Process reduces waste in titanium manufacture

Traditional methods for manufacturing large titanium aerospace parts can waste 95 per cent of raw material, but an international consortium is commercialising a new technique that 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. As reported in The Engineer in 2008, researchers involved in the international Rapid Production of Large Aerospace Components (RAPOLAC) project have been working on developing a way to automate the entire process, as the system requires continuous monitoring and control by a human operator.

The results of the three-year project involving eight academic and industrial partners from the UK, Belgium, Italy and Argentina have been pronounced a success.

Project manager Rosemary Gault said the team now has a proof of concept SMD cell running at Sheffield University’s Advanced Manufacturing Research Centre (AMRC). They have produced test parts for six different aerospace companies and have also been approached by companies from other sectors, including medical and motorsports.

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.

As Gault explained to The Engineer in 2008: ‘It welds by continuously depositing wire on to the plate inside the machine. The wire is fed from a continuous reel next to the machine. It’s basically like icing a cake or squeezing toothpaste out.’

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.

While the original Rolls-Royce technique permitted the movements of the robot welding arms within the SMD cell to be automated, it still required a skilled technician to control the welding parameters such as current and wire feed.

With help from research partners, from the University of Catania in Sicily, the RAPOLAC researchers were able to develop an automated control system that lets the cell run itself without continual supervision.

Gustavo Escobar, a research associate at Sheffield, said the control system uses a range of sensors for measuring the gap between the plasma discharge arc and the wire it intends to melt and fuse.

This data is processed in real time, to compute the exact amount of wire needed to be fed in at each process cycle. It also guarantees the stability of the process.

The automated control was integrated with process models developed by Samtech, an engineering software company based in Liège, Belgium.

The Catania team has applied for a patent on their work.

Stable parameters

Researchers at the AMRC looked at how changing welding parameters such as travel speed affects material properties such as hardness. 

This knowledge was then used to develop a stable parameter window with known mechanical properties for the desired part size.

Meanwhile, researchers at Intec, part of the Universidad Nacional del Litoral in Santa Fe, Argentina, studied the behaviour of the metal weld during the SMD process and predicted the way temperatures affects stresses on the microstructure.

Gault said the microstructure and mechanical properties of the components produced by SMD were studied closely by a team at the Katholieke Universiteit Leuven in Belgium where parameters such as fatigue, tensile strength, and oxygen and nitrogen levels, which can cause brittleness, were assessed.

Tim Chapman, communications manager at the AMRC, said tensile testing was a specific concern. Many believed that parts built up by layers, rather than carved out of a solid block of material, could be weaker, he said. 

Gault added, ‘All the parts have to be certified if you’re going to fly them. You have to be as good as what’s already available.’

According to the team in Belgium, the material strength and ductility of parts produced by SMD exceed the standards for cast material, compete with the standards for wrought material and meet the current aerospace quality standard for additive layer manufacturing.

Gault said that, surprisingly enough, the researchers found that parts produced manually could not exceed the quality of those manufactured by automated controls. 

‘We can now prove the properties are good,’ she said. ‘It’s a matter of selling it now.’

Footprint Sheffield, the leading industrial partner in RAPOLAC, aims to adopt the technology in its own factory.

Gault said the company, which formerly specialised in hand tool manufacturing, is looking to diversify into the high-value aerospace sector.

‘Once you get into the aerospace supply chain, you’ll pretty much stay for the next 10 years,’ she said. 

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