S.L. Cockcroft

Advanced light metals casting development: solidification of aluminium alloy A356

Abstract Microstructural reactions during solidification of aluminium alloy A356 were analysed using cooling curve analysis to determine the evolution in fraction of solid. The experiments involved recording the temperature/time data under different cooling rates so that the various phase changes associated with solidification could be identified in both unmodified and strontium modified versions of the alloy. Once collected, the cooling curve data were processed to calculate the first derivative together with a baseline cooling curve (cooling curve in the absence of any transformations). Comparison of the baseline cooling curve with the first derivative data allowed accurate identification of start temperatures of the various liquid to solid transformations and characterisation of fraction solidified during the cooling process. Strontium modification altered the morphology of A356, decreased the precipitation temperature of new phases (2°C to 11°C), and increased the solid fraction of the beginning of eutectic and Mg2Si precipitation (0.01 to 0.05). Cooling rate appeared to have no effect on solid fraction evolution but an increase in cooling rate reduced solidus temperature (48°C to 75°C).

Modeling of microporosity formation in A356 aluminum alloy casting

A numerical model for predicting microporosity formation in aluminum castings has been developed, which describes the redistribution of hydrogen between solid and liquid phases, the transport of hydrogen in liquid by diffusion, and Darcy flow in the mushy zone. For simulating the nucleation of hydrogen pores, the initial pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas pore growth is based on the assumption that hydrogen activity within the pore and the liquid are in equilibrium. One of the key features of the model is that it uses a two-stage approach for porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than the liquid encapsulation point, the fraction porosity is calculated based on the volume of liquid trapped within the continuous solid network, which is estimated using a correlation based on the Niyama parameter. The porosity model is used in conjunction with a thermal model solved using the commercial finite-element package ABAQUS. The parameters influencing the formation of microporosity are discussed including a means to describe the supersaturation of hydrogen necessary for pore nucleation. The model has been applied to examine the evolution of porosity in a series of experimental samples cast using unmodified A356 in which the initial hydrogen content was varied from 0.048 to 0.137 (cc/100 g). A comparison between the model predictions and the experimental measurements indicates good agreement in terms of the variation in porosity with distance from the chill and the variation resulting from initial hydrogen content.

Effect of cooling rate on solidification characteristics of aluminium alloy AA 5182

Abstract Microstructural reactions during solidification of aluminium alloy AA 5182 were investigated using cooling curve analysis to determine the start temperature of the various transformations and the overall evolution in solid fraction. Three cooling rates were studied: 0.5 K s-1, 1 K s-1 and 2 K s-1. The key finding of the study is that the main eutectic reaction and subsequent reactions, including Mg2Si formation, occur at much lower temperatures and higher solid fractions than previously published. The primary eutectic was found to precipitate between 575 and 588 °C, which corresponds to a solid fraction between 0.87 and 0.91, and Mg2Si was found to precipitate between 551 and 560 °C, which corresponds to a solid fraction between 0.96 and 0.97. Increasing cooling rate was observed to result in a slight increase in solid fraction for primary eutectic precipitation from 0.87 - 0.88 at the low cooling rate to 0.89 - 0.91 at the intermediate cooling rate and 0.91 at the highest cooling rate. The highest cooling rate also resulted in a drop in solidus temperature to 461 °C from 500 - 510 °C (at the low and intermediate cooling rates) that led to an increase in the solidification interval from 123 K to 151 K.

Simulation of microporosity in A356 aluminium alloy castings

Abstract A numerical model for predicting microporosity formation in aluminium castings has been developed, in which the redistribution of hydrogen between solid and liquid phases, and Darcy flow in the mushy zone were taken into account. For simulating the nucleation and growth of hydrogen pores, the pore radius is assumed to be a function of the secondary dendrite arm spacing, whereas the pore growth assumes equilibrium hydrogen activity between pore, liquid and solid. One of the key features of the model is that it uses a two-stage approach to porosity prediction. In the first stage, the volume fraction of porosity is calculated based on the reduced pressure, whereas, in the second stage, at fractions solid greater than grain coalescence f SC, an increment in the fraction porosity is calculated based on the volume of liquid trapped within a continuous solid network, which is estimated using a correlation based on the Niyama criterion. The model has been implemented within the commercial software package, ABAQUS, which has been used as a platform to solve the thermal field. The numerical model has been applied to a simple cylinder-shaped test casting, and the simulated results have been evaluated by comparing with the experimental results.

The effect of water ejection and water incursion on the evolution of thermal field during the start-up phase of the direct chill casting process

Abstract A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the direct chill casting process. The model, based on the commercial finite element package ABAQUS, includes primary cooling to the mould, secondary cooling to water and ingot base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of surface temperature, water flow rate and position relative to the point of water impingement. In addition, the secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat extraction rates). The algorithm used to describe ingot base cooling, includes the drop in contact heat transfer due to base deformation (butt curl) and also the increase in heat transfer due to water incursion between the ingot base and the bottom block. The model has been verified against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different conditions (non-typical “cold” practice and non-typical “hot” practice). Ingot base deflection data has also been obtained for the two test castings. Comparison of the model predictions with the data collected from the embedded thermocouples indicates that a 2-D longitudinal model is capable of describing the flow of heat in the early stages of the casting process in a region close to the centre of the rolling face. A sensitivity analysis completed with the model has clearly identified the link between ingot base cooling and secondary water-cooling heat transfer during the start-up phase.

Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.

Development of a 3D Filling Model of Low-Pressure Die-Cast Aluminum Alloy Wheels

A two-phase computational fluid dynamics model of the low-pressure die-cast process for the production of A356 aluminum alloy wheels has been developed to predict the flow conditions during die filling. The filling model represents a 36-deg section of a production wheel, and was developed within the commercial finite-volume package, ANSYS CFX, assuming isothermal conditions. To fully understand the behavior of the free surface, a novel technique was developed to approximate the vent resistances as they impact on the development of a backpressure within the die cavity. The filling model was first validated against experimental data, and then was used to investigate the effects of venting conditions and pressure curves during die filling. It was found that vent resistance and vent location strongly affected die filling time, free surface topography, and air entrainment for a given pressure fill-curve. With regard to the pressure curve, the model revealed a strong relation between the pressure curve and the flow behavior in the hub, which is an area prone to defect formation.

Effect of chill cooling conditions on cooling rate, microstructure and casting/chill interfacial heat transfer coefficient for sand cast A319 alloy

Abstract A combination of experiments and numerical analyses were used to examine the cooling conditions, solidification microstructure and interfacial heat transfer in A319 cast in a chilled wedge format. Both solid copper chills and water cooled chills, with and without a delay in water cooling, were examined in the study. Various chill preheats were also included. The goal of the investigation is to explore methods of limiting heat transfer during solidification directly beside the chill and increasing cooling rates during solidification away from the chill. Within the range of conditions examined in the study, chill preheat was found to have only a small effect on cooling rates between 5 and 50 mm from the chill/casting interface, pour superheat a moderate effect and water cooling a significant effect. In comparison to the results for the solid chill, the solidification time at 5 mm with water cooling applied at the beginning of mould filling is reduced from 56 to 15 s and at 50 mm from 588 to 93·5 s. Furthermore, the average cooling rate during solidification is increased from 1·9 to 7·06°C s−1 at 5 mm and from 0·18 to 1·13°C s−1 at 50 mm. At 50 mm, for example, the increased cooling rate achieved with water translates into a reduction in secondary dendrite arm spacing from 40 to 25 μm or ∼40%. Delaying the water cooling by 10 s facilitated slow cooling rates at 5 mm (similar to those achieved with a solid chill) and high cooling rates 50 mm from the chill. A temperature based correlation was found to be suitable for characterising the behaviour of the interfacial heat transfer coefficient in the solid shill castings, whereas a time based correlation was needed for the water cooled castings.

Development of an Optimization Methodology for the Aluminum Alloy Wheel Casting Process

An optimization methodology has been developed for the aluminum alloy wheel casting process. The methodology is focused on improving the timing of cooling processes in a die to achieve improved casting quality. This methodology utilizes (1) a casting process model, which was developed within the commercial finite element package, ABAQUS™—ABAQUS is a trademark of Dassault Systèms; (2) a Python-based results extraction procedure; and (3) a numerical optimization module from the open-source Python library, Scipy. To achieve optimal casting quality, a set of constraints have been defined to ensure directional solidification, and an objective function, based on the solidification cooling rates, has been defined to either maximize, or target a specific, cooling rate. The methodology has been applied to a series of casting and die geometries with different cooling system configurations, including a 2-D axisymmetric wheel and die assembly generated from a full-scale prototype wheel. The results show that, with properly defined constraint and objective functions, solidification conditions can be improved and optimal cooling conditions can be achieved leading to process productivity and product quality improvements.

Modeling and Optimizing Ti-6Al-4V Ingot Production

Control of chemistry and shrinkage void in the final stages of the consolidation processes employed to produce Ti alloy ingots is critical from the standpoint of productivity as there is a direct correlation to the amount of material that must be removed prior to further downstream processing. The application of power to the top surface during this stage allows the raising of the depth of shrinkage voids; however, it can also cause excessive evaporation of volatile elements within the alloy. The balancing of these two factors represents a classic optimization problem. A mathematical model describing the final stage of a commercial consolidation process has been developed to assist in the optimization of the process. The model solves the coupled thermal-fluid flow problem including solute conservation and evaporation. Experimental measurements consisting of the sump depth, pool profile marking, and local composition analysis have been used to validate the model predictions under various casting conditions.