An Investigation on the effects of Exp-ECAP process on mechanical properties of a light alloy MMC

نوع: Type: Thesis

مقطع: Segment: PHD

عنوان: Title: An Investigation on the effects of Exp-ECAP process on mechanical properties of a light alloy MMC

ارائه دهنده: Provider: Reza Tahmasbi

اساتید راهنما: Supervisors: Faramarz Fereshteh-Saniee (Ph. D)

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Rahman Seifi (Ph.D)- hamed azodi deilami (Ph.D)-Mohammad javad Mirnia (Ph.D)

زمان و تاریخ ارائه: Time and date of presentation: 2026

مکان ارائه: Place of presentation: آمفی تئاتر دانشکده مهندسی

چکیده: Abstract: Abstract: Owing to the expanding use of lightweight materials in industries increasing the strength-to-weight ratio of materials has gained particular importance. The relatively recent expansion equal-channel angular pressing (Exp-ECAP) process has demonstrated considerable potential for improving the properties of pure metals and alloys. However, the effects of this process on composite materials have not yet been systematically investigated. The effects of Exp-ECAP process on the microstructure, crystallographic texture, dislocation density, porosity, mechanical properties, and corrosion behavior of AA6063/Al₂O₃, AA6063/SiO₂, and biodegradable Mg–2.5Zn–0.5Si/HA composites were investigated. In addition to experimental and statistical analyses, finite element simulation was used to evaluate the strain distribution in the product, and machine learning was employed to predict the flow curves. Finite element simulations revealed a heterogeneous effective-strain distribution across the product section. The calculated high-strain regions were consistent with the experimentally observed material-flow pattern and microstructural variations. Conducting the process through route C, (180° rotation between the passes), produced a more symmetric and relatively uniform strain distribution than route A. For flow-stress prediction, numerically corrected ring-compression data obtained at three temperatures and three strain rates were used to train an XGBoost model. For a completely unseen condition, the model achieved an RMSE of 1.49 MPa and a MAPE of 0.85%. For the aluminum-based nanocomposite, design-of-experiments analysis identified processing temperature, number of passes, and nanoparticle type as the most influential parameters affecting the strength and hardness. The best processing condition consisted of a temperature of 25 °C, a ram velocity of 5 mm/min, three passes, and 2 wt.% Al₂O₃. Under these conditions, the yield stress, maximum stress (the highest point of the stress–strain curve), and microhardness increased by 126.04%, 72.67%, and 53.59%, respectively, compared with those of the homogenized specimen. These improvements were results of increased dislocation density, reduced porosity, fragmentation of nanoparticle agglomerates, and breakup and redistribution of the β-Al₅FeSi phase. Al₂O₃ nanoparticles also provided greater strengthening effect than SiO₂ because of its more favorable dispersion and load-transfer capability. For the magnesium-based nanocomposite, the first pass simultaneously consolidated the powder compact and initiated the grain refinement mechanisms, while the second pass promoted grain refinement, pore reduction, and fragmentation of HA agglomerates and Si-rich regions. The specimen containing 1 wt.% HA after two passes of the operation exhibited the best combination of grain refinement and strength, with a median grain size of 1.62 µm and a fracture strength of 403.78 MPa. The highest fracture strain was obtained in the HA-free specimen, whereas the highest microhardness was observed in the specimen containing 2 wt.% HA, both after two passes of the process. Corrosion behavior was governed by the combined effects of porosity, secondary-phase distribution, and HA agglomeration or dispersion. Overall, Exp-ECAP was effective in consolidation, pore reduction, phase modification, particle redistribution, and control of microstructural heterogeneity. The integration of experimental, statistical, numerical, and data-driven approaches in this research work provided a unified framework for understanding and predicting the performance of the Exp-ECAP operation on the behaviors of lightweight nanocompositesstrain curve), and microhardness increased by 126.04%, 72.67%, and 53.59%, respectively, compared with those of the homogenized specimen. These improvements were results of increased dislocation density, reduced porosity, fragmentation of nanoparticle agglomerates, and breakup and redistribution of the β-Al₅FeSi phase. Al₂O₃ nanoparticles also provided greater strengthening effect than SiO₂ because of its more favorable dispersion and load-transfer capability. For the magnesium-based nanocomposite, the first pass simultaneously consolidated the powder compact and initiated the grain refinement mechanisms, while the second pass promoted grain refinement, pore reduction, and fragmentation of HA agglomerates and Si-rich regions. The specimen containing 1 wt.% HA after two passes of the operation exhibited the best combination of grain refinement and strength, with a median grain size of 1.62 µm and a fracture strength of 403.78 MPa. The highest fracture strain was obtained in the HA-free specimen, whereas the highest microhardness was observed in the specimen containing 2 wt.% HA, both after two passes of the process. Corrosion behavior was governed by the combined effects of porosity, secondary-phase distribution, and HA agglomeration or dispersion. Overall, Exp-ECAP was effective in consolidation, pore reduction, phase modification, particle redistribution, and control of microstructural heterogeneity. The integration of experimental, statistical, numerical, and data-driven approaches in this research work provided a unified framework for understanding and predicting the performance of the Exp-ECAP operation on the behaviors of lightweight nanocomposites.