Effects of Carbon Content on Intermetallic Compound Layer and Joint Strength in Friction Welding of Al Alloy to Steel

The microstructure and bond strength of the friction-welded interface of Al-Mg alloy A5052 to carbon steel S45C have been investigated, to establish their dependence on C content of steel by comparison with those observed in the joint of A5052 alloy to low C steel S10C. TEM observations revealed that an intermetallic compound layer 100–1 000 nm thick was formed at the interface, consisting of Fe2Al5 and Fe4Al13, similar to that observed at the A5052/S10C interface. The thickness of the intermetallic compound layer was increased almost in proportion to friction time in both joints, while that of the A5052/S45C joint grew at a lower rate than that of the A5052/S10C joint. In the intermetallic compound layer, granular Fe2Al5 and Fe4Al13 were distributed almost randomly, in contrast to those observed at the interface of the diffusion couple or diffusion-bonded joint, where intermetallic compounds formed as layers distributed in the order of their chemical compositions. This suggests that the formation of the intermetallic compound layer was significantly influenced by a factor other than the diffusion of Al and Fe. In the steel adjacent to the intermetallic compound layer, a very fine grain zone was observed, suggesting that the steel surface underwent heavy plastic deformation during the friction process. The thickness of the fine grain zone was also increased in proportion to friction time. It was found that the thickness of the intermetallic compound layer increased with that of the fine grain zone, obeying a relation almost independent of the C content of the steel and chemical composition of the Al alloy. These results suggest a significant contribution of mechanical intermingling of Fe with Al in the formation of the intermetallic compound layer. On tensile tests using specimens with a circumferential notch at the interface, the A5052/S45C joint was fractured at the interface region, showing higher fracture strengths than the A5052/S10C joint, probably because of the lesser thickness of the intermetallic compound layer.