Steven Cockcroft
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
This thesis investigates the key factors required to efficiently predict distortion using numerical methods in two metal Additive Manufacturing (AM) processes: Laser-based, Powder-fed Directed Energy Deposition (LP-DED) and Electron Beam Powder Bed Fusion (EB-PBF).For LP-DED, a part-scale thermomechanical model was developed to predict distortion, incorporating layer agglomeration and time-averaged heat input for computational efficiency. A new approach was developed in which the deposited material was activated at an initial temperature above the solidus to generate plastic strain. The energy input from the beam was adjusted for the enthalpy of deposited material, ensuring a correct system heat input. Additionally, a material-centric approach accounted for the thermal strain mismatch between the deposited material and the start plate, contributing to part distortion. The model was validated against experimental data.Three models were developed for EB-PBF. First, a novel system-scale thermal cavity radiation model was formulated, which included component fabrication to predict heat transport within the built chamber and component. The model incorporated layer agglomeration and time-averaged heat inputs for each computational layer, calculated based on EB-PBF system data considering beam efficiency and enthalpy associated with activating layers at preheat temperature. Experimental data was used to validate the model. The second model conducted a thermomechanical analysis of a component with overhang features fabricated using EB-PBF. It incorporated layer agglomeration, time-averaged heat input, and an Inherent Strain (IS) strategy. Using a new approach, the IS values were applied as initial anisotropic thermal strains at layer activation, establishing static equilibrium layer by layer. By adjusting IS values, the model was tuned to predict the experimentally measured distortion.Lastly, a sub-domain thermomechanical model was developed to explore the role of substrate temperature in plastic strain generation. It described a small region, including the substrate and four powder layers. The thermomechanical properties were defined as a function of temperature and material form. Additionally, the yield stress was defined as a function of strain rate, which was found to be important. A linear regression model was proposed to link substrate temperature to plastic strain, which may be used to estimate in-part variation in the IS.
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Counter pressure casting (CPC) process is a relatively new, emerging technology developed for producing load-bearing parts. It is believed that CPC produces parts with superior quality compared to those produced by the conventional low pressure die casting (LPDC) process since the die cavity is placed within a pressurized chamber (2-3 bar) in CPC. This feature is claimed to benefit the process with respect to both filling and solidification. However, few studies are available in the literature providing data to support this claim. Therefore, this research program is aimed to improve the fundamental understanding of the transport phenomena occurring in the CPC process with a focus on the heat transfer through the die/casting interface, using a combination of experimental and modelling techniques.A series of experiments were conducted on a commercial CPC machine to produce a custom-designed “H-shaped” aluminum casting. Three process conditions, where the chamber pressure was varied, were tested. Results showed that in-die temperatures at various locations, and the secondary dendrite arm space (SDAS) were not significantly affected by the chamber pressure in the range tested (1200-3000 mbar). However, die filling was delayed at a higher counter pressure, possibly due to the increased viscosity and density of the air in the die cavity.A thermal model and a coupled thermal-stress model of the CPC process have been implemented within the commercial finite element (FE) package ABAQUS™ to simulate the process conditions in the experiments. The coupled thermal-stress model was developed using a novel modelling methodology established in the research. The model is able to utilize the deformed state of the hot die and update the casting geometry based on the hot die geometry at the beginning of a casting cycle. Thus, the stress-strain evolution of the die and the casting, the die/casting interface behaviour, and the associated heat transfer can be fundamentally described. A thermal-only model was also formulated and utilized to develop a second interfacial heat transfer coefficient that is a function of interface temperature. The results of the comparison indicated a slight improvement in accuracy obtained with the thermal-stress model in areas prone to gap formation.
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Ti-N inclusions, classified as Type I defects in titanium alloys, are nitrogen-enriched areas that locally embrittle and harden the material. The presence of Ti-N inclusions in titanium alloys significantly degrades the fatigue performance, and hence cannot be tolerated in rotor-grade applications. Both reducing the potential for the introduction of these inclusions and removing them in melt-refining processes are therefore critical. The research herein is aimed at understanding: (1) the diffusional transport of nitrogen in Ti and the associated solid-state phase evolution – sub-task 1, and (2) their subsequent dissolution of Ti containing ~ 2 wt. % nitrogen in liquid titanium – sub-task 2. In the first sub-task, nitrogen was introduced to solid commercially pure (CP) titanium rods at 1650 °C in an electric induction furnace. An effective way to avoid the formation of a nitride layer (TiN and Ti₂N) was developed. Microstructure and microhardness were examined on the cross-section of the nitrided samples. Multiple phase layers were observed, and each layer was identified using X-ray diffraction. The effects of temperature and nitriding time on the kinetics of nitrogen diffusion were investigated. Results showed that nitrogen diffusion was accelerated with increasing temperature and nitriding time. Correlations between microhardness and nitrogen concentration were developed for the core and outer layers, respectively. A numerical model has also been developed to simulate nitrogen diffusion. The predicted nitrogen concentration profiles and the displacement of the phase interfaces showed good agreement with experimental observations.In the second sub-task, the nitrided rods were immersed into a molten CP Titanium pool produced by an electron beam button furnace. The evolution of the rod profile over various time periods was observed. Generally, the volume fraction of dissolved Ti-N solid increases with increasing immersion time. A numerical model has also been developed to aid in understanding the transport phenomena involved in the dissolution process. Overall, the predicted dissolved volume fraction across different immersion times agrees well with experimental measurements. Finally, an effective mass transfer coefficient in the range of 4.2×10⁻⁵ to 4.9×10⁻⁵ m/s was derived based on model results, which can be used for evaluating the dissolution kinetics in industrial applications.
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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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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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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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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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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
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
In this research, a 3D heat transfer model incorporating cavity radiation was developed in ABAQUS version 2017 to approximate the thermal field within the build environment in an Electron Beam Powder Bed Fusion (EB-PBF) Additive Manufacturing (AM) Process. The build environment, also referred to as the "pseudo build environment, was fabricated in an Electron Beam Button Furnace (EB BF) using an ARCAM Q20Plus heat shield (with the top section removed). The “build plate” was fabricated from a commercially pure titanium disk, which was surrounded by a stainless-steel plate. A circular beam pattern with a diameter of 50 mm was used to heat the titanium disk in the absence of powder. The experimental set-up was instrumented with type-K thermocouples to record the evolution in temperature on the heat shield walls, within the titanium disk and stainless-steel plate during the experiment. To record and store the temperature, an autonomous data acquisition system was developed for in-situ instrumentation within a vacuum environment. The model was validated with respect to the temperature data extracted from the EB BF.Overall, the results of the heating experiment and the numerical model suggest that the radiative heat exchange between various surfaces within the build environment is complex. The model results indicate that the portion of heat transferred via cavity radiation and absorbed by the heat shield walls was found to be a strong function of the titanium disk temperature. Additionally, four simple numerical case studies were developed to evaluate the effect of heating pattern, initial preheat, the heat absorption by the powder deposition sequence and post powder deposition preheat on the thermal behaviour in the pseudo build environment. The results of the numerical cases provide guidance into future model development, which can potentially aid in better understanding the heat transfer within the build environment leading to better AM process control.
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Components manufactured using the Electron Beam Powder Bed Fusion (EB-PBF) Additive Manufacturing method are often prone to deformation and residual stress caused by the repeated heating, melting, solidification, and cooling that occurs during the process. The presence of residual stress can reduce the service life of the parts. An estimation of the magnitude, state, and distribution of residual stress can aid in maintaining the dimensional accuracy of the component. Although effort has been made to understand the residual stress development in EB-PBF, understanding the complicated interaction between a newly deposited powder layer and the consolidated layer is still in its infancy. In this study, a coupled thermomechanical model was built to examine the buildup of stress and inelastic strain during the layer-by-layer processing of a part at the mesoscale level. A small mesoscale domain was developed to represent a volume extracted from within a much larger component. The sub-domain dimensions were chosen to include the total thickness of four powder layers and a section of previously deposited material equivalent to approximately eight consolidated layers. The model uses a novel approach to capture the transition in material response when the material changes from powder to liquid to solid. A user-defined subroutine was developed to correctly describe the evolution of thermal strain as the material solidifies and contracts. The mesoscale model developed in this work has been used to examine different scenarios. The effect of substrate temperature, electron beam power, and scan speed on the residual stress and deformation were examined. The numerical results show that a compressive plastic strain field forms in proximity to the melt pool. The model also indicates that within the temperature range of 630 ℃ to 730 ℃, a 50 ℃ increase in substrate temperature leads to a ~21% decrease in the in-elastic strain magnitude. Within the beam power range of 740 W to 940 W, the in-elastic strain decreased by ~9% with a 100 W increase in the beam power; and a ~23% increase in the in-elastic strain was observed with a 200 mm s⁻¹ increase in the beam speed.
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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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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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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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