Influence of the Chemical Composition on the Solidification Path, Strengthening Mechanisms and Hardness of Ni-Cr-Si-Fe-B Self-Fluxing Alloys Obtained by Laser-Directed Energy Deposition

Abstract

Nickel-based Ni-Cr-Si-B self-fluxing alloys are excellent candidates to replace cobalt-based alloys in aeronautical components. In this work, metal additive manufacturing by directed energy deposition using a laser beam (DED-LB, also known as LMD) and gas-atomized powders as a material feedstock is presented as a potential manufacturing route for the complex processing of these alloys. This research deals with the advanced material characterization of these alloys obtained by LMD and the study and understanding of their solidification paths and strengthening mechanisms. The as-built microstructure, the Vickers hardness at room temperature and at high temperatures, the nanoindentation hardness and elastic modulus of the main phases and precipitates, and the strengthening mechanisms were studied in bulk cylinders manufactured under different chemical composition grades and DED-LB/p process parameter sets (slow, normal, and fast deposition speeds), with the aim of determining the influence of the chemical composition in commercial Ni-Cr-Si-Fe-B alloys. The hardening of Ni-Cr-Si-Fe-B alloys obtained by LMD is a combination of the solid solution hardening of gamma nickel dendrites and eutectics and the contribution of the precipitation hardening of small chromium-rich carbides and hard borides evenly distributed in the as-built microstructure.

Chemical Composition Effects on the Microstructure and Hot Hardness of NiCrSiFeB Self-Fluxing Alloys Manufactured via Gravity Casting

“…Due to the small size of these precipitates, which were most likely smaller than the EDS analysis volume, certain amounts of the γ-Ni matrix could be included in the EDS analysis result; therefore, only the spectra obtained at the low voltage of 10 kV are shown in Figure 8. These precipitates were not observed by Pereira et al [18] when they were studying an alloy with a similar chemical composition that was fabricated via laser metal deposition (LMD); it is most likely that the faster solidification rate suppressed the formation of these precipitates. Furthermore, in both alloys, nano-sized precipitates (100-300 nm) with a butterfly or cube shape were detected at the borders of the γ-Ni matrix, which was close to the eutectic phases (Figures 6b and 7b).…”

“…At 100 K/s, they observed a eutectic floret-shape structure consisting of Ni and borides. However, in the LMD samples of Alloy C, which were produced at a rapid solidification rate, only fine and blocky primary borides were observed [18]. Furthermore, large eutectic borides (boride/γ-Ni) and primary carbides were observed in Alloy C, and both precipitates were rich in Cr.…”

“…The common coating techniques used for these alloys are flame spraying [3][4][5][6], plasma spraying [7][8][9], high-velocity oxygen fuel spraying (HVOF) [10][11][12], laser cladding [13][14][15][16][17], laser metal deposition (LMD) [18,19], non-vacuum electron beam cladding [20,21], laser powder bed fusion (LPBF) [22], and the plasma transferred arc welding (PTAW) processes [23]. All of these techniques have high solidification rates that can lead to different types of defects, and they can also enhance the porosity and cracking susceptibility of these materials [24].…”

“…In laser metal depositions, a strong relation between the internal porosity of the powder particles and the resulting porosity in the deposited material has been observed [18]. Furthermore, in deposition technologies such as laser cladding and plasma-transferred arc welding, and also after remelting at elevated temperatures, the dissolution of the phases and dilution can be observed [2,[32][33][34].…”

“…On the other hand, the localized heating, rapid cooling, thermal cycles, and large thermal gradients induce severe residual stresses that can lead to cracks, especially at high alloy grades [24]. There have been many attempts to tackle the cracking problem by reducing the cooling rate of the deposits via pre-heating and post-heating [18,36,37], or via increasing the toughness of the alloys via compositional modification [38,39]. The gravity casting process could be a good alternative to the conventional manufacturing processes, since it shows several advantages, as shown in the SWOT analysis in Figure 1.…”