Mg-Zn-Cu합금의 열적, 전기적 특성에 관한 연구

Abstract

This study has been investigated thermal and electrical properties of Mg-Zn-Cu alloys according to the amount of Zn and/or Cu. In this study, Al which is generally added to improve mechanical properties and castability, was excluded since it significantly decreased the thermal and electrical conductivity. On the other hand, Zn was added as a major element, which had less influence on reduced conductivity and could complement to mechanical properties. And Cu was also added, which improved heat transfer characteristics as the amount was increased. The composition range of Zn and Cu are 4 ～ 6wt.% and 0 ～ 1.5wt.% respectively. In the analysis result of XRD in the as-cast condition of the Mg-Zn alloy, α-Mg peak was only detected and the other phases were not detected because their amount was too insignificant. Then, it was found that Mg7Zn3, Mg2Zn and MgZn phases were observed by SEM and EDS. In the Mg-Zn-Cu alloys, MgZnCu phase was observed, and it was found that the peak increased as the contents of Cu increased. The grain size refined as the amount of Cu increased, and there was no influence of Zn alloying contents. In particular, even if a small amount of Cu was added, the grain refinement appeared larger. However it significantly slowed when added with more than 1.0wt.%. For that reason, alloying Cu was affected by raising the solidus temperature, thus creating an effect on grain refinement by improving variation of super-cooling, and the effect of raising the solidus temperature slowed in more than 1wt.%. Electrical conductivity was a linear change, which exhibited decreasing characteristics as the amount of Zn increased, and increasing as the amount of Cu increased. Therefore, this research was able to calculate an impact value, in which Zn and Cu were the alloying elements, affected on the electrical conductivity in the Mg-Zn-Cu alloys of the as-cast condition. An impace value of the electrical conductivity of Zn and Cu were –1.4 ～ -1.6 and 2.3 ～ 2.7, respectively. This showed that the improvement of the electrical conductivity with increased Cu was bigger than the reducing influence of the electrical conductivity with increased Zn. This study was investigated the changing of the thermal conductivity(λ) in Mg-6Zn alloy by adding Cu. By measuring the specific gravity(ρ), specific heat(Cp) and thermal diffusivity(α), the thermal conductivity was calculated with the equation λ=ρ․Cp․α, hrough an indirect measurement method. As the amount of Cu increased in the Mg-6Zn alloy, heat transfer properties were improved, and it showed a slow synergistic effect when added with Cu 1wt.%. This research also was investigated the changing of the electrical conductivity in Mg-5Zn-xCu alloy, which was prepared by various aging treatment up to 240 hours at 200℃. Eutectic phase, which had been in Mg-5Zn alloy of the as-cast condition, was completely removed by solid-solution treatment. On the contrary, MgZnCu phase, which was observed in the ternary Mg-Zn-Cu alloys, was separated from the coarsening and was replaced with round shape as well. However, the fraction was not greatly reduced. After the aging treatment, the precipitation phase of MgZn2 was observed in α-Mg matrix, which lead to the improvement of hardness. Mg-5Zn without the addition of Cu showed maximum hardness in about 30 hours during the aging treatment, but Mg-5Zn-1.5Cu alloys showed maximum hardness in 8 hours. Until the time of the maximum aged condition, the precipitation phase of fine MgZn2 formed continuously, but to be maintained more than the time of the maximum aged condition, it was determined that the over-aging effect appeared from the combination with nearly precipitate rather than the newly formed due to the consumption of Zn element in α-Mg matrix. The electrical conductivity was increased until the time of the maximum aged condition, which showed a significant change even with over-aging. For that reason, it could be inferred that Zn element in α-Mg matrix was reduced as it was continuously consumed until it reached the maximum aging effect, and there was no change more than that time. Therefore, effects such as grain refinement, improvement of hardness and acceleration of aging can be expected through the addition of Cu in the Mg-Zn-Cu alloys. On the contrary, thermal and electrical conductivity showed that the amount of Zn in α-Mg matrix was greatly affected than the grain refinement and the effect of the second phase. In order to investigate the relative thermal conductivity and thermal emission of Mg-6Zn-xCu alloy, AZ91 and AZ31 were experimentally evaluated and compared using a separate test equipment. As a result, Mg-6Zn-1.5Cu alloy compared with AZ91 showed that there was more than 60% improvement in the thermal conductivity and about 20% improvement in thermal emission.