Laser powder bed fusion of SS316L-IN718 functionally graded materials: Processing, microstructure, and properties

Abstract

Functionally Graded Materials (FGMs) development has recently accelerated thanks to Additive Manufacturing (AM) technology, as its layer-wise manner flexibly enables the production of complex high-performance FGMs. More recently, the Powder Bed Fusion (PBF) process has been preferred over other AM techniques owing to its unique advantages in discovering a new generation of FGMs. Therefore, herein, an innovative approach for the composition control has been introduced to fabricate SS316L-IN718 FGMs using a standard laser powder bed fusion (L-PBF) process and then investigated to realize the effects of the design and processing conditions on the FGM's microstructure and mechanical properties. The results demonstrated that the high density of the samples contributed to the reliability and reproducibility of the process. Nevertheless, the FGM with 25 wt% gradient steps (F25) suffered from severe solidification cracking in the 75-25 composition region, formed around oxide inclusions at continuous micro-segregations along grain boundaries leading to the low-temperature eutectic reaction of L → $\gamma$ + Laves. Furthermore, the dynamics of the residual stress variations along the building direction were effectively modified by the FGM design from a sharp change in the direct transition structure (F0) to a smooth change in the F25 structure. However, the sample with a 50 wt% mixed intermediate region fabricated by optimum processing parameters (F50IN.Opt) showed the best mechanical properties (610 MPa tensile strength, 31.5 % elongation) against the F25.Opt structure (580 MPa tensile strength, 11 % elongation) with a dominant brittle fracture mechanism due to rapid propagation of the pre-existed solidification cracks in its susceptible region.