MECHANICAL BEHAVIOUR OF ULTRAFINE GRAINED Al-Mg-Si ALLOY PROCESSED THROUGH SPD

Abstract

The thesis presents the effect of various thermo mechanical treatments on the microstructure evolution and its influence on the precipitation hardening behavior of Al-Mg-Si alloy. The starting material with grain size of several microns has been processed through various thermo mechanical routes to realize ultrafine grained (UFG) structure in it. To ensure the UFG structure in the processed alloy, Electron back scattered diffraction (EBSD), Transmission electron microscopy (TEM), X-ray diffraction (XRD) techniques are used. The mechanical properties of UFG material and its bulk coarse grained counterpart are evaluated through hardness testing and tensile testing at room temperature. The precipitation evolution in UFG material is monitored through differential scanning calorimetry (DSC), hardness testing and TEM. Al-6061 alloy after solution treatment (ST) was processed through cryorolling (CR) and room temperature rolling (RTR) up to ~90% thickness reduction. Effect of low temperature ageing (at 125 °C) on microstructure and mechanical properties was investigated. The results evidenced that, in as-rolled conditions, RTR material has shown higher hardness than the CR material, which can be attributed to the formation of nanoclusters due to dynamic ageing effect during RTR. Low temperature ageing has resulted simultaneous increment in the strength and ductility in both CR and RTR alloys. However the hardening behavior of RTR material is found to be superior to the CR material. The natural ageing behavior of CR and RTR alloy is found to be similar as observed through hardness testing. Transmission electron microscopy analysis revealed the formation of ultrafine grains (UFG) filled with dislocations and nanosized precipitates in the CR and RTR conditions after ageing treatment. The size of the nano sized precipitates found in optimized condition of RTR material is finer than that in CR material. To investigate the effect of warm rolling temperature and % deformation after cryorolling on the mechanical properties and microstructural evolution of Al 6061 alloy, the alloy was subjected to cryorolling followed by warm rolling at 100°C, 145 °C and 175 °C with various % reductions (67%, 75%, 80%). The thermal behavior of processed alloy was investigated through DSC. The microstructural features were characterized by adopting Electron back scattered diffraction (EBSD) and Transmission electron microscopy (TEM) techniques. It was observed that the combination of cryorolling and warm rolling is more effective than cryorolling alone. With increasing WR temperature and % of deformation, the alloy has shown significant II improvement in tensile strength (415 MPa) and partial improvement in ductility (6%) as measured from tensile testing. After subsequent low temperature ageing of CR +WR samples at 125 °C has resulted simultaneous improvement in strength and ductility. Dynamic ageing effect during warm rolling has resulted by the formation of nanosized precipitates (clusters) which is evident from the DSC thermograms. Remarkable improvement in hardness and strength of CR +WR sample is attributed to the combined effect of precipitation hardening, dislocation strengthening and partial solid solution strengthening. To investigate the effect of pre-ageing temperature on the hardening behavior of Al-Mg- Si alloys processed by cryorolling, the alloy was subjected to natural ageing for 2days and preageing at 100 °C, 130°C and 170 °C for 4 hours, 2 hours and 30 minutes respectively. The present investigation revealed that, the natural ageing and pre-ageing before cryorolling is useful to introduce solute clusters in the material to enhance the dislocation density during cryorolling. However artificial ageing of cryorolled samples is not influenced much with pre-ageing. It is also observed that, maturing of CR samples at room temperature for 30 days has resulted better hardening response in subsequent artificial ageing. The optimum heat treatment condition for better mechanical properties may be preferred as “Natural ageing for 2 days + cryorolling 90% + Natural ageing for 30 days followed by artificial ageing at 125 °C for 48 hours. To understand the evolution of microstructure at larger strains and its influence on the precipitation behavior and mechanical properties, Al-Mg-Si alloy was deformed through cryoforging followed by cryorolling. The bulk Al-Mg-Si alloy, with initial grain size 400 μm, was subjected to solid solution treatment (ST) followed by water quenching at room temperature. The ST treated alloy was subjected to ageing at 100 ° C for 4 hours and 8 hours prior to cryoforging. The cryoforged alloy was subjected to cryorolling up to 2.4 true strain for producing long sheets. Finally, the deformed alloy was subjected to low temperature ageing at 120 °C to improve the tensile properties of the alloys. Microstructure and mechanical properties were evaluated through Vickers hardness testing, tensile testing and electron back scattered diffraction (EBSD). The results indicate that combined cryoforging + cryorolling followed by ageing led to remarkable improvement in strength (UTS- 452 MPa) and ductility (8%). The average grain size the alloy was found to be 240 nm, with increased fraction of high angle grain boundaries. Low temperature differential scanning calorimetry (DSC) was used to study thermal behavior of bulk and severely deformed alloy. The thermal behavior of this particular alloy is different as III compared the behavior of the Al-Mg-Si alloys reported in literature. Employing pre-ageing (100 °C for 4 h and 8h) has resulted significant improvement in strength and hardness in the forged material. The combination of cryoforging and cryorolling can be used as a simple and cost effective technique to produce high strength Al alloy sheets with UFG structure. Scheduling suitable heat treatments in precipitation hardenable alloys aids to achieve proper combination of properties.