2. Thesis and Dissertations
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Item Studies on Mechanical Properties and Corrosion Behavior of Ze41 Magnesium Alloy Subjected to Equal Channel Angular Pressing(2022) Sekar, Prithivirajan; S, Narendranath; Desai, VijayAn 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.Item Severe Plastic Deformation of Copper-Titanium Alloys Using Multi Axial Cryo-forging(National Institute of Technology Karnataka, Surathkal, 2020) S, Ramesh.; Nayaka, H Shivananda.Severe plastic deformation (SPD) is a technique where high strains are induced into the material to produce fine-grained structural materials, thereby improving the wear resistance and corrosion resistance. There is an increase in scientific and industrial interest in the development of bulk ultra-fine-grained (UFG) alloys, intended for structural applications. UFG materials offer vastly improved mechanical and physical properties. They also exhibits superplastic properties at elevated temperatures. SPD is done using Equal Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Repetitive Corrugation and Straightening (RCS), Accumulative Roll Bonding (ARB) and Multi Axial Forging (MAF). In MAF, materials are forged repeatedly in a closed die along three orthogonal directions, sequentially. It allows processing of relatively ductile material, because it can be performed at cryogenic temperature. Literature review shows that by using MAF technique, grain refinement phenomena can be observed in some ferrous and non-ferrous metals. MAF is one of the simple and most effective methods of SPD to improve material properties. MAF 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. MAF process leads to ultrafine-grained microstructure in the material which may show superplastic deformation at low temperature and high strain rate. In FCC structured metals, grain refinement also leads to textural changes i.e. high strengthening at cryogenic condition deformation. Copper-Titanium (Cu-Ti) alloy is the nontoxic substitute for Cu-Be and it showed good mechanical and electrical properties and can be used for the production of high strength spring, corrosion-resistant elements, and electrical connections like contact, relay, gears and electrical wires. Hence, in the present study, three alloys of Cu-Ti, namely, Cu-1.5%Ti, Cu-3%Ti and Cu-4.5%Ti, have been processed by MAF. Microstructural evolution in different MAF cycles is studied and it is correlated to the mechanical properties observed. As UFG materials have much higher hardness, they are expected to have higher wear resistance. MAF processed material exhibits higher wear and corrosion resistance, than the asreceived material. Hence MAF processed samples find wider engineering applications.viii Literature review consists of features of various SPD Techniques, advantages, and limitations. MAF process, parameters which influence MAF process, advantages and applications of MAF processed Cu-Ti alloys are discussed in details. Works of different researchers on MAF processed Copper alloys, with respect to, mechanical properties, wear and corrosion behavior are reported. Motivation from literature survey and objectives of the present work are highlighted. Details of the experimentation performed, right from the process adopted for the development of the UFG Cu-Ti to their characterization, are given in chapter three. Microstructural analyses were performed using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Tensile tests were performed on both as-received and MAF processed samples. Dry sliding wear testing was performed using Pin on disc testing machine for both unprocessed and MAF processed samples. For the study of corrosion behavior, electrochemical polarization studies were performed and tofel extrapolation technique was used to obtain the corrosion rates. Chapter 4, Chapter 5 and Chapter 6, explain the results and discussion of various experiments carried out on three alloys Cu-1.5%Ti, Cu-3%Ti and Cu-4.5%Ti Microstructural characterization by OM, SEM, TEM, EBSD and XRD analysis has been discussed. Mechanical properties which includes hardness, tensile followed by fractography has been analyzed. Wear test with different loading conditions and sliding distances has been explained. Corrosion studies by electrochemical measurements test method has been highlighted.Item 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 corrosionItem Mechanical Properties and Microstructural Characterization of Multi Directional Forged and Heat Treated Zn-24al-2cu Alloy(National Institute of Technology Karnataka, Surathkal, 2018) P. C, Sharath; G. V, Preetham Kumar; Rajendra Udupa, K.Multi directional forging (MDF) is one of the techniques to achieve severe plastic deformation (SPD) which yields ultrafine grained metallic materials. In this technique, the material is compressed in a channel die to a fixed strain, retaining its shape by appropriately rotating it over the three axis. In the present investigation, the MDF process was carried out on selected Zn-24Al-2Cu alloy at two different temperatures, viz., 100 °C for three passes and 200 °C for six passes. Further, the MDF process was subjected to age hardening treatment by following the sequence of solutionizing, MDF processing and post aging. The aging studies were carried out by the choice of aging temperature between 100 to 250 °C and aging curves were generated. Further, all mechanical characterizations of the material were carried out on the test samples which had been aged at 100 °C for one hour as this combination of parameters yielded highest hardness in all the categories of sample. Techniques like optical microscopy, electron microscopy and X-Ray diffractometry were used to characterize and analyze the microstructure of MDF processed and age hardened materials. Microstructural analysis revealed that the MDF processing reduces the grain size, yielding fine grained materials, apart from well-distributed phases. The microstructure is observed to be consisting of three phases, viz., Al-rich α phase, Zn-rich η phase and CuZn4 ε phase. Mechanical characterization of the materials revealed an important fact that strength and hardness increased along with the ductility. It was attributed to fineness of grain and evolution of phases in a favorable manner because of MDF and aging treatment meted out to the material. The creep behavior of the material is investigated by conducting the indentation creep test with 2 kg and 2.5 kg load at different temperatures from 30 to 150 °C. The MDF processed material is found to lose its creep resistance substantially mainly because of microstructural refinement. It was possible to infer from the determination of value ofactivation energy that creep occurred by various mechanisms at different temperature regimes. Those mechanisms include dislocation creep and diffusional creep. Wear behavior of the materials was assessed by conducting the test on pin on disc machine resorting optimization of parameters using Taguchi method. It was proved that processing the materials by MDF followed by post-aging improved the wear resistance of the materials. Wear mechanism was observed to be abrasive type in the case of solutionized samples and adhesive type in MDF-processed samples as revealed by SEM investigations on worn surface.