Robot on the wire

Traditional methods for the manufacture of large titanium aerospace parts can waste up to 95 per cent of raw material, but a technique being developed by an international research project promises little or no scrap.

The process, shaped metal deposition (SMD), manufactures components by building them up from welded wire - typically titanium alloy or aerospace-grade steel which reduces the machining needed to achieve final component design.

Rolls-Royce patented and first demonstrated the technology several years ago. Now researchers are looking for a way to automate the entire process as the system requires continuous monitoring and control by a human operator.

The three-year Rapid Production of Large Aerospace Components (RAPOLAC) project involves eight academic and industrial partners from the UK, Belgium, Italy and Argentina.

Now at its halfway point, the project is using a prototype SMD cell at Sheffield University's Advanced Manufacturing Research Centre (AMRC).

The system involves a robot arm carrying a Tungsten Inert Gas (TIG) welding head, operating in a sealed cell filled with argon gas. The robot welder follows a path derived from a CAD model of the component.

'It welds by continuously depositing wire on to the plate inside the machine,' said Rosemary Gault, research project manager at Sheffield. The wire is fed from a continuous reel next to the machine. 'It's basically like icing a cake or squeezing toothpaste out,' she added.

The robot then runs a continuous plasma discharge over the wire to melt and fuse it. Although the robot uses a welding head, Gault stressed that the process is different to normal welding.

'People mostly use welding for joining two parts together, whereas with this process you use it to basically make a part from scratch,' she said.

The movements of the robot-welding arm within the SMD cell are automated, but a skilled technician controls the welding parameters such as current and wire feed. This is a tedious job and an expensive waste of staff resources.

RAPOLAC researchers believe an automated control system that lets the cell run itself without continual supervision is vital for the commercial viability of the technology.

There has been progress on this with research partners, from the University of Catania in Sicily, who recently demonstrated fully automated control on a simplified version of the SMD cell, constructed at their own laboratory.

Giacomo Spampinato, a Catania researcher, said: 'We have found stable working points for the welding process. By adjusting the effective voltage of the welding torch and the speed of the wire feed around these points, we can use the feedback to control the welding process. We now aim to implement the system we have developed on our machine on the full SMD cell in Sheffield.'

Other RAPOLAC partners are testing and analysing the sample parts being produced by the Sheffield team.

A team at the Catholic University of Leuven studying the microstructure and mechanical properties of the components, found that they have similar strength to parts produced by established processes.

Meanwhile, researchers at Intec, part of the Universidad Nacional del Litoral in Santa Fe, Argentina, are studying heat transfer and the behaviour of the metal weld during the SMD process.

Such refinement and analysis is essential if SMD is to move from the laboratory to the factory floor. Footprint Tools, the leading industrial partner in RAPOLAC, aims to adopt the technology to help it expand its markets and offer higher-value services.

Gault estimates producing parts for commercial use in a year or two. 'Any sort of sacrificial aerospace components would be our first target because it is quite difficult to certify critical engine parts,' she said.