Machining of a high-alloy tool steel produced by electron beam powder bed fusion

Abstract

This thesis investigates the machinability of an additively manufactured high alloyed tool steel. The metallic powders were melted with help of the powder bed fusion electron beam (PBF-EB) process. As sample specimens, ring shaped geometries were produced to study the machinability in terms of grinding and turning. The hardness of the printed and heat-treated specimens was about 68 HRC. After the printing, the specimens were subjected to post-treatment including a combined hot-isostatic-press and heat treatment. Initial material characterization provided insight of the as-printed and post-processed specimens, showing the fine and evenly dispersed carbides with estimated elemental composition. The grinding tests were done using an external centerless shoe setup, which were found forgiving with reference to a rough surface texture and deviating geometry of the as-printed rings. Grindability was evaluated based on a conventional Al₂O₃ and a super abrasive cBN wheel, where both as-printed initial outer diameter and bulk regions where investigated. The material was grindable using both wheels, however, the cBN wheel provided better process stability and should thus be further evaluated in cases of industrialization. The machinability in terms of turning was evaluated by testing different insert grades, including cemented carbides, ceramic, pcBN and PCD. The cleaning of the intermittent as-printed surface, and the continuous machining after cleaning, was evaluated. The cemented carbide inserts performed better in terms of tool wear and surface roughness. However, chipping occurred for the inserts at various cutting conditions. A “hook” shaped compressive stress profile, as function of depth in the material, was obtained after machining with the cemented carbide, ceramic and pcBN inserts. The importance of a rigid machine during hard turning was observed when using pcBN and PCD inserts due to the high cutting forces and the challenges of reaching the desired dimensions (outer ring diameter).