The Effect Of Deformation Processing On The Microstructure And Superplastic Properties Of 7475 Al Alloy

7xxx group Al alloy is the high strength Al alloy. It has wide applications in aircraft and aerospace industry because of the high strength and comprehensive properties. 7475 is developed on the base of 7075 by reducing the Fe, Si content. 7475 Al alloy has both the high strength and the superplastic properties. It is used to make complex parts of aircrafts and rockets, which need high strength。A thorouth research has been made about the superplastic of 7475. But all of the deforming temperature of them is above the initial melting point. When deforming, there is a liquid-like substance existence along the grain boundaries, which can regulate the grain boundaries sliding effectively. The research about the superplastic below the melting point of 7475 is few. The superplastic deformation is carried on at the temperature range of 380℃-470℃. The superplastic deformation mechanism of 7475 is studied at the aspects of grain boundaries sliding (GBS), crystal orientation, dislocation slip, dynamic recrystallization and facture mechanism. The superplastic deformation mechanism above and blow the melting point of 7475 is compared in the paper. The conductions is follow:(1) Product 7475 Al alloy with the thickness of 2mm by the thermomechaical treatments of annealing, overaging, rolling at about 250℃and recrystallization annealing. Obtain the elongation of 295%-405% during the temperature of 380℃-470℃, and accomplish the low temperature superplastic.(2) Obtain the elongation of 405% and 295% at the initial strain rate of 1×10-2/s and 1×10-1/s, and accomplish the high strain rate superplastic.(3) The microstructure and crystal orientation before and after superplastic deformation of 7475 Al alloy is studied. The traces of GBS, dynamic recrystallization and dislocation slip are observed. So it is believed that the superplastic deformation mechanism of 7475 is grain boundaries sliding with the accommodation of dynamic recrystallization and dislocation slip.(4) Fracture surfaces are compared between the temperatures above and below the melting point of 7475. No liquid-like substance is observed. There is not liquid phase accommodation when the deformation is carried on below the melting point of 7475. It is the dynamic recrystallization and dislocation slip accommodate the GBS that we can obtain the high elongation at low temperature and high strain rate.(5) Cavity characteristics and fracture surface of the specimens strained to fracture are analyzed. When GBS cannot be accommodated by other mechanisms, stress concentration at certain sites may cause the development of cavitations. The cavities nucleated at the triple junction of grains, particles and even ledges on grain boundary is the real reason of specimen fracture.