마그네슘 합금 개재물의 명도 평가법에 관한 연구

Abstract

Magnesium and its alloy are used in various industrial fields including parts for automobiles and electronic devices. These parts are manufactured mainly through die-casting process and magnesium ingots are used as raw materials. In these days, recycled ingots are often used by commercial purpose. However the quality of primary magnesium and recycled ingots are not secured, and massive casting defects which damage the final products can occur. This study was conducted to evaluate inclusions of primary magnesium, recycled ingot and die-casting melts for AM50A, AM60B, AZ91D alloys, and to evaluate pure Mg added MgO and C powder with various contents. And it includes composition analysis by spectrometer, measurement of inclusion contents by SEM & EDS, brightness test on fractured surface and etc. Especially, the brightness test method is very easy and obtains results quickly and the brightness is low when a lot of oxides or inclusions are present on the surfaces, so the method is suitable for fabricating field of Mg parts. The result of brightness test have been compared with those obtained from various test methods and obtained satisfactory result in evaluation of inclusion and oxide. A composition analysis of AM50A, AM60B, AZ91D alloy with various conditions satisfies ASTM B 94 specification, and the brightness of fractured area was 30-50 for die-casting melts, 40-50 for virgin and recycled ingots. Generally, brightness decreased with increasing alloying elements(Al, Zn etc), it resulted in increasing β-phase, intermetallic compound. After constant temperature and humidity test for fractured area, and the brightness is lower as the test is longer. The brightness are 56.49 for pure Mg, 49.04 for AM50A, 49.59 for AM60B, 43.60 for AZ91D after 3 hours of constant temperature and humidity test. After degassing and filtering treatment for Mg melts, the brightness was increased. For pure Mg added MgO & C powder, a specimen with MgO is brighter than C powder in optical inspection for fractured area when the content is constant, but the brightness was similar in constant contents of MgO & C. A grid counting method is executed in die-casting melts of AM50A and AZ91D, virgin ingots of AM50A and AZ91D, and the area fraction is 0.566%, 0.572%, 0.660%, 0.670% respectively. Automatic EDS analysis for MgO & AlN is executed in various specimens. The brightness values are lower as inclusion contents is higher. When the brightness value is over 50 in AM50A and 47～48 in AZ91D, the mechanical properties is expected to be good.