Enhancement of mechanical and microstructural properties of friction stir welded AA2024 by eccentric square tool pin

Abstract

Grain size significantly affects joint strength and mechanical properties after friction stir welding (FSW). The tool pin shape/feature is an important factor influencing weld quality and microstructural features. The current work investigates the FSW of AA2024 with eccentric (distance between the axis of tool pin and shoulder) square tool pin and its influence on mechanical and microstructural characteristics. A square tool pin with three different eccentricities (0.0, 0.1, and 0.2 mm) is considered with a tilt angle of zero, one and two degrees. A defective weld is observed for the zero eccentricity, while the defect is reduced for the eccentricity of 0.2 mm with the reduction in grain sizes. Ultimate tensile strength enhanced to 407 MPa, contributing to a weld efficiency of 87 % obtained with a 0.2 mm pin eccentricity and two-degree tool tilt angle. Also, the average grain size and hardness at the stirred zone are 3.4 μm and 164 HV, respectively. Tool eccentricity produced 50, 14.7, and 11.2 % increase in tensile strength for 0o, 1o, and 2o tool tilt angles respectively, while increasing eccentricity from 0.0 to 0.2 mm. It is also found that eccentricity results in a higher dynamic area of 8 and 16 % for 0.1 and 0.2 mm, leading to more plastic deformation and better material mixing, resulting in enhanced mechanical and microstructural features. Also, higher rotational paths due to eccentricity cause interacting material to deform more with enhanced plasticization. Average grain size reduces by 25 %, 18.5 %, and 46 % when tool pin eccentricity is increased from 0 to 0.2 mm for a tilt angle of zero, one and two degrees, respectively. thermomechanical and mechanistic models are developed to explain the grain refinement and defect reduction mechanism due to eccentricity.