PSI - Issue 69

Roman Karelin et al. / Procedia Structural Integrity 69 (2025) 35–40

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5]. Also NiTi tubes are used for the production of connecting elements, that are simple in loading and reliable, providing generation of high stresses and excellent mechanical performance [6, 7]. Limited technological plasticity of titanium nickelide in combination with the structural sensitivity [8-10] of operational characteristics complicate existing NiTi tubes production technologies. Most of the world's manufacturers focus on the production of NiTi seamless tubes for medicine [11, 12]. Common production technologies involve deep drilling to create an initial shell, followed by cold drawing with intermediate annealing stages, and various finishing operations. [13-16]. This approach is suitable for obtaining seamless small and medium tubes for medical application. However, for technical applications this approach has some limitations including high cost, low metal utilization rate, lack of the possibility to produce tubes with a diameter of more than 20 mm and a tubes schedule of more than 1 mm because of the high deformation force required and very high load on the equipment. Therefore, the development of an alternative technological scheme for the production of NiTi seamless tubes is at a great interest. In the present study a new technological scheme for the production of seamless tubs from Ti-50.7 at.% Ni shape memory alloy is performed. Analysis of the structure, mechanical and functional properties of seamless tubes at various plugging stages were also conducted. The suggested technological scheme of NiTi seamless tubes production is promising in terms of simplicity of applied technological processes and tubes price reduction because of minimization of metal losses and higher efficiency. 2. Materials and methods Hot-rolled rods of Ti-50.7 at. % Ni alloys with a diameter of 40 mm was used as the initial blank. SC 703 electrical discharge super drill with a copper electrode and a high-precision SCT32-ST electrical discharge machine with a molybdenum wire were used for the obtaining of shells. Screw rolling was carried out on an SVP 70 screw rolling mill. Samples for the investigation were cut out from the resulting tubes by the electrical discharge method. Samples for light microscopy were ground on abrasive paper with a grain size from P120 to P4000 with subsequent polishing. These samples after mechanical grinding and polishing, as well as samples before conducting X-ray analysis, were etched in a solution of 1HF:3HNO 3 :6H 2 O 2 . The phase composition and microstructure of the obtained tubes were studied by optical microscopy using a Versamet-2 Union optical microscope with a magnification of 50 to 100 and X-ray diffraction analysis using a DRON-3 X-ray diffractometer in CuKα radiation in a 2θ angel range from 40 to 44°. Mechanical properties were studied by hardness and tensile tests on a LECOM 400-A hardness tester under a load of 1 N and INSTRON 2253 universal testing machine at a strain rate of 2 mm/min, respectively. Flat samples with dimensions of 1 x 2 x 50 mm were applied for tensile tests and three samples for each condition were tested. The functional properties were studied using differential scanning calorimetry using a Mettler Toledo DSC 3+ calorimeter with a heating and cooling rate of 10°C/min in the temperature range of -100…+100 °C to determine the characteristic temperatures of martensitic transformations (MT). 3. Results and discussion 3.1. Experimental plugging of a NiTi SMA shell on a screw rolling mill Shells from NiTi SMA were obtained by electrical discharge cutting using, at the first stage, an SC 703 electrical discharge super drill for burning through holes, and a high-precision SCT32-ST electrical discharge machine, at the second stage, for cutting the required sleeves for subsequent rolling on screw rolling and rotary forging mills. The general appearance of the obtained shells is shown in Figure 1.