Residual Stress Formation in Laser-Based Powder Bed Fusion (PBF-LB) of Invar 36

Laser-based powder bed fusion (PBF-LB), also known as selective laser melting (SLM), is a metal additive manufacturing process associated with residual stress formation due to rapid heating and cooling. This paper aims at investigating residual stresses and deflections of Invar 36 parts produced using a selective laser melting machine that is equipped with a 400-W ytterbium fiber laser source. Invar 36 has been used in the aerospace industry for the past decade because it is known for its ferromagnetic property, high strength, and improved toughness. Invar 36 has a very low coefficient of thermal expansion below its Curie temperature (279ºC); therefore, it is a good candidate for the PBF-LB process because it shows low thermal stresses and small deflections. Parts manufactured by the PBF-LB process usually experience void formation, internal cracks, metallurgical changes, vaporization of alloying elements, and residual stress formation. In this paper, evolution of residual stresses and deflections of Invar 36 parts is analyzed using a coupled thermal-mechanical finite element model in ANSYS Additive 19.2 software. The numerical results are validated experimentally. Residual stresses are measured using an X-ray diffraction (XRD) instrument, and part deflections are measured using a coordinate measuring machine (CMM). Parts are produced at the optimum process parameters for Invar 36 in order to eliminate the formation of excessive residual stresses during the PBF-LB process. These optimum process parameters that give the smallest deflections and lowest residual stresses are tested using the finite element model. In addition, the relationships between the thermal properties of Invar 36 and the process-induced residual stresses and deflections are discussed.