J. Srinath

Corrosion and Multi-Scale Mechanical Behaviour of Plasma Electrolytic Oxidation (PEO) and Hard Anodized (HA) Coatings on AA 2219 Aluminum Alloy

The electrochemical corrosion and mechanical properties of ceramic coatings fabricated by plasma electrolytic coating (PEO) and hard anodizing (HA) methods comparatively examined for AA2219. Potentiodynamic polarization results revealed that the corrosion resistance of both coatings are comparable to each other. However, the indentation and scratch testing indicated that the hardness and modulus of the PEO coating was significantly higher when compared to HA coating. The critical load (Lc2) causing adhesive failure of the PEO coating was much high (19N) when compared to HA coating (10N) showing better adhesive strength of the PEO coating.Key words: Plasma electrolytic oxidation (PEO), potentiodynamic polarization, nanoindentation, hard anodizing

Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle

High-strength aluminum alloy AA7075 forgings are widely used in various control system components of liquid engines of satellite launch vehicles. In one such application, a forged component was found to crack under storage. Multiple cracks were noticed on the surface of the component. Fractographic features indicated intergranular mode of failure. Optical microscopic examination revealed dendritic coring in the material and crack propagation to be along the grain boundaries. Anodic dissolution of grain boundaries was also noticed. Based on detailed metallurgical investigations, it was concluded that the failure was due to ‘stress corrosion cracking.’

Effect of Cooling Medium on Solution Treatment Response of Titanium Alloy Ti-5Al-5V-2Mo

Titanium alloys are widely used in aerospace industry in the areas of pressure vessels, airframe structures, landing gears, aeroengine compressor blades etc. The principal qualities of titanium alloys required for these applications are high specific strength, low density and high specific modulus. Among the families of Ti alloys, high strength titanium alloys come under martensitic α + β and metastable β alloys. Titanium alloy Ti-5Al-5V-2Mo (BT-23) is an important example of martensitic α + β alloy similar to the work horse Ti6Al4V alloy which exhibits good combination of strength and ductility in solution treated and aged conditions. But due to quenching from solution treatment temperature, the alloy tends to retain good amount of residual stresses. The severity of residual stress increases with increase in solution treatment temperature as well as severity of quench. An attempt has been made to study the effect of air cooling subsequent to solution treatment to compare the strength of the alloy vis-à-vis that achievable during water quenching. An attempt has also been made to correlate the microstructure evolution, hardness with variation in solution treatment temperature and quench severity in titanium alloy Ti-5Al-2Mo-5V. Samples subjected to air cooling subsequent to solution treatment exhibited higher microhardness when compared to water quenched samples. It is proposed that dynamic aging and/ or stress relieving occurs during air cooling from solution treatment temperature down to room temperature. Also the fine α precipitates formed during air cooling may be resulting in higher hardness compared to the α’’/α’ formed during water quenching. The same has been supported by thermal analysis of air cooling and water quenching processes employed subsequent to solution treatment.

Characterization of the Electrical Discharge Machined M300 Maraging Steel Flexures Using Nanoindentation

Gyroscope is an orientation determining sensor whose performance is mainly dependant on one of the critical element called the gimbal flexure. Thinner the flexure, better is the performance. To have sufficient strength, M300 maraging steel is used to fabricate the thin flexures machined by Electrical Discharge Machining (EDM) process down to the required thickness of 0.05 mm in various stages. After the EDM processing, the recast layer formed can cause degradation in the microstructure and mechanical properties. Micro-abrasive machining was carried out to remove the recast layer. An attempt was made to understand the changes that occurred after EDM and after micro-abrasive machining with respect to the microstructure using optical and Scanning Electron Microscopy (SEM) and with respect to mechanical properties using nanoindentation. Nanoindentation technique was adopted as the recast layer was just few microns thick. Indentations were carried out on the base material, as EDM cut flexure, and the micro-abrasive machined EDM cut flexure to obtain elastic modulus and hardness values for each condition and the results were analyzed.

Axial and Bending Fatigue Testing of AISI 304 L Plumbing Tubes Used for Launch Vehicles Control System

Gimbaling the engine nozzle within a specified angle through hydraulic actuator modules is commonly used to control the trajectory path of satellite launch vehicles. During one of the hot test of a liquid engine stage, a drop in system pressure in actuator module was noticed due to cracking of case drain plumbing inside the ferrule. The tensile and fatigue (axial and bending) test of the tube material was carried out to understand the cause of failure, simulating the actual service conditions such as strength and bending moment in assembled tube. Findings of the metallurgical investigation along with the fatigue test results were presented in this paper.

Microstructural and Mechanical Characterization of Alumina Coating on 07X16H6M Substrate

Alumina coating is given to a 07Х16Н6M stainless steel substrate, to withstand high frictional loads at elevated temperatures. Properties like high hardness, good adhesion and low thermal mismatch are essential for the coating. An 80%Ni-20%Cr bond coat is given for better adhesion of the alumina coating to the substrate. Air plasma spray technique was used for bond coat as well as the alumina coating.The substrate, bond coat and the alumina coating were characterized using optical microscopy, scanning electron microscopy (SEM) and nanoindentation. Hardness was evaluated using nanoindentation in all the three regions at loads varying from 2 mN in the bond coat to 50 mN in the alumina coating. Alumina coating showed the highest hardness of 11.5 GPa, followed by the bond coat – substrate interface having a hardness of 7.4 GPa. Substrate exhibited the least hardness of 6.3 GPa.

Metallurgical Analysis of a Failed 17-4 PH Stainless Steel Pyro Bolt Used in Launch Vehicle Separation Systems

Precipitation hardenable stainless steels are widely used in aerospace applications because of their excellent combination of strength and corrosion resistance. In view of this advantage, Fe-17Cr-4Ni (17-4 PH) steel is used in pyro bolts in space ordnance systems for stage separation in a satellite launch vehicle. These systems typically consist of a pyro charge to sever a given material resulting in stage separation, and these materials are exposed to high strain rates. In one such application, although perfect severance took place, secondary cracks were noticed longitudinally, which are not desirable. This steel, in view of its composition, has delta ferrite in a martensitic matrix in the hardened and tempered condition. In wrought products worked unidirectionally, delta ferrite gets oriented along the direction of thermomechanical processing. The effect of delta ferrite orientation on the impact properties of 17-4 PH stainless steel was studied in various heat-treatment conditions with a view to understand the effect of microstructure on impact properties. It was found that the transverse impact properties are severely affected by the presence of highly oriented delta ferrite stringers for any given heat-treatment condition. This has been attributed to the weak delta ferrite–martensite interface, which facilitates easy crack propagation. Finally, it was suggested that the anisotropy in impact properties need to be carefully considered in the design of components for applications that involve high strain rates.