Abstract:
In the present work, AA2219-T87 plates (40 mm thickness) in two different pre-deformation conditions: without solution treatment (WST) and solution treatment (ST) are cryo rolled (at 77K) into sheets (imparting reduction ratio of ~97.2%) and rods (reduction ratios of 30, 50 and 70%). The mechanical performance and ageing kinetics of the cryo rolled sheets are compared with cold rolled sheets, to assess any beneficial effect. The as-rolled sheets and rods were subjected to duplex ageing at 175 degree celsius/3min+125degree celsius/8h, which reduces the effective ageing time by several hours compared to industrially practiced ageing time for peak aged tempers. The microstructures of the ST sheets and rods in as-rolled and peak aged conditions consist of very fine structure with high dislocation density in the form of tangles. Peak ageing treatment generated uniform distribution of semi-coherent precipitates with slight decrease in dislocation density due to partial recovery of the structure. Tensile properties are evaluated for the sheets and rods in as-rolled and peak aged conditions at ambient (300K) and cryogenic (77 and 20K) temperatures. Significant improvement in the strength and ductility values were observed in as-rolled and peak aged conditions for ST sheets and rods, whereas tensile properties in WST condition were significantly lower. The combined effect of suppression of dynamic recovery (DRV), grain size refinement, dislocation strengthening and precipitation hardening from the precipitates are responsible for achieving optimum strength and ductility balance at temperatures up to 20K. Based on the outcomes of the present work, severe deformation through cold/cryo/caliber rolling of the ST sheets and rods followed by peak ageing is very effective in improving the cryogenic strength and ductility over conventional processed routes for AA2219 alloy.