Formation of metallurgical bonding interface in aluminum-steel bimetal parts by thixotropic-core compound forging

Compound forging is a promising method for fabricating aluminum-steel bimetal parts. However, metallurgical bonding interfaces, which are important for the mechanical performance, are usually difficult to be obtained due to Al₂O₃ oxide film. In this work, a thixotropic-core compound forging technology was proposed for fabricating aluminum-steel bimetal parts with a reliable metallurgical bonding interface. 7075 aluminum rods and 304L stainless steel shells were used as billets. Before compound forging, the 7075-T6 alloy and 304L steel were heated to the desired temperatures respectively. The influences of the compression displacement and the initial temperature of steel shell on the formation of metallurgical bonding interface were studied. The results showed that a metallurgical bonding interface was formed in the bimetal part. The interface thickness increased gradually as the initial temperature of steel shell increased. However, if the initial temperature of steel shell was higher than 900 °C, an overgrown interface was formed, resulting in substantially deterioration of the shear strength. An increase in the compression displacement enabled tearing of the oxide film. However, when the compression deformation was excessively large, microcracks could be induced near the interface. When the initial temperature of steel shell and the compression displacement were 900 °C and 20 mm, respectively, the optimal metallurgical bonding interface with a thickness of 3.8 μm and shear strength of 108.2 MPa was obtained.