Dissertations / Theses on the topic 'Semi Solid Processing (SSP)'

Twin Screw Rheo Extrusion (TSRE) is a novel semisolid extrusion process developed at BCAST for producing simple profiles such as rods and wires of light alloys directly from melts with refined microstructures and improved mechanical properties. The process represents a shortened manufacturing route with great savings in investment, energy consumption and operation space. Research was carried out to investigate the feasibility of processing magnesium and aluminium alloys, to obtain the operations for the optimized microstructures and mechanical properties of the final product and to understand the mechanisms governing the evolution of microstructures. Experiments were conducted using an AZ91D magnesium alloy and several aluminium alloys on two specially made twin screw rheo extrusion machines and a range of conditions were tested. Results showed that the TSRE process was feasible for the AZ91D magnesium alloy and aluminium alloys, although modifications were required for processing aluminium alloys as the twin screw material used was found to react with aluminium. Analysis revealed that the extruded samples of both alloys had a uniform fine microstructure in both transversel and longitudinal directions and liquid segregation was limited, due to the application of intensive shearing during slurry making and extrusion. Low extrusion temperature was found to refine the structure and suppress the formation of the eutectic. The eutectic was easily dissolved upon heat treatment resulting in reasonable mechanical properties. Numerical analysis on thermal management was carried out and the results showed that a steady state thermal profile with a temperature gradient between the slurry feeding point and extrusion die could be established, promoting nucleation and preventing the formed solid particles from extensive growth during extrusion, which was confirmed by microstructural observations.

Orientador: Antonio de Pádua Lima Filho<br>Resumo: A liga Al-Si A413 foi utilizada para fabricação de tiras metálicas fundidas por cilindro único e por cilindro duplo, aqui chamado processo de reolaminação. Esta liga não é adequada para obtenção de tiras metálicas por laminação convencional devido às partículas frágeis de Si. Por outro lado, a reolaminação é um processo viável para obter tiras metálicas dessa liga. Neste trabalho, a liga Al-Si A413 foi fundida e vazada com diferentes temperaturas em uma calha metálica inclinada a 20º numa vazão de 14 cm³/s para se obter um material semissólido que alimenta um bocal cerâmico (150 cm³) junto ao cilindro inferior. Na reolaminação, o espaçamento entre os cilindros foi de 1,5 mm. Os cilindros na cadeira de laminação são feitos de aço ao carbono comum e têm aproximadamente 105 mm de diâmetro e 100 mm de largura. A região coquilhada/colunar formada no cilindro inferior arrasta a lama metálica a uma velocidade de 0,2 m/s para ser processada tanto por cilindro único como por reolaminação. Para ambos os processos as tiras metálicas fundidas têm uma espessura de 2 mm, aproximadamente. Na saída da cadeira de laminação, as tiras são resfriadas até a temperatura ambiente por chuveiros de água. O equipamento utilizado para a fabricação das tiras metálicas, chamado de “Strip Caster”, passou por inovações durante este trabalho de mestrado: molas de alívio de pressão foram instaladas no cilindro superior e o cilindro inferior foi substituído por um outro cilindro com refrigeração interna. O “S... (Resumo completo, clicar acesso eletrônico abaixo)<br>Abstract: The Al-Si A413 alloy was used in order to produce metallic strips with the single roll and twin roll processes, this last one is known as rheolamination. This alloy is not suitable to obtain metallic strips with the conventional lamination due to the brittle particles of silicon. On the other hand, the rheolamination is a suitable process to obtain metallic strips of this alloy. In this work, Al-Si A413 alloy was melted and poured at 680 ºC on a cooling slope at 20º with a flow rate of 14 cm³/s in order to obtain a semisolid material feeding the ceramic nozzle (150 cm³) at the lower roll. The two rolls of the roll stand are separated with a gap of approximately 1.5 mm, have approximately 105 mm in diameter and are made of carbon steel. The chill/columnar layer formed at the lower roll drags the metallic slurry at a rate of 0.2 m/s to be processed by single roll or twin-roll. Both processes obatin a metallic cast strip with a thickness of 2 mm, approximately. At the exit of the stand roll, the strips are cooled to the room temperature with water showers. The equipment used to produce metallic strips, called “Strip Caster”, suffered some inovations during this work: springs was attached to the upper roll in order to reduce the lamination pressure and the lower roll was substituted with an other roll with internal cooling. The Strip Caster was instrumented with two load cell in order to measure the forming forces and with thermocouples to measure the temperatures during the fabr... (Complete abstract click electronic access below)<br>Mestre

Les polyimides sont des polymères a hautes performances souvent utilisés dans des environnements dits hostiles (hautes températures et hautes pressions, forts frottement, …). Ils sont connus pour leur forte résistance thermique et chimique mais aussi pour leur faible processabilité. La technique de synthèse la plus utilisée industriellement est celle utilisant les poly(acides amiques) comme intermédiaires, ce qui nécessite des solvants couteux et dangereux pour l’homme et l’environnement comme le crésol ou la N-methyl-2-pyrrolidone. Les polyimides semi-aromatiques ont eux des températures caractéristiques (fusion, transition vitreuse) plus basses que les polyimides aromatiques, ce qui permet d’augmenter leur processabilité (extrusion, injection). Une nouvelle technique de synthèse basée sur la polymérisation de sels précurseurs permet de synthétiser des polyimides (aromatiques ou non) en utilisant des solvants moins coûteux et moins dangereux. De nombreuses recherches ont été menées sur ce thème ces cinquante dernières années. À notre connaissance, cette technique n’est pas être utilisée industriellement. L’objectif de ces travaux de thèse consiste en la synthèse de polyimides semi aromatiques via la polymérisation à l’état solide de sels précurseur. Une étude préliminaire sur des molécules de faible masse molaire a permis de mettre en avant les paramètres critiques de l’étape de salification et le comportement thermique lors de l’imidification. Un protocole de salification et de polymérisation à l’état solide a été mis au point puis utilisé pour synthétiser trois polyimides semi-aromatiques présentant une solubilité accrue. Cette solubilité a permis une caractérisation complète des polymères, nous conduisant à un meilleur contrôle la synthèse. Des éléments de réponse au sujet des mécanismes réactionnels ont été proposés. Deux techniques visant le contrôle de la masse molaire des polymères synthétisés ont été testées puis comparées en termes d’efficacité et sur leur caractère industrialisable. Enfin, deux polymères synthétisés ont étés mis en œuvre puis caractérisés mécaniquement<br>Polyimides are known as high performance polymers, they are used in harsh environments (high temperatures, high pressure, …). They also have low processibility. The most used industrial synthesis process is using poly(amic acid) as reaction intermediate. This process requires high prices solvents that are harmful for human being and environment such as n-methyl-2-pyrrolidone and cresol. Semi-aromatic polyimides have lower characteristic temperatures and can thus be processed easily. A new way of synthesis based on salt polymerisation can be used to synthesize polyimides (aromatic or not), this process only needs simple solvents such as water or ethanol. Numerous researches have been made on this subject in the past 50 years. To our knowledge, this technique is not used at industrial scale. The goal of this work is to synthetize semi-aromatic polyimides using solid-state salt polymerisation. A preliminary examination on low weight molecules allowed us to highlight critical parameters on every step of reactions and thermal behaviour. Both salification and polymerization protocols are been made with the view to synthetize 3 soluble polyimides. This solubility allowed us to characterize our polymers and to enhance polymerization control. Those characterizations provided us answers on salification and polymerization mechanisms. Two molar mass control techniques have then been compared in terms of industrialization and efficiency. At the end, polymers have been synthetized, processed and then characterized in physic-chemical and mechanical ways

Gradientes térmicos induzidos no interior de componentes sujeitos a variações de temperatura durante o período de funcionamento podem provocar a ocorrência de tensões e deformações internas. A repetição destes ciclos térmicos pode causar a nucleação e a propagação de trincas por um processo denominado fadiga termomecânica. Este trabalho apresenta um estudo sobre o comportamento da liga de alumínio A356-T6, processada nas condições fundida e tixoextrudada, sob fadiga isotérmica e termomecânica. Foram realizados ensaios de fadiga de baixo ciclo isotérmica para as temperaturas de 120 e 280°C, e ensaios de fadiga termomecânica em-fase e fora-de-fase para a faixa de temperatura de 120 a 280°C. O material tixoextrudado apresentou melhor desempenho em fadiga nas condições isotérmica e anisotérmica (termomecânica) devido a uma microestrutura globular com menor nível de porosidade.<br>Thermal gradients induced in components during service under temperature changes can cause internal stresses and strains. This cyclic thermal behavior can cause crack nucleation and propagation under a process denominated thermomechanical fatigue. Permanent mold casting and tixoextruded A356-T6 aluminum alloy behavior under isothermal and thermomechanical fatigue was study in this work. Isothermal low cycle fatigue tests were performed in temperatures of 120 and 280°C. In-phase and out-of-phase thermomechanical fatigue tests were carried out in temperature range from 120 to 280°C. The tixoextruded material presented better isothermal and thermomechanical fatigue performance due to a globular microstructure and lower porosity level.

In most casting applications, dendritic microstructure morphology is not desired because it leads to poor mechanical properties. Forced convection causing sufficient shearing in the mushy zone of the partially solidified melt is one of the means to suppress this dendritic growth. The dendrites formed at the solid-liquid interface are detached and carried away due to strong fluid flow to form slurry. This slurry, consisting of rosette or globular particles, provides less resistance to flow even at a high solid fraction and can easily fill the die-cavity. The stated principle is the basis of a new manufacturing technology called “semi-solid forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has numerous advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. Among all currently available methods available for large scale production of semisolid slurry, the cooling slope is considered to be a simple but effective method because of its simple design and easy control of process parameters, low equipment and running costs, high production efficiency and reduced inhomogeneity. With this perspective, the primary objective of the present research is to investigate, both experimentally and numerically, convective heat transfer and solidification on a cooling slope, in addition to the study of final microstructure of the cast billets. Some key process parameters are identified, namely pouring temperature, slope angle, slope length, and slope cooling rate. A systematic scaling analysis is performed in order to understand the relative importance of the parameters in influencing the final properties of the slurry and microstructure after solidification. A major part of the present work deals with the development of an experimental set up with careful consideration of the range of process parameters involved by treating the cooling slope as a heat exchanger. Subsequently, a comprehensive numerical model is developed to predict the flow, heat transfer, species concentration solid fraction distribution of aluminum alloy melt while flowing down the cooling slope. The model uses a variable viscosity relation for slurry. The metal-air interface at the top during the melt flow is tracked using a volume of fluid (VOF) method. Solidification is modeled using an enthalpy based approach and a volume averaged technique. The mushy region is modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed or fragmented grains. In addition, the solidification model also incorporates a fragmentation criterion and solid phase movement. The effects of key process parameters on flow behavior involving velocity distribution, temperature distribution, solid fractions at the slope exit, and macrosegregation, are studied numerically and experimentally for aluminium alloy A356. The resulting microstructures of the cast billets obtained from the experiments are studied and characterized. Finally the experimental results are linked to the model predictions for establishing the relations involving interdependence of the stated key process parameters in determining the quality of the final cast products. This study is aimed towards providing the necessary guidelines for designing a cooling slope and optimizing the process parameters for desirable quality of the solidified product.

In most casting applications, dendritic microstructure morphology is not desired because it leads to poor mechanical properties. Forced convection causing sufficient shearing in the mushy zone of the partially solidified melt is one of the means to suppress this dendritic growth. The dendrites formed at the solid-liquid interface are detached and carried away due to strong fluid flow to form slurry. This slurry, consisting of rosette or globular particles, provides less resistance to flow even at a high solid fraction and can easily fill the die-cavity. The stated principle is the basis of a new manufacturing technology called “semi-solid forming” (SSF), in which metal alloys are cast in the semi-solid state. This technique has numerous advantages over other existing commercial casting processes, such as reduction of macrosegregation, reduction of porosity and low forming efforts. Among all currently available methods available for large scale production of semisolid slurry, the cooling slope is considered to be a simple but effective method because of its simple design and easy control of process parameters, low equipment and running costs, high production efficiency and reduced inhomogeneity. With this perspective, the primary objective of the present research is to investigate, both experimentally and numerically, convective heat transfer and solidification on a cooling slope, in addition to the study of final microstructure of the cast billets. Some key process parameters are identified, namely pouring temperature, slope angle, slope length, and slope cooling rate. A systematic scaling analysis is performed in order to understand the relative importance of the parameters in influencing the final properties of the slurry and microstructure after solidification. A major part of the present work deals with the development of an experimental set up with careful consideration of the range of process parameters involved by treating the cooling slope as a heat exchanger. Subsequently, a comprehensive numerical model is developed to predict the flow, heat transfer, species concentration solid fraction distribution of aluminum alloy melt while flowing down the cooling slope. The model uses a variable viscosity relation for slurry. The metal-air interface at the top during the melt flow is tracked using a volume of fluid (VOF) method. Solidification is modeled using an enthalpy based approach and a volume averaged technique. The mushy region is modeled as a multi-layered porous medium consisting of fixed columnar dendrites and mobile equiaxed or fragmented grains. In addition, the solidification model also incorporates a fragmentation criterion and solid phase movement. The effects of key process parameters on flow behavior involving velocity distribution, temperature distribution, solid fractions at the slope exit, and macrosegregation, are studied numerically and experimentally for aluminium alloy A356. The resulting microstructures of the cast billets obtained from the experiments are studied and characterized. Finally the experimental results are linked to the model predictions for establishing the relations involving interdependence of the stated key process parameters in determining the quality of the final cast products. This study is aimed towards providing the necessary guidelines for designing a cooling slope and optimizing the process parameters for desirable quality of the solidified product.

The mechanical properties achieved with high performance wrought aluminium alloys are superior to cast aluminum alloys. To obtain an intricate shaped component, wrought alloys are commonly subjected to forging followed by subsequent machining operation in the automobile industry. As machining of such high strength wrought aluminium alloys adds to cost, productivity gets affected. Shortening the process by near net shaped casting would tremendously enhance productivity. However, casting of such alloys frequently encounter hot tear defect. Therefore, circumventing hot tear to successfully die cast near net shaped wrought alloy components is industrially relevant. A recent advanced casting process, namely ‘Semisolid Die casting’, is proposed as a likely solution. Hot tearing originates due to lack of liquid flow in the inter-dendritic region. To reduce hot tear susceptibility, fine and non-dendritic grain structure is targeted, amenable for processing by semisolid route. For semisolid processing an adequate freezing range for processing is required. Accordingly A6061 wrought alloy whose composition is tuned with higher silicon and magnesium content within the grade limits, is chosen for the study. With the objective of obtaining fine and non-dendritic microstructured billets, electromagnetic stirring (EMS) and cooling slope (CS) methods are employed. On conducting a parametric study with EMS, a finest possible primary α-Al grain size of about 70 μm is obtained at low stirring time at stirring current levels of 175 A and 350 A, with the addition of grain refiner. CS, on the other hand, rendered a grain of 60 μm at a slope length of 300 mm at a slope angle of 45° with grain refiner addition. Of the two methods, CS billets are chosen for subsequent induction heating. A 3-step induction heating cycle has been devised to attain a temperature of 641°C in the billet on the basis of factors including coherency point, viscosity of the slurry and solid fraction sensitivity with temperature. The billet microstructure is found to be homogenous throughout after quenching in water. The characterization of phase along primary α-Al grain boundary and its composition analysis is done by SEM and EPMA respectively, after billet casting as well as induction heating. In addition, the bulk hardness is determined in BHN. The induction heated billets are semisolid die cast to produce an engine connecting rod used in automobiles. The microstructure is characterized at various locations, and is found to consist of smooth α-Al grains in a background matrix of fine grains formed due to secondary solidification. The component hardness is found to be 66 BHN comparable with A6061 alloy under T4 heat treated condition. X-ray radiography does not confirm presence of surface hot tear, which is the normal defect associated with casting of wrought aluminium alloys. No defects are observed along the constant cross-sectional area of the connecting rod, suggesting that the processing could be suitable for semisolid extrusion.

The mechanical properties achieved with high performance wrought aluminium alloys are superior to cast aluminum alloys. To obtain an intricate shaped component, wrought alloys are commonly subjected to forging followed by subsequent machining operation in the automobile industry. As machining of such high strength wrought aluminium alloys adds to cost, productivity gets affected. Shortening the process by near net shaped casting would tremendously enhance productivity. However, casting of such alloys frequently encounter hot tear defect. Therefore, circumventing hot tear to successfully die cast near net shaped wrought alloy components is industrially relevant. A recent advanced casting process, namely ‘Semisolid Die casting’, is proposed as a likely solution. Hot tearing originates due to lack of liquid flow in the inter-dendritic region. To reduce hot tear susceptibility, fine and non-dendritic grain structure is targeted, amenable for processing by semisolid route. For semisolid processing an adequate freezing range for processing is required. Accordingly A6061 wrought alloy whose composition is tuned with higher silicon and magnesium content within the grade limits, is chosen for the study. With the objective of obtaining fine and non-dendritic microstructured billets, electromagnetic stirring (EMS) and cooling slope (CS) methods are employed. On conducting a parametric study with EMS, a finest possible primary α-Al grain size of about 70 μm is obtained at low stirring time at stirring current levels of 175 A and 350 A, with the addition of grain refiner. CS, on the other hand, rendered a grain of 60 μm at a slope length of 300 mm at a slope angle of 45° with grain refiner addition. Of the two methods, CS billets are chosen for subsequent induction heating. A 3-step induction heating cycle has been devised to attain a temperature of 641°C in the billet on the basis of factors including coherency point, viscosity of the slurry and solid fraction sensitivity with temperature. The billet microstructure is found to be homogenous throughout after quenching in water. The characterization of phase along primary α-Al grain boundary and its composition analysis is done by SEM and EPMA respectively, after billet casting as well as induction heating. In addition, the bulk hardness is determined in BHN. The induction heated billets are semisolid die cast to produce an engine connecting rod used in automobiles. The microstructure is characterized at various locations, and is found to consist of smooth α-Al grains in a background matrix of fine grains formed due to secondary solidification. The component hardness is found to be 66 BHN comparable with A6061 alloy under T4 heat treated condition. X-ray radiography does not confirm presence of surface hot tear, which is the normal defect associated with casting of wrought aluminium alloys. No defects are observed along the constant cross-sectional area of the connecting rod, suggesting that the processing could be suitable for semisolid extrusion.

碩士<br>國立海洋大學<br>機械與輪機工程學系<br>86<br>The development of semi-solid metal forming technology that avoid porosity and shrinkage exist in traditional metal forming. This bring the metal forming field into a new situation. Because the semi-solid processing is different with past, the mechanic behavior are also different with fore. The viscosity is salient and important particularly. So, we cannot use the bypass forming parameters. Thereby, this treatise simulate filling process of semi-solid aluminum alloy A356 and traditional aluminum alloy A356. Finding the most flow modes of traditional alloy is turbulence, but the semi-solid alloy is laminar flow. One of the reasons owing to Reynolds number of semi-solid alloy is lower. This result decreasing porosity.If filling velocity is too fast, the viscosity is not enough time to get down. Therefore, the filling velocity of semi-solid metal forming is according as the material properties and object size. We should fit other factors, too. Then we can find the optimal filling velocity wherewith numerical simulation.

In view of the significant advantages offered by semi-solid processing, such as reduction in number of intermediate processing steps and energy input, and the potential for improving component complexity, it is of paramount interest to develop indigenous technology for semi-solid forming of steels, especially nuclear grade steels. For adopting semisolid processing as an alternative method of manufacturing of steels, it is essential to study the amenability of the steel for the process, understand the fundamental mechanisms of micro structural evolution and evaluate the mechanical properties of the steel after processing. To achieve this goal, the present work attempts to appraise the amenability of a low-carbon variant of 18%Cr-8%Ni austenitic stainless steel (304L SS) for semi-solid processing. Among the many requirements of the feedstock in semi-solid processing, a key feature that makes it amenable for semi-solid processing is the unique microstructure containing solid spheroids in a liquid matrix, thereby enabling thixo-tropic behaviour in the alloy. To understand the micro structural evolution in the steel, during major steps of semi-solid processing (partial melting, soaking and solidification), several experiments are carried out by varying the key parameters such as temperature, soaking time and cooling rate. Experimental results are analyzed in details to specify the effects of these parameters on the microstructure of semi-solid processed steel. The analysis indicates different phase transformation sequences during solidification of the steel from its semi-solid state. On the basis of experimental results, mechanism for micro structural evolution during partial melting and subsequent solidification of 304L SS is proposed. The effect of soaking time on the size and shape of the solid globules is analyzed using the theory of anisotropic Ostwald ripening. The semi-solid processing parameters, such as soaking time and temperature, are found to have significant influence on the globule distribution, globule shape, ferrite distribution and dislocation density, which in turn govern the tensile behaviour and mechanical properties of the steel after processing. Semi-solid processed 304L SS exhibits lower yield strength, ultimate tensile strength and higher strain hardening in temperature range 303–873K compared to as-received (rolled and subsequently annealed) 304L SS. However, semi-solid processed steel shows higher uniform elongation and fracture strain compared to the as-received steel. A pronounced effect of semi-solid processing is also found on the high temperature plasticity and dynamic recrystallization pattern. This work demonstrates the amenability of 300 series austenitic stainless steels for semi-solid processing. The investigation provides the significant insight into the mechanism of micro structural evolution in austenitic stainless steels during semi-solid processing and the important information on the mechanical properties and plastic flow behavior of the semi-solid processed steel. The results give crucial inputs for the optimization of processing parameters for obtaining the desired property in the product, and also for deciding the potential industrial application of the process.

In view of the significant advantages offered by semi-solid processing, such as reduction in number of intermediate processing steps and energy input, and the potential for improving component complexity, it is of paramount interest to develop indigenous technology for semi-solid forming of steels, especially nuclear grade steels. For adopting semisolid processing as an alternative method of manufacturing of steels, it is essential to study the amenability of the steel for the process, understand the fundamental mechanisms of micro structural evolution and evaluate the mechanical properties of the steel after processing. To achieve this goal, the present work attempts to appraise the amenability of a low-carbon variant of 18%Cr-8%Ni austenitic stainless steel (304L SS) for semi-solid processing. Among the many requirements of the feedstock in semi-solid processing, a key feature that makes it amenable for semi-solid processing is the unique microstructure containing solid spheroids in a liquid matrix, thereby enabling thixo-tropic behaviour in the alloy. To understand the micro structural evolution in the steel, during major steps of semi-solid processing (partial melting, soaking and solidification), several experiments are carried out by varying the key parameters such as temperature, soaking time and cooling rate. Experimental results are analyzed in details to specify the effects of these parameters on the microstructure of semi-solid processed steel. The analysis indicates different phase transformation sequences during solidification of the steel from its semi-solid state. On the basis of experimental results, mechanism for micro structural evolution during partial melting and subsequent solidification of 304L SS is proposed. The effect of soaking time on the size and shape of the solid globules is analyzed using the theory of anisotropic Ostwald ripening. The semi-solid processing parameters, such as soaking time and temperature, are found to have significant influence on the globule distribution, globule shape, ferrite distribution and dislocation density, which in turn govern the tensile behaviour and mechanical properties of the steel after processing. Semi-solid processed 304L SS exhibits lower yield strength, ultimate tensile strength and higher strain hardening in temperature range 303–873K compared to as-received (rolled and subsequently annealed) 304L SS. However, semi-solid processed steel shows higher uniform elongation and fracture strain compared to the as-received steel. A pronounced effect of semi-solid processing is also found on the high temperature plasticity and dynamic recrystallization pattern. This work demonstrates the amenability of 300 series austenitic stainless steels for semi-solid processing. The investigation provides the significant insight into the mechanism of micro structural evolution in austenitic stainless steels during semi-solid processing and the important information on the mechanical properties and plastic flow behavior of the semi-solid processed steel. The results give crucial inputs for the optimization of processing parameters for obtaining the desired property in the product, and also for deciding the potential industrial application of the process.