Microstructure and Mechanical Properties of TIG-Assisted Friction Stir Welded Joints of 3 mm-Thick Q235B Steel
摘要
Friction stir welding (FSW) presents a promising solid-state alternative to fusion welding for steels, offering potential advantages in mechanical properties and defect reduction. However, its application to high melting point materials like Q235 steel faces challenges such as high process forces, tool wear, and insufficient heat input leading to defects. This study investigates the implementation of a TIG welding torch for auxiliary preheating to enhance the FSW process of 3 mm-thick Q235B steel. The effects of transverse speed (100 and 150 mm·min−1) and TIG preheating current (0-30 A) on microstructure, tensile properties, microhardness, and impact toughness were systematically evaluated. Results indicated that TIG preheating significantly improved material flowability, significantly reducing internal voids and lack-of-fusion defects observed in conventional FSW joints at higher speeds when the preheating current reached 30 A. All joints exhibited tensile strengths matching the base metal (~464 MPa), with fractures predominantly occurring in the base metal. Moreover, the application of preheating current also significantly improved the plasticity of the joint, and the elongation increased by approximately 5%. Notably, the impact toughness along the transverse direction of the WNZ for most joints (23-30 J at − 20 °C) surpassed the base metal requirement (15 J). The optimal combination of mechanical properties was achieved at a transverse speed of 100 mm·min−1 with a 30 A preheating current. This study demonstrates that TIG-assisted FSW is a cost-effective strategy for producing high-integrity, defect-free Q235 steel joints with superior plasticity and enhanced impact toughness, while simultaneously addressing productivity constraints through enabling higher traverse speeds.