Effects of Temperature Distribution on Microstructure and Mechanical Properties of Hot Extruded Al–Zn–Mg–Cu Alloy Pipe with Variable Cross-Section

The microstructures and mechanical properties of a hot extruded Al–Zn–Mg–Cu alloy pipe with variable cross-section are investigated by transmission electron microscopy, electron backscattering diffraction, hardness testing and finite element method. The results suggest that the variable temperature distribution at different positions along the extrusion direction after hot extrusion attribute to the changes in hot deformation degrees. The extruded profile at billet temperature of 410 °C and a ram speed of 1.5 mm/s shows a gradual increase in temperature from 416 to 467 °C at strains from 3.7 to 8.6 along the extrusion direction. The dynamic recovery (DRV) and partial dynamic recrystallization (DRX) are observed in the hot extruded profile, and high-angle grain boundaries fraction exponentially rises along the extrusion direction. The hardness at different positions of the hot extruded profile shows a nearly exponential decline along the extrusion direction, mainly ascribes to the decrease of the dislocation density that caused by the dynamic softening.