| AA6061 has good formability,weldability,machinability and excellent corrosion resistance,it is widely used in the production and manufacturing of automobile body,engine parts and heat exchanger industry,etc.Commonly used Al-12 Si brazing alloy,the melting point range is 559-591,suitable for 590～605℃ brazing,close to or even higher than the melting point of many 6xxx series aluminum alloy,and the temperature is easy to cause the growth of the base metal grain or overburning.Therefore,it is of great significance to develop new filler metals with lower melting point,lower cost and high quality.In order to solve this problem,Al-Si-Zn filler metal which is the appropriate composition of medium-temperature suitable for brazing 6061 aluminum alloy was found in this paper,and the properties of brazing joints were optimized by adding trace element Sr or Ti.In addition,the influence mechanism of trace element was studied through first-principles simulation calculation.In this paper,the model of Sr adsorption on the surface of Si(111)was established to explore the morphologic regulation mechanism of metamorphic elements.Then,the influence of Sr elements on dissolution potential of Al-Si-Zn alloy was explored by constructing the model of Sr-Si to replace the atoms of Al-Zn(100)surface layer.The influence of Sr/Ti content on Al-Si-Zn filler metal properties was studied by DTA,XRD,SEM and EDS.The main conclusions are as follows:(1)The vacancy of Sr adsorpted on Si(111)surface is the most stable,and its adsorption energy increased with the increase of coverage.The larger the coverage is,the smaller the surface force between Sr and Si is.On the contrary,the larger the surface force between Sr and Si is,the larger the influence on the morphology of Si is when a small amount of Sr is adsorbed on the Si(111)surface,and the smaller the influence on the morphology of Si is when Sr increases.(2)When the coverage of Al-Zn(100)surface containing Sr increases from 1/4 m L to 3/4 m L,the chemical potential of Al-Zn on the surface increases by 0.13～0.93 e V compared with that without Sr.The potential of the dissolved electrode of Al-Zn atom on the surface also shifted to a negative direction,indicating that Al-Zn(100)containing Sr becomes more easily corroded.Mulliken charge population analysis shows that the total negative charge number of Al atoms increased with the increase of Sr coverage.The negative charge on the surface of Al indicated that the surface potential decreased and the surface work function decreased,which made Al more active and more prone to electrochemical corrosion reaction.(3)The microstructure mainly includes α-Al solid solution,fibrous Si,and η-Zn solid solution.After Sr was added to Al-6Si-40 Zn filler metal,its melting range remained unchanged,and the spreading wettability was improved.The corrosion performance of filler metals decreased compared with that without Sr.Generally speaking,increasing Sr content could improve the average microhardness of brazed joints.The shear strength of the brazed joint containing Al-6Si-40Zn-0.8Sr filler metal was 134.23 MPa,which was 39.3% higher than that of the brazed joint without Sr.(4)The microstructure mainly includes α-Al solid solution,fibrous Si,and η-Zn solid solution.After Ti was added to Al-6Si-40 Zn filler metal,its melting temperature of increased,and the spreading property increased first and then decreased.The corrosion performance of Al-6Si-40Zn-xTi filler metals decreased compared with that without Ti.Generally speaking,increasing Sr content could improve the average microhardness of brazed joints.The shear strength of the brazed joint containing Al-6Si-40Zn-0.8Sr filler metal was 116 MPa,which was 20.1%higher than that of the brazed joint without Ti. |
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