Avaliação morfológica, microestrutural e eletroquímica da liga Ti-6Al-4V soldada por fricção linear

Abstract

MORPHOLOGICAL, MICROSTRUCTURAL, AND ELECTROCHEMICAL EVALUATION OF THE Ti-6Al-4V ALLOY WELDED BY FRICTION STIR WELDING - Reprocessing is an important step in the nuclear fuel cycle and basically consists of dissolving the fuel (UO2) in a boiling medium (6.0 or 11.5 mol L‒1 HNO3). In order to do so, the choice of appropriate materials for the construction of the reprocessing infrastructure is required. From the economic point of view, stainless steel would be the best option; however, it does not exhibit good corrosion resistance against the oxidant medium mentioned above. Therefore, other materials must be employed to mitigate corrosion problems. In this sense, valve metals and their alloys (Ti, V, Nb, Zr, etc.) are a good alternative due to the spontaneous formation of a stable oxide film on the metal surface. Thus, one way to fulfill these requirements might be the union of those two materials through welding. In the present work, a plate of a Ti alloy (Ti-6Al-4V) was welded to a 304 L stainless steel plate using the friction stir welding process. So, the aim of this work was to particularly study the welded region of the Ti alloy, which will be held in contact with the oxidizing solution of HNO3. Thus, morphological (metallography, scanning electron microscopy coupled to elemental analysis), microstructural (X ray diffraction - XRD), mechanical (Vickers microhardness), electrochemical (open circuit potential, electrochemical impedance spectroscopy - EIS - linear and cyclic voltage scans), and mass loss assays (immersion in boiling HNO3 11.5 mol L–1) were investigated along the weld. The results indicated a microstructural (increase of the β phase confirmed by XRD) and morphological (modification of size and shape of grains detected by metallography) change in the mixing zone (MZ) of the welded joint. Vickers microhardness tests showed an increase in hardness of the welded region due to grain refinement and plastic deformation caused by the welding process. Linear and cyclic scanning electrochemical assays (6.0 and 11.5 mol L–1 HNO3) indicated a higher corrosion susceptibility of MZ, which was confirmed by: i) a narrow potential window of the passivation region and ii) high anodic values of the reconstruction charge, caused by contamination of the Ti alloy by Fe and Cr from the stainless steel. EIS tests also confirmed that MZ is more susceptible to corrosion attack since this region showed lower values of charge transfer resistance than the base metal. The appearance of an inductive looping in the complex plane can be associated to the formation and dissolution of the passive film of Ti oxide, as well as adsorption of species from the electrolyte. Finally, mass loss assays also confirmed the higher corrosion susceptibility of the MZ region, mainly due to Fe and Cr contamination.