The Effects of Welding Conditions and Post-Weld Heat-Treatment on the Corrosion Performance of Friction Stir Welded High Strength Aluminium Alloys
By H Zhang and M Gittos
High strength aluminium alloys, such as 7050 and 7475, have been used extensively for aerospace applications. Joining 7xxx alloy components, using friction stir welding (FSW), is attractive compared with other joining technologies. However, there are important microstructural changes due to the heat and deformation cycles associated with FSW. These microstructural changes have implications for the performance of the FSW joints:
- Welds in 7xxx alloys show different corrosion behaviour to the parent metal due to microstructural differences, eg the thermomechanically-affected zone (TMAZ), the heat-affected zone (HAZ), or the stir zone;
- Laboratory tests have shown that the HAZ and/or stirred regions display some sensitivity to stress corrosion cracking (SCC);
- SCC behaviour varies considerably for different alloys and with the use of different welding conditions.
Only a limited number of corrosion studies have been performed, despite the fact that potential applications require an understanding of environmental degradation.
This report describes research to produce corrosion properties data for FSW joints in aerospace aluminium alloys that can be used by manufacturers to facilitate the introduction of this new technology and to produce practical recommendations for improving the corrosion resistance of high strength aluminium FSW joints by post-weld heat-treatment (PWHT) at moderate temperatures, ie within the aging range.
- The range of welding parameters employed had some effect on microstructures, tensile properties and weld formation in friction stir welds in 7050 and 7475 aluminium alloys, but no effect on corrosion behaviour of the welds was detected.
- FSW of the aluminium alloys creates some sensitivity to SCC and intergranular corrosion (IGC), using both low and high heat inputs. The changes in welding parameters did not affect the corrosion sensitivity of friction stir welds in high strength aluminium alloys. Without PWHT, the narrow boundary zone at the edge of the TMAZ was the region most susceptible to IGC.
- Eight PWHT procedures were applied to FSW aluminium welds, with one being particularly effective in improving SCC resistance for both alloys.
- Alternative PWHTs were found to be effective in improving SCC resistance, at either 50 or 90% of the parent metal proof stress, for 7475 welds.
Section of 7050 aluminium alloy tested SCC coupon etched in Kellers’s reagent
Transverse section of aluminium alloy 7475 SCC coupon etched in Keller’s reagent: cracking in the TMAZ