The role of substrate preheating on the microstructure, roughness, and mechanical performance of AISI 316L produced by directed energy deposition additive manufacturing

Directed energy deposition (DED), one of the additive manufacturing (AM) technologies, was used to realize AISI 316L specimens. The role of substrate preheating on the microstructure, hardness, surface roughness, and mechanical properties of AISI 316L samples was studied using two groups of deposited samples on a cold substrate and a preheated one. The cooling rate in each specimen was determined using primary cellular arm spacing (PCAS). It is found that the thermal gradient and cooling rate in the samples produced on the preheated substrate are lower. However, in both cases, the cooling rate decreased as the deposition height increased. The phase composition analysis confirmed that the lower cooling rate in the preheated samples resulted in a lower residual δ-ferrite content. The deeper microstructural analysis also confirmed the formation of non-metallic inclusions during the building process. However, the quantity of these inclusions was lower in the samples realized on the cold substrate. Preheating the base plate had a negligible effect on the surface roughness of the AISI 316L cubes. The tensile results demonstrated that the samples produced using the cold base plate had better mechanical performance than those deposited on the preheated base plate. All in all, using a preheated substrate led to a lower thermal gradient, lower cooling rate, larger PCAS, higher inclusion content, higher oxygen pick-up, and lower nitrogen pick-up that strongly affected the mechanical characteristics of the AISI 316L samples.