Improving Formability of Al-Li Alloys

Via this NASA innovation, a product is first heated to a temperature within the range of 204 to 343 degrees C for an extended soak of up to 16 hours. The product is then slowly heated to a second temperature within the range of 371 to 482 degrees C for a second soak of up to 12 hours. Finally, the product is slowly cooled to a final soak temperature of 204 to 343 degrees C before cooling to room temperature. The product so treated will exhibit greatly improved formability. To date, the low formability issue has limited the use of lightweight Al-Li alloys for large rocket fuel tank dome applications. Manufacturing a dome by stretch forming typically requires multiple panels as well as multiple welding and inspection steps to assemble these panels into a full-scale fuel tank dome. Complex tensile and bending stresses induced during the stretch forming operations of Al-Li alloys have resulted in high rates of failure for this process. To spin form a large rocket dome, the spin blank must be prepared by joining smaller plates together using friction stir welding. However, friction stir welding produces a distinct metallurgical structure inside and around the friction stir weld that makes it very susceptible to cracking during spin forming. NASA scientists have designed a novel heat treatment process that significantly improves the formability of high-performance aluminum-lithium (Al-Li) 2195 alloy plate stock. The heat treatment process dramatically reduces cracking and also improves the yield and range of product sizes/shapes that can be spin/stretch formed. The improved yields also provide lower costs. The NASA innovation enables the use of Al-Li 2195 alloy for large structures like rocket domes or fuel tanks of much lighter weights than if made using a conventional aluminum-copper (Al-Cu) alloy. While specifically designed to address Al-Li 2195 alloy, similar heat treatments may possibly improve the formability of other high-performance Al-Li alloys as well.