SiCp/Al Composites Fabricated by Modified Squeeze Casting Technique

A cost effective method was introduced to fabricate pure aluminum matrix composites reinforced with 20% volume fraction of 3.5 μm SiC particles by squeeze casting followed by hot extrusion. In order to lower volume fraction of the composites, a mixed preform containing pure aluminum powder and the SiC particles was used. The suitable processing parameters for the infiltration of pure aluminum melt into the mixed preform are： melt temperature 800℃, preform temperature 500℃, infiltration pressure 5 MPa, and solidification pressure 50 MPa. Microstructure and properties of the composites in both as-cast and hot extruded states were investigated. The results indicate that hot extrusion can obviously improve the mechanical properties of the composite.     

Squeeze-casting conditions of Al/Al2O3 metal matrix composites with variations of the preform drying process

The characteristics of the preform play a role in determining the final properties of MMCs. Effects of organic binder and microwave drying on preform microstructure have been examined by SEM. In the preform with organic binder, flocking processes are observed during drying. The preform has a uniform distribution of binder and dries quickly with microwave drying owing to its internal and volumetric heating patterns. The fundamental manufacturing process and controlling parameters of squeeze casting, including preform temperature, mould temperature, applied pressure and molten metal temperature, have been studied in Al/Al₂O₃ composites. MMCs have poor mechanical properties with too high temperatures of preform and molten metal due to thermal shocking of the preform, oxidation of the matrix and thermal damage to the fibers. Mould temperature barely affects the tensile strength of MMCs. High applied pressure reduces voids and solidifies the matrix faster. Conditions for squeeze casting to achieve optimal processing, are suggested. The tensile strength of MMCs can be improved by up to about 20% compared with the unreinforced matrix alloy.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

Fabrication of gradient distribution alumina short fibre reinforced aluminium alloy

Gradient distribution alumina short fibre reinforced 6061 aluminium alloy have been fabricated by taking advantage of preform compressive deformation during squeeze casting. Pressure was applied mechanically by a punch. Velocity of the punch, pre-heat temperature of the preforms and pouring temperature were controlled during the infiltration of molten 6061 alloy into alumina short fibre preforms. The distribution of hardness along the infiltration direction in the composites was measured and the distribution of volume fraction along the infiltration direction was calculated by the hardness. Velocity of the inflow, pre-heat temperature of the preform, pouring temperature of the molten metal, binder content of the preform and volume fraction of fibres, all have a very great effect on the gradient distribution of alumina short fibres in the aluminium alloy composites.     

压铸法制造SiCw/Al复合材料的渗透过程分析

本文深入分析了用压铸法制造SiCw/Al复合材料过程中液态铝渗入SiC晶须预制块中的渗透过程。通过理论计算得到液态铝渗入晶须预制块的临界渗透压不超过2MPa。对不同晶须含量的预制块所进行的模拟渗透过程的压缩试验结果表明,随外力的增加,预制块被压缩的程度增大,从而使预制块的晶须相对含量增大。通过对渗透过程的分析,认为液态铝渗透晶须预制块需要一定时间,因此当外力以较大的速度达到最大值时,液态铝不能完全渗入预制块中,这时预制块将被压缩,导致所得复合材料晶须相对含量提高。研究结果表明,复合材料晶须体积分数主要取决于预制块晶须体积分数和复合压力。      

Influence of processing route on electrical and thermal conductivity of Al/SiC composites with bimodal particle distribution

Al/SiC composites with volume fractions of SiC between 0.55 and 0.71 were made from identical tapped and vibrated powder preforms by squeeze casting (SC) and by two different setups for gas pressure infiltration (GPI), one that allows short (1–2 min) liquid metal/ceramic contact time (fast GPI) and the other that operates with rather long contact time, i.e., 10–15 min, (slow GPI). Increased liquid metal–ceramic contact time is shown to be the key parameter for the resulting thermal and electrical conductivity in the Al/SiC composites for a given preform. While for the squeeze cast samples neither dissolution of the SiC nor formation of Al₄C₃ was observed, the gas pressure assisted infiltration led inevitably to a reduced electrical and thermal conductivity of the matrix due to partial decomposition of SiC leading to Si in the matrix. Concomitantly, formation of Al₄C₃ at the interface was observed in both sets of gas pressure infiltrated samples. Longer contact times lead to much higher levels of Si in the matrix and to more Al₄C₃ formation at the interface. The difference in thermal conductivity between the SC samples and the fast GPI samples could be rationalized by the reduced matrix thermal conductivity only. On the other hand, in order to rationalize the thermal conductivity of the slow GPI a reduction in the metal/ceramic interface thermal conductance due to excessive Al₄C₃-formation had to be invoked. The CTE of the composites generally tended to decrease with increasing volume fraction of SiC except for the samples in which a large expansive drift was observed during the CTE measurement by thermal cycles. Such drift was essentially observed in the SC samples with high volume fraction of SiC while it was much smaller for the GPI samples.     

挤压铸造硼酸铝晶须增强Al基复合材料浸渗过程理论分析

本文作者采用挤压铸造法,制取了硼酸铝晶须增强Al基复合材料,并根据Laplace方程及多孔体的渗流物理基本原理,从多孔渗流动力学角度分析了挤压浸渗过程中金属液在多孔体中的流动,认为紊流才是其主要的表现形式。并较详细地分析了晶须长度对预制件孔结构及浸渗的影响。     

Optimization of chemical reactions between alumina/silica fibres and aluminium-magnesium alloys during composite processing

An Al-Si-;Cu-Mg alloy reinforced with alumina/silica fibres (Fiberfrax®, alumina/silica ratio=45/55) has been extensively characterized in terms of microstructure, interfacial chemical reactions and mechanical properties. The composite was fabricated by squeeze casting. The above characteristics were measured as a function of (a) calcination temperature of the fibre preform before infiltration, and (b) subsequent composite heat treatment. The main reaction that occurs during the processing of aluminium alloy matrix composites is the reduction of silica in the binder and fibres by magnesium from the matrix. When calcined below 1000°C, the fibres remain amorphous with a coating of porous silica binder. In this condition, the reinforcement reacts strongly with the matrix during heat treatment of the composite. In contrast, at high calcination temperatures (1200°C), the fibres transform partially into mullite and the silica binder densifies; these fibres are somewhat less reactive with the matrix. In both cases, the matrix/reinforcement reactions are very strong during high-temperature heat treatment, leading to a complete reduction of silica in some cases. The degradation caused by chemical reactions adversely affects the mechanical properties of these composites. Therefore, in order to optimize the mechanical properties of this composite, the fibre preform first must be calcined at high temperature, then the composite heat treatment limited to low temperature.     

熔融渗透工艺制备SiC-TiSi2复相陶瓷的反应机理

熔融Si渗透过程伴随着复杂的化学反应及多组分扩散,对该过程进行研究有助于更好地理解熔渗反应机理。本工作采用熔融渗透工艺制备SiC-TiSi2复相陶瓷,在生成SiC基体的同时原位生成TiSi2。通过扫描电子显微镜(SEM)、X射线能谱分析(EDS)和微区X射线衍射(micro-beam XRD)分别对熔融硅区域、Si/SiC界面以及SiC基体的微观结构和相组成进行表征和分析,研究了熔渗工艺制备SiC-TiSi2的反应机理。结果表明:高温下液Si渗入C-TiC预制体,发生化学反应生成SiC、TiSi2以及少量副产物Ti5Si3,其中Ti5Si3主要集中于Si/SiC界面处。随着反应进行,液Si与TiSi2形成液态Ti-Si共晶。该液态共晶通过流动扩散在Si区域中析出TiSi2。而预制体中的少量固态C在液Si中溶解、扩散,并在Si区域生成均匀分布的孤立SiC颗粒。     

Fabrication process of carbon nanotube/light metal matrix composites by squeeze casting

Multi-walled carbon nanotubes (MWCNTs) should be attractive for the reinforcement of metal-matrix composites, because of their high strength, high modulus and high thermal conductivity. However, the fiber diameter of MWCNTs is hundreds of times smaller than that of carbon fiber. This causes difficulty in infiltration into the MWCNT preform. Moreover, the threshold pressure which was applied to the preform will cause preform deformation. Therefore, knowledge of preform compressive properties which are the buckling strength and elastic modulus are necessary to fabricate the composites. In this study, at first, wettability of the basal plane of graphite by molten aluminum or magnesium was measured using the sessile drop method. Moreover, trial fabrication of MWCNT-reinforced aluminum or magnesium alloy composites was carried out by squeeze casting. As a result, these composites were fully infiltrated. An order-of-magnitude agreement was found between the estimated threshold pressure and the applied infiltration pressure to the MWCNT preform.     

Processing of B4C Particulate-reinforced Magnesium-matrix Composites by Metal-assisted Melt Infiltration Technique

In fabricating magnesium-matrix composites, an easy and cost-effective route is to infiltrate the ceramic preform with molten Mg without any external pressure. However, a rather well wettability of molten Mg with ceramic reinforcement is needed for this process. In order to improve the wettability of the metal melt with ceramic preform during fabricating composites by metal melt infiltration, a simple and viable method has been proposed in this paper where a small amount of metal powder with higher melting point is added to the ceramic preform such that the surface tension of the Mg melt and the liquid-solid interfacial tension could be reduced. By using this method, boron carbide particulate-reinforced magnesium-matrix composites （B4C/Mg） have been successfully fabricated where Ti powder immiscible with magnesium melt was introduced into B4C preform as infiltration inducer. The infiltration ability of molten Mg to the ceramic preform was further studied in association with the processing conditions and the mechanism involved in this process was also analyzed.     

真空压渗铸造铝基电子封装复合材料研究

叙述了真空反压渗透铸造法制铝基电子封装复合材料的过程：测试了ＳｉＣｐ，镀铜碳短纤维，Ｐ１３０石墨短纤维，Ａｌ２Ｏ３短纤维，石墨磷片，石墨颗粒增强铝硅铸造合金复合材料的密度，孔积率和增强体的体积分数；给出了每种复合材料的金相照片，分析了不同增强体，预制件制造方法和混杂增强对铝基复合材料的体积分数，孔积率等方面的影响。     

PROCESSING OF METAL AND CERAMIC MATRIX COMPOSITES WITH THREE-DIMENSIONAL BRAIDED REINFORCEMENT

This paper summarizes some of the pioneering work done in developing the processing techniques to consolidate metal and ceramic matrix composites with 3-D braided fiber architecture. The 3-D braided fiber architecture features a fully integrated structure with multidirectional reinforcement and allows for the braiding of complex shapes. For metal matrix composite, a 3-D braided AI₂O₃ fibers preform reinforced Al-Li matrix composite has been successfully fabricated by liquid vacuum infiltration methods. For ceramic matrix composites, chemical vapor infiltration technique has been used to densify a 3-D braided Nicalon fibers preform with SiC matrix composite. A significant improvement in composite performance can be achieved through the architecture of 3-D integrated reinforcement geometry. The future needs and directions for developing viable methods for fabricating strong, tough high temperature composites with 3-D reinforcements is also discussed.     

Microstructure, thermal behavior and mechanical properties of squeeze cast SiC, ZrO2 or C reinforced ZA27 composites

ZA27 alloy based composites were synthesized by stirring method, followed by squeeze casting. Stir casting was employed successfully to incorporate 5 vol.% of various reinforcement particulates, namely, SiC, ZrO₂ or C. The porosity in the composites was decreased by squeeze pressure. The presence of particles and/or application of squeeze pressure during solidification resulted in considerable refinement in the structure of the composites. The microstructures, X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDXA) results indicated that no significant reactions occurred at the interface between the SiC or C particles and ZA27 alloy. However, in case of ZrO₂ reinforced ZA27, the ZrO₂ reacted with Cu present in the molten ZA27 alloy, forming Cu₅Zr. Thermal analysis showed that both α and β nucleation and growth temperatures of the composites were lower than those of the ZA27 alloy. The presence of particles in the as-cast or squeezed composites led to not only an accelerated age hardening response, but also an increase in the peak hardness of the composites. The values of coefficient of thermal expansion (CTE) of the composites were drastically lower as compared to those of the ZA27 alloy. The tensile properties of the composites decreased as a result of the addition of the particles. Scanning electron microscope (SEM) pictures of the composites indicated that cracks mainly initiated at particle-matrix interface, propagated through the matrix and linked up with other cracks leading to failure of the composites.     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

Ti基非晶合金及其SiC陶瓷骨架复合材料动态压缩力学行为研究

为了验证Ti基非晶合金和陶瓷两种材料三维连通网状结构的复合优势,制备出具有优良抗冲击性能的复合材料,本文采用铜模吸铸法制备了Ti基非晶合金,并用渗流铸造法制备出孔隙率分别为30.86%、18.14%和15.28%的Ti基非晶合金/SiC陶瓷骨架复合材料。采用X射线衍射仪对纯Ti基非晶合金以及SiC陶瓷骨架复合材料进行相分析,确认了试件材料的非晶状态;在不同应变速率下,用分离式霍普金森压杆(SHPB)实验装置对试样进行室温轴向动态压缩力学性能测试,并利用能谱型场发射扫描电镜(SEM)等设备观察了试件的微观组织和断面特征,对比分析了Ti基非晶合金和SiC陶瓷骨架复合材料的动态压缩力学性能和失效机理。研究表明,Ti基非晶合金/SiC陶瓷骨架复合材料内部的微裂纹最初萌生于应力集中的两相界面处,并在SiC相内部或两相界面处扩展,继续加载,SiC相失效后,Ti基非晶合金相在远超过其动态压缩强度的应力下迅速失效,复合材料整体失效。SiC相内的断裂形貌主要有微裂纹与解理台阶,Ti基非晶合金相内的断裂形貌有脉状花样、多重脊状条带、蜂窝状花样与光滑无特征区,其中以光滑无特征区为主。复合材料的抗压强度随Si...     

Influence of atmosphere and carbon contamination on activated pressureless infiltration of alumina–steel composites

Al₂O₃/steel metal–matrix composites (MMCs) were fabricated by activated pressureless infiltration at atmospheric pressure in different gas atmospheres; N₂, Ar and Ar–5%H₂. The infiltration quality was evaluated with examination of the microstructure, infiltrated area and remaining porosity. The atmosphere with the best infiltration quality was chosen for improvement of infiltration by varying infiltration parameters such as temperature, holding time and heating and cooling rates. Further improvement was achieved by addition of Si or SiO₂ powder to the preform in order to reduce the effect of the residual carbon. The results show that the activated melt infiltration can be successfully done at atmospheric pressure in inert gas.     

Synthesis of SiC/Al Co-Continuous Composite by Spontaneous Melt Infiltration

1. IntroductionBecause of the adVantages such as ease of fabrication, low cost and near net shape forming capability,liquid metal infiltration has become a popular technique to fabricate metal matriX composites(MMCs)with high contellt reinforcements. In the process ofmelt infiltration, liquid metal penetrates into the continuous porosity of a packed ceramic filler or preform either by aPPlied positive or negative pressuresuch as in vacuum infiltration[1], pressurized liquldmetal infiltration…     

Effects of sintering temperature of whisker preform on the microstructures and tensile properties of Bi(OH)3-coated Al18B4O33 whisker-reinforced aluminum matrix composites

A alumina borate whisker with Bi(OH)₃ coating was prepared by a chemical method. The coated whiskers were sintered at various temperatures. The coated whisker-reinforced pure aluminum matrix composite was fabricated by squeeze casting method. The microstructures of the coated whiskers and coated composites with the different sintering temperature of whisker preform were studied, and the tensile properties of the coated composites at room temperature were also investigated. It can be found that the microstructures of coatings on the whisker surfaces and at the interface in the coated composites are strongly dependent on the sintering temperature of whisker preform. The ultimate tensile strength and elongation to fracture of the coated composites increased with the increasing of sintering temperature of the whisker preform.     

SiC泡沫／Al双连续相复合材料连续性的研究

运用挤压铸造法制备了SiC泡沫／Al双连续性复合材料，研究了SiC泡沫、复合压力和合金成分对复合材料连续性的影响。结果表明，SiC泡沫陶瓷的加入阻碍了基体合金流动，降低了复合材料的连续性。随着复合压力的增加，复合材料的连续性逐渐增强，当压力为150MPa时，复合材料的连续性最好。随着含硅量的增加，基体合金的热膨胀系数逐渐降低，基体和增强体之间的热膨胀匹配增强，复合材料中残余应力降低，复合材料的连续性增强。