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Solidification behavior and texture of laser-wire directed energy deposition of 316L austenitic stainless steel
DeNonno, Olivia L.
DeNonno, Olivia L.
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2023
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Abstract
Austenitic stainless steels have a combination of good weldability, mechanical, and corrosion properties, making them suitable candidates for additive manufacturing (AM) and use in Army ground vehicle systems. During AM processes, the 316L feedstock material undergoes melting followed by fast solidification and partial remelting of previous build layers, resulting in unique final microstructures and textures throughout AM builds. In this study, the solidification behavior and crystallographic texture of AM 316L produced using a laser-wire directed energy deposition (DED-LB) process was investigated and delta-ferrite morphology was utilized to determine the solidification pathway of AM 316L.
Variations in shielding gas set-up and build type, single versus multi-track walls, resulted in differences in primary solidification mode and final microstructure morphology among the DED-LB builds. Three delta-ferrite morphologies were formed in the as-built microstructure as a result of variations in the solidification sequence for which distinct orientation relationships and crystallographic texture between delta-ferrite and austenite developed. Skeletal and interdendritic delta-ferrite exhibited a (001) austenite // (001) delta-ferrite relationship with austenite and a {001} solidification texture aligned with the build direction was observed for both austenite and delta-ferrite. Lathy delta-ferrite exhibited a Kurdjimov-Sachs orientation relationship due to austenite nucleation in the solid state.
The build type and shielding gas set-up directly impacted the solidification conditions and element vaporization of the build process, resulting in the range of final AM 316L microstructures. Primary ferrite solidification is predicted based on feedstock material but variations in solidification mode from primary ferrite to primary austenite occurred due to increased solidification velocity at the top of the melt pool stabilizing the austenite dendrite tip over the delta-ferrite dendrite tip as well as due to compositional variations between builds. The initial feedstock composition cannot accurately represent all the solidification mode variations of AM 316L indicating the need for the as-built wall composition.
A dendrite growth model was paired with Rosenthal’s analytical heat transfer model for the development of solidification maps, which were used to predict solidification microstructure morphology, size, and shift in solidification mode. The solidification maps were assessed against experimental dendrite arm spacing data for which there was good agreement between the experimental data, the dendrite growth model, and the solidification path generated from Rosenthal’s analytical heat transfer model.
Heat treatment of the DED-LB AM 316L material at 800°C and 900°C resulted in an increase in hardness due to sigma phase formation. Variation in hardness between samples heat treated at the same conditions is attributed to differences in the volume fraction of sigma phase due to differences in the initial delta-ferrite volume fraction in the as-built material. Recrystallization and grain growth were observed after one hour and four hours at 1200°C, respectively.
Overall, AM316L produced from the DED-LB process highlighted the variability within AM processes that results in the range of mechanical and material properties reported for AM materials. Specifically, variations in composition and solidification conditions from adjustments in AM process set-up or part geometry were shown to greatly impact the as-built microstructure. Solidification and heat transfer modeling provide a method for making predictions about as-built microstructure size, morphology, and solidification mode without needing in-depth metallurgical analysis. The results from this work can be used to understand the impact of composition and solidification conditions on the as-built microstructure of 316L produced using other additive processes.
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