EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Laser based powder bed fusion (PBF-LB) of metals enables the fabrication of geometrically complex structures, which are difficult or impossible to manufacture with conventional subtractive processing methods. Therefore, economic advantages in terms of enhanced cutting velocities can be facilitated by implementing internal cooling channels in drills or milling cutters made of tool steel. However, this microwelding process is restricted to small batch sizes for special applications as a consequence of low build-up rates. Additionally, severe cracking of these high-alloyed tool steels may occur due to a combination of process-related stresses and a susceptible material class.
The aim of this work is to illuminate the crack initiation and growth mechanism in a 0.85C-4.25Cr-2.46W-2.72Mo-2.01V-4.35Co (weight%) tool steel with extremely fine microstructure manufactured by the PBF-LB process. Primarily, multiscale correlations of process-related residual stresses and individual microstructural constituents, both formed as a result of rapid cooling during the fabrication process, were investigated using high-resolution techniques. Scanning and transmission electron microscopies were used to analyze the morphology of crack surfaces and their immediate proximity. These reveal dendritic structures on the surfaces and also crack branching according to a connected dendrite network, which was formed during rapid solidification. In order to investigate the chemical composition of microstructural features determining the crack formation, atom probe tomography measurements are performed. The spatial occurrence of particular cracks was correlated with residual stress concentrations and relaxations evaluated using synchrotron X-ray diffraction. The findings show accumulations of tensile stresses at the sample edges, which obviously promotes the crack initiation and growth.