Welding makes major impact on 3D printing technology.
Since 3D printing was introduced there have been a number of developments, but the more recent use of fusion welding as a deposition source has opened up wide ranging possibilities in manufacturing.
The process is one in which metal is deposited layer-by-layer to form a three dimensional shape (Fig 1). Various melting techniques have been used to achieve this aim including electron beams and lasers but one being actively pursued currently is Wire and Arc Additive Manufacture (WAAM) using a GTAW (TIG) power source.
Titanium alloy airframe wing spar created using robot control of GTAW.
Driving Forces Behind the Development of WAAM
The primary driving force behind the development is the potential to make huge savings in materials and therefore costs.
One specific area of application is in airframe manufacture. The component in Fig 1. is made currently by machining from a solid billet or forging but over 50% of the original stock is lost as swarf. Another area under consideration is landing gear production where a cost saving of 70% is expected by using additive manufacturing.
Because of the high cost of titanium there are clearly huge financial incentives: the aerospace industry estimates requirements of about 20 million tonnes of billet material over the next 20 years. Conventional manufacturing strategies need reconsideration.
BAe Systems in the UK is working at RAF Marham to engineer ready-made parts for four squadrons of Tornado GR4 aircraft, including protective covers and guards. The WAAM process has been used by BAe Systems to produce a 1.2 metre long titanium alloy wing spar.
Electron beam and laser technology has been used with considerable success but this route to manufacture involves capital equipment that is expensive to purchase and to operate. Further work using these techniques is being undertaken in the UK, Germany and Sweden. A much more practical approach has been to use a standard arc welding procedure, usually with a GTAW (TIG) torch.
Early work at Cranfield University for Rolls-Royce targeted aero engine applications. Researchers here developed the wire + arc deposition process to examine the use of Inconel, titanium, aluminium and various nickel alloys. Since then the focus has shifted to airframes. Although laser and powder methods are useful for certain applications such as rapid prototyping or for small highly complex parts, this technology is limited by its speed and the size of component it can accurately manufacture. In contrast, the processes being developed at Cranfield are designed for high deposition rates.
To put this difference into context, the centre is currently targeting a deposition rate of 10kg an hour for titanium, compared with a typical 0.1kg using laser + powder methods, which can also potentially carry the risk of the material not being fully consolidated if fusion has not occurred between grains. Additive arc + wire systems are also capable of producing parts several metres in size and simplify the process of producing single piece linear intersections.
One of the main projects at Cranfield’s Welding Engineering Research Centre takes the technology one step further. The programme began in 2007 with funding from both the University’s Innovative Manufacturing Research Centre and 15 industry partners. The idea is to simplify the process of complete product within a single one-hit additive manufacturing system incorporating a fully integrated robot.
Additive layer manufacturing offers several advantages for certain structural airframe components such as a vast reduction in material wastage, especially when producing many heterogeneous parts, and the ability to produce a great variety of part designs for prototype work quickly.
There is also the key benefit that it allows the consideration of unconventional designs that otherwise would not be practical because of manufacturing or cost constraints due to, for example, complex or unusual geometries, bringing with it many different opportunities and challenges.
Overcoming the problem of oxygen contamination during WAAM
Many alloys may be used during the WAAM process simply by using the welding torch inert gas shroud as protection. However, some materials are much more prone to reaction with residual oxygen and this can lead to fusion zone and surface oxidation. Titanium alloys are particularly sensitive and demand additional inert protection.
With the electron beam process, protection is assured since operations are carried out in a vacuum. Nevertheless this is an expensive alternative to arc welding.
Huntingdon Fusion Techniques HFT® has worked with the Cranfield team to resolve the issue of adequate protection by developing flexible enclosures (Figs 2 and 3). These can accommodate the entire welding equipment and robot and provide inert gas protection throughout the process.
27 cubic metre flexible enclosure used to prevent oxidation of parts during robot controlled WAAM operations.
Small flexible enclosure used during development exercises.
The robot/enclosure interface is effectively sealed against leaks using an adaptable occlusion.
Flexible Enclosure Technology
There have been considerable advances in enclosure development since the concept was introduced over two decades ago. Huntingdon Fusion Techniques® for example has spearheaded a drive to design systems specifically for the welding industry. The company has been at the forefront in developing these enclosures for many years and has exploited the opportunities offered by advanced engineering polymers.
These innovative products offered significant attractions over both vacuum and glove box alternatives; a significant reduction in cost, very small floor footprint and availability of a range of sizes up to 27 cu m. The HFT® product has rapidly become the preferred alternative enclosure globally.
A combination of translucent material and optically clear sheet is used depending on the viewing requirements of the customer. Ultra violet stabilized engineering polymers are used throughout during manufacture. Material thickness is nominally 0.5 mm (480 microns).
Principle large access, leak tight zips are fitted and additional entry points can be provided for operators gloves. A service panel incorporates access ports for welding torches and for electrical leads and cooling water supplies. A purge gas entry port and an exhaust valve to vent displaced gas to atmosphere are incorporated into each enclosure.
If necessary, repairs can be carried out by the user on site and a kit is supplied for this purpose.
Size for size the HFT® range costs less than 10% of a metal glove box and only 2% that of a metal vacuum system.
Size and shape can be made to meet customer requirements. Standard models from 0.3 to 3.0 cubic metres are available from stock. Weight is very low and the enclosures occupy little space – the collapsed volume of a 1.25 metre diameter system is less than 0.2 cubic metres and weighs only 8 kg. They can thus be moved easily and stored efficiently so floor footprint is minimised.
Large viewing area
Large sections can be manufactured from optically transparent ultra-violet stabilized engineering polymers. This offers the opportunity for use by several operators at the same time – ideal for training purposes.
Multiple access points
Systems can be manufactured with numerous access locations for personnel gloves and gas/electrical entries. Large leak-tight zips afford easy access for components.
The largest facility supplied to the Cranfield Welding Engineering Research Centre has a volume of 27 cu m, adequate to accommodate all work-pieces, welding equipment and even a programmable robotic system. The research team ensure that the optimum gas environment during welding of titanium alloys is achieved by evacuating the enclosure prior to admitting high purity argon gas. This gives an operating oxygen content well below 100 ppm (0.01%) and for the Cranfied team this is considered low enough to prevent significant oxidation of titanium alloys during welding and cooling.
One of the largest 3D metal parts in the UK has now been produced as a result of the Cranfield research. Designed by BAE Systems engineers, the part measures 1.2m in length and is made from titanium alloy. Known as a ‘spar section’ this forms a main structural element in the aircraft wing and took just 37 hours to build from a digital model, where previously this process would have taken many weeks.
- P.A. Kobryn, U.S. Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLMP, Wright-Patterson AFB, OH 45433 USA
- P. Edwards, A. O'Conner and M. Ramulu
J. Manuf. Sci. Eng 135(6), 061016 Nov 18, 2013
- Current activity at TWI and Warwick University (UK), Trumpf (D), ARCAM (S) and University of Texas (USA)
- Design for Wire and Arc Additive Layer Manufacture. J. Mehnen, J. Ding, Lockett, P. Kazanas. Manufacturing Department, Cranfield University
- Williams, S.W., Martina, F., Addison, A.C., Ding, J., Pardal G., and Colegrove, P., 2015. Wire + Arc Additive Manufacturing, , Materials Science and Technology
- Huntingdon Fusion Techniques Ltd, United Kingdom. www.huntingdonfusion.com
- Brooks R. Siemens Now Producing Gas Turbine Parts by Metal 3D Printing. American Machinist Feb 4, 2016
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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