Satish Kumar Singh

Development and Characterization of Ti5Al2.5Sn-ELI Alloy Hemispherical Domes for High-Pressure Cold Helium Tanks

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.

Metallurgical Studies on Explosive Welded Aluminium Alloy-Stainless Steel Bimetallic Plates

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.

Effect of Cold Working and Heat Treatment on Recrystallization, Mechanical Properties and Microstructure of High Strength Ti15V3Al3Cr3Sn (Ti-Beta) Alloy

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.

Heat-Treatment Studies on Dissimilar Ti-Alloy EB Weld Joints

Ti3Al2.5V alloy is lean alpha plus beta titanium alloy having two phase microstructure at room temperature (RT) and has beta transus temperature (Tβ) about 9350C and Ti6Al4V-ELI (extra low interstitials) alloy has two phase structure at RT and has Tβ about 9750C. The samples were electron beam welded (EBW) after the optimization of weld parameters on bead-on plate of 6mm thickness of Ti6Al4V-ELI alloy. The samples were heat treated (solution treated, ST at 9100C for 20 minutes water quenched, WQ and aged at 5100C for 8 hours and air cooled, AC to RT). The specimens were tested to evaluate mechanical properties in as-welded, solution treated and aged conditions. The improvement in the tensile strength was 810 MPa to 897 MPa in as weld to ST conditions, and comparable in STA condition (887 MPa). The improvement in yield strength was 760 MPa to 782 MPa and 848 MPa in as-welded, ST and aged conditions respectively. While there is a decreasing trend in the ductility (%EL) of the joint in three conditions (as weld, ST and STA). In all the specimens evaluated at different heat treated conditions the failure location was within parent Ti3Al2.5V alloy (parent metal away from the weld interface) as confirmed through OM (optical microscopy).

Thermo Mechanical Working and Heat Treatment Studies on Meta-Stable Beta Titanium Alloy (Ti15V3Al3Sn3Cr) Plates

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.