Effect of Intermetallic Compound Layer on Bond Strength of Friction-welded Interface of Commercially Pure Aluminum to Mild Steel

The microstructure of friction-welded interfaces of commercially pure aluminum A1070 to mild steel S10C has been investigated mainly by TEM observations to reveal metallographic factors controlling the bond strength. The bond strength was estimated from the tensile strength of a specimen with a circumferential notch at the interface. A maximum strength of 124 MPa was obtained at a friction time of 0.5 s (rotation speed = 20 s-1, friction pressure = 20 MPa, and forge pressure = 100 MPa). At this friction time, no IMC(Intermetallic compound) layer could detected at the interface, and fracture occurred in the aluminum base metal. With an increase in friction time, the bond strength was decreased, and brittle fracture at the interface came to occupy the almost whole area of the fractured surface at friction times more than 2 s. At a friction time of 2 s, an interfacial layer about 30-100 nm wide that consisted of Fe2Al5 was formed at the interface. TEM observations of the cross-sectional microstructure of fractured surfaces revealed that the brittle fracture occurred in the IMC layer. The bond strength of these joints decreased with an increase in the width of the IMC layer. The width of the IMC layer was increased almost linearly with an increase in friction time. The kinetics of the growth of the IMC layer suggests that it is controlled by mechanical mixing of Al and Fe at the interface as well as their diffusion.