Studys Of Anodic Film Surface State Of Aluminum Alloys And Its Adhesive Performance

Adhesive bonding joints of aluminum and its alloys is widely used in many fields such as aviation, aerospace, defense, automotive and so on. Anodizing is one of the important pre-treatments to aluminum alloys for structural bonding. Anodic films, which are anodized in chromic acid and phosphoric acid, have both excellent initial adhesion and better durability performance to adhesive joints. But many works need to do further, for example, the effects of macro-roughness of aluminum alloys surface, deoxidizer technique and the relationship between structure of anodic films and adhesion performance. Although chromic acid anodizing as a pre-treatments for structural bonding joints can provide both excellent adhesive properties and good resistance, harmful pollution existed in the anodic films and bath. So the study on replacement of chromic acid anodizing is becoming hot.The influence of surface macro-roughness finished by series of sandpaper on the performance of bonding joints of aluminum alloys has been studied in this paper. The experiment results show that the adhesive bonding strength of LY12aluminum alloy is influenced by the surface roughness. The samples finished with abrasive papers of moderate grade sizes have higher bonding strength, which may result from the composite effects of increased surface area and decreased mechanical interlocking effect as the surface is getting rougher. The surface finishing with abrasive papers before phosphoric acid anodizing may influence the bonding strength of samples. Finishing with a moderate grade paper before phosphoric acid anodizing shows a better bonding strength. However, no such influence is observed when alkali etching is applied between finishing and anodizing.The micro-feature of aluminum alloys surface was infected by the etched and active pretreatment. And then, adhesive performance is depend on the micro structure. Lap-shear and wedge tests showed sulphate active (P2) and low voltage phosphoric acid anodizing dipping (LVPAA) are ideal replacement of chromic-sulphuric acid (FPL) etching process which is harmful to human being and nature.With the help of re-anodizing technology and SEM, effects of thichness, porosity, cell diameter and pore diameter of anodized film on bonding performance have been studied. The results indicated film thichness has limited influence on joints’initial adhesion and durability. However, the film is too thick to make the adhesive performance decrease because of gas-filling defects formation in the interfacial region between epoxide and oxidized film. Porosity and pore diameter are key factors lead to adhesive performance undulation. In terms to PAA film, when porosity decreased and pore diameter increased, the adhesive performance enhanced. Formation voltage is key contral parameter to oxidized film structure, so the film having excellent adhesive performance can be obtained by adjustment of formation voltage.Based on relationship of structural with performance, a phosphoric acid modified boric/sulphric acid anodizing process has been developed. The film obtained by the process has excellent adhesive performance and corrosion resistance, hence the anodizing process is an ideal choice to replacement of CAA technology.