Additive Manufacturing of Duplex Stainless Steel by Laser Metal Deposition with Wire
Amir Baghdadchi 1, Vahid A Hosseini1, Maria Asuncion Valiente Bermejo1, Björn Axelsson2, Ebrahim Harati3, Mats Högström1, Leif Karlsson1
1Department of Engineering Science, University West, 461 86 Trollhättan, Sweden, 2Alfa Laval Tumba AB, 147 80 Tumba, Sweden, 3ITW Welding AB, 433 25 Partille, Sweden
Duplex stainless steels (DSS), with ferritic-austenitic microstructure, have received much attention in various industries thanks to their high corrosion resistance and excellent mechanical properties. Additive manufacturing (AM) has provided new opportunities to fabricate near-net-shape DSS components with low waste of material, tailored properties, and complex geometries. A microstructure with an approximately equal amount of ferrite and austenite provides the best combination of properties of DSS and is controlled by the chemical composition and thermal cycles. An excessive amount of ferrite or/and formation of detrimental secondary phases therefore have to be avoided in AM of DSS.
This study aims at the production of a full-size 25 kg additive manufactured cylinder using Laser Metal Deposition with DSS Wire (LMDw). A four-stages methodology (single bead, one-row wall, snickers, and cylinder) was employed to evaluate effects of deposition parameters, shielding gas and heat treatment on microstructure and properties before fabrication of the full-size cylinder. In each stage, a subsequent 1h heat treatment at 1100℃ was applied to homogenize the microstructure and to control the phase balance. The nitrogen content, due to its role in promoting austenite formation, was measured and correlated with the as-deposited and heat treated microstructure. The results showed that high cooling rates and complex AM thermal cycles resulted in an inhomogeneous microstructure in the as-deposited condition, including almost highly ferritic regions with more than 80% ferrite as well as regions with less than 30% ferrite, resulting in low mechanical properties. Heat treatment successfully balanced the ferrite and austenite fractions and improved the mechanical properties. Finally, the implementation of this stepwise methodology made it possible to achieve a stable and consistent LMDw process and provided very useful input for future fabrication of full-size high performance DSS AM components.