【作者】 李莹；

【导师】 何立子； 乐启炽；

【作者基本信息】 东北大学 ， 材料工程（专业学位）， 2020， 硕士

【摘要】 宽幅镁合金板带材在交通运输和航空航天等领域具有广阔的应用前景。镁合金铸锭-热轧法是目前宽幅镁合金板带材生产的主要方法。但是,由于镁合金的HCP晶体结构特点,一般只能采用热变形方式,而其低热容和低导热能力特点又很容易导致其塑性变形过程中的温度不均,进而导致不均匀变形。基于以上原因,宽幅镁合金板带材的组织与力学性能的均匀性需要轧制变形过程中的较苛刻的工艺控制,而目前有关轧制工艺对宽幅镁合金板带材组织与力学性能的均匀性的相关研究缺乏。因此,开展宽幅镁合金板带材组织与力学性能均匀性研究对轧制工艺优化与控制具有重要的指导意义。本文以铸锭-热轧法生产宽幅为1500mm的AZ31镁合金板材为研究对象,数值模拟研究了宽幅镁板厚度在3mm～30mm范围内热轧过程中的板材的温度场、应力场和应变场的变化规律;并通过工艺试验考察了四种典型厚度宽幅板材的微观组织、力学性能与织构在厚向与宽向的均匀性,在此基础上,分析了宽幅板材微观组织与力学性能的关联性。研究取得以下主要结论:(1)数值模拟结果表明,轧制过程中板材厚向与宽向温度分布不均匀,总体表现为表层温度低于中心层,边部温度低于中部。随着轧制道次的增加,板材厚向上表层与中心层的温度差距逐渐减小为20℃,板材宽向上边部与中部的温度差逐渐增大,最大温差达30℃。轧制变形区的等效应力在宽度方向上从中部到边部逐渐增大,中部的等效应力比边部小约15MPa,等效应力分布不均匀。随着轧制的进行,边部与中部的应变差由0.05mm/mm 增大到 1mm/mm。(2)板厚(13mm和18mm)较大的宽幅镁板微观组织和力学性能在宽度和厚向均存在不均匀性。厚度为18mm和13mm的宽幅中板,轧态时基本上为部分再结晶的组织状态。厚度为18mm板材的平均晶粒尺寸与抗拉强度在厚向上的均匀性均好于宽向,其厚向最大差别分别为7μm和35MPa,而宽向则为12μm和40MPa;而厚度为13mm时,板材宽向组织与力学性能均匀性均显著提高,且组织均匀性宽向优于厚向,两向的抗拉强度最大差别均在10MPa以内。(3)板厚(3mm和6mm)较小的宽幅镁板平均晶粒尺寸差别均很小,但微观组织在宽向上边部具有较多的剪切带和孪晶,且晶粒尺寸离散程度也明显大于中心部位;板材厚度为6mm时,宽向抗拉强度均匀性较差,伸长率差别较小;板材厚度为3mm时,宽向抗拉强度均匀性较好,伸长率差别较大。(4)根据现有显微硬度与拉伸性能之间的关系模型,通过消去中间变量显微硬度,建立了抗拉强度与屈服强度之间的数学关系,并通过Hall-Petch关系式,建立了拉伸性能与平均晶粒尺寸之间的数学关系。更多还原

【Abstract】 Wide magnesium alloy plates have broad application prospects in transportation,aerospace and other fields.Magnesium alloy ingot hot rolling is the main method to produce wide magnesium alloy plates.However,due to the HCP crystal structure characteristics and its poor plasticity,magnesium alloy can only be processed by hot deformation.And its low heat capacity and low thermal conductivity can easily lead to uneven temperature in the process of plastic deformation,which leads to uneven deformation.For the above reasons,the uniformity of microstructures and mechanical properties of wide magnesium alloy plates require more stringent process control during rolling deformtion.However,there are a lack of researches on the uniformity of microstructures and mechanical properties of wide magnesium alloy plates about rolling technology.Therefore,the research on the uniformity of microstructures and mechanical properties of wide magnesium alloy plates has important guiding significance for the optimization and control of rolling process.This paper takes the 1500mm AZ31 magnesium alloy plates produced by ingot-hot rolling method as the research objects.The variation of the temperature,stress and strain of the wide magnesium plates with thickness ranging from 3mm to 30mm during the hot rolling process were studied by numerical simulations.The microstructures,mechanical properties and texture uniformity of four typical thickness wide plates were investigated through process tests.On the above basis,the relationship between the microstructures and mechanical properties of the wide plates is analyzed.The main conclusions are as follows:(1)Numerical simulation results show that the temperature distribution of plates in thickness and width directions are uneven during rolling.The overall performance is that the surface temperature is lower than the center layer,and the edge temperature is lower than the middle.With the increase of rolling passes,the temperature difference between the surface layer and the central layer of the rolling plate decreases to 20℃.The temperature difference between the edge and the middle part of the rolling plate increases gradually,and the biggest temperature difference reaches 30℃.The equivalent stress in the rolling deformation zone gradually increases from the middle to the edge in the width direction,and the equivalent stress in the middle is about 15MPa smaller than the edge.The equivalent stress distribution is uneven.With the progress of rolling,the strain difference between the edge and the middle increased from 0.05mm/mm to 1mm/mm.(2)The microstructures and mechanical properties of wide magnesium plates with larger thickness(13mm and 18mm)are uniform in both width and thickness directions.Microstructures are incompletely recrystallized structures.The uniformity of average grain size and tensile strength of the 18mm thickness plate are better in the thickness direction than that in the width direction.The biggest difference of grain size and tensile strength in thickness direction is 7 μm and 3 5MPa respectively.The biggest difference of grain size and tensile strength in width direction is 12μm and 40MPa respectively.The uniformity of average grain size and tensile strength of the 13mm thickness plate are significantly improved.And the microstructures uniformity in width direction is better than that in thickness direction.The biggest difference of tensile strength in both width and thickness directions are within 10MPa.(3)The difference in average grain size of wide magnesium plates with smaller thickness(3mm and 6mm)is very small.There are more shear bands and twins at the edge in width direction.And grain size dispersion is also significantly larger than that in the middle.The uniformity of the tensile strength of 6mm thickness plate is poor in width direction,and the difference in elongation is small.The uniformity of the tensile strength of 3mm thickness plate is better in width direction,and the difference in elongation is lager.(4)Based on the existing models for the relationship between microhardness and tensile properties,a mathematical relationship between tensile strength and yield strength was established by eliminating the intermediate variable of microhardness.Combined with Hall-Petch relationship and the relationship between tensile properties and average grain size was obtained.更多还原