Eliminating metallurgical defects and achieving strength-ductility combination in selective laser melted martensitic stainless steel through high-pressure annealing

Abstract

Due to high difficulty in fabricating and processing high-strength metals, additive manufacturing has been considered to have broad prospects compared to the traditional casting means. However, unavoidable metallurgical defects and strength–ductility trade-off dilemma have become Achilles' heels for their practical applications. In this work, one high-pressure annealing (HPA) strategy was introduced to process the selective laser melted (SLM) martensitic stainless steel. The microstructures and mechanical properties of as-built, high temperature annealing and HPA and as-rolled SLM samples were systematically investigated. It was found that the HPA treatment makes the stainless steel achieve the significant yield strength of 1828 MPa and the comparable plastic strain of 36.2% compared to other samples. The pores and laminar structures from the SLM process were eliminated by HPA. What is more, the HPA treatment results in uniform carbide precipitation, grain refinement and nanoscale heterogeneous structure comprising a series of coarse and fine grains. The physical mechanism for nanoscale heterogeneous structure induced by HPA was discussed from the perspective of the cooperative effect of temperature and pressure. Additionally, the strengthening contributions from grain refinement, carbide precipitation, dislocation hardening, heterogeneous structure and pore closure were quantitatively determined. The current study not only reveals the comprehensive effect of temperature and pressure on microstructure and mechanical properties, but also establishes HPA as an effective method to evade the strength-ductility trade-off in alloys fabricated by additive manufacturing.