Metal matrix composites: production by the stir casting method

A ceramic-reinforced aluminum matrix composite is formed by contacting a molten aluminum-magnesium alloy with a permeable mass of ceramic material in the presence of a gas comprising from about 10 to 100% nitrogen, by volume, balance non-oxidizing gas, e.g., hydrogen or argon. Under these conditions, the molten alloy spontaneously infiltrates the ceramic mass under normal atmospheric pressures. A solid body of the alloy can be placed adjacent a permeable bedding of ceramic material, and brought to the molten state, preferably to at least about 700° C., in order to form the aluminum matrix composite by infiltration. In addition to magnesium, auxiliary alloying elements may be employed with aluminum. The resulting composite products may contain a discontinuous aluminum nitride phase in the aluminum matrix and/or an aluminum nitride external surface layer.

Metal matrix composites

Metal Matrix Composites (MMCs) have evoked a keen interest in recent times for potential applications in aerospace and automotive industries owing to their superior strength to weight ratio and high temperature resistance. The widespread adoption of particulate metal matrix composites for engineering applications has been hindered by the high cost of producing components. Although several technical challenges exist with casting technology yet it can be used to overcome this problem. Achieving a uniform distribution of reinforcement within the matrix is one such challenge, which affects directly on the properties and quality of composite material. In the present study a modest attempt has been made to develop aluminium based silicon carbide particulate MMCs with an objective to develop a conventional low cost method of producing MMCs and to obtain homogenous dispersion of ceramic material. To achieve these objectives two step-mixing method of stir casting technique has been adopted and subsequent property analysis has been made. Aluminium (98.41% C.P) and SiC (320-grit) has been chosen as matrix and reinforcement material respectively. Experiments have been conducted by varying weight fraction of SiC (5%, 10%, 15%, 20%, 25%, and 30%), while keeping all other parameters constant. The results indicated that the ‘developed method’ is quite successful to obtain uniform dispersion of reinforcement in the matrix. An increasing trend of hardness and impact strength with increase in weight percentage of SiC has been observed. The best results (maximum hardness 45.5 BHN & maximum impact strength of 36 N-m.) have been obtained at 25% weight fraction of SiC. The results were further justified by comparing with other investigators.

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Development of Aluminium Based Silicon Carbide Particulate Metal Matrix Composite

Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, or continuous fiber. In this chapter, we first describe important techniques to process metal matrix composites, then we describe the interface region and its characteristics, properties of different metal matrix composites, and finally, we summarize different applications of metal matrix composites.

Metal Matrix Composites

Metal matrix composites (MMCs) have become attractive for engineering structural applications due to their excellent specific strength property and are increasingly seen as
alternative to the conventional materials particularly in the automotive industry. In this study, a historical background on the development and application of metal matrix composite for automotive brake rotor is presented. The discussion also includes analysis of the product life cycle with stir casting as a case study. The historical review analysis revealed that gradual development of material and processing technique have lead to a lighter weight, lower cost, and higher performance brake rotor as a result of the better understanding of the mechanics of metal matrix composite. It emerged from the study that stir casting technique provides ease of operation, sustainability and most significantly very competitive without sacrificing quality relative to other techniques;
and as such is the most attractive manufacturing process in the industry. These findings can be used for future design and manufacture of an efficient and effective aluminium
matrix composite brake rotor for automotive and other applications.

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Metal matrix composite brake rotor: Historical development and product life cycle analysis

A method and apparatus for preparing cast composite materials of nonmetallic particles in a metallic matrix, wherein particles are mixed into a molten metallic alloy to wet the molten metal to the particles, and the particles and metal are sheared past each other to promote wetting of the particles by the metal. The mixing occurs while minimizing the introduction of gas into the mixture, and while minimizing the retention of gas at the particle-liquid interface. Mixing is done at a maximum temperature whereat the particles do not substantially chemically degrade in the molten metal during the time required for processing, and casting is done at a temperature sufficiently high that there is no solid metal present in the melt. Mixing is preferably accomplished with a dispersing impeller, or a dispersing impeller used with a sweeping impeller.

Process for preparation of composite materials containing nonmetallic particles in a metallic matrix, and composite materials made thereby

A method for preparing cast composite materials of nonmetallic carbide particles in a metallic matrix, wherein the particles are roasted and then mixed into a molten metallic alloy to wet the molten metal to the particles, and the particles and metal are sheared past each other to promote wetting of the particles by the metal. The particles are roasted in air or other source of oxygen to remove the carbon from the near-surface region of the particles and to produce an oxide surface diffusion barrier, resulting in a reduction of carbide formation in the molten matrix. The mixing occurs while minimizing the introduction of gas into the mixture, and while minimizing the retention of gas at the particle-liquid interface. Mixing is done at a maximum temperature whereat the particles do not substantially chemically degrade in the molten metal during the time required for processing, and casting is done at a temperature sufficiently high that there is no solid metal present in the melt.

Process for preparation of composite materials containing nonmetallic particles in a metallic matrix

Silicon carbide particulate reinforced aluminum alloy matrix composites are formed using techniques which include agitation of a melt of aluminum alloy, containing magnesium, and silicon carbide particulates in a manner whereby the silicon carbide particles are maintained, during agitation, within the body of the melt; the agitation, which involves shearing or wiping of the particles in the liquid, is carried out under vacuum; and may involve incorporation into the melt of an additional amount of magnesium such that that amount compensates for the amount of magnesium which segregates to the carbide surfaces, and is sufficient to effect strengthening of the resulting composite. Aluminum alloy matrix composites, containing copper, are produced using similar agitation and mixing procedures, with the copper being incorporated in such a way as to discourage reaction between the copper and SiC particles.

Process for production of metal matrix composites by casting and composite therefrom

A cast composite material is prepared from a modified aluminum-containing matrix and reinforcement particles mixed into the matrix. From about 15 to about 130, preferably from about 20 to about 50, parts per million of an element, preferably beryllium, that forms a more stable oxide than magnesium oxide is included in the matrix alloy. The stable-oxide-forming element reduces the amount and thickness of the aluminum oxide and other oxides formed at the surface of the melt, which otherwise may be mixed into the melt to cause microstructural irregularities in the matrix of the cast composite material.

Cast composite material having a matrix containing a stable oxide-forming element

The development of viable techniques for the recycling and reclamation of metal-matrix composites (MMCs) is critical to the commercialization of these advanced materials. The recycling of both MMC wrought alloy (6061) scrap and foundry alloy (high-silicon) returns has been studied. The MMC extrusion alloy scrap has been recycled back into direct-chill cast logs and the resulting billet has been tested to determine whether the composite properties are degraded by repeated recycling. Similarly, fluxing and degassing techniques have been developed so that MMC foundry alloy gates and risers produced in shape-casting may be recycled back into useful castings. These fluxing and degassing processes have been tested commercially. Ultimately, when either type of MMC scrap can no longer be recycled, the alumina particles in the wrought alloys or the silicon carbide particles in the foundry alloys may be removed by common salt or other fluxing techniques. Rotary salt furnace technology has been shown to be effective for this approach, and the results of large-scale trials are reported here.

Michael D. Skibo

Papers

Process for preparation of composite materials containing nonmetallic particles in a metallic matrix, and composite materials made thereby

Process for preparation of composite materials containing nonmetallic particles in a metallic matrix

Process for production of metal matrix composites by casting and composite therefrom

Cast composite material having a matrix containing a stable oxide-forming element

The recycling and reclamation of metal-matrix composites