Residual Constitutive Behavior of Aluminum Alloys after Fire Exposure

Aluminum alloys are increasingly being used in lightweight transportation applications such as naval vessels and light passenger rail. The aluminum alloys considered include 5xxx-series (Al-Mg) and 6xxxseries (Al-Mg-Si) alloys due to their mechanical strength, corrosion resistance, and weldability. A major concern in the use of aluminum alloys in lightweight structural applications is fire exposure. Aluminum mechanical properties are significantly reduced at 300°C. After fire exposure, structural damage will vary due to the local thermal history which is governed by the fire size and proximity and other environmental factors. It is paramount to understand the nature of this structural damage in terms of residual (post-fire) constitutive behavior so as to allow for informed damage assessment. AA5083-H116 and AA6061-T651 residual constitutive behavior was characterized using quasi-static tension tests. The alloys were exposed to elevated temperatures at controlled heating rates using an induction heater to simulate the varying conditions in a fire environment. The thermal history dependence of residual constitutive behavior was elucidated in terms of the microstructural strengthening mechanisms. The primary strengthening mechanisms of AA5083 and AA6061 evolve at elevated temperatures due to recrystallization and precipitate coarsening, respectively. Strengthening mechanism evolution has been shown to be kinetically dependent which explains the thermal history dependence of residual constitutive behavior. Structural damage from a wide range of fire scenarios may be understood using these underlying strengthening mechanisms which govern residual constitutive behavior.