Study On Modification Effects Of Er On The Microstructure Of As-Cast And Semi-Solid A356 Alloys

Semi-solid metal (SSM) processing, as a new near-net shaping technology, takes advantages of both liquid and solid forming processes. The size and morphology of the primaryα-Al phase and the eutectic silicon influence the properties of finished products in hypoeutectic aluminum-silicon alloys. The modification effects of Er on the microstructure of as-cast and semi-solid A356 hypoeutectic aluminum-silicon alloys were examined by using optical microscopy (OM), quantitative metallography, X-ray diffraction (XRD), thermal analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that:The secondary dendrite arm spacing (DAS) of as-cast A356 alloy was reduced and the grain was refined with addition of Er, which lead to equiaxed microstructure. The primaryα-Al percentage was increased due to the higher number of heterogeneous nuclei by adding Er, and a widerα-Al solidification range would be obtained. From SSM standpoint, a largerα-Al mushy zone can reduce the sensitivity of temperature during the preparation of semi-solid slurries, and save the cost for the production and controlling of semi-solid slurries.The morphology change of eutectic silicon phase can improve the flowability of semi-solid slurries. Er has great modification effect on eutectic silicon. The morphology of eutectic silicon can be changed from acicular and flake-like to fibrous with addition of 0.3%Er. The modification of Er was correlated with the critical cooling rate, and the modification effect of Er was enhanced with the increasing of cooling rate. The eutectic nucleation and growth temperature were decreased while the eutectic recalescence was increased with addition of Er. The nucleation of eutectic silicon become very difficult, and the eutectic aluminum nucleate before eutectic silicon, which force eutectic silicon can only nucleate between eutectic aluminum, and the morphology of eutectic silicon was changed.Semi-solid aluminum slurries containing fine and spherical particles are obtainable via effectively control of nucleation and growth by adding Er. The solid fraction can be varied in a wide range from 0 to 0.45. The sphericity of primaryα-Al phase was increased with the increasing of Er content, and the sphericity obtained maximum when adding 0.5% Er. Er also has great effect on the primaryα2-Al phase, which was originated from the secondary solidification of remaining liquid in semi-solid slurries. The primaryα2-Al phase was changed from dendrite crystal to refined spherical particles.The mechanical properties show significant increase in as-cast A356 alloys with addition of 0.3% Er. With further increasing of Er, however, the microstructure of alloys appear needle-like and bulk Er-riched phase, which is considered as a disadvantage for mechanical properties. Thus, the optimum addition level of Er is 0.3%. The alloy could obtain the maximum hardness when the aging treatment regime was 180oC/6h. The main strength effect of the alloy was also contributed by Mg2Si at the aging treatment process and there were few effects of Al-Er intermetallic for improving strength of the alloy.