ADDITIVE MANUFACTURING OF AISI D2 TOOL STEEL USING DIRECTED ENERGY DEPOSITION

Abstract

The main objective of the current thesis is to optimise direct energy deposition (DED) process parameters to fabricate parts with good mechanical and geometrical properties and to determine the best of postprocessing heat treatment schedule in order to enhance the the mechanical properties of the as-printed samples. Firstly, the impact of laser-directed energy deposition (DED) process parameters and post-deposition heat-treatment cycles on the microstructural characteristics and hardness were evaluated for cladding AISI D2 tool steel onto annealed AISI D2 substrates. The influences of powder feed rate, scanning speed and layer height were assessed. After laser deposition, the DED samples were subjected to various tempering heat-treatment cycles. In addition, the wear resistance, indentation, and scratch hardness responses were assessed to evaluate the heat-treatment effect compared to wrought D2. It was demonstrated that the hardness values of the DED fabricated parts were higher than an annealed wrought D2, but lower than the air-cooled D2. Subsequent tempering of the DED printed parts resulted in a final hardness, essentially equivalent to the air-cooled level. Furthermore, the geometrical characteristics and surface roughness of the DED-processed specimens were assessed. It was concluded that the use of higher laser scanning speeds decreases both the size of single clads and the level of sample over-building. However, there was no clear trend for powder feed rate on either single-clad size or dimensional accuracy. Furthermore, the surface roughness was also assessed using CLSM, to evaluate the influence of the various DED operating conditions. It was demonstrated that the amount of partially un-melted powder particles adhered to the sample surface significantly affects the surface roughness of the DED processed specimens. The ability to deposit overhang structures using DED was also investigated by depositing inclined thin walls, under various process parameters and angles of inclination. The impact of the process parameters and the angle of inclination on both angle accuracy and side surface roughness were analysed. These favourable findings contributed to the scientific knowledge of DED of AISI D2 tool steel by optimising the process parameters and reducing the post-processes to obtain high wear resistance, better dimensional accuracy, and improved surface roughness.