Comprehensive understanding of the mechanical properties and microstructure evolution of aluminum alloy/steel laminates during friction stir-assisted incremental forming with synchronous bonding process

Aluminum alloy/steel laminated parts can be fabricated by separated sheets based on a friction stir-assisted incremental sheet forming with synchronous bonding (FS-ISF&SB) method. The current work aims to realize comprehensive understanding of microstructure evolution during FS-ISF&SB and its effect on mechanical properties of the formed parts. Three typical sets of experiments with different combinations of process parameters are carried out, and three different regions are classified by grain feature, defined as deformed region, loading region and undeformed region. Recrystallization and recovery behaviors occur at all of the three regions on both sheets, which generate softening effect. However, the effect of work hardening keeps a dominant role on mechanical property compared with softening effect on steel, while the influence level of these effects on aluminum alloy is opposite during FS-ISF&SB. Besides, stratified structure and gradient distribution of grain size from the outer surfaces to the interface are observed. The microstructure evolutions greatly accelerate the atomic diffusion and subsequently promote the metallurgical bonding of the laminates. It is also found that Fe–Al intermetallic compounds (IMCs) consisting of FeAl3 and Fe2Al5 are generated on the dissimilar interface of the fabricated parts. The thickest IMCs are usually found at the loading regions. Moreover, the maximum nominal bonding stress obtained by peeling test of the laminated parts can reach 100.0 MPa with the moderate thickness of IMCs as of 13.1 μm. •Investigated effect of microstructure evolution on mechanical performance during FS-ISF&SB.•Explored stratified structure and grain gradient distribution through thickness direction.•Revealed recovery & recrystallization behaviors in Al alloy and steel across thickness.•Discussed competing behavior of work hardening and softening effect during FS-ISF&SB.•Revealed atom diffusion bonding mechanism accelerated by recovery and recrystallization.