Microstructure Evolution On Thermal Conductivity And Corrosion Behavior Of Mg-Zn-Mn(Ag) Alloy

Modern society has entered the era of high information,high data and high intelligence.An amazing amount of data exchanges at every moment.The electronic components used for data exchange can generate a lot of heat in the work,which has a detrimental impact on their operation.Therefore,a good heat dissipation ability is particularly important to the electronic components,and it is of practical significance to develop a kind of Mg alloy with light weight and high thermal conductivity to be the raw material of the heat conducting components.In this paper,a negative pressure suction casting device was developed to make as-cast bars which could be directly used in the directional solidification process.In the experiment,Mg-x Zn-Mn alloy and Mg-Zn-x Ag alloy were taken as the research objects to study the influence of different micro-structures and precipitation behavior produced by different process（casting process,directional solidification process,cryogenic treatment,aging treatment and cryogenic+aging treatment）to the thermal conductivity of Mg alloy.The process parameters to acquire the best thermal conductivity were obtained and the mathematical relationship between the columnar grains’morphology and the alloy’s thermal conductivity was simulated.The experiments were also made to study the effect of different processing methods to the corrosion resistance of Mg alloy,and to finally obtain the Mg alloy with high thermal conductivity and good corrosion resistance.In this paper,the effect of solute content on the thermal conductivity of as-cast Mg alloy was studied.With the increase of the solute Ag or Zn content,the solute segregation was enhanced.The proportion of the second phase Mg_25.04Ag_7.96 in the Mg-Zn-x Ag alloy increased,the grains were refined and the grain size increased from grade 0.5 to grade 2.In Mg-x Zn-Mn alloy,Mg₂Zn₁₁and Mg Zn₂ were increased,a large number of eutectic structures were formed on the grain boundaries,and the grain size was refined from grade 1 to grade 3.The excessive Zn could lead to the severe composition under-cooling in the solid-liquid interface during the solidification process,so thatα-Mn particles would precipitate betweenα-Mg dendrites.Thus,the solute content has an obvious effect on precipitation and nucleation rate,which can change the grain size and the morphology and quantity of the second phase.For these reasons and the radius difference between Mg and the other solute atoms,the thermal conductivity of as-cast Mg-Zn x Ag alloy is higher than that of Mg-x Zn-Mn as-cast alloy.Because the effect of solid solution inα-Mg matrix on the thermal conductivity of alloy was greater than the segregation of the second phase,the alloy’s thermal conductivity decreased by power function with the increase of the solute content.The microstructures of Mg-Zn-x Ag alloy and Mg-x Zn-Mn alloy could be changed by the directional solidification process.For the columnar grains,the lower the pulling speed was,the greater the ratio B（the average length L in the axial direction to the cross-section grain size G’）was,the smaller the angle was between the growth direction of the columnar grains and the pulling direction,and the better the morphology was.Thus the thermal conductivity of the alloy could be improved.The thermal conductivity of Mg-Zn-1Ag alloy made under the pulling velocity V=15μm/s was about 28%higher than that of its as-cast alloy.Therefore,by controlling and adjusting the pulling velocity,we could obtain better grain morphology and higher thermal conductivity.In this paper,by analyzing and fitting the experimental data of directional solidified Mg-x Zn-Mn alloy,the mathematical relationship between alloy’s thermal conductivity and grain morphology was obtained:λ=9.0043ln（B）+78.484,which had been verified then by bringing values of thermal conductivity and grain morphology of the directional solidified Mg-Zn-x Ag alloy,and the results showed that the average deviation between the calculated value and the measured value was less than 3%.In this paper,experiments were made to find that the cryogenic treatment could affect the alloy’s thermal conductivity by affecting its microstructure,precipitation behavior and internal defects.In cryogenic environment,the driving force of Mg-x Zn-Mn alloy’s precipitates was greatly reduced.The precipitation rate of the spherical and rod-shaped Mg₂Zn₁₁ and Mg Zn₂ second phases became slow,and a small amount of eutectic structure was formed at the grain boundaries.Thus,the purification of theα-Mg matrix was limited.But meanwhile,the deformation energy caused by cold contraction was greatly increased,the（101 0）,（0002）and（101 2）planes of the close-packed hexagonalα-Mg crystals steered to the（1011）plane,and the grains were broken,which could lead to the refinement of the grains and form a small amount of sub-crystal structure,and the inner defects would congregate and contract.After48 hours cryogenic treatment,the thermal conductivity of Mg-1Zn-Mn and Mg-3Zn-Mn alloys increased by 6.3%and 5.6%respectively compared with their as-cast alloys.In this paper,the influence of microstructure and precipitation behavior on the thermal conductivity of Mg alloy after aging process was further analyzed.The increasing of the aging temperature had obviously accelerated the precipitation rate of the second phase and quickly changed the morphology of the precipitates.With the increase of the aging temperature,the solution element in Mg-3Zn-Mn alloy was rapidly consumed,the second phase was greatly increased,and theα-Mg matrix was greatly purified.At 190℃,the Zn in Mg₂Zn₁₁ phase was activated to precipitate and combine with theα-Mg around it and transform to Mg Zn₂ phase.Theα-Mg+Mg Zn₂eutectic structure was obviously increased on the grain boundaries.The aging process for improving the thermal conductivity of Mg-3Zn-Mn alloy has been optimized.After2h*200℃aging treatment,the thermal conductivity of Mg-3Zn-Mn alloy increased by 11.7%compared with its as-cast alloy.In order to develop Mg alloy with high thermal conductivity and good corrosion resistance,the effects of different processes on the corrosion resistance of Mg-x Zn-Mn alloy were studied in this paper.The cryogenic process can effectively improve the corrosion resistance of the Mg alloy with low solute content,and the aging process has double-sided impact on the corrosion resistance of Mg alloy.So the ideal process was obtained as this:directional solidification+4h cryogenic treatment+2h*160℃aging treatment,which had been verified by doing experiments to Mg-3Zn-Mn alloy and the results showed that its thermal conductivity increased by 8.3%,and its corrosion resistance increased by 32%.Finally,the Mg alloy with high thermal conductivity and good corrosion resistance is obtained.