Regulatory mechanism of medium characteristics on defect evolution and mechanical properties in friction stir welded CFRP joints

The engineering application of carbon fiber-reinforced polymers (CFRP) in lightweight structural systems is constrained by joining defects in friction stir welding (FSW). Current research lacks systematic comparison and mechanism analysis of different auxiliary media for CFRP FSW. This study comparatively investigates the regulation mechanism of aqueous medium and cutting fluid on defect evolution and mechanical properties of CFRP FSW joints. Results demonstrate that cutting fluid achieves a precise balance between welding heat input and cooling rate via synergistic cooling–lubrication effects, overcoming the defect sensitivity caused by the single cooling function of aqueous medium. The intrinsic regulation mechanism of auxiliary media on defect formation is revealed, and cutting fluid is verified to significantly improve joint compactness, suppress interfacial degradation, and enhance mechanical performance. These findings fill the knowledge gap in medium-assisted CFRP FSW and support high-quality joining of high-performance CFRP components.