【作者】 翟秋亚；

【导师】 袁森； 井晓天；

【作者基本信息】 西安理工大学 ， 材料加工工程， 2003， 硕士

【摘要】 汽车业和3c产业的轻量化趋势为镁合金应用提供了广阔的空间。作为零件近净成形的半固态加工技术，SIMA法特别适合于易挥发、易燃烧的镁合金零件生产。本文使用500T挤压机，对铸态AZ91镁合金进行热挤压变形，然后二次加热熔化激活形成半固态合金，水淬激冷制备试样。应用SEM和XRD分析了挤压变形组织特征，采用Quantmet-500定量分析仪测定半固态组织固相率和颗粒尺寸，研究了AZ91镁合金SIMA半固态组织演变规律。 试验结果表明，热挤压变形消除了镁合金中粗大的树枝晶，获得细小的再结晶等轴晶组织，组织处于较高的应力状态，为后续SIMA法半固态等温处理奠定了能量和物质条件。随着变形量的增大，挤压条纹细密，等轴晶细小；随挤压温度的提高，挤压组织有明显的粗化趋势。推荐的挤压工艺参数：T≈380℃，λ≈10。 AZ91镁合金的SIMA法半固态组织不仅取决于加热温度和等温时间，而且在很大程度上受合金原始应变状态的影响。随加热温度升高，f_S单调减小，晶粒平均直径有一定程度的长大；随等温时间延长，较小的颗粒或熔化消失、或聚合成较大的颗粒，较大的颗粒发生一定程度的长大，在7min～9min内固相率达到稳定。对挤压态、铸态和镦粗态三种不同加工状态下的半固态组织演变进行对比研究，发现挤压态镁合金在565℃等温5min，即可获得大约56％固相率，颗粒细小、形态圆整、分布均匀的半固态浆料。铸态镁合金二次加热过程中缺乏必要的应变诱发熔化激活，等温12min组织才开始粒化，其半固态组织形状不规则、分布不均匀。镦粗态西安理工大学硕士学位论文镁合金半固态组织球状化程度大大高于铸态合金，但因其应变诱发熔化激活存在不均匀性，组织不及挤压态的均匀。 推荐的挤压态AZgl镁合金半固态等温参数为:T尧565℃，t七5一7min。 挤压态AZgl镁合金SIMA法半固态组织中固相率f、与温度T的回归关系式为: 关=一2568.86917+10.58326T一0.01052T’更多还原

【Abstract】 The lightening tendency of mobile industry and 3C industries provides a widely application space for Mg alloys. As a near-forming technique, SIMA semi-solid forming is particularly suitable for those manufacturing parts, which are easy to vaporize and oxidize. In this paper, as-cast AZ91 alloy is processed by heat extrusion using a 5007 extruder at first, and then the alloy is re-melted into a semi-solid state, and rapidly quenched in water to obtain the final samples. The microstructure characteristics of extrusion deformation have been analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Then using a quantimet-500 apparatus, the solid phase fraction and particle diameter d in semi-solid-state microstructures are measured in details. Finally, the microstructure evolution of semi-solid AZ91 Magnesium alloy based on SIMA process has been studied.The results show that the coarse dendrites are eliminated and the fine equiaxed microstructure resulting from recrystallization has been obtained via heat extruded deformation. Therefore, the extruded microstructure is of a high stress state and hence provides a mass and energy condition for subsequent holding temperature treatment based on SIMA semi-solid-state technique. With the increase of deformation degree, the extrusion bends become compact and the recrystallized equiaxed grains become fine. And with the increase of extrusion temperature, the extrusion microstructure has a remarkable tendency of coarsening. Experimentally, the optimal extrusion parameters are determined as: T=380℃, λ=10The microstructures of AZ91 Magnesium alloy at semi-solid-state depend not only on the heating temperature and holding time, but also on the initial stress states at a great extent. With the increase of the heating temperature, the solid phase fraction decreases steadily and the average diameter of particles increases in some extent. With the increase of the holding time,some small particles tend to dissolve and disappear and the others coalesce into slightly larger particles, whose size reaches a steady state within 7 to 9 minutes.The semi-solid-state microstructure evolution under extrusion, cast and forge processing conditions is compared. For as-extrusion alloy, it is found that fine spherical particles are distributed uniformly in the continuous liquid phase of the slurry with a solid fraction of fs= 56%, held at the temperature of 565℃ for 5 minutes. Whereas the microstructures of as-cast alloy do not particularize until the holding time attains 12 minutes at 565℃ and irregular and large particles are distributed inhomogeneously in the slurry, for lacking of the necessary S1MA. The as-forge semi-solid-state microstructure is finer and more uniform than the as-cast one, but is not so homogeneous as the as-extrusion microstructure.The optimal holding temperature T and holding time t at semi-solid-state for as-extrusion AZ91 magnesium alloy are 565℃ and 5~7min, respectively.The relationship between the holding temperature and the solid phase fraction under SIMA semi-solid-state condition for extruded AZ91 Magnesium alloy is determined as:fs =-2568.86917+ 10.58326T-0.01052T2更多还原