Fatigue crack propagation characteristics of aluminum/steel laser-metal inert gas fusion-brazed joints in different micro-zones

Research on fatigue damage of aluminum/steel dissimilar joints is critical to ensure the reliability of multimaterial lightweight structures. In this study, the fatigue crack propagation (FCP) characteristics of the interface, weld, and heat-affected zone (HAZ) of aluminum/steel laser-metal inert gas (MIG) fusion-brazed joints were investigated. Additionally, the influencing role of the microstructure of different micro-zones on the FCP behaviors was discussed. Single-edge notched tensile (SENT) specimens of the interface region exhibited a higher crack growth rate and crack threshold than SENT-weld and SENT- HAZ specimens due to the highly hard and brittle nature of the intermetallic compounds (IMC) at the aluminum/steel interface. Moreover, the fatigue fracture surface of SENT-interface specimens showed a brittle fracture pattern without fatigue striation characteristics, which was different from aluminum/steel solid-state and traditional aluminum alloy welded joints. Electron backscatter diffraction (EBSD) analysis results revealed that the synergistic effects of grain size, subgrain boundaries, and micro-performance all contributed to the crack deflection of SENT-weld at a large angle to the HAZ, thereby resulting in more optimized crack resistance in the weld zone.