基于热模拟实验和晶体塑性理论的轻合金流变行为研究

发布时间：2018-12-31 12:01

【摘要】：随着航空航天工业的发展以及对产品轻量化日趋严格的要求,以铝、钛以及镁合金等为代表的轻合金在工业领域的应用越来越广泛。热塑性成形是轻合金产品制造的重要途径,而充分认识宏观成形条件和细观组织结构对热成形力学行为的综合影响,具有重要的理论与实践价值。本文利用Gleeble热模拟试验机对Al 7050和Ti-6Al-4V两种材料在宽泛的变形条件下进行了热模拟实验,得到材料在高温塑性变形过程中的应力应变曲线;以此为基础,建立了Johnson-Cook(JC)本构模型和带有应变修正的Arrhenius(A-type)双曲正弦本构模型;将两种本构模型耦合到DEFORM 3D有限元软件中,从应力情况、应变分布、整体变形等方面综合分析了不同模型在有限元模拟中的表现。其次,利用晶体塑性理论和有限元模拟结合的方法,分析和探讨了多晶体材料在细观角度下的力学行为表现。主要结论如下:①两种材料在高温压缩变形中,流变应力随着变形温度的升高而减小,随着变形速率的提高而增大,具有明显的热致软化现象和速度敏感性。②JC模型对流变应力的直接解析式预测精度较低;而带有应变补偿的A-type本构模型具有更高的预测精度,即使在实验拟合数据之外,也能很好地反映实际流变行为。③针对热模拟实验的圆柱体镦粗过程,现有理论方法计算得到的试样表面正应力σz分布存在很大差异,但利用上述本构模型进行数值模拟的应力值和解析计算结果具有很好的一致性;不考虑应变补偿的A-type本构模型的应力直接预测结果和数值模拟结果的吻合程度低于JC模型。此外,本构方程对圆柱体热镦粗过程金属材料的整体流动变形的影响相对较小。④通过对高径比为1.5的圆柱体镦粗过程的晶体塑性有限元模拟,发现在不考虑再结晶的前提下,晶粒总界面的变化趋势是随着变形量的增加先减小而后增大。对于晶体取向具有明显方向性的多晶材料,由于相邻晶粒位向较接近,塑性变形时晶粒之间更容易相互协调,使得晶界面积增长更快,变形抗力增大;当晶粒以等轴晶为主时,随着变形量增加,晶界界面的面积增加幅度大,位错运动在晶界处受阻的可能性增大;晶粒度较大时,晶界处的应力集中较为明显,晶体的变形表现出不均匀性,而晶粒度越小,晶粒界面增加、材料强度提高。
[Abstract]:With the development of aerospace industry and the increasingly stringent requirements of lightweight products, light alloys, such as aluminum, titanium and magnesium alloys, are more and more widely used in the industrial field. Thermoplastic forming is an important way to manufacture light alloy products. It is of great theoretical and practical value to fully understand the comprehensive influence of macro forming conditions and microstructure on the mechanical behavior of hot forming. In this paper, the thermal simulation experiments of Al 7050 and Ti-6Al-4V were carried out under a wide range of deformation conditions by using Gleeble thermal simulation test machine, and the stress-strain curves of the materials during plastic deformation at high temperature were obtained. On this basis, the Johnson-Cook (JC) constitutive model and the Arrhenius (A-type) hyperbolic sinusoidal constitutive model with strain correction are established. The two constitutive models are coupled to DEFORM 3D finite element software, and the performance of different models in finite element simulation is analyzed synthetically from the aspects of stress, strain distribution and global deformation. Secondly, the mechanical behavior of polycrystalline materials in meso-angle is analyzed and discussed by combining the theory of crystal plasticity with finite element simulation. The main conclusions are as follows: 1 during high temperature compression deformation, the flow stress decreases with the increase of deformation temperature and increases with the increase of deformation rate. It has obvious thermal softening phenomenon and velocity sensitivity. The 2JC model has low precision in predicting rheological stress directly. The A-type constitutive model with strain compensation has higher prediction accuracy, and it can reflect the actual rheological behavior well even outside the experimental fitting data. 3 for the upsetting process of cylinder in thermal simulation experiment, The normal stress 蟽 z distribution on the surface of specimens calculated by the existing theoretical methods is very different, but the stress values obtained by numerical simulation using the above constitutive model are in good agreement with the analytical results. The results of direct stress prediction and numerical simulation of A-type constitutive model without strain compensation are lower than those of JC model. In addition, the constitutive equation has relatively little effect on the overall flow deformation of the metal material during the hot upsetting of a cylinder. 4 the crystal plastic finite element simulation of the upsetting process of the cylinder with a ratio of height to diameter of 1.5 is carried out. It is found that the change tendency of the total grain interface decreases first and then increases with the increase of deformation without considering recrystallization. For the polycrystalline material with obvious directionality, the grain boundary area increases faster and the deformation resistance increases because of the close orientation of adjacent grains and the easier coordination between the grains during plastic deformation. When the grain is mainly equiaxed, with the increase of deformation amount, the area of grain boundary interface increases greatly, and the possibility of dislocation movement being blocked at grain boundary is increased. When the grain size is larger, the stress concentration at grain boundary is obvious, and the deformation of crystal shows inhomogeneity. The smaller the grain size is, the more the grain interface increases and the strength of the material increases.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：V252.2

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