A novel high-strength Al-Li alloy printed by laser powder bed fusion: Microstructural evolution and strengthenin mechanism

This paper obtained an additively manufactured high-strength Al-Li alloy by laser powder bed fusion (LPBF) based on a third generation AA2195 alloy powder raw material. The relationship between the process optimization, microstructure evolution, and mechanical properties of the as-printed (AP) and heat-treated (HT) specimens was established for the first time to explain the intergranular fracture sensibility during LPBF and reveal the dominant precipitation strengthening mechanism induced by the subsequent heat treatment. The precipitation order of AP Al-Li alloy at the last stage of the solidification process was: L (Liquid phase) -*T2 (Al6CuLi3) + theta '(Al2Cu) + delta '/beta' (Al3(Li,Zr)) + T (LiAlSi) + Omega (AlCuMgAg). The micro-cracks caused by a relatively high grain boundary coverage of interconnected film-like eutectic phases, as well as micro-voids caused by the localized slip between the coarse T2-phases and soft precipitate-free zones, resulted in the high intergranular fracture sensibility. The well-designed T6 heat treatment of 515 degrees C/60 min solution treatment and 170 degrees C/6 h aging treatment was conducted to maximize the precipitation strengthening by T1-phases. The precipitation order of HT Al-Li alloy was supersaturated solid solution -* GP zone + delta '/beta' -* theta ' + T1 (Al2CuLi) + omega (Al7Cu2Fe) + beta '. The presence of continuously distributed T1-cells along the grain boundaries was not only capable of providing a pinning effect on dislocations movement and boundary migration, but also able to shorten the pile-up distance on the slip plane. Such phenomena improved the resistance ability of Al-Li alloys to mechanical damage and permitted a significant strength enhancement.