Formation mechanisms and control strategies of FQZ softening in Al–Li alloy welded joint

Abstract

This study briefly reviews the progress on microstructure and performance control of Al–Li alloy welded joints. The formation mechanisms and softening effect of the fine equiaxed zone (FQZ) in fusion welded joint are summarized, and the improvement of different welding processes on the properties of Al–Li alloy is analyzed. Finally, the development directions of new welding methods and control strategies of Al–Li alloy are also prospected. The main viewpoints are as follows: During the welding process, a non-flowing residual thin layer exists near the boundary of molten pool. The Al3(Lix, Zr1-x) phases, which act as the heterogeneous nucleations, promote the formation of FQZ microstructure. A large number of eutectic phases form at the grain boundaries of FQZ, which lead to the initiation of microcracks in this region. At the same time, grain boundary segregation results in the decrease of alloying elements and precipitated phases in the interior of the grains, which further weakens the precipitation strengthening effect and makes the FQZ become the weakest area of the welded joint. External energy field (ultrasonic field and electromagnetic field) assisted arc oscillation or scanning mirror laser can effectively improve the flow behavior of the molten pool, refine the weld microstructure, promote the uniform distribution of alloying elements and precipitated phase, and further improve the mechanical properties and fatigue performance of the joint. The energy field assisted welding methods are also expected to become the ideal welding process for Al-Li alloys.

Keywords

• Al–Li alloy
• Welding
• FQZ
• Microstructure control
• Performance improvement