Sushant K. Manwatkar

Effect of micro arc oxidation treatment on localized corrosion behaviour of AA7075 aluminum alloy in 3.5% NaCl solution

Alumina coating was formed on AA7075 aluminum alloy by micro arc oxidation (MAO) method and its corrosion and stress corrosion cracking (SCC) behaviors were examined in 3.5% (mass fraction) NaCl solution. Electrochemical impedance spectroscopy (EIS) was used to evaluate the degradation of the coating as a function of immersion time and was modeled with appropriate equivalent circuits. Constant load stress corrosion cracking (SCC) results followed by post-test metallographic observations demonstrated the usefulness of MAO coating to avoid the premature failure of the alloy due to severe localized corrosion initiated by Cu- and Fe-rich intermetallic phases.

Effect of Welding Processes (GTAW & EBW) and Solutionizing Temperature on Microfissuring Tendency in Inconel 718 Welds

Inconel 718 is widely used superalloy in the Indian space program for high temperature application. Some of the newer applications envisage use of this alloy in very critical high pressure oxygen carrying vessels. The alloy is frequently used in welded condition which requires extensive characterization of various types of welds viz Electron beam welding (EBW) and Gas tungsten arc welding (GTAW). In many cases the weldability of Inconel 718 is found to be limited by microfissuring phenomenon in the weld heat affected zone. Microfissures are fine intergranular cracks and their severity strongly depends on pre-weld solution treatment temperature (grain size), weld heat input and concentration of impurities (B, P and S) in the base metal. In the present work, a study was undertaken to compare the microfissuring tendency in EBW and GTAW processes using two pre-weld solution treatment temperatures. The samples were solution treated at 970°C and 1050°C to generate different grain sizes. Amount of heat input and cooling rate were calculated since they are known to affect the microfissuring and an effort was made to understand their role on the microfissuring. It was observed that microfissuring susceptibility is more at coarser grain size. Severity is more in EBW. The reasons for this phenomenon have been discussed in this paper correlating microfissuring with microstructures and other factors. Procedures to achieve minimal microfissuring during welding have also been brought out.

Stress Corrosion Cracking of a Maraging Steel Shear Bolt Used in the Interstage Structure of a Satellite Launch Vehicle

M250 grade maraging steel is used widely in satellite launch vehicle structures due to its excellent combination of strength and toughness. One of the maraging steel bolts used in interstage structures was found broken during the visual inspection of the hardware. The failed bolt was in assembled condition for several months. The bolt has failed at the stepped region before the start of threads. Crack initiated at the corrosion pits located at the stepped region and propagated inward in an intergranular mode. Fractography revealed brittle intergranular features and corrosion products were noticed on the outer periphery of the fracture surface. EDS analysis of the corrosion products near the fracture edge indicated the presence of chlorine. However, at the center of the bolt, the fractographic features correspond to transgranular mode of failure. Based on detailed metallurgical analysis, the failure of the bolt was attributed to ‘stress corrosion cracking.’

Effect of Prior and Post-Weld Heat Treatment on Electron Beam Weldments of (α + β) Titanium alloy Ti-5Al-3Mo-1.5V

Titanium alloy Ti5Al3Mo1.5V is used in the fabrication of critical engine components for space applications. Double vacuum arc re-melted and (αxa0+xa0β) forged blocks were sliced into 10-mm-thick plates and subjected to electron beam welding (EBW) with five different variants of prior and post-weld heat treatment conditions. Effects of various heat treatment conditions on the mechanical properties of the weldments have been studied. The welded coupons were characterized for microstructure, mechanical properties, and fracture analysis. An optimized heat treatment and welding sequence has been suggested. Weld efficiency of 90% could be achieved. Weldment has shown optimum properties in solution treated and aged condition. Heat-affected zone adjacent to weld fusion line is found to have lowest hardness in all conditions.

Role of Metallographic Analysis in the Identification of Location of Crack Initiation in a Burst Tested AA 2219 Propellant Tank

Owing to its excellent combination of weldability and strength at low temperatures (up to 4xa0K), aluminum alloy AA 2219 is the material of choice for the design of propellant tanks for cryogenic engines of satellite launch vehicles. In order to assess the design margins available, one propellant tank has been subjected to burst test. This paper highlights the role of microstructural analysis conducted on the burst tested propellant tank in locating the origin of crack initiation. Specimens collected from different locations of the burst tested tank have been subjected to optical microscopy and scanning electron microscopy to locate the origin of crack initiation.

Microstructural Characterisation of AA2219 Weldments

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.

Promising anode material for lithium-ion cells based on cobalt oxide synthesized by microwave heating

High-performance anode material for lithium-ion cell based on cobalt oxide was synthesized through a combination of sol-gel route and subsequent microwave heating. The influence of microwave irradiation temperature of the precursor on the characteristics of the active materials formed was studied. The physicochemical, structural, and morphological properties of the materials were studied in addition to the electrochemical performance by cyclic voltammetry and charge-discharge cycling vs. Li+/Li. Microwave heating at 350xa0°C resulted in the formation of Co3O4, whereas at 450 and 550xa0°C, a mixture of Co3O4, CoO, and Co was formed. Co3O4 synthesized at 350xa0°C possessed porous morphology with high specific surface area and exhibited superior electrochemical performance with initial specific capacity of 982xa0mAhxa0g−1 and coulombic efficiency of ~75% along with good cycle performance retaining ~87% of initial capacity after 60xa0cycles.

Metallurgical Analysis of Failed AISI 304L Stainless Steel Tubes Used in Launch Vehicle Applications

Two numbers of AISI 304L stainless steel tubes with Ø6 and Ø4xa0mm used in pressurisation system module of a liquid engine of a satellite launch vehicle were found to crack after the qualification level random vibration test. The module has undergone resonance survey, sinusoidal vibration, and random vibration along two different axes before the occurrence of failure. The tubes failed during third-axis random vibration test. The failed tubes were subjected to detailed metallurgical analysis to understand the reasons for failure. The location of failure in both tubes was at the cross-section change area in the nipple. Cross-section change was provided in the design to reduce the diameter of the nipple to suit to tube diameter for welding. The cross-section change in the nipple acted as stress concentration resulting in the initiation of fatigue cracks. The presence of sharp-cornered inclusions in the material helped easy crack initiation. Smaller radius and angle at the weld–parent interface resulted in additional stress concentration. The presence of striations on the fracture surface and slip bands near to the fracture edge confirm that the failure of tubes to be due to vibration induced fatigue.

Failure Analysis of AISI 302 Steel Compression Spring Used in Flush and Purge Valve of Liquid Engine

AISI 302 stainless steel is used for making compression springs for launch vehicle programmes. One such AISI 302 stainless steel compression spring used in flush and purge valve of liquid engine of a satellite launch vehicle failed during testing. The failure was at the second round of spring and it failed in a slanted type fracture. Detailed metallurgical analysis indicated that the failure was due to fatigue.

Adhesive Joining of Metal to Metal and Metal to Ceramic by Ceramic Precursor Route

Inorganic ceramic adhesives (geopolymers) based on aluminosilicate matrix are versatile candidates for bonding metals to metals or metals to ceramics. On curing, they result in an amorphous, crosslinked, impervious, acid resistant 3D-structures. Alkali activated aluminosilicate based ceramic adhesive was developed for bonding metals to ceramics and metal to metal, for high temperature applications. The bonding is achieved at 175°C for 3 hrs, by solid state reaction of alkaline solution of allkalisilicate precursor with the refractory filler, contributing to the bulk aluminosilicate matrix. Lap shear strength of 2-4 MPa was obtained for bonding stainless steel. The XRD patterns show the amorphous nature of the aluminosilicate matrix, with mullite formation at higher temperatures. Thermogravimetric analysis shows that the weight loss is only due to the removal of water from the system by means of evaporation and polycondensation of Si-OH groups and Al-OH groups. This is followed by structural reorganisation in which aluminium ions are incorporated into the silicate chains forming the Si-O-Al network resulting in the bonding with the metallic surface. The system can withstand the maximum operational temperatures of the substrates and can be used for bonding different metallic or ceramic, joints/interfaces for RLV-TD/TSTO.