Laser directed energy deposited SS316L/IN625 bimetallic structure with gradient transition composition: Microstructure and local performance studies

Laser-directed energy deposition (L-DED) enables the fabrication of functionally graded materials (FGMs) with tailored compositions and properties, offering advantages for high-performance applications. In this study, L-DED was adopted to produce FGMs combining stainless steel (SS316L) and nickel-based superalloys (IN625) with graded chemical composition, make it serve as the transition zone in SS316L/IN625 bimetallic structures. The effects of composition gradients under as-deposited (AD) and hot isostatic pressing (HIP) conditions on microstructure and mechanical properties were investigated. A dilution-based model accurately predicted compositional profiles, validated by energy-dispersive X-ray spectroscopy. More Laves phase mixed with carbides were observed in IN625-rich regions (≥75%) under AD condition with the HIP treatment promoting the dissolution of Laves phase. Meanwhile, the AD FGMs exhibited a transition from weak Goss and Copper textures (25% IN625) to a stronger rotated Cube texture (≥50% IN625), while HIP led to coarse equiaxed grain formation and texture homogenization. As a result, the local microhardness peaked at ∼280 HV in 100% IN625 region, while HIP reduced average hardness by 30% across the transition zone and mitigated hardness variation in the initial layers. Micropillar compression tests demonstrated increasing maximum resolved shear stress (MRSS) with IN625 content, with HIP causing 15.8–33.7% reductions from Laves phase dissolution and dislocation density reduction. These results demonstrate the feasibility of L-DED for producing high-performance FGMs and highlight the role of HIP in tailoring microstructure and properties for industrial applications.