Effect of Stirring Needle Length on the Microstructures and Properties of A380/6061 Dissimilar Aluminium Alloy FSW Joints

In this study, FSW experiments were conducted on a 4 mm-thick A380 aluminium alloy plate and 6061 aluminium alloy using four different needle lengths (2.5 mm, 2.8 mm, 3.0 mm, and 3.5 mm) of H13 steel. The experiments were conducted at the same welding parameters (1000 rpm and 120 mm/min) to explore the effects of different stirring needle lengths on the microstructures and properties of the FSW joints. The experimental results show that FSW joints with varying lengths of needles have a significant effect on the microstructures and properties of welded joints, and the tensile strength increases and then decreases with increases in the needle length. A 2.8 mm needle length can achieve the maximum tensile strength of 203 MPa, about 85.3% of the base material. Too long or too short a needle leads to a decrease in joint performance. Furthermore, different needle lengths have a significant influence on the flowability of the weld core zone, and a suitable needle length will lead to better flowability in the weld core zone. With increases in the needle length, the heat production also increases, and the area of the RS-HAZ will increase with the heat production. When the joints achieve appropriate heat production, the weld core zone will experience grain refinement. At the same time, the grain will grow in the RS-HAZ; the hardness cloud diagram shows that, in the RS-HAZ, the material properties are weaker, and the tensile specimens are mainly fractured in the RS-HAZ. Finally, the tensile specimens exhibit mixed fracture.