Study of Innovative Processing to Reduce Corrosion Cracking and Improve Heat Treatment Efficiency of Aluminum Alloys

This study will investigate the effects of cold working and thermal processing of aluminum alloys AA 6061 and AA 7075 on the corrosion cracking susceptibility. These two alloys are heat treatable aluminum alloys, which attain their strength via aging heat treatment. AA 6061 may suffer from the intergranular cracking (IGC), which adversely affect the quality and service life of products made of AA 6061. Previous research indicates that factors affecting the IGC include copper concentration, quench rate, and aging heat treatment. These factors determine the microstructure and electrochemical condition in the grain boundaries, and ultimately determine the occurrence and severity of the IGC. The major purposes of this study are to improve the IGC and SCC of AA 6061 and AA7075, respectively, and to increase the efficiency of aging heat treatment simultaneously. In order to reduce both the corrosion cracking susceptibility of AA 6061 and AA 7075 and the aging time, this study will use cold working following the solution heat treatment to introduce numerous dislocations within the matrix and grain boundary regions. The existence of dislocations will assist the nucleation of precipitates and prevent the formation of precipitate free zone near the grain boundary regions which may form due to inadequate quench rates. Consequently, more uniform grain boundary morphology may form and SCC susceptibility can be reduced. In addition, large quantity of dislocations may also assist the nucleation and local diffusion of solute atoms. Therefore, a shorter aging time may be realized. The feasibility studies of this project clearly proved that the use of cold working following the solution heat treatment could significantly reduce the IGC susceptibility of AA 6061. The T8 heat treatment can shorten the aging time in AA 7075 from 32 hours to 10 hours. The SCC susceptibility of AA 7075 is positively reduced by the degree of cold work. This study will use transmission electronic microscopy, optical microscopy, and corrosion test to quantitatively study the effect of cold work and aging heat treatment on the corrosion cracking susceptibility and aging response of alloys AA 6061 and AA 7075. The result of this study is anticipated to be used in numerous industrial applications.

Project ID:PB10107-0945
External Project ID:NSC101-2622-E182-003-CC3