Papers by Author: P. Ramkumar

Abstract: Material with ultra-low coefficient of thermal expansion (CTE) is required for mounting camera and other optical elements in satellite systems. Invar (64 Fe 36Ni) has been the work-horse material for this purpose. In recent years, modified version of conventional invar i.e. Superinvar with 5% cobalt (replacing 5% nickel) is being used to further bring down the errors in camera mountings due to thermal expansion. Processing of this alloy poses many challenges due to its requirement of ultra-low CTE. In the present work, melting and thermomechanical processing parameters were selected to meet the specified requirement of the alloy. The alloy was melted through vacuum induction melting process to obtain uniform and homogeneous chemistry and properties. Virgin raw material was used to achieve lowest carbon and manganese contents. Chemical composition thus obtained is found to be within the specification. Material was hot worked to refine the microstructure. Three different sizes of forged blocks were produced. Hot worked material was heat treated to obtain desirable and stable microstructure. Heat treatment cycle for stabilization was selected and used to retain carbon in the solution and minimize temporal growth. Mechanical properties (tensile strength and modulus of elasticity) and physical properties (CTE, thermal conductivity) were evaluated. Properties were found to be meeting the specification. It is observed that the material shows uniform single phase austenitic microstructure. The paper presents details of the process selection and challenges in processing of this alloy to obtain the targeted CTE < 0.6x10^-6 per °C in the temperature range of 25°C to 150°C along with other desired mechanical properties.

Abstract: Titanium alloy Ti6Al4V is used for manufacturing pressure vessels of launch vehicle in view of its excellent corrosion resistance and fabrication characteristics. This material also offers advantage of good strength to weight ratio necessary for space application. This paper describes the experience gained in the technology development of hemispherical forgings of titanium alloy Ti6Al4V. To achieve α-β microstructure and mechanical properties in the final product, hot working of the alloy from the stage of forging of cast ingots to finish working was carried out in the (α-β) region. The manufacturing parameters are established through state-of-the-art technology and capabilities. Process technology for processing of the hemispherical dome has been established and the same has been presented here.

Abstract: The present investigation is on synthesis of in situ Al-alumina composite and to evaluate the effect of equal channel angular pressing on the refinement of the grain structure and enhancement in the hardness and the strength. The billets pressed in as cast condition has shown cracks during first pass. The billets pressed immediately after solution treatment for one pass and followed by ageing treatment immediately after pressing exhibited very high hardness of 125BHN against 95 BHN to that of the T6 condition of 6061 aluminium alloy. The microstructural refinement from 35 µm to 11 µm is obtained in annealed and ECAP 2 pass condition.

Abstract: 0.3C-CrMoV(ESR) steel is an ultra-high strength low alloy steel indigenously developed by ISRO for space applications. The steel is used in the form of rings of 2.8 m diameter also. In this paper, the effect of tempering temperature on ring rolled steel for the best combination of fracture toughness and strength properties is studied. The tensile properties and fracture toughness of the steel were evaluated in the as quenched and tempered conditions through the specimens drawn in radial direction of the ring segment. Five tempering temperatures were used in the study: 200, 450, 475, 500 and 510°C. Tensile strength of the steel showed continuous decrease with increasing tempering temperature, but yield strength increased reaching maximum when tempered at 450°C and further decreased with increasing tempering temperature. The elongation was higher for higher tempering temperature. The strain hardening exponent decreased with increasing tempering temperature. The fracture toughness test results showed that tempering between 475 and 510°C exhibited better combination of fracture toughness and strength.

Abstract: Medium carbon low alloy Ni-Cr-Mo steel is used in the fabrication of aerospace fasteners. It finds application in different heat treated conditions to meet the desired strength level. The alloy was realized through double melting route. Heat Treatment studies have been carried out by following different tempering temperatures to obtain varying strength levels ranging from 1200MPa to 1400MPa. Microstructural analysis has been carried out to find out reasons for variation in mechanical properties. Tempering cycle has been suggested to obtain fully tempered martensitic structure. This paper presents the different hardening and tempering cycles studied to obtain the desired strength level for the intended application.

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.