Magnesium and its alloys have been considered as alternatives to aluminum alloys and steels for structural applications in automotive and aerospace applications due to their superior specific strength and light-weight. However, they have hexagonal-close packed (hcp) structure, and thus have a small number of deformation systems resulting in low ductility and formability near room temperature, anisotropic thermo-mechanical response and strong deformation textures. The aim of this work is to investigate experimentally the effect of crystallographic texture generated during severe plastic deformation (SPD), on the subsequent formability and mechanical flow anisotropy in AZ31B Mg alloy. The proper control of grain size and texture through SPD is expected to result in better low temperature formability and better control of mechanical flow anisotropy. AZ31B Mg alloy has been successfully processed using equal channel angular extrusion (ECAE) following different processing routes, multiple passes, and different processing temperatures, in order to obtain samples with a wide variety of grain sizes, ranging from ~370 nm up to few microns, and crystallographic textures. Low temperature processing of the AZ31B Mg alloy was successful after initial high temperature processing. Smaller grain sizes were achieved using the temperature step-down method leading to incremental reduction in grain size at each ECAE pass. The temperature step-down method was utilized to develop hybrid ECAE routes to obtain specific crystallographic textures. Optimized hybrid ECAE routes were developed which resulted in a high strength/high ductility material with the average grain size of ~370 nm. The ECAE processed alloy showed a high tensile yield strength of ~380 MPa that has never been reported so far in AZ31 ingot metallurgy Mg alloys. The influence of grain size on the critical stress for the activation of individual deformation mechanisms was also investigated by systematically controlling the texture and grain size, and assuming the activation of mainly a single deformation mechanism through the careful selection of the loading direction on the processed samples. It was revealed that the Hall-Petch slope for the basal slip was much smaller than those of prismatic slip and tensile twinning.

There have been numerous efforts in processing metallic alloys into fine-grained materials, so as to exhibit high strain rate superplasticity (HSRSP) and/or low temperature superplasticity (LTSP). The current study is to apply the most simple and feasible one-step extrusion method on the commercial AZ31 magnesium billet to result in low temperature and high strain rate superplasticity (LT&HSRSP). The one-step extrusion was undertaken using a high extrusion ratio at 250-350oC, and the grain size after one-step extrusion became ~1-4 mm. The processed AZ31 plate exhibited high room temperature tensile elongation up to 50%, as well as superior LTSP and/or HSRSP up to 1000%. Meanwhile, the AZ31 alloy was also conducted by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of 461% may be attained at a temperature of 150oC, equivalent to 0.46 Tm where Tm is the absolute melting temperature. This result clearly demonstrates the potential for achieving low temperature superplasticity. A detailed investigation, using x-ray diffraction (XRD), electron back scattering diffraction (EBSD), and transmission electron microscopy / selected area diffraction (TEM/SAD), revealed different textures in the as-extruded and as-ECAP bars. These dominant textures were characteristic of <10 0>//ED in the extruded bars and < 76>//ED in the ECAP condition, where ED is the extrusion direction. The results show that the basal planes tend to lie parallel to the extrusion axis in the extruded bars but there is a rearrangement during ECAP and the basal planes become reasonably aligned with the theoretical shearing plane. As to the extruded plates, the {0002} planes tended to lie on the plane that contains the extrusion axis. At different tensile temperatures, different deformation mechanisms would be dominant. Over the lower loading temperatures within 150-200oC, the true strain rate sensitivity, mt, after extracting the threshold stress is determined to be 0.28, suggesting that power-law dislocation creep but the Qt value is not related to any creep mechanism. It should be partly due to thermal activated dislocation slip mechanism. However, more data need to be tested systematically this part in the future study in order to define the correct deformation mechanism. As to the loading temperatures over 250-300oC, the mt value and the true activation energy for the extruded specimens are calculated to be ~0.4-0.5 and ~90-100 kJ/mol, implying that the major deformation mechanism is grain boundary sliding plus minor solute drag creep, with the rate controlling diffusion step being the magnesium grain boundary diffusion.

AM60B magnesium alloy is being increasingly used in auto industry in applications that usually involve various formats of cyclic loading scenarios. Therefore, the fatigue response of this alloy is investigated in this thesis. Our investigation is focused on characterization of the influence of compressive stress cycles within a given cyclic loading scenario on alloy’s crack propagation response. In the first part of this dissertation, fatigue crack growth rate (FCGR) of AM60B alloy subject to cyclic loadings with various stress ratios (both positive and negative) is investigated and a modified model is proposed to predict the FCGR under a wide range of stress ratios. Subsequently, using the modified model, the experimental results of the crack propagation tests are condensed into a single line in a logarithmic scale and the integrity of a proposed FCGR model is investigated. The investigation is continued by studying the influence of compressive stress cycle (CSC) on FCGR. Constant and random amplitude loadings with several magnitudes of CSCs are applied, leading to considerable acceleration in FCGR. The stress distribution ahead of the crack tip is also studied using the finite element method. The tensile residual stress and plastic zone are characterized upon the removal of the CSCs. The acceleration in the crack propagation is shown to be governed by the tensile zone ahead of the crack tip. Furthermore, application of an overload within an otherwise constant amplitude loading (CAL) has been known to retard the crack propagation, thus increase the fatigue life. This retardation would be a function of the affected zone and retardation magnitude. It is shown in this thesis that the affected zone would be influenced by the “sensitivity” of the material to overload. Moreover, it is also demonstrated that the nature of baseline CAL loading would also affect the retardation response and dimension of the affected zone. Therefore, modification to the Wheeler model is proposed, thereby enabling the model to account for material’s sensitivity and nature of the baseline loading. The integrity of the proposed model is verified by the experimental results obtained in this project, as well as those reported by other investigators for other alloys.

O presente trabalho estuda a conformação da liga de magnésio AZ61 pelos processos de extrusão direta e forjamento, ambos a quente. No processo de extrusão, são submetidos à conformação corpos-de-prova da liga de magnésio AZ61 nos estados fundido e recristalizado. As geometrias de ferramenta são variadas e a influência destas variações sobre a força de processamento e propriedades mecânicas e microestrutura do material são analisada. Demonstra-se que a liga no estado inicialmente recristalizado tem propriedades mecânicas finais superiores à liga inicialmente fundida, embora demande de maior força de processamento. Demonstra-se também que a geometria de ferramenta tem influência sobre as respostas mecânicas obtidas das peças após extrusão. Referente ao processo de forjamento, a liga de magnésio AZ61 é processada apenas no estado recristalizado, variando-se a temperatura e o número de tapas de forjamento. Foi avaliada a influência destas variações de processo nas propriedades mecânicas finais da peça. Ao fim, percebe-se que ao se forjar as peças com múltiplas etapas de forjamento e temperaturas decrescentes, impede-se a recristalização do material, alcançando alto encruamento dos grãos, o que gera, na peça final, propriedades mecânicas superiores ao processo convencional. / This thesis studies the deformation of a magnesium alloy AZ61 by forward extrusion and forging processes, at elevated temperatures. In the extrusion process, the samples were deformed in as cast and recrystallized conditions. The tools geometries were variated and the influences on processing force, mechanical properties and material microstructure were analyzed. Was found that the recrystallized samples have better mechanical properties than as cast samples, after extruded, although it requires more processing force. Also was found that the tool geometry influences on mechanical properties and microstructure of extruded magnesium alloy. Relatively to the forging process, magnesium alloy AZ61 was processed only in the recrystallized state, varying the process temperature and the number of forging steps. Aimed to evaluate the process variations influences on the mechanical properties and microstructure of the final parts. At the end, was observed that when forging parts with multi-step forging and decreasing temperatures, to prevent recrystallization of the material, achieving high strain hardening of the grains, which results in final part with superior mechanical properties to the conventional process.

O presente trabalho estuda a conformação da liga de magnésio AZ61 pelos processos de extrusão direta e forjamento, ambos a quente. No processo de extrusão, são submetidos à conformação corpos-de-prova da liga de magnésio AZ61 nos estados fundido e recristalizado. As geometrias de ferramenta são variadas e a influência destas variações sobre a força de processamento e propriedades mecânicas e microestrutura do material são analisada. Demonstra-se que a liga no estado inicialmente recristalizado tem propriedades mecânicas finais superiores à liga inicialmente fundida, embora demande de maior força de processamento. Demonstra-se também que a geometria de ferramenta tem influência sobre as respostas mecânicas obtidas das peças após extrusão. Referente ao processo de forjamento, a liga de magnésio AZ61 é processada apenas no estado recristalizado, variando-se a temperatura e o número de tapas de forjamento. Foi avaliada a influência destas variações de processo nas propriedades mecânicas finais da peça. Ao fim, percebe-se que ao se forjar as peças com múltiplas etapas de forjamento e temperaturas decrescentes, impede-se a recristalização do material, alcançando alto encruamento dos grãos, o que gera, na peça final, propriedades mecânicas superiores ao processo convencional. / This thesis studies the deformation of a magnesium alloy AZ61 by forward extrusion and forging processes, at elevated temperatures. In the extrusion process, the samples were deformed in as cast and recrystallized conditions. The tools geometries were variated and the influences on processing force, mechanical properties and material microstructure were analyzed. Was found that the recrystallized samples have better mechanical properties than as cast samples, after extruded, although it requires more processing force. Also was found that the tool geometry influences on mechanical properties and microstructure of extruded magnesium alloy. Relatively to the forging process, magnesium alloy AZ61 was processed only in the recrystallized state, varying the process temperature and the number of forging steps. Aimed to evaluate the process variations influences on the mechanical properties and microstructure of the final parts. At the end, was observed that when forging parts with multi-step forging and decreasing temperatures, to prevent recrystallization of the material, achieving high strain hardening of the grains, which results in final part with superior mechanical properties to the conventional process.

O presente trabalho estuda a conformação da liga de magnésio AZ61 pelos processos de extrusão direta e forjamento, ambos a quente. No processo de extrusão, são submetidos à conformação corpos-de-prova da liga de magnésio AZ61 nos estados fundido e recristalizado. As geometrias de ferramenta são variadas e a influência destas variações sobre a força de processamento e propriedades mecânicas e microestrutura do material são analisada. Demonstra-se que a liga no estado inicialmente recristalizado tem propriedades mecânicas finais superiores à liga inicialmente fundida, embora demande de maior força de processamento. Demonstra-se também que a geometria de ferramenta tem influência sobre as respostas mecânicas obtidas das peças após extrusão. Referente ao processo de forjamento, a liga de magnésio AZ61 é processada apenas no estado recristalizado, variando-se a temperatura e o número de tapas de forjamento. Foi avaliada a influência destas variações de processo nas propriedades mecânicas finais da peça. Ao fim, percebe-se que ao se forjar as peças com múltiplas etapas de forjamento e temperaturas decrescentes, impede-se a recristalização do material, alcançando alto encruamento dos grãos, o que gera, na peça final, propriedades mecânicas superiores ao processo convencional. / This thesis studies the deformation of a magnesium alloy AZ61 by forward extrusion and forging processes, at elevated temperatures. In the extrusion process, the samples were deformed in as cast and recrystallized conditions. The tools geometries were variated and the influences on processing force, mechanical properties and material microstructure were analyzed. Was found that the recrystallized samples have better mechanical properties than as cast samples, after extruded, although it requires more processing force. Also was found that the tool geometry influences on mechanical properties and microstructure of extruded magnesium alloy. Relatively to the forging process, magnesium alloy AZ61 was processed only in the recrystallized state, varying the process temperature and the number of forging steps. Aimed to evaluate the process variations influences on the mechanical properties and microstructure of the final parts. At the end, was observed that when forging parts with multi-step forging and decreasing temperatures, to prevent recrystallization of the material, achieving high strain hardening of the grains, which results in final part with superior mechanical properties to the conventional process.

Magnesium alloy tubes have good formability at elevated temperatures. In this paper, firstly, uniaxial tensile tests are conducted to evaluate the flow stress of AZ61 magnesium alloy at different temperatures and strain rates. Secondly, a hydraulic warm forming machine with axial feeding, counter punch and internal pressure is designed and manufactured. Using this testing machine with the FEM results, experiments of hydraulic forming of AZ61 magnesium alloy tubes at different temperatures are carried out. The effect of loading paths on the product shape and formability at different temperature are discussed.

In an effort to improve the fuel efficiency of automobiles, car designers are investigating new materials to reduce the overall vehicle weight. Magnesium alloys are good candidates to achieve that weight reduction due in part to their low density and high specific strength. To support their introduction into vehicle body structures, the dynamic behavior of magnesium alloys must be determined to assess their performance during a crash event. In this work, the tensile high strain rate behavior of AZ31B magnesium alloy sheets was characterized. Two different temper conditions were considered: AZ31B-O (fully annealed) and AZ31B-H24 (partially hardened). Three different sheet thicknesses were considered for the O temper condition, 1.0, 1.6 and 2.5 mm, while the H24 temper was 1.6 mm in thickness. The sheet condition of the magnesium alloys implies an in-plane anisotropy induced by the rolling process. Therefore, both the rolling and transverse directions were investigated in the current research. In order to characterize the constitutive behaviour of AZ31B-O and AZ31B-H24 magnesium alloy sheets, tensile tests were performed over a large range of strain rates. Quasi-static experiments were performed at nominal strain rates of 0.003s-1, 0.1s-1 and 1s-1 using a servohydraulic tensile machine. Intermediate strain rate experiments were performed at 30s-1 and 100s-1 using an instrumented falling weight impact (IFWI) apparatus, and high strain rate experimental data at 500s-1, 1000s-1 and 1500s-1 was collected using a tensile split Hopkinson bar (TSHB) apparatus. Elevated temperature experiments (up to 300°C) were also performed at high strain rates using a radiative furnace mounted on the TSHB apparatus. The tensile experiments show a significant strain rate sensitivity of the constitutive behavior of both the O and H24 temper conditions. The two tempers exhibit an average increase of stress level of 60-65 MPa over the range of strain rates considered. As the strain rate increases, the strain rate sensitivity of both tempers also increases. The strain rate has a different effect on the ductility of the two material conditions. The ductility of AZ31B-O is significantly improved under high strain rate deformations, whereas the AZ31B-H24 exhibits similar ductility at low and high strain rates. Both material conditions presented a strong in-plane anisotropy, with an average stress level in the transverse direction higher than in the rolling direction by 15 MPa and 35 MPa for the O and H24 tempers, respectively. The thermal sensitivity for both tempers at high strain rates was obtained. The two material conditions exhibit a clear thermal softening. From room temperature to 250°C, the loss in strength at 5% plastic strain was found to be 55 MPa and 125 MPa for the AZ31B-O and AZ31B-H24 materials, respectively. The thickness of the AZ31B-O sheets has a mild effect on the measured constitutive behavior. The flow stress increases with increasing thickness. An average difference of 10-15 MPa was seen between the flow stress of the 1.0mm and 2.5mm sheets. However, similar strain rate sensitivity was seen for the three thicknesses. The experimental data was fit to three constitutive models: the Johnson-Cook model, its modified version with a Cowper-Symonds strain rate sensitivity formulation, and the Zerilli-Armstrong model. The three models were evaluated by numerical simulation of the TSHB experiment under various testing conditions. It was found that the Zerilli-Armstrong model was the most accurate in predicting the flow stress of the different material conditions. However, finite element models incorporating the three constitutive fits failed to predict necking in the specimen.

Because magnesium alloy tube has good formability at elevated temperature, uniaxial tensile tests were employed to evaluate the warm properties of AZ31 magnesium alloy. After that, this paper has designed and manufactured a hydraulic bulge warm forming machine which can be used to experiment with fixed tube length. Using this testing machine, the experiments of hydraulic bulge forming of AZ31 magnesium alloy and 6061 aluminum alloy tube at elevated temperatures were carried out. According to experimental results, the hydraulic bulge forming properties of tubes at different temperatures are discussed.

In this research, one solid state processing technique, friction stir processing, is applied to modify the AZ61 magnesium alloy billet and to incorporate 5-10 vol% nano-sized ceramic particles SiO2 into the AZ61 magnesium alloy matrix to form bulk composites, using the characteristic rotating downward and circular material flow around the stir pin. The microstructure and mechanical properties of the modified alloy and composite samples are examined and compared. The FSP modified AZ61 alloy could be refined to 3-8um via the dynamic recrystallization during processing. However, the one-pass FSP modified alloy appeared the inhomogeneous grain structures to influence the tensile ductility along the welding direction at elevated temperatures due to the onion splitting. In contrast, the multi-pass FSP could improve the inhomogeneous grain structures to reduce the influence of the onion-splitting to the deformation at elevated temperatures. The FSP modified alloys show the lower yielding stress due to the unique texture of (0002) basal planes, with roughly surround the pin column surface of the pin tool in the stirred zone. In addition, it is suggested that the second processing of the subsequent compression along the normal direction might be necessary to alter the texture and to improve the lower yielding stress after modifying the grain size by FSP. Friction stir processing could successfully fabricate bulk AZ61 Mg based composites with 5 to 10 vol% of nano-sized SiO2 particles. The nano-sized SiO2 particles added into magnesium matrix could be uniformly dispersed after four FSP passes. The average grain sizes of the composites varied within 0.5-2um, and the composites nearly double the hardness as compared with the as-received AZ61 cast billet. This composite exhibited high strain rate superplasticity, with a maximum ductility of 470% at 1x10-2 s-1 and 300oC or 454% at 3x10-1 s-1 and 400oC while maintaining fine grains less than 2um in size.