Fundamental role of vertical building sequence and its thermodynamic mechanisms during multi-material additive manufacturing of 18Ni300-CuSn10 structures with IN718 interlayer

Multi-material laser powder bed fusion (LPBF) innovates design opportunities by integrating multiple materials into predetermined 3D shapes. However, for satisfying the diverse performance/functional layouts, understanding the fundamental role of material building sequences in interfacial bonding is still insufficient. This work prepared multi-material structures of immiscible CuSn10-18Ni300 with an interlayer IN718 vertically, using two reversed building sequences by LPBF, to compare their interfacial segregation and microstructure, resulting mechanical properties, and behind mechanisms. Sequence I printed CuSn10 onto 18Ni300 via IN718; Sequence II printed 18Ni300 onto CuSn10 via IN718. Both sequences produce three shapes of interfacial macrosegregation (shape of beaches, peninsulas, and islands), but following different thermodynamic mechanisms. The microstructure highlights the traces related to those macrosegregation shapes, verifying the mechanism, and interestingly, the cracks are even healed by upper CuSn10 under Sequences I, benefiting interfacial bonding. Besides, a transition of cellular-columnar grains from 18Ni300 to IN718 is observed under both sequences, while less visible molten pool boundaries are seen in CuSn10. Those IN718 columns are either 〈001〉 oriented along the building direction under Sequence I, or randomly oriented under Sequence II with reduced temperature gradients. Finally, microhardness evolution and tensile behaviours successfully evaluate the structural reliability under two building sequences. A favourable ductile fracture within the CuSn10 region is produced with strength of 395 ± 29 MPa and elongation of 16.6 ± 2.6 % under Sequence I, benefiting from the healed cracks and enhanced interfacial bonding. Conversely, Sequence II causes a brittle fracture within the interlayer with lower tensile properties, due to the remaining cracks and microstrain. Such high microstrain even impacts the detection of clear Kikuchi bands. These results could enrich theoretical bases for selecting building sequences and matched structural designs for manufacturing multi-materials by LPBF.