Influence of the L-PBF Process Atmosphere on the Microstructure and Tensile Properties of AISI 318LN Duplex Stainless Steel

Abstract

Duplex stainless steels (DSSs) have excellent mechanical properties, owing to their austenitic–ferritic microstructure. The phase equilibrium strongly depends on solidification conditions and chemical composition, where elemental nitrogen significantly stabilizes the austenitic phase. When DSSs are processed by laser powder bed fusion (L-PBF) under an argon atmosphere, the rapid cooling rates result in an undesirable fully ferritic microstructure. To better understand the microstructure formation, this study examined the influence of the L-PBF process atmosphere on the porosity, microstructure, and mechanical properties of DSS AISI 318LN. Gaseous argon and nitrogen were used as a protective atmosphere, and specimens were analyzed in the as-built and post-processed conditions via optical and electron microscopy, electron backscatter diffraction, and tensile testing. Specimens processed under a nitrogen atmosphere showed a lower initial density in the as-built conditions, and tended to form more lack-of-fusion and gas pores compared to specimens processed under argon. The different defect types in nitrogen-processed specimens were still present after solution-annealing and quenching, leading to a 13% lower tensile strength and 43% lower elongation at fracture. Differences in phase equilibrium caused by the process atmosphere could not be established. All differences in porosity can be minimized by hot isostatic pressing, thus resulting in comparable mechanical properties of argon- and nitrogen-processed specimens.

Crack‐Free Tungsten Fabricated via Laser Powder Bed Fusion Additive Manufacturing

“…Researchers have equated the local solidification velocity, R loc in LPBF to the maximum solidification velocity R max using the relation R loc = R max · cosα, where α is the angle between the laser scanning direction and the maximum heat flow direction. [ 68 ] The high cooling rates attained in the LPBF process allow for the trapping of a larger amount of these interstitials. Similar results have been reported in the LPBF processing of steel under the N 2 build atmosphere.…”

“…Similar results have been reported in the LPBF processing of steel under the N 2 build atmosphere. [ 67,68 ]…”

“…Similar results have been reported in the LPBF processing of steel under the N 2 build atmosphere. [67,68] Figure 7a shows the phase diagram of the W-N system in black lines and indicates the very low solubility of N in W. The red lines indicate the calculated paraequilibrium diagram (T0 lines) for the system under the assumption of no elemental diffusion occurring between the phases. The paraequilibrium diagram suggests that the theoretical solubility of N in W is higher at 8.25 at.% than the low value indicated by the equilibrium phase diagram.…”

Influence of PBF-LB Process Atmosphere on the Fatigue Strength of Hot Isostatically Post-Densified Duplex Steel Parts Produced via the Shell Core Approach

“…The mechanical properties obtained from the tensile tests are compared with the specifications of standard DIN EN 10088-3 [39] and with values determined in preliminary tests [40] on fully dense specimens. Figure 3 values for porosity and roundness of the pores were determined using ImageJ 1.53 software [37].…”

“…The mechanical properties obtained from the tensile tests are compared with the specifications of standard DIN EN 10088-3 [39] and with values determined in preliminary tests [40] on fully dense specimens. Figure 3 The investigation of the specimens prepared under argon atmosphere shows a clear influence of the initial porosity on both the static strength and the elastic behavior.…”