Effect of pulsed fiber microcutting on the microstructure and the functional properties of NiTiCu shape memory alloy

Laser microprocessing of NiTi based Shape Memory Alloys (SMAs) is one of the most suitable methods for the manufacturing of products in several industrial applications, such as the biomedical, sensor and actuator fields. The nature of material removal in laser material processing is generally thermal; this means that the thermal effect of the processing can influence the performances of the machining itself in terms of quality of the results and productivity. As a consequence, the modification of the microstructural and functional properties of these functional materials has to be expected; its evaluation is consequently needed for a better comprehension of the real performances of the SMA final device. In this work, 150 μm thick sheet of the ternary SMA Ni40Ti50Cu10 (at. %) was cut by means of a nanosecond fibre laser. The effect of some process conditions (process speed and number of laser passes), having the goals of producing through and stable cut edges, was investigated on the quality of the laser machining result and on the SMA properties. As reported in literature, laser material processing produces some thermal damages, such as melted material, heat affected zone, oxidation, possible cracks and so on, depending mainly on the pulse duration. Scanning electronic microscopy was used for observation of the laser cut edges' morphology, at high magnifications. The presence of significant amount of melted material has been detected, whose chemical composition can be slightly changed during the laser machining. The presence of surface oxides was seen, mainly on the bottom surfaces, because of the lack of shielding effect of the assist gas. Nanoindentation measurements were performed on the transversal section of the laser cut edges for estimating the extent of the areas subjected to thermal affection (melted and heat affected zones). It was observed that the range 20-40 μm can be a reasonable estimation of the width of the heat affected zone along the thickness. Moreover, calorimetric and thermo-mechanical properties has been also investigated in function of the process speed. It was seen that the amount of heat can strongly modify the SMA functional response. In fact, the calorimetric analysis showed that the shape of the DSC signal was changed in function of the process conditions, mainly when compared with the one of the base material; the presence of multi-peaks at the martensitic transformation can be associated to the presence of different microstructures, due to some thermal treatments established by the heat flow of the laser beam. Tensile test at different temperatures showed that the laser machining modifies the mechanical response as well; the martensite showed a lower value of the recovered strain, if higher heat flow was adopted during its machining as well as the austenite a lower Young modulus. It can be concluded that residual effects, associated to the laser processing, can be detected on the functional response of the SMA. Post-processing could be required, if the properties of the base material have to be completely replaced, for the use of the final device. However, the thermal affection, even if it is present using fibre lasers, can be considered quite small if compared with other types of traditional lasers.