Microsoldagem por pulsos de GTAW em fios de ligas com memória de forma Ni-Ti.

Abstract

The welding of Shape Memory Alloys (SMA) remains one of the biggest challenges to expanding the applications of these materials that have high cost due to the noble elements used and its hard manufacturing. The welded joint can lead to the development of products with complex geometry that would be infeasible to be manufactured by conventional processes. In this work, several tests were carried out in order to evaluate the effects of parameters on the properties of SMA Ni-Ti wires using welding by pulses of GTAW (Gas Tungsten Arc Welding). For this, using a GTAW welder, PUK U4 model, supplied by Lampert (German company), several welding tests were performed initially evaluating the conditions that promote full penetration in actuators and superelastics wires made of SMA Ni-Ti with diameters of 0.9 mm (900 µm). The penetration tests parameters were obtained through experimental design by using the central composite rotatable design (CCRD) method. Thus, it was possible to obtain three great settings that allow full penetration of the joint, with values close to 900 µm. These configurations were tested to evaluate their effects in similar joints (actuators and superelastics) and dissimilar joints. The evaluation tests of the thermomechanical properties of welded joints were performed through optical microscopy (OM), thermal analysis by differential scanning calorimetry, microhardness, tensile testing until failure at room temperature and isothermal ranging from 40° C to 90°C at intervals of 10 ° C, fracture surface analysis by scanning electron microscopy and dynamic mechanical analysis. The results show that all three configurations exhibit good correlation of properties in the joint when compared to each other. Larger grains were observed in the center joints near the welding pulse application area. Narrower columnar grains were observed in the area where the pulse intensity was more intense. The welded joint suffers changes in the phase transformation temperature of the material, as well as the profile of the peaks, compared to the whole material. The increase in test temperature promotes the SMA phase transformation of the actuator welded joints that becomes more brittle and tend to fracture prematurely from 70 °C. At this temperature, there is a mixed phase zone in actuators joints. While the failure of superelastic joints occurs in strain of 2.5% regardless of the isothermal test temperature. The fracture of the whole wire at low and high temperature seemed ductile, while, in general, welded wire showed from brittle fracture, with the presence of faceted planes (cleavage), to a fracture of ductile-brittle type (mixed). The dissimilar joint showed intermediate properties between both bases material. Based on these results we can say that the welding by pulse GTAW has great potential for application on permanent union of SMA Ni-Ti wires in order to develop complex structures for application in several fields.