Abstract
The objective of the present work was to evaluate welding of thin sheets (thickness < 1 mm) of similar and dissimilar NiTi alloys with AISI 304 stainless steel using the Gas Tungsten Arc Welding (GTAW) process and to study the mechanical and metallurgical properties of the joints with and without post-welding heat treatment (PWHT). The GTAW process was chosen because it is more economical than the usual welding processes for NiTi alloys, such as the Laser Beam Welding (LBW). The welded joints were characterized by the techniques of SEM, OM, electrical resistance in temperature (ERT), tensile test and Vickers microhardness. It was observed that the dissimilar joints presented a brittle behavior due to the formation of brittle elements along the weld metal by the excessive increase of the hardness in this region with peaks of hardness higher than 900HV. Similar joints presented superior mechanical behavior, with extensive plastic deformation before rupture and fracture surface with ductile appearance. The PWHT in the similar joints promoted a decrease in the rigidity of the material by minimizing the thermal stresses from the welding process, the PWHT did not influence the dissimilar joints mechanical behavior.
Highlights
Shape Memory Alloys (SMA) are metal alloys for which reversible phase transformations induced by appropriate temperature and/or stress changes are observed
The electrical resistance in temperature (ERT) curves especially that of the base metal (NiTi sheet without weld), are typical of the two-stage martensitic transformation of NiTi SMA, austenite, cubic structure, to the rhombohedral R phase with great distortion at α angle, and to the monoclinic martensite, forming a peak in the cooling phase that begins at Rs temperature . 2,24 The S11 joint was submitted to a welding thermal cycle and this acted as a post-welding heat treatment (PWHT)
The S12 joint was submitted to a welding thermal cycle in addition it was further heat treated by the real PWHT
Summary
Shape Memory Alloys (SMA) are metal alloys for which reversible phase transformations induced by appropriate temperature and/or stress changes are observed. A phase transformation of an SMA is unique, accompanied by high data recovery capacity and stress as a function of temperature changes. There is still precipitation of deleterious phases in the heat affected zone (HAZ) and the weld metal (WM), resulting in severe reduction of mechanical resistance 4. These phenomena are responsible for limiting the applicability of the NiTi SMA in multiple areas of interest
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