Post-heat treatment effect on the microstructure and mechanical properties of directed energy deposited IN738LC alloy

IN738LC alloy is one of the representative Nickel-based superalloys for high-temperature components, including gas turbine systems. As the lifetime of gas turbine systems getting longer, maintenance, repair and overhaul of Ni-based superalloy components becomes important issues. However, a conventional welding procss has a limitation to apply high-grade Ni-based superalloys due to their high crack susceptibility. To overcome this problem, laser-based additive manufacturing (AM) has been developed but the application of high-grade Ni-based superalloy remains one of challenging issues. Because the biggest obstacles for depositing crack-free high-grade Ni-based superalloy are liquation cracking during solidification, a small laser-beam sized directed energy deposition (DED) was applied in this study. To explore the optimal processing window, response surface methodology was conducted to find the highest dimensional accuracy – relative density – hardenss combinations. Based on the optimal DED parameters, crack-free IN738LC components were successfully built by laser-based DED. To control the microstructure and mechanical properties of DED IN738LC components, various post-heat treatments were conducted. Although the solution treatment was conducted at 1190 ℃, homogenization was not completed in the DED IN738LC due to the solute drag resistance and pinning force from fine MC carbides in the as-built state. Also, the heat treatment step increment causes microstructural changes, including γ′ phase growth and MC carbide decomposition in the IN738LC samples, and such microstructural changes affect both tensile and creep properties. This result represents that crack-free IN738LC components can be achieved by reducing heat affected zone during laser-based AM, and appropriate post-heat treatment required for AM components due to their different microstructure with the conventional IN738LC alloy.