Carl Reilly

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.

Application of Numerical Optimization to Aluminum Alloy Wheel Casting

A method of numerically optimizing the cooling conditions in a low- pressure die casting process from the standpoint of maintaining good directional solidification, high cooling rates and reduced cycle times has been developed for the production of aluminumalloy wheels. The method focuses on the optimization of cooling channel timing and utilizes an open source numerical optimization algorithm coupled with an experimentally validated, ABAQUS-based, heat transfer model of the casting process. Key features of the method include: 1) carefully designed constraint functions to ensure directional solidification along the centerlineof the wheel; and 2) carefully formulated objective functions to maximize cooling rate. The method has been implemented on a prototype production die and the results have been tested with plant trial test.

Evaluation of the Casting/Chill Interface Thermal Behaviour During A319 Alloy Sand Casting Process

One approach to improve fatigue resistance of A319 alloy cast components is to reduce the size of microstructural discontinuities by increasing cooling rates during solidification. A combination of experimental and numerical methods were used to explore the efficacy of using a water-cooled chill to increase cooling rates during solidification in A319 alloy cast in a chilled-wedge format. A solid and a water-cooled chill were designed and installed in a bonded sand mould package instrumented with thermocouples to measure cooling rates and two Linear Variable Differential Transducers to measure the displacement of the casting/chill interface. A mathematical model was developed to evaluate the casting/chill interface heat transfer behaviour in the casting produced with the two chill configurations. The experimental results show that for the casting format studied, the water-cooled chill is able to significantly increase cooling rates during solidification up to 50mm from the chill and the macro-scale gap that develops at the interface is significantly larger for the case of the solid chill compared with the water-cooled chill.

An Examination of the Thermally Related Factors Influencing the Melting/Dissolution of Solids in Liquid Titanium

The melting/dissolution behaviour of a solid in liquid titanium has been investigated with the aid of an Electron Beam Button Furnace (EBBF) to understand the phenomena contributing to the melting/dissolution of exogenous solids introduced into liquid titanium during melt processing and casting. To begin, cylindrical rods of commercial purity (CP) titanium were dipped into a molten CP titanium pool for various periods of time to investigate the melting/dissolution behaviour in the absence of compositional effects. The dimensions of the dipped rods were measured before and after various immersion times allowing quantification of the evolution of the solid/liquid interface and melting rate. The temperature within the dipped rod was also measured during melting to provide additional quantitative data on heat transport for analysis. The results show that the molten titanium initially solidifies onto the cold rod. The solid/solid interface formed between the rod and solidified titanium was found to significantly reduce the heat transfer to the rod. After a short period of time, the solidified titanium re-melts followed by melting and dissolution of the rod. Analysis with the model has confirmed that both thermally induced buoyancy and, in particular, thermally induced Marangoni forces contribute significantly to the melting/dissolution process. A numerical model has been developed to describe the solidification and melting process, the results of which are shown to correlate well with experimentally obtained data.

Study of Microporosity Formation under Different Pouring Conditions in A356 Aluminum Alloy Castings

In this work, the formation of microporosity has been examined under different casting conditions aimed at manipulating the tendency to form and entrain oxide films in small directionally cast A356 samples. Porous disc filtration analysis (PoDFA) 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 (XMT) analysis. By fitting a pore formation model to the experimental results, an estimate of the pore nucleation population has been made. The results from the model predictions indicate that increasing the tendency to form and entrain oxide films not only increases the number of nucleation sites but also reduces the supersaturation necessary for pore nucleation in A356 castings.