Investigation in the use of plasma arc welding and alternative feedstock delivery method in additive manufacture.

摘要

The work conducted for this thesis was to investigate the use of plasma arc welding (PAW) and steel shot as a means of additive manufacturing. A robotic PAW system and automatic shot feeder were used to manufacture linear walls approximately 100 mm long by 7 mm wide and 20 mm tall. The walls were built, layer-by-layer, on plain carbon steel substrate by adding individual 2.5 mm diameter plain carbon steel shot. Each layer was built, shot-by-shot, using a pulse of arc current to form a molten pool on the deposit into which each shot was deposited and melted. The deposition rate, a measure of productivity, was approximately 50 g/hour. Three walls were built using the same conditions except for the deposit preheat temperature prior to adding each new layer. The deposit preheat temperature was controlled by allowing the deposit to cool after each layer for an amount of time called the inter-layer wait time. The walls were sectioned and grain size and hardness distribution were measured as a function of wall height. The results indicated that, for all specimens, deposit grain size increased and hardness decreased as wall height increased. Furthermore, average grain size decreased and hardness increased as interlayer wait time increased. An analytical heat flow model was developed to study the influence of interlayer wait time on deposit temperature and therefore grain size and hardness. The results of the model indicated that as wall height increased, the rate of deposit heat removal by conduction to the substrate decreased leading to a higher preheat temperature after a fixed interlayer wait time causing grain size to increase as wall height increased. However, the model results also show that as wall height increased, the deposit surface area from which heat energy is lost via convection and radiation increased. The model also demonstrated that the use of a means of forced convection to rapidly remove heat from the deposit could be an effective way to boost productivity and maintain smaller grain size and therefore higher hardness and strength in the deposit. It was concluded that the use of PAW welding with small shot as feedstock could offer a means to additively manufacture components with reasonably small geometric details.