Wetting Behavior And Interfacial Structure Of Molten Sn On Al-steel Multilayered Composite Plates

The metallic multilayered composite plates?MMCP?usually containing two or more different metallic layers,are fabricated by the accumulative roll bonding method or the solid-liquid reactive bonding method,which can obtain hybrid-performance,such as high strength,corrosion resistance,electric conduction,magnetic conductive and etc.,and has a wide application prospect in industry.For the multilayered plate was fabricated from different metals,however,the huge differences in the physical and chemical properties in the layer of MMCP lead to the poor weldability by using the traditional fusion welding method,as well as due to the precipitation of brittle phases at interface.Soldering or brazing,as one of the candidate joining methods for MMCP,can process at a relatively low temperature,may avoid the brittleness in the joints.However,as a prerequisite,the wettability of MMCP by the molten filler metal should satisfy?_f<90^o.In this study,the sessile drop method was used to investigate the wetting behavior of Sn on Al,mild steel and Al-steel MCP,and the effect of coupling of heterogeneous surfaces with interfacial reaction was discussed as well.It is expected that this work not only provide a primary guidance for soldering of A l-steel MCPs but could enrich the reactive wetting theory in heterogeneous surface systems.Firstly,we investigated the wetting behavior?typical reactive and dissolutive wetting systems?of pure Sn on Q235 steel and pure aluminum at temperatures between623K and 723K under high vacuum,the interfacial microstructures were also analyzed in detail.The final contact angle of pure Sn on steel is approximately 39°and less affected by experimental temperature.IMCs?Fe Sn,Fe Sn ₂?were formed at the interface which can disrupt the oxide film outside the surface of steel.Moreover,the precursor film was emerged in the vicinity of triple line due to reaction between Sn and Fe.Linear and non-linear spreading stages were appeared during the spreading process and th e activation energies for the two spreading stages are 26.1 k J/mol and 24.6 k J/mol,respectively,which are corresponding to the summation of melting heat of Fe and Gibbs free energy for the formation of Fe Sn₂;Sn-Al,is a typical dissolutive wetting system.Dissolution of Al into liquid drop at the interface has significant effect to remove the oxide film covered on substrate and the characteristic of Sn on Al surface presents linear spreading?R_d=kt?.Secondly,the wetting behavior of Sn on Al-steel multilayer plate was studied to shed new light on the effect of surface heterogeneity on final wettability.The results showed that the morphology of molten metal in equilibrium condition is in semi-ellipsoid shape,and the final contact angle in front view and side view is not equal.The final contact angle in 623K,673K and 723K is 59°,35°and 32°,respectively.The wetting behavior in side view,however,is blocked by non-wettable surface.Surface tension was changed due to the dissolution of Al into molten metal,and thus Fe₂Al₅ was formed in the case of solute Al react with Fe.The final contact angle cannot satisfy Cassie equation due to the surface tension was changed by the dissolution of Al.In the end,we calculated the surface tension at different temperatures,and obtained a modified equation by coupling the changed surface tension with Naidich's model which is perfectly suitable for the experimental results.Finally,the fluid dynamics software of Fluent was used to modelling the wetting behavior of liquid drop on heterogeneous surfaces,and the calculated results have good agreement with experimental results.