Study On Pore Structure Controlling And Properties Of Porous Metals Based On Preparation Of NaCl Beads

Foam metal is widely used in the fields of aerospace,electrochemistry,construction engineering,machinery,medicine,environmental protection due to its many advantages such as large specific surface area,light weight,low modulus,high specific strength,high specific stiffness,energy absorption,etc.In this paper,the open-cell foam metal was prepared by space holder method with NaCl beaded as space holders and the mechanical properties of the metal foam were investigated based on preparation of NaCl beads.The space holder method solved common problems of other methods for preparing foam metal with high cost,long cycle,unsatisfactory structure and poor controllability of mechanical properties.Distilled water,starch and salt were used as raw materials to fabricate the green part of NaCl particles.Afterwards,the green part was oil bath treatment under stirring then it was calcinated to obtain the spherical and hardness NaCl particles.The size of NaCl particles was varied from 0.6 to 2.5mm.The salt spheres observed by SEM showed ellipsoid in shape and a large number of microporous of 13-18?m were distributed on the surface.The results indicated that these micropores produced capillary action when the salt spheres were dissolved,which helped the space holder to rapidly dissolve from the inside of the foam green part,and the porous body were not damage.The aluminum foam and copper foam green body were prepared by space holder method.The space holder inside the green body is dissolved to obtain a porous body,and the foam metal was sintered at 0.7-0.8 T_m under a protective atmosphere to obtain a dense skeleton.Preparation process,structural characteristics and physical properties of the foam metal were discussed.Mechanical adjustment for Cu foam by different forms was carried out to explore the effects of different strengthening forms on the improvement of mechanical properties and buckling behavior of composite porous structures.The following results were obtained:?1?Constructing a porous metal body by filling powders into a salt bed with a auto-vibration machine has the advantages of good process stability,high structural reproducibility,high preparation efficiency,good three-dimensional interconnectivity,less internal defects,good universality,etc.The cells of the foam were arranged closely and the density in the radial direction of the porous body decreases from the edge to the center.SEM observation shows that the size of the connected pores is larger than 150?m,and the connected window size increases slightly with the increase of the size of the space holder.The volume shrinkage of Al foam is different from that of the Cu foam.The porosity of the obtained Al foam is 77.15⁷8.28%,and the porosity of Cu foam is 83.56⁸5.37%.?2?The compression curves of Al foam and Cu foam undergo three stages:linear elasticity,plateau and densification.The whole deformation process is characterized by buckling of the pore edge till fracture and collapsing of the pores layer-by-layer.The energy absorption ranges of Al and Cu foam with different porosity are 5.04⁵.42MJ·m^-3 and 3.47³.59MJ·m^-3.?3?The increase in the number of copper tube implanted in the foam matrix leads to an increase in the porosity of the composite structure.The mechanical properties of the triple copper tube reinforced structure are slightly higher than those of the single copper tube reinforced structure.The single solid copper cylinder reinforcement structure is strongly hindered by the matrix foam during compression,so the composite structure still maintains the deformation characteristics of the foam metal.The composite structure strengthened by the three solid copper cylinders that breaks away from the constraint of the matrix foam during compression produces and exerts shearing effects on the foam body exhibits a combination of dense metal and foam metal properties.?4?The average permeability coefficients of Al foam with porosity of 77.15%,77.72%and78.28%are 4.4404E^-11,3.2018E^-11,2.4439E^-11,respectively.The average permeability coefficients of Cu foam with porosity of 83.56%,84.05%,85.37%are 5.3527E^-11,4.4772E^-11,and 3.9841E^-11.?5?The thermal expansion coefficient of the Al foam decreases with increasing porosity in the axial direction during thermal expansion.In the radial direction,however,the change in porosity of the sample has no significant effect on the thermal expansion coefficient.The thermal expansion coefficient curves of the Cu foam along the axial and radial directions show an increasing-stabilizing-falling trend with temperature increasing.