Influence of Heat Treatments on a NiTi Shape Memory Alloy Obtained Using Vacuum Induction Melting and Reprocessed by Plasma Skull Push-Pull

Abstract

Shape memory alloys (SMA) are metallic attractive engineering materials due to their capacity to store pre-defined shapes through a thermally induced phase transition from a solid state. This paper aims to evaluate the influence of solubilization thermal treatments on a NiTi shape memory alloy originally fabricated by vacuum induction melting and then reprocessed by plasma melting followed by injection molding (Plasma Skull Push Pull process) into different metal molds (steel, aluminum, brass and copper) in order to compare the thermal properties regarding to its raw state. The thermal treatments of solubilization were carried out at 850°C in different times (2n function, n = 0, 1, 2 and 3, in hours). The influence of solubilizing treatments in the NiTi shape memory alloy was analyzed using the following characterization techniques: Differential Scanning Calorimetry (DSC) and Electrical Resistance as a function of Temperature (ERT). The results demonstrate that the solubilization heat treatments applied on the reprocessed NiTi shape memory alloy through the plasma skull push pull process, provides important changes in the phase transformation of the material. Therefore, it was demonstrated that it is necessary to solubilize the material after melting or remelting the NiTi shape memory alloy via this process to obtain mini-actuators products with homogeneous properties.

Thermomechanical Behavior of CuAlMn SMA Cellular Structures Obtained by Rapid Investment Casting

“…The structures presented final temperatures of martensite phase transformation (M f ) above the temperature of 20 • C. The forward and reverse transformation enthalpy values ranged from 4.1 J/g to 13.2 J/g for structures with 0.5 mm thickness, while for structures with 1.0 mm thickness, the values ranged from 4.1 J/g to 7.5 J/g. Similar values were reported by [31], corroborating the aspect of sufficient homogeneity of the structures mentioned previously.…”

“…Indirectly, this thermal behavior shows that induction casting in a plaster mold followed by quenching in water at room temperature already allows for homogeneous structures to be obtained. When the alloys and/or products are not sufficiently homogeneous, flat or disturbed DSC peaks, or even the non-appearance of peaks, are verified, as reported in the work of Simões et al [31] for NiTi SMA injected into metallic molds. This aspect can even be considered a novelty of this work, as it eliminates a step in the manufacturing process of these structures, since they can leave the process and go directly to performance tests.…”