Abstract:The effects of Mn and Sn on the microstructure of Al-7Si-Mg alloy modified by Sr and Al-5Ti-B were studied. The results show that the columnar dendrites structure is observed with high content of Sr, indicating a poisoning effect of the Al-5Ti-B grain refinement. In addition, Sr intermetallic compounds distribute on the TiB₂ particles, which agglomerate inside the eutectic Si. The mechanism responsible for such poisoning was discussed. The addition of Mn changes the morphology of iron intermetallic compounds from β-Al₅FeSi to α-Al(Mn,Fe)Si. Increasing the amount of Mn changes the morphology of α-Al(Mn,Fe)Si from branched shape to rod-like shape with branched distribution, and finally converts α-Al(Mn,Fe)Si to Chinese script shape. The microstructure observed by transmission electron microscopy (TEM) shows that Mg is more likely to interact with Sn in contrast with Si under the effect of Sn. Mg₂Sn compound preferentially precipitates between the Si/Si interfaces and Al/Si interfaces.

Key words: Al-7Si-Mg aluminum alloy; grain refinement; intermetallic compound; manganese; tin; Al-5Ti-B; Sr