Reseach On Brazing Technology For Aluminum-lithium Alloy

The composition of the oxide film on the surface of Al-Li alloy was analyzed after accelerated oxidation at 500℃and 540℃respectively, and the effect of removing oxide film was researched by analyzing the residua of CsF-AlF3 flux and KF-AlF3 flux with the oxide film on the surface of Al-Li alloy, the reaction mechanism between CsF-AlF3 flux and the oxide film on the surface of Al-Li alloy was also discussed. The results show that the oxide film on the surface of Al-Li alloy was composed of Li2CO3 and amorphous Al2O3 at 500℃～540℃. CsF-AlF3 flux and KF-AlF3 flux can evenly remove the oxide film effectively and promote wetting and spreading of the liquid state filler metal on the surface of Al-Li alloy, but the working temperature of KF-AlF3 flux is above 580℃and unsuitable to braze the Al-Li alloy. The manner of removing oxide film is that the CsF-AlF3 flux reacted with and/or dissolved the oxide film, which the CsF played an important role in the course of removing the oxide film. The key step is the generation of HF, which induced and accelerated the reaction of removing oxide film, the generation of H2O also accelerated the reaction at 500℃～540℃. The study on the mechanism of removing oxide film by CsF-AlF3 flux will provide theory base for brazing Al-Li alloy.Al-Li alloy was brazed in furnace by use of Ag-Al-Cu-Zn filler metal matching CsF-AlF3 flux at 530℃, and the holding time was three minutes. The results indicate that under the condition of N2 atmosphere, the tensile strength of butt joints was up to about 390MPa with the strength factor of 0.89 and the shear strength of lap joints was up to about 380MPa with the strength factor of 0.86, however the strength factors of butt joint and lap joint were respectively only 0.13 and 0.15 without N2 atmosphere. The theoretical analysis shows that destroying and removing the complex oxide films on the surface of Al-Li alloy effectively under the temperature of about 530℃was the key to the brazing process, and the protection of N2 was an important way to improve mechanical properties of the braze metal.The changing rule of strength of brazed joints was studied by way of analyzing microstructures of brazed joints by means of metallographic microscope and SEM. The results indicate that there were few defects such as gas pores, slag inclusions, cracks and so on in brazed joints with N2 atmosphere, which effectively improves mechanical properties of the brazed joints, however there were a few defects in brazed joints without N2 atmosphere which the mechanical properties of brazed joints were poorer. These sufficiently show that the protection of N2 is very important to obtain good brazed joints without defects.