Solidification Of Al-Si Alloys Research Articles

Effect of Multi-Scale Thermoelectric Magnetic Convection on Solidification Microstructure in Directionally Solidified Al-Si Alloys Under a Transverse Magnetic Field

The influence of a transverse magnetic field (B < 1 T) on the solidification structure in directionally solidified Al-Si alloys was investigated. Experimental results indicate that the magnetic field caused macrosegregation, dendrite refinement, and a decrease in the length of the mushy zone in both Al-7 wt pct Si alloy and Al-7 wt pct Si-1 wt pct Fe alloys. Moreover, the application of the magnetic field is capable of separating the Fe-rich intermetallic phases from Al-7 wt pct Si-1 wt pct Fe alloy. Thermoelectric magnetic convection (TEMC) was numerically simulated during the directional solidification of Al-Si alloys. The results reveal that the TEMC increases to a maximum (\( u_{\rm{max} } \)) when the magnetic field reaches a critical magnetic field strength (\( B_{\rm{max} } \)), and then decreases as the magnetic field strength increases further. The TEMC exhibits the multi-scales effects: the \( u_{\rm{max} } \) and \( B_{\rm{max} } \) values are different at various scales, with \( u_{\rm{max} } \) decreasing and \( B_{\rm{max} } \) increasing as the scale decreases. The modification of the solidification structure under the magnetic field should be attributed to the TEMC on the sample and dendrite scales.

An Evaluation of Primary Dendrite Trunk Diameters in Directionally Solidified Al-Si Alloys

The primary dendrite trunk diameters of Al-Si alloys that were directionally solidified over a range of processing conditions have been evaluated. The empirical data are analyzed with a model that is an extension of one used to describe the ripening of dendrite arms. The analysis, based primarily on an assessment of secondary dendrite arm dissolution in the mushy zone, fits well with the experimental data. It is suggested that the primary dendrite trunk diameter is a useful metric that correlates well with the actual solidification processing parameters, and complements the conventionally used primary, secondary, and tertiary arm spacings.

Flow Control during Solidification of AlSi-Alloys by Means of Tailored AC Magnetic Fields and the Impact on the Mechanical Properties

This paper presents an experimental study which in a first stage is focused on obtaining quantitative information about the isothermal flow field exposed to various magnetic field configurations. Melt stirring has been realized by utilizing a rotating magnetic field. In a second step directional solidification of AlSi7 alloys from a water-cooled copper chill was carried out to verifythe effect of a certain flow field on the solidification process and on the resulting mechanical properties. The solidified structure was reviewed in comparison to an unaffected solidified ingot. Measurements of the phase distribution, the grain size, the hardness and the tensile strength were realized. Our results demonstrate the potential of magnetic fields to control the grain size, the formation of segregation freckles and the mechanical properties. In particular, time–modulated rotating fields show their capability to homogenize both the grain size distribution and the corresponding mechanical properties.

The columnar to equiaxed transition of horizontal unsteady-state directionally solidified Al-Si alloys

Experiments were conducted to investigate the influence of thermal parameters on the columnar to equiaxed transition during the horizontal unsteady-state directional solidification of Al-Si alloys. The parameters analyzed include the heat transfer coefficients, growth rates, cooling rates, temperature gradients and composition. A combined theoretical and experimental approach is developed to determine the solidification thermal variables considered. The increasing solute content in Al-Si alloys was not found to affect significantly the experimental position of the CET which occurred for cooling rates in the range between 0.35 and 0.64 K/s for any of three alloy compositions examined. A comparative analysis between the results of this work and those from the literature proposed to analyze the CET during upward vertical solidification of Al-Si alloys is reported and the results have shown that the end of the columnar region during horizontal directional solidification is abbreviated as a result of about six times higher thermal gradient than that verified during upward unidirectional solidification of alloys investigated.

Effects of strontium impurity on the structure and dynamics of Al88Si12 liquid

The effects of strontium modification on the structure and dynamics of Al88Si12 liquid are studied by means of ab initio molecular dynamics simulations. By replacing 0.5% and 4.0% of Al with Sr, we show that the addition of Sr lowers the self-diffusion of Al and Si of the liquid and reduces the nearest-neighbor correlation between Si atoms. The simulation provides an explanation for the change in morphology of the eutectic phases observed in rapidly solidified Al–Si alloys modified with Sr.

Effect of solidification conditions on fractal dimension of dendrites

Dendrites are complex, three-dimensional structures that have conventionally been characterized by measuring the secondary dendrite arm spacing or the primary spacing in a dendritic network, but these global measures do not adequately describe the branched appearance of secondary and tertiary arms. This work focuses on the integral measurement of fractal dimension, a measure of complexity relatively unexplored in dendrites, in addition to specific surface area. Measurements were made on aluminum dendrites in directionally solidified Al–Si alloys of varying composition and solidification velocity, with and without induced convection currents. Contrary to expectations, average fractal dimension was found to be relatively insensitive to changes in solidification velocity and fluid flow within the ranges observed, compared to the variation in fractal dimension measured within any individual data set. Specific surface area was found to increase linearly with solidification velocity.

Comparison of Thermal Analysis and Differential Thermal Analysis for Evaluating Solid Fraction Evolution during Solidification of Al-Si Alloys

Both thermal analysis (TA) and differential thermal analysis (DTA) have been used since long to evaluate latent heat release and solid fraction evolution during solidification of metallic alloys. TA makes use of cooling curves recorded under "natural" cooling while DTA consists in recording the temperature difference between the sample temperature and an inert reference during a controlled cooling, i.e. at imposed constant cooling rate. In both cases, the solid fraction evolution is deduced from a calculation of the latent heat release as estimated by means of a heat transfer model. This paper provides a comparison of such evaluations performed on one Al-Si alloy.

Functionally Graded Al Alloy Matrix In-Situ Composites

In the present work, functionally graded (FG) aluminum alloy matrix in-situ composites (FG-AMCs) with TiB2 and TiC reinforcements were synthesized using the horizontal centrifugal casting process. A commercial Al-Si alloy (A356) and an Al-Cu alloy were used as matrices in the present study. The material parameters (such as matrix and reinforcement type) and process parameters (such as mold temperature, mold speed, and melt stirring) were found to influence the gradient in the FG-AMCs. Detailed microstructural analysis of the composites in different processing conditions revealed that the gradients in the reinforcement modify the microstructure and hardness of the Al alloy. The segregated in-situ formed TiB2 and TiC particles change the morphology of Si particles during the solidification of Al-Si alloy. A maximum of 20 vol pct of reinforcement at the surface was achieved by this process in the Al-4Cu-TiB2 system. The stirring of the melt before pouring causes the reinforcement particles to segregate at the periphery of the casting, while in the absence of such stirring, the particles are segregated at the interior of the casting.

Effects of Fe content and cooling rate on the phase formation during the solidification of Al-Si alloys

Effects of Fe content and cooling rate on the phase formation during the solidification of Al-Si alloys

Influence of forced convection on the directional solidification of AlSi alloys: comparison of experiments and simulation

In order to predict the formation of dendritic microstructure the software package CrysMAS, developed at Fraunhofer IISB, is extended using a multi-scale model based on the work of Wang and Beckermann (1996). The model is validated, by applying it to the directional solidification of AlSi7 under the influence of a Rotating Magnetic Field (RMF). The calculated results are compared to experiments conducted by MICAST partners (Steinbach, 2005). A good correlation between simulation and experiment is found for the macrosegregation caused by the RMF. However, the calculated results for the primary dendrite spacing (PDS) do not correlate well with experimental results.

Modelling of grain size transition with alloy concentration in solidified Al–Si alloys

The transition in grain size with Si content in Al-Si alloys has been systematically investigated by the Cellular Automaton-Finite control Volume Method (CAFVM) to understand the operating mechanisms for this behavior. Three aspects: growth restriction factor (GRF), the chemical driving force (CDF) and the constitutional undercooling (Delta T-C) have been demonstrated to affect the microstructure formation, and among them the Delta T-C plays the most important role. Furthermore, it is also shown that the surface modification of the nucleant particles by silicon significantly influences the grain formation. However, the combined effects of the investigated factors on the grain size were not sufficiently strong to cause a grain size change similar to that observed experimentally. This implies that there could be other mechanisms that control the transition.

Characterization of tensile properties and microstructures in directionally solidified Al–Si alloys using linear roughness index

The tensile properties in directionally solidified Al–Si alloys and in gravity die casting of the same composition are presented. Examples of relating both the tensile and the microstructural properties of these alloys with the fracture roughness index are indicated. The roughness index was measured on vertical sections cut through the tensile fracture surface. The tensile properties examined were the yield and ultimate tensile strengths, strain at fracture, and Young's modulus. The analyzed microstructural features were porosity, dendrite area fraction, secondary dendrite arm spacing, and Si particle spacing. In almost all cases an unambiguous correlation was found between the roughness index and the tensile or the microstructural properties. A marked improvement in ductility was observed for directionally solidified samples over their gravity die casting processed counterparts. The roughness index diminished going from die cast to direction solidification, and this is likely accompanied by change in the fracture mechanisms.

Solidification of AlSi Alloys in the ARTEMIS and ARTEX Facilities Including Rotating Magnetic Fields – A Combined Experimental and Numerical Analysis

The solidification microstructure is the consequence of a wide range of process parameters, like the growth velocity, the temperature gradient and the composition. Although the influence of these parameters is nowadays considerably well understood, an overall theory of the influence of convection on microstructural features is still lacking. The application of time dependent magnetic fields during directional solidification offers the possibility to create defined solidification and flow conditions. In this work, we report about solidification experiments in the ARTEMIS and ARTEX facilities including rotating magnetic fields (RMF). The effect of the forced melt flow on microstructural parameters like the primary and secondary dendrite arm spacing is analyzed for a wide range of magnetic field parameters. The experimental analysis is supported by a rigorous application of numerical modeling. An important issue is hereby the prediction of the resulting macrosegregation, i.e., differences in the composition on the scale of the sample (macroscale) due to the RMF. For the considered configuration and parameters an axial enrichment of Si is found beyond a certain magnetic field strength. The results are compared to available theories and their applicability is discussed.

Modeling of Columnar-to-Equiaxed Transition in Solidified Al-Si Alloys

A Cellular-Automaton Finite-Volume-Method (CAFVM) algorithm has been developed, coupling with macroscopic model for heat transfer calculation and microscopic models for nucleation and growth. The solution equations have been solved to determine the timedependent constitutional undercooling and interface retardation during solidification. The constitutional undercooling is then coupled into the CAFVM algorithm to investigate both the effects of thermal and constitutional undercooling on columnar growth and crystal selection in the columnar zone, and formation of equiaxed crystals in the bulk liquid. The model cannot only simulate microstructures of alloys but also investigates nucleation mechanisms and growth kinetics of alloys solidified with various solute concentrations and solidification morphologies.

Quantitative Validation of Microstructure Simulation in Case of Unidirectionally Solidified Al-Si Alloys

The paper focuses on the microstructural investigation of Al-Si binary alloys solidified under steady- and non-steady state conditions using a unidirectional heat removal. A brief description of the developed solidification simulation software and its validation by computer aided image analysis are also given.

Chemical inhomogeneity of intermetallic phases precipitates formed during solidification of Al-Si alloys

In this work, the results of an examination of the solidification process of the AlSi6 and AlSi12 alloys as influenced by the content of the transition metals, Fe, Mn and Cr, are presented. The concentration of these elements in alloys prepared from pure components has been changed in the following range: Fe 0.5–3 wt.%, Mn 0–2 wt.% and Cr −0.3 wt.%. The thermal effects during solidification of alloys at a cooling rate of 3.3 K/min (between liquidus and solidus) connected with forming of structural components of alloy were determined. The temperature of the phase transformations was evaluated and correlated with chemical composition of alloys. It was stated that, on an average, an increase in Fe and Mn content of about 1 wt.% increases the temperature of primary proeutectic iron-bearing phase precipitation by about 40 K (in the examined range of the alloys compositions). By means of microscopic observations of the alloy microstructure, the morphology of the primary phase precipitates frozen at a chosen temperature was estimated. On the basis of the results of X-ray microanalysis (EDS), the chemical composition of the primary iron-bearing phases was analysed, and a relation among the components (i.e., Al, Si, Fe, Mn and Cr) was established. The sequence of microstructural component formation during solidification of the alloys examined was presented.

A model of the interfacial heat-transfer coefficient for the aluminum gravity die-casting process

Interfacial heat-transfer coefficients were measured during the solidification of Al-Si alloys against coated die steel chills with varying chill temperature, coating thickness and coating type. Two principal resistances to heat transfer across the casting-chill interface were identified, namely, (1) the resistance to heat transfer of the coating itself and (2) the resistance to heat transfer of a layer of gas, (assumed to be air), trapped between the coating and casting surfaces by virtue of their roughness. These thermal resistances were evaluated by measurement of the coating thermal conductivity and determination of the thickness of the applied coatings and the thickness of the layer of air between the coating and casting surfaces. This produced a simple equation to predict the interfacial heat-transfer coefficient during the solidification of Al alloy die castings, which produced values that were found to agree well with the experimentally determined results. This equation was used to interpret the experimentally measured heat-transfer coefficients and to explain their variation with the different experimental conditions employed. A simple modification of the equation can also take into account the formation of an air gap, where the casting locally retreats away from the die surface, leading to a local reduction in the heat-transfer coefficient.

Numerical study on directional solidification of AlSi alloys with rotating magnetic fields under microgravity conditions

A global thermal model of the ARTEX facility, developed with the modeling tool CrysVUn, is presented. The model is validated with experimental data obtained under microgravity conditions during the TEXUS39 mission. Numerical studies are then reported, in which the effects of a rotating magnetic field are investigated during directional solidification of binary AlSi7 alloys. It appears that beyond a certain magnetic field strength a macrosegregation effect is resulting, leading to the development of a liquid channel inside the mushy zone.

Evolution of primary phase in solidification of Al-Si alloys

The morphology of primary phase in solidification of hypo-eutectic Al-Si alloys has been investigated by both experiment and numerical simulation using the modified cellular automaton(MCA) method. In order to generate the globular structure without any thermo-mechanical stirring, the effects of pouring temperature, cooling rate and inoculation on the evolution of globular structure were examined. The optimum condition for obtaining uniform and fine globular structure is that the pouring temperature should be as closely low as to the liquidus and the cooling rate should be less than 0.3°C/sec. As the pouring temperature and the cooling rate increase, the primary phase transforms from globular to rosette-like, and finally to dendritic structure. The inoculation with nucleation potency is also very effective in forming a globular structure. The dominant mechanism responsible for the evolution of globular structure was found to be the increase of nucleation sites due to the lower pouring temperature, inoculation, and uniform solute distribution due to the lower cooling rate.

Microstructures and tensile properties of an A1-12 wt pct Si alloy produced by reciprocating extrusion

A reciprocating extrusion (RE) process has been developed for producing A1-12 wt pct Si bulk alloys with fine and uniform microstructures and superior properties. Two starting forms were used: disks produced by the hammer-and-anvil method and cast billets produced by casting. Variations of micro-structure and mechanical properties with the number of extrusion passes are investigated for these two starting forms. The results show that the porosity along the interfaces between the rapidly solidified layers could be completely eliminated to give a sound matrix. The Si-phase particles in both cases could be refined and distributed uniformly. The strength and ductility of all specimens are also enhanced, until the microstructure reaches an optimum state, as the number of extrusion passes increases. The tensile properties of the rapidly solidified Al-Si alloys are found to be superior to those of ingot-processed alloys, due to the inherent finer particles produced by rapid solidification. The mechanism for the improvement of the microstructures and properties is also discussed.