Laser induced forward transfer of NiTi deposits for functionally graded SMA components

Laser-induced forward transfer (LIFT) is a 3D micro-fabrication tool wherein laser pulses are used to sequentially print thin sub-voxels of metal onto a substrate. We are developing a LIFT-based process to fabricate micron-scale parts with shape memory alloy (SMA) properties exploiting its high degree of spatial and temporal control. SMAs exhibit shape memory effect; during which they generate a substantial amount of strain or force, and hence can be used as the basis of actuators such as micro-grippers and fibre-optic manipulators, for surgical and other in-vivo medical applications. We are particularly interested in nickel-titanium (NiTi) SMAs given their biocompatibility and a transition temperature (the temperature at which the material returns to its initial state) close to body temperature. Small variations in chemical composition can be used to tune their transition temperature. However, compositional, and spatial control of these SMAs is limited to macroscopic manufacturing techniques. In this paper, we explore a novel approach to locally control the composition of NiTi alloy using LIFT. The donor is a multilayer comprising nickel and titanium thin films. During the transfer, the laser pulse melts and diffuses the metals forming a composite droplet. We demonstrate the possibility of obtaining NiTi deposits with equiatomic composition (50- 50 atomic %). Conventional SMAs have a narrow range of control parameters which makes it difficult for actuator design. However, by locally altering the composition, it would be feasible to locally tune the transformation window, providing a more complex response to temperature and hence a wide range of SMA based micro-actuator applications.