Process of Prefabricated Vortex Flow-Based Friction Stir Welding for Low Carbon Steel

At present, the demand for hard-to-weld steels, such as high-nitrogen stainless steel, oxide-dispersion-strengthened steel, and twinning-induced plasticity steel, in the high-end equipment manufacturing industry has gradually increased. Low-cost and reliable joining is a prerequisite for meeting the diverse application requirements of these steels. Conventional fusion welding of hard-to-weld steels often produces metallurgical defects, including pores and cracks. In contrast, friction stir welding (FSW), a solid-state process performed entirely below the materials melting point, effectively avoids such defects. Moreover, its combined thermal-mechanical action promotes the formation of high-performance joints. However, the high temperatures and contact stresses associated with FSW of hard-to-weld steels can lead to tool wear and fracture. To address this limitation, this study proposes a novel prefabricated vortex flow-based FSW (PF-VFSW) process. A systematic investigation was conducted using 3-mm-thick Q195 steel to evaluate the effects of holder material, rotational speed, welding speed, and tool tilt angle on the joints macroscopic morphology, microstructure, and mechanical properties. For a WC-Co holder with 0° tilt, the optimal rotation and welding speeds were 500 r/min and 20 mm/min. However, kissing- bond defects were observed at the bottom of the joint, with oxide distribution and unbonded regions increasing with distance from the top of the joint. Using a W-Re holder with a 1° tilt and adjusted process parameters eliminated these defects, with the optimal rotation and welding speeds being 300 r/min and 20 mm/min. Severe plastic deformation in the stir zone induced dynamic recovery and both continuous and discontinuous dynamic recrystallization, reducing the recrystallization fraction compared to the base material. The proportion of low-angle grain boundaries increased markedly, accompanied by pronounced grain refinement. The minimum average grain size in the stir zone was 3.8 μm, representing an 80.51% reduction relative to the base material. The microhardnesses of joints produced with WC-Co and W-Re holders increased by 6.44% and 18.90%, respectively, compared to the base material. Tensile strength improved by 1.74% to 317.26 MPa and 5.91% to 330.25 MPa, respectively, achieving a joint efficiency of 100% in terms of tensile strength relative to the base material. These results demonstrate that PF-VFSW is an effective, low-cost method for producing high-quality joints in Q195 low-carbon steel.