Abstract The directional solidification technique allows the study of growth of the solid phase, as-cast structure and, finally, its mechanical characteristics as a consequence of thermal parameters. On the other hand, in the last decade, a process called equal-channel angular pressing (ECAP) has emerged as a widely-known technique in fabrication of ultrafine-grained metals and alloys. Applicability of the ECAP technique affords an excellent potential for changing, in a controlled and beneficial manner, the resulting properties of metals and alloys. For this paper, an experimental research has been conducted to study the effects of solidification parameter (cooling rate) on resulting microhardness in hypoeutectic Al-Si alloys, upon use of an ECAP procedure. The influence of cooling on the scale of the dendritic patterns is presented and discussed with recourse to equations. The resulting microhardness variation with position throughout the as-cast materials and cooling rate were characterized by experimental power laws. Results determined after the solidification experiments have revealed microhardness as a function of both cooling rate and position (P) of the as-cast materials to be dependent on alloy composition. In the ECAP process via route C with three passes, “as-solidified” microstructures have been found to be distorted and fragmented during the severe plastic deformation. This deformation imposed on the billets during the ECAP process facilitated obtaining a fine microstructure and high levels of microhardness were observed. However, even with the ECAP process, it was shown that microhardness is strongly dependent of the cooling rates.

Al-Fe alloys are usually used as packaging and structural materials, but in the recent years, there have been considered for possible applications in aerospace field. The solidification sequence in pure aluminum containing 1 wt.% Fe is described in term of the formation of macrostructure, microstructure, microsegregation, porosity and mechanical properties. This material was studied in the upward unidirectional solidification system under transient heat flow conditions. Differences in microstructure, microsegregation, porosity and mechanical properties such as ultimate tensile strength, elongation and microhardness, due to the thermal parameter effects were observed and discussed. Experimental growth laws relating cellular spacing to the cooling rate and solidification speed have been determined, indicating that the increase in thermal parameter have induced a refinement effect on cell morphology. Microsegregation profiles of Fe solute were experimentally determined from the central region of the cell to the intercellular region under different solidification speeds. The Fe microsegregation determined from central region of the cell (FS = 0) to the intercellular region (FS = 1) show a growing profile, in any case considered. However, the profiles move upward with the increase in solidification speed, which indicates that Fe solubility in solid, increases with the increase in solidification speed. The effect of the solidification thermal parameters and cellular spacing on the porosity content were experimentally investigated. The value of porosity content increased along the casting. These results have pointed out that porosity content is affected by solidification parameters and cellular patterns. Further, measurable effects of the thermal parameters, cellular spacing and porosity content on the mechanical properties were experimentally determined. It stands out among experimental results the influence of porosity on the mechanical properties of as-cast material. In any case analyzed, mechanical properties increase with decreasing porosity content.

Abstract Virtually all metals used industrially undergo a solidification process during their production. Depending on the material and its manufacturing process, its physical/mechanical properties are affected to a greater or lesser extent by the microstructure and microsegregation obtained during the phase change. Aluminum casting alloys are good examples of products where this microstructure is vital for obtaining the desired properties. A sequence of experiments to analyze the upward vertical unidirectional solidification with transient heat transfer conditions in Al-Si-Cu ternary alloys was developed in the present work. The experimental results obtained were compared with classical microsegregation models. Discrepancies related to their use for ternary alloys were raised. Since the calculated results by these models do not take into account the influence of one alloying element on the solubility of the other element, disparities were founded between experimental and numerical results. A microsegregation model was proposed based on the solubility limits of the Si in the alloy as a function of the Cu concentration present in the liquid. The model, combined with the concepts of classical microsegregation theory, allows a realistic description of the microsegregation phenomenon. The model showed an excellent agreement between microsegregation profiles of solute experimentally measured and calculated.

Abstract The solidification control is of extreme importance, because it strongly affects the final casting quality sanity. The structure obtained is generally not homogeneous and gives rise to great variations in composition, with position at small and large scales, which is known as segregation. An understanding of the way segregation occurs in continuous casting is of great importance for steels and in designing post-casting processes. As-cast structures are responsible the reduction in both scale and extension of segregation, because mass transport is dependent on the time required to diffuse a solute over a characteristic distance, e.g., the dendrite spacing that the characterizes the solidification structure. In this work, the effect of pouring temperature in steel slabs on the continuous casting processes are systematically investigated. Relationships between pouring temperature (PT) and center macrosegregation was qualitatively examined. Photomicrographs of specimen taken from transverse sections of steel slabs, shows that macrosegregation is strongly affected by pouring temperature (PT). For solute of carbon, phosphorus and sulfur, has been shown that the pouring temperature (PT) has a significant role on the resulting macrosegregation profiles, while that the elements as such silicon, manganese and aluminum, the said thermal parameter seems not able to affect its macrosegregation profiles. This is due to the fact that the solutes with lower partition coefficients favors segregation during the continuous casting process. It is shown for considered steels, the pouring temperature (PT) influences the position of the columnar to equiaxed transition (CET). Experimental results show that the end of the columnar region is abbreviated when lower pouring temperatures is used in continuous casting process. One can observe that as the pouring temperature (PT) increase in continuous casting process, the secondary dendritic arm spacing (λ2) increase, i.e., the dendritic morphology became coarsen.

Abstract Hypoperitectic Cu − 20 wt.% Sn alloy was solidified under different cooling rates and solidification growths using directional solidification system. The effects of cooling rate and solidification growth on the microsegregation profiles and tertiary dendritic arm spacing (λ3) were experimentally investigated along the casting. A mathematical model known as phase-field was applied to simulate microstructure and microsegregation during solidification in system Cu-Sn liquid. In this paper the applicability of the phase-field model to the solidification problem in a real alloy system was systematically explored. Microsegregation profiles and realistic dendritic structures were obtained using the phase-field model. The results calculated by phase-field model show various solidification features consistent with our experiment. The calculated tertiary dendritic arm spacing (λ3) and microsegregation profiles were compared with experimental values from directional solidification system. Since the calculated microsegregation profiles using the equilibrium partition coefficient (keq) can yielded discrepancies from the experimental results, an effective partition coefficient (kef) as a function of solidification growth, is proposed in phase-field model, showing a good agreement with the experimental data for any case examined.

Abstract The solidification of metals and alloys and the resulting microstructures, which as a function of thermal and solutal parameters can evolve as planar, cellular and dendritic, are important from a practical point of view, since they strongly influence the properties and quality of the final product. In many practical situations it is impracticable to develop analytical solutions permitting reliable predictions of microstructural growth during unsteady-state solidification conditions. The Phase Field method has become very popular and effective in modeling complex solid/liquid interfaces due to its ability to simulate the interface kinetics and the formation and evolution of different morphologies along the solidification process. In this work, a numerical analysis of the microstructural evolution during the transient solidification of dilute alloys of the Al-Cu-Si system is developed, which uses a phase-field approach for the simulation of ternary alloys. The phase-field, energy and solute concentration equations were numerically solved for the correspondent ternary system, varying the mesh parameters, temperature and alloy composition. The analysis performed were confronted with existing theoretical models and the results obtained are in agreement with the solidification theory.

The microstructure and mechanical properties of cast materials are strongly dependent on the thermal history during solidification process. The global casting industry has faced major challenges, i.e., customers increasingly demand completely finished cast parts as well as complexity of casting alloys are rising. Therefore, the previous knowledge the solidification conditions effects on the as-cast ingot microstructure and mechanical properties resulting is very useful in the casting industry in order to improve the casting quality. In this present paper, the thermal parameters effects on the microstructure, microhardness and microsegregation of a Cu- 20 wt.% Sn alloy under transient heat flow conditions were experimentally investigated. The experimental observations indicate that the tertiary dendritic arm spacing (λ3), microsegregation and microhardness are affected by the thermal parameters (solidification speed and cooling rate). Solidification speed (SS) associated with increasing cooling rate (RC) are found to contribute to the decreasing tertiary dendritic arm spacing (λ3). However, cooling rates from 11.36 °C/s to 0.65 °C/s were not found to affect significantly the microhardness along the ingot length. The solute concentration (Sn) in solid region were calculated by Scheil and Clyne-Kurz equation, and used in the predictions of microsegregation profiles. The results calculated by equations, have shown deviations from the experimental data. It is well known that it is very difficult to calculate these concentration profiles using the equilibrium partition coefficient, since frequently castings solidify under non-equilibrium conditions and the solidification process is known as non-equilibrium solidification. For this reason, effect of solidification speed (SS) was considered into equations through effective partition coefficient (kef) that has been determined for the range of solidification speed experimentally examined between 0.19 and 0.89 mm/s. However, results calculated by Scheil and Clyne-Kurz equation using the effective partition coefficient (kef), yielded discrepancies from the experimental results. Because of its deviations between calculated and experimental data, an experimental equation with effective partition coefficient (kef), is considered in present work, showing an excellent agreement with the experimental data.

Ternary Al-9.0wt%Si-4.0wt%Cu alloy was solidified in a vertical directional solidification system under unsteady-state heat flow conditions. The resulting dendritic morphology and microsegregation were investigated. A more detailed analysis was dedicated to the microsegregation phenomena where a multielement interaction was observed. The solidification parameters such as: solidification speed (VL) and cooling rate ( T ˙) were determined from the cooling curves obtained during the solidification process. The thermal variables effect on the dendritic morphology is presented. The measurements of tertiary dendrite arm spacing (λ3) and microsegregation were performed for different positions along the casting. The experimental curves for microsegregation were obtained for Si and Cu from the center of dendritic tertiary arm to the next nearest tertiary arm. The solidification speed (VL) influence is "built into" the effective partition coefficient (Kef_Cu and Kef_Si) that has been determined for the range of VL and microsegregation curves are calculated by Scheil's equation for comparison with experimental data. Good agreements of the Scheil's equation with experimental data on microsegregation curves of the Si and Cuwere obtained when effective partition coefficient (Kef_Cu and Kef_Si) is taken into consideration. The multielement interaction effect on the Si microsegregation is investigated. Experimental results show that, Cu-rich dendrites were accompanied by minute amounts of Si. The concentration profiles obtained experimentally point to a strong negative correlation between Si and VL on ternary Al-9.0wt%Si-4.0wt%Cu alloy.

Aluminium with silicon as alloying element, form a class of material providing the most significant part of all material casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries. It is widely recognized that moderate addition of silicon to aluminum, significantly improves the resulting mechanical properties. Hypoeutectic Al-Si alloys were used in the present experimental study to investigate the solidification parameters effect and Si 3-5 wt% addition on the microstructural features and resulting hardness in a vertical directional solidification system. The hypoeutectic alloys were directionally solidified under transient heat flow conditions in a range of cooling rates from 0.3 to 4 ºC/s. Characterization analyses by optical microscopy indicate clearly that secondary dendritic arm spacing (λ2) increases significantly with the decrease in solidification speed (SP) and cooling rate (Ṫ). On the other hand, experimental growth laws relating the secondary dendritic arm spacing (λ2) to the solidification speed (SP) and cooling rate (Ṫ) indicate that Si addition from 3 wt% to 5 wt% has induced a thickening effect leading to increase in λ2 . While, experimental results have shown that the resulting hardness increase as solidification processing parameters (SP and Ṫ ) increase. Results of Vickers hardness test for Al-5 wt% Si alloy has hardness increase of 18 % from that of Al-3 wt% Si alloy with mean values of 26 and 22 HV, respectively.

Prediction of microstructure evolution and microsegregation is one of the most important problems in materials science. The dendritic growth and microsegregation provide a challenging simulation goal for computational models of solidification, in addition to being an important technological feature of many casting processes. The phase-field model offers the prospect of being able to perform realistic simulation experiments on dendrite growth in metallic systems. In this paper, the microsegregation and dendritic growth of hypoeutectic Al-Cu alloys under constant cooling rate was simulated using a phase-field model. The main new feature of the present model is based on the fact that the effect of the growth rate is incorporated via an effective partition coefficient that has been experimentally determined for a range of growth rates. It is shown that both models (Phase-field model and Scheil) have significant deviations from the experimental data when the equilibrium partition coefficient is considered in the calculations. Since the predicted results using the models yielded discrepancies from the experimental data, an experimental equation is adopted for calculating the effective partition coefficient from experimental data. The experimental equation is then adopted in the calculations of phase-field model and Scheil's equation, showing a good agreement with the experimental data.

Mechanical properties and performance of metallic materials depend on their microstructures. In order to develop engineering materials that match prescribed criteria and to enable design with multifunctional materials, it is essential to be able to predict their microstructural patterns, such as size, shape, and spacing of the dendritic structures observed in solidified metals. In the cases of metallic alloys, which present dendritic structure, the mechanical properties of foundry products depend mainly on the primary- and secondary-arm. Therefore, it is important, in a computational simulation of the solidification processes, to use reliable methods to correlate the thermal parameters with secondary-dendrite arm spacing. This study presents a numerical model for prediction of secondary-arm spacing as a function of thermal parameters (cooling rates and local solidification time). Spacing of the arms for a binary alloy is numerically predicted using a phase-field model. Secondary dendrites calculated by phase-field model, they are similar to the ones found in experiments investigation of solidification in Al-Cu alloys. Arm spacing predicted in the present work, when compared with the experimental results, showed good agreement. Its estimation takes place at the late stage of growth. The effect of physical properties (partition coefficient (ke), diffusion in the liquid (DL) and diffusion in the solid phase (DS)) on secondary-arm spacing is systematically investigated by phase-field model. With the help of numerical results for Al-4.5wt%Cu alloy, the applicability of the phase-field model to the estimation of secondary-dendrite arm spacing during unidirectional solidification is demonstrated.

A phenomenological model to predict the multiphase diffusional decomposition of the austenite in low-alloy hypoeutectoid steels was adapted for welding conditions. The kinetics of phase transformations coupled with the heat transfer phenomena was numerically implemented using the Finite Volume Method (FVM) in a computational code. The model was applied to simulate the welding of a commercial type of low-alloy hypoeutectoid steel, making it possible to track the phase formations and to predict the volume fractions of ferrite, pearlite and bainite at the heat-affected zone (HAZ). The volume fraction of martensite was calculated using a novel kinetic model based on the optimization of the well-known Koistinen-Marburger model. Results were confronted with the predictions provided by the continuous cooling transformation (CCT) diagram for the investigated steel, allowing the use of the proposed methodology for the microstructure and hardness predictions at the HAZ of low-alloy hypoeutectoid steels.

The phase-field methods were developed mainly for studying solidification of pure materials, being then extended to the solidification of alloys. In spite of phase-field models being suitable for simulating solidification processes, they suffer from low computational efficiency. In this study, we present a numerical technique for the improvement of computational efficiency for computation of microstructural evolution for both pure metal and binary alloy during solidification process. The goal of this technique is for the computational domain to grow around the microstructure and fixed the grid spacing, while solidification advances into the liquid region. In the numerical simulations of pure metal, the phase-field model is based on the energy and phase equations, while, for binary alloy, the said model is based on the concentration and phase equations. Since the thermal diffusivity in the energy equation is much larger than the diffusivity term in phase equation in pure metal system, about twenty eight times the difference between them. The computational domain growth around the microstructure is controlled according with the thermal diffusivity for pure material in the liquid region. In the numerical simulation of dendritic evolution of Fe-C alloy, the idea is similar, i.e., the solute diffusivity in concentration equation is larger than the diffusivity term in phase equation in the liquid region, in this case eleven times the difference in Fe-C alloy system. The computational domain growth is controlled via solute diffusivity in the liquid region. Hence, phase-field model is proposed with an adaptive computational domain for efficient computational simulation of the dendritic growth in a system for both pure metal and binary alloy. The technique enables us to reduce by about an order of magnitude the run time for simulation of the solidification process. The results showed that the microstructure with well-developed secondary arms can be obtained with low computation time.

In this paper, we present a phase-field model with a grid based on the Finite-Difference Method, for improvement of computational efficiency and reducing the memory size requirement. The numerical technique, which is based on the temperature change of the pure material, enables us to use, in the initial steps of the computation, a very small computational domain. Subsequently, in the course of the simulation of the solidification process, the computation domain expands around the dendrite. The computation showed that the dendrite with well-developed secondary arms can be obtained with low computation time and moderate memory demand. The computational efficiency of this numerical technique, the microstructural evolution during the solidification, and competitive growth between side-branches are discussed.

A phase-field model is proposed for the simulation of microstructure and solute concentration during the solidification process of Fe-C-P ternary alloys. A relation between material properties and model parameters is presented. Two-dimensional computation results exhibit dendrites in Fe-C-P alloys for different phosphorus concentrations. Alterations in the phosphorus concentration appear to affect the advance speed of the solid-liquid interface. Such an alteration is due to the small diffusivity of phosphorus during the solidification process.