摘要
镁合金作为一种新型轻质金属结构材料,在汽车制造、通讯电子、航空航天等工业领域具有广阔的应用前景。由于镁是密排六方(HCP)结构材料,其塑性变形在室温下仅限于基面{0001}<11(?)0>滑移及锥面{10(?)2}<1011>孪生,因此,镁合金的室温塑性加工能力较差。目前大多数镁合金制品的加工局限于铸造,特别是压铸成型,然而,铸件的力学性能不够理想且容易产生组织缺陷,极大地限制了镁合金的应用范围。变形镁合金在铸造后往往通过热变形方式(如挤压、轧制等)细化晶粒、改善合金的组织结构来提高合金的力学性能。与铸造镁合金相比,变形镁合金的综合力学性能优异;但常规变形镁合金在热变形后一般会产生强烈的{0002}基面织构,而该织构的存在是导致变形镁合金低的室温塑性和高的各向异性的主要原因。良好的室温塑性是变形镁合金广泛应用的前提之一,而如何通过织构控制及晶粒细化法有效地改善和提高镁合金的室温塑性成为变形镁合金工业发展中的重要方向。
针对上述问题,本论文开展了如下研究工作:(1)铸态纯镁热轧变形过程中{0002}基面织构的演变规律;(2)异步轧制AZ31镁合金板材的形变织构及退火织构;(3)非对称热挤压AZ31镁合金板材的显微组织、织构特征及力学性能;(4)晶粒尺寸及织构对AZ31镁合金室
温压缩变形行为的影响。主要结论如下:
铸态纯镁在400℃热轧过程中发生了明显的动态再结晶,伴随晶粒细化和{0001}基面织构的形成。随着轧制道次的增加,晶粒逐渐细化,晶粒大小趋于均匀,孪晶数量减少;织构由初始态的无规则取向逐渐转化为{0002}基面织构,且基面织构的强度随着热轧变形量的增加而增加。经多道次热轧后(ε=78%),纯镁板材内部形成均匀的等轴晶组织和较强的{0002}基面织构。热轧纯镁中动态再结晶的形核机制主要为基于孪生的动态再结晶形核机制。由于动态再结晶过程中具有基面取向的晶粒不易产生滑移,位错密度低,畸变能小,对动态再结晶不敏感,随着热轧变形量的不断增加,具有基面取向的晶粒数量增多,最终在板材内部产生较强的{0002}基面织构。
热挤压态镁合金板材内部的主要织构组分为(10(?)7)[0772](90°,15°,0°)织构和((?)26)[(?)02(?)](60°,10°,30°)织构;经异步冷轧后,板材内部的织构类型保持不变,但织构强度变化明显。慢辊速侧织构强度较快辊速侧变化幅度大,随着异步冷轧形变量的增加,织构强度关于中心层不对称的趋势增加。不同变形量异步轧制AZ31镁合金板材经不同温度退火处理后,镁合金板材的主要织构组分保持不变,强度发生变化,(10(?)7)[0(?)72]、((?)26)[(?)02(?)1]织构在退火过程中弱化明显。镁合金织构弱化与其在退火过程中发生再结晶有直接关系,对于异步冷轧形变量16%的AZ31镁合金,其织构强度及显微组织在300℃及以上保持稳定,表明再结晶充分完成的最低温度为300℃。同步轧制与异步轧制工艺的结果比较表明异步冷轧退火态AZ31镁合金板材的屈服强度和抗拉强度与同步冷轧退火态基本保持一致,但室温延伸率有显著提高。
铸态AZ31镁合金经400℃非对称热挤压,晶粒尺寸由原始态的75μm细化至-4μm。非对称热挤压制备的AZ31镁合金板材厚度方向上存在晶粒尺寸梯度,上表层(有倒角侧)平均晶粒尺寸最小(2.50μm);下表层最大(3.67μm);中间层晶粒尺寸介于两者之间(3.13μm)。铸态AZ31镁合金经400℃非对称热挤压,板材内部形成织构。上表层的织构为{0002}基面织构,但强度较弱,且发生ND向RD方向约15°倾转。中层织构表现为漫散弱化的{0002}基面织构,TD方向的漫散程度大于RD方向。下表层织构为典型的{0002}基面织构。非对称挤压AZ31镁合金板材各层的力学性能差异明显:上表层的屈服强度明显低于中层及下层,但延伸率有较大提高;上表层的抗拉强度与下表层抗拉强度保持一致。三点弯曲试验表明上层的力学性能优异。有限元模拟结果表明:非对称挤压过程中,45°倒角的存在引入较大的切应变,改变金属材料的流变,从而形成了非对称挤压板材特殊的织构特征。点追踪的结果表明:非对称挤压过程中,倒角的存在使挤压板材内部出现应变速率梯度;应变速率梯度是导致晶粒尺寸梯度的重要原因。
热挤压AZ31镁合金压缩及拉伸的Hall-Petch关系分别为:σ_(0.2)=22+390d~(-1/2);σ_(0.2)=80+303d(-1/2)。热挤压AZ31镁合金室温下发生塑性变形过程中,除了基面
滑移开动,非基面滑移及孪生发挥了重要作用。具有{0002}纤维织构的热挤压AZ31镁合金棒材在平行于挤压方向承受压应力过程中,基面滑移及{10(?)2}孪生是主要变形方式,{10(?)2}孪晶的发生使得晶粒取向发生显著变化。
Magnesium alloys are the lightest metallic structural materials with high specific stiffness and strength,good electromagnetic shielding capability,good damping,therefore,they are very attractive in various applications in automotive,communication,electronics,and aerospace industries.Due to their hexagonal close packed crystal structure and limited deformation mechanisms at room temperature:{0001} < 11(?)0 > basal slip and {10(?)2} <10(?)1> twinning; magnesium alloys usually have poor formability at room temperature.At present,most magnesium alloy products are limited to forming in cast;especially die casting,which limits the application scope of magnesium alloy in a great extent.Comprehensive mechanical properties of cast magnesium alloys can be improved by grain refining during hot deformation (such as extrusion,rolling).However,primary processing such as conventional hot rolling and hot extrusion generally gives rise to a strong basal texture,and this leads to a very limited ductility near the room temperature.Improved room formability is the foundation and necessity of the wide applications of wrought magnesium alloys.Therefore,enhancing the room temperature formability by texture controlling and grian refineing during hot deformation became the research focus in the development of wrought magnesium alloys.
Aiming at some aspects mentioned above,research works were focused on following aspects:(1)Evolution of {0002} basal texture during hot rolling of cast pure Mg;(2)Evolution of texture in AZ31 magnesium alloy sheet during DSR and following annealing; (3)Examination of mechanical properties,characteristics of microstructure and texture in AZ31 magnesium alloy fabricated by asymmetric hot extrusion;(4)Effect of grain size and texture on compression behavior of AZ31 magnesium alloy at room temperature.The primary conclusions were as follows:
Dynamic recrystallization,accompanying grain refinement and formation of {0002} basal texture,occurred during hot rolling of cast pure magnesium at 400℃.With the increasing of reduction,the amount of twins was reduced while grains were refined step by step.As the rolling was conducted,grain orientation of cast pure magnesium became orientation with {0002} basal texture,and the intensity of the basal texture increased with the inceasing of hot rolling reduction.When the rolling reduction achieved to 78%, microstructure of pure magnesium became homogeneous and typical {0002} basal texture was formed.The present study shows that the nucleation of DRXed grains was related to the specific DRX mechanism associated with twinning.Grains with basal texture orientation with low dislocation density and energy of distortion were not favorable for basal slips,which were not easy to recrystallize.With the increasing of the amount of grains with basal texture orientation,intensive {0002} basal texture formed in the hot rolled pure magnesium sheet.
The main texture components of hot extruded AZ31 magnesium alloy sheets were (10(?)7)[0(?)72](90°,15°,0°) and((?)(?)26)[(?)02(?)](60°,10°,30°) textures.After cold differential speed rolling(DSR),the texture components remained while the intensity of texture changed. Compared with the texture intensity of the layer closed to the higher speed roller,the intensity of texture closed to the lower speed roller changed in a larger degree.Cold DSRed AZ31 magnesium alloy sheets were annealed at different temperature.After annealing,there was no change in texture types,but intensity had changed.(10(?)7)[0(?)72]and((?)(?)26)[(?)02(?)] textures were weakened,but was not sensitive to the annealing temperature.The weakening of texture during annealing was dependent to the recrystallization process.As for cold DSRed AZ31 magnesium alloy with 16%reduction,microstructure and texture intensity became stable at 300℃and above,indicating that the recrystallization completed at temperature 300℃or higher.Compared with conventional rolling,yield stress and ultimate tensile stress in the annealed DSRed AZ31 magnesium alloy sheet were almost the same to the conventional rolling,while the room elongation of annealed DSRed AZ31 magnesium sheet was improved.
Cast AZ31 magnesium alloy was asymmetrically hot extruded at 400℃,grains were refined from 75μm to~4μm.Asymmetrically extruded AZ31 sample showed a fine grained microstructure and some grain size gradient.Average grain size of top surface(with chamfer) was smallest(2.50μm);grain size of bottom surface was largest(3.67μm),while average grain size of center was larger than that of top surface and smaller than that of bottom surface (3.13μm).There also exists an inhomogeneous distribution of textures throughout thickness in AZ31 magnesium alloy sheet produced by asymmetric hot extrusion.Asymmetric extrusion AZ31 sample showed the basal plane inclined by~15°from the normal direction toward the extrusion direction in the top surface,and weakened and scattered basal texture in the center layer,and strong basal texture in the bottom surface of the sheet.Ductility can be improved by inclined and weakened basal texture in top surface,while yielding stress was brought down. Three point bending test results indicated that mechanical properties of top surface were excellent.FEM analysis indicated that the shear strain was introduced by the asymmetrical die and,which changed grid distorations during asymmetric hot extrusion and special texture characteristics were formed.Point tracking results showed that the gradient of grain size was ascribed to the gradient of strain rate in the thickness direction during asymmetric hot extrusion.
Hall-Petch relationship of hot extruded AZ31 magnesium alloy during compression and tension were:σ_(0.2)=22+390d~(-1/2) andσ_(0.2)=80+303d~(-1/2),respectively.Besides basal slip,non-basal slip and twinning activated during deformation of hot extruded AZ31 magnesium alloy at room temperature.Since the existing sharp fiber texture favored activating of {10(?)2} <10(?)1> twin, while loaded in the extrusion direction during compression,basal slip and {10(?)2} twinning were the main deformation mechanism,and grain orientations were alerted by the {10(?)2} twinning.
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