Electrochemical Performances Of Recycled Magnesium Alloys And Development Of High Thermal Conductivity Magnesium Alloys

With the wide application of magnesium alloys in automobiles,electronics,aerospace and other fields,under the social theme of"energy saving,emission reduction,green environmental protection",the recycling of waste magnesium alloys has attracted more and more attention.In addition,magnesium alloys are required to have good heat dissipation and mechanical properties when they are used in radiators of electronic devices such as LED lamps.Therefore,it is necessary to develop high strength and high thermal conductivity magnesium alloys.This Dissertation focuses on the recycling of waste magnesium alloys and the development of high strength and high thermal conductivity magnesium alloys.Firstly,the recycled AZ31 magnesium alloys（RMA）were obtained by recycling and melting waste magnesium alloys in crucible furnace.Secondly,ternary magnesium alloys with nominal components of Mg-1.5Zn-0.3Mn and Mg-2.0Zn-0.3Mn and quaternary alloys with nominal components of Mg-1.5Zn-0.3Mn-mY and Mg-2.0Zn-0.3Mn-nY（m=0.3,0.6,n=0.4,0.8）were prepared by melting method.The compositions of the alloys were determined by inductively coupled plasma spectrometer（ICP-OES）.The structure characteristics of the samples were characterized by X-ray diffraction（XRD）,metallographic microscope,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.In the aspect of alloy properties,the electrochemical properties of RMA in NaCl and AlCl3 electrolyte solutions were analyzed,and compared with commercial magnesium alloys（CMA）.For Mg-Zn-Mn and Mg-Zn-Mn-Y alloys,the thermal conductivity in as-cast,as-extruded and T6 heat-treated conditions was analyzed in the temperature range of 25oC-300oC.The mechanical properties at room temperature were also investigated.The results are as follows:For the research on RMA,firstly,ICP test shows that the range of RMA components is in line with the national standard AZ31.Compared with CMA,its Al content is a bit higher.XRD and microstructure observation show that RMA and CMA have similar crystal structure characteristics.Their phase compositions are similar,mainly consisting ofα-Mg matrix and a small amount of Mg₁₇Al₁₂2 second phase.Because of the high Al content in RMA,more Mg₁₇Al₁₂2 second phase is formed.Secondly,the corrosion experiments,AC impedance and polarization curves in NaCl and AlCl₃ solutions show that the corrosion rate of RMA is higher than that of CMA,because RMA contains more Mg₁₇Al₁₂2 second phases.The corrosion rates of RMA and CMA increased with the increase of electrolyte concentration.The corrosion rate of the alloys in AlCl₃ solution is higher than that in NaCl solution,mainly because there are more Cl-in AlCl₃ solution.Thirdly,the discharge performance of RMA and CMA were tested respectively.The results show that the discharge capacity of RMA is higher than that of CMA in the same electrolyte solution.The discharge capacity increases with the increase of solution concentration.The discharge capacity of the two alloys in NaCl solution is higher than that in AlCl₃ solution.RMA has the highest discharge capacity in 0.9 M NaCl solution,reaching2.44 mAh/cm².For the research on Mg-Zn-Mn ternary magnesium alloys,firstly,the ICP analysis of components shows that the actual zinc content in Mg-1.5Zn-0.3Mn and Mg-2.0Zn-0.3Mn alloys is 1.71%for the former alloy and 2.37%for the latter alloy.XRD and microstructural observation displays that the phases of all alloys in as-cast,extruded and T6 heat-treated state are mainlyα-Mg matrix and a certain amount of MgZn₂ second phases.There are more dislocations in as-cast alloys.Under extrusion,the grain size is obviously refined.The dislocation density is greatly reduced,and the precipitation of MgZn₂ phase is increased.Under T6 heat treatment,dislocations almost disappeared and MgZn₂ phase precipitation in the matrix further increased.Secondly,the measurements on the relationship between the thermal conductivity of the alloy samples and the temperature reveal that the thermal conductivity of all alloys increases with the increase of temperature.The thermal conductivity of alloy samples with low Zn content is higher than that of alloys with high Zn content,which is related to the high lattice distortion of alloys with high Zn content.After extrusion and T6 heat treatment,the thermal conductivity of the alloys increases,which is due to the decrease of dislocation and the increase of precipitation of the second phases after extrusion and heat treatment.Thirdly,the mechanical properties of the alloys under cast,extrusion and T6 heat treatment show that the yield strength and tensile strength of the alloy with high Zn content are higher than those of the alloy with low Zn content,but the elongation is lower The corresponding strengthening mechanism is mainly solution strengthening.Compared with the as-cast state,the yield strength and tensile strength of as-extruded alloy markedly increased.Compared with the extruded state,the yield strength and tensile strength of T6 heat treatment alloy increased,but the elongation decreased.The strengthening mechanism mainly depends on the second phase strengthening.For the research on Mg-Zn-Mn-Y quaternary alloys,firstly,the ICP analysis of component shows that the content of Zn in the actual alloy sample is 10%or more higher than that of nominal component,and the content of Y is slightly higher than that of nominal component.XRD and microstructure observation displays that the phases of as-cast alloys are mainly a-Mg matrix,and a small amount of MgZn₂ second phases and Mg₃Y₂Zn₃ second phases.There are a lot of dislocations in the as-cast samples.Under extrusion and T6 heat treatment,the precipitation of the second phase increases,and the dislocation density decreases until it almost disappears.The grain size of the alloy was refined remarkably by adding rare earth Y.The measurements on the relationship between thermal conductivity of quaternary alloys and temperature shows that the thermal conductivity of quaternary alloys increases with the increase of temperature.After extrusion and T6 heat treatment,the thermal conductivity of the alloy increases,mainly due to the reduction of dislocation in the alloy and the increase of precipitation of the second phase after extrusion and heat treatment.In addition,with the increase of rare earth Y content,the thermal conductivity of the alloy increases first and then decreases.The mechanical properties of as-cast,as-extruded and T6 heat treated quaternary alloys show that the yield strength and tensile strength of T6 heat treated samples increase,compared with as-cast and as-extruded alloy.But the elongation decreases.The strengthening mechanism is related to the second phase strengthening.With the increase of Y content in the alloy,the yield strength and tensile strength of the alloy increase to a certain extent,and the strengthening mechanism depends on solution strengthening.Combined with the above research,it is found that T6 heat treated Mg-2.0Zn-0.3Mn-0.4Y alloy has the best comprehensive properties and meets the application requirements of high strength and high thermal conductivity.Its thermal conductivity reaches141.2/m·K,tensile strength reaches 308.19 MPa and elongation reaches 9.11%.