Experimental Study of Tungsten Inert Gas Pulsed Welding Applied to Ni-Ti Shape Memory Alloy Wires
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Experimental Study of Tungsten Inert Gas Pulsed Welding Applied to Ni-Ti Shape Memory Alloy Wires Luiz F.A. Rodrigues1,2, Fernando A. Amorim2, Francisco F.R. Pereira2, Carlos J. de Araújo2 1 Federal Institute of Education, Science and Technology of Paraiba (IFPB), Av. Tranquilino Coelho Lemos, 671, Dinamérica, CEP: 58432-300, Campina Grande – PB, Brazil 2 Federal University of Campina Grande (UFCG), Av. Aprígio Veloso, 882, Universitário, CEP: 58429-140, Brazil. ABSTRACT Shape memory alloys are functional materials that can recover plastic strains between 2 and 6%. This property can be used to produce actuators for many areas as medicine, robotic, aeronautic and others. Recently, it has been observed the particular interest for shape memory alloys welding, especially to obtain Ni-Ti similar and dissimilar joints and fabricate simple or complex structures. In this sense, this work present an experimental study of tungsten inert gas pulsed welding applied to Ni-Ti shape memory alloy wires with 0.9 mm in diameter, previously heat treated at 450 °C for 20 minutes and air cooled. For that, it was carried out tensile tests at isothermal temperatures from 40 °C to 90 °C (steps of 10 °C) for welded and unwelded wires. The transformation temperatures obtained from differential scanning calorimetry were compared to verify the effect of welding process. It was also performed a stabilization process by mechanical cycling in some welded and unwelded Ni-Ti wires. The results showed a low strength and strain capacity of the weld joint at higher temperatures. Although, at lowest temperature, close to 40 °C, it was observed higher values of maximum stress and strain for welded Ni-Ti wires. INTRODUCTION Shape memory alloys are special metallic materials that have an interesting property of recover plastic strains between 2 and 6 % introduced at a low temperature and followed by heating above a critical temperature. This phenomenon, called shape memory effect, is closely associated with a phase transformation of the martensitic type, crystallographically reversible [1]. On the other hand, welding process can often become a good way to obtain shape memory alloy actuators and structures that probably should be harder to manufacture by conventional methods. The welding processes of shape memory alloys may facilitate the development of more complex smart actuators. The method that has been more effective and consequently has been most studied, is the laser welding process due to its high accuracy and small size of the heat affected zone (HAZ) [2-4]. However, there are few studies [5,6] concerning the Ni-Ti shape memory alloy welding using the tungsten inert gas process, which is simpler and cheaper compared with laser welding. Thus, this work aims to present some new results for welding of Ni-Ti shape memory alloy wires in the martensitic state. For this, it were employed Ni-Ti wires with 0.9 mm in diameter, supplied in a cold worked state. The Ni-Ti wires were welded by tungsten inert gas pulses and analyzed the welding influence on its
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