In-situ duplex structure formation and high tensile strength of super duplex stainless steel produced by directed laser deposition

Abstract

Additive manufacturing of 25Cr super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (L-PBF) in recent years. The as-built L-PBF 25Cr SDSS exhibits a microstructure that is close to 100% ferritic (due to the fast cooling in L-PBF) and requires a high temperature thermal treatment to obtain the desired duplex structure. Directed laser deposition (DLD), as a variant of the additive manufacturing methods, is characterized by a much slower cooling rate than L-PBF and is potentially beneficial to achieve the duplex phase fraction in the as-deposited condition. DLD of 25Cr SDSS is new, the microstructure development and the mechanical properties remain unclear. The microstructure and mechanical properties of additively manufactured 25Cr SDSS produced by metal powder based DLD are reported in this work. The as-deposited microstructure revealed a balanced duplex structure, with a near 50/50 ferrite-austenite phase ratio, differences to the previously reported microstructure of SDSS produced by L-PBF are discussed. The mechanical properties of DLD 25Cr SDSS were investigated using uniaxial tensile testing and compared with the commercial wrought alloy. DLD 25Cr SDSS showed superior tensile strength to the commercial wrought alloy, the origin of the strength and strain hardening is discussed, and the flow strength for DLD and wrought 25Cr SDSS are described using a simple rule of mixtures model. The modelling supports the notion that the main difference between the mechanical response of the DLD 25Cr SDSS and the commercial alloy arises from the finer scale of the phases in the DLD SDSS. The findings demonstrate the feasibility of additive manufacturing using DLD as applied to 25Cr SDSS.