Use of vacuum for counter-gravity casting of metals

 The use of vacuum to fill molds to form investment castings in a countergravity casting fashion was started in the early 1970s. This paper describes how the processes work and the reasons for their advantages in cost, quality, and process control. Countergravity casting is being done in sand, permanent, and investment (lost wax) ceramic molds and alloys are cast in air and inert atmospheres. The process started with common steels, was extended to heat resistant superalloys, and recently even titanium alloys are being cast routinely using ceramic crucibles. The use of a unique mold design enables a great reduction in metal cast to yield a given part, thereby greatly reducing the energy use in making such castings. Computerized control of the rate of vacuum application enables complete control of filling rates, permitting casting of very high quality castings in very viscuous metals, such as metal matrix composites of aluminum. These processes are used to produce more than $200  million worth of castings annually and this will greatly increase in the next few years due to the production of automotive exhaust manifolds in stainless steel. Received: 21 July 1998 / Accepted: 10 September 1998     

Hot tensile and fatigue behaviour of zinc–aluminum alloys produced by gravity and squeeze casting

The tensile and fatigue properties of zinc–aluminum alloys (ZA-8, ZA-12 and ZA-27) in squeeze and gravity cast forms have been investigated. Tensile tests were conducted at ambient and elevated temperatures up to 150 °C. At low temperatures, the ultimate tensile strength and yielding strength of the squeeze cast alloys have been found to be superior those of the gravity-cast alloys, as the temperature increased they decreased. In the same way, Brinell hardness of the squeeze cast alloys were obtained at higher values than gravity castings. The fatigue tests were performed at a constant speed of 400 rev/min and under a number of stress levels ranging from 100 to 150 MPa. The fatigue behaviour results of the ZA alloys were similar to obtained from the tensile testing. The squeeze cast alloys exhibited good fatigue resistance in proportion to the gravity castings. Metallography examinations showed that the microstructure of the castings differed according to the method of casting used. It was considered that the mechanical properties of the alloys were affected from these micro-structural changes.     

Quantitative metallographic assessment of the electromagnetic casting influence on the microstructure of 7075 Al alloy

This study presents an attempt to obtain the better quality of an aluminum super-high strength alloy by application of electromagnetic field during the casting process. The conventional continuous casting process of aluminum alloys causes many defects, such as surface imperfections, grain boundary segregation, non-uniform grain size, and porosity. The better ingot surface along with the homogeneous fine-grained microstructure, and hence the better mechanical properties of the ingot, can be achieved by applying the electromagnetic casting process. The microstructure characterization, accompanied by quantitative metallographic assessment, reveals that it is possible to avoid or decrease many defects of as cast ingots during electromagnetic casting process. In this article, the microstructure of the samples of as cast 7075 aluminum alloy, obtained with and without electromagnetic field influence, was analyzed by optical microscope and the variation of key alloying elements content, i.e., Zn and Mg, through the ingot cross section was examined by chemical analysis. Besides, the microstructural parameters such as dendrite arm spacing, interdendritic space width, as well as eutecticum and intermetallic phases volume fraction, were measured using linear method. The electromagnetic field influence on the microstructure of the as cast 7075 Al alloy was evaluated based on measured quantitative metallographic data.     

ZL205A热壳精密铸造工艺研究

ZL2 0 5A合金以其优良的综合力学性能在航空航天工业中得到广泛的应用 ,但ZL2 0 5A合金的铸造工艺性能较差 ,一般适用于带冷铁的砂型铸造 ,在热壳精密铸造生产铸件时 ,对浇注工艺的要求很高 ,否则很容易出现疏松、缩松、气泡、表面缩陷等铸造缺陷 ,导致铸件质量和力学性能不合格 .本研究结合ZL2 0 5A的铸造性能特点 ,进行了ZL2 0 5A合金的热壳精密铸造研究 ,总结了浇注工艺参数和加压凝固工艺对ZL2 0 5A热壳精密铸件质量的影响 .     

Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329     

Development of ultra light magnesium-lithium alloys

Magnesium-lithium alloys are among the lowest density metallic materials. Addition of lithium, with a relative density of 0·53, in magnesium reduces the density of the alloy significantly. Furthermore, addition of nearly 11 wt.% lithium converts hexagonal close packed structure of pure magnesium to a body centered cubic lattice, markedly improving formability of the alloy. The development of these alloys, however, had been hampered due to the high reactivity of lithium and magnesium in the molten state and also, due to poor creep resistance and instability of mechanical properties at room temperature. In an attempt to indigenize these ultra light alloys for possible applications in Indian satellite programme, detailed research work was initiated in DMRL. The difficulties associated with producing sound cast ingots have been overcome by controlling melting and casting parameters of these alloys. Extensive work has been done on structure-property correlation of alloys with varying lithium content and minor alloying additions. Based on these work, advanced magnesium-lithium alloys have been developed with improved tensile properties, room temperature stability and creep resistance. Wrought products (plates/sheets) of magnesium-lithium alloy have been supplied to ISAC, Bangalore and are being used in their INSAT-2 programme. This paper describes the systematic studies carried out in the laboratory to indigenize these ultra light alloys.     

Magnesium – der Zukunftswerkstoff für die Automobilindustrie?

Magnesium – future material for automotive industry? Magnesium alloys show a very high potential in automotive applications as constructive metal, whereas the main focus lies on die cast parts. Electronic industry is the major commercial consumer for die castings besides the automobile industry. Room temperature applications like steering wheels and frame components in cars as well as mobile phone‐ or notebook housings are well established. These castings are produced with AZ‐ or AM‐magnesium alloys, which show good room temperature properties and a good castability. The great alloy development challenge in extending the use of magnesium cast alloys are application for higher temperatures. The application in powertrain components is considered to be the benchmark here. Besides alloy development there are also further research activities in development of casting processes. Semi‐solid processes like New‐Rheocasting (NRC), Thoxomolding ? or Thixocasting (TC) are adapted to the requirements of newly developed alloys. Not only cast alloys but also magnesium wrought alloys have moved to the centre of interest in the last decade. Alloy development for improving the formability on the one hand as well as process development in extrusion or rolling has to be done in order to find optimum parameters for deforming magnesium alloys properly.     

Alloy and process development of TiAl

The development of cast TiAl-based components for the automotive industry and the possibility of wrought components for applications in gas turbines are clear indications that these alloys are maturing as materials for engineering components. There are however problems in the cost of manufacture and in the properties of these alloys. In this paper it is argued that recent developments, which have resulted in a totally new microstructure, taken together with improved casting technology, open the way for wider application of these alloys. The new microstructure has been obtained by tempering massively transformed TiAl alloys so that a highly convoluted fine scale microstructure is obtained by simply heat treating cast samples. This fine microstructure can thus be obtained in cast components, without the need to add boron and problems associated with the extreme plastic anisotropy of fully lamellar samples are also eliminated.     

The microstructural control of cast and mechanical properties of zinc-aluminium alloys

The zinc-aluminium alloys containing 8, 12, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%Al, Zn-12%Al, Zn-27%Al alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%Al alloy, which provided the highest mechanical properties, was very suitable for the squeeze-casting technique. The mechanical properties were increased sharply in these squeeze-cast bars.     

Electroslag crucible melting of age hardening copper–chromium alloy

Abstract

The electroslag crucible melting process, developed on the well established principles of electroslag refining, has been used to cast age hardening Cu–0·5Cr alloys using copper scrap. Initial trials have demonstrated the viability of electroslag crucible melting for producing cast alloys with low impurity content and superior tensile properties. The castings produced by electroslag crucible melting are of high quality in terms of surface finish and casting defects. The cast alloy could be successfully drawn into wire of diameter 3 mm.

MST/3368     

Ti321钛合金轴箱铸件石墨型铸造工艺研究

目的 研究钛合金高铁轴箱铸件采用石墨铸型工艺时,铸件尺寸、成形及内部质量控制技术。方法 通过对高铁轴箱铸件的结构特点进行分析,针对铸件结构复杂、尺寸比较大、铸件成形比较困难、容易变形的特点,开展了铸型结构设计、铸件尺寸控制技术和铸件成形工艺等方面研究。采用多活块组合的石墨铸型,设计底注开放式浇注系统,利用Procast软件对高铁轴箱铸件的浇注方案进行了模拟分析。选用Ti321合金,设定合理的浇注参数,利用真空自耗电极凝壳炉对铸件进行浇注。结果 铸件成形良好,经过热等静压和后精整后的铸件,铸件内外部冶金质量和性能指标,满足客户标准要求,验证了方案的合理性。结论 铸件的计算机模拟对铸件工艺的设计提供了技术支撑,高铁轴箱铸件采用的石墨型浇注工艺比较合理。     

Preparation of Cu-Nb alloys for multifilamentaryin situ superconducting wire

A study has been made of two techniques, chill casting and consumable arc melting, for preparing ingots of Cu-Nb alloy for production of multifilamentary Nb₃Sn superconducting wire. It was found that Y₂O₃, ThO₂ and graphite all make excellent crucible materials for melting Cu-Nb alloys at 1850° C. Some difficulty was found with Nb segregation in chill-cast 5 cm diameter ingots. The consumable arc casting technique was shown to produce a uniform Nb dendrite distribution with little macrosegregation in 5 cm diameter castings and is regarded as having excellent potential for scale-up production of uniform Cu-Nb ingots of 25 to 30 cm diameter.     

Study on casting composite ingot of 4045/3004/4045 aluminum alloys

Two kinds of equipments were designed for casting composite ingots, and three‐layer ingots of 4045/3004/4045 aluminum alloys were prepared by direct‐chill (DC) semi‐continuous casting process, macro‐morphology, microstructure, temperature distribution near the interface, elements distributions and tensile strength of the interface were investigated. The equipment suitable for casting composite ingots was determined, and results show that solid supporting layer with uniform temperature distribution forms near the interface, which ensures the stable casting process. The two different aluminum alloys are well bonded. The criterion for temperature control of solid supporting layer is founded based on the relationship between the interface bonding strength and the temperature of solid supporting layer when contacted by 4045 alloy melt.     

Prediction of elastic properties of precipitation-hardened aluminum cast alloys

A methodology is proposed for predicting the elastic properties of precipitation-hardened alloys by combining different modeling techniques: the CALPHAD method, first-principles calculation, and elasticity models. The proposed procedure was applied to conventional aluminum cast alloys to predict their elastic moduli. The predicted Young’s moduli are in reasonable agreement with values reported in the literature, which verifies the potential applicability of the methodology to the development of high-stiffness aluminum cast alloys.     

Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters

For almost half a century the catastrophic failure of direct chill (DC) cast high strength aluminum alloys has been challenging the production of sound ingots. To overcome this problem, a criterion is required that can assist the researchers in predicting the critical conditions which facilitate the catastrophic failure of the ingots. This could be achieved at first glance by application of computer simulations to assess the level and distribution of residual thermal stresses. However, the simulation results are only able to show the critical locations and conditions where and when high stresses may appear in the ingots. The prediction of critical void/crack size requires simultaneous application of fracture mechanics. In this paper, we present the thermo-mechanical simulation results that indicate the critical crack size distribution in several DC-cast billets cast at various casting conditions. The simulation results were validated upon experimental DC-casting trials and revealed that the existence of voids/cracks with a considerable size is required for cold cracking to occur.     

Improved properties of A319 aluminum casting alloy modified with Zr

The beneficial effects of 0.15 wt.% Zr addition on mechanical properties and wear resistance of A319 aluminum casting alloy were investigated. The cast alloys were given a solutionizing treatment followed by artificial aging in the temperature range 175 to 235 °C for different period of times. Mechanical properties and wear behavior of the Zr-containing material were determined and compared to those of the base A319 alloy in both as-cast and age-hardened conditions. It is shown that minor addition of Zr results in the precipitation of Al₃Zr particles in the aluminum matrix. These particles are stable upon heating due to the low solubility of zirconium in aluminum matrix. The main effects of such particles are an increase in hardness, strength, quality index and wear resistance. This is very promising where these aluminum cast alloys are to be used at relatively high temperatures.     

Verbund‐Gießschmieden hybrider Aluminiumbauteile

In compound‐cast‐forging of hybrid aluminum parts the positive characteristics of casting and forging processes as well as the different materials are combined. This makes it possible to produce components with complex geometries and different local characteristics. Component areas with high complexity (e. g. with an undercut) are cast, areas that are exposed to high mechanical stresses, are forged. In the conducted investigations a preformed, massive formed semi‐finished part out of a wrought aluminum alloy was joined with a die‐cast aluminum alloy by casting and forming from the casting temperature in one cast‐forging process. The results of the studies show a good joining quality of cast‐forged components made of different aluminum alloys.     

Computer simulation of the solidification of cast titanium dental prostheses

Temperature distributions in titanium dental castings and molds are of great influence on the quality of titanium dental castings, and few efforts have been made in the numerical simulation of heat transfer in the process for casting titanium for dental applications. A finite difference scheme of the component-wise splitting method, which is unconditionally stable, was developed to solve the three-dimensional heat transfer problem for titanium dental casting during the investment cast and centrifugal cast process. 4 kinds of runner system were simulated and the computational efficiency were analyzed by the component-wise splitting method and the explicit finite difference method, the results shown that the techniques used in the current research can greatly improve the computational efficiency of the simulation system. The porosity predictions of 4 kinds of runner system were carried out with the simulation program. The predicted results were in good agreement with those of literatures.     

电磁铸造5182铝合金锭的组织与性能研究

采用电磁铸造技术铸造了易拉罐用5182变形铝合金，采用光学显微镜和扫描电镜分析了其显微组织，而且对其进行了均匀化处理和对比实验。结果表明，电磁铸造锭有更加良好的内部组织和优良的力学性能，电磁铸造试样的硬度大约是普通连续铸造试样的两倍，疲劳性能是普通连续铸造铸锭的3倍，电磁铸造铸锭还有良好的耐磨性，电磁铸造锭的优良性能得宜于电磁搅拌的作用使整个铸锭获得均匀细小的晶粒组织，电磁铸造技术是一种无模铸造技术，它依靠电磁力约束液体金属成型，液体金属不与铸模接触，铸锭表面光滑如镜；相反，普通连续铸造锭表面因存在振动痕和亚表面偏析等铸造缺陷，力学性能大大降低。     

Directional pressure quench casting of aluminium alloys

Abstract

A novel method of casting is described in which liquid metal in permeable moulds is subjected to high ambient pressure and quenching simultaneously. A small pressure quenching chamber was used and shapes of two different geometries cast into sand moulds. Two non-heat treatable alloys, LM6M and LM21M, comprised the raw material. The results show that increasing ambient pressure alone up to a value of 2 MPa (20 bar) can reduce porosity of castings, has a limited effect on tensile strength, and little effect on dendrite arm spacing. Casting under pressure with quenching increased tensile strength by up to 30% compared with conventional castings and virtually eliminated porosity. The dendrite arm spacing is also considerably reduced. Pressure quench casting is a potentially inexpensive means of improving the mechanical and microstructural properties and integrity of a wide range of aluminium alloys made by existing sand casting methods. For heat treatable alloys it offers the possibility of solution treating in the mould, thus eliminating one operation in a production process.

MST/3099