Research On Microstructure And Properities Of High Strength And High Thermal Conducivity Mg-La-Zr/Mn Alloys

Magnesium alloy has the advantages of high specific strength/specific stiffness,good damping and shock absorption,good electromagnetic shielding,low elastic modulus and easy cutting.The research of high strength and high thermal conductivity magnesium alloy is of strategic significance with the development of light weight of the automotive industry,the miniaturization of 3C electronic products and the LED energy-saving industry.In this paper,the light rare earth element La was selected as the strengthening element,and the effects of the addition of Zr and Mn on the microstructure,mechanical properties and thermal conductivity of Mg-La alloy were investigated.The alloy ingot was obtained by permanent mold water-cooling casting method.The as-cast alloy was observed and tested.Then the reverse extrusion processing method was used to analyze the influence of extrusion treatment on the microstructure and properties of the alloy.Finally,a low-cost high-strength and high-conductivity magnesium alloy with both mechanical properties and thermal conductivity is prepared to expand the commercial application range of the magnesium alloy.In this paper,the effects of different La contents on the microstructure,thermal/electrical conductivity and mechanical properties of as-cast and extruded Mg-La binary alloys were investigated.The main second phase in Mg-La alloy is Mg₁₂La phase.With the increase of La content,the volume fraction of Mg₁₂La increases,the grain size of Mg-La alloy refines,the strength of as-cast Mg-La alloy increases,but thermal conductivity and electrical conductivity reduce.After extrusion,the mechanical properties of Mg-La alloy are significantly improved,the thermal conductivity is anisotropic,and the thermal conductivity in the//ED direction is lower than the thermal conductivity in the?ED direction.The Mg-8.0 La alloy undergoes brittle fracture due to the formation of the coarse second phases.The best comprehensive performance of the extruded Mg-3.1La is 332 MPa,elongation 7%,thermal conductivity 135 W/m·K,and conductivity 19.5 MS·m^-1.Through the study on Mg-La-Zr alloys,it is found that Zr can refine the grain size of as-cast Mg-La alloy and improve the strength and plasticity of as-cast Mg-La alloy.Zr has a great influence on the thermal conductivity of magnesium alloy.By adding0.3wt.%Zr,the thermal conductivity of as-cast Mg-La alloy is reduced by 20 W/m·K.After extrusion,the strength of the Mg-La-Zr alloy is further improved,and the thermal conductivity/conductivity exhibits anisotropy.When the extrusion temperature is lowered,the recrystallized volume fraction of the extruded Mg-La-Zr alloy decreases,and the strength is higher than the extruded alloy of 350°C.Otherwise,the extrusion temperature has a few effect on the thermal conductivity of Mg-La-Zr alloys.Mg-5La-0.3Zr has the highest strength,yield strength of 388 MPa,tensile strength of421 MPa,elongation of 6.8%,thermal conductivity of 120 W/m·K,and electrical conductivity of 14 MS·m^-1.Mn can improve the corrosion resistance and strength of magnesium alloys.Finally,the effects of different Mn contents on the microstructure and properties of as-cast and extruded Mg-La alloys were investigated.Mn can not refine the as-cast Mg-La alloy grains,so the addition of Mn can not improve the mechanical properties of the as-cast Mg-La alloy.Mn has a great influence on the thermal conductivity of as-cast Mg-La alloy.Adding 0.5wt.%Mn,the thermal conductivity of magnesium alloy is reduced by20 W/m·K and adding 2.0wt.%of Mn,the thermal conductivity of magnesium alloy is reduced by 70 W/m·K.After extrusion,the strength of the Mg-La-Mn alloy is increased by 4-5 times,and the strength of the extruded Mg-La-Mn alloy at 300°C is higher than350°C.A large number of rod-shaped and spherical nano-Mn phases are precipitated during the preheating process before extrusion,which hinder the movement of dislocations and the growth of crystal grains in hot extrusion deformation.The finely dispersed spherical nano-Mn phase significantly increases the thermal conductivity of the as-cast Mg-La-Mn alloys,especially the thermal conductivity of the Mg-La-2Mn alloy increased by 50 W/m·K after extrusion.Mg-2.6La-2.3Mn has excellent thermal and mechanical properties,yield strength of 373 MPa,tensile strength of 388 MPa,elongation of 9.8%,thermal conductivity of 130 W/m·K,and electronical conductivity of 16 MS·m^-1.