Wetting Behaviors And Numerical Simulation Of Al Alloys On The Surface Of Metallic Substrate Under Cold Metal Transfer Condition

Dissimilar metal joints of Al/steel, Al/Ti and Al/Cu are widely used in automotive, aerospace, military and home appliance industries, etc. However, the brittle intermetallic compounds may easily produced in practical dissimilar metal joining processes, which would degrade the tensile strength of the joint. Meanwhile, the good performance of joints is closely related to the wetting and interfacial reactions of Al/steel, Al/Ti and Al/Cu systems in brazing process, while the control and characterization of wettability is not easy in practice. Therefore, on the premise of reducing amount of intermetallic compound, it is important to improve the wettability of molten AlSi₅ alloy on the surface of bare steel, titanium and copper, as well as analysis the wetting behaviors of molten AlSi₅ drop quantitatively.Both experimental and numerical simulation methods are used to study the wetting behavior of molten AlSi₅ alloy on the surface of bare steel, titanium and copper under the condition of cold metal transfer（CMT）. First of all, using laser backlight and high resolution camera in experimental method, the wetting behavior of molten AlSi₅ alloy on the surface of titanium and copper was analyzed by the dynamic sessile drop method. The results showed that: the variation of wetting behaviors（contact angle and contact radius vary with time） and the final wettability of molten AlSi₅ alloy on the surface of titanium or copper under the condition of CMT are related to the cleaning degree of oxide film covered on the surface of titanium or copper sheets respectively. In the case of only mechanical cleaning oxide film on the titanium or copper sheets, the spreading wetting was hindered in initial time by the small amount of oxide film, which showed the triple line was blocked. And the reactivity at interface was indirectly reduced by the small amount of oxide film at the same time, which results in the interfacial reaction of only mechanical cleaning oxide film was not severe as the interfacial reaction of both mechanical and chemical cleaning oxide film. Compared with the wettability of only mechanical cleaning oxide film on the surface of titanium or copper sheets, the wettability of AlSi₅ alloy on the surface of titanium or copper sheet under both mechanical and chemical cleaning oxide film of titanium or copper plate was better and the triple line was not blocked during the wetting process.Secondly, the numerical simulation were performed by the volume of fluid and the solidification-melting models, the model of the spreading wetting of molten AlSi₅ alloy on the surface of bare steel, titanium and copper sheets were established in the first cycle of different wire feed speed under the condition of CMT. The results showed that: the distribution of temperature field is the main factor influencing the spreading of AlSi₅ alloy on the surface of base metal. When the molten AlSi₅ alloy was wetting on the surface of titanium or bare steel sheet, the solidification of AlSi₅ alloy firstly appears in the triple line, and then spreads to the center of itself, due to heat exchange at interface. However, when the molten AlSi₅ alloy wetting on the surface of copper plate, the AlSi₅ alloy directly solidified from solid-liquid interface, and then gradually expanded upward. The spreading of molten AlSi₅ alloy on the surface of base metal would be hindered by the solidified AlSi₅. Besides, the wettability of AlSi₅ alloy on the surface of base metal becomes better with the increase of the wire feed speed.In the end, compared with the wettability of AlSi₅ alloy on the different surface of base metal, the temperature of AlSi₅ alloy is higher; the solidified AlSi₅ alloy is less and the wettability of AlSi₅ becomes better with the decrease of the base metal thermal conductivity（copper > bare steel > titanium）. By analyzing the relationship between the capillary number and the final contact angles of AlSi₅ alloy on the different surface of base metal, the contact angles are almost linear decrease with the increase of the capillary number.