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A Study on Microstructure and Mechanical Properties of High Pressure Die-cast Magnesium Alloys
고압 다이캐스팅으로 제조한 마그네슘합금의 미세조직 및 기계적 특성에 관한 연구

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Authors
소태일
Advisor
신광선
Major
공과대학 재료공학부
Issue Date
2015-08
Publisher
서울대학교 대학원
Keywords
Die-castingMg-Al-Sn AlloysMicrostructureMechanical PropertyMicroporosityConstitutive Model
Description
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 신광선.
Abstract
Recent years, magnesium alloys have been one of the attractive metals among structural materials owing to their several advantages such as high strength to weight ratio, high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping capacity and good machinability to use in various applications. Magnesium alloys have generally been produced by various manufacturing technologies including high/low pressure die-casting, extrusion, strip-casting etc. Among them, high pressure die-casting is one of the representative casting processes and has several advantages such as fine microstructure due to fast cooling, good surface quality and high strength. Therefore, over 95% of magnesium applications have been produced on a high pressure die-casting process in the world. In high pressure die-casting, a molten metal is injected into die-cast mold to produce various products for automotive and electronic applications. One of the major issues in high pressure die-cast magnesium alloys has been focused on the development of heat resistant alloys by the addition of various alloying elements for automotive applications which require heat resistance.
In the first chapter, in order to improve the high temperature mechanical properties of high pressure die-cast magnesium alloys, Mg-Al-Sn based alloys have been developed by using relatively inexpensive alloying elements. At first, Sn was added to Mg-6 wt.% Al alloy as a second alloying element, and then various alloying elements including Ca, Sr and Mm were also added to the Mg-Al-Sn alloy. These alloys were produced on a 320 ton die-cast machine with vacuum system. The microstructure is mainly consisted of ?-Mg, Mg17Al12 and Mg2Sn phases in Mg-6 wt.% Al-4 wt.% Sn (AT64) alloy. The Mg2Sn phase in dendrite boundaries is disappeared by the addition of Ca, and coarse CaMgSn and Al2Ca phases newly appeared in Mg-6 wt.% Al-4 wt.% Sn-1 wt.% Ca (ATX641) and Mg-6 wt.% Al-4 wt.% Sn-2 wt.% Ca (ATX642) alloys. The additional alloying elements such as Sr and Mm to ATX alloys reduced the size of CaMgSn phase, also thermally stable precipitates fractions were increased. The yield strength of each alloy was significantly improved due to the addition of alloying elements at room temperature and 150oC. Creep tests of high pressure die-cast magnesium alloys were performed under conditions of the applied stress of 70 MPa at a temperature of 150oC. From these tests, creep properties were also improved by the addition of alloying elements. The dominant creep mechanisms for Mg-6 wt.% Al-4 wt.% Sn-1 wt.% Ca (ATX641) and Mg-6 wt.% Al-4 wt.% Sn-2 wt.% Ca (ATX642) alloys were investigated. From this study, the dominant creep mechanism for the ATX641 alloy is inferred as dislocation climb in the range of the applied stress from 30 to 90 MPa. On the other hand, in the ATX642 alloy, grain boundary sliding was dominant in the range of the applied stress from 30 to 70 MPa at 150oC, and dislocation climb in the range of the applied stress from 70 to 90 MPa at 150oC. It was possible to suggest that the theoretical background to develop improved alloy systems through the understanding of creep mechanisms for the developed magnesium alloys.
In the second chapter, the relationship between casting defects and mechanical properties of high pressure die-cast Mg alloys were investigated. High pressure die-cast magnesium alloys showed variation in the mechanical properties depending on internal defects such as microporosity and brittle precipitates. In order to investigate tensile elongation to microporosity variation in high pressure die-cast magnesium alloys, we suggested theoretical model to predict tensile elongation having microporosity which takes into account strain related factors such as the strain hardening exponent, strain rate sensitivity and microporosity distributions. The decrease level of mechanical properties due to microporosity is defined as the defect susceptibility, and this is strongly dependant on its microstructural factors. The Mg17Al12 phase in Mg alloys gives negative effect on their ductility due to crack initiation. The increase in the total fraction of precipitates and the decrease in the spacing between precipitates were existed by combined addition of Sn and Ca. The yield strength of die-cast Mg-6Al base alloys increased by the addition of alloying elements such as Sn, Ca, Sr and Mm, however, the tensile strength and elongation were decreased in Mg-6Al-X (Sn, Ca, Sr and Mm) alloys due to precipitates at dendrite boundaries. The defect susceptibility of tensile elongation to microporosity variation slightly increased by the addition of alloying elements. The strain hardening exponent of each alloy was decreased by the addition of Sn and Ca. Constitutive prediction proposed by A. K. Ghosh was used to verify the experimental results based on the strain hardening exponent and the strain rate sensitivity of investigated alloys. From the calculations, the strain within void region is deviated from the iso-strain line, and the degree of deviation from the iso-strain line is strongly dependant on the strain hardening exponent. However, predicted tensile elongation in the magnesium alloys using the original Ghosh`s model did not agree with the experimental values. In this thesis, the modified constitutive model which considers microporosity distribution factors agrees well with the experimental results than the original Ghosh`s constitutive model. In the design of high strength and high ductility die-cast Mg alloys which have casting defects such as microporosity or brittle precipitates, the use of the modified constitutive model may provide useful directions from the view of optimization of the high pressure die-casting process for removal of casting defects, or alloy design by the addition of alloying elements for modification of the microstructural factors.
Language
English
URI
https://hdl.handle.net/10371/118011
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
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