Effect of processing on texture and microstructure in Aluminium-Lithium alloys and its consequences on mechanical properties

Abstract

Lightweight Aluminium -Lithium based alloys have high potential for use in aerospace structural components. However, large anisotropy in mechanical properties restricts the use of these alloys. The alloy AA2195 (Al-Li-Cu-Mg-Ag-Zr) is the third generation Al-Li based alloy which is viewed as the most promising choice for structural aerospace applications among the other Al-Li alloys. However, even in this alloy, the combined effect of crystallographic texture and heterogeneous distribution of precipitates leads to mechanical property anisotropy, which is highly undesirable for the formability of this alloy. This underscores the need for developing suitable processing strategies for overcoming the problem of anisotropy. The present thesis aims at exploring the possibilities to improve the mechanical properties and reduce anisotropy in the material by tailoring texture and microstructure in the alloy AA2195 also by designing newer processing schedules and by suitable alloying addition. For this, in the present investigation, texture and microstructure of an already hot rolled plate of AA2195 was modified by employing various types of cross rolling in the processing schedule vis-à-vis an identical processing schedule involving normal unidirectional rolling and the consequent evolution of mechanical properties was examined. The change in strain path led to the formation of weak texture, and a reduction in the degree of anisotropy from 24 % in the as-received hot rolled material to as low as 5 % in the materials processed through routes involving cross rolling. Further, the differently textured sheets were subjected to incremental forming. It was observed that texture weakening by change in the strain path during rolling led to a significant improvement in the formability of the alloy during incremental sheet forming. In the subsequent chapter, further attempts for weakening the texture though a route involving multi-axial forging (MAF) of the cast material were made and consequent evolution of anisotropy in mechanical properties was evaluated. It has been observed that incorporation of multi-axial forging in the processing schedule has led to further weakening of texture. In the next chapter, the effect of severe plastic deformation by Equal Channel Angular Pressing (ECAP) has been examined on the evolution of texture and microstructure and the effect of this process on improvement in mechanical properties has been reported with the detailed explanation of the mechanism. In the next chapter, which aims at suitably modifying the chemistry of the alloy AA2195 by alloying, the effect of scandium (Sc) addition to AA2195 has been studied with special emphasis on the evolution of texture and microstructure during calibre rolling. Addition of Sc has led to enhancement in the degree of grain refinement compared with AA2195 alloy without Sc, when processed using calibre rolling. Finally, it has been concluded that suitable modifications in the processing schedule and the composition modulates the microstructure and texture thereby, improving the mechanical properties