Interfacial microstructure and property of 6061 aluminium alloy/stainless steel hybrid inertia friction welded joint with different steel surface roughness

Considering that the friction welded aluminium (Al) alloy to stainless steel (designated as steel) hybrid structures are of great potential to be served as high-performance light-weight pipelines, spindle, etc., this paper highlights the importance of steel surface roughness (Sa) by characterising its effect on the interfacial microstructure and mechanical properties of the resultant 6061 Al alloy/304 stainless steel inertia friction welded joint. The insufficient frictional heat and material flow could easily cause the appearance of un-bonded area in the central region of the interface when the steel surface roughness was higher than 0.68 mu m (subjected to grinding), whereas, in the joint produced by Al alloy and polished steel (Sa = 0.02 mu m), an amorphous Fe-Al layer was observed in the central region indicating an metallurgical reaction was achieved. In the outer region of the interface, a reaction layer was observed in all the joints consisting of steel with varying degrees of surface roughness. Further inspection of such region revealed a partially crystallised Fe2Al5 layer and an amorphous layer were formed. The energy calculation indicated that the amorphous phase might form when the composition deviated from that of Fe2Al5 owing to a negative formation enthalpy and Gibbs free energy. Upon decreasing steel surface roughness, the joint strength showed a noticeable increasing trend. The variation of tensile properties was then clarified by studying the correlation between the interfacial microstructure and fractography. Therefore the steel surface condition is never too over-emphasised when joining with other metal with lower stiffness by inertia friction welded.