Effects of Alloy Composition and Solid-State Diffusion Kinetics on Powder Bed Fusion Cracking Susceptibility

Laser powder bed fusion (LPBF) has demonstrated its unique ability to produce customized, complex engineering components. However, processing of many commercial Al-alloys by LPBF remains challenging due to the formation of solidification cracking, although they are labelled castable or weldable. In order to elucidate this divergence, solidification cracking susceptibility from the steepness of the solidification curves, specifically |dT⁄dfs1⁄2|, as the fraction solidified nears 1 towards complete solidification, was calculated via Scheil–Gulliver model as a function of solute concentration in simple binary Al-Si, Al-Mg, and Al-Cu systems. Introduction of “diffusion in solid” into Scheil–Gulliver model resulted in a drastic reduction in the cracking susceptibility (i.e., reduction in the magnitude of |dT⁄dfs1⁄2|) and a shift in the maximum |dT⁄dfs1⁄2| to higher concentrations of solute. Overall, the calculated solidification cracking susceptibility correlated well with experimental observation made using LPBF AA5083 (e.g., Al-Mg) and Al-Si binary alloys with varying Si concentration. Cracking susceptibility was found to be highly sensitive to the composition of the alloy, which governs the variation of |dT⁄dfs1⁄2|. Furthermore, experimental observation suggests that the contribution of “diffusion in solids” to reduce the cracking susceptibility can be more significant than what is expected from an instinctive assumption of negligible diffusion and rapid cooling typically associated with LPBF

In situ TEM Characterization of Microstructure Evolution and Mechanical Behavior of the 3D-Printed Inconel 718 Exposed to High Temperature

This in situ transmission electron microscopy work presents a nanoscale characterization of the microstructural evolution in 3D-printed Inconel 718 (IN718) while exposed to elevated temperature and an associated change in the mechanical property under tensile loading. Here, we utilized a specially designed specimen shape that enables tensile testing of nano-sized thin films without off-plane deformations. Additionally, it allows a seamless transition from the in situ heating to tensile experiment using the same specimen, which enables a direct correlation of the microstructure and the mechanical property of the sample. The method was successfully used to observe the residual stress relaxation and the formation of incoherent γ′ precipitates when temperature was increased to 700°C. The subsequent in situ tensile test revealed that the exposure of the as-printed IN718 to a high temperature without full heat treatment (solutionizing and double aging) leads to loss of ductility

Microstructural Development in As Built and Heat Treated IN625 Component Additively Manufactured by Laser Powder Bed Fusion

The RL10 engine program is exploring the use of IN625 Ni-base superalloy components that are additively manufactured using laser powder bed fusion (LPBF). IN625 alloy powders are commercially available for LPBF to produce dense, complex parts/components. In this study, IN625 components, with both simple and complex geometries with overhangs, were manufactured via LPBF, and subjected to a heat-treatment consisting of a stress relief, hot isostatic pressing (HIP), and a solution anneal. The microstructure was examined with optical, scanning electron, and transmission electron microscopy. Changes in phase constituents and microstructure were documented as a function of heat treatment and component geometry (i.e., bulk section built on support structure versus thin, overhang section built on top of the previous powder bed). The as-built microstructural features included large columnar grains, a sub-grain cellular-solidification structure, approximately ~ 1 µm in diameter, and solute enriched cell boundaries decorated with A2B Laves phases. After heat treatment, the bulk section consisted of recrystallized equiaxed grains with annealing twins, and the sub-grain cellular-solidification structure was found to be completely dissolved. However, in the thin, overhang section, the sub-grain cellular-solidification structure persisted within columnar grain structure, which exhibited no recrystallization. An alternate HIP cycle with a higher temperature was employed to produce desired microstructure (i.e., recrystallized grains without sub-grain cells and Laves phases) in components with geometrical complexity for successful testing of RL10 engine

Phase Field Modeling Of Interdiffusion Induced Microstructure Evolution Under Different Driving Forces

Interdiffusion induced microstructure evolution in binary multi-phase alloys was investigated using a phase field model under both isothermal condition and temperature gradient. First, the model was used to simulate microstructure evolution in diffusion couples of Ni-Al alloy system containing P(fcc) vs. P+PQ(L12) phases under isothermal condition. Dissolution of the PQ phase in the two-phase region was well predicted by the model. Second, a new phase field model was devised and employed to investigate the effect of thermotransport or diffusion under a temperature gradient in single-phase and multi-phase alloys of a binary system. Simulation results show that an applied temperature gradient can cause significant redistribution of constituents and phases in the alloy. In multi-phase alloys, the thermomigration effect can cause the formation of single-element rich phases at the cold and hot ends of the alloy

Degradation Of (Ni,Pt)Al Coatings By Mixture Of Sodium And Potassium Sulfate At 950°C

Environmental stability of β-(Ni,Pt)Al coatings due to combustion by-product of fuel impurities has been examined owing to recent interests in bio-derived (e.g., algae-derived) fuels. Pure sodium sulfates (Na 2SO4), potassium sulfates (K2SO4) and three of their mixtures with different weight ratio were prepared by mechanically grinding, and their high temperature interactions with diffusional Pt-modified β-NiAl coatings were investigated in a laboratory furnace at 950°C. The corroded samples were characterized by X-ray diffraction and scanning electron microscopy equipped with X-ray energy dispersive spectroscopy. The results showed severe damages occurred in the β-(Ni,Pt)Al coatings when the salts were in molten state at 950°C via fluxing mechanism, and accelerated oxidation also occurred when the pure solid state K 2SO4 was in contact with β-(Ni,Pt)Al

Electrophoretic Deposition Of Environmental Barrier Overlay Coatings For Yttria-Stabilized Zirconia Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are increasingly susceptible to environmental degradation due to molten deposits of fuel impurities and air ingested sand. At elevated temperature, these deposits adhere, melt and degrade the TBCs via repeated freeze-thaw action and, to a certain extent, direct chemical reaction with TBC constituents. In order to protect TBCs from such melt ingression, a thin, dense and continuous environmental barrier overlay coating can be applied on the surface of TBCs. In this study, dense, continuous and crack-free overlay coatings of YSZ, Al2O3 and MgO were fabricated by electrophoretic deposition (EPD) on air-plasma sprayed (APS) YSZ. APS YSZ specimens of 300 μm thickness were prepared from 8YSZ powders on graphite substrates. A stable colloidal suspension of YSZ, Al2O 3 and MgO powders in an organic solvent mixture of acetone and ethanol with Iodine additive was used for EPD. At a controlled applied DC voltage with deposition time within minutes, a crack-free green compact can be obtained. After drying and controlled sintering at high temperature, thin, dense, crack-free overlay coatings of controlled thickness on free-standing YSZ were produced. The coatings were characterized by x-ray diffraction and scanning electron microscopy. The degradation resistance of overlay coatings fabricated by EPD and sintering for YSZ TBCs against melt ingression of CMAS and V 2O5 is highlighted. Copyright © 2009 by ASME

The Effect Of Bond Coat Grit Blasting On The Durability And Thermally Grown Oxide Stress In An Electron Beam Physical Vapor Deposited Thermal Barrier Coating

Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y2O3-ZrO2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0-2.2 GPa in the as received state to 2.8-3.3 GPa at 10 cycles, then gradually decreased to 1.2-1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600-750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness. © 2003 Elsevier B.V. All rights reserved

Long-Term Oxidation Behavior Of Aluminized Cmsx-4 Superalloys

Thermally grown oxide (TOO) scale on aluminized CMSX-4 single crystal superalloy has been examined after long-term oxidation testing to investigate the spallation behavior using photo-stimulated luminescence spectroscopy (PSLS) and microscopy. Disk-shaped (2.54 cm dia. and 0.32 cm thickness) specimens were oxidized in air at 788°C (1450°F), 871°C (1600°F), 954°C (1750°F) and 1010°C (1850°F) for 1000, 2500, 5,000, 7500 and 10,000 hours corresponding to a total of 20 specimens. Spallation of TOO scale along the NiAl (B2) grain boundary ridges was observed for specimens exposed to higher temperature and longer time. At the lowest temperature of 788°C, a significant amount of metastable γ- and θ-alumina was observed by PSLS along with some spallation. This type of spallation did not occur along the grain boundary ridges, but within the grains themselves