Papers by Author: S.V.S. Narayana Murty

Abstract: Mg-3Al-1Zn (AZ31) alloy was caliber rolled at 250, 300, 350, 400 and 450 °C. The effects of caliber rolling temperature on the microstructure and tensile properties were investigated. The room temperature tensile tests were carried out to failure at a strain rate of 1 x 10 ^-4 s^-1. The nature of stress-strain curves obtained was found to vary with the temperature employed in caliber rolling. The yield strength and tensile strength followed a sinusoidal behaviour with increasing caliber rolling temperature but no such trend was noted in ductility. These variations in tensile properties were explained by the varying grain sizes obtained as a function of caliber rolling temperature.

Abstract: Fine grains were developed in Mg-3Al-1Zn (AZ31) alloy by isothermal caliber rolling at five different temperatures in the range of 250-450°C. The samples of different grain sizes were deformed by constant strain rate and differential strain rate test techniques over the temperature range of 220-450 °C and strain rate range of 10-5 to 10-1 s-1. The effects of grain size, test temperature and strain rate on flow st^{SuperscSuperscript text}ript textress were analysed to develop the constitutive relationship for sup^{Superscript text}erplastic deformation. The parameters of the constitutive relationship obtained from the constant strain rate tests and differential strain rate tests were used to find out the material constant A of the constitutive relationship.

Abstract: Al-Cu-Mg (Russian grade V65) alloys are used for riveting applications in aerospace industries due to their relatively high shear strength of the order of 25 kg/mm2 combined with a high plasticity. This paper presents the processing of V65 aluminium alloy rivets from wire rods. It was observed that wire rods which have not been intermittently annealed and having more than 25% cold working prior to heading operation, cracked on the free bulged surface of the head. In view of this, it is recommended that a maximum of 25% of cold work is allowed in the wire rods to be used for rivet fabrication to successfully realize defect free rivets.

Abstract: High shear strength of 25 kg/mm2 combined with a high plasticity makes Russian grade V65 aluminum alloys as potential candidate compared to AA2024 for riveting applications in aerospace industries. The lower content of magnesium over the AA 2024 alloy decreases the aging kinetics of this alloy because of the decreased vacancy concentration in the as-quenched alloy which in turn gives flexibility to carryout riveting operation even after 4 days after solution heat treat¬ment and natural aging at room temperature. This paper presents the processing and characterization of V65 aluminium alloy wires processed from hot and warm rolled rods. Mechanical properties of the wires have been evaluated in T6 and T4 tempers and correlated with microstructures. Detailed microstructural examination using optical microscopy and fractography of the tensile tested samples using scanning electron microscopy were carriedout.

Abstract: While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine-grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensite steels for all materials. Formability of ultrafine-grained steel is examined by applying to form a M 1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews.

Abstract: Copper - multiwall carbon nanotubes (MWCNT) composite was processed by powder metallurgical processing technique. Pure copper powder and MWCNT were mechanically alloyed by high energy milling to produce Cu-MWCNT composite powder. The composite powder was subsequently consolidated by vacuum hot pressing. Characterization studies were conducted along axial (hot pressing direction) and radial (transverse) directions. Microstructural observations of the processed composite revealed random distribution of MWCNT in axial direction and aligned distribution in radial direction. The structure property correlation was established and it revealed certain degree of anisotropy in mechanical and electrical properties of the composite.

Abstract: In the present study, thermal treatments for homogenizing cast structures of Al-Cu-Li alloy AA2195 for improved workability are developed chiefly by empirical methods and detailed Differential Scanning Calorimetry (DSC) and microstructural characterization. DSC has been carried out on as-cast samples to establish the homogenization temperatures and avoid incipient melting. Homogenization time has been calculated empirically and microstructural characterization and DSC has been carriedout to after each cycle to validate the empirically established homogenization cycle. Homogenization cycle (435°C/8hrs+495°C/12hrs+525°C/32hrs) has been established for AA2195 alloy having an average grain size of 500μm based on calorimetric studies and microstructural examination.

Abstract: Al-Cu-Mg (Russian grade V65) alloys are used for riveting applications in aerospace industries due to relatively high shear strength of the order of 25 kg/mm² combined with a high plasticity. The main advantage of using V65 aluminum alloy for rivet application comes from its significantly slower natural aging response, which gives flexibility to carryout riveting operation even after 4 days after solution heat treatment and natural aging at room temperature. This very valuable feature is not found in its counterpart alloy AA2024 where due to rapid aging kinetics of this alloy, the riveting operation must be completed within 30 min of the solution treatment, which is many times impractical. This paper presents the processing of V65 aluminium alloy by Direct Chill (DC) casting followed by subsequent secondary processing through extrusion and forging to realize feedstock for caliber rolling. These ingots were caliber rolled to 12 mm diameter rods at different temperatures and mechanical properties were evaluated in T6 and T4 tempers. Detailed microstructural examination using optical microscopy and fractography of the tensile tested samples using scanning electron microscopy were carriedout to correlate microstructure and mechanical properties.

Abstract: Silver reinforced with carbon nanotubes, instead of graphite, would increase both the electrical conductivity, hardness and wear resistance of the electrical brush materials. The effective utilization of carbon nanotubes in the Ag/CNT composite depends strongly on its uniform distribution and strong interfacial adhesion to the silver matrix and thus demands for its surface modification. In order to carry out the surface modification of carbon nanotubes, electroless coating was given to them after liquid phase oxidation, sensitization and activation process. The room-temperature chemical treatment results in a nominally complete coating over the entire outer surface of multiwall carbon nanotube. The surface morphology of the carbon nanotubes after each step has been studied using TGA, DSC, XRD, FTIR and SEM.

Abstract: The objective of the present study was to melt and cast AA2195 alloy in Vacuum Induction Melting (VIM) under dynamic inert atmosphere. These billets were homogenized and subsequently hot forged and rolled to sheets. The products in the form of sheets were subjected to T8 (Solution Treatment +WQ+CW+Aging) temper condition. Mechanical properties were evaluated at room temperature and correlated with microstructure. Highest mechanical properties obtained in T87 temper have been reported.