| Forming technology of magnesium alloy is the key to develop its high-end products, for which the semi-continuous casting technology can provide high-quality magnesium alloy billets. In the present study, semi-continuous casting method was used to prepare the alloy ingot. The element distribution in the different part of the ingot was examined and the relationship between the microstructures and mechanical properties was studied. The extrusion processes of AZ31profiles with hollow thin wall and ZK60profiles with I shape were discussed. The effects of extrusion parameters on macro-qualities, microstructures and mechanical properties of the profiles were investigated. All the results were listed as follows:1) φ110mm AZ31alloy ingots undertaken DC and cp300mm AZ31alloy and ZK60alloy ingots fabricated by low frequency electromagnetic continuous casting were successfully obtained, and the large ZK60profiles with I shape were prepared as well as the AZ31profiles with hollow thin wall, which had bright surface and exact dimensions;2) The secondary cooling water and casting speed had a great effect on the organization and properties of the semi-continuous casting ingot. The average grain size and second phase particle size reduced as the secondary cooling water and casting speed elevated. The microstructure of the ingot undertaken optimization parameters was made up of equiaxed grains and discontinuous second phases. And the white block phase in the organization was β-Mg17Al12, some of which may be the expansion phase (3-Mg17(AlZn)12. There also had a small amount of AlMn phase. Slight improvements of the strength and ductility were observed from the center to the surface the alloy ingot, being up to200MPa,196MPa and11.6%,10.8%, respectively;3) Electromagnetic had dual effects on the molten metal which was mechanical effects and thermal effects. Under low frequency electromagnetic field, the segregation and the grain size of the cp300mm AZ31ingot reduced as well as the frequency increased, while the uniformity and mechanical properties had been elevated. Differences in the mechanical properties between the edge and center of the alloy ingot also showed a decreasing trend. When the electromagnetic frequency was equal to20Hz, the ingot organization was very homogeneous. There was no significant segregation about the solute elements and a number of small grains. Compared with the ingot fabricated by DC, the UTS and ε of the center part of the ingot was increased by12.6%,58.8%, which reached225MPa and15.03%. What’s more, the φ300mm ZK60ingot had no significant segregation, whose organization was composed of equiaxed grains, discontinuous distributed eutectic phase and a few lumps or needle-shaped second phases. In addition, the performance of its mechanical properties was really good. The UTS and ε of the center part were up to250MPa,13.4%;4) The degree of dynamic recrystallization increased with the extrusion velocity and temperature elevated in the extrusion process, so the grain was refined significantly. When the value of extrusion velocity or temperature was higher than the critical value of complete dynamic recrystallization, the fine grains had began to grow. As the extrusion velocity and temperature kept increasing, the UTS, TYS as well as elongation to failure increased first then started to drop. The AZ31hollow thin-walled profiles with good quality were obtained under the extrusion parameters of T=400℃and V=0.4～4.0m·min-1;5) The large ZK60magnesium alloy profiles with I shape occurred incomplete dynamic recrystallization during the extrusion process, whose organization was composed of mixed grain with different sizes. The profiles exhibited apparent anisotropic character according to the tensile results at room temperature. In particular, the samples of45ED direction exited highest elongation to failure (23%), and the ED direction presented highest strength value (σb=325MPa), respectively. |
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