Effect of the energy input on the microstructure and mechanical behavior of AA2024-T351 joint produced by friction stir welding

Butt joints of AA2024-T351, with 6 mm thickness, were friction-stir-welded. The objective was to investigate the effects of the energy input on the microstructure and mechanical properties of the joints. The welded joints were tested employing a Mo–Va tool, with shoulder and pin diameter of 15 and 6 mm, respectively. The parameters were selected according to visual inspection of the welds, bending test, and microstructural analysis. Constant values of rotational speed and axial force were employed: 1000 rpm and 13 kN, respectively. The welding speed varied from 3 to 6 mm/s. Temperature measurements were performed along the welding process, analysis by optical microscope, microhardness, and bending tests. The process parameters selected resulted in butt joints with good characteristics, no microvoids, and full penetration. The base metal presented mainly S-type compounds, while in the HAZ thickening and loss of coherence of this precipitates are found. A more refined microstructure was found in the stir zone, based on the CDRX mechanism, with evidences of GDRX in the SZ/TMAZ (AS). As far away from the weld the indentation line is, lower is the hardness of the stir zone. The joints presented a similar increase in the mechanical resistance, with loss in ductility.