CORROSION STUDIES ON ULTRAFINE GRAINED Al ALLOYS FOR AEROSPACE APPLICATIONS

Abstract

The development of high strength Aluminium alloys for aerospace and automobile applications is ever growing due to the importance of realizing extended life period of the structural components fabricated from these alloys. Among various Al alloys, Al 2014 alloy is extensively used for structural applications due to its high strength to weight ratio and reasonable high fracture toughness. The mechanical properties of these Al 2000 series alloys can be enhanced further by refining the grain structure to ultrafine regime. To enhance the strength of Al alloys further, rolling at cryogenic temperature is one of the best options as it produces ultrafine grained structures. It is well known that when the material undergo sever deformation, yield strength and fracture strength will increase. However, this increase in strength is usually accompanied by a loss in ductility. Achieving a combination of high strength and ductility is still a challenge. Also the corrosion behavior of UFG materials has received only limited attention. Therefore, the present work has been envisaged to investigate the mechanical and corrosion properties of ultrafine grained Al alloys (Al 2014) produced by cryorolling. The optimum ageing conditions to improve both strength and ductility of Al 2014 alloys has been identified. In the first part of the present work, the solutionised plates of Al 2014 alloy was subjected to rolling at cryogenic temperature using liquid nitrogen to true strain of 2.2.The effect of cryorolling on microstructure and mechanical properties were investigated by using tensile tests, hardness tests, Optical micrograph study, OIM study(EBSD) ,DSC and TEM. A substantial improvement in strength (536 MPa) has been observed in ultrafine grained Al 6061 alloys as compared to its bulk counter part (378 MPa). The solution treated cryorolled combined with post-cryorolled ageing treatment (100 °C for 30h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with substantial improvement of ductility (13.5%) without any loss in tensile strength. In the second part of this work the electrochemical behavior of UFG 2014 alloy was compared with its coarse grained counterpart by using Linear polarization, Tafel polarization, Potentiodynamic polarization and EIS techniques. It has been found that there is no significant change in corrosion rate of UFG 2014 alloy with its coarse grained counterpart. The potentiodynamic results shows that the UFG Al 2014 alloy exhibits less passive current compared with the coarse grained Al 2014 alloy.