Laser additive manufacturing of bulk shape memory alloy structures : numerical modeling and experimental investigation

Abstract

Shape memory alloys (SMA) are of great research interest for applications in various areas, such as active layers in thin film actuator, Micro Electro-Mechanical Systems (MEMS) devices, damping systems and vibration control systems. The current market for SMA is dominated by NiTi with maximum deployment in the form of thin films. The use of SMA in the form of bulk structures is very limited due to various defects, including high hysteresis and reducing in life cycle. The alloying with a third element Cu to the binary NiTi alloy overcomes these defects with the same system efficiency. However, building tailored structures of bulk binary and ternary SMA with same shape memory effect is a challenging task. Complex structures of SMA are very difficult to built using conventional tooling technologies. In order to address many of the existing issues, laser additive manufacturing (LAM) may deployed to build tailored complex structures of various compositions. Porosity and brittleness are two major concern in LAM built SMA structures and these can be addressed by post processing techniques, like- laser shock peening and laser annealing. To the best our knowledge, there are not published reports on laser based post processing on LAM built SMA structures. Hence new and interesting findings are expected In the present study, LAM is deployed to build bulk structures of NiTi with three different compositions (Ni rich NiTi45, Ti rich NiTi55 and equiatomic composition of NiTi50) and TiNiCu ternary alloy with six different compositions (Ti percentage is maintained at 50% and the Cu is varied from 5-30% in the step 5%.) The built samples are subjected to two different types of post processing, i.e., laser shock peening and laser annealing is demonstrated using a solid state laser. A numerical simulation is carried out to estimate the temperature and residual stress developed on the samples during LAM and the results are compared with the experimental results. Also, the post processing effect of laser shock peening and laser annealing are simulated and validated with the experimental results. The built samples are also investigated for surface morphological properties, mechanical properties, crystalline properties and phase transformation properties in the as manufactured state and after post processing technique. The samples are finally selected to study the shape memory properties to figure out the best combination of LAM sample.