| 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. |
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