Powder bed fusion of two-functional Cu-Al-Ni shape memory alloys utilized for 4D printing

Transforming a 3D-printed part into a pre-programmed shape over time is called four-dimensional (4D) printing. Shape memory alloy (SMA) is most commonly used material for 4D printing. However, metal-based SMAs exhibit lower recovered strain during 4D printing compared to shape memory polymers, thereby requiring enhanced recovery in metal-based SMAs. In this study, a novel two-functional Cu-Al-Ni SMA consisting of two separate structures fabricated by powder bed fusion process was investigated to enhance the recovery in SMAs. The binding morphology of the two structures and subsequent consequences on the 4D printing behavior were analyzed. The first structure recovered at a lower temperature, whereas the second structure recovered at a higher temperature. In the case of the two-functional material, the difference in the hardness values and recovery temperature favored the recovery ratio due to the damping behavior of the harder structure and the generation of a temperature gradient across the deformation section. Additionally, the higher residual stresses obtained in the harder structure by simulation proved the mismatch between the two structures, leading to resistance against plastic deformation. Meanwhile, the formation of secondary martensite plates was observed in the second structure of the two-functional material. These secondary martensite plates enhanced the overall recovery by (i) the attenuation of thermal conductivity across the deformation section and generation of the temperature gradient, and (ii) the increase in residual stresses and mismatch between the two joined structures.