Materials Science Forum Vols. 830-831

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: Transformations of hot-pressed partially recrystallized structure of the magnesium alloy ZK60 into ultrafine-grained one with the (sub) grain size of ~1 μm under multi-step isothermal forging were analyzed. The mechanisms of recrystallization, as well as the role of the processing temperature and second phases in the development of new grains are discussed.

Abstract: Aluminium based FGM rings, reinforced by in-situ primary Si and primary Si/ Mg₂Si hybrid reinforcement were successfully fabricated by centrifugal casting and micro structural, chemical, hardness and corrosion characteristics were evaluated. It was observed that in Al-20Si ring the primary Si particles were present mostly in inner region and few in outer region where as in Al-20Si-3Mg ring both the primary silicon and Mg₂Si were completely found in the inner region only resulting in a graded FGM structure. The hardness values were measured along radial direction of samples and variations corresponding to micro structural variation were analysed. Optical Emission spectroscopic studies have revealed the remarkable compositional changes along radial direction. Corrosion characteristics were also evaluated both in particle rich and depleted regions.

Abstract: 15Cr-5Ni-1W precipitation hardening (PH) stainless steel is a martensitic PH stainless steel finding extensive use in semi-cryo engine applications. The alloy was developed through Vacuum Induction Melting (VIM) + Electro slag refining (ESR) under argon cover route. The alloy contains heavy elements like Mo, Nb, V and W totalling ~ 2 % by weight. Since the alloy is martensitic, stringent gas levels were also specified. Hence it was a challenging task to realise it without any segregation and stringent gas levels. The alloy was successfully melted through two different melt routes – (C). Electric Arc melting followed by Vacuum Oxygen decarburization (VOD) - vacuum degassing (VD) followed by secondary melting by ESR and also by melt route (V) vacuum induction melting (VIM) + ESR route. It was then forged into bars, rods and rings. The samples from the alloy were subjected to two different heat treatment cycles. Both the heat treatment cycles involved hardening at 1000°C for 2 hrs followed by air cooling to room temperature. In one of the cycle, sub-zero heat treatment at-70 °C was done prior to tempering while in the other cycle; direct tempering was carried out after hardening operation. Tempering was carried out at 2 different temperatures of 490 and 500 °C to achieve the specified mechanical properties. It was found that the alloy could meet the specified strength and ductility with both the heat treatment cycles mentioned above. However samples subjected to subzero heat treatment showed marginally higher strength with slight compromise in ductility. The alloy also exhibited similar impact toughness in both the heat treatment conditions. Delta ferrite was also found to be within 2% for both the heat treatment cycles employed in this study. The alloy also exhibited excellent strength and ductility at elevated temperature of 500 °C with just 25% reduction in yield strength compared to room temperature yield strength without much change in ductility.

Abstract: Hot isostatic pressing (HIPping) technology is used for healing the casting defects for aerospace applications. Castings used for aerospace applications like turbo-pumps need to meet very stringent quality requirements. Complexity of the castings used in these applications, makes it difficult to meet the quality requirements in all the areas. Defects like gas holes, shrinkages, cavities etc. may occur in few locations and need to be repaired by welding or healed by HIPping. In the present study, we attempted to simulate the defect healing capability of HIP in a systematic manner. Artificial defects were created in Austenite-Martensite grade stainless steel cast rod. These rods were then subjected to HIP prcoss cycle at 1150 °C and at a pressure of 1620 bar. Healing of the defects was ensured through X-ray radiography. Detailed microstructural analysis using optical metallography and scanning electron microscopy (SEM) with EDX was carried out before and after HIPping, to understand the defect healing mechanisms. These results are discussed in detail here.

Abstract: Austenitic stainless steels are extensively used as structural materials for various aerospace systems. Nitrogen containing stainless steels have special role due to their austenite stabilization tendency down to subzero temperatures, improved strength and resistance to sensitization. Primary processing of nitrogen containing cryogenic grade stainless steel 202 has been carried out through two different melting routes viz. (1). conventional melt route of electric arc furnace (EAF) melting followed by vacuum oxygen decarburization (VOD) & vacuum degassing (VD) and other one through (2). vacuum induction melting (VIM) followed by ESR. Chemical analysis and macrostructure analysis was carried out on the samples drawn from these billets. Homogenization and thermomechanical processing parameters were selected and the same were followed for the ingots made through both the melt routes. Mechanical properties evaluation (including tensile properties at subzero temperature of 77K) and micro structure characterization of the products realised from all the two melt routes were carried out. It is observed that, both the melt routes could result in achieving the required aerospace quality of alloy with respect to the chemical composition, metallurgical and mechanical properties. This paper confirms that any of the melt routes studied herein can be adopted according to availability of the facilities. The process development and characterization of the steels processed by conventional EAF+ VD & VOD and VIM+ESR melt routes is presented in this paper.

Abstract: Rheocasting refers to a process in which a liquid metal is vigorously agitated during initial stage of solidification to produce globular semisolid slurry which remains highly fluid. This slurry is directly fed into any shaping operation to obtain SSM parts. Rheocasting is an energy saving as well as cost saving, good surface finish and negligible porosity for a specialized casting, in which heating step of thixocasting is completely eliminated. Rheocasting also offers shot size flexibility and amenable for processing of wrought alloys and metal matrix composites, which are otherwise difficult to process by thixocasting route. A slurry with globular shape of the solid phase particles and hence injected directly into the die. In the present development, Al base thin wall components for electronic packaging applications were produced by rheocasting process. The paper presents the die designing aspects, optimization of rheocasting process parameters to achieve the sound casting and mechanical property. Thin walled Al-Alloy components lot was realized by the novel processing technology, meeting dimensional accuracy, desired microstructure, mechanical property, value for high integrity component needed for space applications.

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: This study was undertaken to investigate the effect of different concentrations of magnesium (0.2%, 0.3%, and 0.4%) on the modifying efficiency of Sr in aluminium alloy A356. Both sand casting and gravity die castings were taken for the same composition. Modifying efficiency was studied quantitatively by measuring the size and shape of Si eutectic particles by using ImageJ image analyser and giving a Modification Rating (MR), developed by American Foundrymen’s Society (AFS), for each microstructure ranging from 1 to 6, 1 being unmodified and 6 being completely modified very fine structures. Characterisation was done with optical microscope and SEM equipped with EDS and XRD. Mechanical testing involved hardness test in both as cast and standard T6 condition.