Carbide size control in high-speed steels and hardmetals using electron beam powder bed fusion process parameters

Abstract Creating a functionally graded material, where the material properties are varied locally, can be done by various methods e.g. coating or case hardening for tools. As most additive manufacturing technologies create objects layer-by-layer, allowing access to and control of process parameters for each voxel in the object, this is a possible route to spatial control of microstructure and material properties. Using electron beam powder bed fusion of metals (PBF-EB/M), this paper demonstrates the possibility of controlling the carbide size in high-carbon Fe and Co alloys, in particular the high-speed steel grades Vibenite® 150, 280, and 290, and the hardmetal Vibenite® 480. By adjusting the energy input locally, it is possible to tailor the carbide size. By increasing the energy input during the PBF-EB/M process, variation in the average carbide sizes of around 2–5 times for HSS, in particular 0.5–2.6 µm for Vibenite® 290, and 22 times, 0.3–31.7 µm, for the hardmetal was measured. Transitions between coarse and fine carbide domains were sharp and domains could be made as small as a few millimetres. The creation of complex-shape domains of varying carbide size was also demonstrated. The effects of process parameters were persistent, surviving heat treatments. Varying the carbide size did not impact the macro-scale hardness of the material, but wear properties were affected with coarser carbide high-speed steel samples showing lower wear in both a ball-on-disc and an unlubricated sliding wear test. These findings indicate the possibility to partially decouple wear resistance from hardness.