Microstructure and flow behaviour during backward extrusion of semi-solid 7075 aluminium alloy

The microstructure and flow behaviour during thixo backward extrusion of 7075 aluminium alloy were investigated. Reheating the steel die and the aluminium billet placed into the die at the same time using an induction furnace provides rapidly a very homogeneous microstructure suitable for thixoforming. During thixoextrusion, despite the high solid fraction, the solid globules are weakly connected and slide over each other without any plastic deformation. The flow remains quasi homogeneous resulting in homogeneous induced microstructure of the component.

► Two different reheating processes are studied to get the semi-solid microstructure.
► Larger grains by isothermal heating at 580 °C over 30 min in the resistance furnace.
► Inductive reheating of a 7075 billet in a die offers rapidly a more homogeneous microstructure.
► The grains size does not exceed 70 μm after inductive reheating of the billet in a die.
► fs = 0.77 is a good compromise to get lower load and a good surface quality of the thixoextruded.