Solidification sequence and crystal growth during laser welding stainless steel to copper

Abstract

In the present study, solidification and growth features were systematically studied during fibre laser welding 304 stainless steel to T2 copper. Via orientation-composition combination detection, Cr-rich ferrite (δ), Ni-rich austenite (γ) and pure Cu phases were precisely clarified. Close to the fusion line on the steel side, strict epitaxial growth from γ substrate grains produced continuous and uniform crystalline orientations for both γ and Cu phases. In the vicinity of α/ε junctures, similar orientation identities of γ and Cu were maintained until the presence of δ. Morphologically, δ presented globular and dendritic forms in the ε structure with dispersive γ spheres and Cu grains. These outcomes indicated that δ particles produced direct or immediate interruptions, thus leading to localized refined γ and Cu grains. By metallurgical analyses, such refinement was primarily attributed to initial δ dispersion and its subsequent chemical or mechanical blocking-up on γ and Cu by forming K-S, N-W and G-T relationship orientations. Additionally, solidified γ also exerted accordant growth of Cu. In summary, the substantial method to obtain identical refinements of δ, γ and Cu grains depended naturally on the dispersed and uniform precipitation of priorly precipitated δ.