Impact behaviour of 6061 aluminum alloy prepared by friction stir processing under extreme temperature conditions

The 6061 aluminum alloy is widely utilised because of its excellent weldability and extrudability; however, enhancing its mechanical and impact properties – especially across a broad temperature range – remains a significant challenge. This study examined Al6061 subjected to friction stir processing (FSP), assessing its microstructural and mechanical properties at temperatures ranging from −196 to 125 °C. FSP led to a 50% reduction in grain size and an 81% increase in maximum hardness in areas adjacent to the processed zone. When compared to the as-received alloy, the absorbed impact energy increased by 123%, 100%, 78%, and 94% at −196, −70, 25, and 125 °C, respectively. Fractographic analysis indicated a shift from predominantly brittle fracture at lower temperatures to more ductile fracture at elevated temperatures. Despite the improvements in hardness and impact energy, FSP resulted in an average reduction of 16.7% in fracture toughness, with a maximum decrease of 21%. This reduction in toughness was especially pronounced at −196 °C, emphasising the impact of microstructural refinement on cryogenic fracture behaviour. In summary, while FSP enhances impact resistance, it may compromise flaw tolerance, suggesting a trade-off that should be carefully considered when designing FSP-treated Al6061 components for applications in extreme temperature conditions.