Himanshu Sahasrabudhe

This article explores the use of ultrasonic nondestructive evaluation for qualification of laser-DED IN718 samples. The main goal of this article is to identify potential ultrasonic parameters which have highest sensitivity to microstructral changes that result from fabrication of DED samples. The ultrasonic qualification parameters were extracted from ultrasonic testing including velocity and attenuation measurement, and C-Scan imaging. These measurements were further used to extract parameters that quantify the anisotropy, microstructural heterogeneity, and grain scattering. Two laser-DED IN718 samples fabricated with slightly different processing parameters were evaluated to observe the influence of the laser power and scan speed on the qualification parameters. The identified qualification parameters were compared for these two samples, along with a hot-rolled sample that was also used as reference. The results suggest that the anisotropy, attenuation, and heterogeneity were highest in the DED samples compared to the reference sample. The identified qualification parameters seem to capture these changes, suggesting they could be potentially used for qualification of AM parts.

This article reports on the characterization of the “as-fabricated” state of Inconel 718 samples fabricated using laser directed energy deposition (DED). Laser-DED is known to produce complex metastable microstructures that can significantly influence the baseline ultrasonic response compared to conventional processing methods. The present work uses three parameters to characterize the samples: (a) ultrasonic velocity, (b) an attenuation coefficient, and (c) a backscatter coefficient. The baseline ultrasonic response from the DED sample was compared against the ultrasonic properties of conventional IN718 samples reported in the literature. The results suggest that strong grain boundary scattering from large macrograins can lead to attenuation and backscatter values that are significantly higher than conventional samples. Additionally, the results including velocities, attenuation and backscatter coefficients were found to be dependent on the fabrication direction, with the build direction being different from the transverse directions. Finally, destructive analysis was used to develop conjectures to explain the experimentally observed ultrasonic response.

Ni and Co based MCrAlY (M = Ni and/or Co), dual phase β and γ/γ'; β = NiAl, γ = Ni (Co, Cr), and γ' = Ni₃Al) superalloys exhibit several beneficial high-temperature (>1000°C) (2012°F) strength and corrosion resistance properties. Our study investigates the feasibility of fabricating a commercially available NiCoCrAlY alloy (tradename “Amdry 386”) using laser directed energy deposition (laser-DED). A significant challenge for laser fabrication of bulk structures with Amdry 386 (A386) stems from the presence of relatively higher amounts of brittle β phase than ductile γ/γ' phases. While prior studies report laser fabrication of these materials in coating and cladding configurations, bulk structures have not been realized. Fabrication of A386 bulk structures using laser-DED was unsuccessful due to hot cracking and delamination during and after deposition. Laser-DED fabricated A386 alloy exhibited a metastable, rapidly solidified β-rich microstructure typical to laser processes. To alleviate these challenges, this study systematically evaluates the effects of modifying the A386 alloy by adding 10, 20 and 30% by weight of a Ni-20%Cr (NiCr) alloy (mainly γ/γ'). The A386 + NiCr mixtures were processed using laser-DED, and small coupons from each composition were evaluated using SEM, XRD, and microhardness to characterize the as-processed microstructure. Thermodynamic modeling was performed to determine the phase evolution as a function of the alloy composition. Our results show that the alloy chemistry can be successfully modified to improve the structural integrity of laser processed structures.

Cobalt superalloys such as Tribaloys are widely used in environments that involve high temperatures, corrosion, and wear degradation. Additive manufacturing (AM) processes have been investigated for fabricating Co-based alloys due to design flexibility and efficient materials usage. AM processes are suitable for reducing the manufacturing steps and subsequently reducing manufacturing costs by incorporating multi-materials. Laser directed energy deposition (laser DED) is a suitable AM process for fabricating Co-based alloys. T800 is one of the commercially available Tribaloys that is strengthened through Laves phases and of interest to diverse engineering fields. However, the high content of the Laves phase makes the alloy prone to brittle fracture. In this study, a Ni-20%Cr alloy was used to improve the fabricability of the T800 alloy via laser DED. Different mixture compositions (20%, 30%, 40% NiCr by weight) were investigated. The multi-material T800 + NiCr alloys were heat treated at two different temperatures. These alloy chemistries were characterized for their microstructural, phase, and mechanical properties in the as-fabricated and heat-treated conditions. SEM and XRD characterization indicated the stabilization of ductile phases and homogenization of the Laves phases after laser DED fabrication and heat treatment. In conclusion, the NiCr addition improved the fabricability and structural integrity of the T800 alloy.

Laser Directed Energy Deposition (DED) is an additive manufacturing (AM) process where 3D structures are fabricated layer by layer and where the metallic powder is deposited and simultaneously melted by the incidence of a laser beam. One of the advantages of the laser DED process, which at the same time makes this process more attractive, is the possibility of fabricating