Beryllium Effects on the Microstructure and Mechanical Properties of A356 Aluminium Casting Alloy

초록

Microstructure of A356 aluminium alloys cast in the 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(AlfFeSi). Morphologies of the β-phase was found to change gradually with the Be:Fe ratio like these. In be-free alloys,β-phase with needlelike morpholgy was well developed, but script hpase was appearde when the Be:Fe ratio is above 0.2:1. With the Be:Fe ratio of 0.4:1∼1:1, script phase as well as Be-rich phase was also pbserved. In case of higher Be addition, above 1:1, Be-rich phase coarse. It was also pbserved 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. Beryllium addition improved tensile properties and impact toughness of the A356 aluminium alloy, due to the formation of a script phase or a Be rich phase or a Be-rich phase instead of a needlelike β-phase. The DSC tests indicated that the presence of Be could increase the amount of Mg which is available of MgsSi precipitate hardening, and enhance the preipitation kinetics by lowering the ternary eutic temperture.