Influence of FSW process parameters on thermal cycles and mechanical properties of welded joints of AA7075 T651

Resumo

Aluminum alloys are widely used in the transportation sector due to their favorable characteristics and mechanical properties, which meet the growing demand for high-performance vehicles with enhanced autonomy. The 7XXX series is particularly notable among these alloys, especially in the aeronautical industry. The AA7075-T651 alloy was chosen for this study because of its high mechanical strength, low density, and superior corrosion resistance in comparison to other aluminum alloys typically recommended for aeronautical and aerospace applications. However, its use is constrained when employing conventional welding techniques, mainly due to weldability challenges. To broaden the applicability of this alloy to other structural components, the development of solid-state joining methods is essential. The low weldability of these alloys under fusion-based techniques is linked to the rapid vaporization of zinc and magnesium, key alloying elements, that results in pore formation in the weld metal, thereby compromising joint integrity. Friction Stir Welding (FSW), a solid-state joining process, offers a promising alternative, effectively addressing these limitations. Since the mechanical properties of welded joints in heat-treatable aluminum alloys are significantly influenced by thermal exposure during welding, it is crucial to assess the temperature distribution throughout the FSW process and establish correlations with the thermal cycles experienced by the joints. In this study, AA7075-T651 alloy plates were welded under various process parameters: welding speeds of 117 mm/min and 47 mm/min, along with tool rotation speeds of 1585 rpm and 470 rpm, using a threaded cylindrical tool made of H13 steel. Uniaxial tensile testing, macro- and microstructural characterization, Vickers hardness measurements, and thermal cycle monitoring using K-type thermocouples were performed. The highest peak temperatures, nearing 375 °C, were recorded under elevated welding and tool rotation speeds (117 mm/min and 1585 rpm, respectively). In contrast, joints produced at lower welding (47 mm/min) and tool rotation speeds (470 rpm) displayed lower peak temperatures (~324 °C), reduced cooling rates (~3 °C/s), and enhanced mechanical performance, with ultimate tensile strength (UTS) reaching 353 MPa. Conversely, joints manufactured under higher energy input conditions exhibited brittle behavior and significantly lower UTS values (~176 MPa), attributed to excessive heat input and the accelerated thermal cycles involved.