Showing papers by "Sushanta Kumar Panigrahi published in 2021"

Microstructure dependent electroplastic effect in AA 6063 alloy and its nanocomposites

Abstract: The flow stress reduction during plastic deformation superposed with electric current, commonly referred as ‘electroplasticity’ has been actively researched over the past few decades. While the existence of an electron–dislocation interaction, independent of Joule heating is established, the exact rate controlling mechanism of the observed behaviour lacks consensus. Understanding the governing mechanism is complex due to the combined effect of Joule heating and electron–dislocation interaction. The present work attempts to establish the electroplastic mechanism in AA 6063 alloy and its nanocomposites. The role of microstructure on the electron interaction is investigated by preparing four distinct microstructure from the base alloy. All the samples were subjected to constant amplitude direct current during plastic deformation. The Joule heating effect is decoupled using the experimentally measured temperature history. The potential electroplastic mechanism for the alloy is elucidated by analysing the trend of flow stress reduction with strain and strain rate. It is inferred that micro Joule heating and electron wind effect cannot completely explain the observed electroplastic behaviour in AA 6063. The SiC particles in nano-composites suppressed the electroplastic effect. The observed mechanical behaviour under electric current is in agreement with the trend predicted assuming magnetic depinning mechanism. The reduction of dislocation density quantified using X-ray diffraction is found to concur with the inferred mechanism.

Twin evolution and work-hardening phenomenon of a bulk ultrafine grained copper with high thermal stability and strength-ductility synergy

Abstract: In this paper, Cu is severely cryorolled by employing low deformation strain per pass, large roller diameter, and moderate deformation speed. This process leads to the evolution of a unique homogenous ultrafine grained (UFG) microstructure enriched with deformation nanotwins and dislocation-rich structures. Excellent thermal stability combined with high strength-ductility synergy is observed during isochronous annealing of the UFG material. Accelerated recovery consumes the thermal energy imparted during low temperature (150–200 °C) annealing and averts premature grain and twin coarsening. Microstructural homogenization prevents disproportionate recrystallization and grain growth. Dense clusters of deformation nanotwins reduce the grain boundary mobility and arrest the grain growth. At 200 °C, twin complexes comprising a crisscross network of fine deformation and annealing twins are formed, further enhancing the thermal stability at higher annealing temperatures (200–250 °C). However, large-scale growth of these annealing twins at 300 °C devours the nanotwin complexes and reduces their stabilizing efficiency, thereby compromising the thermal stability. Ultimately at 350 °C, a complete transformation of the strained UFG microstructure into a fully recrystallized bimodal microstructure is observed. Large concentration of thermally stable, recovered nanometric grains and twins reduces the dislocation mean free path and promotes rapid work-hardening in UFG Cu. They help in retaining good strength-ductility synergy of the material up to a very high annealing temperature.

Modified Method of Characteristics for Analysing Cold Flow in Bell-Type Rocket Nozzle

Abstract: To achieve higher thrust force, numerical investigation of flow behaviour of bell-type rocket nozzle has been carried out in the present research work. Analysis of thermodynamic properties is performed by using ANSYS Fluent software. K-ω shear stress transport model has been used to study the turbulent components of thermodynamic properties. Overall performance gain of a bell-type nozzle and nozzle geometric modifications are discussed. The nozzle geometric modifications are highlighted by executing a C-code with concepts of the method of characteristics (MOC) and modified method of characteristics (MMOC). With modified geometry as per the modified method of characteristics (MMOC), the present numerical study has ensured the fully expanded cold flow in nozzle. The results obtained by the method of characteristics (MOC) are compared with the calculations of the thrust force for the modified geometry.