Regulating metallurgical continuity and fracture-path selection through interfacial coupling in multi-material additive manufacturing

The growing demand for multifunctional components requires structural and functional properties to be spatially tailored within a single architecture, a capability difficult to achieve using monolithic materials. Multi-material additive manufacturing offers a promising solution, yet its implementation is constrained by the difficulty of forming reliable interfaces between dissimilar alloys. Here, we reveal how interfacial melt-pool coupling governs interface evolution and fracture-path selection in 316L stainless steel/CuSn10 multi-material architectures fabricated by multi-material laser powder bed fusion. Four programmed coupling conditions were produced by varying the cross-boundary scan extension and were operationally classified as decoupled, partially coupled, stably coupled, and over-coupled according to the resulting interfacial morphologies. Stable coupling transformed the sharp and discontinuous boundary into a continuous, chemically heterogeneous transition zone with broadened Fe/Cu redistribution, refined interfacial grains, and locally identified BCC α-Fe within the Cu-rich matrix. This transition suppressed boundary-dominated failure and redirected fracture across both constituent materials, increasing the ultimate tensile strength by 61.7%, from 161.1 to 260.5 MPa, although the elongation decreased from 8.33% to 5.23%, indicating a strength–ductility trade-off. Insufficient coupling preserved lack-of-fusion defects, whereas over-coupling generated pore-bearing Cu-rich regions and shifted failure to a pore-assisted path. Time-resolved synchrotron X-ray imaging further revealed keyhole fluctuation, partial collapse, and pore retention as a plausible pore-formation pathway. Application-oriented multi-material demonstrators with complex geometries were successfully fabricated, supporting the geometric implementation of interface-controlled MM-LPBF. These findings establish a coupling-dependent interface-control principle for balancing metallurgical continuity against defect-assisted damage in multi-material additive manufacturing.