The effect of heat treatments on the microstructure of Inconel 718 produced by additive manufacturing: a comparative study on LPBF, LDED and casting

Additive manufacturing (AM), popularly known as 3D printing, has been increasingly popular in recent times, drawing attention from both academia and industry. It has been developing at a fast pace, and in recent years it has left its original purpose of being a prototyping technology, and it has become a true manufacturing technique, suitable for the production of end parts, especially through metal additive manufacturing. The industry that currently uses additive manufacturing to the greatest extent is the aerospace sector, in which cost is not a strong constraint, and the benefits of additive manufacturing are profoundly valuable (optimized design, weight reduction, internal channels for cooling, improved mechanical properties). In the aerospace industry and energy industry alike, a key material is Inconel 718, a precipitation hardened nickel superalloy used on turbine blades and disks, due to its extraordinary mechanical properties and high temperature resistance. Inconel 718 is difficult to process through traditional operations, due to its high liquidus temperature, high hardness and yield strength and low thermal conductivity, making it a great candidate for AM routes. For metal additive manufacturing, the process that is used the most in industry is laser powder bed fusion (LPBF). The second most used metal additive manufacturing process (after powder bed fusion technologies) is laser powder directed energy deposition (LDED), with different benefits and challenges. Regardless of the manufacturing technique, the condition of the parts in the as-produced state is not ideal. Thermal treatments are necessary in order to optimize the microstructure, dissolve brittle precipitates and promote the formation of strengthening precipitates, relief residual stresses and obtain better mechanical and metallurgical properties. The most commonly used heat treatment for AM Inconel 718 is the standard heat treatment AMS 5662 for wrought Inconel 718. In this work, Inconel 718 parts produced by LPBF, LDED and casting (the traditional process for this alloy) are submitted to different heat treatments (no heat treatment, solubilization, solubilization plus aging and solubilization plus double aging) based on AMS 5662 and characterized in terms of microstructure and microhardness, in order to evaluate whether the AMS 5662 heat treatment is suitable for parts fabricated through AM. The effect of the different heat treatments on the microstructure and microhardness is assessed and compared with the as-produced state, through OM, SEM, EDS and microhardness tests. Both AM techniques yield superior hardness in comparison to cast samples, and LPBF leads to higher numbers than LDED, due to the finer microstructure. Despite the high cooling rates, microsegregation of Nb and Mo was detected even in LDED and LPBF samples. Aging treatments successfully promoted increased hardness when compared to the non-treated samples, but the solubilization treatment failed to dissolve Nb-rich phases such as Laves and delta, and did not eliminate microsegregation. As a consequence, the second aging proved ineffective, with inconsistent effect on hardness, due to the low Nb availability. This highlights the importance of the starting microstructure, and evidences the need of developing an adequate heat treatment specifically tailored to additive manufacturing processes, since the standard heat treatment for wrought parts did not produce the desired results on AM parts.