Material characterization of AA7075-T651 deformed at different temperatures and strain rates

• Main Author: Dalai, Biswajit
• Format: Others
• Language: English
• Published: Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik 2021
• Subjects: Metallurgy and Metallic Materials; Metallurgi och metalliska material
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-84325; http://nbn-resolving.de/urn:isbn:978-91-7790-860-9; http://nbn-resolving.de/urn:isbn:978-91-7790-861-6

In this licentiate thesis, the mechanical response and the microstructure evolution in aluminum alloy (AA) 7075 in its T651 heat treated state have been studied through mechanical tests at varied conditions of strain rate and temperature. First, compression tests were performed on AA7075-T651 samples at room temperature (RT), 100, 200, 300, 400 and 500 °C (melting temperature of AA7075 is ~635 °C) with two comparatively low strain rates of 0.01 and 1 s-1 using a Gleeble thermo-mechanical simulator. The effects of softening mechanisms, such as adiabatic heating, dynamic recovery (DRV) and dynamic recrystallization (DRX), on the flow stress level, strain rate sensitivity (SRS) and temperature sensitivity were analyzed over the entire range of deformation temperatures. Additionally, the extent of DRX with respect to temperature, strain rate and local strain was demonstrated and explained through electron back-scattered diffraction (EBSD) analysis. The deformation behavior of AA7075-T651 was then studied at high strain rates in the order of 102-103 s-1 at RT, 200, 300 and 400 °C using a Split-Hopkinson pressure bar (SHPB) setup complemented with an induction coil. Unlike in case of deformation at low strain rates, the unavailability of sufficient time for the dissipation of heat energy during deformation at high strain rates resulted in the formation of adiabatic shear bands (ASBs). Two types of ASB, namely distorted shear band (DSB) and transformed shear band (TSB) were characterized by their distorted-elongated and ultra-fine grain structures, respectively. A deformation path was established following the evolution of ASBs, micro-voids and micro-cracks leading to the fracture of the material. The effects of strain, strain rate and deformation temperature on the formation of ASBs and cracks were also discussed.