In the intervening years, TWI has worked, both directly for its members and in collaboration with wider industry, to successfully manufacture a wide range of thin-walled (0.5 mm -1.8 mm thick) axis-symmetric components up to 550 mm in height and 1500 mm in diameter. This includes a number of different sized gas turbine components and rocket nozzles; all of which exhibited similar challenges and outputs to the helicopter casing discussed previously, but principally showed that large scale is feasible and in process distortion was manageable.
One variation worthy of note was an increase in material deposition rate to 280 g/h for the manufacture of the rocket nozzle shown in Figure 2, forming a weld bead 1.3 mm in width and 300 µm in depth. This increase became possible because of a thicker wall and a steeper wall orientation meaning a larger pitch of the helical toolpath could be used with little detriment to surface finish.
When moving away from thin-walled components, but still in the context of additive manufacturing, higher material deposition rates of 1-2 kg/h are achievable with laser and powder, particularly when used with a suitable nozzle. But no matter what the geometry, a higher material deposition rate will again come at the expense of lower feature resolution and surface texture, and will run the risk of greater levels of slumping, thermally-induced distortion and generally larger microstructure grain growth (through the need to use a high laser power).
Through continued use of TWI’s CAM software, large scale and more feature rich components have been developed and manufactured. One example is a representative geometry of an aero engine casing (see Figure 3). This particular part contains walls and flanges ranging in thickness from 1-2 mm for a single pass track weld, and >12 mm using a combination of spiral and helical tool path forming tacks side by side as well as one on top of the other. This approach creates a good surface consistency by allowing gradual changes in nozzle position relative to the rotating substrate.
During the manufacture of this part, a number of observations and challenges were observed which are worthy of note here. Firstly, it was difficult to predict distortion behaviour of the part during manufacture because of a lack of robust design tools for DED. Hence, a balance had to be found through iterative experimentation between the deposition rate and heat input and the required feature resolution (e.g. wall thickness) to minimise both distortion and finish machining e.g. the uppermost flange on the part shown in Figure 3, which was deposited onto a 1.4 mm thick wall, had to be built with a low material deposition rate of 130 g/h. Ultimately, all walls within the part that had surface deposited flanges and bosses had to be built with a generous oversizing to ensure a suitable resistance to any thermally induced distortion; an approach clearly detrimental with attempts to minimise post machining and material waste.
For reference, the maximum material deposition rate used in the manufacture of the part was 500 g/h (measured at the nozzle), the average being 350 g/h. TWI has estimated around 25-30% of material was removed during finish machining to achieve net shape of selected features.
With regards to machining, a single stage finish machine was not possible because access for the cutting tool was difficult or blocked by certain features. Hence, an appropriate sequence of material deposition and machining steps had to be choreographed. This had to take place with additional intermediary stress relief heat treatment steps, to prevent tool chatter during machining, that would otherwise be caused by warping and distortion of the part as material is removed.
Despite the obvious challenges, DED will no doubt increase its market share in metal AM, driven by its process flexibility and large-scale capability. It is also very clear that industry are currently confronting these issues with great vigour, continually introducing new and exciting advances in hardware, software and processes. In support of this, TWI is working on DED training programmes, CAM software and much needed design rules alongside industrially driven projects geared towards part production at multiple meters scale. This includes current projects developing DED maturity into a viable alternative manufacturing method for cast, forged and fabricated parts.