Abstract: It was shown from experimental results that excellent brazability of aluminium alloy could be obtained with magnesium activator in vacuum brazing.It was the purpose of the present study with the help of Auger electron spectroscopy and Scanning electron microscopy surface analysis to research for variation of magnesium constituent and surface configuration of brazing filler metal layer on composite sheet specimens.At different temperature the concentrated evaporation of magnesium constituent atmelting temperature of brazing filler metal was indicated during experiments.Thiseffect may play a positive factor in form of fine fillet.In addition,by adding of magnesium pieces,the effect of different magnesium vapour concentrations on the abilityof brazing filler metal to fill the joint gap was studied.The results of these testsindicate that proper concentration of magnesium near the joints is the key to obtainfair brazing joints.While the concentration of magnesium is excessive,the brazingfiller metal flows away and when the concentration of magnesium is low,the brazingfiller metal does not spread and the fillets do not form.The tests of annular T-typejoint show that due to the different degree of opening of structure (for example,external and internal fillets),the effect of magnesium is different.By adjusting theadding magnesium amount or putting on an accessory cover are able to get excellentjoints.On the basis of experiments above mentioned and making use of obtainedexperiment results and technological measures,the blade wheels were brazed successfully.