Emil Strandh

A high-alloy (Cr-Mo-V) cold-work tool steel was manufactured by laser powder-bed fusion (PBF-LB) without preheating and by electron-beam powder-bed fusion (PBF-EB) with the build temperature set at 850 °C. The solidification rates, cooling, and thermal cycles that the material was subjected to during manufacturing were different in the laser powder-bed fusion than electron-beam powder-bed fusion, which resulted in very different microstructures and properties. During the solidification of the PBF-LB steel, a cellular–dendritic structure was formed. The primary cell size was 0.28–0.32 µm, corresponding to a solidification rate of 2.0–2.5 × 10⁶ °C/s. No coarse primary carbides were observed in the microstructure. Further rapid cooling resulted in the formation of a martensitic microstructure with high amounts of retained austenite. The high-retained austenite explained the low hardness of 597 ± 38 HV. Upon solidification of the PBF-EB tool steel, dendrites with well-developed secondary arms and a carbide network in the interdendritic space were formed. Secondary dendrite arm spacing was in the range of 1.49–3.10 µm, which corresponds to solidification rates of 0.5–3.8 × 10⁴ °C/s. Cooling after manufacturing resulted in the formation of a bainite needle-like microstructure within the dendrites with a final hardness of 701 ± 17 HV. These findings provide a background for the selection of a manufacturing method and the development of the post-treatment of a steel to obtain a desirable final microstructure, which ensures that the final tool’s performance is up to specification.

During laser powder bed fusion (L-PBF), the protective atmosphere, normally argon, can become trapped in cavities formed during the process. We have demonstrated this by introducing ø0.5 mm pores in the CAD file (totalling 0.5% by volume) and measuring a threefold increase in argon content, versus an identical IN718 build-job without pores. For critical applications where the material is subjected to cyclic loading, any defects can act as stress raisers and initiate cracks. The pores that were introduced drastically reduced the fatigue performance of said material. However, after Hot Isostatic Pressing (HIP), the material regained its properties as if the pores were never introduced. Still, the argon remains after HIP according to measurements as it lacks solubility and thus cannot leave the material. Worth noting is that a small quantity of argon originates from gas-atomisation of the powder and thus does not leave entirely during the remelting in L-PBF.