Daan Maijer
Relevant Thesis-Based Degree Programs
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Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
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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Aluminum-copper casting alloys have relatively high strength and hardness, fatigue and creep resistances and good machinability, all of each are dependent on the copper content of the alloy. The Al-Cu casting alloy (4.2-5.0 wt% Cu), known as B206, is a potential candidate material for use in marine applications where good mechanical properties and high strength to weight ratio is desired. These properties are ideal for components of tidal-based energy generating systems. However, corrosion continues to be an issue. This dissertation presents and discusses the results of several electrochemical and microstructural investigations conducted on B206, contributing to a further understanding of the fundamental corrosion processes. Applications of this research are strongest within the marine industry field, yet are extendable to other infrastructural and engineering applications such as aerospace and military.Results of this work elucidate the mechanism of localized corrosion of B206 alloy in seawater. Focused ion beam (FIB) used to determine the subsurface microstructure at local attack sites within the corroded area reveals that localized corrosion is propagated where continuous particles are buried beneath the surface. Propagating away from the initiation sites, corrosion develops preferentially along the grain boundary network beneath the alloy surface. Retrogression and re-aging (RRA) of the alloy to modify the grain structure and render uniform the distribution of the second phase is revealed not to have a substantial effect on the corrosion susceptibility of the alloy. However, Electrochemical Impedance Spectroscopy (EIS) and Mott-Schottky tests support the feasibility of implementing anodizing and possibly anodic protection systems for B206 in specific service environments. EIS was also used to determine the effect of cathodic protection (CP) on coated B206 and reveals that its corrosion resistance with CP is superior to the situation without CP and, therefore, that the coating is compatible with CP. Due to its use in the as-cast state, the effect of casting porosity on the corrosion of B206 was investigated using a pencil electrode method. Results reveal that the corrosion can be attributed to the local chemistry inside the pores (conductivity and potential at the bottom of pores).
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A combined experimental / numerical approach has been applied to investigate thebulk transfer of solute due to liquid metal feeding during shape casting of aluminumalloy A356 (Al-7Si-0.3Mg). A series of dumbbell-shaped experimental casting geome-tries have been developed, which promote solute redistribution due to liquid metalfeeding. Three of the castings were produced in small moulds with natural cooling,forced cooling and insulated conditions and one casting was made in a large mouldwith natural cooling. The redistribution of solute in the castings has been evaluatedusing a novel image processing technique based on the area fraction of silicon. Theresults show that the casting with the forced cooling configuration exhibited a largerdegree of macrosegregation.In the numerical model, silicon segregation during solidification is calculated as-suming the Scheil approximation, and is coupled with a macro-scale transport modelthat considers resistance in the mushy zone and feeding flow. The model has beenimplemented within the commercial CFD software, FLUENT, which simultaneouslysolves the thermal, fluid flow fields and species segregation on the macro-scale. Theresults from the simulation agree with the experimental results, except for the caseswhere significant liquid encapsulation occurs. The model predicts high levels of enrich-ment when liquid encapsulation is present in the joint section of the dumbbell-shapedcastings.Finally, a constitutive behaviour relationship was developed based on the Ludwik-Hollomon equation to predict the flow stress of Al-Si-Mg alloys with varying siliconcomposition and Dendrite Arm Spacing (das) in the as-cast (ac) or T6 conditionwith high accuracy. This model was then used with the results of the segregationmodel to predict yield strength distribution in the aforementioned dumbbell-shapedcasting. The results show that silicon segregation has a more significant effect on the yield strength than das.
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In metallurgical reactors, the thermal stress field of refractories always changes with the heat transfer conditions at the hot-face. It is suggested that ‘thermally induced refractory cracking’ is often the primary cause of in-service refractory failure but quantitative support for this is lacking. The current work is focussed on studying this aspect by developing an experimentally validated thermomechanical model that considers refractory strength degradation under repeated thermal cycling.A thermo-mechanical model has been developed with ABAQUS to predict thermal stress and damage in a refractory specimen subjected to thermal cycling. An experiment based on the “contact-conduction method” that uses a hot/cold metal block to heat/cool a refractory specimen was carried out to validate the model. The experiments were run for up to 3-cycles starting from cold- and hot-refractory specimens. Thermocouples were used to gather temperature data from refractory and steel block. An inverse heat conduction model was developed to predict the heat flux applied to the refractory specimen by the steel block based on the temperature history from the steel block. Ultrasonic testing was carried out on the refractory specimens before and after the thermal cycling tests. The contact-conduction method was successful in creating significant thermal gradients in the refractory specimens. Thermocouples on refractory located at 1cm from the steel-refractory show temperature variation of about 500°C and 575°C for cold- and hot-refractory specimen, respectively after 3-cycles. The model was capable of predicting the temperature changes and damage in the refractory material after multiple cycles. Ultrasonic velocity tests show significant change in the sound velocities in the areas experiencing thermal cycling, indicating significant micro-cracking damage in those areas. It was seen that with multiple cycles the damage penetrated further into the specimen, however the magnitude of the damage does not increase significantly. Application to an example tundish operation indicated that the model was capable of analyzing an ideal preheating schedule and was capable of predicting the effect of idle time and multiple thermal cycles on the damage in refractories. However, to predict thermal spalling more precisely, an integrated model that considers the effect of thermal gradients, chemical reactions and mechanical loads is needed.
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Aluminum alloy wheel manufacturers face on-going challenges to produce high quality wheels and increase production rates. Improvements are generally realized by modifying the wheel and die designs and continually improving the manufacturing processes. Conventionally, these improvements have been realized by trial-and-error, building on past practice or experience. This approach typically results in long design lead times, high scrap rates and less than optimal production rates. The work presented in this study seeks to reduce the reliance on trial-and-error techniques by developing a new methodology to optimize the wheel casting process through the combination of a casting process model and open-source numerical optimization algorithms. The casting process model utilized in this method was developed in the commercial finite element package Abaqus™ and was validated through plant trials. An open source optimization module Python Scipy.optimize has been employed to perform the optimization. The work focuses on optimizing the cooling conditions in a low-pressure die-casting (LPDC) process used to produce automotive wheels. Specifically cooling channel timing was selected because of the critical role heat extraction plays on casting quality, both in terms of dendrite cell size and the formation and growth of porosities. The methodology was first developed with a series of test problems ending with an L-shaped geometry that employed the major features of the wheel casting process. The most suitable approach, based on the test problems, was then applied to the optimization of a 2-D axisymmetric prototype wheel die structure. The outcome revealed that numerical optimization coupled with a state-of-the-art process model has the potential to dramatically improve the method of determining cooling channel timings while also improving the product quality and process performance. The utility of the optimization methodology was found to depend on the accuracy of the casting process model. Significant challenges remain before widespread implementation of this methodology can occur in industry. Possible directions for further developments have been identified. In summary, this study represents one of the initial applications of a numerical optimization methodology to wheel casting, and that with further development; it will become an effective tool for process and die design optimization.
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Both experiments and numerical modeling work have been carried out to understand the phenomena contributing to the melting of solid condensate in liquid titanium alloys during Electron Beam Cold Hearth Re-melting (EBCHR). To begin, ice/water and ethanol/water analogue physical models were adopted to study the melting of a low melting point solid introduced into liquid and to provide data suitable for developing a comprehensive numerical-based modeling framework. The results revealed that thermal and compositional driven buoyancy and surface tension (Marangoni) flows, when present, can have a significant impact on solid melting in a system where forced convection is not significant. In work that followed, the melting behavior of Commercial Purity Titanium (CP-Ti) rods in liquid CP-Ti was investigated with the aid of an Electron Beam Button Furnace (EBBF) to examine the melting kinetics in the titanium system in the absence of compositional effects. The results showed that the liquid titanium initially froze onto the cold rod when it was immersed, resulting in the formation of a solid/solid interface that acted to retard melting when present. Data collected from the experiments included the evolution in the solid profile of the rod with time and the evolution in temperature obtained from a thermocouple embedded in the rod. The numerical modeling framework developed for the ethanol/water system was modified and applied to support analysis of the experimental results including the determination of an effective interfacial heat transfer coefficient (EIHTC). A similarity solution was also developed to assess the numerical model derived EIHTC. In the final phase of the study, work was conducted on Ti-Al solid rods partially immersed in liquid CP-Ti and liquid Ti-6wt%Al-4wt%V (Ti64) as a means of approximating the behavior of condensate in industry. The melting behavior of Ti-Al was observed to differ significantly from that of CP-Ti rods. Despite having a lower melting point, the Ti-Al rod was found to heat up and melt at a much slower rate. Metallographic examination of partially melted rods and a sensitivity analysis conducted with the numerical model has been able to partially, but not fully explain this difference.
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The application of rotary forming to A356 offers a potential improvement in material use, simplified castings and ameliorated fatigue resistance. To investigate the utility of adopting this process industrially, an extensive characterization and modelling effort was undertaken.The constitutive behaviour of A356 in the as-cast condition was assessed with compression tests performed over a range of deformation temperatures (30-500°C) and strain rates (~0.1-10/s). The flow stress as a function of temperature and strain rate was quantified via an extended Ludwik-Hollomon and Kocks-Mecking framework.The through-process microstructural effects on A356 subjected to rotary forming at elevated temperatures was also investigated. This was conducted on material at 350°C with an industrially-scaled, purpose-built apparatus, inducing varying levels of spinning deformation. This was also conducted on commercially flow formed material with high levels of deformation at the same temperature. Macro and micro-hardness testing was used to track the changes from the as-cast and as-formed states, as well as following a T6 heat treatment. Further EDX analysis indicate that precipitation aspects of heat treatment is not appreciably affected by forming. Forming was found to principally affect the eutectic-Si particle size, resulting in a finer particle post heat treatment.An explicit finite element rotary forming model reciprocating experimental forming conditions was developed incorporating the Ludwik-Hollomon description. This forming model was found to be computationally expensive; however, demonstrated reasonable agreement with experimental geometry and phenomena.In evaluating the effect of forming on fatigue, multiaxial testing of A356-T6 was conducted to apprehend the basic fatigue mechanisms. Endurance limits are found to be generally governed by porosity and maximum principal stress for high cycle fatigue. Uniaxial fatigue tests of both experimentally and commercially formed material showed a 30% increase in endurance limits over unformed material, principally through mitigating porosity.
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Controlling and eliminating defects, such as macro-porosity, in die casting processes is an on-going challenge for manufacturers. Current strategies for eliminating macro-porosity focus on the execution of pre-set casting cycles, die structure design or the combination of both. To respond to process variability and mitigate its negative effects, advanced process control methodology has been developed to dynamically drive the process towards optimal dynamic or static operational conditions, hence minimizing macro-porosity in the casting.In this thesis, a Finite Element heat transfer model has been developed to predict the evolution of temperatures and the volume of encapsulated liquid in a casting with a high propensity to form macro-porosity. The model was validated by comparison to plant trial data. A virtual process has then been developed based on the model to simulate the continuous operation of a real process, for use as a platform to evaluate a controller’s performance.Since macro-porosity cannot be measured during casting, die temperature has been used as an indirect indicator of this defect. A model-based methodology has been developed to analyze the correlation between die temperature and encapsulated liquid volume, a precursor to the formation of macro-porosity. This methodology is employed to assess the suitability of different in-cycle die temperatures for use as indicators of macro-porosity formation. The optimal locations have then been determined to monitor die temperatures for the purpose of minimizing macro-porosity.A nonlinear state-space model, based on data from the virtual process, has been developed to provide a reliable representation of this virtual process. The control variable-driven portion exhibits linear dynamic behavior with nonlinear static gain. The resulting MIMO state-space model facilitated the design of a controller for this process.Finally, the performance of the nonlinear model-based predictive controller was evaluated using the virtual process. Independent of the initial state of the process - i.e. steady state or startup, the controller exhibited the capability to automatically adjust the process toward the dynamic or static optimal operational condition during disturbances examined. The advanced control methodology developed for LPDC provides a novel solution to improve the operational conditions in die casting process.
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The interdendritic permeability is a critical parameter that defines the feedability of the mushy zone during solidification. In this study, a theoretical expression to describe the evolution of permeability throughout the complete solidification range (from dendritic to dendritic/eutectic) of hypoeutectic aluminum alloys has been derived, verified and validated through physical and numerical modeling. The permeability of the primary, equiaxed, dendritic phase has been characterized using geometries obtained by X-ray microtomographic analysis of Al-4.5wt%Cu alloy samples quenched at different temperatures after the start of solidification. The permeability during equiaxed eutectic solidification was characterized on simulated dendritic/eutectic microstructures predicted using a Cellular Automaton technique. For both the dendritic and dendritic/eutectic structures, the permeability was characterized i) physically using large-scale analogues of the characterized microstructures and ii) numerically by predicting the flow through the simulated microstructures. The microstructural parameters were then linked to more practical parameters available in solidification models through i) developing an inverse analysis technique to characterize eutectic solidification and ii) development of a geometric model for dendritic solidification. The permeability values determined through physical and numerical modeling are in good agreement with each other and are consistent with the mathematical expression. The proposed permeability expression is valid over the complete solidification range and for a wide range of compositions. The expression reduces to the conventional Carman-Kozeny expression during dendritic solidification and/or dendritic/eutectic solidification with low density of eutectic grains. However, it deviates from the conventional Carman-Kozeny expression as the density of eutectic grains increases.
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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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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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Additive Manufacturing simulations for thermal fields are computationally expensive because of the highly disparate length and time scales involved and can sometimes take days to run. Improving the speed of these simulations enables multiple virtual experiments to be run to understand the effects of various process parameters on heat buildup and can even be useful for in situ process control based on sensor measurements from the build area. The goal of this work is to reduce the computational time of such simulations while maintaining sufficient physics fidelity to yield reliable results. The approach taken is to replace the FEM model with a Fast-to-run (FTR) model which exploits the cyclic nature of the process to predict the thermal fields during AM. In this approach, peak temperatures and melt pools dimensions in a substrate melted by a moving heat source are modelled. The dependence of the heat transfer patterns on the heat source location and characteristics and the initial conditions of the substrate is modelled using data from the FEM simulation. Simulation time using the FTR model has been reduced significantly compared to the FEM simulation based on the domain size and time simulated. Finally, the FTR model is run on various complex scenarios. The effects of various hatching strategies are modelled and their maximum temperatures and melt depths are compared. Additionally, a slice of an impeller model is simulated using the FTR model to generate maximum temperature and melt depth maps, allowing the identification of hotspots and undermelted regions.
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Plasma-arc cold hearth melting (PAM) is an important technology used in the melting process for titanium alloys. Compared to the more common, electron beam cold hearth re-melting process, PAM allows an inert gas environment which significantly reduces the evaporation rate of alloying elements. To develop a better understanding of the effects of the plasma torch in the PAM process, a numerical model is being developed. However, this model requires an accurate description of the torch heat flux distribution. This research presented in this thesis focused on developing and verifying an inverse heat transfer analysis methodology to characterize the heat flux distribution from a plasma torch. A test block trial was conducted with in an industrial scale plasma arc furnace to measure the temperature history in a test block during heating and cooling. Following the trial, the test block was sectioned to get the liquid pool profile. The distribution of heat flux calculated from the inverse analysis assumed a Gaussian-like distribution, decreasing radially from the centerline to the edge of the block. Predictions for temperature history and liquid pool profile are in good agreement with the measured results from the experiment. Sensitivity analysis was performed to find some key factors that influence the prediction.
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The focus of this project is to develop a methodology to quantitatively describe the heat transfer in the cooling channels of the low-pressure die casting process, which is the dominant commercial technology for the production of aluminum automotive wheels, and to successfully implement the methodology in a numerical model of the casting process. Towards this goal, an algorithm capable of calculating heat transfer coefficient (HTC) based on process parameters and surface temperature within the cooling channel is developed. The algorithm was implemented in the form of a user-defined subroutine in a 3-D thermal model of the Low Pressure Die Casting (LPDC) process developed in the commercial finite element analysis package in ABAQUS.The cooling channel HTC’s are often input into thermal models as an average constant value derived based on trial-and-error. The trial-and-error process to obtain the HTCs in the cooling channel involves, prescribing a trial set of HTC values and comparing the results of the casting simulation with thermocouple measurements. The trial cooling channel HTCs are then adjusted until a reasonable fit to the temperature measurements are achieved. The trial-and-error process is generally time consuming and does not accurately describe the physical phenomenon occurring in the cooling channel during casting. The constant cooling channel HTCs obtained through the trial-and-error process are tuned to a given set of operating conditions, compromising the utility and generality of the model.To provide data necessary for model validation, casting plant trials were performed at Canadian Autoparts Toyota Inc. in Delta, British Columbia. The trials included temperature measurements at pre-determined locations within the top, side and bottom dies. The validity of HTC calculations have been assessed by comparing the predicted temperature history of the subroutine-based model with the measured thermocouple data collected during the casting cycle and also comparing the model predictions with the base-case model with constant cooling channel HTC.
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The focus of this project is to improve the understanding of the interfacial heat transfer behavior within the Low-Pressure Die Casting (LPDC) process, which is the main manufacturing process for A356 aluminum alloy wheels, and to develop an improved methodology/expression for calculating the heat transfer across the wheel/die interface. To formulate and assess expressions for the interfacial behavior, a 2D-axisymmetric coupled thermo-mechanical model has been developed in the commercial finite element package, ABAQUS. The model was capable of predicting the thermal history, deformation and the variation of the air gap and pressure along the wheel/die interface. The temperature predictions of the coupled thermo-mechanical model were compared with temperature measurements obtained at Canadian Auto Parts Toyota Inc obtained on a production die. A displacement measurement setup using a high temperature eddy current displacement sensor was designed and tested in a lab setting but not employed in a plant trial due timing issues.Initially, the coupled thermo-mechanical model was run with a temperature dependent interfacial heat transfer coefficient to obtain preliminary air gap and pressure behavior at various locations. Comparisons with the thermocouple measurements suggest that the model is able to generally qualitatively, and at some locations quantitatively, predict the temperature changes from the main physical phenomena occurring during the casting process. The preliminary air gap and pressure predictions were used to develop a temperature, gap size and pressure dependent interfacial heat transfer coefficient based on literature review. The interfacial heat transfer coefficient was implemented in the model, and was found to improve the agreement between the model predictions and measured temperatures, but was prone to numerical convergence issues.A new methodology of incrementally changing the interfacial heat transfer coefficient has been proposed to solve the issues. The methodology was implemented in an EXCEL spreadsheet to test it and the calculated interfacial heat transfer coefficients were found to be continuous, reflecting the effects of air gap and pressure evolution. The methodology and corresponding algorithm should be further developed for use in an ABAQUS model in the future.
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The Electron Beam Cold Hearth Remelting (EBCHR) process has emerged as a key process in producing high quality Ti-6Al-4V ingot and electrode as it is able to effectively consolidate both sponge and scrap material while removing undesired impurities and inclusions, such as Low Density Inclusions (LDIs) and High Density Inclusions (HDIs). However, the challenge of composition control arises in processing alloys such as Ti-6Al-4V where evaporative loss of elements with higher vapor pressure (Al in this case) cannot be ignored. Therefore, in order to cast a product of specified composition, a thorough understanding of the evaporation mechanism and melt flow conditions becomes crucial in process control and optimization. This research presents a comprehensive model of the melt pool produced during Electron Beam Button Melting (EBBM) which has been developed to serve as an intermediate step in the development of a comprehensive tool for analysis and optimization of the industrial EBCHR process. With proper geometry and boundary conditions, the EBBM model can be readily applied to an industrial EBCHR furnace to minimize costly experiments in optimizing process parameters. A thermal-fluid-compositional model has been developed that includes Al evaporation, thermal and compositional buoyancy, thermal and compositional Marangoni flow and flow attenuation in the mushy regime. Experiments on Ti-6Al-4V and CP titanium with a circular electron beam pattern were conducted in a laboratory scale EBBM furnace in order to study the evaporation process and fluid flow in the liquid pool. The data obtained from the experimental work was used to tune the thermal boundary conditions and validate the model predictions. The temperature, surface velocity, pool profile and concentration profile have been experimentally quantified and used for validation of the mathematical model.
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Electron beam cold heart melting (EBCHM) is a consolidation and refining process capable of consolidating titanium scrap and sponge material into high quality titanium alloy ingots. Unlike other consolidation processes for titanium, EBCHM is efficient in removing both high and low density inclusions. During the final stage of casting in EBCHM, operators must balance the potential to form large shrinkage voids, caused by turning off the electron beam heating, against the tendency to evaporate alloying additions, which occurs if the top surface remains molten. To this end, a comprehensive understanding of the evaporation and fluid flow conditions occurring during the final stage of EBCHM is required in order to optimize ingot production. This research focused on developing a coupled thermal, fluid flow and composition model, capable of predicting the temperature, fluid flow and composition fields within an EBCHM cast, Ti-6Al-4V ingot. The physical phenomena of thermal and compositional buoyancy, mushy zone flow attenuation and aluminum evaporation were incorporated in the model formulation. Industrial scale experiments were carried out at the production facilities of a leading industrial producer of titanium to provide data and measurements used for model verification. The model has been used to study the effects of variation of electron beam power input and hot top time duration on the evaporative losses and position of solidification voids. Model predictions for liquid pool profile, last liquid to solidify and composition fields are in good agreement with the industrially measured results. Sensitivity analysis was performed by varying electron beam power and hot top duration independently and observing the effect on the composition fields and last liquid to solidify. For the cases examined, there was a strong correlation between electron beam power and alloying element losses, while hot top duration variation results indicated a stronger dependence on last liquid to solidify than on alloying element losses. Therefore a classic optimization problem arises between balancing hot top duration with alloying element losses.
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The difference of die filling, which can be characterized by the free surface flow behavior, has a strong influence on the quality of casting components. In the case of cast aluminum alloy wheels, an undesired filling pattern with excessive turbulence can cause portions of the surface oxide film to be entrained within the bulk liquid resulting in defects such as cosmetic paint-pops, hot tears, porosity and rim-leaks. To investigate the influence of die filling on defect formation in low-pressure die-cast aluminum wheels, a water analogue physical model was built, instrumented and tested to simulate the free surface behavior during die filling of a low-pressure die-cast (LPDC) wheel. The physical model contains a transparent planar die section which was manufactured out of the geometry of a production die, and an automatic pressure control system that achieves liquid feeding conditions similar to the industrial process. A set of die filling tests with different venting conditions was carried out to explore the role of venting on the free surface behavior of water and to produce data for validation of a computational model. The computational model was developed, based on the commercial computational fluid dynamics code ANSYS CFX, for the purposes of predicting the flow conditions during die filling, providing qualitative and quantitative flow information that are otherwise not possible to obtain through experimental measurement, and identifying key features that influences the flow during die filling at a lower cost of time and labor. Comparison between the experimental and numerical data has shown that the computational model was able to qualitatively reproduce the flow behavior observed in the water model in the conditions tested. Both the experimental and the model results indicate that the entrainment of surface oxide films and air bubbles could occur at the outboard rim flange during the filling of the flange, below the free surface of the returning waves in the spoke and at the junction of the hub and the spoke during the filling of the hub. Venting conditions have been proved crucial and the importance of vent design in commercial die design was highlighted.
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During the production of titanium products, the presence of aluminum-rich regions can cause Type II alpha stabilized defects which are deleterious to down-stream performance. Al-rich material can enter the melt via ballistic transfer from the melting hearth at various stages during electron beam cold hearth re-melting (EBCHR) of Ti-6Al-4V (Ti-6wt%Al-4wt%V) alloy. If this material is not fully dissolved and homogenized when solidification occurs, the ingot will contain Al-rich regions. Thus, in order to produce high-performance components for aerospace applications, titanium producers must understand the dissolution process for alloying elements entering the melt. To study and characterize the phenomena associated with the dissolution and homogenization of alloying elements during EBCHR processing of Ti-6Al-4V, a water-ethanol physical analogue model has been developed to simulate the thermal, compositional and fluid flow behavior that are active in the dissolution process. The physical model consists of a hot water solvent contained in a transparent cell (beaker) in which solidified ethanol or ice solute is dipped. The data generated from the physical model was used to validate a coupled thermal- fluid flow-composition model (developed in the commercial CFD code ANSYS CFX).The analogue model focuses on characterizing the effects of thermal and compositional variations on surface tension driven fluid flow (Marangoni flow) and buoyancy driven flow during the dissolution of a low density, low surface tension and low melting point solid material (frozen ethanol) in a high density, high surface tension and high melting point liquid (water), which was found to be analogous to the dissolution of solid Al in liquid Ti. In addition, the analogue model was also capable to predict the dissolution behavior when there was no compositional difference between the solute and the solvent. Based on a comparison of fluid flow pattern and interface shape, and temperature data obtained at discrete locations in the experimental and computational results, the numerical model has been shown to quantitatively and qualitatively predict the dissolution behavior observed in the physical process.
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Electron Beam Cold Hearth Remelting (EBCHR) and its associated casting process is an important consolidation technique for the treatment of virgin titanium sponge and scrap. The development of robust models to describe the casting process hinge on accurately capturing heat transfer phenomena within the ingot and fluid flow phenomena within the liquid pool. The flow field that develops within the liquid pool is influenced by several factors including buoyancy driven flow due to thermal gradients within the pool, surface tension, or Marangoni, driven flow due to the large thermal gradients induced on the surface by the Electron Beam and the ability of the mushy, or semi-solid, zone to attenuate the flow. A mathematical model describing fluid flow and heat transfer in a Ti6Al4V button sample during electron beam melting has been developed to examine the relative contribution of the three factors cited above on the pool profile and flow field within the pool. The model has also been used to compare the steady state solution for a time averaged circular beam pattern with a transient solution obtained for the case where the beam pattern is comprised of a series of discrete points scribing the same circle. The latter, in which the beam spot is periodically stationary for small but finite periods, is intended to more closely mimic the industrial process.The model is also used to examine the sensitivity of the predictions to changes in numerical and process parameters. The results indicate that the electron beam power and heat transfer coefficient have the largest influence on the liquid pool profile while the surface tension coefficient has little effect (i.e. 25% change in electron beam power results in ~25% liquid pool profile while 100% change in time step results in less than 1% in prediction).
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Horizontal direct chill (HDC) casting is a continuous process used to produce extrusion billet and re-melt aluminum ingot. As in vertical DC casting, secondary cooling, where water directly impinges on the cast surface, is an important process that can affect cast quality and production rates. During HDC casting, secondary cooling is further complicated by horizontal water flow and the water spray conditions. Characterizing the heat transfer during the secondary cooling process is necessary for improved understanding of the process. Since the accessibility of the HDC casting machine is limited and the direct measurement of heat transfer in secondary cooling are difficult, numerical modeling thus becomes a good approach for process development.In this research, the heat transfer occurring in secondary cooling of an HDC ingot has been studied. Water spray conditions on three different casting surface were simulated separately by quenching the blocks of HDC cast A356 aluminum alloy which was cut from a T-ingot. The temperature history during the cooling within the blocks was recorded by sub-surface thermocouples. An inverse heat transfer model was developed and used to calculate the heat fluxes on the casting surfaces using measured temperature data. The heat fluxes were characterized via boiling curves, which are the functions of surface temperatures, in each spray configuration.The effects of operational parameters, including the casting speed and the water cooling rate, were investigated by comparing the characteristic features of the calculated boiling curves. The spray configuration effect was also studied with the calculated results from the stationary tests in a qualitative fashion.Then a fitting technique was developed to idealize the calculated boiling curves. The idealized boiling curves were summarized into the functions,which provide practical database for application of the results in this research.All in all, the simulation apparatus and the IHC model provide the ability of characterizing the heat transfer occurring in secondary cooling region of HDC casting with lab-scale experiments. Consequently, the expensive and risky plant trials can be avoided.
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