Up and away: Four squadrons of Tornado jets will be supplied with parts made by 3D printing
These are interesting times in the aerospace sector, as advanced manufacturing techniques promise to replace traditional methods such as milling or forging for the production of big components.
Only last month, BAE Systems announced that a military jet had flown with a metallic 3D printed part on-board for the first time. Exciting as that was, more sophisticated additive techniques are in the pipeline, bringing the possibility of much larger printed components such as ribs and spars.
Potentially the new production methods deliver important advantages over traditional techniques: 3D printing is cheaper than most subtractive methods which entail the milling away of a lot of waste material. And 3D machines could one day be positioned at the front in wartime, significantly reducing lead times for vital components needed in theatre.
The BAE project has offered a glimpse into the future – powder deposition technology was used for the printing of a stainless-steel bracket. Other non-metallic ground equipment parts for the Tornado jet have also been produced, including protective covers for cockpit radios, support struts on the air intake door, and protective guards for power take-off shafts.
Matt Stevens, who is looking at how 3D printing might be applied across BAE Systems, explains the technology. “We have developed a number of capabilities around metallic and non-metallic 3D printing processes,” he says. “The part on the Tornado was manufactured in stainless steel using a metal powder bed machine. We started off with a base plate, then the machine laid down an 80-micron layer of metal powder. This was then selectively melted with a laser and consolidated where we wanted it to be. Another layer was put down, and repeated, until the part was built.”
Fit to fly: This stainless-steel bracket made by 3D printing went airborne on a Tornado last month
Making the bracket for the Tornado was a relatively straightforward build process, he says. “For a first-off development, we didn’t want to attempt a high-risk, critical-structure part.
“From first design to having it in our hand took approximately two weeks. And then we completed the standard qualification process that any component would have to go through in order to be fitted onto a fast jet.”
BAE’s combat engineering team is using 3D printing to produce ready-made parts for supply to four squadrons of Tornado GR4 aircraft. Some of the parts are costing less than £100 per piece to manufacture. Stevens says that 3D has already resulted in savings of more than £300,000 and will offer further potential cost reductions of more than £1.2 million by 2017.
Those levels of returns mean the company is looking at using 3D printing on a wider array of parts, and across other airframes. “There are quite a range of parts that we are looking at,” says Stevens. “On the non-metallic side of things we are looking at ECS-type ducting, and almost any non-structural application where currently we use any sort of polymers. We’ve already got non-metallic parts that have flown this way, having produced a component for a customer on the VC10 last year and for our regional jet arm.
“This is a technology that allows us to print one-offs at purely the costs of the printing process itself – you can save significant amounts of money compared with injection moulding on one-off or small-batch production.”
On the metallic side, using titanium and aluminium, Stevens says there is an ambition to one day apply 3D printing technology to the production of safety-critical parts. He says: “We want to develop the technology to a point where it will replace traditional forgings or castings. We estimate that will take somewhere between five and seven years, which isn’t too long in terms of airframe design.”
And he sees no reason why the technology should be restricted to the air systems side of the business. “Potentially it’s a real game-changer. Initially we saw a possibility on the air side of the business, but we are also working with other BAE divisions to look at opportunities within their areas. It might, for instance, be feasible in the future to house 3D printers on aircraft carriers or any other type of maritime vessel. You have the opportunity to print where the parts are being used. That means we will be able to turn things around much quicker.”
There are, however, limitations with the powder deposition technology, specifically in terms of component size restrictions caused by having to carry out the printing process inside a cabinet. At the moment, BAE works to a maximum 300mm3 build envelope, although companies are starting to produce machines with 500mm3 capability. This limitation is driving research into freeform processes that come without constraint. The technology is at a lower state of readiness, but it is developing fast.
Box of tricks: The 3D printing machine at BAE Systems
Much of this work is being done at Cranfield University, which is looking at a potentially disruptive technique known as wire plus arc additive manufacture (WAAM). This allows components to be printed in weld metal. The basic hardware comprises four parts: CNC controller, motion system, heat source for melting the metal, and a method of adding material, usually a robotic arm.
The process uses an electric arc as the heat source, and material is added in the form of wire, producing freeform components.
WAAM effectively delivers freespace unrestricted weld metal deposition – it can essentially be thought of as a very high-speed micro-casting process. Virtually any shape can be created by adding successive layers of material. The process can produce vertical, horizontal and angled walls, mixed-material conic sections, enclosed sections, crossovers and intersections.
Crucially, the WAAM process is not constrained within a cabinet, so larger components can be produced. Indeed, Cranfield and BAE Systems have already used it to print a 1.2m-long titanium spar section. WAAM can put down weld metal at up to 4kg an hour. The process uses off-the-shelf aerospace-grade wire. Cranfield has produced components in titanium, aluminium and steel, so far.
The Cranfield research is being led by Stewart Williams, professor of welding science and technology. He says: “WAAM parts are built up in layers – it’s a bit like icing a cake. You take a CAD drawing and then programme the robot to deposit the layers. The robot follows the pattern, increasing the layers, until you get to the desired part.
“Much of our work on WAAM so far has been around ensuring that we get as close to the net shape as possible. We have developed very detailed process algorithms that can accurately relate the process parameters to the feed shape geometry, meaning we can control the height and width of the bead being put down. We can now control it to less than half a millimetre.”
Scaling up: WAAM can be used to make sizeable parts such as this 1.2m titanium
spar section
One of the key advantages of WAAM over traditional subtractive production methods is cost. Milled parts often result in huge amounts of material waste. Williams says this isn’t the case with WAAM, which can deliver a buy-to-fly ratio – the cost associated with the amount of raw materials required to produce a finished part – of less than 1.5:1. Some aerospace components milled in titanium using traditional methods can have a buy-to-fly ratio of up to 40:1, says Williams.
“The main driver of WAAM is cost reduction. Other wire-based processes can achieve a buy-to-fly ratio of 3:1. We generally build to 1.2:1, and we guarantee to achieve less than 1.5:1.”
The process is very scaleable, says Williams. “You can make whatever you want,” he says. “Most of the parts we have made to date have been on a robot-based system. The scale of the part is down to whatever size robot you have. The other process is a machine tool-based approach – we have a 5m x 3m x 1m system that enabled us to build some fairly big parts.
“Next month we are building a 2.5m wing rib for Bombardier in aluminium. We will be using two robots building symmetrically on either side, so we will be doubling the build rate. You can scale the process by adding more power sources, although there are some limits related to residual stresses and heat build-up that need to be dealt with.”
Williams thinks the development of WAAM will have huge implications for the aerospace sector. So far, studies at Cranfield have shown that the quality of the WAAM parts is inherently high, and better than that produced by normal welding procedures. Additionally, WAAM parts don’t need any post-processing such as hot isostatic pressing.
He says: “What we have found for titanium is that we can introduce what is known as inter-pass rolling. We put a layer down, then we cold roll it, and that transforms it to a very highly equiaxed anisotropic microstructure that essentially has the same properties in both directions. Normally you get different properties in different directions. Our research shows that we are able to get higher strength and better ductility, and that’s due to producing a very highly refined grain structure.”
Williams thinks that in the not-too-distant future WAAM will be used to print safety-critical parts that form main structural elements of aircraft wings.
“That’s what we are planning to do,” he says. “We are hoping to get it qualified for flight use for safety-critical parts by 2019.
“There’s a lot of hype about powder deposition technology, but the parts being produced are so small scale and expensive that I can’t see them being used. WAAM technology is so simple. Our industry partners think it’s going to be a viable route forward, with cost saving the big difference.”
Those industry partners include the defence and aerospace giant Lockheed Martin which has established a WAAM test laboratory at its Ampthill facility in Bedfordshire. Lockheed Martin is interested in using 3D printing, but has not been convinced by the material quality offered by traditional laser sintering and powder deposition technologies.
Steve Burnage, the engineer responsible for the development of WAAM at Lockheed Martin UK, says: “We feel it is a better approach for our applications. From what we have done so far, the resultant properties that we have got from steel and particularly titanium are looking very good.
“They have given us the same sort of material properties that you would expect from a basic billet of those materials.”
Lockheed Martin also thinks that WAAM offers huge potential in terms of better buy-to-fly returns. Burnage says: “With conventional techniques, on something like a wing spar as much as 90% can be machined away. It’s very expensive. With WAAM we are aiming for 10% because it gets such a good net shape, and then you can skim back in the areas you need.
“The lay-down rates are also very good. WAAM can really throw down an awful lot of material – up to four times as much compared to sintering.”
The facility at Ampthill gives Lockheed Martin the capability of producing fairly large, thin-walled conical structures using the process. Such components might one day be used on military space platforms, says Burnage. “We are producing parts that are typically 0.5m in diameter with 2mm wall thickness by welding. We can now start to consider WAAM rather than traditional methods of manufacturing.”
The Ampthill facility has a fully air-conditioned welding booth with multi-core robot arm which is being used for collaborative research. “We are getting good, stable products out of it,” says Burnage. “The facility is state-of-the-art: so much so that Cranfield is coming over and using it. They helped us spec it, and now we are helping them with their research.”
The laboratory will be used in the coming months to overcome certain challenges associated with the WAAM process.
“One issue is optimising the lay-down rate to the structure we are manufacturing, and understanding how we control the thermal wicking to minimise build-
up of thermal stresses,” he says.
Overall, Lockheed Martin believes that WAAM is an exciting technology that will have a big impact on its business. “We can see how it might be used for very large pressure vessels for space applications in titanium, for instance, and we would love to get it into military aircraft.”
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