The development of aluminium-zinc-magnesium alloys for superior stress corrosion resistance

Abstract

A thorough literature survey has been undertaken to provide the necessary understanding of: i) the general metallurgy and microstructure of Al-Zn-Mg alloys, ii) the stress corrosion cracking (SCC) of Al-Zn-Mg alloys (including accelerated SCC test methods), and iii) the influence of composition, microstructure and heat treatment on SCC properties. Three accelerated SCC test methods were evaluated using existing commercial alloys in different temper conditions. These were the notched rod load relaxation, the electrochemical acceleration and the slow strain rate SCC tests. The slow strain rate method gave the most reliable and reproducible results. This was therefore chosen for all subsequent testing. Baseline SCC test data was obtained from existing alloys in order to facilitate comparison of new alloy compositions * The microstructure of a representative Al-Zn-Mg alloy was examined using optical, scanning and transmission electron microscopy. The effects of quench rate from solution heat treatment, and ageing time and temperature on both the microstructure and SCC properties were investigated. Decreasing quench rate produced a moderate increase in resistance to SCC. The characteristic increase in resistance to SCC found by overageing was, however, associated with an unacceptable loss of mechanical properties, ABSTRACT A thorough literature survey has been undertaken to provide the necessary understanding of: i) the general metallurgy and microstructure of Al-Zn-Mg alloys, ii) the stress corrosion cracking (SCC) of Al-Zn-Mg alloys (including accelerated SCC test methods), and iii) the influence of composition, raicrostructure and heat treatment on SCC properties. Three accelerated SCC test methods were evaluated using existing commercial alloys in different temper conditions. These were the notched rod load relaxation, the electrochemical acceleration and the slow strain rate SCC tests. The slow strain rate method gave the most reliable and reproducible results. This was therefore chosen for all subsequent testing. Baseline SCC test data was obtained from existing alloys in order to facilitate comparison of new alloy compositions.. The raicrostructure of a representative Al-Zn-Mg alloy was examined using optical, scanning and transmission electron microscopy. The effects of quench rate from solution heat treatment, and ageing time and temperature on both the raicrostructure and SCC properties were investigated. Decreasing quench rate produced a moderate increase in resistance to SCC. The characteristic increase in resistance to SCC found by overageing was, however, associated with an unacceptable loss of mechanical properties. Melting, casting and hot working techniques were developed in order to fabricate defect-free small scale experimental alloy compositions. Seven experimental casts were made to cover a wide compositional variation {2n:4~6 wt*t, and Mg:0.8-2.5 wt.%). Slow strain rate SCC testing revealed the beneficial effects of having a zn:Mg ratio of 3:1 (wt.%).