Papers by Author: Bhanu Pant

Abstract: Titanium alloys play a major role in launch vehicle due to its high specific strength, good weldability, excellent corrosion resistance and retain toughness at cryogenic temperatures. In the present paper we discuss on the electron beam welding studies carried on alpha-beta titanium alloys for cryogenic applications in launch vehicle programme. The Ti6Al4V having extra low interstitial elements and Ti5.4Al3Mo1V alloys were joint individually through electron beam welding on 6.8mm thickness coupon. Carried microstructural observations of the weld by optical microscope and are reported. Further tensile properties evaluated in welded condition at ambient and cryogenic (20K) temperatures are compared with parent metal properties. Micro hardness plot was generated across the weld in the transverse direction for both the alloys. Showing highest hardness in the weld region and drops towards the parent metal, due to formation of very fine martensitic alpha with high dislocation densities within coarse beta grains of the weld and HAZ region. The failure location of all the weld specimens is in parent metal far away from the weld region.Keywords: Ti6Al4V-ELI, Ti5.4Al3Mo1V, electron beam welding, cryogenic, microstructure, micro hardness and tensile properties.

Abstract: The beta titanium alloys are highly cold workable in annealed condition, due to presence of single phase bcc structure (beta) at ambient temperature. The Ti15V3Al3Sn3Cr alloy is a metastable beta alloy retains single beta phase at ambient temperature by beta annealing. The beta alloys are most hardenable among titanium alloys, due to the formation of hard alpha (hcp) precipitates in beta (bcc) grains in solution treated and aged (STA) conditions. The present paper brings out the hot forging and rolling studies carried above beta transus temperature and correlating microstructure with mechanical properties in heat treated conditions (a. 800°C for 30 minutes and b. 800°C for 45 minutes, subsequent water quenched from single phase beta region plus aged at 482°C/538°C). The results conclude that solution treatment carried for 45 minutes and aged at 482°C/538°C achieved high tensile strength with improvement in ductility. This is due to less nucleation sites of alpha precipitates along the grain boundaries for the 45 minutes solution treated specimens. The Young’s modulus evaluated for solution treated (78GPa), aged at 482°C (105GPa) and 538°C (103GPa), the increase in aged conditions is due to the formation of alpha precipitates throughout the matrix and makes the alloy two phase alpha-beta system.Keywords: Metastable beta, alpha precipitates, solution treatment, tensile strength, Young’s modulus.

Abstract: Cu-Cr-Zr-Ti alloy is being used widely in aerospace engines, due to its synergetic combination of high strength and thermal conductivity. Brazing is the preferred process being adopted to realize intricate shapes and complex dimensions. In the present work, Cu-Cr-Zr-Ti alloy was brazed using Cu-Mn-Ni-Sn-Fe base brazing foil. This braze foil exhibits liquidus temperature of ~980°C. Brazing experiments were carried out at 1030°C under high vacuum condition. The effect of varying load (0.5-2 kg) has been studied in the current experiment. Microstructural study of the parent materials and joints were carried out using optical microscope (OM). Lap shear testing (1T configuration) of the brazed joints was evaluated to obtain shear strength values. Also, micro-hardness traverse has been carried out across the brazed joint. Applied load plays a significant role in obtaining defect free braze joint.

Abstract: The penetration characteristics of EBW are primarily dependent on the main beam current (Iw), potential difference between the cathode and anode (voltage, V) and the welding speed (S). There are other influencing parameters like weld focus current (If), welding gun to work distance (GW) and beam oscillation.In the present study, the effect of work distance on focus current and on penetration of Ti6Al4V weld is studied using a 60kV, 30kW EB welding machine. Weld focus current is measured over a range of work distance and variation in its pattern is studied. Similarly, the variation of depth of penetration by varying the work distance and varying the focus current at a particular work distance constant are also analyzed.It is observed that, focus current is inversely proportional to the work distance and the variation in focus current per unit change of work distance is high in the shorter work distance region. The change in focus current and work distance affects the weld penetration and fusion zone geometry. For optimum penetration at a given work distance, the beam focus should be below the surface in keyhole welding. As the work distance increases, the penetration capacity of the beam decreases and maximum penetration is obtained in lesser work distance region.

Abstract: Transition metal diborides, especially zirconium and hafnium diboride are potential ceramic material for ultra high temperature applications above 1800°C. These borides are characterized by high melting point, formation of high melting point oxides, good oxidation resistance and excellent thermo-mechanical properties. In this present exploration, zirconium diboride (ZrB₂) has been selected for its moderate density (6.09 gm/cc) and better oxidation resistance compared to high density hafnium diboride (11.2 gm/cc). The developed ZrB₂ composite in the present study contains 10 wt. % SiC and 10 wt. % MoSi₂ as sintering additives. SiC and MoSi₂ were added to improve the thermal shock resistance and sinterability of the ultra high temperature ceramics (UHTCs). Vacuum hot pressing was carried out at 1800°C for a holding period of 30 minutes and applied pressure of 30 MPa. Attractive feature of this ZrB₂ composite is good machinability due to better electrical conductivity and complicated shapes can be realized easily through electro discharge machining (EDM) process. Detailed XRD phase analysis and microstructural investigation of the polished and fractured composites was carried out using SEM. Mechanical and thermal properties tests have been carried out for the optimized ZrB₂ composite material.

Abstract: In the present paper, Haynes 214 plate of 7mm thick was welded by Gas tungsten arc welding (GTAW) in two passes. The welded plates were subjected to post-weld heat treatment such as direct aging (DA) and solution treatment + aging (STA). The aging cycle was optimized using hardness by generating isothermal aging curve. The weld tensile properties, hardness test, microscopic analysis and fractographic studies were carried out in as-welded condition and at two different post-weld heat treatment conditions. The results show a significant increase in both yield and tensile strength after post weld heat treatment.

Abstract: Among the copper alloys, the Cu-3Ag-0.5Zr alloy is one of the potential candidates for combustion chamber of liquid rocket engine because of its optimum combination of high strength with thermal conductivity. The present study is a detailed characterization of microstructure, strength, and electrical conductivity during the aging treatment. The aging cycle for Cu-3Ag-0.5Zr alloy after the solution treatment (ST) was optimized to obtain higher hardness without compromising on electrical conductivity. The precipitates responsible for strengthening in aged samples are identified as nanocrystalline Ag precipitates with an average diameter of 9.0±2.0 nm.

Abstract: Titanium alloys are used for high-pressure gas bottles / propellant tanks and structural applications owing to their high specific strength, good fabricability / weldability and compatibility with various working fluids. For these applications at ambient temperature, the workhorse Ti6Al4V alloy is extensively used. For the applications at low temperatures, two ELI grades of titanium alloys namely Ti6Al4V and Ti5Al2.5Sn are used as these retain toughness down to 77K and 4K respectively. Due to this inherent advantage, Ti5Al2.5Sn-ELI alloy has been selected as high pressure helium gas bottle submerged in liquid hydrogen (20K temperature). The gas bottle is spherical in shape and is made by electron beam welding of two machined hemispherical shells of 500 mm nominal diameter. The hemispherical shells for the difficult-to-forge Ti5Al2.5Sn-ELI alloy are developed through controlled closed-die forging operations. Shells are subsequently characterized for microstructures and mechanical properties at ambient temperature. Substantial increase in tensile strength with reasonably good ductility with respect to ambient temperature is achieved at 20K temperature. Multi point necking is observed at 20K. The present paper briefly outlines the process control devised for development of these domes and discusses the various characterization results obtained on forged hemispherical shells.

Abstract: Aluminium alloys and austenitic stainless steels are often used for construction of cryogenic pressure vessels owing to their attractive properties at cryogenic temperatures. Indian space programme requires AA2219/ICSS1218-SS321 bimetallic components which are machined from explosive welded plates. Pure aluminium sheet is used as an interlayer between aluminium alloy and steel to achieve a satisfactory bond. Internal soundness of the joint is evaluated through ultrasonic testing (UT). The present paper discusses bonding trials carried out by varying the explosive parameters using facilities and expertise of Terminal Ballistic Research Laboratory (TBRL), Chandigarh and M/s Giridhari Explosives Private Limited (GEPL), Hyderabad. The welded joint is extensively characterised with respect to Lap Shear and Ultimate Tensile Strength at ambient temperature and for metallographic analysis.

Abstract: Ti15V3Al3Cr3Sn (Ti-15-3-3-3) alloy is a highly cold formable metastable beta alloy and attains very high strength by proper selection of cold working and heat treatment cycles. Due to the presence of softer BCC beta phase at ambient temperature in solution treated condition, this alloy is highly cold workable; to as much as 90% without need of any intermediate annealing. In the present work, cold working up to 83% was imparted to the sheet. Subsequently, cold worked sheet was solution annealed at various temperatures. Cold worked and solution annealed samples were provided with two types of aging cycles: 482°C/16hrs/AC (STA-1) and 538°C/8hrs/AC (STA-2). Hardness measurement, tensile property and microstructural evaluation in cold worked (CW), CW plus solution annealed (CWSA) and CWSA plus aged conditions were carriedout. Recrystallization temperature has been observed to be dependent on extent of cold working. Tensile properties are higher for STA-1 cycle as compared to STA-2 cycle. Hardness values in cold worked condition are higher than solution annealed samples whereas for cold worked and STA samples, an increase in hardness w.r.t. to annealed values has been observed. Corresponding volume fraction of alpha precipitates is found to be more with STA-1 cycle than STA-2 cycle. Also, distribution of the precipitates with STA-1 cycle is observed to be more uniform than STA-2 cycle. The present paper discusses the effect of cold working on recrystallization, mechanical properties and microstructure in Ti-15-3-3-3 beta alloy.