The Effects of Rare Earth Elements on High Pressure Die Casting Mg Alloys

Abstract

마그네슘합금은 높은 열전도도, 비강도 및 기계 가공성 등 우수한 특성을 가지고 있으며 일반적으로 고압 다이캐스팅(HPDC) 공정을 통해 제조되고 있다. 현재 스티어링휠, 엔진블록, 오일팬 등의 자동차 부품들은 다른 부품에 비해 기술적 어려움과 원가의 문제점을 가지고 있음에도 불구하고, 전체 자동차 무게에 있어 큰 비중을 차지하여 경량화 효과를 크게 얻을 수 있다. 본 연구에서는350 ton 진공 다이캐스팅 장비를 이용하여 Mg-Al 합금에 Sn, Ca, Sr, Y 및 Mm 의 원소들을 첨가하여 다양한 합금계를 제조하였으며 첨가 원소에 따른 다이캐스팅 시편의 미세조직 변화와 기계적 특성을 평가하였다. 또한 120oC에서 0.5, 1,2,4,8,12시간동안 열처리하고 기계특성 및 미세조직에 대해 고찰 했다. 본 연구에서 SEM 및 XRD등 분석 기계를 통해서 모두 시편들의 미세조직 하고 석출물을 분석하였다.

Magnesium is the 12th element in the element pool, the 8th abundant element in the earth. It attracted many researchers applied on structural materials because of its low density. As the lightest structural materials, magnesium alloys are very suitable for the applications of automotive industry where vehicle weight reduction and consequently energy saving are becoming the word focus. It is well known that the possible application of AZ91 and AM60 alloys is still restricted due to some problems. For example, the two alloys are unsuitable for manufacturing parts operating at temperatures higher than 120 °C. In addition, the strength and ductility of the commercial AZ91 and AM60 alloys do not simultaneously satisfy the requirement of some important parts. For example, AZ91 alloys strength is relatively high

however, the ductility of the alloy is not so good due to the high Al content. Although AM60 alloy has a high ductility, its strength is relatively poor. Various alloying and/or micro-alloying elements, such as Ce, Nd, Y, Si, Ca, Ti, B, Sr, Sb, Bi, Pr and so on, have been chosen to further improve the mechanical properties of AZ91 alloy. With the addition of yttrium, the AZ91D alloy is found to have good mechanical properties with the tensile strength of 270 MPa and elongation of 11%. For the modified-AM60 alloys, various alloying and/or micro-alloying elements, such as Ca, Sn, Y, Si, Ti, Ag, Nd, B, RE, Sr, Ce and so on, have been chosen to further enhance the mechanical properties of AM60 alloys. In this article, the effect of rare earth elements on commercial Mg alloys were observed. For developing high strength die casting Mg alloys, various RE elements were used for precipitating new phase to achieve the goal. Tensile test, corrosion test and other mechanical test was carried out, and SEM and EDS was used for the analysis of section part. In the first part, the process of developing new alloys will be introduced and the influence of different elements on Mg alloys will be compared as well. Sr, Y, Sn, Ca and Zn were added into Mg-Al based alloys. The specimen was produce 350t cold chamber die-casting machine. In the second part, the practical die casting process and parameters will be introduced. The strength and elongation was observed. Microstructure was observed by SEM and OM. Then phase and precipitate were observed by EDS and XRD. Compared with simulated results, the phase was observed as expcted. At last, the specimen were heat treated at 120 oC for 0.5h to 12h. After heat treatment, the mechanical properties was improved, and due to different phase in different alloys, some properties was improved significantly.