Metal Additive Manufacturing Conference - MAMC 2020

Laser Powder Bed Fusion (LPBF) provides advantages compared to conventional manufacturing methods, such as forging or casting, in terms of design freedom. New opportunities regarding tool geometries are introduced. Due to the layered build-up of parts, internal cooling channels can be manufactured in tools enhancing cutting speed and consequently productivity for special applications. Since tool steels exhibit high carbon equivalents, certain problems may occur during LPBF. Besides pore formation, which is mainly a function of the applied laser energy, severe cracking due to a combination of susceptible materials and thermal stresses caused by a high process-related thermal gradient is very likely. This work aims to shed light on the evolution of defect structure and microstructure of an additively manufactured powder metallurgical cold-work tool steel in dependence of the applied volumetric energy density. Investigations were performed on so-called step experiments in which a varying number of welding bead layers were manufactured. Defect surfaces, such as cracks and different types of porosity, were characterized by means of light optical and scanning electron microscopy. Additionally, the microstructure as well as the solidification structure, especially in the immediate vicinity of cracks or within pores, were analyzed in order to determine possible correlations to defect formation mechanisms. The results show that with increasing energy input porosity changes from lack-of-fusion to so-called keyhole porosity. Irrespective of the chosen energy input, crack surfaces exhibit freely solidified dendritic structures and thus hot-cracking can be assigned as dominating fracture mechanism. The microstructure in the as-built condition revealed a martensitic matrix with retained austenite. In contrast to conventionally manufactured and hardened cold-work tool steel, no primary carbides are formed and the material exhibits a very fine solidification structure with a dendritic carbide network.