Avaliação da união de chapas finais de ligas com memória de forma NiTi pelos processos de soldagem GTAW, PAW E LBW.

Abstract

This work aims to study NiTi shape memory alloy (SMA) welded joints of thin sheets, in the austenitic and martensitic states, by the GTAW, PAW and LBW welding processes using the factorial design technique. It was verified the influence of the process parameters and the influence of the post-weld heat treatment on the mechanical strength of the welded joints by tensile tests, at room temperature, on the phase transformation temperatures via ERT, on the Vickers microhardness and also in microstructure. In order to carry out welding by GTAW and PAW processes, a multiprocessing welding source of IMC-SOLDAGEM, model Digiplus A7, was used in the direct current or pulsed mode, in the micro GTAW or micro PAW mode. The welds by the LBW process used an IPG fiber laser, model YLR-2000. The results indicate that after welding of NiTi austenitic SMA the phase transformation temperaturesincrease, but the welding by itself can not “unblock” the phase transformations, requiring a post- welding heat-treatment (PWHT) for this purpose. On other hand, for the NiTi martensitic SMA, the welding acted as a localized heat treatment, being able to “unblock” the phase transformations and the PWHT promoted slight increases in the phase transformation temperatures. The joints welded by the GTAW process havehigher hardness than those of the PAW and LBW processes and the microstructure was sensitive to current pulsation and PWHT. The joints welded by the GTAW process were also able to withstand high deformations, greater than 60% of the deformation supported by the base metal (BM) in the austenitic state and greater than 90% in the martensitic state. These results was higher than those obtained by PAW and LBW processes, which supported about 60% of the maximum deformation by the BM in the austenitic state and 40% in the martensitic state. The overall appearance of the fracture surface was ductile, regardless of the welding process used.