Study On Electron Beam Evaporation Characteristics Of Metals And Alloys

Metal vacuum evaporation is an important physical process,which is widely used in electronic industry,aerospace,waste recycling and other fields.At present,the experimental study of vacuum evaporation is limited to the slow evaporation of metals with low melting point,such as alkali metals,mercury,lead,zinc,etc.,but it lacks the study of the fast evaporation process of metals and alloys with high melting point.The electron beam evaporation characteristics of metals and alloys are studied in this paper.The thermogravimetric analysis device suitable for the characterization of electron beam evaporation process is designed and manufactured,and the performance is verified.The test temperature could reach 3000℃,the test sample mass could reach 300 g,the mass accuracy is 0.01 g,and the evaporation rate deviation is about 1.5%and the interference of electron beam could be shielded to maintain the evaporation for a long time.The electron beam evaporation characteristics of aluminum,copper,titanium,iron,tin,cobalt and other metal are studied by this device.On this basis,the variation of electron beam evaporation characteristics and the interaction between alloy elements are studied.By means of molecular dynamics simulation,the electron beam evaporation characteristics of metals which cannot be tested are further studied.By analyzing and calculating the experimental data of electron beam evaporation of each metal,the transition temperature,evaporation rate,evaporation coefficient of slow evaporation to fast evaporation are obtained.The transition temperatures of aluminum,copper,titanium,cobalt,tin,iron and nickel are about 1242℃,1250℃,2250℃,1919℃,1706℃,1981℃ and 1996℃,respectively.The evaporation coefficient is the ratio between the actual evaporation rate and the theoretical evaporation rate.Although the evaporation coefficient of each metal is always less than 1,the variation is different.The analysis of its microscopic mechanism is related to the intermolecular collision and the form of evaporation.The evaporation coefficients of aluminum and copper decrease with increasing temperature,which is caused by collisions between metal vapor molecules.The evaporation rate increases rapidly,the molecular number density of metal vapor increases sharply,the number of intermolecular collisions increases,and the number of molecules returning to the evaporation surface increases,and the macroscopic performance is that the evaporation coefficient decreases monotonously.The evaporation coefficients of tin,titanium,iron and cobalt firstly increase and then decrease with the increase of temperature,which may be caused by the existence of evaporation form of molecular clusters formed by multiple atoms.The molecular cluster form evaporates faster than the monatomic form without significantly increasing the molecular number density.This form of evaporation occurs only at relatively low electron beam evaporation temperatures of transition metals with strong metallic bonds,and molecular clusters cannot be maintained at higher temperatures.On the basis of metal elements,the vacuum evaporation characteristics of Cu-Sn alloy,Ti-Al alloy and V-Al alloy and the interaction between elements were studied.The microscopic mechanism of the influence is analyzed,which is determined by the microstructure of the alloy in liquid state.The microstructure of the alloy determines the size of the evaporation barrier of the alloy elements.During the evaporation process of Cu-Sn alloy,the transition temperature of Cu increases with the increase of Sn,while the transition temperature of Sn decreases with the increase of Cu content,and finally stabilizes between 1577～1600℃.At the same temperature,the evaporation rate of the alloy has a minimum value with the change of composition,and the composition of the alloy is always close to Cu-50Sn,indicating that there may be more stable intermediate compounds in the liquid CuSn alloy,which reduces the evaporation rate of the alloy,and the maximum reduction is near Cu-50Sn.During the evaporation process of Ti-Al alloy and V-Al alloy,titanium and vanadium almost do not evaporate,and the transition temperature of aluminum element increases to 1750℃,and increases with the increase of the content of titanium and vanadium in the alloy.In this study,we use molecular dynamics simulation to study the electron beam evaporation characteristics of metal which cannot be tested,and establish a suitable molecular dynamics model.The reliability of the model is verified by electron beam evaporation simulation of copper.Compared with the experimental results,the LJ potential is more suitable for the molecular dynamics simulation of metal vacuum evaporation than the EAM potential.The simulation results are closer to the experimental results,indicating that the microstructure of liquid metal at high temperature is closer to the hard sphere model.