Directed energy deposition repair of dissimilar ultra-high strength steels: Martensite variant selection and dynamic deformation mechanism

Laser directed energy deposition (DED) for dissimilar material reinforcement repair of ultra-high strength steel (UHSS) components after localized damage is a cost-effective method that significantly improves the performance of the components. In this study, high-alloy UHSS AerMet100 was used for DED repair on 30CrMnSi steel, and the microstructure, as well as the quasi-static and dynamic mechanical properties of the repaired samples, were systematically evaluated. The aim is to achieve restoration of base material properties without post-heat treatment. The results indicate that the microstructure of the repair zone is predominantly composed of fully martensitic, devoid of carbides. Owing to grain refinement and increased carbide precipitation, the heat affected zone of the 30CrMnSi substrate showed no signs of weakening or degradation in mechanical properties in various mechanical tests. The ultimate tensile strength of the repaired sample is 965 MPa, which corresponds to 99 % of the substrate strength (970 MPa). Additionally, the lath martensite in the repair zone exhibited a strong variant preference for V7 to V12, while the six variants within the same habit plane were nearly evenly distributed, indicating that martensite variant preferential transformation is primarily governed by the specific habit plane of the parent austenite, rather than the habit direction. Moreover, the tendency of dislocation slip to follow longer paths to avoid premature obstruction at lath boundaries, combined with the greater number of available slip systems in the body-centered cubic structure, makes the shape factor of martensite the key determinant in slip system activation under high strain rate deformation.