Oxide bifilms in aluminium alloy castings – a review

Abstract

Aluminium alloy castings are most widely used in automobile industry because of their light weight, better castability and improved properties. The liquid aluminium surface easily oxidises during melting, transferring and pouring operation which may entrain oxide films into the casting. Research work has shown that the entrainment of this surface film and formation of bifilms in castings appear to be the source of most of the casting defects leading to a significant reduction in the mechanical properties of aluminium alloy castings. In this paper, the phenomenon of formation of oxide bifilms in aluminium alloy castings, effect of these bifilms on casting properties and their assessment techniques are discussed. For enhancing the quality of casting, research should focus towards development of process techniques for healing of bifilms in liquid metal during solidification.     

The behaviour of double oxide film defects in Al–4.5 wt% Mg melt

The possibility of bonding of the two layers of a double oxide film defect when held in a liquid Al–4.5 wt% Mg alloy was investigated. The defect was modelled experimentally by maintaining two aluminium oxide layers in contact with each other in an Al–4.5 wt% Mg liquid alloy at 750 °C from 2 min to 16 h. Any changes in the composition and morphology of these layers were studied by SEM, EDX and XRD. The results showed that in contrast to previous studies reported in the literature on Al–0.3 wt% Mg in which the two layers bonded to each other after a holding time of 5 h, no bonding took place between the two oxide layers even after a holding time of 16 h. Based on the comparison between the two studies, it was concluded that a transformation involving rearrangement of atoms at the interface between the two oxide layers is essential for the bonding to take place between the two oxide layers. This criterion could be used to predict the bonding behaviour of oxide film defects when held in different liquid aluminium alloys, or when subjected to a HIPping process.     

Ti-3Al-2.5V钛合金精密铸造浇注系统优化研究

利用数值模拟方法,研究了重力场和立式离心场下,不同浇注系统（顶-底、螺旋、径向和树形系统）Ti-3Al-2.5V钛合金铸件的充型和凝固过程及缩孔缺陷分布规律,并通过实际铸造实验,对铸件的机械性能及缩孔缺陷进行了检测。结果表明,重力场下的顶-底浇注系统氧含量波动范围大,不同浇注系统对合金力学性能影响不大,缩孔的模拟结果与实际铸造情况都吻合较好。相比而言,重力场下由于金属液的紊流更容易形成夹杂、钛豆、缩孔等缺陷,而离心场下由于离心力的作用,形成缩孔等缺陷的数量相对较少。     

铝(镁)合金消失模铸造近净成形技术研究进展

阐述了铝(镁)合金消失模铸造技术的研究现状,着重介绍了铝(镁)合金消失模铸造在金属液充型、振动凝固、压力凝固以及消失模壳型铸造等技术方面的最新研究进展。研究表明,铝(镁)合金在消失模铸造过程中,需重点解决针孔、缩松等缺陷,提高液态合金的充型能力和铸件的力学性能;通过采用振动凝固和压力凝固的手段,可以提高金属液充型能力、细化组织、提高组织致密性,明显提高铸件力学性能。真空低压消失模壳型铸造技术,可以解决普通消失模铸造易于出现的孔洞和夹杂等缺陷以及浇不足和浇注温度高等问题,是一种生产复杂薄壁高质量铝、镁合金精密铸件的新方法。     

Gating System Design for a Magnesium Alloy Casting

The gating system of a cylindrical magnesium casting has been designed by using multiple objective optimiza- tion and Taguchi method.Mold filling and solidification processes were simulated by using MAGMASOFT（？）. The simulation results indicate that the gating system design has a significant effect on the quality of magne- sium castings.In an effort to obtain the optimal design of gating system,the signal-to-noise （S/N） ratio was used to analyze the effect of various gating designs on cavity filling and casting quality by using a weighting method based on the design of an orthogonal array.Four gating system parameters,namely,ingate height, ingate width,runner height,runner width,were optimized with a consideration of multiple objective criteria including filling velocity,shrinkage porosity and product yield.     

铝合金汽车转向节精密铸造工艺研究

目的 对某铝合金汽车转向节的精密铸造工艺进行设计与优化研究,以得到合格的铝合金汽车转向节的精密铸造工艺方案。方法 结合铝合金转向节铸件的结构特征、铸件材料特性和铸造经验,在转向节铸件主体部和鹅颈部各开设一个内浇口,设计了铝合金转向节初始浇注方案;通过在初始工艺方案中铸件缺陷较严重的区域设置补缩冒口、在铸件顶部增设排气道等措施给出了铝合金汽车转向节的优化浇注方案,基于ProCAST软件建立了铝合金转向节精密铸造2种浇注方案的有限元模型,对铝合金转向节精密铸造的充型过程、凝固过程及缩孔缩松特性进行了数值模拟与分析。结果 铝合金转向节铸件初始浇注方案的充型过程相对稳定流畅,铸件在凝固过程中有孤立液相区的形成,完全凝固后铸件中间部位存在大面积缩松缩孔缺陷;优化浇注方案能够控制金属液的流动、充型顺序及凝固特性,铸件的整个凝固过程基本呈中间对称分布,最后凝固区域位于补缩冒口内部,最大缩孔缩松率控制在2%以下。结论 优化浇注方案的设计合理且有效,能够有效地消除铝合金转向节铸件的缺陷。     

Optimised running system design for bottom filled aluminium alloy 2L99 investment castings

Abstract

This research has evaluated the effect of surface turbulence introduced during the filling of moulds on the reliability of Al-7Si-Mg alloy (2L99)investment castings. Four different running systems were designed to provide different amounts of surface turbulence: top filled, uncontrolled bottom filled, and controlled bottom filled with and without a filter. Computer modelling has been used to simulate the filling of the different designs and the results have been compared with the actual flow behaviour established using real time X-ray radiography. Castings have been produced using the different running systems, tested in four point bending, and the results analysed using the Weibull statistical technique. It has been shown that top filling produces significant turbulence, which results in the least reliable castings (Weibull modulus of 25.5). There was no significant difference between this and the Weibull modulus of 27 found for uncontrolled bottom filled systems. This indicates that a poorly controlled bottom filled system is no better than a top filled system. Both were inferior to the controlled bottom filled systems without and with filters, which had Weibull moduli of 38 and 54 respectively. SEM examination and oxygen analysis of representative fracture surfaces has provided supporting evidence for the important role that oxide films played in reducing properties.     

镶铸管铸造工艺铸件无损检测分析

目的 研究镶铸工艺成形钛合金铸件镶铸部位冶金缺陷形成原因及无损检测表征,为镶铸工艺铸件的无损检验判定提供依据,并为此类产品冶金质量评价提供依据,建立质量评价体系标准。方法 通过对采用镶铸钛管工艺成形铸件的镶铸部位进行X射线检测及微观形貌、成分检测,结合铸造成形工艺的数值模拟过程进行对比分析。结果 预埋钛管表面在铸造工艺过程中出现表面氧化,浇注过程中,在镶铸部位存在多股金属液充型交汇,镶铸管部位结构融合效果存在差异,存在未融合现象。结论 采用镶铸工艺制备的铸件在无损检测方面存在不同于传统的冶金评判特征,镶铸部位的未融合缺陷可以通过X射线检测发现,在X光底片显示为沿钛管界面的黑线。     

铝合金曲轴后端盖压铸模浇注与排溢系统优化设计

为满足某发动机曲轴后端盖铸件高质量的外观与密封要求,针对该铝合金零件进行了高压铸造模具的浇注与排溢系统优化设计。首先根据铸件形貌和尺寸特征进行浇道、内浇口和排溢槽的理论设计,随后采用ProCAST软件模拟高压铸造充型、凝固过程,对铸件的卷气、缩松、缩孔及组织进行预测分析。研究发现,方案1的平直型横浇道浇注方案,金属液冲击型芯容易产生紊流和飞溅,且设置在铸件中间和下部的排溢系统不能减少紊流,易形成裹气和大的缩孔缺陷;而方案2为钳形浇道,去除方案1铸件中部的溢流槽并将下部连通溢流槽改为两个,明显改善铸件内裹气和缩孔现象,确定方案2为最终方案。试模铸件外观质量良好,经X射线探伤发现铸件无裂纹,厚大缩孔位置与缺陷仿真预测良好吻合;金相显微镜(OM)、扫描电镜(SEM)和电子背散射衍射(EBSD)观察组织发现,铸件晶粒细小、组织致密,晶粒取向差小,内部应力小,与组织和应力仿真结果一致;能谱分析(EDS)、X射线衍射(XRD)分析表明,铸件成分均匀,组织内部无杂质。本研究可为端盖类铝合金压铸件模具及工艺开发提供参考。     

Healing of double oxide film defects in A356 aluminium melt

The possibility of the bonding of the two layers of a double oxide film defect when held in liquid A356 Al alloy was investigated. The defect was modelled experimentally by maintaining two aluminium oxide layers in contact with each other and the atmosphere trapped between them in the A356 liquid alloy for varying lengths of time of between 30 s and 48 h. Any changes in the composition and morphology of these layers were studied by SEM, EDX and XRD. The results showed that the two layers of a double oxide film defect might bond to each other by two different mechanisms, one during the transformation of Al₂O₃ to MgAl₂O₄ spinel in relatively short holding times, which would cause the layers to bond at several points and the other during the gradual transformation of spinel to MgO in longer times, which would cause strong bonding between the layers. It was shown that any bonding can take place essentially only after the oxygen and nitrogen of the atmosphere within the defect are consumed.     

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。     

A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing,microstructure, and properties

Manufacturing businesses aiming to deliver their new customised products more quickly and gain more consumer markets for their products will increasingly employ selective laser sintering/melting (SLS/SLM) for fabricating high quality, low cost, repeatable, and reliable aluminium alloy powdered parts for automotive, aerospace, and aircraft applications. However, aluminium powder is known to be uniquely bedevilled with the tenacious surface oxide film which is difficult to avoid during SLS/SLM processing. The tenacity of the surface oxide film inhibits metallurgical bonding across the layers during SLS/SLM processing and this consequently leads to initiation of spheroidisation by Marangoni convection. Due to the paucity of publications on SLS/SLM processing of aluminium alloy powders, we review the current state of research and progress from different perspectives of the SLS/SLM, powder metallurgy (P/M) sintering, and pulsed electric current sintering (PECS) of ferrous, non-ferrous alloys, and composite powders as well as laser welding of aluminium alloys in order to provide a basis for follow-on-research that leads to the development of high productivity, SLS/SLM processing of aluminium alloy powders. Moreover, both P/M sintering and PECS of aluminium alloys are evaluated and related to the SLS process with a view to gaining useful insights especially in the aspects of liquid phase sintering (LPS) of aluminium alloys; application of LPS to SLS process; alloying effect in disrupting the surface oxide film of aluminium alloys; and designing of aluminium alloy suitable for the SLS/SLM process. Thereafter, SLS/SLM parameters, powder properties, and different types of lasers with their effects on the processing and densification of aluminium alloys are considered. The microstructure and metallurgical defects associated with SLS/SLM processed parts are also elucidated by highlighting the mechanism of their formation, the main influencing factors, and the remedial measures. Mechanical properties such as hardness, tensile, and fatigue strength of SLS/SLM processed parts are reported. The final part of this paper summarises findings from this review and outlines the trend for future research in the SLS/SLM processing of aluminium alloy powders.     

Role of Double Oxide Film Defects in the Formation of Gas Porosity in Commercial Purity and Sr-containing Al Alloys

The role of double oxide film(bifilm) defects in the formation of gas porosity in commercial purity and Srcontaining Al alloys was investigated by means of a reduced pressure test(RPT) technique.The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt.The melt was then subjected to different "hydrogen addition" and "holding in liquid state" regimes before RPT samples were taken.The RPT samples were then characterized by determining their porosity parameters and examining the internal surfaces of the pores formed in them by scanning electron microscopy.The results indicated oxide defects as the initiation sites for the growth of gas porosity,both in commercial purity and Sr-containing Al alloys.The results also rejected reduction of the surface tension of the melt,increase in the volumetric shrinkage and reduction in interdendritic feeding as the possible causes of an increase in the porosity content of the Al castings modified with strontium.The change in the composition of the oxide layers of double oxide film defects was suggested to be responsible for this behaviour.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

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.     

Investigation on mechanical properties and tribological behaviour of stir cast LM13 aluminium alloy based particulate hybrid composites

LM13 aluminium alloy with boron carbide (0 wt.%–7.5 wt.%) and fly ash (2.5 wt.%) reinforced particulate hybrid composites were fabricated using liquid metallurgy route. Microstructure and mechanical properties viz., hardness, ultimate tensile strength and ductility were investigated. Wear behaviour of composites was tested by varying sliding distance and load. Fracture surface and worn surface of composites were examined using field emission scanning electron microscope. Microstructure of hybrid composites revealed uniform dispersion of particles in LM13 aluminium alloy. Hardness and tensile strength of composites increased with increasing wt.% of boron carbide and fly ash particles. Wear test results showed that addition of particles significantly decreased the weight loss and coefficient of friction. Also cumulative weight loss decreased up to 47.2 % for 10 wt.% of hybrid composites as compared to LM13 aluminium alloy. Fracture surface of composites showed dimples with particle cracking on the surface. Worn surface of LM13 aluminium alloy showed continuous grooves due to ploughing with delamination. However, worn surface of composites showed fine grooves due to the presence of hard reinforcements on the surface. Boron carbide and fly ash reinforced LM13 aluminium hybrid composites exhibited superior mechanical properties with excellent wear resistance as compared to LM13 aluminium alloy.     

Mechanical properties of In-based eutectic alloy solders used in Josephson packaging

InBiSn and InSn eutectic alloy solders are used in the packaging of the cryogenic Josephson processor. The ductile behaviour of these alloys is important to the application of joints which experience large thermal stresses. In order to characterize the mechanical behaviour at cryogenic temperatures, tensile and shear strengths of bulk solders were measured at and below room temperature. It is found that the ultimate tensile and shear strengths of bulk solders increase as temperature decreases. At low temperatures, the ultimate tensile strength of InBiSn alloy is about three times less than that of InSn alloy. The ductility of both alloys reduces at low temperatures. The fracture surfaces of both bulk solders and solder joints having an interface material (Pd and Au thin films) to the electrical contact pads (Nb thin film) were examined using SEM. Ductile fracture mode was observed for all the specimens down to liquid nitrogen temperature.     

Ti-6Al-4V熔模精密铸造充型及凝固过程计算机模拟

应用自行开发的基于微机上运行的铸件凝固 /充型计算机模拟软件 ,对Ti-6Al-4V钛合金薄壁件精密铸造的充型及凝固传热过程进行了模拟分析 .应用自行安装的多通道钨铼热电耦温度数据计算机采集、分析系统 ,测定了该钛合金起吊接头精密铸件的凝固冷却曲线 ,获取了该合金有关的凝固参数 .对包括上述零件在内的钛合金薄壁件精密铸造的充型过程及凝固传热的温度分布进行了数值模拟 ,模拟计算与实测结果合理吻合 .基于该研究可对其精密铸造工艺进行优化设计 .     

铝合金上等离子喷涂金属镍层的研究

利用金相显微镜和扫描电镜对活塞铝合金表面等离子喷涂镍层(厚度为0. 1～ 0. 2mm ) 的金相组织、形貌和界面进行了观察。发现在铝合金与带状喷涂镍层之间存在较厚氧化膜形成的不规则熔合线, 阻碍焊合效果。因此在现有工艺条件下, 镍层与铝合金间难以实现冶金熔合效果, 并对它们之间的连接机理进行了探讨。      

The influence of porosity on the fatigue strength of high-pressure die cast aluminium

Aluminium is a lightweight material with high strength and good corrosion resistance among other beneficial properties. Thanks to these properties, aluminium is more extensively used in the vehicle industry. High‐pressure die casting of aluminium is a manufacturing process that makes it possible to attain complex, multi‐functional components with near‐net shape. However, there is one disadvantage of such castings, that is, the presence of various defects such as porosity and its effect on mechanical properties. The aim of this work was to investigate the influence of porosity on the fatigue strength of high‐pressure die cast aluminium. The objective was to derive the influence of defect size with respect to the fatigue load, and to generate a model for fatigue life in terms of a Kitagawa diagram. The aluminium alloy used in this study is comparable to AlSi9Cu3. Specimens were examined in X‐ray prior to fatigue loading and classified with respect to porosity level and eventually fatigue tested in bending at the load ratio, R, equal to ?1. Two different specimen types with a stress concentration factor of 1.05 and 2.25 have been tested. It has been shown that the fatigue strength decreases by up to 25% as the amount of porosity of the specimen is increased. The results further showed that the influence of defects was less for the specimen type with the higher stress concentration. This is believed to be an effect of a smaller volume being exposed to the maximum stress for this specimen type. A Kitagawa diagram was constructed on the basis of the test results and fracture mechanics calculations. A value of 1.4 Mpa m^1/2 was used for the so‐called stress intensity threshold range. This analysis predicts that defects larger than 0.06 mm² will reduce the fatigue strength at 5 × 10⁶ cycles for the aluminium AlSi9Cu3 material tested.