2. Thesis and Dissertations

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    Studies on Mechanical Properties and Corrosion Behavior of Ze41 Magnesium Alloy Subjected to Equal Channel Angular Pressing
    (2022) Sekar, Prithivirajan; S, Narendranath; Desai, Vijay
    An exceptional combination of properties such as high specific strength, good damping capacity and abundant availability confirm magnesium alloys as a potential candidate for applications in automobile, biomedical, aerospace as well as electronics industries. However, the limited ductility of Magnesium and its alloys as well as their relatively lower corrosion resistance still remains a hindrance to extend their applications. Equal channel angular pressing (ECAP) has immense potential to attain remarkable grain refinement thereby improving mechanical properties such as yield strength, ultimate tensile strength and percentage of elongation of Mg alloys. In the present work, ZE41 Mg alloy is subjected to two step ECAP. The microstructure and mechanical properties of ZE41 Mg alloy before and after two step ECAP are investigated. Further, the corrosion and galvanic corrosion behaviour of ZE41 samples are evaluated in 0 M, 0.1 M and 1 M NaCl solutions to mimic conditions encountered in automobile applications. Two step equal channel angular pressing carried out on as received ZE41 Mg alloy resulted in a remarkable grain refinement. As compared to grain size of 46μm in as received sample, refinement upto 2.5μm is achieved after 8th pass equal channel angular pressing (ECAP). The combined effect of crystallographic orientation and grain refinement is investigated by analysing the mechanical properties and corrosion behaviour of ZE41 Mg alloy using electron back scattered diffraction (EBSD). The first stage comprises of 1st, 2nd, 3rd and 4th passes at a processing temperature of 300 °C while the 5th, 6th, 7th and 8th passes are ECAPed at 275 °C in second stage. The mechanical properties of ZE41 Mg alloy 158 MPa yield tensile strength (YTS), 230 MPa ultimate tensile strength (UTS) and 7 % elongation in as received condition is enhanced to 236 MPa YTS, 295 MPa UTS and 19.76% respectively after first stage ECAP. The yield tensile strength deteriorated due to the effect of texture predominating grain refinement during the second stage ECAP. The corrosion resistance of ZE41 Mg alloy is significantly enhanced by ECAP and is inferred from electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation results. The role of microstructure is minimal on corrosion behaviour of ZE41 Mg alloy due to extra resistance when tested in 0M NaCl. However, the influence of grain refinement greatly influenced the improvement in corrosion resistance of ZE41 Mg alloy rather than crystallographic orientation observed from EBSD analysis. In contrast, the crystallographic orientation predominated the effect of grain refinement during ZE41 Mg alloy corrosion in chloride containing 0.1M and 1M NaCl solutions. From the observation of results it is found that equal channel angular pressing has the dual advantage of improving mechanical properties and corrosion resistance of ZE41 Mg alloy. Further, the galvanic corrosion behaviour of as received and ECAPed ZE41 Mg alloy coupled with Al7075 alloy is investigated using zero resistance ammeter (ZRA) in three different corrosive environments 0 M, 0.1 M and 1 M NaCl to mimic the conditions experienced in engineering applications. The mechanism of galvanic corrosion for ZE41 Mg alloy –Al7075 Aluminium alloy is explained. It is observed that a robust surface film containing composite layer of oxide/hydroxide of magnesium and aluminium is established in 0 M NaCl solution. However, only a single layer of magnesium oxide/ hydroxide is detected in chloride containing environments. ECAP improved the resistance to galvanic corrosion by 58% and 54% when compared with as received ZE41 Mg alloy in 0 M and 1 M NaCl solution respectively. In contrast, galvanic corrosion resistance decreased by 26% in 0.1 M NaCl after equal channel angular pressing while the as received samples evinced pits unfavourable to be used in engineering applications. ECAP is a promising method to combat galvanic corrosion encountered by ZE41 Magnesium alloy used in automobiles and components of military vehicles.
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    Effect of Equal Channel Angular Extrusion on Microstructure Mechanical Properties and Corrosion Behavior of Wrought AZ-Magnesium Alloys
    (National Institute of Technology Karnataka, Surathkal, 2020) Naik, Gajanan M.; S, Narendranath.
    Wrought magnesium alloys are lightest engineering material and it has quite special properties which lead to particular applications. In specific, their highest strength to weight ratio, good machinability and high damping capability makes magnesium alloys tremendously attractive in aerospace, electronics, marine and automobile industries. Indeed, Magnesium alloys have poor tensile strength, ductility and corrosion resistance properties associated with other engineering materials like aluminium alloys, steels and super alloys etc. Therefore, many researchers worked on equal channel angular pressing of magnesium alloys to improve the mechanical properties and corrosion resistance. In this work, the effect of channel angles on material properties were investigated during equal channel angular pressing of AZ80/91 magnesium alloy using processing route-R at 523K, 598K and 663K processing temperature. Channel angles of 90⁰ and 110⁰, common corner angle of 30⁰ have been considered for the study. It has been revealed that the channel angle has a significant influence on deformation homogeneity, microhardness, ultimate tensile strength, ductility and corrosion behaviour of AZ80/91 magnesium alloys. Specifically, AZ80/91 Mg alloys processed through 90⁰ channel angle i.e die A is considered as optimal die parameter to improve above-said material properties. Investigation showing with reference to as-received AZ80 and AZ91 Mg alloy indicates 11 %, 14 % improvement of UTS and 69 %, 59 % enhancement in ductility after processing through 4P through die A (90º) at 598K respectively. Also, the corrosion rate reduces to 97 % and 99 % after processing the sample with 4P-ECAP die A (90º) at the same processing temperature for AZ80 and AZ91 Mg alloys respectively. This is mainly due to grain refinement and distribution of Mg17Al12 secondary phase during ECAP. Further, this work investigates the effect of annealing and aging treatment on microstructure and corrosion behaviour of as-received and ECAPed AZ80/91 Magnesium alloys. Here, annealing at 523K, 623K, and 723K were accomplished, meanwhile samples were cooled in the furnace after 6 h and 12 h of diffusion annealing treatment. In this study, samples were characterized by using optical microscopy (OM) and scanning electron microscopy (SEM) and electrochemical corrosion behavior of annealed AZ80/91 Mg alloy has beeninvestigated. With this, an attempt has been made to enhance the corrosion resistance of the AZ80/91 Mg alloy by changing its microstructure and re-distribution of secondary phase during annealing and aging treatment. It was found that corrosion rates are minimum at higher annealing temperature and aging time because of uniform distribution of secondary β-phases in Mg matrix, evidently shown in the microstructure of the heat-treated AZ80/91 Mg alloy. As a result, the annealing treatment at 723K for 12 h aging is desirable to enhance the corrosion resistance. Further enhancement of asreceived and ECAPed AZ80/91 Mg alloys were observed after High Velocity Oxy-Fuel (HVOF) coating of 316 stainless steel powder. Our results revealed that 316 stainless steel coating on ECAP-4P AZ80/91 Mg alloys were uniform and compact on substrate with a thickness of 80±5 µm. Furthermore, HVOF-coating process of 4P-ECAP significantly reduce corrosion rate at 3.5wt.% NaCl solution making it promising for industrial applications. The corrosion behaviour and effect of the ECAPed fine-grained magnesium alloy and coarse-grained as-received AZ80/91 Mg alloy was investigated in a 2.5wt.% NaCl, 3.5wt.% NaCl solution and Natural Sea Water (NSW) in order to explore the corrosion performance of ECAPed magnesium alloys in various environments. From, electrochemical corrosion experiments and surface morphology observations evidently shown that grain refinement exhibited improved corrosion resistance of the AZ80/91 alloy in all environments, also which shown a protective passive film on the surface to shield corrosion
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    Severe Plastic Deformation of Magnesium Alloys by Equal Channel Angular Pressing
    (National Institute of Technology Karnataka, Surathkal, 2017) K. R, Gopi; Nayaka, H Shivananda
    Magnesium and its alloys possess good mechanical properties like low density, good castability, high specific strength and low cost. Requirement of magnesium alloys is more towards industrial applications, particularly in aerospace and automotive industries. The main limitation is the poor room temperature formability, with limited number of slip systems, due to their hexagonal closed packed (HCP) structure. Formability at room temperature in Mg alloys can be improved by grain refinement, as fine-grained structure improves ductility. Severe plastic deformation (SPD) is a technique where high strains are induced into the material to produce fine grained structural material. They also have few limitations, like high wear rate and low corrosion resistance, which hinders the usage of magnesium in many applications. Wear rate and corrosion resistance can be improved by subjecting the material to SPD, thereby, increase the applications of magnesium alloys. Equal channel angular pressing (ECAP) is one of the simple and most effective methods of SPD to improve the material properties. ECAP is a process in which the workpiece is subjected to shear deformation and thus, severe plastic strain is induced into the material without any change in the cross-sectional dimension of the sample. Microstructure has major effect on mechanical properties. ECAP process leads to ultrafine-grained microstructure in the material which may show superplastic deformation at low temperature and high strain rate. In HCP structured metals, grain refinement also leads to textural changes i.e. high strengthening in some particular directions. Magnesium alloys are available in various systems – (a) Mg-Al system where aluminum addition improves the mechanical property as well as the castability. Addition of aluminum up to 6% and more makes the alloy age-hardenable. Manganese (Mn) addition plays a vital role in grain refinement for magnesium alloys. (b) Aluminum-manganese (AM) series magnesium alloys are widely used in manufacturing of various automobile components such as seat frames, instrument panels etc., due to their better damping, better toughness, impact absorption and elongation properties compared to aluminum-zinc (AZ) series alloys.In the present study, ECAP was performed on AM70, AM80 and AM90 alloy with varying percentage of aluminum and manganese. In spite of various applications of AM series magnesium alloy, limited work has been done to improve the physical properties of AM series magnesium cast alloys. We may further improve these properties by using ECAP, so that its application can be expanded in different areas of engineering. ECAP was carried out using hot die steel (HDS) die with channel angle (Φ) as 110° and outer arc of curvature (Ψ) as 20° using route BC. Unprocessed and ECAP processed samples were subjected to microstructural studies and tested for mechanical properties. Strength and hardness values showed increasing trend for the initial 2 passes of ECAP processing and then started decreasing with further increase in the number of ECAP passes, even though the grain size continued to decrease in all the successive ECAP passes. However, the strength and hardness values still remained quite high when compared to the initial condition (as-cast and homogenized). This behavior was found to be correlated with texture modification in the material as a result of ECAP processing. Wear and corrosion tests were conducted to study tribological and corrosion behavior of ECAP processed samples. Results showed reduction in wear mass loss for the ECAP processed samples. Coefficient of friction (COF) was studied for different loads and improvement in COF values was observed for ECAP processed samples compared to initial condition. Potentiodynamic polarization and electrochemical impedance spectroscopy test showed improvement in corrosion resistance of ECAP processed samples. Immersion test showed similar trend with increased corrosion resistance of ECAP processed samples with low hydrogen evolution.