Low Pressure Die Casting (LPDC) process is one the most common casting process to produce structural automotive components, such as alloy wheels and suspension components. It has been identified that cavity filling and solidification process are two most critical aspects to produce premium quality casting components.During the solidification process of casting alloy, it is a well known phenomenon that metal experiences volumetric shrinkage due to its density difference between liquid and solid phase. When this volumetric shrinkage is not properly compensated, then a casting defect commonly known as solidification shrinkage occurs. The solidification shrinkage has very detrimental effects on structural integrity required for premium quality casting such as aluminium alloy wheels.Literature and practical experiences of foundry men show that it is critical to achieve unidirectional solidification pattern by avoiding an isolated hot spot in order to minimise the solidification shrinkage. However, it is found that the geometry of industrial casting applications is often constrained by other design factors that would not naturally avoid these isolated hot spots. The subject of this research, aluminium alloy wheels, is not excluded from this constraint.In aluminium alloy wheels, an isolated hot spot is commonly observed in an area known as rim and spoke junction due to its geometry constraints. Consequently, the solidification shrinkage is commonly experienced, which is undesirable due to its detrimental effects for the structural integrity of alloy wheels. In order to minimise the solidification shrinkage, forced cooling method is applied to avoid an isolated hot spot. The control of this forced cooling is achieved by cooling media, flow rate of cooling media and duration cooling circuit. Foundry experiments in industrial environment were conducted producing aluminium alloy wheels using commercially treated A356 (Al-Si) alloy, where different durations of cooling circuit were used to understand the sensitivity of solidification shrinkage formation to the duration of cooling circuit. This was followed by metallurgical structure analysis and numerical modelling to suggest the sensitivity of cooling circuit duration in controlling solidification shrinkage.The major finding conclusion of this research is that control of the shrinkage formation is not very sensitive to the duration cooling circuit. It is suggested that as casting solidifies initially from the mould wall, it retracts away from the cast-mould interface due to thermal contraction. Consequently, air gap is formed between casting and mould interface, creating an effective thermal resistance layer. Thereafter, heat transfer across the cast-mould interface is not sensitive to the change in the cooling channel which is a distance to the cast-mould interface.Some limitations of numerical modelling and metallurgical analysis were also identified during this research and recommendations were made to improve the accuracy of local hot spot prediction in production of aluminium alloy wheels. More specifically, numerical modelling of the effect of grain refinement and use of non homogeneous material property (particularly fraction of solid) for rapidly chilled area. Fraction of eutectic rather than secondary dendrites arm spacing is a proper microstructure parameter that can be used to locate the hot spot.

Low Pressure Die Casting (LPDC) process is one the most common casting process to produce structural automotive components, such as alloy wheels and suspension components. It has been identified that cavity filling and solidification process are two most critical aspects to produce premium quality casting components.During the solidification process of casting alloy, it is a well known phenomenon that metal experiences volumetric shrinkage due to its density difference between liquid and solid phase. When this volumetric shrinkage is not properly compensated, then a casting defect commonly known as solidification shrinkage occurs. The solidification shrinkage has very detrimental effects on structural integrity required for premium quality casting such as aluminium alloy wheels.Literature and practical experiences of foundry men show that it is critical to achieve unidirectional solidification pattern by avoiding an isolated hot spot in order to minimise the solidification shrinkage. However, it is found that the geometry of industrial casting applications is often constrained by other design factors that would not naturally avoid these isolated hot spots. The subject of this research, aluminium alloy wheels, is not excluded from this constraint.In aluminium alloy wheels, an isolated hot spot is commonly observed in an area known as rim and spoke junction due to its geometry constraints. Consequently, the solidification shrinkage is commonly experienced, which is undesirable due to its detrimental effects for the structural integrity of alloy wheels. In order to minimise the solidification shrinkage, forced cooling method is applied to avoid an isolated hot spot. The control of this forced cooling is achieved by cooling media, flow rate of cooling media and duration cooling circuit. Foundry experiments in industrial environment were conducted producing aluminium alloy wheels using commercially treated A356 (Al-Si) alloy, where different durations of cooling circuit were used to understand the sensitivity of solidification shrinkage formation to the duration of cooling circuit. This was followed by metallurgical structure analysis and numerical modelling to suggest the sensitivity of cooling circuit duration in controlling solidification shrinkage.The major finding conclusion of this research is that control of the shrinkage formation is not very sensitive to the duration cooling circuit. It is suggested that as casting solidifies initially from the mould wall, it retracts away from the cast-mould interface due to thermal contraction. Consequently, air gap is formed between casting and mould interface, creating an effective thermal resistance layer. Thereafter, heat transfer across the cast-mould interface is not sensitive to the change in the cooling channel which is a distance to the cast-mould interface.Some limitations of numerical modelling and metallurgical analysis were also identified during this research and recommendations were made to improve the accuracy of local hot spot prediction in production of aluminium alloy wheels. More specifically, numerical modelling of the effect of grain refinement and use of non homogeneous material property (particularly fraction of solid) for rapidly chilled area. Fraction of eutectic rather than secondary dendrites arm spacing is a proper microstructure parameter that can be used to locate the hot spot.

The primary focus of the present work is to model the entire casting process from filling stage to complete solidification. The model takes into consideration any phase change taking place during the filling process. An implicit volume of fluid (VOF) based algorithm has been employed for simulating free surface flows during the filling process and the model for solidification is based on a fixed-grid enthalpy-based control volume approach. Solidification modelling is coupled with VOF through User Defined Functions (UDF) developed in commercial fluid dynamics (CFD) code FLUENT 6.3.26. The developed model is applied for the simultaneous filling and solidification of pure metals and binary alloy systems to study the effects of filling process on the solidification characteristics, evolution of mushy zone and the final macrosegregation pattern in the casting. The numerical results of the present analysis are compared with the conventional analysis assuming the initial conditions to be a completely filled mould cavity with uniform temperature, solute concentration and quiescent melt inside the cavity. The effects of process parameters, namely the degree of superheat, cooling temperature and filling velocity etc. are also investigated. Results show significant differences on the evolution of mushy zone and macrosegregation between the present analysis and the conventional analysis. The application of present model to simulate three dimensional sand casting is also demonstrated. The three dimensional competetive effect of filling generated residual flow and the buoyancy-induced convective flow pattern cause significant difference in macrosegregation pattern in casting.

Submitted by Cassia Cavalcanti da Silva null (cassiacavalcanti@terra.com.br) on 2016-08-01T22:46:23Z No. of bitstreams: 1 Silva_Cássia_2016.pdf: 9369604 bytes, checksum: d9e7dedc0ffe91324f392628a9e5c12f (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-08-03T16:09:39Z (GMT) No. of bitstreams: 1 silvia_cc_dr_guara.pdf: 9369604 bytes, checksum: d9e7dedc0ffe91324f392628a9e5c12f (MD5) / Made available in DSpace on 2016-08-03T16:09:39Z (GMT). No. of bitstreams: 1 silvia_cc_dr_guara.pdf: 9369604 bytes, checksum: d9e7dedc0ffe91324f392628a9e5c12f (MD5) Previous issue date: 2016-07-07 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A liga AA 356 é uma liga Al-Si amplamente utilizada tanto na indústria automobilística como na aeronáutica. Para melhorar suas propriedades mecânicas, são usados inoculantes para o refino de grão e para modificação eutética. Os componentes solidificados em moldes metálicos, que são mais eficientes na extração de calor, em geral, têm propriedades mecânicas superiores àqueles solidificados em moldes de areia que apresentam uma menor taxa de extração de calor. A curva de resfriamento que é uma curva da temperatura em função do tempo obtida da extremidade de um termopar localizado no centro do molde e conectado a um sistema de aquisição de dados é uma ferramenta que permite avaliar tanto o refino de grão quanto a modificação eutética. Essa ferramenta permite observar também a formação de fases intermetálicas, e é influenciada por fatores como a taxa de resfriamento. A primeira derivada da curva de resfriamento que representa a taxa de resfriamento é parte da ferramenta, pois melhora a precisão dos dados obtidos com a curva de resfriamento. Para o desenvolvimento desse trabalho foram preparados lingotes variando-se o material do molde (areia e aço), o teor de modificador de eutético e a adição de refinador de grão, ambos na forma de ligas-mãe (ante-liga) Al-10Sr e Al-5Ti-1B, respectivamente. Os valores para traçar as curvas de resfriamento foram obtidos monitorando a temperatura do metal desde o vazamento até sua completa solidificação e posterior resfriamento. Amostras do material foram preparadas para metalografia e obtidas imagens de macroestrutura e microestrutura. Da microestrutura foram obtidas as imagens com o ataque químico convencional e o ataque químico profundo; foram realizadas medidas de fração de porosidade por área, por técnica de microscopia, além de medidas de dureza Vickers e Brinell. Os resultados confirmam que a técnica da análise térmica da curva de resfriamento é uma excelente ferramenta, pois além de fornecer as temperaturas solidus e liquidus, as temperaturas de transformações de fases e informações sobre a eficácia do refino de grão e modificação eutética, pode ser utilizada na avaliação da presença de impurezas e na identificação de fases presentes nas ligas, tanto da liga primária quanto da liga reciclada. / The alloy AA 356 is an Al-Si alloy widely used in the automotive and aeronautics industry. To improve the mechanical properties of this alloy inoculants are used to grain refine and eutectic modification. The solidified components in metal mold, with a higher heat extraction rate, generally have mechanical properties superior to those solidified in sand mold with a lower heat extraction rate. The cooling curve is a curve of temperature versus time obtained in the end of a thermocouple located in the center of the mold and connected to a data acquisition system. It is a tool to evaluate both the grain refinement as the eutectic modification. This tool also can also be used to observe the formation of intermetallic phases, and is influenced by factors such as the cooling rate. The first derivative of the curve representing the cooling rate is part of the tool because it improves the accuracy of the data obtained from the curve. For the development of this work ingots were prepared varying the mold material (sand and steel), the content of eutectic modifier and the grain refiner addition , both in the form of master alloys Al-10Sr and Al-5Ti-1B, respectively.The data for plotting the cooling curves were obtained experimentally during solidification; Samples of the material were prepared for metallography and macrostructure and microstructure images were obtained. Through microstructure images obtained with conventional etching and deep etching. Porosity fraction area was measured by microscopy technique and hardness measurements were performed by Vickers and Brinell metdhod. The results confirm that the thermal analysis of the cooling curve is an excellent tool because provides the solidus and liquidus temperatures and the phase transformations on the effectiveness of the grain refining and eutectic modification and it can be used to assess the presence of impurities in the phase identification, for both the primary alloy and the recycled alloy.