Analysis of tool traverse speed in Friction Stir Welding of AZ31 Magnesium alloy thin plates

Abstract

The automotive industry's great interest in lightweight materials, such as magnesium alloy AZ31, reflects the growing importance of reducing cars' weight. One of the challenges of using such alloys is that their welding using conventional fusion methods does not result in good mechanical properties. The low corrosion resistance typical of these alloys makes it necessary to search for new welding methods, such as solid-state processes as Friction Stir Welding (FSW). However, the literature about AZ31 magnesium alloys welded by FSW revealed the absence of a consensus on the best process parameters for thin plates' welding as plates used in automotive seats. Therefore, this work aims to analyze the best combinations of parameters to weld a 2 mm thick AZ31 magnesium plate by FSW to obtain welds without defects and with good mechanical performance. To meet this objective, AZ31 plates were welded in two configurations (butt - BW and overlap – OW), varying only the tool traverse speed, and their mechanical properties were analyzed by tensile, shear, three-point bending tests, and microhardness, and the microstructure was observed by optical microscopy. About the surface finish, the lowest speeds resulted in flashes formation due to high-temperature development. For BWs, the UTS average increased when the speed increased until 2 m/min, becoming constant for speeds upper to 2m/min. For OWs, shear stress values initially increased due to grain refining, however decreasing with high speeds due to the formation of the large voids. The hardness profiles were not affected a lot by speed, since its increase resulted in grain refinement.