The Effect of the Melt Temperature and the Cooling Rate on the Microstructure of the Al-20% Si Alloy Used for Monolithic Engine Blocks

The Al-20%Si melt heated to 785°C (1445°F) and 850°C (1562°F) exhibited refinement of the primary Si while heating to 735°C (1355°F) produced coarse and heterogeneous primary Si crystals following the solidification process at approximately 1.3, 4.5, 15 and 35°C/s. The primary Si crystals were 40% finer for the samples heated to 850°C (1562°F) as compared with those heated to 735°C (1355°F). Higher cooling rates produced better primary Si refinement and minimized its variation caused by the melt temperature. The secondary dendrite arm spacing (SDAS) was not affected by the melt temperature and was a function of the cooling rate for the given experimental conditions. The SDAS changed from approximately 32 to 22μm for a 1.3 and 4.5°C/s cooling rate and was reduced to approximately 11μm for a 35°C/s cooling rate. Cooling curve analysis was used to analyze the sequence of the metallurgical transformations and fraction liquid development during alloy melting and solidification. The non-equilibrium thermal characteristics under cooling rate up to 15°C/s were analyzed as well. The experimental results were used to optimize the casting process and improve the service characteristics of the vacuum assisted high pressure die casting (HPDC) motorcycle engine blocks.