Interfacial layer in friction-bonded joint of low carbon steel to Al-Mg alloy (AA5083) and its influence on bond strength

The metallographic factor controlling the strength of friction-bonded interface of low carbon steel (approximately 0.10mass%C) to aluminum-magnesium (Al-Mg) alloy (equivalent to AA5083) has been investigated by TEM observations. The bond strength, estimated from the tensile strength of a specimen with a circumferential notch at the interface, rose rapidly with an increase in friction time, and then reduced. A maximum strength of 306 MPa was obtained at a friction time of 2 s (rotation speed = 20 s(-1), friction pressure = 40 MPa, and forge pressure = 230 MPa). At a friction time of 1 s, an IMC layer about 100 nm wide that consisted of (Fe,Mn)Al-6 and Mg2Si was formed at the interface, and an Al-oxide layer of a width less than 10 nm was observed between this IMC layer and low carbon steel substrate. In a joint showing the highest bond strength (friction time = 2 s), no Al-oxide layer could be detected between the low carbon steel substrate and IMC layer which consisted of (Fe,Mn)Al-6, Fe4Al13, Fe2Al5, and Mg2Si. The width of the interfacial layer was increased to about 300 nm. At a friction time of 4 s, a layer of MgAl2O4 was observed in addition to intermetallic compounds of (Fe,Mn)Al-6, Fe4Al13, Fe2Al5 and Mg2Si. The width of this layer was about 700 nm. Thus the phases formed in the interfacial layer as well as its width were altered depending on the friction time. The change in the bond strength with friction time was discussed in view of these differences in the interfacial microstructure.