Improved interface bonding in thixotropic-plastic compound forming of aluminum alloy and steel by Ni interlayer addition

In the thixotropic-plastic compound forming (TPCF¹) of aluminum alloy and steel, a tough and strong interface is helpful to not only the decrement in the risk of shrinkage crack, but also the metallurgical bonding. In this work, a Ni interlayer was introduced in the TPCF of 7075 Al alloy and 304 L steel. The effects of the Ni interlayer thickness, and a bonding-strengthening diffusion heat treatment before forming on the interface microstructure at two typical strained areas were studied. Moreover, the microstructure and shear strength of the interface after the heat treatment followed by TPCF were further studied, and the relationship between the microstructures and mechanical properties were discussed. Results showed that a Ni interlayer with a thickness of ~ 10.5 μm is helpful to improve the bonding interface by the formation of Ni-Al intermetallic compounds (IMCs²). Most of them are tougher than brittle Fe-Al IMCs obtained as the Ni layer was not introduced, and thus playing key roles in the improvement in interface bonding by releasing the stress concentration near interface. A bonding-strengthening diffusion heat treatment under 750 °C/ 2 h improved the formation of Ni-Al IMCs and contributed to decrease the growth rate of IMCs layer in subsequent heat treatment. The optimal interface shear strength was tested to be 42.4 MPa (a ~ 60% increment) as a ~ 10.5 μm Ni interlayer followed by the bonding-strengthening diffusion treatment was introduced. The optimized heat treatment regime after forming was 460 °C for 4 h.