Understanding the deformation mechanisms of horizontal internal channels during the LPBF of 18Ni300 maraging steel

Abstract

The 18Ni300 maraging steel is arguably the standard material for mould and die production employing laser powder bed fusion (LPBF). Owing to its good processability via the LPBF process and high strength after the ageing treatment, 18Ni300 maraging steel has been widely employed for producing complex moulds with internal conformal channels. In order to fully take advantage of the cooling channels within the mould, these channels should respect the geometrical and dimensional requirements. An apparent issue concerning the internal channels is the geometrical deformations occurring during the process. With other alloys used in LPBF such as stainless steel, the circular horizontal holes tend to deform in tension around the upper arch, resulting in hole enlargement around this region. However, the 18Ni300 is highly sensitive to the thermal history during the LPBF process and can deform differently according to the channel geometry. The use of supports in horizontal channels as a remedy is not suitable due to the difficulty of their removal. Alternatively, the use of non-circular profiles may act as stress accumulation zones during the lifetime of the die. Accordingly, this work studies the deformation mechanisms during the LPBF of 18Ni300 maraging steel concerning two types of channel profiles, namely circular and diamond shaped. Samples with different channel widths and lengths were produced and geometrical deviations were measured. Powder bed monitoring data and microstructural analyses were made to construct a phenomenological description of the deformation mechanism. The results indicate that the final form of the channel depends also on the thermal history of the successive layer once the horizontal channel is closed. The influence of volumetric expansion in martensitic transformation was found to be relevant on the generation of an apparent compression field. An analytical model was used to estimate an equivalent stress value starting from the experimental data showing a compression state. The results confirmed that the 18Ni300 deformation depends on the channel form and dimensions, moreover occurs differently compared to other LPBF alloys such as stainless steels, Al-, Ni-, and Ti-alloys, which deform outward in a tension state.