Enhanced mechanical performance of AA7075-boron carbide/blast furnace slag hybrid composite through friction stir welding with cryogenically treated tool: A microstructural and mechanical characterization

AluminIum 7075 hybrid metal matrix composites are widely utilized in aerospace, automotive, defense and marine applications due to their superior strength, stiffness, hardness and wear resistance properties. Friction stir welding (FSW) is a promising technique for joining such composites, but its industrial application is limited by the challenges such as excessive tool wear, reduced weld strength and weld defects caused by too degradation. This study focuses on development of an H13 tool with enhanced wear resistance for the FSW of AA7075 hybrid metal matrix composite (MMC) reinforced with 5%, 10%, 15% of boron carbide (B₄C) particles and 8% blast furnace slag (BFS) by weight. A cryogenically treated H13 tool was proposed to improve tool durability and minimize the wear. The treated tool shows a remarkable 336% improvement in wear resistance under in situ welding compared to untreated tool. Micro-mechanical characterization of the welded joints was performed, revealing improved weld properties, particularly in plates with higher reinforcement percentages. Mechanical testing revealed that the ultimate tensile strength increased with reinforcement content with a maximum of 266.37 ± 3.17 MPa (50.14% higher than the unreinforced sample) for the sample containing 15 wt% of B₄C + 8 wt% of BFS, while microhardness improved by 12% achieving a peak value of 164.42 Hv. These findings demonstrate that the use of cryogenically treated H13 tools enables the FSW of highly reinforced AA7075 MMCs, producing defect-free joints with superior strength and hardness, thereby extending the potential of FSW in advanced structural applications.