Study of Microstructure and corrosion effects of precipitation hardened high strength AA-7xxx aluminium alloy subjected to cryo rolling

Abstract

The strength and life of an alloy is the primary parameter taken into consideration while building hulls and superstructures for marine applications. Usually AA-7xxx is widely used for these applications and hence is chosen for the present investigation. Cast AA-7xxx alloy reveals distribution of soluble intermetallic compounds in the α-aluminium matrix. In order to improve the mechanical properties and condition the microstructural features, precipitation hardening (solutionizing and ageing) treatment was carried out by varying time and temperature. Ageing treatment was carried out at 120°C varying time interval. Thermo-mechanical treatment, in particular, cold rolling and cryo rolling(-190ºC) is a unique technique to produce super high strength aluminium alloys with elongated grained structure. In order to conduct the rolling at room temperature, the alloy ingot was rolled from 6mm to 1mm with 85% reduction in thickness. Detailed microscopic analysis was done using optical and electron microscopy to understand the phase and structural evolution during ageing. Evolution of coherent precipitates and their morphology were studied using transmission electron microscope. A potentio-dynamic polarization study was performed to evaluate the corrosion behaviour of AA-7xxx series aluminium alloy processed in 3.5% NaCl solution at different conditions. The aged, cold rolled and cryo rolled alloy is found to exhibit better corrosion resistance characteristics in comparison with other alloys.

Keywords: AA 7xxx alloys, Precipitation Hardening, Cold rolling, Cryo rolling (-190ºC), Micro structure correlation, TEM analysis and Corrosion behaviour.