Search

Laser powder bed fusion (LPBF) has great potential for the fabricating complex geometries with improved functionality. In combination with nickel alloys such as Hastelloy X, parts produced with this technology find usage in high-temperature applications. Many studies have focused on the microstructure of Hastelloy X fabricated via LPBF technology, but most have been performed on simple cubic geometries. Considering the aforementioned applications, the parts are often characterized by fine features, like very thin-walled structured, often in the sub-1 mm range. However, little is known about the LPBF fabrication of such structures. The study presented in this paper shows that wall thickness plays an important role in microstructure formation. In situ temperature measurements and thermal simulations showed a difference in thermal history. Local heat accumulation in the vicinity of the scanned tracks was observed for the sample consisting of multiple tracks. Moreover, this effect was enhanced with the number of tracks, leading to different melt pool morphologies. Significantly, coarser solidification cells were found near the sample edges. In addition, larger grains oriented parallel to the build direction were observed for the samples consisting of 3 and more tracks, while for the thinner samples, a very fine microstructure with random orientation was found.

This work demonstrates the tailoring of the microstructure of two FeMnSi-based shape memory alloys during laser powder bed fusion. Significant variations in the microstructure of the parts can be induced by modifying the scanning speed or the scanning strategy during the process. Specifically, the volume phase fractions, texture, and grain size and morphology change with the applied processing parameters as the thermal history experienced by the material is modified. Additionally, the generally undesired Mn evaporation during laser melting can be deliberately exploited to manipulate the phase composition in the final microstructure. Taking advantage of this, graded-microstructure samples with tailored amounts of the two phases (bcc-δ and fcc-γ) and distinct microstructure at different locations can be fabricated. A good combination of pronounced shape memory effect and ultrahigh strength can be achieved in this way. The findings of this study offer valuable insights that can drive future research and advancements in additive manufacturing, opening exciting opportunities for the fabrication of multifunctional and high performance materials.

The additive manufacturing (AM) of the γ` precipitation strengthened Ni-base superalloys still remains a challenge due to their susceptibility to micro-cracking. Post-processing, such as HIPing, has been shown to heal the micro-cracks but it remains desirable to prevent the micro-cracking from even occurring. Numerous studies highlighting potential mechanisms for micro-cracking exist but few solutions have been demonstrated. The intent of this study was to identify the micro-crack mechanisms and demonstrate how process and alloy modifications can reduce the micro-cracking. The micro-crack surfaces exhibit a dendritic appearance that is indicative of solidification cracking. Additionally, Gleeble experiments, simulating the L-PBF induced Heat Affected Zone (HAZ), were conducted below the γ` solvus temperature and reveal the existence of grain boundary liquation, indicative of liquation cracking. Two cracking mechanisms are thus coexisting during Laser Powder Bed Fusion (L-PBF) of CM247LC. Based on experimental evidence, reduction in the solidification interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. The study also involved two separate and distinct processing conditions to highlight the importance of melt pool geometry on micro-crack density. Samples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with optimized processing conditions exhibit a reduction in crack density of 98 %. This study demonstrates the importance of both processing conditions and alloy chemistry on micro-cracking in L-PBF fabricated γ` hardening Ni-base superalloys.