B. Kumar, D. Cope, C. Widener, B. Tweedy, National Institute of Aviation Research, Wichita State University, Wichita, KS

Summary:

Friction stir welding is a solid state joining process that produces a far less heterogeneous microstructure than conventional fusion welding. Previous research shows that the welds have good mechanical properties but there are concerns about their susceptibility to corrosion. Post weld treatment to improve the corrosion resistance has been advocated by several researchers. The post weld treatments of thick sheets affect not only the resistance to corrosion but also the mechanical and fatigue related properties, including changes in the microstructure of the weld. There is little information in open literature about the aforementioned effects of post weld treatment in the thin 2024 and 7075 Al sheets.

This research effort will determine the effects of the different post weld treatments on the static, fatigue crack growth, resistance to exfoliation corrosion and the changes in the weld microstructure of the 2024 and 7075 Al sheets of 0.040” and 0.125” thickness.

Initial testing of the alloys with no post weld treatment has been completed. Some of the selected results are included in this abstract. The static test results are shown in table 1. The results indicate a Poisson’s ratio of about 0.35. The 7075 specimens have an ultimate strength close to 70 Ksi while the 2024 specimens had an ultimate strength in the lower 60 Ksi range. Ultimate elongations ranged from 6% for the 0.040” 2024 to 14% for the 0.125” 7075 alloy. The results of the fatigue crack growth of 7075 alloy are shown in figure 1. For the most part, the 7075 specimens cracked down the center of the weld. However, the cracks in the 2024 material grew quickly towards the base material. This is similar to the finding of other researchers, and it can be inferred that the fatigue properties of the weldment are not weaker than the base material. The welds of both the 2024 and the 7075 performed poorly in the exfoliation test. The testing solution consisted of salt, potassium nitrate, and nitric acid with an initial pH of ~ 0.4. All of the welds sustained enough attack that a minimum rating of EC was chosen for the entire exposures. It was apparent there was a notable attack on the heat affected zone of each weld. The characterization of the weld microstructure is currently underway.

The various post weld heat treatments that are being considered will be evaluated on the basis of the changes they generate in the corrosion resistance, mechanical and fatigue properties. This will help in determining the suitability of the different post weld treatments for the thin sheet 2024 & 7075 Al alloys.