Beyond the obvious

Metal matrix composites are often overshadowed by carbon fibre composites. However, the materials have distinct advantages that should not be overlooked. Justin Cunningham investigates.

If you think of a lightweight, performance-based material, carbon fibre reinforced plastic is probably not far from your top guess. It is the obvious choice and 'go-to' material for engineers who need to remove weight and improve performance.

Yet lightweight materials are abundant and engineering is all about finding the optimum and using the best material for the job. Engineers should always look beyond the obvious or fashionable and seek out all the options available before deciding on the best.

This was the case for US engine maker Pratt and Whitney. Speaking at a Green Aviation Conference in June, Dr Alan Epstein, vice president of technology and environment at the aircraft engine manufacturer, says: "We need ultra lightweight, extra efficient engine fan blades. The fan blades we are developing need to be extremely lightweight. So naturally, we thought the material that will give the best performance would be a carbon fibre composite material. But it is not. It is a type of metallic. Turns out the hybrid metallic [metal matrix composite] is lighter than composite and much less expensive."

Metal matrix composites (MMCs) are often overshadowed by carbon fibre composites when it comes to lightweighting parts and structures. But MMCs do have a number of unique and interesting advantages over other lightweight materials that have been used by industries such as aircraft, defence, space, oil and gas, nuclear fusion, renewable energy, automotive, marine and rail.

Being metallic, MMCs can withstand very high temperatures, making them a potential lightweighting solution for hot applications such as inside an engine. It also has the advantage of processability. On the whole, manufacturers know how to work with metals and have the tooling.

Farnborough-based TISICS design, develops and manufactures lightweight titanium composites. It was formed in 2005 through a management buyout from Qinetiq's fibre reinforced metal matrix composite business.

Stephen Kyle-Henney, managing director of TISICS says: "Initially the material was focused on the demanding, high-temperature, lightweight needs of hypersonic aircraft developments in Europe and America such as HOTOL and NASP, as well as fighter engine components where very high thrust-to-weight ratios are required."

TISICS produces materials that use silicon carbide monofilament fibres to reinforce metals such as titanium and aluminium. The reinforced region is bonded into the surrounding alloy to ensure that loads are transferred between the component's fixtures and the fibre. This exploits the materials' strength, stiffness and fatigue performance and unlike CFRP, metal composites are more easily exchanged on a like-for-like part basis.

"Fibre-reinforced titanium has the same stiffness and strength as high-strength steel, but is 40% lighter," says Kyle-Henney. "And, where systems can be redesigned to maximise the performance benefits, then 70% weight savings are possible."

TMC is roughly twice the specific stiffness and strength of conventional metals. As it can be more easily used for like-for-like material replacement in parts, it is easier for engineers that do not have room for significant redesign to utilise. The fatigue, temperature (high and cryogenic), low thermal expansion (relative to most metals) and excellent creep performance all provide benefits depending on the application.

One of the major advantages is the compression strength which is higher than landing gear steels and probably only bettered by ceramics. This is an exploitable property because it can be applied to components with tensile or bending loads, which would not be ideal for a ceramic.

"The low weight and corrosion resistance really benefit longer-life applications, as well as the through-life cost savings compared to other materials," says Kyle-Henney. "This helps to offset higher manufacturing costs."

TISICS plans to generate a large database of performance data and associated design modelling tools that any industry can use to develop products so that parts perform as predicted virtually.

"We have some of this data and customers have modelling techniques with very good correlation between designs and prototypes produced," says Kyle-Henney. "But these are not yet open access. We hope to gain UK or EU R&D programme support with academia and industry to generate this data and the associated design tools but £5million programmes are difficult to initiate when you are an SME."

Like much of UK industry, TISICS has greatly benefited from help by The UK Technology Strategy Board (TSB). This is not just in funding but also the networking opportunities that the board is able put together.

"The TSB has been extremely supportive to TISICS to achieve the goals above," says Kyle-Henney. "The ability to both develop innovative technology and work with potential industrial partners and customers is critical to an SME like TISICS."

Also in Farnborough is Materion Aerospace Metal Composites (AMC). The company was spun out of technology originally developed by oil and gas giant BP in the early 90s, and recently acquired by US-based materials giant Materion.

AMC produces a particle-reinforced aluminium alloy that takes superfine ceramic particles – usually a silicon carbide or boron carbide – to reinforce the aluminium alloy. It uses a powder metallurgy approach to distribute the ceramic particles evenly in the metallic matrix.

Though the material itself has not changed all that much in 20 years, the technology to produce it has evolved significantly to bring the material to a price point where it can broaden applications beyond F1 and fastjet aircraft.

Andrew Tarrant, director at Materion AMC, says: "MMCs offer very high stiffness to weight ratios, but in a form that is essentially isotropic. Our standard 25% ceramic grade metal matrix composites called AMC225xe, has an elastic modulus of 115GPa but with a density that is very close to aluminium alloys."

The materials isotropic property gives it distinct advantages over CFRP for acting as a load path in to a main structure, particularly if loads are experienced off-axis. Being metallic, MMCs also offer other significant advantages. The first is processability: Materion AMC usually sells its material as billets, shaped billets, plate, net-shaped forgings and extrusions, which can be machined to produce parts with very high surface finishes and tolerances.

"You also get secondary benefits such as thermal stability, capability to operate at temperature and the wear properties," says Tarrant. "The material has good tribological behaviour, but with lightweight characteristics."

The material's unique combination of high specific stiffness, thermal stability, enhanced fatigue endurance, corrosion resistance and lightweight properties has seen it used in aircraft and automotive engine components such as structures, engines, brake systems and gearboxes.

"We work mostly in specialist applications, but those are broadening," says Tarrant. "For Formula One and aerospace applications we tend to apply the techniques to give us the absolute maximum performance."

There are circumstances, however, where AMC produces lower-cost parts by manufacturing components at 80-90% of possible potential performance and these still offer a 50-60% performance advantage over conventional aluminium alloy.

The company is continuing to work on achieving a cost-effective manufacturing technique for these parts to continue to open up the opportunities and broaden the spectrum.

Metal matrix composites are, like carbon fibre, high-end, performance-driven materials. This makes it unlikely that they might be used as structural parts on a Ford Mondeo. However, there are examples of where that is changing, especially for efficient and lightweight engines. For connecting rods and valve train components, MMCs can replace steels with more than 50% weight saving.

"If you use a large lump of material the cost may become prohibitive for mainstream applications," says Tarrant. "But, if you are using the material efficiently and combining net shape techniques to save on finishing costs, it is not dissimilar to using a coating technology."

Justin Cunningham

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