Study On Thermal Conductivity Of AZ91D Magnesium Alloy

At present, 3C products, aerospace, communications and electronics are facing increasing pressure to lightweight. Meanwhile, in order to improve stability and service life, high thermal conductivity is necessary to some components, especially in heat-sink device. Thermal conductivity of pure magnesium is rank only second to copper and aluminum in common commercial metal material. However, thermal conductivity of magnesium alloy is low. For instance, thermal conductivity of commercial casting AZ91 D is only 51.2 W(m·K)-1. This problem hinder the application of magnesium alloy in Thermal field. Therefor, it is necessary to research on thermal conductivity of magnesium alloy.The magnesium alloy specimens of various deform process, heat treatment and different thickness of die-cast plate were preparated in this article. On the one hand, thermal conductivity of specimens preparated were measured by laserflash thermal analyzer. On the other hand, the microstructure of magnesium alloy specimens were investigated by optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffractometer(XRD) and Energy Dispersive Spectrometer(EDS). The influence factors of thermal conductivity AZ91 D magnesium alloy were analyzed subsequently.The main research results and conclusions are as follows:(1) Thermal conductivity of die-casting AZ91 D magnesium alloy is higher than that of sem-icontinuous casting AZ91 D alloy, and extrusion has a negative influence on thermal conductivity of AZ91 D alloy. The main mechanism are: 1) semi-continuous casting alloy has abundant Mg17Al12 phase, and has more Al atoms and Zn atoms in Mg-matrix compared with die-casting alloy, which reduces alloy thermal conductivity as a whole. 2) a huge number of grain boundries and defects are formed and plenty of Al atoms dissolve in Mg matrix during extrusion process, which increase lattice distortion in alloy and decrease alloy thermal conductivity.(2) Mg17Al12 phase in sem-icontinuous casting and die-casting AZ91 D alloy dissolved entirely during solid solution treatment and a amount of Al atoms dissolve in Mg-matrix acting as the scattering centers, which limit the mean free path of electrons and phonons and reduce thermal conductivity of AZ91 D alloys. Mg17Al12 phases that dissolved in T4 treatment are formed again during artificial aging treatment, which consume the solute elements in Mg matrix, leading to an increase thermal conductivity of alloy.(3) As for extrusion AZ91 D alloy, the dislocation line formed in extrusion recover during solid solution treatment. Meanwhile, grain growth and defect increase in solid solution treatment, which increase thermal conductivity. Thermal conductivity of alloy increase again during subsequent artificial aging treatment.(4) With the increase of thickness during die-cast, the grain size and grain boundaries of die-casting AZ91 D alloy increased, acting as the scattering centers of electrons and phonons. As a result, thermal conductivity of alloy decreased. Meanwhile, with the decrease of grain size, more Al atoms dissolve in Mg-matrix, which has a negative effect on thermal conductivity.(5) With the temperature increased, the heatsink transfer capacity of electrons and phonons strengthen and will trigger aging behaviours, leading to the increase of thermal conductivity.