Mg계 합금의 유동성과 부식특성에 미치는 첨가원소 및 제조공정의 영향
(The) effect of additional elements and manufacturing process on corrosion and fluidity characteristics of Mg-alloy systems
마그네슘 합금 유동성;
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Calcium was added during the casting of magnesium alloy to minimize the frequently occurring ignition problem and its optimal alloy composition was determined for the calcium effect. The casting characteristics of magnesium melt was also investigated. Three compositions of magnesium-calcium casting alloys were sought regarding both the variety of microstructure and the electrochemical properties with and without heat treatment. The microstructural difference between two compositions of AZ91D and AZ31 was observed with various processing methods and the corrosion behavior was also investigated with the variety of compositions and processing methods. The rolled material was fabricated from AZ31 which is known to have superior casting property by many researchers, and the effect of heat treatment of rolled material on the microstructure and the corrosion behavior was investigated. It was observed that the fluidity was improved by the catalyst effect of silicon from the result of silicon addition of 0.2∼0.4 weight percent into melt of AZ91D magnesium standard alloy. Followings were concluded expecting improvement of industrial application; 1) Though calcium addition was effective in suppressing the ignition during melting of magnesium alloy, the effectiveness was not linearly dependent on the calcium content nor occurred above 700℃. 2) Magnesium-calcium(0.22 weight percent) alloy, where most calcium was verified to dissolve into α-Mg matrix after casting, had both much lower corrosive electric potential and much higher efficiency property than pure magnesium despite of observation of homogeneous distribution of nano-sized fine Mg_(2)Ca phase in the matrix phase. 3) Regardless of fabrication method, AZ91D showed superior corrosion durability to AZ31. Mg_(17)Al_(12) phase precipitated at magnesium grain boundary was thought to contribute to improvement of corrosion durability. 4) The corrosion durability of chromated AZ91D was twice as that of die-casted AZ91D, as confirmed from the observation of microstructure and corrosion characteristic. 5) As proved from the phase analysis of AZ31 alloy by microstructure observation and x-ray diffraction, the alloy of Mg-3wt%Al-1wt%Zn alloy showed no precipitation of Mg_(17)Al_(12) up to 150℃. 6) While the microstructure of rolled AZ31, which was annealed, was coarse, that of hot-processed AZ31 was much finer. The corrosion durability of hot-processed AZ31 was higher than that of the annealed. The microstructure formed during hot-processing was thought to improve durability. 7) For addition of silicon into the melt of AZ91D magnesium standard alloy, the fluidity increased a little for 0.2∼0.4 wt% of silicon, but decreased slightly at 0.6 wt%. 8) For addition of silicon into the melt of AZ91D magnesium standard alloy, the fineness effect of crystal grain was appeared to improve a little for 0.2-0.4 weight percent of silicon.