Severe Plastic Deformation of Copper-Titanium Alloys Using Multi Axial Cryo-forging

Abstract

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.