Study On Preparation, Microstructure And Properties Of Thermal Conductivity Mg-Zn-Zr Alloys

As the promising new material in 21st century, magnesium alloys possess the advantages of light-weight, high specific strength, high specific stiffness, good damping performance,good electromagnetic shielding performance and good thermal conductivity. LED lighting is the most widely used lighting fixture at present, but the heat dissipation problem restricts its further development. Compared to the traditional heat dissipation materials of copper and aluminum alloy, magnesium alloys not only possess good thermal conductivity but also possess the advantages of light-weight and low-cost. Accordingly, magnesium alloys have very good application prospect in heat dissipation materials. It is very important to develope a new kind of magnesium alloy for heat dissipation material.The most two important physical parameters characterizing the thermal conductivity property of the materials are the thermal conductivity and the thermal diffusivity. The thermal conductivity can be obtained by measuring the thermal diffusivity, density and specific heat capacity of the material. Inside the materials, the two ways of thermal conduction are electronic thermal conduction and phonon thermal conduction. Phonon thermal conduction is through the vibration of lattice particle. Distortion of crystal lattice, grain boundaries,dislocations and other defects are unfavorable for the thermal conductvity. Improving the thermal conductivity property and the mechanical property are contradictory if there is no phase transformation in alloy. The main purpose of this thesis is to obtain good mechanical property under the premise of good thermal conductivity and to improve the thermal conductivity property and mechanical property of the alloy at the same time by aging treatment.A kind of Mg-Zn-Zr alloy with good thermal conductivity is successfully designed.Mg-2wt.%Zn-xZr (x=0,0.5,1,1.5wt.%) alloy was chosen as the research object. The microstructure of the alloy is characterized by optical microscopy, scanning electron microscopy and X-ray diffraction, and the alloy is characterized with the thermal conductivity test, tensile test and microhardness test. First, the effects of Zr content on the microstructure and properties of alloy are studied. Then the solid solution and aging treatment are conducted on the Mg-2Zn-Zr alloy. The effects of different aging temperature and time on the microstructure and properties of the alloy are investigated. The optical aging treatment is obtained to improve the thermal properties and mechanical properties at the same time. Next,the alloy is rolled and subsequently annealed. The effects of the rolling and annealing on the microstructure and properties are studied. The optical properties of the alloy are obtained by the solid solution and aging treatment.Due to the dispersed fine Zn-Zr intermediate phase precipitation during aging treatment process, the thermal conductivity and strength of Mg-2Zn-Zr alloy are improved at the same time. On the one hand, the Zn-Zr precipitating consumes Zn in Mg matrix, reducing the matrix lattice distortion, thereby reducing the scattering effect of lattice particle on the phonon thermal conduction and electronic thermal conduction in the thermal conduction process. On the other hand, the Zn-Zr precipitating has the precipitation strengthening effect. The optical aging treatment of Mg-2Zn-Zr alloy is 175℃ for 24h. Compared to the as-cast Mg-2Zn-Zr alloy, the thermal conductivity is lower after rolling. The thermal conductivity of the alloy increases after annealing, but still lower than that of the as-cast alloy. The results show that the distortion of crystal lattice, grain boundaries, dislocations of the alloy are harmful to the thermal conductivity.The final rolling and heat treatment of Mg-2Zn-Zr alloy are homogenizing annealing at 370℃ for 12h first, then rolling at 400℃ of 6 passes under the reduction rate of 50%, next recrystallization annealing at 400℃ for 1h, and solid solution treatment at 420℃ for 4h,finally aging treatment at 175℃ for 24h. In the end, the thermal conductivity and thermal diffusivity are. 132.1Wm-1K-1 and 68.9mm2s-1, respectively, increasing by 11.9% and 3.0%than that of the as-cast.The tensile strength and yield strength are 279.0MPa and 196.4MPa,respectivel, increasing by 32.3% and 1.7 times than that of the as-cast alloy, the elongation is 25.2%, which has little difference with that of as-cast alloy.