Printed titanium aircraft parts challenge forging strength with reduced cost

This is the aim of RAWFEED, a £995,000 R&D project, with £630,000 support from Innovate UK that aims to deliver pre-forms that are equivalent to Class One forgings whilst simultaneously reducing waste from 80-90 per cent to 30-35 per cent and significantly increasing production efficiency.

The RAWFEED (Rolling Assisted Wire Feed Direct Deposition for Production of High Value Aerospace Components) process uses a welding torch to deposit a continuous bead of material on a titanium baseplate. This first layer cools down and is then cold rolled to improve the material’s properties.

The consortium is developing and validating the performance of sub-system hardware and software behind the process, with Cranfield using a large friction stir welding machine to provide the forces and motion control required for cold rolling.

Adrian Addison, a senior research fellow at Cranfield University explained that wire and arc ALM can lead to parts that with large columnar grains that grow vertically through the structure as it is deposited.

He said: ‘It…gives you an isotropic property, so you get different strength depending on which direction you examine the material in, and that can be quite significantly different - perhaps 20 per cent - which is not what the aerospace guys want. They want something much more consistent.’

Addison explained that Cranfield had already worked on reducing residual stress in welds, which can be alleviated by applying a high-pressure roller across the top of the material.

He said: ‘We thought that perhaps this would be good on additive manufactured parts because we were trying to reduce - or eliminate - some of the residual stress, the shrinkage stress, that’s in each layer as you build up the part.

‘We started doing the high pressure rolling and…we did see that distortion was significantly reduced. When we did metallurgical sections we found that the grain had been refined as well. So, we did some investigation and what we found [is that] when you do the rolling – because it’s very high pressure and very local – it distorts. So it strains the material a bit and stores some energy in the crystal lattice which, when you put the next layer on the layer underneath, re-crystalizes into a fine grain structure.

‘Therefore, the large columnar grains that we had been seeing are refined. What we get then, in the structure itself, is a more fine grained equiaxed, multidirectional grain structure so the properties become anisotropic and improved as well because of the finer grain structure.’

To date, Cranfield has applied this rolling method on straight-line walls with a machine that that moves in one direction. To fabricate 3D and 2.5D parts, the Cranfield team is incorporating steerable deposition and rolling functions into its three-axis CNC friction-welding machine.

Addison said: ‘Starting early next year we’re going to move into the 2.5D parts. We’re also working with Delcam to produce control algorithms with their PowerMILL software so that instead of having to programme everything by hand we can actually generate tool paths using software, which is also quite a challenge both for deposition and for rolling.

‘As we go through we will be generating information which confirms the business case for the overall process…and also generating information like specifications…so that at the end of the project we are able to specify the machine tool which could be taken forward and built into production.’

In a statement, Curtis Carson, head of Research & Technology - Industrial Strategy and System at Airbus said: ‘Airbus currently procures £250m of these [titanium alloy] components every year, so the savings potential in terms of waste and production efficiency are enormous.’