A comparison between FSW and TIG welding techniques: modifications of microstructure and pitting corrosion resistance in AA 2024-T3 butt joints

Abstract

An experimental investigation has been carried out, in present paper, on microstructure and corrosion resistance of weld butt joints of AA 2024-T3. Two different welding processes have been considered: a conventional tungsten inert gas (TIG) process and an innovative solid state welding process known as friction stir welding (FSW). Micro-hardness measurements allow pointing out a general decay of mechanical properties of TIG joints, mainly due to high temperatures experienced by material. In FSW joint, instead, lower temperatures involved in process and severe plastic deformations induced by tool motion allow rising of a complex situation: by a general point of view a slight decay of mechanical properties is recorded in nugget zone, flow arm and thermo-mechanically altered zone (TMAZ), while in heat-affected zone (HAZ), due to starting heat treatment of alloy under investigation, a light improvement of such properties is appreciated. In flow arm and in nugget zone, however, a light recovery of hardness, w.r.t. TMAZ zone, is recorded, due to the re-crystallisation of a very fine grain structure. Polarisation curve tests and electrochemical impedance spectroscopy, performed in this paper, allow assessing a generalised nobler behaviour of weld bead with respect to parent alloy. In FSW joint, however, the differences between the three examined zone are not so evident as in TIG joint; what is more, inside FSW weld bead, retreating zone shows a behaviour nobler than advancing one.