| Pure aluminium and3003Al-alloy are usually brazed by Al-Si filler metal. The spreadability ofAl-Si filler metal is excellent, and the strength of the joint brazed by Al-Si filler metal is good.However, the melting point of Al-Si filler metal is too high to handle, for example, the base metalmight be overheated and even melt during the brazing process. Novel filler metals are in great needfor brazing pure aluminium and Al-alloy at the middle temperature. In addition, because the price ofCu soared in recent years, it also becomes a hotspot to substitute Al for Cu. More and moreproblems exist and need to be solved in Cu/Al brazing before new filler metals for brazing Cu/Al jointdeveloped. In order to solve the problems as mentioned above, the Zn-Al alloy is studied for brazingAl/Al joint and brazing Cu/Al joint in this thesis.The range of the melting temperature of Zn-Al alloy may keep within381℃~500℃, which isvery suitable to braze Al/Al joint and to braze Cu/Al joint at the middle temperature. The effects of Aland Ag addition on Zn-Al filler metal were studied deeply and systematically in this thesis and thecomprehensive properties of the Zn-Al filler metal were invesigated including the meltingtemperature of the filler metal, the spreadability and the brazed joint mechanical properties. On thebasis of studing and analyzing the mechanism of Al and Ag effects on the joints including purealuminium brazed joint,3003Al-alloy brazed joint, Cu/Al brazed joint, the compositionof the Zn-Alfiller metal were optimized.The effects of Al on the spreadability of Zn-Al filler metal on pure aluminium surface werestudied, moreover, the mechanical properties and microstructures of the brazed joints also have beenstudied by brazing pure aluminium joints. The results indicate that the spreadability on purealuminium surface enhances with the increase of Al content in the Zn-Al filler metal. The filler metalgets the best spreadability on pure aluminum surface when Al content in the Zn-Al filler metal is15wt.%. However, the spreadability decreases when Al content in the Zn-Al filler metal exceeds15wt.%.Furthermore, the mechanical property tests indicate that all pure aluminium lap joints fracture on thebase metal. But some of pure aluminium butt joints fracture on the brazing seam. The strength of thepure aluminium butt joint enhances with the increase of Al content in the Zn-Al filler metal. Thehighest strength of the pure aluminium butt joint is obtained when Al content is15wt.%. However,when Al content exceeds15wt.%, the microstructures of the brazed joint become coarse, thecorresponding strength of brazed joint decreases. For comprehensive consideration,85Zn-15Al filler metal possesses the best comprehensive properties.3003Al-alloy were brazed by the different Al content Zn-Al filler metals. The effects of Al onthe spreadability of Zn-Al filler metal were investigated on3003Al-alloy surface, and the mechanicalproperties and microstructures of the brazed joints were also studied. Results indicate that thespreadability on3003Al-alloy surface enhances with the increase of Al content in the Zn-Al fillermetal. The filler metal presents the best spreadability on3003Al-alloy surface when Al content inZn-Al filler metal is about at15wt.%. However, the spreadability gradually deteriorates when Alcontent exceeds15wt.%. Furthermore, the mechanical property tests indicate that all3003Al-alloylap joints fracture on the base metal. But some of3003Al-alloy butt joints fracture on the brazingseam. When Al content in the Zn-Al filler metal are less than or equal to15wt.%, the3003Al-alloybutt joint fracture on the base metal. However, when the Al content exceeds15wt.%, themicrostructures of the brazed joint become coarse and the strength of the brazed joint decreases. Thensome butt joints fracture on the brazing seam. According to the spreadability and the brazed jointstrength,85Zn-15Al filler metal possesses the best comprehensive properties for brazing3003Al-alloy.Effects of Al on the spreadability of Zn-Al filler metal on pure copper surface were alsoinvestigated, moreover, the mechanical properties and microstructures of the brazed joints have beenstudied by brazing Cu/Al joints. Results indicate the spreadability on pure copper surface is improvedwith the increase of Al content in Zn-Al filler metal. It also has been found that the spreadability onpure copper surface is much lower than that on pure aluminum surface, so the filler metal flow in thebrazing seam depends mainly on its spreadability on the pure aluminium surface in the courese ofbrazing Cu/Al joints. Furthermore, the mechanical property tests indicate that all the Cu/Al brazedjoints fracture on the brazing seam including lap joints and butt joints. The strength of the Cu/Al jointsincluing lap joints and butt joints enhances with the increase of Al content in Zn-Al filler metal. Thepeak strength of the brazed joints presents when Al content is15wt.%. However, when Al contentexceeds15wt.%in Zn-Al filler metal, the CuAl2phase becomes coarse in the brazing seam. Stressconcenstration is easy to occur on the boundary of the coare CuAl2phase, which leads thecorresponding strength of the brazed joint to decrease, so far as we know,85Zn-15Al filler metal isthe better metal for brazing Cu/Al joint.Some shortcomings of85Zn-15Al filler metal brazing Cu/Al joint have been found, especially,the mechanical properties of the Cu/Al brazed joints are not very high, so on the basis of85Zn-15Alfiller metal, the effects of Ag on the melting temperature of Zn-Al filler metal, spreadability, andmechanical property of the Cu/Al brazed joint as well as the brazing seam microstructure were studied respectively. Results show that the melting temperature rises with the addition of Ag, meanwhile, theaddition of Ag improves the spreadablity on the pure aluminium surface and on pure copper surface.Furthermore, the mechanical property tests indicate that all the Cu/Al brazed joints fracture on thebrazing seam including lap joints and butt joints. The microstructures of the brazed joints areimproved significantly with the addition of Ag. The addition of Ag can refine the CuAl2brittle phasein the brazing seam, which decreases the stress concentration and enhances the brazed joint strength.And the strength of the Cu/Al joints incluing lap joints and butt joints enhances with the increase ofAg content in Zn-Al filler metal. The Zn-Al filler metal with3.3.wt.%Ag shows the best mechanicalproperty when applied in the Cu/Al brazed joint. When the content of Ag exceeds3.3wt.%, themechanical properties of the brazed joints remain stable, the Zn-Al filler metal with3.3wt.%Agpossesses the best comprehensive properties, and the optimal composition of the Zn-Al filler metalhas been successfully applied in the hardware industry, electricity industry, refrigeration industry, andso on. |
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