The need for lighter weight vehicles to improve fuel efficiency is becoming increasingly imperative. Sheet magnesium alloys offer the potential as a light weight material for use in the transportation industry due to their high specific strength and stiffness. In fact, magnesium alloys have the highest strength-to-weight ratio of all the common structural metals. Though the demand for light weight materials is present and sheet magnesium is available, the use of these materials in automotive applications has been rather limited due to high production costs and poor corrosion performance and formability. A promising process to produce wrought magnesium sheet in a more cost effective manner is Twin Roll Casting (TRC). In addition, enhanced corrosion resistance and ductility may be realized in these sheet alloys with the possible introduction of a clad layer during the TRC process thereby producing a laminate sheet where the surface properties are different from the core. The focus of this research was to investigate the potential of cladding magnesium alloy AZ31 material during the TRC process. As part of this research, a thermal fluid mathematical model of the TRC process was developed, which was then further refined to include the addition of a clad layer during the process. The TRC model was validated through experimental work conducted at the Pohang Institute of Science and Technology (POSTECH University), where TRC experiments of AZ31 were conducted under various casting conditions. The as-cast microstructure of the AZ31 sheets were characterized and measurements of the secondary dendrite arm spacing (SDAS) made at the mid-region were compared to predicted microstructures from the TRC model based on solidification history. The predicted SDAS matched with the measured values, thus, validating the model. Using the validated TRC model the feasibility of adding a clad layer was assessed and various simulations were conducted to observe the effects of cast speed, cast thickness, and clad material on the thermal history and temperature profile in both the clad and core domains. The material properties and clad thickness did not seem to impact the temperature profiles significantly, while the cast speed and initial temperature dictated whether or not the cast would be successful. Using these operational parameters a process window was created (based on the CANMET facility) to illustrate the feasibility of casting and cladding during TRC. This window is beneficial for future experimentation and understanding the effects of these casting parameters.

The need for lighter weight vehicles to improve fuel efficiency is becoming increasingly imperative. Sheet magnesium alloys offer the potential as a light weight material for use in the transportation industry due to their high specific strength and stiffness. In fact, magnesium alloys have the highest strength-to-weight ratio of all the common structural metals. Though the demand for light weight materials is present and sheet magnesium is available, the use of these materials in automotive applications has been rather limited due to high production costs and poor corrosion performance and formability. A promising process to produce wrought magnesium sheet in a more cost effective manner is Twin Roll Casting (TRC). In addition, enhanced corrosion resistance and ductility may be realized in these sheet alloys with the possible introduction of a clad layer during the TRC process thereby producing a laminate sheet where the surface properties are different from the core. The focus of this research was to investigate the potential of cladding magnesium alloy AZ31 material during the TRC process. As part of this research, a thermal fluid mathematical model of the TRC process was developed, which was then further refined to include the addition of a clad layer during the process. The TRC model was validated through experimental work conducted at the Pohang Institute of Science and Technology (POSTECH University), where TRC experiments of AZ31 were conducted under various casting conditions. The as-cast microstructure of the AZ31 sheets were characterized and measurements of the secondary dendrite arm spacing (SDAS) made at the mid-region were compared to predicted microstructures from the TRC model based on solidification history. The predicted SDAS matched with the measured values, thus, validating the model. Using the validated TRC model the feasibility of adding a clad layer was assessed and various simulations were conducted to observe the effects of cast speed, cast thickness, and clad material on the thermal history and temperature profile in both the clad and core domains. The material properties and clad thickness did not seem to impact the temperature profiles significantly, while the cast speed and initial temperature dictated whether or not the cast would be successful. Using these operational parameters a process window was created (based on the CANMET facility) to illustrate the feasibility of casting and cladding during TRC. This window is beneficial for future experimentation and understanding the effects of these casting parameters.

Although Twin Roll Casting (TRC) process has been used for almost 60 years in the aluminum industry, TRC of magnesium is relatively new. In TRC, molten metal is fed onto water-cooled rolls, where it solidifies and is then rolled. Solidification of the molten metal starts at the point of first metal-roll contact and is completed before the kissing point (point of least roll separation) of the two rolls. The unique thermo-physical properties inherent to magnesium and its alloys, such as lower specific heat and latent heat of fusion and larger freezing ranges (in comparison with aluminum and steel) make it challenging for TRC of this alloy. Therefore, a comprehensive understanding of the process and the interaction between the casting conditions and strip final quality is imperative to guarantee high quality twin roll cast strip production. A powerful tool to achieve such knowledge is to develop a mathematical model of the process. In this thesis, a 2D mathematical model for TRC of AZ31 magnesium alloy has been developed and validated based on the TRC facility located at the Natural Resources Canada Government Materials Laboratory (CanmetMATERIALS) in Hamilton, ON, Canada. The validation was performed by comparing the predicted exit strip temperature and secondary dendrite arm spacing (SDAS) through the strip thickness with those measured and obtained by experiments. The model was developed in two stages, first a thermal-fluid model was developed followed by validation and then a thermal-fluid-stress model was developed. This is the first time a comprehensive thermal-fluid-stress model has been developed to simulate the TRC process for magnesium alloys. The work has led to new knowledge about the TRC process and its effects on magnesium strip quality including the following: 1) Using ALSIM and ANSYS® CFX® commercial packages a 2D mathematical model of thermal-fluid-stress behavior of the magnesium sheet during TRC was successfully developed and validated. 2) An average value of 11 kW/m2°C for the Heat Transfer Coefficient (HTC) was found to best represent the heat transfer between the roll and the strip during TRC casting of AZ31 using the CanmetMATERIALS facility. 3) Modeling results showed that increasing casting speed, casting thicker strips and applying higher HTCs led to less uniform microstructure through thickness in terms of SDAS. 4) Simulations showed the importance of casting parameters such as casting speed and set-back distance on the thermal history and stress development in the sheet during TRC; higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC. 5) The effect of roll diameter on the thermal history and stress development in the strip was also studied and indicated how larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness. 6) The correlation between the mechanisms of center-line and inverse segregation formation and thermo-mechanical behavior of the strip was performed. The modeling results suggested that increasing the set-back distance decreases the risk of both defects. Moreover, increasing the roll diameter reduces the propensity to inverse segregation but has a minor effect for center-line segregation formation.

Although Twin Roll Casting (TRC) process has been used for almost 60 years in the aluminum industry, TRC of magnesium is relatively new. In TRC, molten metal is fed onto water-cooled rolls, where it solidifies and is then rolled. Solidification of the molten metal starts at the point of first metal-roll contact and is completed before the kissing point (point of least roll separation) of the two rolls. The unique thermo-physical properties inherent to magnesium and its alloys, such as lower specific heat and latent heat of fusion and larger freezing ranges (in comparison with aluminum and steel) make it challenging for TRC of this alloy. Therefore, a comprehensive understanding of the process and the interaction between the casting conditions and strip final quality is imperative to guarantee high quality twin roll cast strip production. A powerful tool to achieve such knowledge is to develop a mathematical model of the process. In this thesis, a 2D mathematical model for TRC of AZ31 magnesium alloy has been developed and validated based on the TRC facility located at the Natural Resources Canada Government Materials Laboratory (CanmetMATERIALS) in Hamilton, ON, Canada. The validation was performed by comparing the predicted exit strip temperature and secondary dendrite arm spacing (SDAS) through the strip thickness with those measured and obtained by experiments. The model was developed in two stages, first a thermal-fluid model was developed followed by validation and then a thermal-fluid-stress model was developed. This is the first time a comprehensive thermal-fluid-stress model has been developed to simulate the TRC process for magnesium alloys. The work has led to new knowledge about the TRC process and its effects on magnesium strip quality including the following: 1) Using ALSIM and ANSYS® CFX® commercial packages a 2D mathematical model of thermal-fluid-stress behavior of the magnesium sheet during TRC was successfully developed and validated. 2) An average value of 11 kW/m2°C for the Heat Transfer Coefficient (HTC) was found to best represent the heat transfer between the roll and the strip during TRC casting of AZ31 using the CanmetMATERIALS facility. 3) Modeling results showed that increasing casting speed, casting thicker strips and applying higher HTCs led to less uniform microstructure through thickness in terms of SDAS. 4) Simulations showed the importance of casting parameters such as casting speed and set-back distance on the thermal history and stress development in the sheet during TRC; higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC. 5) The effect of roll diameter on the thermal history and stress development in the strip was also studied and indicated how larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness. 6) The correlation between the mechanisms of center-line and inverse segregation formation and thermo-mechanical behavior of the strip was performed. The modeling results suggested that increasing the set-back distance decreases the risk of both defects. Moreover, increasing the roll diameter reduces the propensity to inverse segregation but has a minor effect for center-line segregation formation.

A principal motivação para o desenvolvimento do presente projeto de doutorado foi comparar as microestruturas e texturas resultantes da liga de alumínio 3003 obtida por lingotamento contínuo e semicontínuo. O processo “roll caster" (lingotamento contínuo em cilindros) tem sido crescentemente utilizado na indústria de laminação de alumínio. Produtos que antes sofriam altos graus de deformação durante seu processamento são hoje obtidos nas dimensões próximas da espessura final. Com o intuito de entender este processo relativamente novo, as microestruturas e as texturas (macrotextura e microtextura), ao longo da espessura, das chapas produzidas por lingotamento contínuo e por lingotamento semicontínuo (placas fundidas com 250 mm de espessura, seguido de homogeneização e laminação a quente) foram analisadas. Foi possível constatar que os materiais provenientes dos dois processos diferem bastante e que ao longo da espessura também há grandes diferenças. Foi possível observar que durante o processamento “roll caster" há considerável deformação plástica, com a formação de células de discordâncias e subgrãos. Foi possível observar, também, que a microestrutura, a morfologia e o tamanho de grão, da amostra “caster" bruta de fundição são mais homogêneos que na amostra laminada a quente. As duas chapas, a laminada a quente e a obtida por lingotamento contínuo, exibem forte gradiente de textura ao longo da espessura. A textura predominante nos dois processos, ao longo da espessura, é a do tipo latão {011}. A evolução da textura do material proveniente do processo “roll caster" também foi caracterizada após laminação e posterior recristalização, revelando a presença de componentes típicas de laminação e de recristalização de alumínio. Após laminação a frio com redução de 91% e posterior tratamento térmico de 400°C por 1 h, a chapa proveniente do processo “caster" apresentou uma textura de recristalização caracterizada pela presença da componente cubo (recristalização) somada à parcela de textura de deformação. No caso do alumínio e suas ligas, sabe-se que esta textura proporciona os melhores resultados para minimizar o efeito de orelhamento durante a estampagem. Outro ponto relevante do trabalho foi o aperfeiçoamento da técnica de extração de precipitados para a liga 3003, uma vez que, a extração de precipitados em ligas de alumínio é especialmente problemática, pois a maioria das partículas presentes dissolve-se até mais facilmente que a matriz de alumínio. Algumas técnicas foram testadas: 1. dissolução química em solução de iodo em metanol; 2. dissolução eletrolítica em solução de ácido benzóico e hidroxiquinolina em clorofórmio e metanol; 3. dissolução eletrolítica em solução de ácido perclórico, butil glicol, álcool etílico e água; 4. dissolução química em solução de fenol. A técnica que apresentou melhores resultados foi a dissolução química com fenol, onde foi possível separar os precipitados da matriz alumínio. Com o auxílio desta técnica foi possível analisar as transformações de fase do composto intermetálico Al6Mn durante tratamentos térmicos. Obtiveram-se dados da completa transformação da fase Al6Mn em alfa-AlMnSi, também conhecida na literatura como, transformação "six to alfa". A precipitação e os precipitados desempenham um papel importante na cinética de recristalização e no tamanho de grão final. Para se entender este comportamento, comparou-se uma chapa que sofreu tratamento térmico antes da laminação com outra chapa que foi laminada no estado bruto de fundição. Os resultados revelaram que a recristalização foi retardada na amostra, inicialmente, bruta de fundição. A precipitação ocorreu simultaneamente com a recristalização, com isso, os dispersóides precipitaram preferencialmente na microestrutura deformada, em subcontornos ou nas discordâncias isoladas, levando a um considerável atraso no rearranjo das discordâncias e na nucleação da recristalização. / The main reason for the development of this PhD Thesis was to compare the microstructures and textures of the 3003 aluminum alloy produced from continuous and semi-continuous casting processes. The roll caster process (continuous casting) has been increasingly used in the aluminum industry. Products usually obtained by means of heavy rolling operations in the past are nowadays obtained with dimensions close to the final ones using the roll caster technology. To understand this new process, microstructures and textures (macrotexture and microtexture) along the thickness of the sheet produced by continuos casting and semi-continuous casting (plates with 250 mm thickness, followed by homogenization and hot rolling) have been investigated. It was possible to verify that materials coming from these two processes did differ each other a lot mainly across the thickness. During roll casting the plastic deformation was large enough to promote the formation of dislocation cells and subgrains. It was also possible to notice that the microstructure, morphology, and grain size from as-cast sample are more homogenous than the ones found in hot rolled samples. Both the sheets obtained by hot rolling and continuous casting have shown a strong texture gradient across the thickness. The predominant texture in both processes is the Brass component {011} . The texture evolution from sheets obtained by roll casting was also determined after rolling and subsequent recrystallization. It shows the presence of typical components of rolling and recrystallization of aluminum. After 91% cold rolling and subsequent recrystallization at 400°C for 1 h, the cube component (recrystallization texture) and the deformation texture were observed. It is well known that this texture minimizes earing effects during deep drawing of aluminum products. Another relevant point in this work was the development of the technique for the extraction of precipitates for the aluminum 3003 alloy. The extraction of precipitates extraction is particularly problematic in aluminum because most of the particles tend to dissolve more readily than the aluminum matrix. Some of the techniques performed are the following: 1. chemical dissolution with iodine in methanol solution; 2. electrolytic dissolution with benzoic acid and hydroxyquinoline in chloroform and methanol; 3. electrolytic dissolution with perchloric acid, butyl-glycol and ethanol; 4. chemical dissolution with phenol solution. Chemical dissolution with the phenol solution was the technique that provided the best results. It was possible to separate precipitates from the aluminum matrix. With this extraction technique it was possible to analyze phase transformations of the intermetallic compound Al6Mn during heat treating of this alloy. The phase transformation of the Al6Mn compound into alfa-AlMnSi, also known as 6-to-alfa transformation, could be followed in detail by means of this technique. Precipitation plays an important role in the recrystallization kinetics and final grain size. Therefore, to understand this behavior, the roll cast aluminum alloy was cold rolled from two distinct starting conditions: as-cast and heat-treated (homogeneized) conditions. It was shown that recrystallization was delayed in the sheet rolled from the as-cast condition. Precipitation has occurred simultaneously with recrystallization, in such a manner that dispersoids did precipitate in the deformed microstructure, preferentially, at subgrain boundaries or at free dislocations. As a result, the rearrangement of the dislocations and further recrystallization nucleation has been significantly retarded.

High shear mixing offers a promising solution for particle dispersion in a liquid with intensive turbulence and high shear rate, and has been widely used in the chemical, food and pharmaceutical industries. However, a practical high shear mixing process has not yet been adapted to solve the particle agglomeration in metallurgy due to the high service temperature and reactive environment of liquid metal. In this study, the effect of high shear mixing using the newly designed rotor-stator high shear device have been investigated with both Al and Mg matrix composites reinforced with SiC particles through casting. The microstructural observation of high shear treated Al and Mg composites show improved particle distribution uniformity in the as-cast state. Increased mechanical properties and reduced volume fraction of porosity are also obtained in the composite samples processed with high shear. With the melt conditioning procedure developed for twin roll casting process, two distinct solutions has been provided for thin gauge Mg strip casting with advanced microstructure and defect control. The melt conditioning treatment activates the MgO as heterogeneous nuclei of α-Mg through dispersion from continuous films to discrete particles. Thus enhanced heterogeneous nucleation in the twin roll casting process not only refines the α-Mg grain size but also eliminates the centre line segregation through equiaxed grain growth and localized solute distribution. The grain refinement of the α-Mg through SiC addition has also been studied through EBSD and crystallographic approaches. Two reproducible and distinct crystallographic orientation relationships between α-SiC (6H) and α-Mg have been determined: [1010]SiC//[2113]Mg, (0006)SiC//(1011)Mg, (1216)SiC//(2202)Mg and [0110]SiC//[1100]Mg, (0006)SiC// (0002)Mg, (2110)SiC//(1120)Mg.

A principal motivação para o desenvolvimento do presente projeto de doutorado foi comparar as microestruturas e texturas resultantes da liga de alumínio 3003 obtida por lingotamento contínuo e semicontínuo. O processo “roll caster” (lingotamento contínuo em cilindros) tem sido crescentemente utilizado na indústria de laminação de alumínio. Produtos que antes sofriam altos graus de deformação durante seu processamento são hoje obtidos nas dimensões próximas da espessura final. Com o intuito de entender este processo relativamente novo, as microestruturas e as texturas (macrotextura e microtextura), ao longo da espessura, das chapas produzidas por lingotamento contínuo e por lingotamento semicontínuo (placas fundidas com 250 mm de espessura, seguido de homogeneização e laminação a quente) foram analisadas. Foi possível constatar que os materiais provenientes dos dois processos diferem bastante e que ao longo da espessura também há grandes diferenças. Foi possível observar que durante o processamento “roll caster” há considerável deformação plástica, com a formação de células de discordâncias e subgrãos. Foi possível observar, também, que a microestrutura, a morfologia e o tamanho de grão, da amostra “caster” bruta de fundição são mais homogêneos que na amostra laminada a quente. As duas chapas, a laminada a quente e a obtida por lingotamento contínuo, exibem forte gradiente de textura ao longo da espessura. A textura predominante nos dois processos, ao longo da espessura, é a do tipo latão {011}. A evolução da textura do material proveniente do processo “roll caster” também foi caracterizada após laminação e posterior recristalização, revelando a presença de componentes típicas de laminação e de recristalização de alumínio. Após laminação a frio com redução de 91% e posterior tratamento térmico de 400°C por 1 h, a chapa proveniente do processo “caster” apresentou uma textura de recristalização caracterizada pela presença da componente cubo (recristalização) somada à parcela de textura de deformação. No caso do alumínio e suas ligas, sabe-se que esta textura proporciona os melhores resultados para minimizar o efeito de orelhamento durante a estampagem. Outro ponto relevante do trabalho foi o aperfeiçoamento da técnica de extração de precipitados para a liga 3003, uma vez que, a extração de precipitados em ligas de alumínio é especialmente problemática, pois a maioria das partículas presentes dissolve-se até mais facilmente que a matriz de alumínio. Algumas técnicas foram testadas: 1. dissolução química em solução de iodo em metanol; 2. dissolução eletrolítica em solução de ácido benzóico e hidroxiquinolina em clorofórmio e metanol; 3. dissolução eletrolítica em solução de ácido perclórico, butil glicol, álcool etílico e água; 4. dissolução química em solução de fenol. A técnica que apresentou melhores resultados foi a dissolução química com fenol, onde foi possível separar os precipitados da matriz alumínio. Com o auxílio desta técnica foi possível analisar as transformações de fase do composto intermetálico Al6Mn durante tratamentos térmicos. Obtiveram-se dados da completa transformação da fase Al6Mn em alfa-AlMnSi, também conhecida na literatura como, transformação "six to alfa". A precipitação e os precipitados desempenham um papel importante na cinética de recristalização e no tamanho de grão final. Para se entender este comportamento, comparou-se uma chapa que sofreu tratamento térmico antes da laminação com outra chapa que foi laminada no estado bruto de fundição. Os resultados revelaram que a recristalização foi retardada na amostra, inicialmente, bruta de fundição. A precipitação ocorreu simultaneamente com a recristalização, com isso, os dispersóides precipitaram preferencialmente na microestrutura deformada, em subcontornos ou nas discordâncias isoladas, levando a um considerável atraso no rearranjo das discordâncias e na nucleação da recristalização. / The main reason for the development of this PhD Thesis was to compare the microstructures and textures of the 3003 aluminum alloy produced from continuous and semi-continuous casting processes. The roll caster process (continuous casting) has been increasingly used in the aluminum industry. Products usually obtained by means of heavy rolling operations in the past are nowadays obtained with dimensions close to the final ones using the roll caster technology. To understand this new process, microstructures and textures (macrotexture and microtexture) along the thickness of the sheet produced by continuos casting and semi-continuous casting (plates with 250 mm thickness, followed by homogenization and hot rolling) have been investigated. It was possible to verify that materials coming from these two processes did differ each other a lot mainly across the thickness. During roll casting the plastic deformation was large enough to promote the formation of dislocation cells and subgrains. It was also possible to notice that the microstructure, morphology, and grain size from as-cast sample are more homogenous than the ones found in hot rolled samples. Both the sheets obtained by hot rolling and continuous casting have shown a strong texture gradient across the thickness. The predominant texture in both processes is the Brass component {011} . The texture evolution from sheets obtained by roll casting was also determined after rolling and subsequent recrystallization. It shows the presence of typical components of rolling and recrystallization of aluminum. After 91% cold rolling and subsequent recrystallization at 400°C for 1 h, the cube component (recrystallization texture) and the deformation texture were observed. It is well known that this texture minimizes earing effects during deep drawing of aluminum products. Another relevant point in this work was the development of the technique for the extraction of precipitates for the aluminum 3003 alloy. The extraction of precipitates extraction is particularly problematic in aluminum because most of the particles tend to dissolve more readily than the aluminum matrix. Some of the techniques performed are the following: 1. chemical dissolution with iodine in methanol solution; 2. electrolytic dissolution with benzoic acid and hydroxyquinoline in chloroform and methanol; 3. electrolytic dissolution with perchloric acid, butyl-glycol and ethanol; 4. chemical dissolution with phenol solution. Chemical dissolution with the phenol solution was the technique that provided the best results. It was possible to separate precipitates from the aluminum matrix. With this extraction technique it was possible to analyze phase transformations of the intermetallic compound Al6Mn during heat treating of this alloy. The phase transformation of the Al6Mn compound into alfa-AlMnSi, also known as 6-to-alfa transformation, could be followed in detail by means of this technique. Precipitation plays an important role in the recrystallization kinetics and final grain size. Therefore, to understand this behavior, the roll cast aluminum alloy was cold rolled from two distinct starting conditions: as-cast and heat-treated (homogeneized) conditions. It was shown that recrystallization was delayed in the sheet rolled from the as-cast condition. Precipitation has occurred simultaneously with recrystallization, in such a manner that dispersoids did precipitate in the deformed microstructure, preferentially, at subgrain boundaries or at free dislocations. As a result, the rearrangement of the dislocations and further recrystallization nucleation has been significantly retarded.

Title: Microstructure and mechanical properties of lightweight structural AZ31 alloy prepared by twin-roll casting method Author: Mariia Zimina Abstract: Microstructure of AZ31 twin-roll cast magnesium alloy was studied using light optical, electron and atomic force microscopy. The effect of annealing temperature on the microstructure was tested. Mechanical properties of a thin magnesium strip were investigated by means of microhardness tests and tensile tests at a relatively low strain rate 10-3 s-1 . Results show that the ductility of the twin roll cast strip increases with increasing deformation temperature, however, a remarkable decrease was observed at about 200 řC. This effect appears also in a conventionally cast ingot of a master alloy and is caused by a change of a deformation mode occurring at this temperature. Moreover, the effect of severe plastic deformation on the microstructure and mechanical properties was studied. Equal channel angular pressing was applied on magnesium strip samples and lead to a significant grain refinement accompanied by an unfavorable strengthening of the initial basal texture, which is effectively suppressed by a subsequent annealing. A constrained groove pressing was for the first time successfully applied on AZ31 twin-roll cast strip leading to an increase of...

<p dir="ltr"> Most industrial processes are not fully-automated. Although reference tracking can be handled by low-level controllers, initializing and adjusting the reference, or setpoint, values, are commonly tasks assigned to human operators. A major challenge that arises, though, is control policy variation among operators which in turn results in inconsistencies in the final product. In order to guide operators to pursue better and more consistent performance, researchers have explored the optimal control policy through different approaches. Although in different applications, researchers use different approaches, an accurate process model is still crucial to the approaches. However, for a highly transient process (e.g., the startup of a manufacturing process), modeling can be challenging and inaccurate, and approaches highly relying on a process model may not work well. One example is process startup in a twin-roll steel strip casting process and motivates this work. </p><p dir="ltr"><br></p><p dir="ltr"> In this dissertation, I propose three offline reinforcement learning (RL) algorithms which require the RL agent to learn a control policy from a fixed dataset that is pre-collected by human operators during operations of the twin-roll casting process. Compared to existing offline RL algorithms, the proposed algorithms focus on exploiting the best control policy used by human operators rather than exploring new control policies constrained by the existing policies. In addition, in existing offline RL algorithms, there is not enough consideration of the imbalanced dataset problem. In the second and the third proposed algorithms, I leverage the idea of cost sensitive learning to incentivize the RL agent to learn the most valuable control policy, rather than the most common one represented in the dataset. In addition, since the process model is not available, I propose a performance metric that does not require a process model or simulator for agent testing. The third proposed algorithm is compared with benchmark offline RL algorithms and achieves better and more consistent performance.</p>

Im Rahmen dieser Arbeit wurde das Umformverhalten der gieß- und warmgewalzten Magnesiumlegierung des Mg-Al-Zn-Systems (AZ31) unter einachsiger Zugbeanspruchung in Abhängigkeit von Temperatur, Dehnrate und Entnahmerichtung untersucht. Weiterhin wurde das richtungsabhängige Grenzformänderungsverhalten im statischen sowie erstmalig im dynamischen Lastfall bei verschiedenen Temperaturen bewertet. Für die dynamischen Versuche wurde ein eigens konstruiertes Prüfmodul entwickelt und eingesetzt. Die Ergebnisse zeigen, dass sich bei einer Variation der Beanspruchungsbedingungen die Ausprägung der Versetzungsbewegung, der mechanischen Zwillingsbildung sowie der Hochtemperaturmechanismen signifikant verändert. Die Verknüpfung der sich dadurch ändernden mechanischen Eigenschaften mit der Mikrostrukturentwicklung konnte mit der Licht- und Rasterelektronenmikroskopie sowie der Texturbestimmung mit der Röntgenbeugung (XRD) nachgewiesen werden.