Papers by Author: P.P. Sinha

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: 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.

Abstract: Titanium alloys are used for high-pressure gas bottles / propellant tanks and structural applications owing to their high specific strength, good fabricability / weldability and compatibility with various working fluids. For these applications at ambient temperature, the workhorse Ti6Al4V alloy is extensively used. For the applications at low temperatures, two ELI grades of titanium alloys namely Ti6Al4V and Ti5Al2.5Sn are used as these retain toughness down to 77K and 4K respectively. Due to this inherent advantage, Ti5Al2.5Sn-ELI alloy has been selected as high pressure helium gas bottle submerged in liquid hydrogen (20K temperature). The gas bottle is spherical in shape and is made by electron beam welding of two machined hemispherical shells of 500 mm nominal diameter. The hemispherical shells for the difficult-to-forge Ti5Al2.5Sn-ELI alloy are developed through controlled closed-die forging operations. Shells are subsequently characterized for microstructures and mechanical properties at ambient temperature. Substantial increase in tensile strength with reasonably good ductility with respect to ambient temperature is achieved at 20K temperature. Multi point necking is observed at 20K. The present paper briefly outlines the process control devised for development of these domes and discusses the various characterization results obtained on forged hemispherical shells.

Abstract: Severe plastic deformation processes (SPD) are gaining importance as advanced materials processing techniques and hold immense potential in obtaining ultra fine-grained high strength materials. Among the SPD techniques, Equal channel angular pressing (ECAP) has its own merits to produce materials with ultra fine grains in bulk with better mechanical properties. The material deforms with high level of plastic strain inside the channel resulting in grain refinement of the output material with improvement in mechanical properties. A very viable die configuration was conceptualized and die was made with 1200 channel angle. Processing of 25 mm dia. of Al alloy AA2219 at room temperature was successfully carried out and grain refinement was observed. The mechanism of grain refinement has been studied using optical and transmission electron microscopy (TEM). It was observed that low energy dislocation structure (LEDS) forms concurrently with sub-grain structure due to dislocation rearrangements, which provide stability to the evolving sub-grain structure. Dislocation mobility is hindered by the presence of precipitates and / or intermetallic dispersoids present in the matrix and results in presence of dislocations in grain interiors. The pile up of dislocations at intermetallic dispersoids was confirmed from the dark field TEM micrographs. Present paper describes the experimental procedure and followed to attain severe plastic deformation through ECAP. Increase in hardness as well as refinement in the grain size after 5-passes have been discussed in light of extensive optical and TEM. The mechanisms of grain refinement to achieve nano-grained structure and strengthening accrued from the grain refinement through ECAP has been discussed.