Experimental and numerical modelling study on the nucleation and grain growth of inoculated aluminium alloys
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With the aim to achieve an in-depth understanding on the heterogeneous nucleation and grain growth behavior during solidification of inoculated aluminum alloys, an integrated study by in-situ X-radiography, numerical modeling, and thermodynamic calculation was carried out in the present thesis. By in-situ isothermal melt solidification experiments of Al-5Ti-1B inoculated Al-10Cu alloys, the kinetics of heterogeneous nucleation and grain growth under the isolated influence of cooling rate and addition level of inoculant particles has been quantitatively studied. A special focus has been put on the influence of solute segregation around growing grains on the nucleation kinetics and nucleation ceasing mechanism. It proves that, when recalescence is absent, the nucleation process stops due to solute segregation stifling. Moreover, based on novel approaches of image processing and analyzing, the maximum nucleation undercooling and solid volume fraction at nucleation ceasing have been determined. To better understand the heterogeneous nucleation and grain growth behaviors under the insitu isothermal melt solidification experiment conditions, a new grain size prediction model in which both pure globular growth kinetics and dendritic growth kinetics including spherical/globular to dendritic transition (GDT) has been developed. The quantitative agreements between the simulation results and experimental results in terms of grain size, maximum nucleation undercooling and solid fraction at nucleation ceasing, have confirmed the validity of the solute segregation stifling mechanism for castings without recalescence. Furthermore, it is demonstrated that globular growth kinetics is an acceptable approximation for grain size prediction purposes of well grain-refined aluminum alloys. However, for poorly inoculated aluminum alloys with well-developed coarse dendritic grains, an application of dendritic growth kinetics significantly improves the grain size prediction power of the model. To reveal the effect of temperature gradient on the heterogeneous nucleation, an in-situ Xradiographic study on the directional solidification of an inoculated Al-20wt.%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also has the influences of promoting the preferential growth of dendrite arms along the temperature gradient direction and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is also revealed that the propagation velocity of nucleation front during directional solidification is approximately equal to the ratio between cooling rate Ť and temperature gradient G . Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’(INZ) and ‘active nucleation zone’(ANZ). The model has been applied to simulate the in-situ directional solidification experiments and a good agreement was achieved between the predicted grain number density and the experimental results. Solute content is known to have significant effects on grain refinement of Al alloys during solidification. In this thesis, we also investigated the influence of solute content on the thermodynamic nucleation driving force and solid-liquid interfacial energy of binary Al alloys by CALPHAD method. The solute effect on the nucleation barrier and nucleation rate, thus on the grain refinement of Al alloys both with and without high potency nucleation particles, was analyzed based on the heterogeneous nucleation theory and free growth concept. For uninoculated Al alloys, the calculation results indicate that Si has the effect of increasing the nucleation barrier and thus reducing the nucleation rate while Cu and Mg promote heterogeneous nucleation and grain refinement. However, peritectic forming elements, e.g., Ti, Zr, V, has only a marginal effect on the nucleation barrier. For solidification of Al alloys with high potency nucleation particles, it is revealed that alloying elements Cu, Mg, and Si have the influence of promoting the grain refinement by reducing the free growth undercooling. However, this effect is small in comparison to the growth restriction effects of the solute elements.