Avaliação das Propriedades Termomecânicas de Fios de Liga com Memória de Forma NiTi Soldados por Pulsos de Micro TIG

Abstract

Este é um artigo publicado em acesso aberto (Open Access) sob a licença Creative Commons Attribution Non-Commercial, que permite uso, distribuição e reprodução em qualquer meio, sem restrições desde que sem fins comerciais e que o trabalho original seja corretamente citado. Soldagem & Inspeção. 2015;20(4) Evaluation of the Thermomechanical Properties of NiTi Shape Memory Alloy Wires Welded by MICRO TIG PulsesAbstract: Micro welding can be an efficient way for joining shape memory alloys (SMA) due to high accuracy and location of heat, reducing the size of the molten zone and heat affected zone (HAZ) promoting improvements in mechanical properties in these types of joints. This study aimed to determinate variations in thermomechanical properties of welded joints of NiTi SMA wires. For this, NiTi wires (ASTM F2063) with 0.9 mm diameter were divided into two groups: (a) as received wires (NiTiA) and (b) heat treated wires at 400 °C for 20 min (NiTi400). These wires were welded by TIG process, using controlled pulses (spots). The thermomechanical characterization of micro welded wires was performed using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), uniaxial tensile tests, optical microscopy (OM), scanning electron microscopy (SEM), and Vickers microhardness. The results show the efficiency of TIG welding NiTi wires of both groups, even with the reduction of the phase transformation temperatures of the joint with respect to the original base metal. The wires with welded joints showed showed maximum superelastic strain levels of about 8% with martensite induction stresses between 400 and 500 MPa. This performance of welded joints allow the realization of superelastic cycles up to 4% deformation safely. The analysis of welded wire fracture surfaces allowed verifying ductile characteristics at the rupture process.

Mechanical behavior and fatigue life of micro welded joints obtained by TIG spots in NiTi wires

“…This variation of hardness along the welded joint is mostly related to grain growth, since the presence of finer grains in the BM and coarser grains in HAZ and WZ has been previously reported. A similar behavior was observed by other authors in TIG welding of NiTi wires [15,44] and laser welding of NiTi sheets [50]. On the other hand, Oliveira et al [21] reported no significative changes in the hardness measurements of the TIG welded joints in NiTi plates.…”

“…In addition, the transformation temperatures (M s , M f , A s e A f ) of the WZ significantly decreased compared to the BM. A similar behavior was observed by other authors for superelastic NiTi SMA [10,15,21,30]. According to Chan et al [10], such decrease may originate from the removal of cold working effect in the BM and the presence of thermally induced defects in the weld microstructure, such as residual stress and grain growth.…”

“…Micro-welding is an efficient technique for the joining of NiTi because it localizes the heat input and minimizes the thermal distortion by highly restricting the size of the welding zone (WZ) and of the heat affected zone (HAZ). This provides precision and a good hermetic sealing, required for the application of this technique in bio implants and MEMS [11,15]. Furthermore, both SME and SE are applied under cyclic mechanical loads or variable thermal requests in some applications of SMAs.…”

Mechanical Properties of NiTi Shape Memory Alloys Welded Joints: A Comparative Study between the GTAW, PAW and LBW Processes

“…2b, presented less hardness than those that were not submitted to this operation. Qualitatively, the microhardness profiles observed in the GTAW and PAW processes agree with those observed by other authors [11,19,20], as well as those profiles observed in the LBW process [10,[21][22][23][24]. Fig.…”

Avaliação do número de Nusselt local na convecção forçada assimétrica de fluidos não-Newtonianos

“…Trata-se de um processo que é mais simples e apresenta menor custo em comparação com a soldagem a laser. (Ikai et al, 1996;Amorim et al, 2015;Rodrigues, 2016;Oliveira et al, 2016b). Outra característica importante é que muitas das aplicações das LMF ocorrem sob solicitações térmicas e/ou mecânicas cíclicas ou variáveis, tornando o estudo da vida em fadiga deste tipo de material importante para garantir sua funcionalidade estrutural e sob condições de trabalho.…”