Effect of Thermal Exposure on the Performance of Airframe Aluminium Alloy AA7050-T7451

Increased demands for improved aircraft airworthiness and safety coupled with improved affordability and reduced through life support cost have led to substantial improvements in materials, processes and manufacturing technologies for modern aircraft. A potentially undesirable consequence is an operating environment for the airframe not experienced by comparable legacy aircraft such as increased operating temperatures. 

This paper provides the results of a DST Group experimental program intended to assess the effect of thermal exposure on the performance of the high-strength aluminium alloy AA7050-T7451.  Exposure of AA7050-T7451 to temperatures in excess of 120ºC leads to degradation mechanical properties over time.  The program was designed to contribute to a better understanding of the response of this alloy to in-service thermal environments, as well as assess the risks - if any - posed by thermal exposure by evaluating the limits of thermal stability of this aluminium alloy.

The room temperature tensile test results for coupons exposed to temperatures between 100 ºC and 177 ºC for between one and 100 hours were evaluated and compared with the results for baseline coupons that experienced no thermal exposure. The results were used to verify and improve the current parametric time-temperature approach routinely used by aircraft manufacturers. The existing approach transforms the lifetime thermal exposures to a single design temperature in order to provide an estimate of elevated temperature knockdown based on available (published) data. DST Group has identified shortfalls associated with the application of this approach, and, by incorporation of the test data, has improved the current models ability to predict thermal knockdowns for AA 7050-T7451. Metallurgical and microstructural tools including DSC (Differential Scanning Calorimetry) and TEM (Transmission Electron Microscopy) were used to investigate the microstructural changes that were linked to degradation of mechanical performance of this alloy.