Friction stir lap welding of 5456 aluminum alloy with different sheet thickness: process optimization and microstructure evolution

Abstract

The influences of friction stir welding process parameters on microstructure evolution and mechanical properties of lap-welded 5456 aluminum alloy plates with different thickness and temper conditions were investigated. The upper plate was 5 mm thick, cold rolled aluminum alloy 5456-T321 and the lower plate was 2.5-mm annealed sheet (5456-O). Four different pin geometry types (conical thread pin, cylindrical–conical thread pin, stepped conical thread pin, and flared-triflute pin tool) and two rotational speeds (600 and 800 rpm) were used to produce the joints. Microstructures and microhardness values in the weld nugget (WN), thermomechanically affected zone (TMAZ), and the heat-affected zone (HAZ) were examined and correlated with selected processing conditions. Specifically, the influence of tool geometry on the flow of the plasticized material in the nugget zone, extent of hooking defect, and mechanical properties (microhardness) of the FSW joints were documented and quantified. It was found that weld joints made by using the stepped conical thread pin tool produced a homogeneous microstructure with finer grain size (5.4 μm) and higher microhardness levels than the other tools. The optimum processing conditions resulting in sound and defect-free joints with highest mechanical properties were obtained with the stepped conical thread pin and 600-rpm rotational speed. The evolution of the microhardness in each region is characterized and related to processing conditions.