Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions.     

Tensile properties and thermal stability of ZnO-coated aluminum borate whiskers reinforced 2024Al composite

ZnO-coated aluminum borate whiskers reinforced 2024Al composite was fabricated by squeeze casting. Interfacial microstructures and tensile properties of the composite in as-cast and after thermal exposure were investigated. Fracture mechanisms of the composite in as-cast and after thermal exposure were also investigated. The results show that ZnO coating of the whiskers reacts with molten 2024Al and MgAl₂O₄ forms at the interface during squeeze casting. On the one hand, the interfacial reaction between ZnO and 2024Al can improve the wettability of the whiskers by molten 2024Al, increasing the tensile properties of as-cast composite. On the other hand, during thermal exposure, MgAl₂O₄ at the interface can effectively hinder serious interfacial reactions between the whiskers and magnesium in the matrix of 2024Al, improving the thermal stability of the composite at high temperatures.     

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.     

Improvement of the bonding interface in hybrid fiber/particle preform reinforced Al matrix composite

The bonding interface between the reinforcement and the matrix alloy in hybrid AZS fiber/SiC particle preform based aluminum metal matrix composites (Al MMCs) has been investigated as a function of reinforced particle size and the binder content. It is observed that high binder and large particle will result in a poor bonding interface. This has deleterious effects on the mechanical properties of the cast MMCs. Estimation of the binder thickness indicates that there exists a critical particle size above which the particles are not appropriate to be used in fabricating the hybrid fiber/particle preform based MMCs.     

Casting particulate and fibrous metal-matrix composites by vacuum infiltration of a liquid metal under an inert gas pressure

A new method of making metal-matrix composites is reported. This method combines the essentials of three liquid-phase fabrication methods: (i) vacuum infiltration, (ii) infiltration under an inert gas pressure, and (iii) squeeze casting. In this method, the particulate or fibrous preform is placed in a mould and the matrix alloy is placed above the preform. The matrix alloy is heated to the liquidus temperature together with the mould and the preform under vacuum. Then an inert gas like argon is compressed on to the top surface of the matrix-alloy melt, forcing the melt to infiltrate the preform. The pressure is 1000 to 2500 psi. As the melt is just at liquidus temperature, it is much lower than that used in squeeze casting. Moreover, the pressure is an order of magnitude lower than that used in squeeze casting. The low temperature lessens the interfacial reaction between the matrix and the filler, while the low pressure essentially eliminates preform compression. This method has been successfully used to fabricate aluminium-matrix composites reinforced by short ceramic fibres, continuous ceramic fibres, SiC particles, Al₂O₃ particles, graphite flakes and SiC whiskers.     

Microstructure characterization and tensile properties of direct squeeze cast and gravity die cast 2017A wrought Al alloy

Squeeze casting is a pressurized solidification process wherein finished components can be produced in a single process from molten metal to solid utilizing re-useable die tools. This one activates different physical processes which have metallurgical repercussions on the cast material structure. Desirable features of both casting and forging are combined in this hybrid method. 2017A aluminium alloy, conventionally used for wrought products, has been successfully cast using direct squeeze casting. Squeeze casting with an applied pressure removes the defects observed in gravity die cast samples. Tensile properties and microstructures are investigated. The results show that the finer microstructure was achieved through the squeeze casting. Furthermore, higher pressures improved the fracture properties and decreased the percentage of porosity of the cast alloy. The ultimate tensile strength, the yield strength and the elongation of the squeezed cast samples improved when the squeeze pressure increased.     

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.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

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。     

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.     

Preparation of Al matrix nanocomposites by diluting the composite granules containing nano-SiCp under ultrasonic vibaration

In this work, an efficient process by diluting the nano-SiCp/Al composite granules in the molten matrix under ultrasonic vibration(UV) was developed to prepare metal matrix nano-composites(MMNCs).Millimeter-sized composite granules with high content of SiC particle(8 wt%) were specially fabricated by dry high-energy ball milling(HBM) without process control agent, and then remelted and diluted in molten Al alloy under UV. The MMNCs melt was finally squeeze cast under a squeeze pressure of 200 MPa, Microstructure of the composite granules during dry HBM was investigated, and the effect of UV on microstructure and mechanical properties of the MMNCs was discussed. The results indicate that nano-SiC particles are uniformly distributed in the nano-SiCp/Al composite granules, which are covered by vestures of pure Al. During diluting, nano-SiC particles released from the composite granules are quickly dispersed in the molten matrix by UV within 4 min. Microstructure of MMNCs is significantly refined under UV and squeeze casting, eutectic Si phase modified to fine islands with an average length of 1.4 μm. Tensile strength of the squeeze cast MMNCs with 1 wt% of nano-SiC particles is 269 MPa, which is improved by 25% compared with the A356 alloy matrix.     

Mechanical properties and thermal stability of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>-coated aluminum borate whiskers reinforced 2024Al composite

ZnAl₂O₄-coated aluminum borate whiskers reinforced 2024Al composite was fabricated by squeeze casting. Interfacial microstructures and tensile properties of the composite were investigated. The results show that ZnAl₂O₄ coating of the whiskers can improve the wettability of the whiskers by molten aluminum during squeeze casting, resulting in the increase of tensile properties of the composite. During thermal exposure, ZnAl₂O₄ at the interface can effectively hinder harmful interfacial reactions, resulting in the improvement of thermal stability of the composite at high temperatures. Fracture mechanisms of the composite in as-cast and after thermal exposure were also investigated.     

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.     

Improvement of the temperature resistance of aluminium-matrix composites using an acid phosphate binder

The use of phosphate binders instead of the widely used silica binder resulted in improved temperature resistance, increased tensile strength and decreased coefficient of thermal expansion. The effects were largest for the phosphate binder which contained the largest amount of phosphoric acid (P/Al atom ratio = 24 in the liquid binder). These effects were probably due to the protection of the SiC whiskers by the binder phases (aluminium metaphosphate or aluminium orthophosphate), the binder-SiC reaction product (SiP₂O₇) and the binder-aluminium reaction product (AIP) from further reaction between the SiC and aluminium. The tensile strength of the composite containing the SiC whisker preform made with the phosphate binder (P/Al atom ratio = 6 or 24 in the liquid binder) was increased after heating at up to 600 °C for 240 h. The silicon phosphate (SiP₂O₇) acted as an in situ binder and was primarily responsible for increasing the compressive strength of the preform and increasing the temperature resistance of the composite. The carbon fibre composite containing the preform made by using the phosphate binder (P/Al atom ratio = 24 in the liquid binder) with either water or acetone as the liquid carrier during wet forming of the preform had a higher tensile strength than the carbon fibre composite made by using the silica binder. After composite heat exposure to 600 °C for 14 h, the carbon fibre composite made by using this phosphate binder with acetone as the liquid carrier during wet forming of the preform showed the best temperature resistance, while the carbon fibre composites made by using this phosphate binder with water as the carrier showed the second best temperature resistance, and that made by using silica binder was the worst. The reason for the better effect of the phosphate binder than the silica binder is probably due to the ability of the phosphate binder and the binder-aluminium reaction product (AIP) to protect the carbon fibres from the undesirable reaction between the carbon fibres and aluminium. The lack of a binder-fibre reaction contributed to making the carbon fibre composites less temperature resistant than the SiC whisker composites. The use of a higher binder concentration is attractive for increasing the temperature resistance of the composites. The binder concentration in the preform can be increased by increasing the binder concentration in the slurry used in the wet forming of the preform.     

硅酸铝短纤维增强AZ91D复合材料的界面微观结构及力学性能

以晶化的硅酸铝短纤维（Al₂O₃-SiO₂ ）_sf为增强体、用磷酸铝为预制体粘结剂,通过挤压浸渗工艺制备了（Al₂O₃-SiO₂ ）_sf /AZ91D镁基复合材料。通过光学显微镜、TEM和HREM分析研究了复合材料的界面微观结构和界面反应产物。结果表明：用挤压浸渗法制备的硅酸铝短纤维增强AZ91D镁基复合材料的界面厚度约为100 nm,界面上除有一定数量的MgO颗粒和少量的MgAl₂O₄和Mg₂Si颗粒外, 还有少量的MgP₄等反应产物存在;硅酸铝增强纤维与镁合金基体之间形成了较强界面结合,界面微观结构比较理想。力学性能测试表明,与AZ91D基体合金相比,复合材料的室温抗拉强度提高了约18%,弹性模量提高了约58%。     

Pressure infiltration processes to synthesize metal matrix composites – A review of metal matrix composites,the technology and process simulation

Metal matrix composites (MMCs) acquire their improved physical and mechanical properties through the careful reinforcement of their matrices by a variety of light but strong and stable reinforcement materials. The pressure infiltration process (PIP) is one of the most important techniques used for making MMCs with a high reinforcement content in which a molten metal or alloy is injected and solidified in a mold packed with continuous or discontinuous reinforcement materials. Several factors affect the quality of MMCs made by this process. These include, but are not limited to, the reinforcement type, preform geometry, applied pressure and pressure control, as well as the transport phenomena of the molten metal. This paper presents a review of the various aspects of MMCs, the process in terms of the technological details, the latest developments in the reinforcement materials used and the simulation models developed for pressure infiltration manufacturing of MMCs.     

The Performance of Zinc Alloy Based Metal Matrix Composites Produced Through Squeeze Casting

Zinc-aluminum cast alloys (ZA alloys) have good castability, mechanical properties and excellent tribological characteristics. Of all the ZA alloys, ZA-27 (containing 27% aluminum) has the highest strength and optimum wear resistance. However all the ZA alloys, including ZA-27, suffer from lack of creep resistance and high temperature stability. One probable solution to improve these properties is to reinforce the alloys with ceramic particles or fibers to result in metal matrix composites (MMCs). MMCs can be economically produced through squeeze casting which involves infiltration. This paper presents the salient features of an experimental study on ZA-27 alloy based MMCs produced through Squeeze Casting.     

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.     

挤压铸造法制备可变形SiCP/Al复合材料的组织与性能

通过在SiC颗粒预制块中加入铝粉的方法制备了颗粒含量可控的SiC颗粒预制块,并用挤压铸造法制备了可变形SiC_P/Al复合材料。通过对颗粒体积含量为25%的SiC_P/Al复合材料进行热挤压变形,研究了挤压变形的可行性及其对复合材料组织与性能的影响规律。实验结果表明,用本文中提出的新工艺制备的25vol%SiC_P/Al复合材料可以成功地进行挤压比为25∶1的热挤压变形,并且热挤压变形可以明显提高复合材料的强度、刚度和塑性。