AZ91D镁合金导热性能的研究

发布时间：2019-01-17 12:24

【摘要】：当前,3C产品、航空航天、通讯电子等领域均面临着日益增加的轻量化压力,同时,一些零部件对材料的导热性能往往有较高的要求(尤其是散热器件),以保证和提高产品的寿命及工作稳定性。纯镁的热导率在常见商用金属材料中仅次于铜和铝,比热导率与铝相当;然而常用铸造镁合金的导热性能却明显偏低,如AZ系铸造镁合金的热导率仅为50 W(m·K)-1左右,这严重阻碍了镁合金在有导热性能需求的工程领域的应用。因此,对镁合金导热性能研究并提高镁合金导热性能很有必要。本文以AZ91D镁合金为对象,制备了不同成型工艺(半连续铸造、压铸、挤压)、热处理工艺(T4、T6)和不同压铸厚度的镁合金试样。一方面,采用激光导热仪测量所制备镁合金试样的热导率,另一方面,通过光学显微镜(OM)、电子显微镜(SEM)、XRD、EDS等方法分析镁合金试样的微观组织,从组织的角度,研究AZ91D镁合金导热性能的影响因素。本文主要研究结果及结论如下:(1)不同成型工艺下AZ91D镁合金的热导率有一定差异,压铸态AZ91D合金的热导率高于半连续铸造合金,挤压变形使合金热导率显著降低。其主要机理有:1)半连续铸造合金中包含较多发达的第二相,其固溶体中Al原子和Zn原子含量均较高且分布不均匀,使合金整体的热导率降低;2)挤压使合金产生大量晶界和内部缺陷,同时大量Al原子固溶进Mg基体中,降低了合金热导率。(2)固溶处理使半连续铸造和压铸AZ91D合金的第二相充分溶解,大量Al原子固溶进Mg基体,破坏了Mg基体晶格排列的规则性,使合金热导率降低;时效处理中,Al原子从Mg基体中析出并重新形成第二相,该过程消耗了Mg基体中的Al原子,从而热导率增加。(3)对于挤压态AZ91D合金,挤压形成的位错线在固溶处理中发生回复,同时晶粒长大,组织中的缺陷减少,该过程有利于合金导热性能,且其作用大于固溶体中Al含量增加对导热性能的阻碍作用,使挤压态合金固溶处理后的热导率有所增加。经时效处理后,一方面伴随第二相析出,固溶体中Al含量减少,对合金导热的阻碍减小,另一方面,时效处理使挤压态合金晶粒粗化,晶界缺陷减少,有利于合金导热的进行,因此合金热导率上升。(4)随着压铸厚度的减小,AZ91D合金内部晶界增多,增加对电子和声子的散射作用,合金热导率随之越低;另外随着压铸厚度的减小,固溶于Mg基体的Al原子增多,对导热不利,这与固溶处理对合金热导率的影响趋势一致。(5)温度升高一方面使电子和声子的运动能力增强,另一方面会引起缺陷热阻减小并产生一定的时效行为,使AZ91D合金热导率随温度升高而逐渐增大。
[Abstract]:At present, 3C products, aerospace, communication electronics and other fields are facing increasing lightweight pressure, at the same time, some parts often have higher thermal conductivity requirements (especially radiator parts). To ensure and improve the product life and work stability. The thermal conductivity of pure magnesium is second only to that of copper and aluminum in common commercial metal materials, and the specific thermal conductivity is equivalent to that of aluminum. However, the thermal conductivity of common cast magnesium alloys is obviously low. For example, the thermal conductivity of AZ cast magnesium alloys is only about 50 W (m K) -1, which seriously hinders the application of magnesium alloys in engineering fields with thermal conductivity requirements. Therefore, it is necessary to study and improve the thermal conductivity of magnesium alloys. In this paper, AZ91D magnesium alloy samples with different forming processes (semi-continuous casting, die-casting, extrusion), heat treatment (T4T6) and different die-casting thickness were prepared. On the one hand, the thermal conductivity of magnesium alloy samples was measured by laser thermal conductivity instrument. On the other hand, the microstructure of magnesium alloy samples was analyzed by means of optical microscope, (OM), electron microscope and (SEM), XRD,EDS. The factors affecting the thermal conductivity of AZ91D magnesium alloy were studied. The main results and conclusions of this paper are as follows: (1) the thermal conductivity of AZ91D magnesium alloy under different molding processes is different. The thermal conductivity of die-cast AZ91D alloy is higher than that of semi-continuous casting alloy. The main mechanisms are as follows: 1) there are more developed second phases in the semicontinuous casting alloys, and the content of Al atoms and Zn atoms in the solid solution is higher and the distribution is uneven, which reduces the overall thermal conductivity of the alloy; 2) extrusion causes a large number of grain boundaries and internal defects in the alloy, and a large number of Al atoms are dissolved into the Mg matrix, which reduces the thermal conductivity of the alloy. (2) the second phase of semi-continuous casting and die-casting AZ91D alloy is fully dissolved by solution treatment. A large number of Al atoms were dissolved into the Mg matrix, which destroyed the regularity of the lattice arrangement of the Mg matrix and reduced the thermal conductivity of the alloy. During aging treatment, Al atoms precipitate from the Mg matrix and reform the second phase, which consumes the Al atoms in the Mg matrix and increases the thermal conductivity. (3) for the extruded AZ91D alloy, The dislocation line formed by extrusion recovers during solution treatment, while the grain grows and the defects in the microstructure decrease. This process is beneficial to the thermal conductivity of the alloy, and the effect is greater than the effect of the increase of Al content in the solid solution on the thermal conductivity. The thermal conductivity of the extruded alloy increased after solution treatment. After aging treatment, on the one hand, with the precipitation of the second phase, the content of Al in the solid solution decreases and the hindrance to the thermal conductivity of the alloy decreases. On the other hand, the aging treatment results in the coarsening of the grain and the decrease of the grain boundary defect in the extruded alloy, which is beneficial to the thermal conductivity of the alloy. As a result, the thermal conductivity of AZ91D alloy increases. (4) with the decrease of die-casting thickness, the internal grain boundary of AZ91D alloy increases, and the scattering of electrons and phonons increases, the thermal conductivity of the alloy decreases with the decrease of die-casting thickness. In addition, with the decrease of die-casting thickness, the number of Al atoms dissolved in the Mg matrix increases, which is unfavorable to the thermal conductivity, which is consistent with the effect of solution treatment on the thermal conductivity of the alloy. (5) on the one hand, the increase of temperature increases the mobility of electrons and phonons. On the other hand, the thermal resistance of defects will decrease and a certain aging behavior will occur, and the thermal conductivity of AZ91D alloy will increase with the increase of temperature.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG146.22

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