| Large castings and forgings are basic components of electric power, metallurgy, transportation, petrochemical, machinery, and defense industry and so on. Large casting and forging industry is a necessary and key industry for the development of the national economy. Aluminum alloy castings and forgings with big size, which have great advantages in the aspects of mechanical properties, lightweight, recyclable etc., have wide application prospect and great value of the research. Al-Zn-Mg-Cu alloy which belongs to the super-high strength wrought aluminum alloy and can be strengthened by heat treatment is widely used in aviation, aerospace, automotive, transportation, construction and other fields. The "Twelfth Five Year" plan contains the research of large castings and forgings of Al-Zn-Mg-Cu alloy. However, due to the high degree of alloying and big temperature interval between the solidus temperature and liquidus temperature, the ingot is easy to crack and segregation, which greatly reduces the yield and mechanical properties.7075aluminum alloy which is widely used and has excellent comprehensive performance in Al-Zn-Mg-Cu alloy is selected as the research object in this paper. Face with the problem of segregation and cracking in the preparation process of7075alloy, the aim of the research is to put forward a solution, namely developing an advanced coupled stirring technology, which combines electromagnetic stirring and spiral mandrel mechanical stirring, and7075alloy ingots with the diameter size of300mm were prepared use the new technology. The effect of the couple stirring technology on semi-continuous casting process through numerical simulation was studied in this paper. The process of the conventional semi continuous casting and coupling stirring casting process, the as-cast microstructure characteristics and composition segregations, the microstructure evolution during homogenizing treatment, hot deformation behavior, microstructure and properties after solid solution and aging treatment of the ingot prepared by the two type casting methods were compared. The comprehensive and system research have conducted, and the results of the study are show below:(1) The radial melt temperature difference was greatly during the preparation process of7075aluminum alloy ingot with big size, the fluctuation range of the melt temperature in the hot top was30～40℃, the temperature distribution was not uniform, the ingot was easy to crack, the inverse segregation was serious, the grain sizes at the edge,1/2radius, and center position of the ingots was132.2μm,638.3μm and770.5μm, the grains were coarse dendritic grains and the grain size was not uniform. After the use of the chemical method, the average grain size of the ingots was160μm, the grain roundness was improved. But there were still a large number of coarse dendritic grains, and the radial grain size deviation was still up to33%. Under the optimized casting process, the cracking rate of the ingots decreased, the yield ratio increased. However the change of the average grain size of the ingots was not apparent, the radial grain size deviation is30%, and the serious uneven and cracking phenomenon still existed. It shows that7075aluminum alloy was easy to crack, the phenomenon of microstructure inhomogeneous and composition segregation was easy to occur, chemical method and process method was difficult to improve these problems significantly.(2) A new stirring technology coupling electromagnetic stirring technology and mechanical stirring technology was presented and the wherein the mechanical stirring structure was a spiral mandrel. The electromagnetic field was simulated by Maxwell software, and the results showed the electromagnetic inductions and electromagnetic forces from the surface to the center of the ingots decreased gradually. The semi-continuous casting process was simulated by Fluent software, and results showed that the coupling stirring technology reduced the radial temperature difference, improved the uniformity of the temperature distribution, changed the sump shape obviously, made the sump trend become more gentle, the sump depth was reduced by24.5%. In addition, the coupled stirring technology changed the melt flow conditions significantly and forced the melt in the sump strong convection in radial, axial and vertical, and also a strong flow at the crystallization front was produced. The shear force under the coupling stirring was the total of shear force of the electromagnetic stirring and mechanical stirring separately and a strong shear effect was achieved, which realizes forced uniform solidification of7075alloy.(3) The temperature fluctuation range during the conventional semi-continuous casting process was20～50℃, however the temperature fluctuation during the coupling stirring casting process was just1℃, which indicated the coupling stirring technology improved the temperature fluctuation significantly. The coupling stirring technology made the sump trend become shallower, the sump depth changed from128mm to88mm, which reduced by31%. Coupling stirring technology decreased the cracking rate in preparation process of ingots, and broadened the preparation process window of7075alloy ingots with the diameter size of300mm. (4) After the treatment of coupling stirring, the average grain size of the ingots decreased by13%, the grain size deviation in radial of the ingots decreased from30%to9%, the grain shape changed also from dendrites to spherical grains and rosette grains, Zn, Mg and Cu average segregation rate respectively was5.67%,4.52%and5.9%, which indicated coupling stirring reduced the inverse segregation significantly, improved the distribution uniformity of the elements. Put forward A new homogenization technique with470℃×12h+480℃×8h was presented, and was used to treat the7075alloy ingots. The homogenization treatment results showed that the coupling stirring technology is prone to accelerate the solution of the second phase, and improve the homogenization effect and solubility.(5) Before and after coupling stirring, the average flow stresses of the ingots were33MPa and29.6MPa and the radial flow stress deviation were14%and5%which showed the coupling stirring technology made the flow stress smaller and more uniform. The constitutive equations were established use the temperature corrected and friction corrected stresses and the calculation of deformation activation energy showed that the activation energy of the alloy treated by the coupling stirring technology decreased from233KJ/mol to151KJ/mol. According to the hot processing map of the alloy prepared by coupling stirring casting, the safety processing regions were expanded, the instability regions were narrowed, the process window suitable for hot deformation increased, the thermal processing properties of the alloy was improved, the stability of material heat processing performance was improved. Coupling stirring improves the strength of the alloy treated by solution and aging treatment. Using the coupling stirring technology, the tensile strength of7075alloy was increased by29MPa, and the yield strength was increased by41MPa, but the plasticity hadn’t changed. The coupling stirring improved the uniformity of the recrystallized grain size in longitudinal section of the extruded rods, increased the precipitated phase amount, made the grain boundary precipitated phase was small and discontinuous, however the coupling stirring technology had low effect on the intergranular precipitate free zone. |
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