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Abstract The solidification microstructures of plain and inoculated 6061 aluminum builds manufactured with gas metal arc-directed energy deposition were studied with a combination of models and experiments. Electron back-scatter diffraction (EBSD) showed that the plain 6061 build had large, columnar grains with intergranular solidification cracking, while the inoculated build had a near-equiaxed, fine grain microstructure with no solidification cracks. By combining EBSD and energy dispersive spectrometry, the inoculated build has been shown to have exhibited globular growth while the non-inoculated build displayed a dendritic microstructure. A combination of heat transfer and modified grain morphology models were employed to predict the solidification morphology of the 6061 builds, which closely matched experimental results. A modification is proposed to the criterion marking the transition from globular to dendritic growth that better matches experimental results in this work. The results of this study are expected to provide improved methods to predict solidification microstructure for the development of new materials and processing parameters for additive manufacturing. 

Abstract Gas metal arc directed energy deposition (GMA-DED) has potential for the power generation industry to reduce both time and cost since larger and more complex part geometries can be constructed compared to the typical subtractive methods. The performance of GMA-DED builds can be influenced by the deposition method, resulting microstructure, and formation of defects or secondary phases in the final component. Previous work in the literature evaluated the mechanical properties of GMA-DED builds for a range of austenitic stainless steels, however there is limited data on the high temperature mechanical behavior. This work evaluated the high temperature creep properties of GMA-DED builds constructed with type 316H, 316L, 316LSi, and 16-8-2 stainless steels at 650 °C, 750 °C and 825 °C. The alloy with longest time to rupture for a given stress varied depending on test temperature. Creep damage accumulation at grain boundaries was observed along with grain boundary precipitates which likely aided in damage accumulation. Evaluating the creep properties with the Larson-Miller parameter showed the majority of results fell within the scatter band of creep performance for wrought 316 alloys, indicating the GMA-DED process may be suitable for use in advanced energy systems. 

Abstract UNS N06693 is a Ni-base alloy that provides metal dusting corrosion resistance in steam generator pipes with operating temperatures above 500°C. A crack failure occurred in a 6.5mm thick similar weld pipe joint, located at both fusion zone and heat affected zone, after about 10 years in service and 2 months after weld repair in adjacent weld, which warranted an investigation into possible root causes of failure. This study investigates the potential failure mechanisms that may arise during service (such as stress relaxation cracking, stress corrosion cracking, ductility dip cracking, and creep failure) for UNS N06693 in order to understand the observed cracking behavior. In this year, preliminary fractography, metallurgical characterization, thermodynamic and kinetic CALHAD simulations, and investigation into potential contributing factors (e.g., weld procedure specifications (WPS) and post weld heat treatment (PWHT)) to failure have been completed. The fracture surfaces indicate brittle, intergranular failure, such that no shear lips were observed, and radial lines (crack propagation) were primarily observed in weld fusion zone. Metallurgical characterization near the fracture surface is conducted to reveal the contributing factors to failure, such as intermetallic phases (e.g., Cr-rich α-phase) and distribution of carbide particles (e.g., intergranular chromium carbides), that may contribute to reduced cracking and sensitization resistance. Blocky, intergranular Cr-rich precipitates, either Cr-rich α-phase or Cr-rich M23C6., are observed behind secondary cracks. Based on the initial findings, contributing factors for failure considered are increase in tensile residual stresses due to nearby repair field weld and grain boundary embrittlement due to coarse, blocky Cr-rich phase that likely developed during initial PWHT and within the 10-year service window. In the following year, a more in-depth metallurgical characterization, discussion on contributing causes and possible mitigation strategies for improving microstructural stability and performance-based weldability (e.g., weld procedure and PWHT design), and conclusions with root cause analysis will be provided. 

Abstract UNS S34751 and UNS S34709 austenitic stainless-steel alloys contain thermomechanical properties required for use in chemical processing pipe applications with 900–1200°F (482–665°C) operating temperatures. UNS S34751 alloy has demonstrated improved sensitization resistance compared to UNS S34709, a precursor for polythionic acid stress corrosion cracking (PA-SCC), due to lower carbon (C) content (0.01 wt.%) and higher niobium-to-carbon (Nb/C) ratio with lower overall niobium content. The addition of nitrogen (N) in UNS S34751 alloy provides similar thermomechanical properties compared to UNS 34709. Additionally, stress relaxation cracking (SRC) susceptibility in UNS 34709 welds has been documented thoroughly in literature and industry, which poses a problem for long term service life, while UNS S34751 welds have potential for improved SRC resistance without the need for post weld heat treatment (PWHT). In this paper, a literature review of S34751 is explored, and testing matrix of experimental SRC tests using a Gleeble 3500® thermomechanical simulator is developed for S34751 gas tungsten arc welded (GTAW) pipe samples in comparison to S34709 welds. Additionally, initial thermodynamic and kinetic CALPHAD calculations have been completed to analyze potential detrimental phases in S34751 in comparison to S34709, e.g., z-phase. SRC testing has been mostly completed in S34709 welds made with W34710 (E347-16) and S16880 (E16.8.2-15) weld filler, respectively, and SRC comparisons to S34751 are in progress. Current results show higher resistance to SRC in S34751 HAZ and FZ than S34709 FZ and W34710 FZ at 800°C. In the following year, a full comparative analysis between S34709 and S34751 HAZ and FZ, in addition to welds with alternative filler S16880, is planned, including SRC testing at 600–750°C temperatures, metallurgical characterization of intergranular and intragranular precipitates, and additional thermodynamic analyses to complement microstructural observations. Final conclusions on SRC susceptibility comparisons between S34751 and S34709 welds, including alternative fillers, will be made. 

The influence of hatch spacing on tensile properties of high deposition rate gas metal arc-directed energy deposition of 316L and 316LSi was explored 

The weldability of plain and inoculated 6061 aluminum processed with gas metal arc directed energy deposition (GMA-DED) was evaluated and compared to wrought 6061. Autogenous gas tungsten arc welds of varying heat inputs were made, and the degree of solidification cracking was evaluated. The degree of cracking in the inoculated 6061 material was lower than that of plain GMA-DED and wrought 6061. Microstructure characterization revealed that the welds on the inoculated 6061 produced fine, equiaxed grains, whereas the plain 6061 showed coarse, columnar grains. A combination of heat transfer and solidification models were employed to predict the solidification morphology of the 6061 welds, which closely matched the experimental results in all cases. A model was developed to understand the effect of grain morphology on solidification cracking, and it was found that equiaxed grains shifted the critical liquid film range for cracking to lower solid fractions where thermal stresses are the lowest. However, cracking can be caused if sufficient external stresses are applied when the critical liquid film thickness is present during solidification of the equiaxed grain structure. This work provides insight into the role grain size and morphology control can have in suppressing solidification cracking of other aluminum alloys.