Influence of vacuum counter-gravity filling on reliability of 2L99 aluminium investment castings

Abstract

The reliability of 2L99 (Al–7Si–0˙4Mg) alloy investment castings produced using gravity and vacuum counter-gravity mould filling processes has been compared. Large numbers of bars have been cast and tested in four point bending and the scatter in their strengths characterised using the Weibull statistical technique to give the Weibull modulus λ. Both top and bottom gated designs of gravity poured moulds produced unreliable castings (λ=17 for both). This contrasted with previous work which showed much higher moduli for bottom gated moulds (49 and 54); this discrepancy has been attributed to poor control over mould filling and evidence for this was wide variation in filling times in the present work. The standard CLA vacuum counter-gravity filling process significantly improved the reliability (λ=29), but this was marginally reduced (λ=27) when a filter was introduced into the sprue. Possible reasons for this are discussed. A modification to the mould design further increased the reliability (λ=44) but this was again decreased (λ=34) by the addition of a filter. The most reliable castings were produced by the recently developed centrifugal counter-gravity (C3) process, which provided a Weibull modulus of 50.     

Dry vacuum pumping technology outperforms oil-sealed pumps for investment casting

Investment casting is a demanding application for a vacuum pump. Indeed, many traditional oil-sealed pumps cannot maintain the required vacuum performance and have fallen by the wayside, due to the high wear on machinery inherent in this dusty environment. However, in a 20-month trial at an investment casting company in South-West England, a dry pumping system has shown greater reliability, improved vacuum performance and reduced maintenance downtime over the previous oil-sealed technology.     

真空铸造技术的研究现状

通过分析真空铸造在造型、充型、凝固过程中产生的现象，讨论了真空密封造型、实型真空铸造、真空吸铸、真空低压铸造、真空差压铸造等技术的现状，针对国内外发展现状提出今后真空熔模吸铸的研究与发展的方向。     

Fabrication of modeling platform for fused deposition modeling using vacuum casting

This study presents a cost‐effective approach for rapid fabricating modeling platforms utilized in fused deposition modeling three‐dimensional printing system. A small‐batch production of modeling platforms about 20 pieces can be obtained economically through silicone rubber mold using vacuum casting without applying the plastic injection molding. The air venting systems is crucial for fabricating modeling platform using vacuum casting. Modeling platforms fabricated can be used for building rapid prototyping model after sandblasting. This study offers industrial value because it has both time‐effectiveness and cost‐effectiveness.     

Some advances in the casting of metals

Casting is one of the oldest metal shaping methods used by engineers who, therefore, sometimes do not maintain their awareness of progress in the technology. This article provides a rapid update of progress made in the last fifteen years in moulding and casting procedures and the treatment of metal itself.     

Mathematical modelling of dimensional accuracy in vacuum assisted casting

Vacuum assisted casting (VAC) is one of the prototype making processes used to generate replicas of plastic components (PC) for batch production activities (like: investment casting applications). In the present study, the outcome of the Taguchi model has been used for developing a mathematical model for dimensional accuracy (DA) using Buckingham's π-theorem for VAC of PC. Four input parameters, namely material type, mixing ratio (hardener: softener), de-mould time and de-mould temperature, were selected to give output in the form of DA. This study will provide the main effects of these variables on DA and will shed light on the mechanism of DA in VAC for replicas of PC. The comparison with experimental results will also serve as further validation of the model.     

有色金属真空冶金进展

回顾了近三十年来有色金属真空冶金方面的一些进展,扼要说明其作用和应当注意发展的若干方面.     

Gas-plasma flows under femtosecond laser ablation for metals in vacuum

New information on particle distribution over velocities, the mean-mass velocity, and the degree of monochromaticity of gas-plasma flow under femtosecond (τ ～ 45 fs, λ ～ 800 nm) laser ablation is obtained for several metals (Ti, Zr, Mo, Nb, Cu) in vacuum (p ～ 5 × 10^?2 Pa) by means of combined interferometry.     

Platinum metals coatings by vacuum deposition processes

The technique of evaporation by electron bombardment was used to obtain thick coatings of Pd, Pt and Ir on substrates of Ni, mild steel, stainless steel, Ti, Ta, W and graphite. Vapor condensation was effected in three different ways: under vacuum, under a residual pressure of argon and by ion plating (polarization voltage V_c on the substrates 5 kV or less). The temperature range of the substrates was between 100 and 800°C. The coatings were examined for adherence to the substrate and for morphological characteristics. The results obtained allowed us to coat shaped substrates with the platinum metals for various applications.     

钢基耐磨复合材料的真空实型铸造工艺研究

采用真空实型铸造工艺来制备陶瓷颗粒增强的钢基表面复合材料是一种制造选择性增强耐磨复合材料的先进工艺.本文研究了复合材料的制备过程,耐磨涂料配制及涂刷工艺和材料的浇注工艺,并在金相显微镜上对复合材料的组织进行研究.对复合材料微观组织结构分析表明,碳化硅颗粒表面固溶于基体材料组织中,粒形基本保持完整,分布均匀,与基体构成冶金结合.     

真空熔铸工艺对铜铬合金组织与性能的影响

为减少熔炼时合金元素的烧损,采用真空感应熔炼工艺制备了低铬铜铬合金.通过扫描电镜、光学显微镜和能谱分析等手段,研究了真空环境下不同浇注工艺对合金铸锭组织、缩孔及成分偏析的影响.研究结果表明:利用真空直接加铬炼制低铬铜铬合金,采用合适的熔炼和浇注工艺,可以较精确控制合金成分;在低的浇注温度和大冷却速度下可以获得几乎无宏观偏析,铸造缺陷很少的铜铬合金铸件;铸件500℃退火3 h后,硬度可达141HV,电导率提高到86%IACS.     

Preparation of metals in ultra high vacuum by electromagnetic levitation

The conditions for dynamic instabilities of electromagnetically levitated metal specimens have been investigated. Methods which eliminate the instabilities have been developed and were used to decarburize and degas a levitated niobium sphere in ultra-high vacuum for a period of 16 hours.     

Mold-filling characteristics and solidification behavior of magnesium alloy in vacuum suction casting process

A novel semi-solid processing technique, called new vacuum suction casting (NVSC), is used to manufacture high-quality components of AZ91D Mg alloy directly from a liquid metal. The resulting apparent morphologies and microstructures of castings are characterized in detail and linked to the corresponding mold-filling behavior and subsequent solidification behavior. It is revealed that the semi-solid metal (SSM) with higher viscosity can be caused to fill the mold with “solid-front fill”, as compared with the liquid metal “spraying” in the conventional vacuum suction casting (CVSC) process. The smooth filling achieved in the NVSC process diminishes some disadvantages inherent for the CVSC sheets, and generates castings with better surface finish and structures with high integrity. The microstructure of the CVSC sheet consists of the fine and homogeneous supersaturated α-Mg solid solution due to the extremely high cooling rate. In the NVSC microstructure, the “preexisting” primary solid particles, with the morphology of near-globules or rosettes, disperse in the homogeneous matrix consisting of fine near-equiaxed secondary α-Mg grains and fine precipitates of β-Mg₁₇Al₁₂ intermetallics. In addition, owing to rapid solidification, the volume fraction of the β phase in the sheets obtained by both the processes is much lower than that in the as-cast ingot.     

Dissimilar joining of Al/Mg light metals by compound casting process

“Compound casting” was used for production of lightweight Al/Mg couples. In order to prepare the Al/Mg couples using this process, each of the aluminum and magnesium molten metal was cast around solid cylindrical inserts of the other metal. After solidification, the interfacial microstructure and shear strength of the joint were studied. Characterization of Al/Mg interface by an optical microscope and scanning electron microscope showed that in the case of casting aluminum melt around a magnesium insert, a gap is formed at the interface, while in the process of casting magnesium melt around an aluminum insert, a relatively uniform interface composed of three different layers is formed at the interface. The results of the X-ray diffraction, energy dispersive X-ray spectroscopy, wavelength dispersive X-ray spectroscopy, and microhardness analysis of the interface showed that these three layers are mainly composed of high-hardness Al–Mg intermetallic compounds. Furthermore, it was found that the thickness of the interface is not constant throughout Al/Mg joint, and varies gradually from 190 μm at the bottom to 140 μm in the middle and 50 μm at the top of the sample. The results of shear strength tests obviously showed that the strength of the interface depends on the interface thickness and increases by decreasing the thickness of the interface.