【作者】 李乃朴；

【导师】 张兴国；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 镁合金作为轻质金属结构材料,由于其低密度、高比强度、优越的抗震性能和电磁屏蔽性等独特的优点,受到了越来越多的关注。采用传统的铸态轧制工艺对中厚板材进行变形的过程中,加工效率较低,得到的板材存在明显的基面板织构。本文选取电磁连铸AZ61变形镁合金,采用挤压轧制复合工艺进行中厚板材的变形研究,探讨了轧制温度对板材性能的影响并与铸态轧制工艺板材的组织和力学性能进行了对比分析。连铸过程中引入的电磁场使AZ61铸态组织细化,破碎晶界处粗大网状分布的β-Mg17Al12共晶相,使其细小并且弥散分布。连铸过程中加入的电磁场改善了镁合金铸锭的表面质量；与无磁场连铸相比,电磁连铸AZ61镁合金铸锭抗拉强度达到181MPa,提高了25%；伸长率为7.4%,提高了45%。铸态轧制工艺的板材在变形初期会有大量的孪晶组织生成,之后以孪生动态再结晶的方式细化晶粒；75%变形量下的晶粒组织为不完全动态再结晶,力学性能为σb=300MPa,σ0.2=235MPa,δ=8.0%。挤压轧制复合工艺板材在轧制初期围绕着挤压产生的细小晶粒发生动态再结晶,后期以旋转动态再结晶的方式形成局部剪切带来细化晶粒,改善塑性。三种轧制变形量的板材组织均小于铸态轧制。60%变形量时的力学性能等同于铸态轧制板材；75%和80%两种轧制变形量的板材极限抗拉强度、屈服强度以及伸长率为315MPa、283MPa、12.0%,325MPa、306MPa、10.3%,相比于铸态轧制工艺,分别提高了5%、20.4%、50%,8.3%、30.2%、28.7%。挤压轧制复合板材的{0002}基面板织构最大极密度低于同等变形量下的铸态轧制板材。在较低温度下轧制时,两种原因引发裂纹源导致在板材侧边部产生45°斜向裂纹。一是较大的应力集中导致孪晶和晶界的交界处产生裂纹源：二是晶粒之间的变形程度失调而诱发裂纹源。随着轧制温度的升高,裂纹的产生倾向减小,而且微观组织从大量的孪晶逐渐变为完全动态再结晶的细小等轴晶粒,并发生一定程度的粗化：板材的极限抗拉强度和屈服强度逐渐降低,塑性逐渐提高,断裂机制从典型的准解理断裂向混合断裂机制转变。挤压轧制复合工艺下的优化轧制温度为400℃。经过研究对比,可通过较小的轧制下压量得到与传统铸态轧制工艺下大变形量性能相当甚至更高的高性能板材。减小了板材在大变形量轧制时的破坏趋势,提高了板材的加工效率。更多还原

【Abstract】 As a lightweight metal structural material, magnesium alloy has attracted more and more attention because of its low density, high specific strength, excellent shock resistance, electromagnetic shielding and other unique performances. The thick magnesium alloy plate with traditional cast rolling deformation process performed a low processing efficiency and a strong base panel texture. This paper employed a new approach of the extrusion composite rolling technology to research the deformation of the thick electromagnetic continuous casting AZ61wrought magnesium alloy plates. It compared the effect of the new approach with the cast rolling process on the microstructure and mechanical properties of the sheets. And it also explored the influence of rolling temperature on the new approach.The introduction of electromagnetic fields in the continuous casting process make AZ61cast microstructure refinement, broken the coarse mesh distributed β-Mg17Al12eutectic phase at the grain boundaries and making it be fine and dispersed. And it improved the surface quality of the magnesium alloy ingots. Compared with the normal casting, the ultimate tensile strength of the electromagnetic casting AZ61magnesium alloy ingot increased by25%of181Mpa, and the elongation had45%enhanced of7.4%.There will be a large number of twins in the early deformation of cast rolling sheets. Afterwards, the grain refinement mechanism is the twin dynamic recrystallization. The grains organization is incomplete dynamic recrystallization with75%deformation and the mechanical properties is σb=300MPa, σ0.2=235Mpa and δ=8.0%.In the early deformation period, dynamic recrystallization occurred around the fine grains in the extrusion of the extrusion composite rolling process. And then, the local shear zones produced by the rotation dynamic recrystallization bring the grain refinement to improve the plasticity. The average grain size of three different rolling deformations is less than the cast rolling.Mechanical properties of the60%rolling deformation sheets were equivalent to the cast rolling. The ultimate tensile strength, yield strength and elongation of75%and80%rolling deformation sheets were315MPa,283MPa,12.0%,325MPa,306Mpa and10.3%. Compared with the cast rolling process, they were increased by5%,20.4%,50%,8.3%,30.2%, and28.7%respectively. The maximum pole density of {0002} base panel texture of the sheets with extrusion composite rolling is lower than that of the cast rolling sheets under the same deformation.There are two reasons inducing the crack sources lead to the45°diagonal crack on the sheet side rolled at lower temperatures. First, the crack sources generate on the junction of the twin and grain boundaries because of large stress concentration. The other is the disorder of the deformation degree between the grains.As the rolling temperature increased, the cracks tendency reduced, the microstructure gradually turned into slightly coarse and equiaxed grains from a large number of twins because of fully dynamic recrystallization. The ultimate tensile strength and yield strength of the sheet decreased and a gradual increase in plasticity at the same time. And the fracture mechanism turned to mixed fracture from the typical quasi-cleavage fracture. The optimized rolling temperature of extrusion composite rolling process is400℃.It was studied that high-performance sheets can be manufactured under a small rolling deformation with extrusion composite rolling process, and the mechanical properties of them are equivalent to or even higher than the high rolling deformation sheets with traditional cast rolling process. Consequently, this new approach decreased the failure trends with a large rolling deformation and improved the processing efficiency of the magnesium sheets.更多还原