G. Sudarshan Rao

Effect of Simulated Brazing Cycle on the Microstructure and Mechanical Properties of Cu-Cr-Zr-Ti Alloy

Cu-Cr-Zr-Ti alloys are widely used for fabrication of thrust chamber in liquid rocket engines, because of their high thermal conductivity and adequate strength. The alloy should be used in peak-aged condition to achieve the best combination of strength and conductivity. However, realization of final component involves forming and brazing operations which limit the usage of the alloy in peak aged condition. Therefore, an attempt has been made to study the effect of the simulated brazing treatment on the microstructure and mechanical properties using optical microscopy, electron microscopy, mechanical testing and hardness measurement. The high temperature brazing treatments resulted in tremendous grain growth. The measured hardness is observed to be in line with the grain size of the specimens. The hardness and tensile strength decreased in samples subjected to simulated brazing cycle, implying requirement for a post braze treatment. After the post brazing treatment the optimum mechanical properties have been achieved at room temperature. Low ductility was observed at 600°C in the samples subjected to simulated brazing cycle.

Study of LCF Behavior of IN718 Superalloy at Room Temperature

nconel 718 is an age hardenable nickel base supper alloy with high strength at elevated temperatures, and excellent creep properties. It is used extensively in turbine discs, blades where components experience elevated temperatures for prolonged duration, leading to coarsening of the microstructure. To evaluate the life of such components after prolonged exposure to service conditions, LCF properties at such large grain sizes are essential. For this purpose, low cycle fatigue (LCF) behavior of forged Inconel 718 turbine rotor disc having large grain size was studied at room temperature. Total strain controlled fatigue tests were conducted in air at ambient temperature on this alloy in solution treated and aged condition. The results indicated that the material exhibits cyclic strain softening and the cyclic yield strength is lower by 40% compared to the monotonic yield strength. The deformation takes place by multiple planar slip.

Local Deformation Behavior of Inconel 718 TIG Weldment at Room Temperature and at 550°C

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.

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.

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.