Investigation on the weldability of synergistically double-sided friction stir welding 2024-T3 aluminum alloy joints

This study aims to investigate the potential of employing a developed method of Double-sided Friction Stir Welding (DS-FSW) to butt-join (similar joint) 2024-T3 aluminum alloy plates with a reasonably thin thickness. The developed DS-FSW eliminates the utilization of sequential passes that the conventional DS-FSW process requires. Instead, it utilizes two identical tools and symmetrically aligns them in a manner that the pins’ tips face each other perpendicularly. Thus, both tools can perfectly work synergistically, and this approach can be referred to as Synergistic Double-Sided Friction Stir Welding (SDS-FSW). Coupled Eulerian Lagrangian (CEL) method was formulated into a Finite Element (FE) model to predict thermal distribution and material flow patterns, providing insights into good agreement with mechanical tests. The J-C plasticity model defined the material’s elastic-plastic properties during the welding stage. Microstructures of the joints were photographed using a digital Optical Microscope (OM), and mechanical properties were tested to assess the joint’s integrity through tension and microhardness testing. The rotating and welding speeds were optimized using a bottom-up approach with two factors and four levels, resulting in 600 rpm and 200 mm/min as the most ideal process parameters. OM images indicate that the greater rotating and welding speeds result in an increased heat input, which weakens material bonding. As a result, it causes a kissing bond and scribble flimsy line to an inner defect in the stir zone (SZ). Most of the tensile samples exhibit brittle characteristics with pulse-like crack patterns, and many of them fracture in the SZ, with an average Ultimate Tensile Strength (UTS) of ~ 55% of the Base Material (BM). The output of FE model shows a thermal rising discontinuity before reaching (slightly below BM’s solidus temperature). The material flow was profiled like an imperfect vortex due to the weak driving intensity generated by the tool, especially from the pin. The insufficient pin’s height dimension reduces the pitch number and the root cavity of the pin’s thread as well, which has an impact on the swept volume. It’s crucial to make significant improvements in the upcoming studies to maximize the performance of the tool design and rearrange the tools’ alignment to gain the double heat input.