Welding dissimilar aluminium alloys is critical for maritime, automotive and aviation industries. Friction stir welding (FSW) is a promising technique that can achieve this by operating at lower temperatures, lowering intermetallic formation, and preserving the base material properties. However, conventional FSW generally has defects that reduce the overall mechanical properties. This work aims to enhance the weld strength of AA6061-T651 and AA7075-T651 by using an in-line directed jet cooling method using a coolant-water mixture in the weld zone. The key FSW parameters like tool rotational speed (1000, 1200, 1400 rpm), welding traverse speed (60, 90, 120 mm/min) and water-coolant mixing ratio (90:10, 70:30, 50:50). The tensile strength (ASTM E8M-04), microhardness (ASTM E384), and microstructural analysis using optical microscope and scanning electron microscope were conducted on the welded sample. The results indicate that the optimum weld conditions 1400 rpm tool rotational speed, 60 mm/min traverse speed and 50:50 water-coolant mixture yielded the maximum tensile strength of 225.22 MPa, showing a 61.55% improvement over the conventional FSW. A maximum hardness of 168.03 HV with a 56.25% increase was observed for the same process parameters. In addition, the microstructure analysis revealed that the Mg2Si dissolution has significantly reduced, thereby offering grain refinement at the weld zone. These findings prove that using the directed jet cooling method effectively improves the weld strength of dissimilar aluminium alloys, making it suitable for high-performance industrial applications.
