Abstract:
In the first part of the present work, isothermal hot compression tests were conducted on aluminium alloy AA2219 to study the hot workability and microstructure evolution over wide range of temperatures (300-500oC) and strain rates (0.001-100s-1 ). True stress-true strain curves exhibited gradual flow softening at all temperatures except at 300oC where strain hardening was followed by severe flow softening. Optimum processing parameters such as temperature of 450oC and strain rate of 0.001s-1 were proposed, based on contour plots of efficiency of power dissipation and strain rate sensitivity parameter. The activation energy (QHW) for hot deformation of the alloy was found to be around 169 kJ mol-1 . Finally, a constitutive equation for hot deformation of AA2219 was established as: ̇ 109 .exp(0.06149𝜎).exp(-168.958/RT).
Second part of the present work consisted of single pass, hot isothermal plane strain compression (PSC) testing to understand the microstructural evolution during large strain deformation of aluminium alloy AA2219. Hot isothermal PSC tests were conducted in the temperature range of 250oC -400oC and at different strain rates of 0.01s-1 and 1s-1 with 75% thickness reduction. Severely compressed and elongated “ribbon” type grains were found to evolve as a function of strain. Serration of initial boundaries was found in the specimens when deformed at high temperature (400oC) and low strain rate (10-2 s -1 ). Electron Back Scatter Diffraction (EBSD) analysis of the PSC tested specimens at the location of maximum strain confirmed the presence of highly elongated boundaries with very low fraction of new transverse high angle boundaries (HABs).Based on optical and EBSD analysis, the mechanisms of new grain formation was attributed to Geometric Dynamic Recrystallization (GRX). Based on detailed microstructure and micro texture analysis, it was concluded that it is very difficult to obtain large fraction of HABs through uniaxial large strain deformation.
In the third part, aluminium alloy AA2219-T87 bars were cryorolled to various amounts of deformation in two pre-deformation conditions viz. (a) without solution treatment i.e. as received T87 (WST-CR) and (b) with solution treatment (ST+CR). The solution treated and cryorolled bars were further annealed leading to a third condition (c) solution treated, cryorolled and annealed (CR+Annealed). Room temperature mechanical properties have been evaluated for all the three cryorolled conditions. Significant improvement in the 0.2% YS and UTS values was obtained for bars cryorolled to cross-sectional area reduction 4 of more than 50% in the solution treated condition (ST+CR) whereas for bars cryorolled in without solution treated condition (WST-CR) only an improvement in the 0.2% YS was observed. Cryorolling did not enhance the precipitation kinetics or increased the response of the alloy to ageing. The mechanical properties were well correlated to the microstructures obtained using optical and transmission electron microscopy. Microstructural evolution in ST+CR condition indicated gradual progression of the principal restoration mechanism from dynamic recovery (DRV) to dynamic recrystallization (DRX) with increasing amount of plastic deformation. In the present study, cryorolling in the solution treated condition to cross-sectional area reduction of more than 50% (ST+70%CR) was found to impart an optimum combination of strength and percentage elongation