Publicação
Mechanical and fatigue properties of functionally graded aluminium silicon alloys
| Resumo: | Many structural components encounter service conditions and, hence, required materials performance, which vary with location within the component. It is well known that abrupt transitions in materials composition and properties within a component often result in sharp local concentrations of stress, whether the stress is internal or applied externally. It is also known that these stress concentrations are greatly reduced if the transition from one material to the other is made gradual. By definition, functionally graded materials are used to produce components featuring engineered gradual transitions in microstructure and/or composition, the presence of which is motivated by functional performance requirements that vary with location within a part. With functionally graded materials, these requirements are met in a manner that optimizes the overall performance of the component. The research on functionally graded materials (FGMs) is encouraged by the need for properties that are unavailable in any single material and the need for graded properties to offset adverse effects of discontinuities for layered materials. Centrifugal casting is a very common method for obtaining functionally graded materials, mainly composite materials or metallic materials which has high differences of density and low solubility on different phases or different materials of the same alloy. The present work is emphasizing the fact that the centrifugal process could be successfully used for obtaining functionally graded materials also for metallic materials (alloys) with moderate solubility and small differences of density of the different phases, as is the case of most aluminum alloys. The first approach of the problem was to isolate the effects of the centrifugal casting technique (the centrifugal pressure effect, the fluid dynamics and the inherent vibration effects) in order to identify the reason of mechanical properties improving. To have a reference for comparison, castings obtained by both centrifugal casting technique and gravity casting technique were tested. To isolate the vibration effect, experimental equipment was designed and constructed in order to be able to cast within a certain level of vibration equivalent with the vibration level of the centrifugal casting equipment. The results are confirming that there is a correlation of improving mechanical properties with the vibration of the melt during solidification. The difference of the mechanical properties of castings obtained by gravity casting technique and by centrifugal casting technique could be explained by the fact that, the vibration due to the inherently vibration of the equipment, the fluid dynamics and the centrifugal pressure make the melt, during solidification, to initiate more nuclei of solidification. Then, the centrifugal pressure moves the nuclei of solidification to the furthest point of the mould (where the pressure is higher) fact that explains the obtained results which are higher on one side of the ingots which corresponds with the side of the mould where the pressure is higher and smaller on the other side where the pressure is smaller. This causes several differences in microstructures in both sides of the ingot. The mechanical and fatigue properties are largely influenced by microstructure and the presence of material inhomogeneities. Pores, inclusions or secondary phase particles are common sites for fatigue crack nucleation in aluminium alloys. The constituent particle’s size and shape are also important characteristics that influence crack nucleation. This study intends to assess also the problem of fatigue life prediction by establishing a relation within some of the characteristics of the micro structural features of studied aluminium silicon alloys such as: micropores, secondary dendrites arm spacing (SDAS), volume fractions of phases (α-Al phase, eutectic and intermetallic phases), the size of silicon lamellas in interdendritic eutectic regions and the size and shape of silicon particles. This evaluation was performed along the ingots gradients for different aluminum alloys. |
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| Autores principais: | Chirita, Georgel |
| Assunto: | Functionally graded materials Gravity casting Centrifugal casting Vibrating casting Mechanical properties Fatigue life Materiais com gradação funcional Fusão com gravidade Fusão centrífugo Fusão vibratória Propriedades mecânicas Fadiga |
| Ano: | 2011 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Many structural components encounter service conditions and, hence, required materials performance, which vary with location within the component. It is well known that abrupt transitions in materials composition and properties within a component often result in sharp local concentrations of stress, whether the stress is internal or applied externally. It is also known that these stress concentrations are greatly reduced if the transition from one material to the other is made gradual. By definition, functionally graded materials are used to produce components featuring engineered gradual transitions in microstructure and/or composition, the presence of which is motivated by functional performance requirements that vary with location within a part. With functionally graded materials, these requirements are met in a manner that optimizes the overall performance of the component. The research on functionally graded materials (FGMs) is encouraged by the need for properties that are unavailable in any single material and the need for graded properties to offset adverse effects of discontinuities for layered materials. Centrifugal casting is a very common method for obtaining functionally graded materials, mainly composite materials or metallic materials which has high differences of density and low solubility on different phases or different materials of the same alloy. The present work is emphasizing the fact that the centrifugal process could be successfully used for obtaining functionally graded materials also for metallic materials (alloys) with moderate solubility and small differences of density of the different phases, as is the case of most aluminum alloys. The first approach of the problem was to isolate the effects of the centrifugal casting technique (the centrifugal pressure effect, the fluid dynamics and the inherent vibration effects) in order to identify the reason of mechanical properties improving. To have a reference for comparison, castings obtained by both centrifugal casting technique and gravity casting technique were tested. To isolate the vibration effect, experimental equipment was designed and constructed in order to be able to cast within a certain level of vibration equivalent with the vibration level of the centrifugal casting equipment. The results are confirming that there is a correlation of improving mechanical properties with the vibration of the melt during solidification. The difference of the mechanical properties of castings obtained by gravity casting technique and by centrifugal casting technique could be explained by the fact that, the vibration due to the inherently vibration of the equipment, the fluid dynamics and the centrifugal pressure make the melt, during solidification, to initiate more nuclei of solidification. Then, the centrifugal pressure moves the nuclei of solidification to the furthest point of the mould (where the pressure is higher) fact that explains the obtained results which are higher on one side of the ingots which corresponds with the side of the mould where the pressure is higher and smaller on the other side where the pressure is smaller. This causes several differences in microstructures in both sides of the ingot. The mechanical and fatigue properties are largely influenced by microstructure and the presence of material inhomogeneities. Pores, inclusions or secondary phase particles are common sites for fatigue crack nucleation in aluminium alloys. The constituent particle’s size and shape are also important characteristics that influence crack nucleation. This study intends to assess also the problem of fatigue life prediction by establishing a relation within some of the characteristics of the micro structural features of studied aluminium silicon alloys such as: micropores, secondary dendrites arm spacing (SDAS), volume fractions of phases (α-Al phase, eutectic and intermetallic phases), the size of silicon lamellas in interdendritic eutectic regions and the size and shape of silicon particles. This evaluation was performed along the ingots gradients for different aluminum alloys. |
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