Damage Tolerance Characterization of AA2195 and its FSW Joints

The application of aluminium lithium AA2195 and its FSW joints for future European space applications has been investigated within an ESA research and technology program “Damage Tolerance of Cryogenic Pressure Vessels”. Based on 10 years of experience obtained in MT Aerospace R&D programs both on Al-Li alloys and FSW joining of conventional aluminium alloys, an activity with emphasis on the following aspects was performed.

1)        Definition of the necessary fracture control approaches for cryotank structural concepts and characterization of a candidate FSW material from fracture control point of view.

2)        AA2195 was aged to a “near peak ageing condition” to investigate a maximum potential on strength properties. Fatigue, crack growth and fracture testing was performed to evaluate the relation between maximum strength and fracture related properties.

3)        Fracture analysis was focused on elastic plastic fracture (EPFM) behavior to identify the maximum potential for structure assessment.

4)        The results of the investigation have been compared with open literature data available from the Space Shuttle super lightweight tank (SLWT) program.

5)        Further investigations have focused on:

1. dissimilar FSW material (AA2195 / AA2219) which currently must be considered for tank joints of sheet to forging rings
2. FSW repair welds

The main results of the activity are:

• The defined fracture control methodology and associated analytical techniques were successfully applied to AA2195 material and its as-welded FSW joints.
• The AA2195 material and its as-welded FSW joints show relatively high damage tolerance even in the peak aged condition.
• The fracture properties agree well with SLWT test results despite the reported difference in strength. This may indicate that fracture properties are less sensitive to heat treatment parameters.
• The surface crack fracture test results, where cracks tend to grow in ST direction, cannot be explained from standard LT or TL fracture toughness tests.
• The applied FSW parameters resulted in a highly efficient weld strength ratio.
• The AA2195 material shows a decrease of fracture toughness with decreasing temperature. Residual strength may however increase with decreasing temperature due to increased yield and ultimate strength properties.