Fabrication of porous NiTinol shape memory alloy by using self-propagating high temperature synthesis with laser ignition

Self-propagating High temperature Synthesis (SHS) can be considered as a potential method for producing porous structures, starting from initial powder of the pure elements, saving both time and energy. Among the porous materials, which can be produced by SHS process, NiTinol Shape Memory Alloys (SMAs) can be considered as one of the most interesting one, because they show both functional properties, such as shape memory effect and pseudo-elasticity, and high level of biocompatibility. The mix of these properties makes the NiTinol SMAs very suitable for biomedical applications, where the porosity is usually an important requirement for improving the osseo-integration. In this contest, the present work has been approached for studying the SHS process, ignited by using a laser beam, on Ni and Ti elemental powders for the realization of samples having controlled porosity. The study can be divided into two main parts: first the preheating of the process and second the SHS process have been investigated respectively. A fundamental step of the SHS process in production of porous NiTi is the preheating of the powder, which allows the next weak exothermic reaction between Ni and Ti. For this reason, the control of the preheating is needed to obtain a homogenous temperature distribution in all over the sample; hence, a set of experiments were proposed to control the preheating temperature by using four thermocouples, placed in different positions of the sample. The prediction of the final preheating temperatures in the powder and in the furnace was obtained in function of the current, controlled for heating the sample holder. In this manner, later there would be no more need of using thermocouples inside the powder. In the second part of the work the effect of the preheating temperature and of the ignition conditions, such as laser power and exposition time, was studied on pore characteristics, on microstructure and on martensitic transformation of the porous NiTinol samples. It was found that the porosity levels varied within the range from 42% up to 48%, as function of the preheating temperature. The main phases realized in the samples were NiTi matrix and small amounts of secondary phases, including NiTi2, Ni3Ti and Ni4Ti3. The characteristic temperatures of the martensitic transformation were not significantly influenced by the process conditions, investigated in this work.