Showing papers by "Ravi Kumar published in 2019"

Microstructure and Mechanical Characterization of Friction-Stir-Welded 316L Austenitic Stainless Steels

Abstract: Fusion welding of austenitic stainless steels (ASSs) causes undesirable metallographic changes. An attempt has been made to join 3-mm-thick 316L ASS plates using friction stir welding and correlate the microstructural evolution and the mechanical properties under various welding conditions. Tungsten alloy (W-Re) tool was employed for welding. Successful joints were obtained within a range of traverse speeds (45-85 mm/min) used in this work. Various zones were observed across the joint line. The stir zone exhibited considerable grain refinement and traces of delta ferrite phase. EBSD images showed a decreasing grain size with increased traverse speed. XRD patterns and TEM images confirmed the existence of delta ferrite. The mechanical properties of the joints including microhardness, tensile strength, impact toughness and bending were further reported.

Friction Welding of Electron Beam Melted γ-TiAl Alloy Ti–48Al–2Cr–2Nb

Abstract: In the current work, rotary friction welding of electron beam melted (EBM) γ-TiAl alloy Ti–48Al–2Cr–2Nb (at%) was investigated. Welding experiments were conducted using cylindrical bars of 12 mm diameter and 70 mm height in as-fabricated and heat treated (1275 °C/2 h/furnace cooling) conditions. No cracking problems were encountered during friction welding of as-fabricated EBM samples. However, in as-welded condition, the joints did not perform well in tensile tests and failures were observed to invariably occur at the weld interface. Friction welded joints produced in heat treated EBM samples were also found to suffer from the same problem. In both the cases, the weld region showed fine, fully lamellar γ + α2 microstructure and relatively high hardness. In order to overcome the problem, a post-weld heat treatment (PWHT) was carried out at 1275 °C/2 h/furnace cooling. After the PWHT, the weld region showed a more desirable duplex microstructure consisting of equiaxed primary γ and relatively coarser lamellar γ + α2. The PWHTed joints showed satisfactory tensile properties, and no failures were observed at the weld interface.

Thermodynamic modelling of Al-B-N system

Abstract: In this work we report the thermodynamic modelling of the Al-N, B-N and Al-B-N systems by the Calphad approach, making use of the ab initio calculations and the available experimental thermochemical and phase equilibria data. Gibbs energy description for the Al-B system is taken from the literature, with a modification for the model for the liquid phase. The enthalpy of formation and the heat capacity of various polymorphs of AlN and BN are estimated using the DFT and the phonon calculations, respectively. The enthalpy of mixing of the mixed nitride (Al,B)N in wurtzite and cubic structures are obtained using the SQS method. Gibbs energy of formation of the mixed nitride from the binary nitrides is also computed using the phonon calculations. These quantities along with the critically assessed thermochemical and constitutional data are used for obtaining the Gibbs energy functions of various phases in the Al-N, B-N, and Al-B-N systems. Calculated phase equilibria and thermochemical data are compared with the experimental data.

Thermal Shock Resistance of Precursor Derived Si-Hf-C-N(O) Foams

Abstract: Thermal shock resistance of precursor derived Si-Hf-C-N(O) foams at temperatures varying from 800o-1000oC subjected to multiple thermal cycles was investigated. The as-synthesized foams possessed i...