Study On Compression Behavior And Thixo-rolling Of Semi-solid 5083 Aluminum Alloy

Since the Semi-solid Metal Forming (SSF) technology was proposed in the 1970s, it has obtained rapid development. And it has also been known as one of the most potential near-net-shape forming technology in the 21st century. The traditional liquid forming and solid plastic deformation are changed by this technology. Because it has unique deformation characteristics and advantages when the semi-solid metal billets are formed between the semi-solid temperature range. Compared with conventional casting process, the forming temperature of SSF is lower and the die life can be extended. Moreover, the production conditions and environment can also be improved by SSF with a better formability. Compared with the conventional solid plastic forming, SSM forming has less deformation resistance, better flow filling properties, energy and cost savings, one-forming for complicate parts.In this work,5083 aluminum alloy with a wide freezing temperature range was prepared by Strain Induced Melt Activated (SIMA) method. The compression deformation behaviors and the microstructure characteristics of cast and semi-solid 5083 aluminum were studied by the Gleebe-3500 thermo-simulation machine. Moreover, the reheating between semi-solid temperature and the thixo-rolling process of this alloy were also studied.The results show that the ideal semi-solid 5083 alloy can be prepared by SIMA with a large extrusion ratio for predeformation. And the optimum parameters are as follows:the extrusion ratio is about 17.36 with the isothermal temperature of 605～610℃and the holding time of 15～20min. The isothermal temperature has almost no effect on the distribution of Mn, but it has great influence on the distribution of Al and Mg in preparing the semi-solid 5083 alloy. And the segregation of a low melting point phase along grain boundary is mainly composed by Mg element. During the formation of a non-dendritic structure in 5083 aluminum alloy, the recovery and recrystallization of grains occur at the heating stage. On the other hand, the low melting point phase segregating along the grain boundary can seep into the small-angle grain boundaries. So that grains may be separated and broken. During the temperature-keeping stage, separated and broken grains gradually spheroidize under the atomic diffusion and surface tension. The mechanism of grain growth is mainly a combination of Ostwald ripening and grain merger. The results of semi-solid compression test show that when other deformation conditions are the same, the compressive stress decrease with increasing of deformation temperature and decreasing of strain rate. And the strain rate has a great influence on the shape of the stress-strain curve. In the semi-solid temperature range, the overall stress level of cast alloy is obviously higher than that of the semi-solid alloy during hot compression deformation. While in the solid temperature range, flow stress curves of the two are similar and there is no remarkable advantage for the semi-solid alloy. There are also three typical deformation region for the two alloys during compression deformation in the semi-solid temperature range. The liquid phase in the semi-solid alloy is uniformly distributed along the grain boundary, but its distribution is very uneven in the cast alloy. The density and uniformity of the semi-solid 5083 alloy are better than those of the cast alloy after hot compression deformation.The results of reheating test show that holding time has little effect on the grain shape and liquid fraction, while the heating temperature has a great influence on them. And holding time has a effect on grain size. Too long holding time can make the semi-solid grains coarse, which may go against semi-solid forming. The ideal holding time for the reheating process is 30～40min.The results of one-pass thixo-rolling experiment show that primary spherical solid-phase particles separate from the liquid phase during thixo-rolling process. And spherical solid-phase particles concentrate in the middle of plates. While the liquid phase flows to the edge of the plate under the rolling pressure. And 5083 aluminum plates with tensile strength of 264.45 MPa and elongation of 27.51% can be obtained through one-pass thixo-rolling. About 600℃is a good temperature for the thixo-rolling. The tensile strength and elongation of the plates improve with the rolling deformation. But the edge of the plates may crack when the rolling deformation reaches 80%. And it can be resolved by side dam in practical production.The results of cold rolling indicate that tensile strength of plates has improved significantly after cold rolling. The tensile strength after cold rolling is about 70MPa higher than that before cold rolling by average. But the elongation has dropped to only 9～10%.