Welding makes its mark in 3D printing

Welding makes its mark in 3D printing
Welding makes its mark in 3D printing

Delta Consultants' Dr Michael Fletcher looks at how welding is making major impact within the 3D printing technology arena.

Since the advent 3D printing there has 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. 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 gas tungsten arc welding (GTAW) power source. The primary driving force behind the development of WAAM is the potential to make huge savings in materials and therefore costs. One specific area of application is in airframe manufacture. A wing spar component is typically made 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 3D printing. Because of the high cost of titanium there are 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. Wing spar goes far BAE Systems 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 1.2m 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. 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 rapid prototyping or for small highly complex parts, it 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. 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. Flexible enclosure technology Huntingdon Fusion Techniques (HFT) has worked with the Cranfield team to resolve the issue of adequate protection by developing flexible enclosures. These can accommodate the entire welding equipment and robot and provide inert gas protection throughout the process. There have been considerable advances in enclosure development since the concept was introduced over two decades ago. For example, Huntingdon Fusion Techniques 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 27m3. 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. Ultraviolet stabilised engineering polymers are used throughout during manufacture. Material thickness is nominally 0.5mm. 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 3.0m3 are available from stock. Weight is very low and the enclosures occupy little space – the collapsed volume of a 1.25m diameter system is less than 0.2m3 and weighs only 8kg. They can be moved easily and stored efficiently so floor footprint is minimised. Large sections can be manufactured from optically transparent ultraviolet stabilised engineering polymers. This offers the opportunity for use by several operators at the same time – ideal for training purposes. The largest facility supplied to the Cranfield Welding Engineering Research Centre has a volume of 27m3, adequate to accommodate all workpieces, 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 of below 100ppm (0.01%) and for the Cranfield team this is considered low enough to prevent significant oxidation of titanium alloys during welding and cooling. www.huntingdonfusion.com

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