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The present work is an effort to enhance the mechanical properties of Aluminium-Lithium alloy AA2195 via a series of thermo-mechanical treatments. This was deemed necessary to help the alloy to compete with a myriad of upcoming high strength Aluminium alloys such as 2xxx and 7xxx alloys for effective use in the aerospace industry. In the first part of the present work, ingots of the alloy were obtained after Vacuum Induction Melting. Cast ingots were homogenized and subjected to hot rolling to reduce the thickness from 44mm to 8mm via large strain deformation. Thereafter, cold rolling was carried out at room temperature to reduce to final thickness. Different types of cold rolling processes such as uni-directional rolling (UDR-1.1 and UDR-2.3), reverse rolling (RR-1.1) and two step cross rolling (TSCR-1.1) were carried out by rotating about the normal direction to plane of rolling, some of the sheets at 0o, 180o, 90o respectively after a pre-determined number of passes. This was done to gauge the effect of strain path change on the mechanical properties of the material. The nomenclature of the sheets is hereby termed as <type of rolling> - <final thickness of the sheet>. Defect free sheets of the following dimensions (1000x300x1.1) mm were successfully obtained. In the second part of the present work, age hardening treatments were designed based on the results from Differential Scanning Calorimetry of fine sized chips obtained from the sheets. This was done to capture the effect of key strengthening phases before they started melting and to avoid the formation of precipitates which were known to have deleterious effects on the mechanical properties of the material. Henceforth, multiple age hardening treatments were carried out such as direct ageing at 139oC and 146oC, solution treatment at 505oC followed by rapid water quenching and ageing at 146oC and 177oC. After measuring Vicker's hardness, blanks were aged for the time duration after which the maximum hardness had been achieved. Tensile tests were conducted to determine the UTS, YS and percent elongation. The best heat treatment procedure was determined and it would be solution treatment at 505oC followed by rapid water quenching and ageing at 177oC. By correlating with Transmission Electron Microscopy results, a uniform dispersion of Al2CuLi (T1) precipitates was found to have major strengthening effect on the alloy. Fractographs were taken from the failed tensile specimens via Scanning Electron Microscopy. It was established that varying degrees of predominantly, ductile fracture had occurred via coalescence of micro-cavities. From EBSD analysis, a high percentage of high angle grain boundaries were observed which proved that the ageing treatments had led to strengthening via dynamic recrystallization. In the third part of the present work, a series of tensile tests were carried out for UDR-2.3 and RR-1.1 across combinations of a high temperature viz. 475oC, 425oC and true strain rates (10-3, 10-2 and 10-1 s-1). Subsequently, strain rate sensitivity 'm' was calculated by plotting ln(true stress) vs ln(true strain) at a particular value of nominal strain and finding the slope across different regions in the strain space. The highest superplastic elongation that was achieved was 262% for UDR-2.3 at temperature 425oC and strain rate 10-3 s-1. The strain rate sensitivity was high (~0.508) between two different low strain rate values viz. 10-3 and 10-4 s-1. |
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