Papers by Author: Chakkingal Uday

Abstract: The low cycle fatigue (LCF) behaviour of aluminium-magnesium-silicon alloy AA6061 processed by equal channel angular pressing (ECAP) was studied. Solutionized specimens of the alloy were subjected to one, two and three passes of ECAP at room temperature using route Bc. Compared to the values of the solutionized material an overall increase in yield strength and ultimate tensile strength with decrease in % elongation is observed for the ECAP processed material. While the solutionized material exhibits cyclic hardening at all strain levels, the ECAP processed material shows different behaviour depending on the number of passes - stable response/cyclic softening/ softening followed by hardening. The ECAP processed material after three passes exhibit superior LCF lives at low strains presumably due to higher strength and large refinement of grains.

Abstract: Friction stir processing was performed on rare earth containing magnesium alloy AE42 with the aim of improving the mechanical properties. The resultant stir zone mechanical and metallurgical properties were analyzed to determine the application temperature of the zone. Stir zone microstructure was refined to 5 micron and the grain morphology was completely changed. Second phase intermetallics Al11RE3 Al2RE were refined to submicron level tiny pieces and homogenously distributed throughout magnesium matrix. As a major achievement, both toughness and strength of the stir zone was improved by 15 to 25%. High temperature properties up to 250°C were better than base metal. The stir zone was capable of withstanding more temperature and stress than the base metal.

Abstract: Magnesium and its alloys are promising candidates for temporary implant applications due to their combination of mechanical properties, biocompatibility and biodegradation. But higher degradation rate restricts their wider applications. Recently friction stir processing (FSP) has emerged as a promising tool to attain near surface fine grain structure in materials. In the present work commercial purity magnesium was processed by FSP to obtain fine grain structure and the effect of the grain refinement on the bioactivity was investigated. The microstructural observations were carried out at different locations of the processed regions, from an original grain size of 1500μm, grain refinement was achieved to a level of 6.2μm at the nugget zone. Microhardness was measured across the processed regions and improvement was observed at the nugget zone. Contact angle measurements were carried out to estimate the wettability of the material and the measurements indicate increased wettability due to the increased surface energy induced by grain refinement. For studying the bioactivity the FSPed samples were immersed in simulated body fluids (SBF 5X) for different intervals of time. The phases formed on the samples were investigated by X-ray diffraction (XRD) method, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The phases on the samples after 72hr of immersion were confirmed as magnesium hydroxide, hydroxyapatite and magnesium phosphate by XRD. Controlled degradation due to formation of these phases was observed. FSPed samples have more deposition of Ca/P than non FSP samples which implies better control over the degradation. Hence grain refinement by FSP can be a simple technique to control the degradation of magnesium for temporary implant applications.

Abstract: Grain refinement studies in titanium have gained significant interest owing to its importance as a biomaterial. Severe plastic deformation techniques have been widely applied for grain refinement in metals and are capable of producing ultra fine and nano sized microstructures. In this study, repetitive shear deformations by constrained groove pressing have been applied to commercial purity titanium sheets of 2 mm thickness at a warm working temperature of 300oC. Microstructure studies reveal the formation of elongated grains with widths of the order of 0.5μm from an initial grain size of 40μm in the annealed condition. An increase in strength is also observed.

Abstract: Commercial purity Ti was subjected to equal channel angular pressing (ECAP) for up to three passes at 400oC using a die with die angle of 120o. Compression testing of the ECAP specimens was carried out to determine the subsequent flow behavior. Two types of compression test specimen orientations, one parallel to the axis of ECAP and the other at 45o to the axis of the ECAP, were prepared from the specimens subjected to ECAP. Anisotropy in flow behaviour (as indicated by values of strength co-efficient, K and strain hardening exponent, n) was observed. The strain hardening rates were also calculated from the experimentally determined flow curves for the specimens tested in compression in the two orientations. The results have been interpreted in terms of the strain path change parameter between the two deformation steps (ECAP and compression). Strain hardening behaviour and microstructure evolution is discussed in terms of strain path change parameter. Specimens compressed in the direction parallel to the ECAP direction had lower strain hardening exponents while exhibiting higher initial flow stresses. The strain hardening rates were lower for specimens compressed at 45o to the ECAP direction compared to specimens compressed parallel to the ECAP direction.

Abstract: Equal channel angular extrusion (ECAE) is a severe plastic deformation (SPD) method for obtaining bulk nanostructured materials. The ECAE die consists of two equal channels that intersect at an angle, usually between 90° and 135°. In the present study, the plastic deformation behavior of the Cu during the ECAE process with 120° die through multiple passes was investigated. Finite element modelling was included in order to analyze the deformation behavior as the material passes through the die. In order to perform the FEM simulations the properties of the commercial purity Cu have been selected.

Abstract: There is increasing interest in using Al alloy sheets for auto body applications. However Al alloys exhibit poor drawability as indicated by low values of the normal anisotropy, rm. Techniques for improving the value of rm rely on developing a favourable shear texture in the sheet. In this study, Al alloy AA 6061 sheets of dimensions 225 mm x 200 mm and 1 mm thick were subjected to severe plastic deformation by repeated groove pressing using a set of grooved and flat dies alternatively. The orientation of the grooves with respect to the rolling direction was also varied. Microstructure characterization and mechanical property measurements were carried out. X- ray diffraction scans were carried out to measure the relative intensities of the (111) and (200) peaks. The r values was measured as per ASTM standard E 517 on strip specimens cut at 0°, 45° and 90° to the rolling direction and the normal anisotropy value (rm) and planar anisotropy value (Δr) values were determined. The limiting drawing ratio (LDR) was determined using the Swift cupping test techniques. It was observed that the rm values increased from 0.72 in the as received condition to a maximum of 0.94 and the LDR increased from 1.93 to 2.06 when the groove pressing was carried out with grooves at to 45° the rolling direction. The improvement in rm values can be correlated to the texture developing in the sheet as a result of severe plastic deformation.

Abstract: Commercial purity Ti is an important candidate material for orthopedic and dental implants because of its high specific strength, good corrosion resistance and excellent biocompatibility. However, for biomedical applications as a replacement for Ti-6Al-4V alloy that is currently used, improvement in strength of CP Ti is necessary. This can be achieved by using severe plastic deformation (SPD) processes like equal channel angular pressing (ECAP) at warm working temperatures followed by conventional processing at room temperatures. This requires adequate workability after ECAP. In this study, the workability of CP Ti after warm ECAP has been investigated. Specimens of CP Ti, 17 mm in diameter, were extruded using processing route Bc through an ECAP die with an angle of 120◦ between the two intersecting channels and at a temperature of approximately 400oC. Workability testing was carried out using collar type compression specimens by upsetting the specimens between flat platens till the onset of cracking. Workability diagrams have been plotted as a function of axial and hoop strains at failure. The results show that processing by ECAP lead to only minor reductions in workability of CP Ti. The tensile strength is enhanced considerably by this process.

Abstract: Groove pressing (GP) is a severe plastic deformation technique for producing ultra fine grain sized microstructures in metals and alloys. In the present study, groove pressing and a two-step process of groove pressing followed by cold rolling was used to investigate the potential of these processes to produce ultra fine grained copper with significantly enhanced strength. Mechanical and microstructure properties were evaluated after groove pressing and after groove pressing followed by cold rolling. The advantages conferred by groove pressing prior to cold rolling on producing copper with enhanced properties has been investigated.

Abstract: Equal channel angular extrusion (ECAE) is a processing method for introducing an ultrafine grain size into a material. In the present study, a two-step severe plastic deformation process was used to produce ultrafine grained copper with significantly enhanced strength. Equal channel angular extrusion was first used to refine the grain size of copper samples. The copper samples were further processed by cold rolling (CR) to a strain of 0.67 (about 50% reduction). This two-step process produced ultrafine grained copper with strengths higher than those of pure copper processed through ECAE only. This paper reports the microstructures and mechanical properties of the copper specimens processed by a combination of room temperature ECAE and CR. The effectiveness of initial processing by ECAE prior to cold rolling is discussed.