Steven Cockcroft


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - April 2022)
An investigation of the efficacy of a water-cooled chill in improving the as-cast structure of the main bearing bulkhead in A319 engine blocks (2018)

In recent years, the automotive industry has been increasing the production of small, high-power gas engines as part of several strategies to achieve the new “Corporate Average Fuel Economy” (CAFE) standards, while at the same time meeting consumer demand for increased performance. This trend requires an improvement in the thermal and mechanical fatigue durability of the aluminium alloys used in the production of the cylinder heads and engine blocks in these engines. In the absence of modifying alloy chemistry, which potentially has significant implications for downstream operations such as heat treating and machining, one viable way to improve fatigue performance is to reduce the length-scales of the microstructural features arising from solidification that limit fatigue life. This, in turn, may be achieved by increasing the cooling rate during solidification (reducing the solidification time). Conventionally, solid chills are employed in industry to achieve this. A potential means of improving the efficacy of these chills is to incorporate water cooling. To assess the effectiveness of this method, a water-cooled chill was designed at UBC and installed in a bonded-sand engine block mould package (1/4 section). Twelve experiments were conducted with both a conventional solid chill and with a water-cooled chill (with and without a delay in water cooling). The moulds were instrumented with thermocouples to measure the evolution of temperature at key locations in the casting, and “Linear Variable Displacement Transducers” (LVDTs) to measure the gap size at the interface between the chill and the casting. A coupled thermal-stress mathematical model was developed in “ABAQUS 2016” to reproduce the experimental conditions and provide insight into interfacial heat transport and gap dynamics. Overall, the experimental and modelling results show the gap dynamics are complex and play a critical role in governing heat transport. If implemented carefully, the adoption of water-cooled chill technology has the potential to improve the cast microstructure, hence, increase the fatigue durability of the engine blocks.

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Experimental and numerical analysis of semi-solid constitutive behaviour of B206 and A356 foundry alloys (2018)

Aluminum foundry alloys are used for a wide range of industrial applications. Hot tearing is often a challenging casting defect in aluminum alloys, occurring in the semi-solid state, which has a substantial impact on the quality of casting products. The constitutive response of the semi-solid material to deformation is crucial for controlling hot tear formation, and thus it is necessary to have a means of assessing a semi-solid’s constitutive behaviour, and the role of microstructure in deforming this two-phase medium. The semi-solid tensile behaviour of two commercially used foundry aluminum alloys was experimentally characterized using a Gleeble thermo-mechanical test apparatus and numerically characterized using a multi-physics numerical model. First, thermo-mechanical testing was carried out on samples prepared by chill wedge-shaped casting. The test results indicated that at relatively high fraction solid (fs=0.95-1), B206 has higher yield stress than A356. However, at lower fraction solids (fs
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Hydrogen pore formation in an Al-Si alloy - an examination of the role of hydrogen macrosegregation (2018)

Porosity related defects are one of the leading causes of cast rejection in casting industry since they are detrimental to the fatigue performance of the cast components, primarily when the pores are located in an area of high cyclic stress or located near the surface. Many efforts have been made to develop computational models that predict pore formation. However, one aspect of microporosity formation that has previously not been considered in detail is the effect of macrosegregation of hydrogen. Towards this goal, the user-defined scalars and corresponding user-defined functions were developed to account for hydrogen macrosegregation during solidification of low pressure die casting (LPDC) of A356 aluminum alloy wheels. Numerical simulation of the LPDC process has been implemented within the commercial CFD software package, FLUENT 16.0. The model has been validated against temperature and microstructural data taken from a commercially cast wheel. The amount of species (silicon and hydrogen) segregation in the wheel has been shown to be significant in the rim/spoke junction. The output data from the FLUENT model were then fed into the in-house microporosity model to predict pore size distribution at discrete locations.The in-house microporosity model has been updated to incorporate the effects of hydrogen macrosegregation, cooling rate and local pressure drop on pore size distribution in the wheel cast. The microporosity model used a Gaussian function of hydrogen supersaturation in the melt to simulate nucleation site distribution and assumed pore growth was controlled by hydrogen diffusion process and besides, the model took into account pore growth associated with liquid encapsulation at a high solid fraction. The samples from a cast wheel have been analyzed using X-rayed microtomography to provide basic validation to the microporosity model. The predicted results showed that the evolution in pressure has the dominant effect on pore growth, but only under conditions where pores have nucleated prior to the abrupt pressure drop. Otherwise, the cooling rate appears to have the dominant effect. The model prediction shows pore size increases with decreasing pressured drop and cooling rate.

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Experimental investigation and numerical modeling of microporosity formation in aluminum alloy A356 (2011)

Microporosity refers to small voids in the material in the size range from a few to hundreds of micrometers. These small voids can reduce the fatigue performance of the cast components. In the foundry industry, numerous efforts have been made to predict and control microporosity formation. The present work studies the formation of microporosity in A356 (Al-7wt%Si-0.3wt%Mg) aluminum alloy castings. The focus is on prediction of pore size distribution, which is a crucial factor in fatigue analysis. This requires precise experimental characterization of pore size and simulation of both nucleation and growth kinetics of the pores. In the initial stage of this work, microporosity formed in directionally solidified tapered cylindrical A356 casting samples were analyzed using high resolution X-ray microtomography (XMT). The results showed that increasing the cooling rate and degassing time yields lower microporosity within the microstructure. These microporosity data were later used to validate a numerical model that simulates microporosity formation in A356 castings. In this model, the nucleation site distribution of the pores is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is a hydrogen-diffusion controlled process. With the model it is possible to evaluate the relative contributions of hydrogen content, cooling rate and nucleation sites to microporosity formation, and to quantify the pore nucleation kinetics at given casting conditions.Furthermore, this model was applied to study the effect of oxide inclusions on pore nucleation kinetics. Castings were prepared under different casting conditions aimed at manipulating the tendency to form and entrain oxides in the melt. Two alloy variants of A356 were tested in which the main difference was Sr content. By fitting the experimental results with the pore formation model, an estimate of the pore nucleation site distribution has been made. It is shown increasing the tendency to form oxide films increases both the number and potency of nucleation sites. Based on the model prediction, Sr-modification impacts both the pore nucleation and pore growth kinetics.

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Strain Evolution During Hot Tearing in Aluminium Alloys (2010)

Hot tearing refers to cracks that frequently occur within the mushy zone during cooling from the liquid to solid state during shape and ingot casting. Both ferrous and non-ferrous alloys may be affected, and there is some evidence to suggest those with long freezing ranges are more susceptible. Due to the nature of this defect the economicimpact is often significant and can result in an immediate productivity loss. It is therefore important for industry to be able to better predict the susceptibility of various alloys to hot tearing.Various theories have been proposed and several different types of experimentalmethods have been developed to interpret the properties of alloys in the semi-solid state.However, many of these techniques do not produce good quantitative data (i.e. strain)that can be used to calibrate a thermal-mechanical computer simulation of casting.Existing experimental methods often measure strain indirectly by means of a load trainfrozen into the end of the casting. However, local strain at the hot tear initiation sitewould be more valuable for computer model calibration. Clearly, the use of traditionalmeasurement techniques, such as strain gauges, is not a viable option and therefore an alternative was investigated. In this work the use of digital image correlation to determine the evolution of strain and strain at the onset of localisation resulting in a hot tear has been evaluated. Data has been determined for aluminium alloys AA6111, AA3104, CA32118, Al-0.5% wt pct Cu under slow cooling conditions and AA3003 under directional solidification using a water cooled copper chill.A new hot tearing experiment has been developed which localises strain topromote hot tearing to occur in only one region of the casting and is cooled by directional solidification. Images of this region were captured during solidification via a glass window embedded in the mould of the experiment. These images were correlated with each other to determine strain accumulated during hot tearing using 3rd party commercial digital image correlation software.

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Master's Student Supervision (2010 - 2021)
A numerical description of nitrogen diffusion in titanium at elevated temperatures (2021)

As part of a program to understand the dissolution of nitrogen-rich titanium solids in liquid titanium, a numerical study of nitrogen diffusion in titanium at elevated temperatures has been carried out. A Landau transformation was applied to the equations governing nitrogen diffusion which were used as the basis for the numerical models developed in this study. To begin a numerical model describing the nitriding of commercially pure titanium was developed. The numerical model was used initially to simulate nitrogen diffusion in a planar geometry and the predicted nitrogen concentration profiles and displacement of Ti-N phase boundaries showed good agreement with analytically derived solutions. The numerical model was then used to simulate nitrogen transport in commercially pure titanium cylinders. The model results were shown to be sensitive to the diffusion coefficients of Ti-N phases present in the system. Based on a sensitivity analysis, diffusion coefficients at 1650 °C of 4.3×10⁻¹¹ m²·s⁻¹, 1.6×10⁻¹¹ m²·s⁻¹ and 1.7×10⁻¹² m²·s⁻¹ for β-Ti, α-Ti and TiN phases, respectively, were back calculated using the model. The model predictions, using the new diffusion coefficients, showed good agreement with previously published data in terms of both the nitrogen concentration profiles and displacements of Ti-N phase boundaries under the conditions examined in the study. The comparison indicates the model framework is capable of accurately approximating the diffusion of nitrogen in titanium at elevated temperatures.In work that followed, the model framework was used to develop an improved numerical model for describing the dissolution of Ti-N particles in liquid titanium. The results of the improved methodology have been compared to a second finite-difference based model formulated using the conventional approach for interface motion. The improved approach accurately accounts for conservation of nitrogen associated with interface motion and hence has the potential to predict particle dissolution times more accurately in commercial melt refining operations.

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Experimental and numerical investigation of the effect of block-type support structure design on the thermal field within components fabricated by selective laser melting (2021)

The successful production of overhang features in the Selective Laser Melting (SLM) process requires additional structures, known as “support structures”. These structures provide mechanical support to overhang features and alter thermal fields within the produced components. Hence, the design of support structures impacts the development of in-situ thermal stress and component deformation. The present research combines experiments and numerical modelling to quantify the contribution of the support structure to the evolution of the thermal field in a cantilevered plate and, in turn, to investigate the relative role of heat transfer on component deformation. Series of build experiments were undertaken to investigate the effect of block-type support structure design on the deformation of the cantilevered plate. Two block-type support structures were designed for the overhang platform of a cantilevered plate. The designed samples were manufactured using SLM®500 machine located at Singapore Centre for 3-D Printing. The deformation of the overhang platform was measured using the Image Analysis technique. Next, a statistical analysis was performed to evaluate the relative impact of each design parameter on component deformation. Finally, a 2-D transient heat transfer model using the “layer agglomeration” approach was developed in the commercial package “ABAQUS” to perform a sensitivity analysis to investigate the impact of design parameters on heat transfer and the evolution of the thermal field in the support structure and the cantilevered plate. Numerical results demonstrated that the total contact area of teeth and the total support base area alter thermal fields within the produced components. It was predicted that increasing the area for conducting heat to the base reduces the peak temperature in the platform. Additionally, the vertical temperature gradient within the overhang platform decreased by increasing the total contact area of teeth. Moreover, increasing the total contact area of teeth produces a more uniform temperature field within the overhang platform, while the total base area was found to have a negligible impact on it. Also, the experimental case study was analyzed with the numerical thermal model suggested that the vertical temperature gradient and the peak temperature were reduced in the sample showing a lower amount of deformation.

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Quantitative assessment of the effect of copper chills on casting/chill interface behavior and the microstructure of sand cast A319 alloy (2013)

Although the demand for A319 alloy has increased in recent years, thermal fatigue resistance of the alloy is still one of the most important challenges in engine applications, especially in the newer generation of engines in which cylinder spacing has been reduced. According to the previous studies there are several parameters that improve thermal fatigue resistance such as: low SDAS, fine grain size, low porosity level, and low intermetallic content.Cooling rate has a direct effect on the shape, size, and distribution of the microstructural phases, as well as on the scale of the dendrites, and pore size. High cooling rates can improve thermal fatigue resistance, as a result of fine microstructure and small pore size. On the other hand, thin sections of a mold may not properly fill and “Cold Shuts” may result, if high cooling rates are applied.One approach to balance these phenomena is to use a water-cooled chill where water cooling is activated part way through the casting sequence. This type of chill causes a lower cooling rate initially, when the filling procedure is occurring, and after filling, the cooling rate increases to reduce the microstructure size. The results show that this method has the potential to both avoid cold shuts and miss-runs and improve the cast microstructure farther into castings remote from the chill. A mathematical model has been developed in “ANSYS CFX 12.0” to evaluate the effectiveness of this concept quantitatively. The model simulates the behavior of the Casting/chill interface and also predicts the cooling rates resulting from different casting conditions when using solid chill and water-cooled chill.

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