Casting of metal matrix composites

Introduction

Metal matrix composites (MMCs) are gaining increasing attention due to their unique characteristics that provide advantageous solutions in terms of weight, mechanical and thermal properties. MMCs are comprised of two or more material components and provide an enhanced performance beyond that achieved by either material alone. The primary component of all MMCs is a metal, typically an aluminium or copper alloy, whilst the second component can be a ceramic, particulate, whisker, blade or any combination thereof.

The use of MMCs in a casting process requires formulation of a suitably matched alloy and the application of additive techniques in order to achieve homogeneous and fully fused results with minimal defects. In addition, the selection of a suitable feeding system, an appropriate pouring temperature, and the assessment of issues such as reaction and shrinkage cavities are vital for an efficient casting process.

Background

MMCs are materials that comprise two or more components, including metals, polymers, ceramics or combinations thereof. They exhibit superior properties compared to their single material counterparts. There are many advantages to the use of MMCs over traditional materials in a casting process. MMCs offer a tool to engineer high-performance components, and the high-performance composite components they produce can often be used in demanding applications. For example, MMCs can be used to increase the hardness, strength, and wear resistance of a component while simultaneously reducing its weight. The ability to create strong components of reduced mass can be beneficial in applications where performance requirements are high and weight must be minimized.

The mechanical properties of MMCs are substantially superior, making them an advantageous choice for components that must endure thermomechanical loading. With their higher corrosion resistance, superior acoustic insulation, and increased resistance to fatigue, MMCs have the potential to outperform traditional materials in terms of application life.

Issues

There are several challenges associated with the casting of MMCs. Due to the nature of MMCs, there is always a risk that the homogenization of the components may not be achieved, particularly in more complex blended matrices. Additionally, the selection of a suitable alloy-feed system, the casting temperature and the issue of shrinkage cavities can cause significant damage to the final product. If a material of a different melting point is included in the MMC, such as a particulate or whisker of a ceramic, the risk of micro- or macro-segregation is increased. Such segregation can generate bi-metallic or heterogeneous regions, reducing the homogeneity of the material and resulting in a product of low quality and strength.

The use of an additive to improve the casting yield and create a homogenous product can be a viable solution to the challenge of successfully casting MMCs. Such an additive must be chosen carefully, however, as inappropriate selections can cause more harm than good.

Techniques

The use of additives has been found to be an effective approach for the casting of MMCs. Additives are compounds that are added to the casting process in order to influence the microstructure of the material and control its mechanical properties.

The most common additive used in the casting of MMCs is a flux material. These materials are used to reduce the surface tension at the molten metal-mold interface, preventing unwanted surface defects and increasing the casting yield. Flux materials can be added to the melt at the mold inlet or used as a coating on the mold itself.

The use of vibration in casting processes has also been proposed as a method to improve material homogeneity. Vibration can aid the pouring process by encouraging uniform delivery into the mold and preserving the MMC’s homogeneous structure by preventing slumping or segregation of phases. It has been demonstrated that the casting yield for MMC can be improved by up to 25% simply through the use of vibration to direct the molten metal into the mold.

Conclusion

MMCs can be effectively cast using a range of techniques including the careful selection of alloy and the application of additives and vibration techniques. Such techniques can improve the material’s homogeneity and improve its mechanical properties. The use of MMCs in casting processes can provide a range of advantages over traditional materials, and the use of such techniques can ensure a successful and efficient casting process.

Metal matrix composites (MMCs) are advanced materials composed of metal and a second material such as ceramics, polymers, and carbon fibers. Their combination of excellent mechanical and electrical properties makes them desirable in many high-performance applications such as aerospace, automotive, and electronics.

The metal matrix of an MMC is typically formed from a pourable liquid alloy. The corresponding reinforcement is added to the metal matrix in the form of powders, fibers, particulates, or flakes in order to achieve a homogenous distribution.

MMCs are formed into components via several casting processes such as centrifugal, sand, and die casting. Centrifugal casting bodies are typically made of solidified metal such as steel or aluminum and can achieve part sizes beyond those achievable with other casting techniques. Sand casting of MMCs is commonly used to produce small and large parts with a variety of shapes, including complex and intricate shapes. Finally, die casting of MMCs is the most cost-effective process for the production of complex components with net-shaped geometries.

The process used to cast MMCs is critical to their ultimate properties and performance. The selection of the appropriate process may involve a trade-off between component cost and the desired performance. In addition, post-process treatments such as heat treatment, wear coating, machining, and inspection may be used to further enhance or modify the components properties.

Overall, MMCs offer a viable solution to manufacturing applications that require components with superior performance characteristics. The selection of the appropriate casting process followed by a post-process treatment plays a key role in tailoring the desired properties of the component.