Effect of Alloying Element Antimony on Macrostructural and Microstructural Development in A356 Alloy Directionally Solidified under Unsteady-state Conditions

Microstructure development is well accepted to have a major effect on mechanical properties during its services.One of the most interesting ways to improve mechanical properties is to reduce secondary dendrite arm spacing (SDAS). SDAS also plays an important role in controlling and providing the well distributed and fine microstructure resulting in better tensile strength and elongation. To reduce SDAS, it is commonly known by increasing cooling rate and increasing interface instability by limited-soluble alloy addition. It is, however,unclear that how both cooling rate and limited-soluble alloy, e.g. Sb, relate to each other. This may be the reason that the limited-soluble alloy may not effectively reduce SDAS. To better understand this phenomenon,influences of Sb on solid/liquid interface instability using columnar to equiaxed transition (CET) were studied in the directionally solidification experiment. From macrographs and micrographs, it was observed that at 0.06-0.18 wt pct Sb the CET min, CET max, CET zone and ?T area gradually increased. The increases of CET max and CET zone in samples with 0.06 to 0.18 wt pct Sb addition results from recalesced zone. On the other hand, the variations of CET max and CET zone in samples with 0.24 to 0.30 wt pct Sb results from equiaxed grain formations that prohibit the growth of columnar grain and latent heat from intermetallic phase solidified.