Layers of manufacturing

Britain’s manufacturers will soon have access to a tool that is likely to take lightweight research to new levels.

Unique to Warwick Manufacturing Group (WMG), the bespoke machine will let the West Midlands team investigate whether plasma transfer arc (PTA) welding and additive manufacturing (AM) can be used to manufacture light-weight 3D components.

Put simply, AM builds parts gradually through the deposition and heating of layers of metal powder and WMG’s machine, delivered on 9 March 2012, is expected to provide fresh insights into exploiting the process.

The machine, described as a ‘hybrid additive subtractive manufacturing system’, will be employed for lightweighting research, for investigating the direct, tool-less manufacture of parts for sectors, including automotive, autosports and medical.

The finished parts created by Warwick’s latest research tool may represent a paradigm shift in Britain’s manufacturing capability, but the machine itself is essentially a CNC device. Dr Gregory Gibbons, a senior research fellow at WMG, told The Engineer that the PTA torch can be simultaneously fed from a powder or a wire feed, making it highly flexible.

The path of the PTA torch is fully pre-programmable in X, Y and Z, with the positional control of the torch integrated with that of the torch metal deposition ― which allows for controlled deposition of metal in three dimensions.

The position of the torch, and therefore the geometry of the weld deposit, is controlled by CNC.

‘PTA gives a high-quality weld at a high rate deposition of material,’ said Gibbons. ‘We want to get a material without any defects, at a significant rate. We’re aiming for perhaps 100g a minute, or up to 6kg an hour. This is high when you consider AM technologies tend to only build at 100g an hour, typically using a laser-based system.’

A CNC machining function is integrated with the PTA process and the X,Y,Z positioning system to enable automatic CNC processing of the deposit between layers ― including the interior of the structure ― which is unreachable using post-manufacture CNC.

“You can make a part that has an internal structure that you wouldn’t be able to machine after you built it”

‘The benefit is you can make a part that has an internal structure that you wouldn’t be able to machine after you’d built it ― you wouldn’t be able to get a smooth surface,’ said Gibbons.

‘We can remove the rough surface to allow better fluid flow, or to improve fatigue properties, so it enhances the quality of the part.’
Gibbons added that the non-bed-based machine also allows for the augmentation or repair of parts, leading to less waste compared with other methods of fabrication.

A decision to acquire the machine was made in early 2011 and it is expected to be operating at the end of March.

Once fully commissioned, manufacturers will be able to work with WMG in developing and evaluating new materials that are sustainable and low-cost, yet deliver highly complex lightweight metallic structures and components.

Gibbons said: ‘We’ve had lots of interaction with companies previously ― particularly large companies ― so we know what the scale of the components is [that] they manufacture.

‘We won’t be able to manufacture all their components as they’ll be off-scale, but for a certain number of their components 500 x 500 x 350mm3 is deemed a useable volume.

‘Compare that to a bed-based machine that only has 250 x 250 x 250mm3 capacity and you’ll see our potential is significantly bigger.’

The WMG team will develop parts in CATIA or Solidworks, and general G-Code will come from a post-processor. ‘It will take in the CAD, generate tool paths and post-process it to G-code, and send it to the machine,’ said Gibbons. ‘The G-Code will operate the PTA and the CNC. The modified G-Code will control the PTA.’

Finished parts have a good, fine microstructure because of the fast cooling rate afforded by AM

Gibbons believes the finished parts, from a metallurgical viewpoint, present a compelling reason for their greater uptake from industry.
He said: ‘Currently with AM, the quality [of the finished part] will beat cast hands down and approaches that of wrought. In some instances, depending on what you’re measuring, it beats wrought properties as well. The fatigue life of some AM is also higher than wrought material.’

Finished parts have a good, fine microstructure because of the fast cooling rate afforded by AM. Similarly, it is possible to generate a microstructure not normally attained by casting a raw ingot. ‘It doesn’t matter if you’re depositing from a wire or a powder, you get 100 per cent melting coalescence to give you very good properties, and the cooling rate can act in your favour as well.’
At WMG, Gibbons said titanium and its alloys will be a key material.
Gibbons believes WMG will also process Inconels (high-temperature nickel super alloys), where high stiffness can be achieved at an elevated temperature.

Other materials for consideration include 316L stainless steel, cobalt chrome molybdenum, and maraging steel.

‘They’re all standard materials you use in ALM that have application for lower-volume components,’ he said.