三维数值模拟

压铸过程中,压室内液态金属卷入的气体是影响铸件质量的原因之一.针对压室压射过程,引入冲头的移动规律以及流动过程中热量传递的计算,实现并开发了压室压射过程的流场耦合温度场的数值模拟程序.采用所开发的模拟程序对压室金属流动及传热过程进行了模拟,模拟结果与FLOW-3D的计算结果以及临界速度理论作比较,两者均吻合.采用压室模拟能够优化冲头的运动规律,使卷气最小.     

New ceramic injection systems for hot chamber die casting of aluminium alloys

Abstract

The quality of hot chamber die cast aluminium alloys can vary, due to differences of casting conditions, in particular the casting pressure. This is caused by the plunger tip galling during the casting process. To solve this problem, a new injection system is proposed using a flexible movable plunger tip and an insert type shot sleeve. With this system, the plunger tip can be turned around and slid into the piston holder, which facilitates smooth insertion of the plunger into the shot sleeve. The proposed insert type sleeve can be made with a strong structure. Moreover, even if the sleeve is partially fractured, it can easily be replaced. With the proposed injection system, high quality hot chamber die cast components can be created. In addition, the maintenance cost of the injection system is dramatically reduced.     

Numerical simulation of molten metal flow under vacuum condition in high pressure and low pressure die casting process

Abstract

The vacuum analysis algorithm was developed to simulate the total system of high pressure die casting process including vacuum vent, cavity and plunger area. The various vacuum degrees (760, 650, 500, 250 and 60 mmHg) were artificially applied in cavity. The filling behaviours of molten metal under the applied vacuum conditions were simulated and compared with those of experiment. The filling amount in cavity was increased with the increase of applied vacuum pressure during partial shot experiments. The simulated filling behaviours of molten metal were relatively well agreed with those of experiment. Through the results of fluid flow simulation, the relationship of filling length and filling velocity with the variation of vacuum pressure was analysed respectively. And it applied to a real die casting product and the internal gas quantity of product was significantly reduced by modification of vacuum gate system.     

压室压射对压铸充型流场影响的研究

为了考察压室压射过程对压铸充型过程金属液流动形态的影响,基于SOLA-VOF算法开发了包含压室在内的压射全过程充型模拟程序和料饼处直接充填模拟程序。并采用这两套程序分别模拟了阶梯块压铸件的充型过程,结果表明:浇道和型腔内的初始流动形态会随压室慢压射过程工艺参数的变化而改变,采用压射全程模拟更能准确的反映金属液在压室内、浇道内和型腔内的流动形态。     

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.     

Numerical simulation of complex multi-phase fluid of casting process and its applications

In simulating complex multi-phase fluid some key problems must be taken into account, namely, turbulence problem, surface tension, heat transfer, phase-change etc. To model the multi-phase fluid exactly, these key problems must be dealt with properly at the same time. In this paper, after reviewing previous works on fluid field simulation, a mathematics model was set up to describe multi-phase fluid, including turbulence, surface tension, heat transfer, phase-change problems etc. Based on a Fi…     

Thermal and flow modelling of ladling and injection in high pressure die casting process

Air entrapment and premature solidification in the shot sleeve of the cold chamber high pressure die casting process are problems affecting the casting quality. These issues are addressed from several viewpoints, including heat loss during metal ladling, turbulence generation and gas escape during pouring, wave formation and propagation during metal pouring and injection, and heat loss and solid creation in the shot sleeve during metal injection. Measurements and simulations have been conducted for a typical industrial case where the fill ratio of the shot sleeve is about 30%. It has been found that the settling time of the aluminium alloy in the shot sleeve has a profound influence on the air entrapment. The settling time also has a negative effect on the metal quality the solid formation (premature solidification or cold flakes) in the shot sleeve. Fast pouring and quick injection (no settling time) are recommended as a good approach to minimise the air entrapment and cold flakes in the shot sleeve in this particular case. IJCMR/474     

压铸镁合金缺陷带的组织特征及形成机理

研究了压铸镁合金缺陷带的组织形貌及分布特征,建立缺陷带与压室预结晶(ESCs)、气缩孔及压铸工艺参数之间的对应关系,在此基础上探讨了缺陷带的形成及演化机理。结果表明,压铸镁合金截面凝固组织以缺陷带为界可划分为3个有明显组织特征差异的区域。随着压铸低速速度提高,镁合金凝固组织中ESCs含量逐渐降低,而缺陷带组织愈加明显并向压铸件中心靠拢,缺陷带宽度减小,其内部孔洞更加聚集;高速速度越大,压铸镁合金凝固组织中ESCs越分散,缺陷带向压铸件中心靠拢,压铸在无高速速度情况下,镁合金凝固组织横截面出现双缺陷带现象。压铸充型过程中金属液流的形态决定了缺陷带的分布位置和发展趋势,在高速金属液的剧烈冲刷及增压压力的作用下,靠近金属液流外轮廓的晶粒发生破碎或转动,在晶粒间形成大于剩余金属液体积的间隙,随着凝固的进行,形成沿液流轮廓分布孔洞聚集的缺陷带组织。     

Comparison of mechanical properties of die cast aluminium alloys: cold v. hot chamber die casting and high v. low speed filling die casting

Abstract

In this paper, the mechanical properties of die cast aluminium alloys made by various die casting technologies were examined. To create high quality aluminium alloy die castings, two die casting processing technologies were employed. These were (a) ultra slow speed filling cold chamber die casting and (b) high speed hot chamber die casting. Significant improvements of the fatigue and mechanical properties were obtained for both die casting systems compared to the normal high speed cold chamber die casting technique. By comparing ultra slow die casting with hot chamber die casting, it was found that the fatigue and mechanical strengths from hot chamber die casting were higher than those for ultra slow filling die casting. The differences in material strength were attributed directly to the material properties, e.g. microstructural morphology and internal defects. Spherical fine dendritic cells in the hot chamber die casting sample gave rise to high fatigue crack growth resistance; the low crack growth resistance for cold chamber die cast aluminium is mostly due to the growth of aluminium rich α phase and the presence of eutectic silicon fibres. The fatigue strength was also related to the number of internal defects, e.g. the lower the defect rate on the fracture surface, the higher the fatigue resistance and mechanical strength. The characteristics of the principal internal defect were different depending on the die casting technology: this showed fine porosity for hot chamber die casting but solidification shrinkage and the scattered chill structure for slow and high speed cold chamber die castings. The reasons for the change of material strength were therefore influenced by the die casting process.     

Effects of process parameters on morphology and distribution of externally solidified crystals in microstructure of magnesium alloy die castings

During the cold-chamber high pressure die casting(HPDC) process, samples were produced to investigate the microstructure characteristics of AM60B magnesium alloy. Special attention was paid to the effects of process parameters on the morphology and distribution of externally solidified crystals(ESCs) in the microstructure of magnesium alloy die castings, such as slow shot phase plunger velocity, delay time of pouring and fast shot phase plunger velocity. On the basis of metallographic observation and quantitative statistics, it is concluded that a lower slow shot phase plunger velocity and a longer delay time of pouring both lead to an increment of the size and percentage of the ESCs, due to the fact that a longer holding time of the melt in the shot sleeve will cause a more severe loss of the superheat. The impingement of the melt flow on the ESCs is more intensive with a higher fast shot phase plunger velocity, in such case the ESCs reveal a more granular and roundish morphology and are dispersed throughout the cross section of the castings. Based on analysis of the filling and solidification processes of the melt during the HPDC process, reasonable explanations were proposed in terms of the nucleation, growth, remelting and fragmentation of the ESCs to interpret the effects of process parameters on the morphology and distribution of the ESCs in the microstructure of magnesium alloy die castings.     

压室内金属液流动形态的二维数值模拟

在普通压铸流场模拟的基础上,通过建立冲头、压室及金属液之间的运动学与动力学控制方程,以及三者之间的边界条件与初始条件,模拟了压室内金属液在冲头推动下的二维流动形态.分析了冲头的不同运动参数对压室内金属液卷气的影响,为实际压铸中慢压射速度的选择提供了参考.     

大型压铸件慢压射工艺优化研究

对于大型复杂压铸件,压铸生产过程中金属液流动复杂,容易在压射室内形成卷气,进而致使压铸件产生气孔等严重缺陷.本研究针对大型铝合金变速箱壳体零件的慢压射过程进行理论分析和仿真模拟,并以理论分析得到的临界慢压射速度作为参考,结合数值模拟软件的模拟结果分析得出了最优的慢压射工艺参数.模拟结果显示,通过优化慢压射冲头速度、冲头加速度等工艺参数,可以有效地减少甚至避免金属液在压射室内形成卷气和困气,有利于提高压铸件的质量,实际生产效果与模拟结果相符.     

The effect of die casting machine parameters on porosity of aluminium die castings

The primary objective of this research is to investigate the effect of machine related parameters on porosity formation in the aluminum die casting process. The amount and distribution of porosity in die castings was examined in relation to plunger velocity during the 1st phase, fast shot set point, plunger velocity during the 2nd phase, die cavity filling time and multiplied pressure during the 3rd phase.

Standard statistical analysis was performed in order to identify the effect of each of the considered parameters and parameter interactions on porosity formation. It was found that, among the parameters considered, multiplied pressure, plunger velocity in the 2nd phase and die cavity filling time have the most significant effect on porosity formation. Using optimal die casting machine parameter levels, the optimum porosity value of aluminum alloy die-castings was predicted. The results were confirmed by further experiments.     

Feasibility of semi-solid die casting of ADC12 aluminum alloy

The feasibility of semi-solid die casting of ADC12 aluminum alloy was studied. The effects of plunger speed, gate thickness, and solid fraction of the slurry on the defects were determined. The defects investigated are gas and shrinkage porosity. In the experiments, semi-solid slurry was prepared by the gas-induced semi-solid (GISS) technique. Then, the slurry was transferred to the shot sleeve and injected into the die. The die and shot sleeve temperatures were kept at 180 °C and 250 °C, respectively. The results show that the samples produced by the GISS die casting give little porosity, no blister and uniform microstructure. From all the results, it can be concluded that the GISS process is feasible to apply in the ADC12 aluminum die casting process. In addition, the GISS process can give improved properties such as decreased porosity and increased microstructure uniformity.     

冷室压铸机最大空压射速度的意义

冷室压铸机最大空压射速度表示压铸机所能提供的压射能量,也表示在较短充型时间内能向型腔内充填大容量的金属熔体,以及与模具特性一起表示工艺灵活性.