Microstructure and mechanical properties of carbon-bearing ultrahigh-strength high Co-Ni Steel (AerMet 340) fabricated via laser powder bed fusion

Abstract

Carbon-bearing high Co-Ni steels, which have superior strength, reasonable ductility, and toughness, are crucial materials for load-bearing parts in the aerospace and defense industries. In this study, laser powder bed fusion (LPBF) was used to fabricate high-density ultrahigh-strength high Co-Ni steel (AerMet 340). The relationship between the microstructure and mechanical properties of LPBF steel samples built by varying the laser scan rates (200 mm/s, 400 mm/s, 800 mm/s) with laser power of 90W was investigated. Nearly defect-free carbon-bearing high Co-Ni steel with a relative density of 99.9% could be fabricated at the scan rate of 400 mm/s. The high Co-Ni steels have a fine columnar microstructure, evidence of rapid cooling during LPBF processing, with effective grain sizes of 1.13∼1.68 μm (martensite) and 8∼12% retained austenite. The fractions and average sizes of Mo-rich carbides increase at higher laser scan rate. The compressive yield strengths of the LPBF-built steels are 1621 MPa (200 mm/s), 1698 MPa (400 mm/s), and 1711 MPa (800 mm/s), respectively. The tensile properties of LPBF bulk sample were measured as 1208 MPa of yield strength (YS), 1791 MPa of ultimate tensile strength (UTS), and 4.5% of elongation. The UTS and strength-elongation combination obtained in this study is the highest level reported for LPBF as-built steels to date. The results of the yield strength prediction and calculation show that solid-solution and martensite with high dislocation density are major hardening mechanisms for the superior mechanical properties of LBPF carbon-bearing high Co-Ni steel.