【作者】 刘新华；

【导师】 谢建新；

【作者基本信息】 北京科技大学 ， 材料加工工程， 2015， 博士

【摘要】 藕状多孔金属是一种力学性能优良的新型多孔金属材料,是近年来多孔材料研究的热点。藕状金属的变形行为和吸能性能的研究可为其在航空航天、高速交通等高技术领域轻质结构和吸能部件方面的应用奠定基础。本文以变形能力差异较大的两种典型藕状多孔金属(铜和镁)为对象,采用GLEEBLE实验和直撞式霍普金森压杆方法,在10-3～2500s-1应变速率范围内,对藕状多孔金属进行了压缩变形测试,系统研究了孔隙结构和变形条件对藕状金属变形行为、力学性能和吸能特性的影响,构建了藕状多孔金属的本构关系,取得的主要研究结果如下。采用自行研制的定向凝固装置,研究了凝固速度、浇注温度、铸型温度等主要制备参数对藕状镁和铜孔隙结构的影响规律,获得了可制备出具有较高孔隙率(40%-65%)、较小平均孔径(Φ0.15-0.55mm)和较好均匀性孔隙结构的藕状多孔镁和铜的条件：采用水冷铜模直接冷却以获得较大的凝固速度,铸型温度分别约为500℃和900℃,浇注温度分别约为760℃和1180℃。采用变形量控制和高速摄像等方法,研究了藕状多孔金属的压缩变形行为,给出了孔隙结构的变化过程和图景,探明了变形条件对多孔金属变形行为的影响。结果表明：藕状多孔金属表现出与致密金属迥异的压缩变形行为,即在压缩过程中应力在较大应变范围内缓慢增加或基本不变,压缩变形过程主要包括以孔壁发生弹性和镦粗变形为主的线弹性阶段、以孔壁发生塑性弯曲变形为主的低应力平台阶段和以孔隙进一步闭合并发生整体流变为主的密实化阶段。研究发现,当平行于气孔压缩时,藕状多孔金属在平台阶段的变形行为与材料特性和应变速率有关。对塑性变形能力良好的纯铜,在低、中应变速率时以孔壁发生的S形弯曲变形为主,在高应变速率时则以C形弯曲变形为主；对塑性变形能力较差的纯镁,在低、中应变速率时主要以孔壁发生局部剪切断裂进而向孔隙内塌陷的方式变形,在高应变速率时主要以孔壁发生倾转进而折断的方式变形。当垂直于气孔压缩时,藕状多孔金属在平台阶段的的塑性变形行为主要与应变速率有关。在低、中应变速率时,以孔壁向气孔内发生弯月形弯曲和塌陷并逐渐填充孔隙部位的变形方式为主,而在高应变速率时以气孔直接压扁闭合的变形方式为主。分别采用力学分析的方法,建立了藕状金属压缩变形平台阶段的本构关系模型,具有较高的精度。研究了孔隙结构与变形条件对藕状多孔金属压缩力学性能的影响,结果表明,孔隙率、平均孔径和孔隙均匀性等孔隙结构参数中孔隙率对压缩力学性能的影响最显著。藕状多孔金属的力学性能具有显著的各向异性,压缩方向与气孔方向夹角越小,平台应力越大：应变速率较低时对力学性能的影响不明显,但对藕状多孔铜当应变速率增大到某一临界值时表现出显著的应变速率硬化效应；变形温度升高,压缩应力下降,平台区的宽度增加。采用线性拟合的方法,获得了孔隙率和变形条件对藕状多孔金属应力应变关系影响的数学模型,建立了藕状多孔金属压缩变形包含孔隙结构参数和变形条件的本构关系,具有较高的精度。研究了典型藕状多孔金属的吸能特性以及孔隙结构和变形条件的影响,结果表明,藕状多孔金属具有较高的单位体积与单位质量吸能能力以及可在较大的应变范围保持基本不变的较高稳定吸能效率；吸能性能具有明显的各向异性,平行于气孔方向压缩与垂直时相比,吸能能力较高,稳定吸能段明显较宽,但稳定吸能效率值稍低；吸能能力随孔隙率的增大而减小,而吸能效率受孔隙率和应变速率的影响不明显。藕状多孔铜的吸能机理主要是孔壁通过发生弯曲、压扁、塌陷等变形将能量转化为塑性功而吸收和消耗能量；藕状多孔镁主要是孔壁通过发生断裂、转动、弯折、塌陷等变形方式耗散和吸收能量。更多还原

【Abstract】 Lotus-type porous metal is a novel porous metal with excellent mechanical properties, and has attracted lots of attention in recent years. The research on mechanical and energy-absorption properties of lotus-type porous metals can lay the foundation for applications of light structure and energy-absorption components in the highly technical areas of aerospace and rapid transit. In the paper, taking the typical high-plasticity and low-plasticity lotus-type porous metals (Copper and Magnesium) as the research target, therefore, in the present thesis the GLEEBLE thermal-mechanical simulation system and a direct impact split Hopkinson pressure bar (SHPB) were used for compression tests of the metals over a wide strain-rate range of10-3～2500s-1in order to systematically investigate the effects of porous structure and deformation condition on deformation behaviors, mechanical properties, and energy-absorption properties of the lotus-type porous metal, and furthermore the constitutive equations of compressive deformation of lotus-type porous metals were established. The main conclusions are drawn as follows:Lotus-type porous Copper and Magnesium were fabricated by self-made unidirectional solidification equipment and the influences of the key parameters including solidification speed, casting temperature and mold temperature on the porous structure were studied, and the appropriate processing parameters for fabricating lotus-type porous copper and magnesium with higher porosity (40%～65%), small average pore diameter (Φ0.15～0.55mm) and homogeneous porous structure were obtained as follows:use of copper mold by water cooling for the maximum solidification speed, mold temperature were500℃and900℃, respectively, and casting temperature were760℃and1180℃, respectively.The methods of deformation controlling and high-speed photograph were used to study compressive deformation of the lotus-type porous metals, showing the porosity structure evolution and prospect, and the effect of deformation condition on the deformation behavior of the lotus-type porous metal was revealed. The results showed that the lotus-type porous metal exhibited different deformation behaviors from their dense metals, i.e., the stress increased slowly or remained unchanged under a wide range of strain, and the compressive processing could be divided into three stages:the linear elastic stage, the plateau stage, and the densification stage.When the compressive direction was parallel to the axial of pores, the deformation behavior of lotus-type porous metal was changed with the properties of material and strain rate. For the lotus-type copper with good performance of plastic deformation, S-shaped bending deformation of pore wall was the main plastic deformation mode during the compressive process at low and intermediate strain rate, but at high strain rate C-shaped bending deformation of pore wall was the main one. For the lotus-type magnesium with low-plasticity, local fracture of pore wall and then collapse into gas pore was the main deformation mode at low strain rate, and at high strain rate the rotation and buckling and then broken of pore wall was the main mode. When compressing vertical to the axial direction of pores, the deformation behavior in the stage of plateau was influenced by the strain rate. The crescent-shaped bending and collapse of the pore wall into gas pore is the main deformation mode, while at high strain rate the main one is the flattening and closing of gas pore. Further, the constitutive equation of the plateau stage of lotus-type porous copper was established by the mechanical model derivation. The calculated results of the established constitutive equation were in a good agreement with the experimental data.The effects of porous structure and deformation condition on the compressive mechanical property of the lotus-type porous metals were studied. The results showed that porosity, average pore diameter and porous homogeneity all had an impact on the compressive properties of the lotus-type metals, and the effect of porosity was maximum one. The smaller the angle between compressive direction and gas pore direction was, the larger both the plateau stress was, indicating apparent anisotropy. The low strain rate had little impact on the mechanical properties, but a significant strain rate hardening effect appeared when the strain rate exceeded a certain critical value. When the deformation temperature increased, the compressive stress decreased, the plateau width increased. The influence rules of porous structure and deformation condition on constitutive equation were obtained by the way of linear fitting, and a universal constitutive equation containing porous structure parameter and deformation condition was established, which has higher accuracy.The energy-absorption property of the lotus-type porous metals and the effects of porous structure and deformation condition were investigated. The results indicated that the lotus-type porous metals had higher energy absorbing capacity per unit mass and per unit volume, and could keep higher and stable energy absorbing efficiency in a wider range of strain. The lotus-type porous metals have anisotropy of energy absorbing efficiency. Compared with compressing vertical to the axial direction of pores, the lotus-type porous metals compressed parallel to the axial direction of pores had higher energy absorbing capacity, lower energy absorbing efficiency and wider stable energy absorbing stage. The energy absorbing capacity decreased with an increase of porosity. However, porosity had little influence on the energy absorbing capacity. The energy absorbing mechanism of the lotus-type porous copper is mainly that the energy is converted into plastic work which is absorbed and consumed, through bending, flattening and collapse of pore wall, for the lotus-type porous magnesium, the converted plastic work is also absorbed and consumed by a variety of models including fracture, rotation, buckling, collapse of pore wall.更多还原