Microstructure Evolution And Formation Machenism Of Semi-Solid 7075 Aluminum Alloys Fabricated By Plastic Predeformation

Semi-solid metal(SSM) processing is a near net shape manufacturing technology that combines the advantages of both forging and casting,this technology is especially suitable for the alloys which have lager solid-liquid rang and high melting point or strength, such as high performance 7075 wrought aluminum alloy. The first step of semisolid methods is to obtain the solid-liquid billet composed of fine equiaxed grain structure. there are many methods to prepare semisolid billet by plastic deformation, however, the microstructure evolution of these semisolid billets has not well generalized, therefore, 7075 aluminum alloy is used in this work, semi-solid 7075 aluminum alloy billets are prepared by extrusion deformation below the recrystallization temperature(RAP) and above the recrystallization temperature(SIMA), and apply quantitative research to microstructure evolution of alloy duing semisolid process, in addition, the effects of different plastic predeformation on microstructure evolution of semisolid alloy are researched.Fibrous tissue along the extrusion direction is obtained by RAP route, the results of semi-solid process are as follow: 1) the elongated structure first recrystallise during remelting, then the recrystallised grains appear growth and spheroidization; 2) the semi-solid billet has smaller grain size and higher shape factor when the compressive temperature decreases and compressive ratio increases; 3) the semi-solid grain sizes increase when isothermal holding time and temperature increase during the remelting process, the rate of grain spheroidization increases with the isothermal holding temperature increased; 4) when the compressive ratio and temperature are 25 and 250℃, by reheating to 620℃ for 10 min holding, the semisolid billet consists of fine spherical grains with size of about 35μm and roundness of 0.74.Partially recrystallization microstructure is obtained by SIMA route, he results of semi-solid process are as follow: 1) the effect of re-melting processes of SIMA route on grain size is as same as that of RAP route, however, the rate of grain spheroidization is slower than that of RAP route under the same condition of re-melting processes; 2) When the billet is remelted, grains obviously occur coarsening and spheroidization; 3) The most suitable re-melting processes are 620℃ and 10 min, the semisolid billet consists of fine spherical grains with size of about 53μm and roundness of 0.72.Compared with the 7075 aluminum alloy semisolid billets obtained by RAP route、SIMA route and FSP(friction stir processing) route, the results are as follow: 1) a high degree of recrystallization microstructure prepared by FSP route can obtain fine and round solid grains in shorter time during semisolid process; 2) The semisolid structure obtained by RAP route is finer and rounder in comparison with SIMA route, owing to obtaining more stored energy after predeformation of RAP route, thus, most of microstructure firstly occur recrystallization; 3) Coarsening of grains and recrystallization of alloyt are closely related, the more recrystallization grains occur during remelting, the more grains grow by role Ostwald ripening mechanism; 4)the quality of semisolid billet and recrystallization of solid billet are closely related, the degree of recrystallization is higher, the solid grains of semisolid billet are finer and ronder.