Materials Science Forum Vols. 790-791

Abstract: Melting of a single-phase polycrystalline material is known to start by the formation of liquid films at the surface and at grain boundaries. The internal liquid films are not necessarily quiescent, but can migrate to avoid/reduce supersaturation in the solid phase. The migration is discussed in the literature to be governed by coherency strains of the solid/liquid interface, by concentration gradients in the liquid or by concentration gradients in the solid phase. A phase transformation model for diffusional phase transformations considering interface thermodynamics (possible deviations from local deviations) has been put up to describe the migration of the solid/liquid (trailing) and the liquid/solid (leading) interfaces of the liquid film. New experimental results on melting in a temperature gradient in combination with simulation calculations reveal that concentration fluctuations in the liquid phase trigger the liquid film migration and determine the migration direction, until after a short time in the order of microseconds the process is governed by diffusion in the solid phase.

Abstract: In solidification experiments of binary eutectic alloys, the eutectic spacing and undercooling are measured as function of the solidification rate. A new theoretical relationship is derived herewith between the Gibbs-Thomson coefficient and the above mentioned values for both lamellar and rod type eutectics. This new equation allows the estimation of the interfacial energy between eutectic solid phases. For the Sn/Pb eutectics the value of about 0.15 N/m is found in this paper using experimental literature data on eutectic solidification experiments. This is consistent with an earlier value obtained by a more complex experimental method of Gündüz and Hunt.

Abstract: Formation of the shrinkage defects in ductile iron castings is far more complicated phenomenon than in other casting alloys. In the presented paper changes the ductile iron density during solidification is analyzed. During the solidification path the influence of the temperature, phase fractions and phase composition is taking into account. Computer model, using cellular automata method, for estimation of changes in density of ductile iron during its solidification is applied. Results of the solidification modeling for Fe-C binary alloys with different composition in the castings with a different wall thickness are presented. As a result of calculations it was stated that after undercooling ductile iron below liquidus temperature volumetric changes proceed in three stages: pre-eutectic shrinkage (minimal in eutectic cast iron), eutectic expansion and the last shrinkage.

Abstract: Quality demands of castings is elemental in recent years and becoming more stringent. Foundries are faced by the need to produce high quality castings, but nonetheless to produce them economically. To fulfill this demand, experimental castings, especially in case of individual production or small runs, are uneconomical. In these cases computer simulation can be the only economical tool for testing and approval. The modulus technique of Chvorinov is an applicable method to examine the local solidification process and to determine the application of chills, geometry modification and feeding. Numerically the control volume method is used to check the solidification process of large steel castings with non-uniform wall thicknesses.

Abstract: Growth restriction plays an important role in grain refinement and is often linked with as-cast grain size of Al-Alloys. It has been suggested that Ti is the most powerful solute element for growth restriction among all the commonly used alloying elements. In this work, the growth restriction effect of Ti on the grain refinement of high purity Al (HP-Al, 99.99%) and commercial purity Al (CP-Al, 99.7%) has been investigated using the Alcan TP-1 tests. Grain refining tests were conducted with the same inoculation of potent TiB₂ at a fixed level and free Titanium addition. The results showed that, when the TiB₂ inoculation was fixed to be equivalent to the particle number density of 0.2% Al-5Ti-1B addition, CP-Al has a fully equiaxed grain structure with only 46 ppm solute Ti, while HP-Al has a fully equiaxed grain structure with much higher Ti addition (960 ppm).

Abstract: DHP-Cu has been modified with small additions of nanosized MgO particles, with and without P and B additions; and Mg, B, Ti and Al additions to investigate their effect on the grain structure under TP1 casting condition. In comparison to a reference DHP-Cu cast sample which exhibited mainly coarse columnar grains, a coarse equiaxed grain structure has been achieved with a single addition of nanosized MgO particles to a residual content of 34 ppm Mg. Whereas no grain refinement has been observed with nanosized MgO particles and P and B additions to a residual content of 109 ppm and 0.15% (nominal), respectively, and with a single addition of Mg to a residual content of 16 ppm. The combined additions of B and Ti, and B and Al to a nominal residual content slightly beyond the DHP-Cu specification have produced excessive coarsening of grains. Keywords: nanosized MgO particles; grain refinement; DHP-Cu

Abstract: The interaction between Zr addition and intensive melt shearing on grain refinement of commercial purity Mg has been investigated experimentally. It was found that, without intensive melt shearing, the grain structure of Mg is changed from columnar grains to equiaxed grains with the increase in Zr concentration. However, with intensive melt shearing, the grain structure of Mg undergoes a complex change as a function of Zr concentration; for instance, the grain structure showed equiaxed grains at 0.1wt. % Zr concentration, while it was changed to columnar grains at 1wt. % Zr concentration. It was noted that, particularly, under the intensive melt shearing condition, the grain size of Mg with minor addition of Zr (0.1%) was further decreased to 134 ± 4 μm compared with that of Mg without Zr addition (217 ± 15 μm).

Abstract: In the present work, ternary Al-1Ti-3B master alloys were prepared in an induction furnace via salt route which involves reaction between preheated halide salts (K₂TiF₆ and KBF₄) and liquid molten Al. During preparation process parameters such as reaction temperatures of 800, 900, 1000°C and reaction times 45, 60, 75 min. have been used to study the influence of these parameters on the morphology of particles present in the master alloy and inturn on the grain refining efficiency of Al-7Si alloy. The indigenously prepared master alloys were characterized by chemical analysis; particles size analysis, XRD and SEM/EDX microanalysis. Results of particle size analysis suggest that the sizes of the particles present in Al-1Ti-3B master alloys increases with increase in reaction temperature (800-1000°C) and reaction time (45-75 min.). However, the population of the particles having sizes less than 10µm decreases with increase in reaction time and temperature. Further, SEM/EDX studies revealed different morphologies of the particles present in the master alloy when processed at different reaction temperatures and reaction times. Further, the performances of the above-prepared master alloys were assessed for their grain refining efficiency on Al-7Si alloy by DAS analysis and by CACCA studies. Results of grain refinement studies and CACCA studies suggest that, Al-1Ti-3B master alloy prepared at reaction temperature of 800°C with a reaction time of 60min. shows better grain refining performance on Al-7Si alloy when compared to the same master alloy prepared under different processing conditions.

Abstract: Prediction of the grain size of inoculated aluminium alloy castings is of great interest for both academic research and industrial application. In this paper, by reviewing the grain size prediction models, the important influence of solute diffusion layer in front of the grain growth front on the final grain size has been addressed. An analytical approach to calculate the thickness of the solute diffusion layer in the melt is proposed, based on which the scale of diffusion layer thickness is evaluated.

Abstract: In this paper, an extendedMaxwell-Hellawell numerical grain size prediction model is employed to predictas-cast grain size of inoculated aluminum alloys. Given melt composition,inoculation and cooling conditions, the model is able to predict maximumnucleation undercooling, cooling curve and final as-cast grain size of multi-componentalloys. The proposed model has been applied to various binary andmulticomponent alloys. Upon analyzing the numerical simulation results, it isfound that for both binary and multi-component alloys, grain size does not havea one-to-one relation with Growth Restriction Factor, Q, but has a clear ubiquitous correlation with the average diffusivity-weightedQ, defined as W in this paper. This founding helps solve the controversy seen inthe recent work on analytical grain size and Q relations. It also has been used to interpret the scatters seenin the measured grain sizes as a function of Q values reported in the literature.