Metal 4D printing of NiTi shape memory alloys

Abstract

Metal three-dimensional (3D) printing, formally known as metal additive manufacturing (AM), has become an increasingly mature processing technology for various types of metals and alloys in recent years. Compared to conventional manufacturing methods, 3D printing has advantages in fabricating complex features and saving materials. 3D printing of shape memory materials, also termed “four-dimensional (4D) printing”, is of particular interest to the biomedical and actuator applications as well as in design fields. One additional dimension, i.e. time, can be added due to the ability of shape memory materials restoring their trained shapes when appropriate stimuli (e.g. temperature) are applied. This project involves metal 3D printing of a NiTi shape memory alloy (SMA) using the Directed Energy Deposition (DED) technique. Microelectronics printing of a microheater by using the Aerosol Jet printer is introduced to provide thermal stimulus for 3D-printed NiTi SMA. Additionally, this project builds a controllable functional device to demonstrate 4D printing. The 3D-printed NiTi samples exhibited a high relative density with less thermal cracks and porosities when using high laser powers during printing. However, the high quantity of secondary phase in microstructure of 3D-printed NiTi resulted in serious brittle behavior, which made it quite impossible to deform thus could not show presentable shape memory effect. Moreover, it is found that number of printing layers is a key factor to achieve dense and well-connected micro circuits for Aerosol Jet printing of microheaters. A controllable microheater device was successfully built, which can provide thermal stimuli for commercial NiTi wires to demonstrate shape memory effect. Further studies are needed to optimize the DED printing parameters or explore other metal 3D printing methods to obtain ductile shape memory NiTi samples.