预变形对镁合金板材冲压性能影响机理的研究

发布时间：2018-05-15 12:43

本文选题：镁合金板材 + 织构　；参考：《重庆大学》2015年博士论文

【摘要】：镁合金作为最轻的商业金属结构材料,被广泛应用于航空航天、汽车、电子通信等领域。然而由于镁合金密排六方的晶体结构,其室温下的滑移系很少,导致其塑性变形能力很差。传统的挤压和轧制镁合金板材具有很强的基面织构,导致其具有很强的各向异性,限制了镁合金板材的广泛使用。本文从改善商业镁合金板材组织结构,提高其力学性能及成型性能的角度出发,首先利用晶体塑性有限元模拟了镁合金板材在拉伸和压缩过程中基面织构的演变,基于模拟结果,分别采用室温大变形预拉伸和预压缩工艺来改善镁合金板材的组织,研究了变形后及退火后镁合金板材的组织转变、织构演变、力学性能及成型性能。最后,通过三种理论模型对比计算了镁合金板材室温下的成型极限图以及n值和r值对成型极限的影响。主要研究内容及成果如下:①通过晶体塑性有限元模型,模拟了AZ31B镁合金板材在拉伸和压缩过程中织构的转变,以及不同的变形方向对织构偏转的影响。拉伸变形处理后的板材基面织构增强,但是其分布更为发散。拉伸板材的基面织构沿着与预拉伸方向垂直的方向呈椭圆形分布,且沿该方向其分布更为发散。沿TD方向预压缩变形使其晶粒的c轴向TD方向发生偏转,导致基面织构强度降低,以及沿TD方向织构的增强。预压缩板材晶粒的c轴总是平行于预压缩的方向,导致平行于预压缩方向基面织构强度的增强。②采用室温大变形量预拉伸及再结晶退火可以有效提高挤压AZ31B镁合金板材的成型性能。经过350°C 1h退火后板材的基面织构强度明显降低,基面织构由ND方向朝ED方向偏转,且分布更加弥散。随着预拉伸变形量的增加,处理后板材的成型性能逐渐提高,屈服强度、塑性应变比r值降低,应变硬化指数n值增大,其Erichsen值为5.6mm,相对于原始板材的2.5mm提高了~124%。③分别沿ED、45°和TD方向对挤压AZ31B镁合金板材进行室温大变形预拉伸及再结晶退火,研究了预拉伸方向对板材力学性能及成型性能的影响。经过三种不同方向预拉伸的板材,其基面织构强度均得以弱化,但是偏转方向均垂直于板材预拉伸的方向,板材沿垂直于预拉伸方向的力学性能得到明显改善,即y沿该方向具有较低的屈服强度、r值以及较高的n值,同时沿预拉伸方向的力学性能降低不明显。④采用预拉伸速度分别为0.3,3,30,120 mm/min,在准静态范围内研究了预拉伸速度对再结晶退火后AZ31B镁合金板材成型性能的影响。随着应变速度的提高,退火后板材的成型性能逐渐提高,当拉伸速度为30 mm/min时,预拉伸板材的erichsen值达到最大值为5.4mm,随着应变速度的继续增大,成型性能呈降低趋势。⑤采用室温预压缩及退火工艺(pca)改善1.0mm厚az31b镁合金薄板成型性能,并研究了其在变形及退火过程中的组织演变及力学性能的变化。沿td方向预压缩及退火后,由于拉伸孪晶及静态再结晶的缘故,晶粒的c轴∥td方向,使pca板材的基面滑移、c+a滑移和拉伸孪晶变得更加容易,使其具有较低的屈服强度、屈强比,较高的n值和较低的r值,pca板材的erichsen值也由原始的2.35mm提高到4.55mm,提高了~94%。同时,当预压缩变形量超过4.8%时,镁合金板材成型性能的改善不再明显。⑥分别沿rd、45°和td方向对az31b镁合金板材进行相同变形量的预压缩,研究了预压缩方向对镁合金板材组织及力学性能的影响。通过对比三种不同压缩方向对镁合金板材组织、力学性能及成型性能的影响研究发现,pca板材晶粒的c轴由nd方向偏向于预压缩的方向,同时沿rd及td方向预压缩可以明显改善该方向及45°方向的性能,而沿45°方向预压缩只能改善rd和td方向的性能,此外,沿垂直于板材基面织构发散的方向进行预压缩可以最大程度的改善镁合金板材的成型性能。⑦采用250°c2h和450°c1h的退火工艺研究了去应力退火与再结晶退火对镁合金板材组织及力学性能的影响。通过对比去应力退火与再结晶退火后预压缩板材组织、力学性能及成型性能可以发现,两种退火方式都可以弱化镁合金板材的基面织构,使其晶粒发生偏转,可以降低镁合金板材的屈服强度、屈强比以及塑性应变比,提高其应变硬化指数,但是保留拉伸孪晶的去应力退火对镁合金薄板的成型性能改善不大,而再结晶退火可以显著改善其成型性能,去应力退火板材与再结晶退火板材的erichsen值分别为2.57mm和5.28mm,分别提高了~9%和~125%。⑧研究了150°c下预压缩及退火对镁合金板材组织及力学性能的影响。通过150°c沿td方向的预压缩及退火处理后的板材,pca板材晶粒的c轴平行于td方向,基面织构弱化,并且沿td方向出现峰值,同时具有其较小的屈服强度、屈强比以及r值和较大的n值,pca板材的erichsen值由原始的2.35mm提高到了5.28mm,提高了近125%。⑨分别基于m-k理论、swift分散性失稳理论和hill集中性失稳理论计算了az31b镁合金板材在室温下的成型极限图(fld)。采用合理的力学性能参数,基于m-k模型的fld可以很好的预测镁合金板材在双向拉伸状态下的成型极限,基于swift分散性失稳理论的fld可以较为准确的预测镁合金板材在拉-压状态下的成型极限,而基于hill集中性失稳理论的fld与实验数据偏差很大,并不适合镁合金成型极限图的理论计算。镁合金板材n值得增大以及r值得降低,都可以有效提高镁合金板材的室温成型极限。
[Abstract]:Magnesium alloy, as the lightest commercial metal structure material, is widely used in aerospace, automotive, electronic communication and other fields. However, because of the crystal structure of six sides of magnesium alloy, the slip system at room temperature is very few, resulting in poor plastic deformation ability. The traditional extrusion and rolling magnesium alloy sheet has a strong base texture, which leads to it. With a strong anisotropy, the extensive use of magnesium alloy plates is limited. From the angle of improving the structure of the magnesium alloy sheet and improving the mechanical properties and forming properties of the magnesium alloy, the evolution of the texture of the base surface of the magnesium alloy sheet in the stretching and compression process is simulated by the crystal plastic finite element, based on the simulation results. The microstructure transformation, texture evolution, mechanical properties and molding properties of magnesium alloy sheet after deformation and annealing are studied. Finally, the molding limit diagram and n value and R value of magnesium alloy sheet at room temperature are compared and calculated by three theoretical models. The main contents and results are as follows: 1. Through the plastic finite element model of crystal, the texture transformation of AZ31B magnesium alloy sheet during tension and compression is simulated, and the influence of different direction of deformation on the deflection of texture. The texture of the base surface after the tensile deformation is enhanced, but its distribution is more divergent. The texture of the base surface of the plate is elliptical along the direction perpendicular to the pretension direction, and the distribution is more divergent along this direction. The pre compression deformation along the direction of TD leads to the deflection of the c axial TD direction of the grain, which leads to the decrease of the texture strength of the base surface and the enhancement along the texture in the direction of the TD. The c axis of the grain of the pre compressed sheet is always parallel to the precompression. The direction of contraction leads to the enhancement of texture strength parallel to the base surface of the precompression direction. 2. The pre tensile and recrystallization annealing can effectively improve the forming properties of the extruded AZ31B magnesium alloy sheet. After 350 C 1H annealing, the texture strength of the base surface of the plate is obviously reduced, and the texture of the base surface is deflected from the direction of ND to the ED direction. With the increase of pre tensile deformation, the molding performance of the plate is increased gradually, the yield strength, the plastic strain ratio R value is reduced, the strain hardening exponent n value increases, the Erichsen value is 5.6mm. Compared with the 2.5mm of the original plate, the ~124%. 3 increases the room temperature of the extruded AZ31B magnesium alloy plate along the direction of ED, 45 and TD, respectively. The effects of pre stretching and recrystallization annealing on the mechanical properties and molding properties of the plate are studied. The texture strength of the base surface is weakened by three different directions, but the deflection direction is perpendicular to the direction of the plate pretension, and the mechanical properties of the plate along the direction perpendicular to the pretension are obvious. The improvement is that y has lower yield strength, R value and higher N value along this direction, while the mechanical properties decrease along the pretension direction at the same time. (4) the pre stretching speed is 0.3,3,30120 mm/min respectively. The effect of pre stretching speed on the molding properties of AZ31B magnesium alloy sheet after recrystallization annealing is studied in the quasi static range. The forming performance of the plate is improved gradually after annealing. When the tensile speed is 30 mm/min, the maximum Erichsen value of the pre stretched sheet is 5.4mm. The molding performance is reduced with the strain speed increasing. 5. The molding property of 1.0mm thick AZ31B magnesium alloy sheet is improved by the room temperature precompression and annealing process (PCA). In the process of deformation and annealing, the changes of microstructure and mechanical properties in the process of deformation and annealing are studied. After precompression and annealing along the direction of TD, the TD direction of the c axis of the grain makes the base plane of the PCA sheet slip, and the c+a slip and the tension twin become more easy because of the tensile twins and the static recrystallization. The yield strength is lower and the yield strength is lower. Strong ratio, higher N value and lower R value, the Erichsen value of PCA plate is also increased from original 2.35mm to 4.55mm, and ~94%. is improved. When the pre compression deformation amount is more than 4.8%, the improvement of molding performance of magnesium alloy plate is no longer obvious. 6. The pre compression of the same deformation amount along Rd, 45 degree and TD direction, respectively, is studied. The influence of precompression direction on Microstructure and mechanical properties of magnesium alloy sheet. By comparing the effects of three different compression directions on microstructure, mechanical properties and molding properties of magnesium alloy sheet, it is found that the c axis of the PCA plate is biased in the direction of Nd in the direction of pre compression, and the direction of this direction and the precompression along the direction of RD and TD can obviously improve the direction and 45. The performance of RD and TD direction can only be improved along the direction of 45 degrees. In addition, the precompression perpendicular to the direction of the texture and divergence of the sheet base can improve the molding properties of the magnesium alloy plate to the greatest extent. By comparing the microstructure, mechanical properties and molding properties of the pre compression plate after the stress annealing and recrystallization annealing, the two kinds of annealing methods can weaken the texture of the base surface of the magnesium alloy sheet and make the grain deflect, which can reduce the yield strength, the yield strength ratio and the strength ratio of the magnesium alloy sheet. The strain hardening exponent is improved by plastic strain ratio, but the improvement of tensile twins' stress annealing has little improvement on the forming properties of magnesium alloy sheet, and the recrystallization annealing can significantly improve the molding properties. The Erichsen values of the annealed sheet and the recrystallized sheet are 2.57mm and 5.28mm, respectively, which are improved by ~9% and ~125%. respectively. The effect of pre compression and annealing on the microstructure and mechanical properties of magnesium alloy sheet under 150 C was studied. The c axis of the grain of PCA plate was parallel to TD direction through the pre compression and annealing treatment of 150 degree C along TD direction. The texture of the base surface was weakened, and the peak value appeared along the TD direction. At the same time, the smaller yield strength, the yield strength ratio, the R value and the relative value were compared. The large n value, the Erichsen value of the PCA sheet is increased from the original 2.35mm to the 5.28mm, and the near 125%. is improved based on the m-k theory, the swift dispersion instability theory and the hill centralized instability theory are used to calculate the forming limit diagram (FLD) of the AZ31B magnesium alloy sheet at room temperature. The molding limit of magnesium alloy sheet under biaxial tension is predicted. Based on the swift dispersion instability theory, FLD can accurately predict the forming limit of magnesium alloy sheet under tension and compression, while the FLD and experimental data based on the theory of hill concentrated instability are not suitable for the theoretical calculation of the molding limit diagram of magnesium alloy. Magnesium alloy sheet n is worth increasing and R is worth reducing. It can effectively improve the room temperature forming limit of magnesium alloy sheet.

【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG306;TG146.22

【参考文献】