Jasmin Saewe

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.

Laser powder bed fusion (LPBF) is an additive manufacturing (AM) process used in many areas – the
aerospace, automotive as well as medical technology - with various metals such as nickel-, aluminumand
titanium-based alloys. But the mechanical engineering industry need wear-resistant steels with high
hardness, like high-speed steels, in combination with the advantages of LPBF: high geometrical
complexity and high degree of individualization. However, when processed with LPBF, these steels
often form cracks, making the process therefore challenging to handle. In this feasibility investigation,
we demonstrate that building up dense and crack-free specimens made from high-speed steel
HS 6-5-3-8 (carbon content > 1.3 wt.-%) with LPBF is possible by preheating the base plate to 500 °C.
The specimens reach a hardness of over 60 HRC (as built). Furthermore, the influence of the LPBF
process parameters on the microstructure of these specimens is evaluated.

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, nickel-, aluminum- and titanium-based alloys are mainly used. In contrast, the mechanical engineering industry is interested in tool steels for many of their applications. Current LPBF research is often limited to hot work tool steel with up to 0.5 % carbon content. However, many applications in tooling need more wear-resistant steel alloys with higher hardness. High-speed steels can meet these requirements. Since these steels are susceptible to cracking because of their high carbon content, preheating temperature needs to be applied during LPBF processing.
In previous studies, we applied a preheating temperature of 500 °C for AISI M50 with 0.8 % carbon content and HS 6-5-3-8 with 1.3 % carbon content. We were able to manufacture dense (ρ > 99.9 %) and crack-free parts for both high-speed steels with a hardness > 62 HRC (as built) by LPBF.
State-of-the-art preheating systems induce heat only into the base plate which, combined with the in-situ heat treatment of the laser, results in complex thermal conditions. Consequently, parts are subject to temperature gradients and different heat treatment effects depending on manufacturing time and part height. In this study, we investigate the influence of preheating temperatures of 200 °C to 600 °C on hardness and microstructure as a function of part height for HS 6-5-3-8. We study the microstructure by light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD).

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, nickel-, aluminum- and titanium-based alloys are mainly used. In contrast, the mechanical engineering industry is interested in tool steels for many of their applications. Current research is often limited to hot work tool steel with up to 0.5 % carbon content using LPBF. However, many applications need 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 temperature needs to be applied during LPBF processing.
In a previous study, we applied a preheating temperature of 500 °C for HS 6-5-3-8 with 1.3 % carbon content. We were able to manufacture dense (ρ > 99.9 %) and crack-free parts from HS 6-5-3-8 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 as a function of part height for HS 6-5-3-8. The analysis of different part height is necessary because state-of-the-art preheating systems induce heat only into the base plate. Consequently, parts are subject to temperature gradients and different heat treatment effects apply depending on part height during the LPBF process. Furthermore, we analyze the microstructure by EBSD for three different part heights.

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, nickel-, aluminium- and titanium-based alloys are mainly used. The mechanical engineering industry is interested in tool steels for many of their applications. Current research often deals with processing of hot work tool steel with up to 0.5 % carbon content using LPBF.
However, many applications need higher 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 often form cracks, when manufacturing with LPBF, preheating temperature needs to be applied. In a preliminary study we applied 500 °C preheating temperature for AISI M50 with 0.8 % carbon content. We were able to manufacture dense and crack-free parts from AISI M50 with a hardness over 62 HRC (as built) by LPBF.
As state of the art preheating systems induce heat into the base plate from below, temperature gradients and different heat treatment effects during the LPBF process can be assumed at different part heights. In this paper we investigate the influence of preheating temperatures up to 650 °C on hardness and microstructure over the part height for AISI M50. Furthermore, we analyze the influence of process parameters on hardness by means of two exemplary parameter sets.