Shaped metal deposition (SMD) is a wire-based additive manufacturing process based on a three-dimensional tungsten-inert gas (TIG) welding system. SMD can produce components in nuclear-grade materials, without the need for special tooling, in a relatively short lead-time when compared to forging or casting routes. The SMD process was developed and patented by Rolls-Royce and was subsequently licensed for use and further development by the University of Sheffield.

The SMD system employs cold wire TIG deposition, using a tungsten cathode to weld the selected material in an inert argon atmosphere to prevent the substrate, electrode and part from reacting with atmospheric gasses. This is a very stable process, chosen for the maturity of process control. The TIG welding head is attached to a six-axis robot and produces weld on to a rotating turntable base. Weld material is built up in an additive way and the construction is monitored throughout the entire process to ensure that the weld parameters are maintained and a robust final component is produced.

Because SMD is a weld-based technique, accuracy in fabrication of the component depends on the thermal stresses induced during the welding process. Control of the thermal transfer during weld deposition can allow reduction in the final component residual stress and hence the distortion. Simulation modelling tools can be employed to predict the distortion levels and this can be fed back in to the SMD process during the deposition phase.

The wall thickness of the component is controlled by the current, travel speed and wire feed rate and also to some extent by the wire thickness. The travel speed is the product of the rotation rate on the turntable, and the rate of movement of the robotic head — the faster the travel speed, the thinner the wall thickness, but the limiting factor is the ability to maintain a robust arc at the weld head.

Cylindrical components are easily produced on the turntable but with the multi­axis robotic head, there is opportunity to produce more complex forms. One such form is shown in Figure 12.6 where two cylindrical components were formed as a prototype exercise.

The complete component is essentially a 100% weld and unlike the previous additive manufacturing systems which use metal powder to produce the component, SMD components do require post-processing heat treatment and final machining to attain the design-intent form. The SMD process is also well positioned to provide

Figure 12.6 Picture of SMD trial piece.

a route for adding material to an existing component such as a large vessel with a boss or nozzle. As a deployable SMD system, this solution could allow for the fabrication of more simple cylindrical forms through the traditional forging route and adding on the external features through an SMD-type technique.