Semi-Continuous Functionally Graded Material Austenitic to Super Duplex Stainless Steel Obtained by Laser-Based Directed Energy Deposition

Abstract

In this work, a semi-continuous functionally graded material (FGM) between an austenitic and a super duplex stainless steel was obtained. These materials are of great interest for the chemical, offshore, and oil and gas sectors since the austenitic stainless steel type 316L is common (and not so expensive) and super duplex stainless steels have better mechanical and corrosion resistance but are more expensive and complex in their microstructural phases formation and the obtention of the balance between their main phases. Using directed energy deposition, it was possible to efficiently combine two powders of different chemical compositions by automated mixing prior to their delivery into the nozzle, coaxially to the laser beam for melting. A dense material via additive manufacturing was obtained, with minimum defectology and with a semi-continuous and controlled chemical compositional gradient in the manufactured part. The evolution of ferrite formation has been verified and the phase fraction measured. The resulting microstructure, austenite/ferrite ratio, and hardness variations were evaluated, starting from 100% austenitic stainless-steel composition and with variants of 5% in wt.% until achieving 100% of super duplex steel at the end of the part. Finally, the correlation between the increase in hardness of the FGM with the increase in the ferrite phase area fraction was verified.