Microstructure and Properties of the LDEDed Cu-Bearing Martensitic Stainless Steel After a Single-Step Tempering Treatment

431 martensitic stainless steels (MSS) alloying with copper (Cu, 0-3.5 wt%) have been fabricated by a laser directed energy deposition technology and treated by a single-step tempering treatment. The microstructure and properties of the tempered MSS specimens have been carefully investigated by various advanced techniques, including XRD, SEM, EBSD, TEM, microhardness tester, universal material testing machine, and electrochemical workstation. The results show that the Cu-free 431 MSS is mainly composed of lath-shaped martensite, a few patches of austenite and carbide, while the martensite matrix is refined with the increased austenite and nano-precipitates by increasing Cu content of the Cu-bearing MSS. Consequently, the microhardness of the Cu-bearing 431 MSS specimens increases firstly and then decreases, reaching the maximum value (472 HV0.2) at the Cu level of 2.5 wt%. In comparison to the Cu-free 431 MSS, the addition of 2.5 wt% Cu could lead to a striking comprehensive performance, including tensile properties with ultimate tensile strength (UTS) of 1576 MPa, yield strength (YS) of 1258 MPa and elongation (EL) of 16.1%, as well as improved corrosion resistance by an order of magnitude. The mechanisms behind the variation in the overall performance of the Cu-bearing 431 MSS specimens were discussed in detail.