Research On Green Preparation In Relatively Low Temperature And Grain Refining Performance Of Al-Ti-C-RE Grain Refiner

Adding grain refiner during the production of aluminum alloy castings is the most economical and practical method for grain refinement. There is some defect in the Al-Ti-B grain refiner which is widely used in industry, such as larger size of nucleation particles, decline of refining effect, and'toxic response'with Cr, Zr and other elements. Al-Ti-C grain refiner which can overcome the above disadvantages is considered to be the new generation grain refiner with most developing potential. However, Al-Ti-C grain refiner has not been widely used at present, mainly because the difficulty in preparation of TiC particles due to the poor wettability between molten aluminum and graphite.In this paper, the author tried to improve the wettability of molten aluminum on the graphite by adding rare earth elements to the melt. Titanium sponge was used to introduce Ti element instead of using Ti fluoride which may cause fluoride gas pollution. The refiner was prepared in relatively low temperature which can avoid oxygenation loss when prepared at high temperature. Four kinds of Al-3Ti-0.2C or Al-3Ti-0.2C-1RE grain refiners were prepared separately at high temperature (1250℃) or low temperature (950℃) with induction furnace or resistance furnace. The phase composition, morphology and distribution of second phase, and the refining performance of the grain refiner was investigated by optical microscopy, X-ray diffraction, scanning electron microscopy with energy disperse spectroscopy. The influence of rare earth on the microstructures and refining performance was analysed, and the synthesis processes of TiC phase and Ti20Al20Ce phase were discussed. So the research results can provide technical support for production and application of Al-Ti-C-RE grain refiner.The results show that the Al-3Ti-0.2C-1RE grain refiner prepared in induction furnace is composed of a-Al, TiAl3,Ti2Al20Ce, and TiC phase. Compared with Al-3Ti-0.2C, the strip TiAl3 phase is relatively small, and Ti2Al20Ce phase formed in the out layer of some TiAl3 phase, and the number of TiC phase was increased. In the preparation process, the RE elements gather on the surface of newly precipitated TiAl3 and suppress the growth of TiAl3 phase. Meanwhile, the rare earth elements can eliminate the oxide film on the reaction interface between graphite and molten aluminum, and improve the wettability between them, and promote the synthesis of TiC phase. In addition, Ti2Al2oCe phase may react with graphite producing TiC particles.Adding 0.4% of Al-3Ti-0.2C refiner prepared in induction furnace, the average grain size of pure aluminum was decreased from 920pm to 151μm, while adding 0.4% of Al-3Ti-0.2C-1RE refiner prepared in induction furnace, the grain size was reduced to 107μm. The rare earth can improve the refining performance significantly.The Al-3Ti-0.2C-1RE refiner prepared in resistance furnace is composed of Al matrix, regular blocky shape TiAl3 phase, irregular blocky shape Ti2Al20Ce phase, and granular TiC phase. The TiAl3 phase is in form of small blocky shape by the effect of RE elements. And the RE neither exist in Al matrix in the form of solution atom, nor precipitate at the grain boundary in the form of simple substance or compound, but all in the form of Ti2Al2oCe phase.Under the preparation condition of resistance furnace, the synthetic process of TiC phase is:when the sponge titanium and graphite were added in the molten aluminum, titanium sponge quickly dissolved into the melt, and became Ti solute atoms. Because the titanium sponge particles were in close contact with the graphite particles, the surface of graphite particles were rich in high concentration of Ti atoms, a large number of TiC particles formed around the graphite particles. The synthetic process of Ti2Al2oCe phase is:rare earth element was in the form of surfactant when added to the molten aluminum. When sponge titanium and graphite mixture was added, the granular TiC phase, small blocky TiAl3 phase and big blocky Ti2Al20Ce phase formed in the melt. With the holding time, solute Ti and Ce atoms kept moving toward the surface of Ti2Al20Ce phase, as a result, a large number of TiAl3 dissolved, and Ti2Al20Ce grew bigger.Adding 0.4% of Al-3Ti-0.2C (C after Ti) refiner prepared in resistance furnace, the average grain size of pure aluminum was just decreased from 755μm to 374μm, and adding 0.4% of Al-3Ti-0.2C (C, Ti mixed) refiner prepared in resistance furnace, the grain size could be refined to 136pm, while adding 0.4% of Al-3Ti-0.2C-1RE refiner, the grain size was reduced to 133μm. Compared with Al-3Ti-0.2C (C, Ti mixed), the refining effect of Al-3Ti-0.2C-1RE is better. The anti-recessionary of Al-3Ti-0.2C-1RE is also better, and no obvious grain growth can be observed when the holding time reached to 60min.Compared with Al-3Ti-0.2C-1RE (reaction time of 1h), the size of Ti2Al20Ce phase in Al-3Ti-0.2C-1RE (reaction time of 2h) increased significantly, which increased the response time and reduced the refining efficiency. By adding 0.1%, 0.2%,0.4% and 0.8% of the Al-3Ti-0.2C-1RE refiner prepared in resistance furnace, respectively, the grain size of aluminum decreased accordingly. But when the addition was more than 0.4%, the further reduction of the grain size was not obvious.