Wire arc additive manufacturing of functionally graded carbon steel - stainless steel 316L - Inconel 625: Microstructural characterization and mechanical behavior

This work utilized a gradient method of joining plain carbon steel to stainless steel 316L and then to Inconel 625 using wire arc additive manufacturing. The research investigated the quality of Functionally Graded Materials (FGM) structure, continuity, defect formation, microstructure, and mechanical properties of gradient regions. The investigation showed a strong, defect-free metallurgical bond between plain carbon steel and stainless steel 316L and stainless steel 316L and Inconel 625. The microstructure of stainless steel 316L resulted from the solid-state transformation of ferrite-austenite (FA), with a significant presence of delta ferrite in the austenite matrix. In Inconel 625, the Laves intermetallic phase formed discontinuously between dendritic arms due to the microsegregation of alloy elements like niobium and molybdenum during solidification. The hardness values of Inconel 625, stainless steel 316L, and plain carbon steel were 194–257 HV, 171–178 HV, and 159–170 HV, respectively. The ultimate tensile strength, yield strength, and elongation were achieved at 487 ± 10 MPa, 300 ± 6 MPa, and 40% ± 0.15, respectively. The tensile test samples failed on the plain carbon steel side, indicating higher tensile strength at the interface and a well-bonded joint between the two alloys. Small, homogeneous dimples on the fracture surface confirmed the ductile fracture mode. The research demonstrates the use of wire-arc additive manufacturing (WAAM) to fabricate gradient materials with the required properties.