Microstructure, phase evolution, and properties of laser powder bed fusion-fabricated Ni50Ti50/Ni-rich NiTi multi-material structures

Multi-material additive manufacturing offers a new approach to creating parts with locally tailored properties. This study focuses on fabricating and characterizing a novel multi-material system composed of equiatomic Nickel (Ni)50Titanium (Ti)50 shape memory alloy–exhibiting shape memory functionality–and Ni-rich NiTi alloy–offering enhanced strength–using laser powder bed fusion (L-PBF). Optimal L-PBF parameters for the Ni-rich NiTi alloy (51.4 at.% Ni) were determined based on density analysis (laser power: 250 W, scan speed: 350 mm/s, hatch distance: 120 μm, layer thickness: 40 μm). Multi-material specimens were printed using a modified L-PBF system with dual powder feeders. Microstructural analysis revealed distinct columnar grains in the as-built Ni50Ti50 zone and melt pool boundaries in the Ni-rich NiTi zone, both evolving after heat treatment (800°C/1 h + 430°C/6 h). Notably, the interface between the two alloys exhibited minimal mixing, with no formation of detrimental secondary phases beyond those intrinsic to each alloy (B19’ martensite in Ni50Ti50; B2 austenite and Ni4Ti3 precipitates in Ni-rich NiTi after aging). Chemical composition analysis confirmed the maintenance of target compositions in their respective zones. Microhardness showed a gradient decrease (from ~220 HV to ~190 HV) from the Ni-rich NiTi zone to the Ni50Ti50 zone. Tensile tests yielded an average ultimate tensile strength of 572.2 ± 57.8 MPa and elongation of 18.8 ± 2.2%. Functional testing demonstrated successful shape recovery upon heating in the Ni50Ti50 region (~5 mm deformation recovered), confirming the preservation of the shape memory effect within the multi-material samples. This work demonstrates the feasibility of L-PBF for producing functional NiTi multi-material structures with combined shape memory and high-strength properties.