The origins and future of Metal Matrix Composites

Uploaded 10 Nov @ 10:24am

Mel Dunkin looks at the use of metal matrix composites (MMCs) in manufacturing. He says it is certainly not a new idea as researchers first explored its possibilities as far back as the early ‘50s. At the time, the many potential benefits of these emerging materials quickly drew the interest of the defence and aerospace industries. Despite this, the prospect failed to stir up much excitement in commercial circles.
Composed of a metal combined with one or more added metallic or non-metallic constituents, the resulting new materials offered a markedly improved strength to mass ratio. This property positioned them as an ideal lightweighting option for use in the construction of airframes and military vehicle bodies. That said, further developments, during the ‘60s, saw the combination of ceramic whiskers with various metals. This combination provided exceptional resistance to high-temperature and thus offered the added prospect of much lighter engine parts.
During the following decade, spurred by the visible success of aircraft manufacturers, the automotive industry also began to investigate the use of composites in its products. At the time, one of this industry’s primary interests lay in ways to reduce fuel consumption by increasing the vehicle’s power-to-weight ratio. For vehicle manufacturers, too, lightweight MMCs offered the perfect solution.
MMCs offered more than just a lightweighting option, however. Some of these new materials displayed other properties that were of value to the automotive industry and others. Among these properties, the high thermal conductivity of aluminium-graphite composites in combination with their low density and thermal expansion has proved invaluable in the manufacture of various electronic modules. For similar reasons, aluminium-silicon carbide and a copper-silver alloy matrix laced with diamond particles, and known as Dymalloy, are now widely employed as substrates in the manufacture of hybrid integrated circuits.
Some of these composites can also be extremely tough. For example, tungsten carbide tools derive their exceptional strength from a matrix composed of cobalt containing the dispersed carbide particles. Likewise, the combination of boron nitride in a steel matrix serves to improve the performance of tank armour.

The Rise of
3D Printing
Research into the potential uses for these materials has intensified worldwide. One result of these efforts has been to open up new opportunities based on their role in 3D printing. Subsequently, this technology has also demonstrated the potential to disrupt the previous methodology used for the manufacture of products from metal matrix composites.
Frequently, the conventional approach to manufacturing with MMCs is to employ molten stir casting. This process most often results in the production of billets, and these must then be subjected to subtractive techniques to arrive at the finished product. Where exceptional hardness is an essential quality of the metal composite, secondary machining of the rough casts can tend to prove difficult, adding both to production times and the cost.
Until recently, the best way around this difficulty was the use of powder metallurgy in a ‘near net shape’ manufacturing process. However, although this means less secondary processing is necessary, machining structures such as cross-bores and threads in these tough, porous composites still poses difficulties.

Additive to the Rescue
Not surprisingly, perhaps, a move to additive technology when working with MMCs seemed likely to offer manufacturers a more practical option. Now, following advances in 3D printer technology, faster and cheaper processing of both lightweight and heat resistant metal matrix composites has become a reality.
While a CAD file remains at the ‘heart’ of the design process and its interpretation, selective laser melting (SLM) is commonly the technique employed for the additive processing of the MMCs. Sometimes referred to as laser powder bed fusion (LPBF) or direct metal laser melting (DMLM), this technology can be applied to mass production or rapid prototyping, as required.
The range of MMCs that can currently be processed using 3D printing technology is already quite extensive and is continuing to expand. The list includes a cobalt-boron carbide composite used to manufacture super-strong cutting tools, as well as the titanium and tantalum powder composites popular in various biomedical applications.
While an electron beam is an alternative means to achieve fusion, despite being the faster option, it is unable to match the detail and quality of finish possible with SLM technology. That said, a rougher finish is desirable for some orthopaedic implants, and so there is a significant demand for products created using the electron beam fusion process.
In summary, it appears that 3D printing will remain an essential manufacturing tool for the foreseeable future. That it could also become the method of choice for processing of metal matrix composites seems equally probable.

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