Shaped metal deposition (SMD) is a DED process patented by Rolls Royce and licensed to the University of Sheffield (Escobar-Palafox, Gault & Ridgway, 2011, p. 622). As a manufacturing concept, using wire and an arc for AM dates back to the 1970s in West Germany where submerged arc welding was used for large scale metal component manufacturing (Kazanas et al., 2012, p. 1043). Another term used to describe arc welding based DED is wire and arc additive manufacturing (WAAM) (Kazanas, 2012, p. 1042).
2.5.1 Processes and applications
Two of the main advantages of using arc welding processes for AM purposes are their ability to reach high deposition rates compared to other AM processes and high wire utilization efficiencies which can go up to 100 % (Almeida & Williams, 2010, p. 25).
While the accuracy and the surface finish of the deposited product is lower compared to laser or EB processes (Baufeld, Biest & Gault, 2010, p. 106) the increased efficiency reduces the amount of scrap metal produced when comparing to traditional machining methods. The use of SMD has especially been investigated for manufacturing parts with the material Ti-6Al-4V (Palafox et al., 2011, p. 623). According to Escobar-Palafox (2011, p. 623) 60 % of titanium production is Ti-6Al-4V and it is widely used in aerospace, medical, sport car and maritime industries. The difficulties of shaping titanium alloys by using traditional methods such as casting, machining and forging as well as the high costs of scrap material caused by machining, have led to the investigation of using SMD. (Baufeld et al., 2011, p. 1) According to Kazanas (2012, p. 1043), when manufacturing aircraft parts in the aerospace industry by traditional manufacturing processes, in some cases, the buy-to-fly ratio, which is the ratio of bought material that ends up going to the aircraft, can even be as high as 10:1. Deposited parts using SMD with the material Ti-6Al-4V can be seen in figure 5.
Figure 5. Deposited parts using SMD with the material Ti-6Al-4V (Baufeld et al., 2010, p.
108).
As with the previously discussed AM processes, SMD also makes manufacturing parts directly from 3-dimensional CAD models possible by using CAD software to design the part and software to create necessary welding paths for the manufacturing process (Baufeld, Biest & Gault, 2009, p. 1536). Compared to traditional methods such as forging or machining, where components are built by subtracting material from the work piece, in SMD the material is added to get the desired shape. When comparing SMD to alternative manufacturing methods the main benefits become apparent. According to Escobar-Palafox et al. (2011, p. 622), the three ways to manufacture a part are by machining a part from a solid block, forging or casting the part to a nearer net shape so less material needs to be machined or by using a powder based AM process. In the first method a large amount of raw material, coolant and energy is wasted. In the second method less coolant and less machining are needed, however large amounts of energy for the forging or casting are required. Additionally when using forging or casting there are high costs and time is consumed if modifications to moulds and dies are done. Therefore forging or casting to a nearer net shape way of manufacturing is not very flexible. The third manufacturing method is good for manufacturing small detailed parts, however manufacturing large parts
with this method is often slow and may not even be possible. (Escobar-Palafox et al., 2011, p. 622.)
The two main welding processes that can be used are GTAW and GMAW. An arc welding based DED system set-up, GTAW for example, can consist of a TIG welding torch that is attached to a 6-axis Kuka robot and a 2-axis table. The welding wire is fed into a controlled atmosphere chamber by a motorized roller guide that ensures the wire deposition rate remains constant. (Baufeld et al., 2010, p. 106.) Additional equipment that can be used are an oxygen monitor to measure and maintain the appropriate atmosphere, a weld monitor to log the current, voltage and wire speed throughout the process, pyrometers to monitor the temperature of the weld, multiple thermocouples and a thermal camera (Escobar-Palafox et al., 2011, p. 624).
2.5.2 Cold metal transfer
One significant benefit of using GTAW over GMAW is that it is free of spattering (Wang
& Kovacevic, 2001, p. 1520). Although spatter free, the deposition rates of GTAW-based DED processes are about 1 kg per hour, which are lower compared to the GMAW-based DED processes, which can reach deposition rates of several kilograms per hour. The drawback for (DCEP)-GMAW-based DED processes when welding titanium alloys however is its poor welding conditions. The poor welding conditions can lead to an unstable, uncontrollable, arc wandering and high spattering process which reduces the overall process efficiency. (Almeida et al., 2010, p. 26.)
Many of the poor conditions that occur during the GMAW-based DED process can however be overcome by combining GMAW with a welding method called cold metal transfer (CMT) (Kazanas et al., 2012, p. 1043). CMT is a welding method that uses digital process-control to retract the welding wire when detecting a short circuit. Retracting the wire helps to detach the droplet from the wire. The wire motion is then reversed and the process is repeated. The basic idea of CMT can be viewed in Figure 6. The main benefits of using CMT are that it has reduced thermal input, it is spatter-free and it has a stable arc.
(Fronius, 2014, p. 3.)
Figure 6. Cold metal transfer (CMT) (Fronius, 2014, p. 3).
With other AM processes, one of the key problems is manufacturing constructions with overhangs or inclined structure walls that have angles less than 45 degrees without the use of support structures. Kazanas et al. (2012, p. 1044) combined GMAW with CMT and managed to build inclined walls with angles of 60, 45, 30 and 15 degrees without using support structures and without tilting the substrate. The walls were 200 mm long with wall thicknesses ranging between 4 mm to 5 mm. The tests were done using a carbon steel S355 substrate of the size 350 mm x 300 mm x 15 mm, a shielding gas mixture of 80 % argon and 20 % CO2, a ER70S-6 grade 0.8 mm diameter welding wire, Fronius CMT, Transpulse Synergic 5000 welding machine and an ABB type MTB 250 6-axis robot. (Kazanas et al., 2012, p. 1044.) The wall angle experiments can be seen in Figure 7. The group also conducted tests of a 50 mm square section (Kazanas et al., 2012, p. 1047), a 200 mm 0 degree horizontal overhang (Kazanas et al., 2012, p. 1048) and 50 mm radius semicircle as shown in Figure 8 (Kazanas et al., 2012, p. 1049).
Figure 7. Wall angle experiments. (Kazanas et al., 2012, p. 1045).
Figure 8. (a) 50 mm square section (b) 200mm 0 degree horizontal overhang (c) 50 mm radius semicircle (Kazanas et al., 2012. p. 1049).