한국주조공학회

Microstructure of A356 aluminium alloys cast in a permanent mold was investigated by optical microscope and image analyzer, with particular respect to the shape and size distribution of iron intermetallics known as β-phase (Al_.,FeSi). Morphologies of theβ-phase was found to change gradually with the Be:Fe ratio like these. In Be-free alloys, β-phase with needlelike morphology was well developed, but script phase was appeared when the Be:Fe ratio is above 0.221. With the Be:Fe ratios of 0.4:1-1:1, script phase as well as Be-rich phase was also observed. In case of higher Be addition, above 1:1, Be-rich phase was observed on all regions of the specimens, and increasing of the Be:Fe ratios gradually make the Be-rich phase coarse. It was also observed that the β-phase with needlelike morphology was coarsened with increase of the Fe content in Be-free alloys. However, in Be-added alloys, length and number of these β-phases were considerably decreased with the increased Be:Fe ratio. It was concluded that Fe impurity element to be crystallized into needlelike interrnetallics was tied up by Be addition element, and new phases were crystallized into script or Be- rich intermetallics.