An Exploratory Study On The Principle Of The Fabrication Of Ultrafine Grained Magnesium Sheets Using Medium-high Strain Rate Rolling Technique And The Related Fundamental Research

Magnesium alloy sheets have many outstanding advantages, such as low density, high specific strength and stiffness, good thermal conductivity, superior damping and electromagnetic shielding properties, and good recycling potential, which make them of great application importance and potentials in the fields such as transport vehicles, aerospace,3C products. Especially, the unltafine grained magnesium sheets present excellent strength and high ductility and toughness except for the lightness; in addition, they show low temperature and high strain rate superplasticity. Therefore, the unltafine grained magnesium sheets have wider application. Some domestic and overseas researches have done some exploratory studies on the fabrication of ultrafine grained magnesium sheets and the associated fundamental theories. However, they have been unable to achieve a breakthrough.Based on the results of the preliminary exploratory experiements of this study, the present dissertation explores a novel methods called medium-high strain rate rolling to produce ultrafine grained magnesium sheets. In order to improve the formability of magnesium alloys, the effects of twinning and dynamic recrystallization (DRX) on the rolling process are strengthened by increasing the rolling strain rate. The ultrafine grained microstucuture is obtained via the extensive DRX in twins.Two series of magnesium alloys were chose in this study, which are AZ31and ZK60. AZ31is the most common magnesium alloy for rolling and ZK60is a high-alloy content sery. The feasibility of the MHSRR process and the deformation principle of magnesium alloys were explored. The thermal rolling simulation experiments were conducted upon the two alloys, and the flow behaviors and micro structural evolution of magnesium alloys during rolling process were studied. The effects of rolling strain rate and rolling temperature on the microstructure, grain orientation and mechanical properties of the magnesium sheets were discussed and the grain refinement mechanisms were focused. The strain-induced precipitation of magnesium alloys during MHSRR was observed and the influence of the precipitation on the rolling deformation behaviors was concentrated.The main conclusions are listed below (1) The feasibility of MHSRR of magnesium alloys depends on the deformation mechanism. The deformation mechanism during MHSRR are of great difference from that of conventional rolling with low strain rate and small strain. During conventional rolling, dislocation sliding dominates the deformation. However, during MHSRR, twinning dominates the deformation and extensive DRX occurs. Twinning and DRX are the most important deformation behaviors successively taking place during the deformation. Since twinning, DRX and the initiation and propagation of cracks are all the ways consuming strain energy during rolling, twinning and DRX compete with cracking. When the deformation strain rate is high enough, the initiation of twinning and DRX is rapid enough to prohibit cracking and guarantee the rolling process, which is the basic reason for the feasibility of MHSRR on magnesium alloys.(2) During the thermal simulation experiments, both at low strain rates (0.01-0.1s"1) and medium-high strain rates (10-50s-1), ZK60and AZ31alloys showed similar overall flow behaviour, i.e., hardening-softening-hardening. But at medium-high strain rates, the change of the flow curves was sharper and the flow curves fluctuated at the early stages. At low strain rate, the strain hardening at early stages was dominated by dislocation multiplication, and the subsequent softening was controlled by the nucleation and extension of DRX at grain boundaries, and the hardening at the later stages was induced by the grain refinement caused by DRX. At medium-high strain rate, the early-stage hardening was dominated by high-density deformation twinning, the subsequent sharp stress drop was due to the high-density DRX nucleation in twins, and the later-stage hardening was also resulted from the grain refinement.(3) The ultrafine grained magnesium sheets with high strength and high ductility were successfully produced by on-pass MHSRR. Rolled at300℃, with a strain-per-pass80%and a strain rate of9.1s-1, the average grain size of the ZK60sheet was0.5μm. The yield strength (YS) was307MPa and the ultimate tensile strength (UTS) reached371MPa and the elongation to failure was28%; the average grain size of AZ31sheet was3μm with the YS of238MPa, UTS317of MPa and elongation to failure of29%. However, the average grain size of the conventionally rolled AZ31sheet was18μm. The YS and UTS were222MPa and262MPa, respectively, and the elongation to failure was17%. The MHSRR is an efficient method to produce ultrafine grained magnesium sheets with a high efficiency and shortened process.(4) The rolling temperature range of magnesium were broadened by MHSRR. Rolled at250-400℃with a strain rate of9.1s-1, the fine grained sheets with superior mechanical properties were obtained in both the two alloys, which breaks the limitation of the narrow process window for conventional rolling. With the increase in strain rate, the grains were futher refined. In the ZK60sheets produced at16.8s-1, grains were refined to nano size.(5) MHSRR produced homogeneous ultrafine grained microstructure via extensive DRX. During MHSRR, a series of microstructural evolution occurred in the materials. First, very high-density deformation twinning took place and high-intensity dislocations formed in these twin lamellae. Subsequently, the dislocations rearranged and dynamic recovery (DRV) occured. The DRV successively formed polygons, cells and subgrains in the twin lamellae. The subgrains turned into the nuclei of continuous DRX. These nuclei transformed to the homogeneous fine DRX grains with low dislocation density and high misorientation by rotation and growth. With further deformation, the nucleation of discontinuous DRX took place at the grain boundaries of the continuous DRX grains, which prohibits the grain growth.(6) Compared with conventionally rolled sheets, the sheets fabricated by MHSRR still presented{0002}basal texture, but the intensity reduced remarkably. Especially with the increase in strain rate, the texture intensity of the sheets split to two peaks. The weakening of the basal texture results from the increase in both of the types and intensity of twins and the increase in the density of DRX nucleation.(7) During MHSRR, the strain-induced precipitation for the nano-sized second phase occurred in both of the ZK60and AZ31alloys. However, the distribution of the precipitates in ZK60alloy was much denser than AZ31alloy, and the composition of the precipitates was more complex. The precipitation processs and various deformation behaviors have mutual effects with each other, and thus influenced the final microstructure of the sheets. The main effects of precipitation were on the dislocation formation, movement and rearrangement, the DRX nucleation and grain growth. The difference of DRX grain size and DRX ratio between ZK60and AZ31sheets was attributed to the difference of the distribution density of the precipitates.