Abnormal grain growth of 2196 Al-Cu-Li alloy weld seams during extrusion and heat treatment

Abstract

The porthole die extrusion processes of 2196 Al-Cu-Li alloy were carried out at billet temperatures of 420–510 °C and ram speeds of 0.25–5.0 mm/s, then the extruded profiles were subjected to the solution and aging treatment. Abnormal grain growth (AGG) was observed along longitudinal weld of profiles during the extrusion and solution treatment. The fine equiaxed grains with Copper orientation were formed in the welding area when extruded at low temperature and low extrusion speed, and some Copper orientated grains transformed into recrystallization textures with the increase of deformation temperature. When extruded at high temperature and speed, AGG occurred in the junction area of Copper texture and recrystallization textures, and some strain-free Cube orientated grains in this area transformed into AGG structures due to the high growth-driving force and growth priority. During the solution treatment, the migration and annihilation of dislocations were repeated inside the equiaxed grains of the welding area, resulting in the coalescence of the subgrains and the formation of AGG. The abnormal grains formed during extrusion were mainly Cube orientated, while those occurred during solution treatment were randomly orientated. The weld seam had higher thermal stability during solution treatment if profiles were extruded at higher extrusion temperature and speed. In the matrix area, most of the dislocations migrated along the same direction, and the dislocations were hard to meet and annihilate. Besides, little driving force for grain growth could be provided by the flat grain boundaries of the matrix area. Hence, the Brass orientated grains in the matrix area possessed high thermal stability during the extrusion and solution treatment. The AGG structure led to the intergranular fracture and poor elongation of samples in the tensile test.