Times have changed in the aerospace sector, with composites stealing a march on metals and winning a much bigger role on new aircraft programmes. Indeed, composites accounted for 50% of content by weight on the new Boeing 787 Dreamliner, a figure that would have seemed highly unlikely a decade ago.
But that doesn’t mean that research into new metallic technologies and materials has come to an end. Far from it – major companies like GKN continue to invest heavily in research into new aluminium and titanium alloys and radically different machining techniques in an attempt to find new applications for metallic components. In some aircraft structures, trade studies show that metals remain a better solution than composites because of their specific characteristics, and GKN is excited by the potential that such new materials and machining and welding approaches are starting to bring.
“It’s too simplistic to think that if you consider composite for one application it’s the right technology for another,” says Richard Oldfield, technical director at GKN. “There is still a lot of interesting research on composites but there are a whole bunch of activities on the metallics side which are also extremely interesting in terms of how they can support future aircraft structures.
“Metallic technology is absolutely not dead – there are some really exciting things happening that will continue to make metallic structures competitive as we move forward. Metallics still account for 50% of our business and will continue to do so. There is a significant growth in composites but the amount of metallics work we have will remain. I think the key message is – metallics will always remain the right material for the right application.”
For example, GKN is involved in a three-year project looking at linear friction welding of aluminium and titanium that could deliver a 40% reduction in material consumption relative to plate machined consumption. A lot of its research effort comes under the banner of closer net shape technologies – trying to improve the fly-to-buy ratio of the metallic structures it produces. At present with certain machining techniques the fly-to buy ratio can be as little as 10%, meaning that 90% of the material is wasted. So for cost and environmental reasons, GKN is keen to achieve nearer net shape technologies so that it is machining away a minimum amount of material.
“An example of one way we might achieve this is through linear friction welding – the ability to join small billets of material together into shapes so that you can produce a freeform before you machine it,” says Oldfield. “Rather than machine something from a solid block, you can weld some of those features onto a blank before you start which gives you the ability to significantly reduce the amount of machining you perform on a particular structure.
“That’s a project we are doing at the moment. We’ve taken a number of example products from Filton and we are now looking at re-engineering those products using linear friction welding to produce an engineered blank. Then we will machine that engineered blank, and test it, look at the material properties, but the expectation is that we are able to move from the conventional solution that we have today towards a linear friction welded engineered blank which will enable us to make dramatic reductions in machining time and improvements in material utilisation.”
First GKN needs to make sure the properties of the welding are adequate and the repeatability of the process is acceptable. And ultimately it would need to prove that any linear friction welded product is fit for purpose, getting it through certification before any fundamental change to manufacturing activities can be made. “But it’s an exciting technology for our aero-structures business and our engine products business,” insists Oldfield. “We make a lot of wing components today that have a lot of features on them.
If we could linear friction weld some of those features onto the products we could potentially bring some quite attractive improvements to our customers. It’s the same with our engine products – if we could linear friction weld the blades onto blisks then that would represent a very attractive technology.”
GKN is also looking at two different additive technologies. The first is producing components through an electron beam additive (EBA) technique, where metal is delivered through a wire feed into the electron beam of the welding head, meaning it is possible to deposit that material in a form most suited to its application. Effectively GKN would be able to “grow” aircraft component features by building them up from welding the wire into the desired shape.
EBA brings the potential of much greater freedom for aircraft part design. Once the part has been grown, all that would be needed is a final machining operation to complete the construction process. Growing features onto products rather than having to machine them out of solid material potentially brings the ability to significantly reduce material utilisation and throughput time. “We are already producing demonstration parts using EBA,” says Oldfield. “I’d expect that in two years time you will be seeing products made like this.”
GKN is also looking at laser additive techniques, effectively laser sintering components using a powder to grow a component from the base to the finished article. This brings the potential of even more design freedom than EBA, because it enables parts to be created “inside-out”, enabling GKN to design structures and build components that would be impossible to make any other way. “It offers absolute freedom,” says Oldfield. “This is one step further away from being ready to use. We are currently carrying out various demonstration activities – and, while simple applications are not far away, full exploitation of the technology will probably take five years. We are involved in something here that we think is really interesting.”
Advanced joining techniques are also creating great interest. Welded structures will have a significant part to play in future aircraft programmes, particularly because they bring the chance to reduce fastener use and therefore cut down on assembly times. GKN is involved in several collaborative projects looking at joining dissimilar materials through technologies such as friction stir welding of aluminium alloys to give different properties in different parts of the structure. “For instance on a wing spar you require different properties in the cap and the web of the spar so there is the opportunity to join dissimilar materials together for further optimisation,” says Oldfield. “We have welded structures on military aircraft like the A400M and we’d like to apply that to civil aircraft.”
Hybrid machining is also something being looked at by GKN. This employs a combination of water-jet cutting and final finished machining. This is about optimising the way in which GKN can produce a component to maximise the use of the high-value assets such as expensive machining units. “We want to find other ways of cutting material closer to form before finishing it using a high-end machine,” says Oldfield. “It’s about improving utilisation of the machine through the use of other techniques, such as water-cutting. It’s all about reducing cost, reducing cycle time and making the maximum use of the installed assets that we have. We have a huge amount of capital equipment at our plants. If we could get an extra 30% in terms of asset utilisation it would be huge in terms of throughput without having to make more capital investment.”
Even the sorts of materials being used are under consideration. GKN is looking at highly machinable titanium and aluminium alloys, with a particular interest in aluminium lithium. In the medium term it will also assess more exotic materials such as nanomaterials, open-cell foam metals and titanium powders. Here Oldfield is confident that advances can be made. “There are new developments of alloys coming on-stream all the time,” he says. “Ultimately we are looking for higher-strength materials. The key trade for advanced metallics is that they have to be weight competitive with composites. That’s the key issue, particularly on large commercial aircraft. If the metallic structure can be competitive from a performance perspective then it becomes very compelling to use metallic structures.”