Enhanced inertia friction welding of aluminum alloy and high-strength steel using CrCoNi interlayer: Microstructural and mechanical characterization

The significant disparities in physical and chemical properties between aluminum alloy and high-strength steel pose substantial challenges for conventional friction joining techniques. To address this issue, this study proposes a novel approach utilizing inertial friction welding with an interlayer to join these dissimilar materials. A CrCoNi medium entropy alloy sheet was selected as the interlayer due to its intermediate melting point, thermal conductivity, strength, and surface hardness between 6061-T6 aluminum alloy and 42CrMo steel, as well as its high element mixing entropy. These properties were deemed crucial for balancing interface heat generation and regulation the formation of intermetallic compounds. The experimental procedure involved embedding the CrCoNi sheet into the end face of the 6061-T6 aluminum alloy, followed by the application of IFW to join the aluminum alloy with 42CrMo high-strength steel. This investigation focuses on examining the effects of three distinct friction speeds (3800, 4000, and 4200 rpm) on the microstructural characteristics and mechanical properties of the regulating joints with the CrCoNi interlayer. Results demonstrate that the CrCoNi enhances the temperature at the steel-side interface through friction with 42CrMo steel and 6061-T6 aluminum, combined with adjustments in the friction sequence and duration, promoting plastic deformation. The axial transfer of heat creates a temperature gradient at the joint, enabling low-temperature welding on the aluminum side and forming a mechanical interlocking structure at the interface. The diffusion of Cr, Co, and Ni elements regulates the type and thickness of interfacial intermetallic compounds, ultimately enhancing the joint's strength. The thickness of the intermetallic compounds AlNi3, FeAl3, AlCo, and Fe2Al5 formed at the interface is less than 2 µm. A phase transformation occurred at the 42CrMo high-strength steel interface, leading to the formation of numerous needle-like martensites, which increased the Vickers hardness in the welding seam to 763.9 HV. The joint's tensile strength initially increased and then decreased with increasing friction speed, reaching a maximum of 168.7 MPa at 4000 rpm, which is more than 60% of the aluminum alloy base material's tensile strength.