Friction stir welding studies on mild steel

The objective of this work was to demonstrate the feasibility of friction stir welding (FSW) for joining of mild steel. Defect-free welds were produced on 0.25-in. plates (6.3 mm) of hot-rolled AISI 1018 mild steel at travel speeds ranging from 1 to 4 in./min (0.42 to 1.68 mm/s) using molybdenum-based and tungsten-based alloy tools. Results for welds made at 1 in./min are reported here. Axial tool loads measured during FSW were approximately 4200 Ibf (18.7 kN), while measured torques were in the range of 500 in.-lb (55 N○m). Peak surface temperatures close to 1000°C (1832°F) were measured on the workpiece and tool during welding using thermocouples and an infrared camera system. Comparisons before and after welding combining both metallographic and metrology techniques suggest changes in tool dimensions stem from both rubbing wear and deformation of the tool. The greatest changes in tool dimensions occurred during the initial plunging stage. Microstructures of the welds were examined using optical and scanning electron microscopy. The weld region displayed several microstructurally distinct regions. Extrapolation of measured temperatures and microstructural evidence suggest peak temperatures of the stir zone exceeded 1100°C (2012°F) and likely surpassed 1200°C (2192°F). Transverse tensile properties of the welds were evaluated at room temperature. Welded samples failed in regions corresponding to the base metal and demonstrated yield and ultimate tensile strengths comparable to those of the base metal. Welded samples passed bend tests to ∼15% strain in the outer fiber. The results have demonstrated the feasibility of FSW of steel with acceptable tensile properties. Based on these results, FSW of transformation-hardenable steels, HSLA steels, and stainless steels appears feasible.