Effect of cellular structure on the mechanical properties of 316L stainless steel fabricated by EBF3

Abstract

The cellular structure formed in additive manufactured 316L stainless steel (316L SS) plays a crucial role in achieving strength–ductility synergy. The cellular structure was also found in the 316L SS fabricated by electron beam freeform fabrication process (EBF3), and its microstructural characterization was analyzed. Interrupted tensile tests were carried out to demonstrate the effect of the cellular structure on the mechanical properties. The results showed that the microstructure of the as-deposited 316L SS consisted of the single austenite phase, and the enrichment of Cr and Mo elements was found in the cellular boundary. The orientation of the cellular structure and boundary was similar, and the obvious {100} preferred orientation was parallel to the building direction. The yield strength and fracture elongation of the as-deposited 316L SS parts were 316 MPa and 37.2%, respectively. Its ultimate tensile strength with a value of 632 MPa, was higher than that manufactured by conventional methods. The hardness and elastic modulus in the cellular interior were 2.88 ± 0.11 GPa and 227.45 ± 10.66 GPa, respectively. The cellular boundary had a higher hardness and a lower elastic modulus than the cellular interior. At the initial stage of tensile deformation, the cellular boundary deforms first due to its lower elastic modulus, causing the dislocation pileup and orientation change. The high dislocation density improved the slip resistance leading to the appearance of deformation twins after necking. The micron-dimples and nano-dimples formed in the fracture surface with the deformation of the cellular boundary and interior. Different from 316L SS fabricated by other additive manufacturing processes, the cellular structure improved the strain hardening capacity of the 316L SS fabricated by the EBF3 process.