In-situ bonding of horizontal bimetallic interface by laser offset during laser powder bed fusion of copper/nickel multi-material structures and underlying thermodynamic mechanisms

Abstract

Achieving robust bonding between dissimilar materials is crucial for both vertical and horizontal interfaces in multi-material structures fabricated via layer-by-layer laser powder bed fusion (LPBF). However, horizontal bonding has been less explored than vertical bimetallic interfaces, largely because of the limitations in the available equipment, process strategies, and research concepts. In this study, we propose a practical horizontal dissimilar laser offset strategy, wherein the laser spot is displaced outwards from the interface profile by a certain distance, to improve horizontal bonding in CuSn10/IN718 structures. A laser offset of 75 μm enhanced metallurgical bonding and reduced microcracks, resulting in a smoother surface. The laser offset area displayed interlocking macro-segregation peninsulas composed of CuSn10 or IN718. The diffusion behaviour between these segregated peninsulas, located at the boundary of the molten pool, was found to be more intense than that at the centre, driven by a steeper temperature gradient at the molten pool boundary (22.61 ×10⁶ K·m⁻¹ at the boundary vs 18.03 ×10⁶ K·m⁻¹ at the centre, representing a 25.4 % increase). Such material heterogeneity shows microstructural differences, characterised by fine columns, reticulations, and discrete pellets. Consequently, hardness and elastic modulus exhibited a smooth transition across the CuSn10/IN718 interface, which was enabled by the homogeneous material distribution obtained using the laser offset technique. The laser offset strategy provides a convenient laser scanning approach for LPBF and other laser-based multi-material AM processes. This study conducted a detailed analysis of segregation and interlocking at the interface, which may be beneficial for studying other material combinations.