Process And Theoretic Study On Semi-solid Extrusion For Aluminium Alloys

The semi-solid billets of ZL116 and 6063 aluminium alloys were prepared by near-liquidus semi-continuous casting process. Based on the reheating test and thermal simulating of the sampling billets, the aluminium billets of ZL116 and 6063 with the homogeneous, fine non-dentritic microstructures, which is suitable to thixoforming in semi-solid state, were extruded in the semi-solid state of high percentage of solid phase in order to obtain the bars with quality, good feasibility, high properties and efficiency, cost saved.The effect of casting velocity, the mechanism and effect of processing parameters on the microstructure of ZL116 and 6063 aluminium alloys billets cast by near-liquidus semi-continuous casting process was studied. Because of small superheat in the melt during the near-liquidus semi-continuous casting process, the temperature distribution in the melt is trend to be uniform, and the quantities of nucleating is obviously increased. The ripening trend of dendritic grain would be increased, and the rosette-shaped grains could be obtained while the growing grains contacted each other because of the decrease of the composition undercooling. The acting time and cooling effect of first and secondary cooling on the melt would be changed with the casting velocity. And the microstructure morphology and distribution in the cross-section of the billets would be affected. The billet with the homogeneous, fine non-dentritic microstructures could be obtained under the condition of the suitable casting velocity and cooling intensity. Under the certain cooling intensity during near-liquidus semi-continuous casting, the semi-solid billets with the smooth surface and homogeneous, fine rosette-shaped or spherical grains could be obtained at 150～200mm/min for ZL116 alloy and 100～170mm/min for 6063 alloy.The reheating test and analyzed calculation of the semi-solid billets were studied. And the evolution mechanism of reheated microstructures with the change of reheating temperature and holding time was studied. The results showed that the spheroidizing process of the grains is fastened, and the holding time is shortened due to the increase of reheating temperature. At the same reheating temperature, the spheroidized grains would be coarse with the increase of holding time. During the eutectic alloy is reheated at the eutectic temperature, eutectic Si would be fused and become the homogeneous, fine granules distributed in the remelted liquid. The Si granules grow and are decreased with the increase of holding time, then disappeared finally. The evolution of remelted microstructure is stable, and the small-sized and spherical microstructure which is suitable for semi-solid thixoforming could be obtained while the sample of ZL116 alloy was reheated at 575℃for 10～15min and the sample of 6063 alloy was reheated at 620℃for 10～15min.The deformation-compressed mechanisms of the ZL116,6063 alloy samples in solid and semi-solid states were studied by the thermal simulation technology. The results showed that: In the semi-solid state of high percentage of solid phase, the deforming stress would be increased with the strain rate at the same temperature, and the increasing extent of the stress is decreased with the increase of the temperature. Compared with the compression in solid state, the effect of strain rate on the stress in the semi-solid state is less. The stable compressing deformation of the ZL116,6063 alloy billets, which were cast by near-liquidus semi-continuous casting could be obtained in semi-solid state at the low strain rateAccording to the analyzed results of reheating test and thermal simulation, the billets of ZL116 and 6063 alloys cast by near-liquidus semi-continuous casting, were extruded in solid and semi-solid state. The microstructures and properties of the extruded bars were analysed. The processing parameters of heat treatment for the semi-solid extruded bars were optimized with the orthogonal design method and artificial neural network method. The results showed that: during the extrusion in semi-solid state, the deformation of billets is mainly composed of sliding of solid particles and its plastic deformation. The energy produced for deforming is little because of the small deforming resistance. The distribution of the stress and the flow velocity in the each part of the billet is uniform during the deformation. And the effect of the bar thick is less. The uniform deformed microstructure could be obtained. During the semi-solid forming, most of solutes are dissolved into the matrix, and distributed uniformly in the matrix as the precipitated phase while the fast cooling. The growth of the recrystallized grains is damped and the matrix is strengthened. The stress concentration in the deformed solid phase for the dislocations packing is decreased because of the liquid remission between the grains. Just some of deformed grains were recovered and recrystallized, and the fine-grain strengthening and dislocations strengthening are promoted. The effect of composition segregation in as-cast microstructure on extruded forming could be avoided due to forming between the solid and liquid zone. So the as-cast billets needn't to be homogenized.Extruded at 575℃in semi-solid state with high percentage of solid phase, the bar surface of ZL116 alloy is finished and the microstructure in the cross-section is uniform. A large number of Si particles and precipitated phases are distributed in the matrix, and the matrix is strengthened. Some of deformed solid particles became more finer because of recrystallizing. Compared with the solid extruded bars, the bar extruded in semi-solid state is more feasible and has the more noticeable properties. The optimized parameters of heat treatment for the semi-solid extruded bar of ZL116 alloy are: solid solution at 545℃for 4h and aging at 178℃for 10h. The tensile strength of the bar is 325Mpa, and the elongation is 14.6%. The plasticity is increased 192% than one of as cast part. And the time of solution is shortened. The energy consumption is decreased. The properties of the semi-solid extruded bars of 6063 alloy are improved due to the solid solution and natural aging on line. The tensile strength of the bar is above 240Mpa, and the elongation is above 19%. The properties index has been approached those of T6 heat treatment for the parts. The parameters of heat treatment for the semi-solid extruded bar of ZL116 alloy optimized by the orthogonal design method are: solid solution at 520℃for 1h and aging at 175℃for 6h. The tensile strength of the bar is 275Mpa, and the elongation is 14.23%.