Powderful allies

Handling powders used to be the preserve of the process sectors, from pharmaceuticals to food. But new manufacturing techniques for handling metals and blending them with other materials are making powders increasingly important in other sectors.

It is becoming clear that powder processing might be the best way to make and work with high-strength alloys — and much of the research is attracting interest and support from the aerospace industry.

The subject is being addressed in the UK by Powdermatrix, one of the industry academia Faraday Partnerships that develops and transfers new technologies from the lab to the marketplace. Among the company’s projects is a method for adapting a continuous rotary extrusion technique that can extrude titanium powder.

Developed with Outokumpu Holton, a Finnish-owned, Bournemouth-based metal processing company, the system could allow the process to handle higher-value materials for more specialised applications.

Known as ConForm, the continuous rotary extrusion process was originally developed by the UK Atomic Energy Authority in the 1970s. Its advantage is that is transforms a traditional batch process of extrusion, where a great deal of pre-processing is needed to make a metal billet in the right form and temperature to be extruded, into a continuous process.

Instead of a billet, the system uses rods or particulate feedstock, fed into a continuously rotating, grooved extrusion wheel where heat and pressure combine to transform the metal into a state that can be extruded. The system can be used to make bars, power cables, wires, strips and shaped profiles.

‘Several people have asked us whether we can work with titanium, but it has turned out to be rather difficult,’ said Outokumpu’s Phil Thomas.

Titanium is becoming less expensive, he explained, thanks to a variety of lower-cost production routes; but these tend to produce a powder or sponge form of the metal. The biggest problem for any material in the ConForm system is persuading it to flow, said Thomas.

‘Powders tend to be sticky — aluminium sticks to itself, the groove and the wheel. And if it doesn’t stick, the particles tend to roll around in the groove. Also, titanium oxidises very rapidly.’

Outokumpu Holton engineers consulted titanium specialists at the Department of Materials, Imperial College London, under lead researcher Martin Jackson. The college’s researchers confirmed that titanium sponge should be suitable for extrusion, and processing it would require the equipment to be set up as though it were extruding a highly alloyed aluminium alloy.

For the ConForm process to work, the grooved extrusion wheel has to be coated in a ‘tyre’ of the material to be extruded. Early attempts to coat it in pure titanium failed, but a 50:50 mixture of aluminium and titanium proved to be successful.

‘The aluminium-titanium mix gave us a product,’ said Thomas. ‘It wasn’t pretty, but it was a good starting point.’

The key to improving performance was the introduction of copper to the mix.

‘Copper has similar physical properties to titanium — especially its strength and melting point — and it improves processability,’ explained Thomas. This gave a better result, although the tooling needed to be modified to raise the temperature inside the wheel groove to improve the ductility of the mixture.

‘This proves to us that extrusion is possible with these lower-cost forms of titanium. It’s early stages, but it’s a good platform for further development,’ he said.

Titanium and its alloys are in increasingly high demand in the aerospace and medical device sectors, because of their light weight and high strength, said Thomas. Their poor processability is a barrier for their use, however, and low-cost processing techniques such as extrusion would allow a wider range of components to be made.

But titanium is not the only metal to be studied by powder specialists. A research consortium led by Xinhua Wu at Birmingham University’s Interdisciplinary Research Centre in Materials Processing is studying a new class of materials which could be crucial for future generations of jet engines.

Currently, jet engine turbine blades are made from a form of nickel known as single crystal. This is stronger than conventional nickel, because there are no ‘fault lines’ between metal grains to weaken the structure. However, because the engines become more fuel-efficient when they run hotter, operating temperatures are increasing.

Aerospace industry observers predict that the limit of single-crystal nickel blades’ temperature tolerance will be reached within the next two decades, so the search is on for new materials which will be able to withstand the mechanical and thermal stresses.

One possibility is intermetallic compounds of niobium and silicon (NbSi). Intermetallics are a sub-class of alloys where the stoichiometry — the proportions of the components — is very strictly defined, and they have structures where atoms of the ‘guest’ metal sit within the lattice of the ‘host’ metal in a highly ordered arrangement.

Although materials theory predicts that they will exist in many metal mixture systems, they are often difficult to make, so have not been studied in detail. Theory predicts that the NbSi compounds should be extremely resistant to temperature, withstanding temperatures several hundred degrees hotter than single crystal nickel’s service ceiling.

They should also be around 25 per cent less dense than current materials, raising the possibility of a significantly lighter engine, with the knock-on effects of reduced noise and improved fuel efficiency.

However, almost nothing is known about the other properties of the materials.

The Birmingham research, which also involves Rolls-Royce, Qinetiq, Alstom Power, Phoenix Scientific, Tetronics and Bodycote HIP, aims to produce large samples of the compounds, and investigate how changing the stoichiometry might alter the properties.

Intermetallics tend to be brittle, because the mixture of different-sized atoms in the lattice structure disrupts the flow of atoms around each other that gives metals their ductility. One goal of the project, therefore, is to see whether increasing the proportion of niobium would improve the ductility, and what effect that would have on strength.

The team is using diffusion bonding to create the materials, where a mixture of niobium and silicon powders are subjected to extremely high temperatures and pressures to force them to diffuse into each other at an atomic level.

Researchers are also using the technique to test whether samples with different proportions of elements can be joined together. In particular, they are trying to join a high-silicon compound, which will be extremely heat resistant, and therefore suitable for use at the tip and edge of a turbine blade, but is likely to be very brittle, to a high-niobium compound, which would be far more ductile and therefore suitable for the root of the blade.

The studies are at an early stage, but the project partners are hoping that by 2007, they will know enough about the materials to decide whether to take the next step.