Evolution Of Surface Oxide Film And Brazeability Of Aluminum Alloy Doped With Mg Under Medium-temperature Brazing

Medium-temperature brazing technical issues of aluminum alloys are related tothe flux, solder, brazing processes and many other aspects of joint quality. The coreproblem is the basic theory of the oxide film removal mechanisms and flux withsolder or base metal interface reaction and activation problems. The surface oxidefilm of aluminum alloy containing magnesium is more complex, and difficult to beremoved. Hence, aluminum alloy containing magnesium brazing is a recognizedproblem. This investigation mainly concentrated on removal mechanism of oxide filmof aluminum alloy containing Mg. Specific studies are as follows.The structure of the surface oxide film along the depth direction of the typicalaluminum alloys under medium-temperature brazing temperature was investigated inthis paper firstly. The result indicated that the alloys with Mg content below2.0wt.%,whether as cold rolling condition or under medium-temperature brazing process, theenrichment of Mg element at the surface is not detected and the oxide-film is pureAl2O3. However, the oxide film grew obviously during medium-temperature brazingprocess, and the thickness is about80nm. For the alloys with Mg content above2.0wt.%, the Mg element is significantly enriched at the outermost surface duringmedium-temperature brazing process, and A MgO-based oxide film mixed with smallamount of MgAl2O4is formed with a thickness about130nm, and crystallineMgAl2O4is dispersed among the amorphous MgO. However, at cold rolling condition,the original surface oxide film is pure Al2O3. The other alloying elements, that is, theMn and Si, didn’t enrich at the surface layer neither at cold rolling condition norduring medium-temperature brazing process for all the selected aluminum alloys, andthe surface oxide film is similar to that of the pure aluminum, which is almost entirecrystalline Al2O3.The exploration is made on interface activity of solder/base metal with e5、E5and b33cesium salt flux after adding ZnCl2、SnCl2and GeO2. Only e5flux addingZnCl2can promote Zn-Al solder spread in the aluminum alloy with Mg content above2.0wt%. For SnCl2, it don’t promote Zn-Al solder spread in the aluminum alloy withMg content above2.0wt%, but if together with ZnCl2it can improve the spreadability.What’s more, when the content of ZnCl2is4wt.%in e5flux the spreadability of Zn-Al solder is best for6061、5052and5083aluminum alloys.5083and6061aluminum alloys were brazed by induction furnace with optimizede5+4wt.%ZnCl2flux and Zn-Al solder. The results indicated that, when the joint gapis200μm, the connection between braze metal and base metal is good and themicrostructure of braze mental is homogeneous without defect. Furthermore, the jointstrength of5083is42%higher than that of6061.