Laser Powder Bed Fusion (LPBF) is an additive manufacturing process employed in the aerospace, automotive, and medical industries. In these sectors, nickel-, aluminium-, and titanium-based alloys are mainly used for various applications. Yet, only few of the commonly used steels have been qualified for the LPBF process in the mechanical engineering industry, which normally uses hot work tool steels with less than 0.5 wt.-% carbon content. However, many applications need high wear-resistant steel alloys with high hardness, both of which can be achieved with a higher carbon content, like in high-speed steels. But when processed with LPBF, these steels often form cracks, making the process very challenging. In this feasibility investigation, we demonstrate that LPBF can be used to manufacture dense and crack-free specimens with a hardness of over 62 HRC (as built) from high-speed steel AISI M50 (carbon content of 0.8 wt.-%). Furthermore, we evaluate the influence of typical LPBF process parameters, especially of preheating temperatures up to 500°C, on the microstructure of the specimens.
The microstructure and mechanical properties of high‐speed steel AISI M50 (80MoCrV42‐16, Mat. Nr. 1.3551), produced by laser powder bed fusion (LPBF), are analyzed. The mechanical properties in hardened and tempered condition are characterized by hardness, fatigue strength, and toughness and compared with the properties of conventionally produced samples. Moreover, the effects of an additional posttreatment by hot isostatic pressing (HIP) on the microstructure and mechanical properties are investigated. Dilatometric testing is used to investigate the influence of the different initial microstructures on hardening. All heat‐treated samples expose a fine martensitic microstructure with high hardness. The conventionally produced samples show a band‐like orientation of carbides due to the production by vacuum induction melting and vacuum arc remelting followed by a hot working process. This carbide structure is bypassed by the rapid cooling in the LPBF process. The LPBF samples show a comparable hardness after hardening and tempering to the conventionally produced material. In heat‐treated state, the LPBF samples show a low fatigue strength. Posttreatment by HIP included in the heat‐treatment chain significantly increases the fatigue strength. Nevertheless, the fatigue strength is still lower compared with the reference material. Both LPBF grades show a low toughness compared with the reference material.
Laser powder bed fusion (LPBF) is an additive manufacturing process employed in many industries, for example for aerospace, automotive and medical applications. In these sectors, mainly nickel-, aluminum- and titanium-based alloys are used. In contrast, the mechanical engineering industry is interested in more wear-resistant steel alloys with higher hardness, both of which can be achieved with a higher carbon content, like in high-speed steels. Since these steels are susceptible to cracking, preheating needs to be applied during processing by LPBF. In a previous study, we applied a base plate preheating temperature of 500 °C for HS6-5-3-8 with 1.3 % carbon content. We were able to manufacture dense (p > 99.9 %) and crack-free parts from HS6-5-3-8 with a hardness > 62 HRC (as built) by LPBF. In this study, we investigate the influence of preheating temperatures up to 600 °C on hardness and microstructure dependent on part height for HS6-5-3-8. The microstructure was studied by light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The analysis of hardness and microstructure at different part heights is necessary because state-of-the-art preheating systems induce heat only into the base plate. Consequently, parts are subjected to temperature gradients and different heat treatment effects depending on part height during the LPBF process.
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