Papers by Author: P. Ramesh Narayanan

Abstract: Limitation in penetration depth is a concern in conventional TIG welding. To improve penetration capability of TIG process, Flux Bounded TIG (FBTIG) has been developed. Stress corrosion cracking (SCC) behavior of FBTIG welds of aluminum alloy AA 2219 T87 is evaluated in 3.5 weight percent NaCl solution using Slow Strain Rate Test technique (SSRT) as per ASTM G129. SCC index defined as the ratio of the elongation of tensile tested specimen in NaCl to that of air is taken as a measure of the susceptibility to cracking. Based on the SCC index, it is concluded that the SCC resistance of FBTIG joints are good and comparable to that of conventional TIG welds.

Abstract: Limitation in penetration depth is a concern in conventional TIG welding process. To improve penetration capability of TIG process, both Activated TIG (ATIG) and Flux Bounded TIG (FBTIG) are investigated in aluminum alloy AA 2219 T87. Undesirable arc wandering and cracking tendency are observed in ATIG welds. Microstructural investigation reveals ATIG welds are prone for liquation cracks. Morphology of the cracks along with the attributable factors are explained with optical and SEM (Scanning Electron Microscope) micrographs. Energy Dispersive Spectroscopy (EDS) results are also presented to explain the solute enrichment in the grain boundaries of the ATIG welds. FBTIG is found to produce good quality welds and is more suitable for welding aluminum alloys. Key words: Flux Assisted TIG; ATIG; FBTIG; Penetration Improvement; Microstructure; AA2219.

Abstract: Metallography technique is used as an important tool in designing and development of materials. The technique is destructive and hence has its own limitations. Non destructive metallography emerged as a solution for its limitations. The microstructure of the component in the field/service can be replicated and observed under microscope. In the recent past, this technique has drawn the attention of quality control personals due to its non destructive nature. This paper explains the use of nondestructive metallography technique for Ti-6Al-4V alloy domes and low carbon steel static test bed exposed to high temperature.

Abstract: Advancement in material science has reached great heights both in terms of development and properties but still failures continue to happen. Failure is often accompanied by economic and human loss. Failures generally occur due to error of knowledge, error of performance and error of intent which comprises improper design and/or material selection, deficiency in processing and assembly. Failure analyst plays an important role in evaluating the causes of failure by using various analytical tools to arrive at the conclusion and suggests remedies to avoid its reoccurrence. This paper highlights the important steps in failure analysis with a few case studies demonstrated. Case studies include the failure of plumbing tube used in engine gimbal control system of liquid propulsion system, where failure was initiated due to process deficiency and its propagation was facilitated under fatigue loading. In the other case study discussed, failure of steel fasteners was due to hydrogen induced stress corrosion cracking (HISCC).

Abstract: Aluminium alloy AA 2219 has been selected for fabrication of both earth storable and cryogenic propellant tanks of launch vehicles due to its high specific strength, compatibility with liquid propellants, good resistance to stress corrosion cracking (SCC), excellent properties at cryogenic temperatures and good fabricability including weldability. Propellant tanks are fabricated by welding sheets in T87 and rings in T851 temper conditions. Microstructural characterization was carried out on the weldments with sheet-sheet and sheet-forge configuration using optical microscopy and electron microscopy with energy dispersive spectroscopy. Microstructure of weld pool had dendritic pattern, typical of weld cast structure. Fine recrystallised grains were observed near fusion line. Heat affected zone on either side of the weld revealed thickened grain boundaries. EDS on these thickened grain boundaries indicated composition of eutectic. Microhardness and tensile strength for both the configurations was evaluated. Fractures surface of tensile tested specimens were examined for fracture morphology and to understand the role of eutectic film in fracture. Presence of the eutectic film along the grain boundaries was explained with the help of liquation mechanism and Al-Cu phase diagram. This paper brings out details of the investigation carried out.

Abstract: Copper to stainless steel brazed components are used in the thrust chamber of cryogenic propulsion system of Indian Space Programme. Temperature and time play a major role in the optimization of brazing cycle. An effort has been made to study the influence of the above parameters and correlate with the microstructure and shear strength achieved. In the present work, brazing was performed using Cu based filler metal. Temperature selected was within the range of 1010-1045 °C for two different time durations (20 minutes and 30 minutes) under high vacuum (10^-5 Torr). Extensive scanning electron microscopy with energy dispersive spectrometry was carried out to understand the kinetics of elemental diffusion and phase formation at and near the brazed joint. Microhardness was evaluated across the joints to facilitate further understanding of the joint chemistry. The optimized shear strength of the joints was as high as 155 MPa for the joint brazed at 1030 °C for 30 minutes against the required strength of 150 MPa. The joint could retain strength at cryo temperature and meet the minimum expected value of 320 MPa. At the same time, it exhibited good mechanical properties at elevated temperatures. This paper brings out details of the investigation carried out.

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: The weld portion will have different mechanical properties in weld bead, HAZ and fusion boundary due to variation in the microstructure, chemistry and internal stresses that arise in the welding process. Inconel 718 TIG welded samples are tested to study the deformation behavior of the weld constituents with the help of non contact laser extensometer. Stress-strain curves for the individual constituents of weld, parent, fusion boundary and HAZ are evaluated at room temperature and at 550°C. Test results show that the welded samples exhibited 50% lower tensile properties compared to the parent material. Parent metal remains elastic through out the test. The proof stress of the weld alone is 100 MPa lower than the average proof stress of the welded sample when considered standard gauge length.

Abstract: This paper covers the influence of crystallographic texture on the mechanical properties in two of the most important high strength Al-Zn-Mg family of aluminium alloys, viz., AA7075 and AFNOR7020 alloys, used in the Aerospace industry. AFNOR7020 Alloy developed a stronger texture compared to the other two alloys. S component of texture is stronger in AA7075 alloy whereas the Bs component is stronger in AFNOR7020 alloy. This is attributed to the shear banding which was found absent in the other alloy. The starting material, AA7075 in T7352 condition and AFNOR7020 in T652 condition show some degree of anisotropy of mechanical properties with regard to yield strength and ultimate tensile strength. Higher degree of deformation leads to more pronounced anisotropy in mechanical properties with regard to yield strength and ultimate tensile strength, with a maxima along the transverse direction. For the alloys, experimentally measured Plastic Strain Ratio, r value, which is a measure of the texture present in the material in the deformation conditions, agree well with the computed values with a maximum at 45^o orientation to the rolling direction.