Discussion of a liquid metal pressure infiltration process to produce metal matrix composites

The two major processes presently used to manufacture metal matrix composites are solid-state diffusion bonding and squeeze casting. The former has shape limitations; it is suitable to produce only flat plates or sheets, and the manufacturing rate is low and expensive. The latter, although suitable for quantity production of complex shapes, requires heavy and expensive tooling. The described pressure infiltration process was developed at Massachusetts Institute of Technology. With this process, fiber, whisker or particulate pre-forms are placed in inexpensive throw-away containers, into which the molten matrix material is forced by gas pressure. Because the gas pressure on the pre-form container is quasi hydrostatic, its strength requirement is minimal, consequently pre-form containers, even those with complicated shapes, are easy to preprare. By virtue of the simplicity of the containers and the absence of the requirement of a heavy mechanical system to effect the infiltration of the liquid metal into the pre-forms, the process is suitable to produce bulk or final-shape composite parts inexpensively. As the processing parameters such as pre-form temperature, infiltrating metal temperature, infiltrating pressure and cooling rate, can be accurately controlled, the process is particularly suitable for research and limited production.     

Thermal spraying of billets and fiber prepregs for semi-solid forming of metal matrix composites

The use of thermal spray processes with subsequent material densification by semi-solid forming can yield advantages for manufacturing of metal matrix composites (MMC) compared to well-established manufacturing methods like spray forming or diffusion bonding. The main challenges of this method lie in well defined process temperatures during material deposition and forming, handling of the material during the processes, microstructure of the semi-finished matrix, and economical efficiency of the process chain.Particulate reinforcement of light metal matrix material is mainly aimed on improvement of mechanical properties at elevated temperatures, e.g. wear behavior, creep resistance, yield and tensile strength. Composite formation for particle reinforced metals (PRM) by thermal spraying with the arc wire system is achieved by simultaneous deposition of matrix and reinforcement by means of cored wires. Unidirectional (UD) fiber reinforced MMC yield highest specific mechanical properties and can be processed by winding of a continuous fiber strand and coating of the fiber layer by arc wire spraying with wires from light alloy matrix material or by plasma spraying with mixed powders. The coating process can be simultaneous to the winding process or in a separate step for prepreg manufacturing with shape and fiber content adapted to the final component geometry and load case.     

基于快速原型的转移涂料法制作模具技术

将快速原型制造与转移涂料精密铸造结合,对复杂曲面金属模具的快速制作方法进行了探索。该方法用三维造型软件设计模具,用分层制作技术快速得到模具原型,再基于原型直接采用转移涂料工艺,制作出近净型金属模具毛坯。该工艺流程短,成本较低,特别适合砂型铸造或消失模铸造用的小型金属模具的快速制作。     

短纤维氧化铝／铝复合材料液态浸渗后直接挤压的试验研究

介绍和分析了液态浸渗后直接挤压成形制作短纤维增强金属基复合材料的工艺原理，特点和用氧化铝短纤维与铝合金为对象进行了有关工艺试验的结果。     

基于SLS原型的快速(重力)铸造工艺

结合SLS快速成形和铸造工艺可以实现金属零件的快速铸造.根据原型材料可熔融和烧失的特性,制定了类似于熔模铸造的快速铸造工艺方案.讨论了快速金属铸件的特点,对快速铸造工艺提出了成功率高、成形性好和性能较高的要求.分析了面向金属零件快速铸造的重力铸造工艺并以实例说明,顶注式浇注系统适用于结构简单且高度不大的快速零件,而阶梯式浇注系统适用于中大型和复杂程度较高的铸件.     

增材制造结合熔模铸造制备亚毫米级金属点阵微结构的工艺研究

基于增材制造方法,采用光固化成形工艺,结合熔模铸造工艺制备出亚毫米直径桁架的金属点阵结构。首先对光固化成形工艺进行优化,最佳工艺参数为50μm的单层厚度条件下,单层固化时间4.52 s,采用优化后的光固化成形工艺制备出树脂基点阵结构,将其嵌入到熔模基体中,加热将树脂材料升华,形成支柱直径为750μm的孔隙模具,分别采用离心铸造和重力铸造两种方法浇注ZAMAK3锌合金,制备出金属点阵结构。压缩试验表明,两种铸造技术制备的金属点阵结构的机械强度相似,承受压力载荷分别达到1.24 kN和1.30 kN。有限元分析的模拟结果与离心铸造的结果更加接近,当变形量超过0.6 mm时,有限元模拟结果与实际变形情况开始出现偏差。     

Two-phase Flow Characteristics in Hollow Shape Fabrication Process of Metal Matrix Composites by Thixoforging

Among advanced manufacturing processes of metal matrix composite parts, thixoforging is one of the most effective forming processes. The investigation of this research article is to provide the proper conditions such as the die shape, the forging velocity, the forging time, the forging pressure, and reinforcement content in the thixoforming process for fabricating hollow shape parts. To investigate the effect of injection velocity and pressure on various defects in thixoforged cylinder liners, filling tests were performed by MAGMA S/W. In order to evaluate the effectiveness of calculated conditions, which are given by computer-aided engineering, A380 and SiCp/A380 cylinder liners were fabricated under the calculated conditions. SiCp/A380 composite billets were fabricated by both the mechanical stirring and electro-magnetic stirring processes. In the case of SiCp/A380 composite cylinder liners, the effect of reinforcement (SiCp) distribution, content (10-20 vol.%) and size (5.5-14 μm) on the mechanical properties was investigated.     

钢纤维增强有色金属基复合材料综述

有色金属基复合材料相对于传统有色金属材料而言,具有更好的抗氧化性、高耐热性、高比强度、高比模量、耐磨损和高使用寿命。在有色金属基复合材料的众多的增强体中,非金属纤维(C/C、SiC)与金属基质结合界面的相容性是制约金属基复合材料性能的关键问题,而金属纤维与金属基质之间良好的相容性能够有效改善金属材料的性能。金属纤维增强有色金属基复合材料的制备工艺主要有扩散粘结法、液态渗透法、压力铸造法、涂层热压法、双辊轧制法。 本文主要总结了钢纤维增强有色金属基(Al、Mg、Cu、Zn和Zr)复合材料的制备方法、微观组织、界面特征和机械性能,指出了钢纤维增强有色金属基复合材料进一步研究发展所需要解决的问题。     

用于铸造金属功能件的快速原型模样的特点分析

介绍几种快速成型系统，并对用于铸造金属功能件的几种快速原型模样进行归纳，总结，针对熔模铸造和陶瓷型铸造对成型模样的特定要求，考察一次性模样使用的几种原型模样的精度、表面粗糙度、热膨胀率、残余灰分等工艺参数，对实际铸造的影响。     

Ceramic-Reinforced MMC Fabricated by Squeeze Casting

Squeeze casting is a well-developed process used to make low cost, high quality parts in almost any material. It has now been shown that it is feasible to insert ceramic reinforcement during casting that will substantially improve the properties of the finished casting. In addition, metal inserts may also be placed in desired locations and a designer using a computer-aided design approach has the flexibility to design castings with specific properties in different areas of the casting. This method of fabricating metal matrix composites should be low cost and well suited to existing industrial methods using conventional manufacturing equipment. It is also amenable to automatic diagnostic quality control procedures. Concepts are tested on a track shoe component (Figure 1)     

Centrifugal casting of functionally graded aluminium matrix composite components

Abstract

Centrifugal casting is one of the potential manufacturing techniques used for producing near net shaped components with improved properties. The emergence of new class of functionally graded materials has made it an important technique for the fabrication of engineering components and structures with graded property. The present paper describes the studies carried out on processing and characterisation of functionally graded Al matrix composites components based on Al–SiC ex situ and Al–Si in situ composites. The microstructural and mechanical characteristics of the composites are evaluated. In the case of Al–SiC functionally graded metal matrix composites discs, the particles are segregated gradiently towards the outer periphery of the casting exhibiting high strength and hardness towards the outer periphery. The Al–Si in situ composite cylinder shows the dispersion of primary Si particles towards the inner periphery of the casting which can lead to higher hardness and wear resistance.     

铝合金消失模模具的快速制造

快速成型工艺结合精密铸造技术是快速制造金属模具的主要途径。研究了一种适用于铝合金消失模模具铸造的最佳工艺，以达到减少铝铸件针孔、提高铝合金铸件质量的目的，解决了新产品开发中金属模具快速制作的关键问题，同时也为快速制造技术在铸造中的应用开辟了一个新领域。     

Influence of T6 heat treatment on A356 and A380 aluminium alloys manufactured by thixoforging combined with low superheat casting

The effects of T6 heat treatment on thixoforged A356 and A380 aluminium alloys were studied. Low superheat casting (LSC) technique was carried out to prepare proper specimens for thixoforging process. The samples were poured at 20 °C above their liquidus temperatures which provided the formation of equiaxed grains instead of dendritic growth. Produced billets were reheated for varied time from 20 to 80 min and thixoforged with 50% deformation rate. After thixoforging process, the samples were T6 heat treated for both A356 and A380 alloys. The microstructural evaluation and hardness alteration of thixoforged, solution treated and aged specimens were examined comparatively by using optical microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy and Brinell hardness equipment. T6 heat treatment provided relatively uniform microstructure with newly formed precipitates that are Mg₂Si and Al₂Cu for A356 and A380 billets, respectively. Accordingly, hardness after artificial aging was increased considerably and reached HB 93 for A356 and HB 120 for A380 alloys.     

碳纤维含量对Al2O3+C/ZL109混杂复合材料摩擦磨损性能的影响

利用挤压铸造法制备了A1203 C／ZLl09短纤维混杂金属基复合材料，并探讨了A1203纤维体积分数为12％时，C纤维含量对该混杂复合材料摩擦磨损性能的影响。结果表明：随着C纤维体积分数的增加，复合材料的摩擦因数和磨损率逐渐降低。12％A1203和4％C短纤维的协同作用使复合材料从轻微磨损到急剧磨损的临界转变载荷比基体合金提高了1倍。当载荷低于临界载荷时，复合材料的主要磨损机制为犁沟磨损和层离，C纤维的加入有利于磨损表面裂纹尺寸的减小。但随着载荷的逐渐增加并发生严重磨损时，基体和复合材料的磨损机制均为严重的粘着磨损甚至局部熔化磨损。     

On the presence of work-hardened zones around fibers in a short-fiber-reinforced Al metal matrix composite

Dislocation densities are investigated in a short-fiber-reinforced Al–11 wt.% Zn–0.2 wt.% Mg metal matrix composite (MMC) with a special focus on regions near the fiber–matrix interfaces. Clear microstructural evidence is provided for the formation of work-hardened zones (WHZs) around fibers during creep using transmission electron microscopy (TEM). The dislocation densities in the WHZs are higher after creep than after squeeze casting, where the plastic strains associated with the thermal stresses that build up during solidification also result in an increased dislocation density close to fibers. The effect of heating and cooling on the dislocation substructure is also considered. The results are discussed in light of previous findings and provide microstructural evidence for the presence of WHZs as predicted by the Dlouhy model of MMC creep.     

Metal matrix composites for fabricating tooling

The objective of this work is to investigate the fabrication, microstructures and, properties of metal matrix composites (MMCs) net-shaped using a four-step hybrid manufacturing method. The manufacturing method consisted of soft tooling fabrication, slurry casting, debinding-sintering followed by metal infiltration. Firstly, silicone rubber molds were fabricated. Subsequently, WC-Co pellets and stainless steel powders were mixed with polymer-based binders and cast into these rubber molds. After solidification, the parts were removed from the rubber molds, and placed in a furnace for debinding and sintering. Finally, sintered parts were infiltrated with bronze and the ensuing microstructures and properties were evaluated. The dimensional changes and mechanical properties of these MMCs as a function of amount of infiltrant were also examined. The results showed that the wear resistance of the MMC parts was comparable to M2 and D2 steels. Demonstrations of the process for net-shaping MMCs were conducted by fabricating wear-resistant tooling. The implications for fabricating MMCs by directly 3D printing rubber molds soft tooling are presented.     

Progress in Metal Matrix Composites

Abstract

Metal matrix composites are produced by several contrasting manufacturing techniques and in many different shapes and forms. Solid state processing routes are generally favoured for putative aerospace applications, whereas liquid metal routes appear more promising for automobile applications. Both groups of production routes can accommodate reinforcements of continuous fibre, short fibre, particulate and hybrid fibre-particulate types and the reinforcement phase can be distributed uniformly throughout the composite structure or, using preform technology, in selected regions of the casting.

The greatest problem arising during manufacture and during post-production thermal processing of MMCs is that of chemical instability of the constituent phases. Thermodynamic incompatibilities restrict the use of certain manufacturing routes for specific composite couples and limit the working environment of others. Commercial aspects of MMC production, either directly to near net shape or by subsequent machining, indicate high cost penalties for specific limited gains.

The mechanical properties of MMCs are inevitably a compromise between the properties of the matrix and reinforcement phases. In particular, ductility and toughness are frequently sacrificed for higher modulus. Measures to improve toughness whilst maintaining stiffness are being explored. Wear properties of MMCs are extremely good; machinability, on the other hand, can be poor.     

Tribological Behavior of Aluminum Alloy AlSi10Mg-TiB<Subscript>2</Subscript> Composites Produced by Direct Metal Laser Sintering (DMLS)

Direct metal laser sintering (DMLS) is an additive manufacturing technique for the production of parts with complex geometry and it is especially appropriate for structural applications in aircraft and automotive industries. Aluminum-based metal matrix composites (MMCs) are promising materials for these applications because they are lightweight, ductile, and have a good strength-to-weight ratio This paper presents an investigation of microstructure, hardness, and tribological properties of AlSi10Mg alloy and AlSi10Mg alloy/TiB₂ composites prepared by DMLS. MMCs were realized with two different compositions: 10% wt. of microsize TiB₂, 1% wt. of nanosize TiB₂. Wear tests were performed using a pin-on-disk apparatus on the prepared samples. Performances of AlSi10Mg samples manufactured by DMLS were also compared with the results obtained on AlSi10Mg alloy samples made by casting. It was found that the composites displayed a lower coefficient of friction (COF), but in the case of microsize TiB₂ reinforcement the wear rate was higher than with nanosize reinforcements and aluminum alloy without reinforcement. AlSi10Mg obtained by DMLS showed a higher COF than AlSi10Mg obtained by casting, but the wear rate was higher in the latter case.     

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting

Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.     

Mathematical Modeling of Particle Segregation During Centrifugal Casting of Metal Matrix Composites

When a metal matrix composite undergoes centrifugal casting, the velocity, deceleration, displacement, and segregation of its particles are modeled according to changes in the centrifugal radius, as well as by variations in the molten metal viscosity as the temperature decreases during the cooling process. A cast aluminum alloy A356 reinforced by 10 V% of silicon carbide particles (SiC), with a median diameter of 12 μm, was used to conduct the experiments, and a mathematical modeling showed that the particles’ volume fraction on the outer casting face varied according to whether the viscosity of the liquid metal used was constant or variable. If variations in viscosity during the cooling process are taken into account, then the volume fraction of the particles for a given time of centrifugation changes on the outer casting face, while it increases if the viscosity was constant. Modeling the particle segregation with variable viscosity produces results that are closer to those obtained with experiments than is the case when a constant viscosity is used. In fact, the higher the initial pouring and mold temperatures, the higher the effect of the viscosity variation on particle segregation.