Al-Si-Mg döküm alaşımlarının niyobyum – boron ilavesi ile tane inceltilmesi

The aim of this thesis is to observe the grain refinement effect of aluminum (Al), niobium (Nb) and boron (B) alloy for silicon and magnesium-based aluminum cast alloys, especially A357. When the silicon content exceeds the 3 % w.t on the aluminum, silicon and magnesium-based alloy, effectiveness of Al-Ti-B grain refiner decreases due to formation of titanium silicide precipitates, called as poisoning effect. The surface of the grain refining constitues are coated with titanium silicide, and number of TiB2 particles decrease. Since the lattice mismatch with aluminum matrix is higher for titanium silicide than Al3Ti and TiB2, grain refinement efficieny decreases. There are other studies to develop the grain refiners for aluminum cast alloys having silicon content higher than 3 %, which are the use of low content titanium instead of Al-5Ti-B, or Al-B in different boron amounts. However, since the commercial aluminum - silicon base alloys include titanium as an impurity, poisoning effect cannot be eliminated. That's why, recently niobium – boron-based grain refiners have been developed. The most important advantage is that niobum is much more stable at 750 - 800 °C than titanium which results in avoiding poisoning. In addition, having nearly same lattice parameters of Al3Nb and NbB2 with Al3Ti and TiB2 is also another advantage as a candidate for grain refiner. In this thesis, Al2.79Nb0.3B alloy is experienced as a grain refiner for A357 alloy. The results show that eutectic structure is changed from acicular to fibrous and transitional lamellar structure. An increase in the amount of eutectic undercooling and decrease in eutectic temperatures supported the microstructural examinations. Moreover, it is observed that grain size is also decreased. However, its effect on grain size is seen as similar with commercially used Al5TiB alloy. So, grain size was decreased by nearly the same amount.