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.     

Water analogue validation of numerical model for fluid flow during slow shot phase in die casting process

Abstract

Air that becomes entrapped in the molten metal during the die casting process has a major effect on the formation of porosity in horizontal cold chamber die castings. In this study, a three-dimensional computational fluid dynamics model was developed to study the flow patterns of liquid metal in the injection chamber, in which the moving boundary conditions of the plunger movement was considered in detail. According to the principle of die casting machines, a water analogue system was designed and built to investigate the slow shot process. A colour high speed camera was used to record the fluid flow patterns under different plunger movement profiles. The numerical simulation results agreed well with the water analogue experimental results, which validated the numerical model of shot processing in the cold chamber of the die casting process.     

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.     

高压开关罐体低压铸造凝固数值模拟研究

根据低压铸造凝固理论,利用Z-Cast数值模拟软件,针对高压开关罐体的低压铸造凝固过程进行了数值模拟研究.确定了合理的浇注系统和工艺参数,并进行了实际生产验证.结果表明:模拟结果与实际生产情况吻合很好,罐体的力学性能、气密性等满足DIN1725标准.     

Computer Simulation of the Process of Solidification in Gravity-diecasting

Abstract

A basic three-dimensional model has been developed to simulate the sequence of solidification in the gravity-diecasting process. The finite difference method was used to analyse the solidification process during all the casting cycles. The prediction of die temperature in the quasi-steady state was analysed using the boundary element method. The influence of die cooling on the solidification pattern of the casting was also investigated. Prediction of computer simulation on temperature profiles and location of shrinkage defects were in good agreement with those observed in experimental diecastings. Models of computer simulation developed in this study can be useful for the design and process control of diecasting.     

镁合金压室模拟工艺参数优化

压铸件产生的缺陷除了在凝固收缩的过程中容易出现,在冷室压铸机压射时,慢压射选取速度不当也是造成铸件缺陷的一个重要原因.通过对镁合金转向管柱支架的慢压射过程进行理论分析和计算机模拟辅助优化,得出加速度为0.6 m/s2、慢压射速度为0.4 m/s情况下,压射速度较为合理.能缩短生产周期,提高效率.     

Numerical simulation of die filling behavior of AZ91D in the semisolid process

In this work, numerical simulation of the die filling and solidification process of AZ91D semisolid alloy was investigated to produce a thin walled connecting rod demanding a high dimensional precision. The Carreau viscosity model was implemented to simulate the flow behavior of semisolid slurry during the filling. The fitted constants for the Carreau model were used to verify the simulation results. The predicted results from this model were in good agreement with the experimental results. Then the verified Carreau model was designed to predict die filling, casting defects and casting process. It was found that the predicted results had a good correlation with those in the experiment. The optimum parameters were obtained with a slurry temperature of 590 ℃, a die temperature of 250 ℃ and an injection velocity of 2 m/s.     

Porosity formation in columnar region of unidirectional solidified Al–4˙5Cu casting

Abstract

In this research, a vertical unidirectional solidification experiment was performed with Al–4˙5 wt-%Cu alloy. The microstructure of specimens obtained from the casting were analysed, which showed that the casting had columnar, columnar to equiaxed transition and equiaxed regions along upward direction. Porosity fractions in the specimens were measured quantitatively. It was found that there exists a decreasing distribution of porosity in columnar region from bottom chill and porosity with a high content in the specimen next to the chill. The general model coupling the Darcy's law with continuous equation was applied to the casting, however, the simulation results did not well agree with the experiment. Furthermore, hydrogen macrosegregation was incorporated into the model to predict the porosity formation, and effect of the macrosegregation was analysed and discussed. The results have shown that hydrogen macrosegregation has an important effect on porosity formation.     

The Horizontal Ohno Continuous Casting Process: Process Variables and Their Effects on Casting Stability

Abstract

Temperature profile measurements within a heated mould have been made during continuous casting of pure tin rod of 8.5 mm dia in an attempt to obtain an understanding of the influence of process variables on the position of the solidification front. It has been established that process variables such as casting speed, mould temperature and cooling position have a sensitive effect on the position of the solidification front. It varies linearly with casting speed for a given cooling position and mould temperature. The change in position of the solidification front in turn exerts a significant effect on the surface quality of the cast strand. It has been demonstrated that the solidification front should be brought well within the mould in order to obtain good dimensional and casting stability.     

箱体低压铸造过程的数值模拟

以计算流体力学和计算传热传质学作为理论基础,利用大型模拟软件ANSYS对箱体的低压铸造过程进行了数值模拟,分析了铝合金在低压铸造充型过程中的流场以及凝固过程中的温度、固相分数的分布情况,预测铸件可能出现的缺陷,为优化铸造工艺和模具设计提供参考。     

Microstructures of super high strength Al–10Zn–2˙5Mg–2˙3Cu–0˙14Zr alloys under electromagnetically stirred direct chill casting

Abstract

In this report solidification and homogenisation structures of a super high strength aluminium Al–Zn–Mg–Cu alloy were discussed. Al–9˙8Zn–2˙5Mg–2˙3Cu–0˙14Zr alloy was produced by low frequency electromagnetic casting. Solidification structure constituents of this alloy cast by low frequency electromagnetic casting was first discussed, in particular, boundary phases formed mostly by eutectic reaction were discussed: MgZn₂ included with Cu is a major eutectic phase in boundaries; T [Mg₃₂(Al,Zn)₄₉] included with Cu and Si, and S (Al₂CuMg) are located with boundaries as eutectics; Al₂Cu is crystallised as eutectic among which a part is found separately as globular; Al₇Cu₂Fe is crystallised as a facet shape. Second microstructure homogenised for 10 h at 460^u C was discussed. Constituents homogenised were MgZn₂ with Cu and Al₇Cu₂Fe with Zn.     

铝合金压铸件凝固过程中的瞬态温度场分析

建立了铝合金压铸件的有限元模型,确定了温度场模拟的初始条件和边界条件;结合材料的热物性,利用AN-SYS软件对A380.0铝合金压铸件凝固过程中的温度场进行了模拟,得到了铸件各点温度随时间变化的规律,模拟结果反映了铸造系统温度的变化过程。结果表明：利用ANSYS模拟温度场运算速度快、结果比较准确,为热应力分析提供了温度场条件,并给压铸优化设计提供了依据。     

铝合金压铸件充型和凝固过程的数值模拟

分析了压力铸造的工艺特点以及对压铸过程数值模拟软件系统的要求,利用基于有限元算法的铸造过程数值模拟软件对铝合金压铸件的充型和凝固过程的物理场进行了数值模拟,模拟结果对实际铸件的生产具有重要指导意义。     

Effect of die-casting conditions on viscoelastic behavior of Mg alloy

Creep and stress relaxation resistance of the most common die-cast Mg alloy AZ91D (Mg-9% Al-1% Zn) are influenced by both casting temperature and injection rate as well as by die temperature and porosity. Relationships between viscoelastic properties of Mg alloy at 150 °C and the parameters of die-casting technology are obtained and presented as simple contour plots (isograms). These properties can be improved by reducing casting temperature and increasing injection rate. When fabricating complicated thin-wall components, castability of the alloy can be diminished due to a fast temperature drop. In this case, casting temperature should be higher, whereas injection rate can be reduced. Porosity seems to be the most important characteristic of die-cast alloy microstructure affecting its viscoelastic properties. Coarse β-phase precipitates do not affect creep and relaxation characteristics of die-cast AZ91D alloy.     

压力对ADC12铝合金超低速压铸件组织及力学性能的影响

采用光学显微镜、扫描电镜、X射线衍射和能谱等显微分析技术,结合力学性能检测,研究超低速压铸条件下压力对ADC12铝合金铸件组织及力学性能的影响。试验结果表明,在超低速试验条件下,持压时间越长、增压延时时间越短,铸件性能越好;铸造压力导致初生α-相形态发生变化,但铸造压力有最佳值,压力较小时,增大铸造压力可使α-相细化,铸件性能升高,过大的铸造压力将导致α-相粗大,使铸件性能下降;在无增压条件下,仅采用较大的压射压力也可使α-相组织细小,获得较好的力学性能。     

压铸铝合金工艺与性能研究

以A380压铸铝合金为研究对象,设计了压铸工艺参数,通过密度测定、力学性能测试和金相观察,分析了压铸工艺参数对铸件性能的影响.研究表明,压力增加,铸件的密度和强度增加;压力一定时,压铸速度对铸件的力学性能有较大影响.使用改进的工艺,铸件抗拉强度达到365 MPa.伸长率达4.2％.     

Derivation and application of time step model in solidification process simulation

The heat transfer during the casting solidification process includes the heat radiation of the high temperature casting and the mold, the heat convection between the casting and the mold, and the heat conduction inside the casting and from the casting to the mold. In this paper, a formula of time step in simulation of solidification is derived, considering the heat radiation, convection and conduction based on the conservation of energy. The different heat transfer conditions between the conventional sand casting and the permanent mold casting are taken into account in this formula. The characteristics of heat transfer in the interior and surface of the casting are also considered. The numerical experiments show that this formula can avoid computational dispersion, and improve the computational efficiency by about 20% in the simulation of solidification process.     

Controlled semisolid microstructures and mechanical properties of Al–Mg–Si–Mn wrought alloy produced by D-SSF

Abstract

The semisolid microstructures and the mechanical properties of Al–1˙35Mg–1˙04Si–0˙67Mn alloy produced by deformation semisolid forming (D-SSF) process were studied. Fine α-Al₁₅Mn₃Si₂ compounds precipitate homogeneously during the homogenisation treatment. These compounds effectively inhibit the coarsening of recrystallised grains during heating to the semisolid temperature. When the liquid fraction is controlled to be ～23%, the complete die filling is not achieved. Therefore, in order to achieve good fluidity, it is necessary to control the liquid fraction to be more than 30%. The average grain size and the liquid fraction at the semisolid temperature influence directly mechanical properties. Therefore, the relationship among the average grain size, the liquid fraction at the semisolid temperature and mechanical properties was evaluated. Furthermore, the optimum semisolid microstructure was determined and the condition for the D-SSF process was established.     

Experimental and numerical modelling of the flow field in a CuxSny direct chill caster

Abstract

To investigate the influence of the casting speed on the flow field and the shape of the solidification front in an industrial bronze caster, numerical calculations have been performed and experimental flow field measurements were used to validate the numerical model. Both numerical and experimental model represented 1:1 the real caster geometry of 820 × 250 × 800 mm³. A steady-state calculation, considering solidification and turbulent flow, was performed. Furthermore, a 1:1 water model of the industrial bronze caster has been built and combined with a Particle Image Velocity (PIV) setup to measure the apparent flow fields. Amongst other parameters, the numerical model provided information on the influence of the casting speed on the solidification front shape. The water model gave the experimentalist the facility to adjust the shape of the solidification front to the one predicted by the numerical model and compare the resulting flow field with the numerical prediction. This work presents a comparison of the results of both numerical and experimental models for different casting speeds with the same numerical parameters and boundary conditions.