A Study On The Pore Structure And Mechanical Property Of Directionally Solidified Porous Cu-Cr Alloy

Directional solidification of metal-gas eutectic, also known as "Gasar", is a revolutionary method for the fabrication of ordered porous materials. The theoretical background of this method is based on the gap of gas (mainly hydrogen) solubility in liquid and solid metals. Garsar porous material is characterized by cylindrical pores embedded in solid matrix. Compared with traditional porous material, the Garsar porous materials not only have advantages like light weight, high specific strength et. al, but also have many special properties like anisotropic, small stress concentration and unique sound, thermal and electrical properties. This makes the Garsar material having great potential application in the industrial field like lubrication, heat exchange, gas dissipation etc.Howerver, it was found that pure metals with directional pore structure could be produced successfully by this process, but alloys with similar pore structure could not be produced with silimlar method. Pure metal generally has low mechanical strength because of the lacking of solution strengthening and precipitation strengthening, and this greatly limits the further application of the Gasar material. It's well known that alloying with strengthening element is an effective way to improve the mechanical properties of the metal material. However, the solidification behavior of Garsar alloy is more complicate than pure metal. There is lack of theoretical study on the gas pore formation, the condition for the cooperative growth of gas and matrix, and so far the gas pore structure can not be effectively controlled in Garsar alloy.In this study, porous Cu-xCr alloy with different mass percentages of chromium and different pore structure have been fabricated in hydrogen atmosphere by using directional mould casting apparatus. Gasar porous single-phase Cu-xCr (x=0?0.3?0.5 wt.%) alloy, hypoeutectic alloy Cu-xCr(x=0.8?1.0wt.%), eutectic Cu-1.3wt.%Cr alloy and hyper eutectic alloy Cu-1.8wt.% Cr are fabricated. The pore structure of the above Cu-xCr alloys can be effective controlled by selection of Cr content, hydrogen pressure and solidification velocity.Based on the law of mass conservation, a theoretical model was developed to predict the solubility of hydrogen in alloy and gas porosity. The results show that the solubility of hydrogen in alloys depends on the solubility of hydrogen in pure metal and the thermodynamic properties of the melt. The experimental results show a quite good correspondence to the model predictions. The gap of hydrogen solubility between liquid and solid phase of the alloy is the main factor determining the porosity. The pore diameter is related to the width of the mushy zone at the front of solid-liquid interface of alloy, and its change rule is the same as the width of the mushy zone.The effects of Cr content, hydrogen pressure and solidification velocity on the pore structure are investigated, and the results show that the porosity and pore diameter first increase and then decrease with increasing Cr content. Increasing the hydrogen pressure can increases the gas porosity and decreases the pore diameter, and makes the pore distribution more homogeneous. The solidification velocity has a minor influence on the gas porosity, but can greatly affect the pore diameter.The effects of chromium content, hydrogen pressure and solidifying velocity on the matrix micro structure were investigated. The results show that an ordered pore structure can be obtained when micro structure is composed of cellular or columnar dendritic grain structure. The ordered pore structure is deteriorated when equiaxed dendritic structure appears. The gas pores nucleate at the eutectic fronts in hypoeutectic alloy, eutectic alloy and hypereutectic alloy. With increasing Cr content, the amount of eutectic microstructure, porosity and roundness increase.The pore coalescence in pure copper was observed by combining directional solidification technique and 3D X-ray microscope. It was found that the small sized pore can be more easily coalescence with a large sized pore. Theoretical analysis shows the gas pore pressure drops significantly during the growth of pore. For a coarse pore, the drops of pore pressure will result in a backflow of the melt from solidification interface into gas pore, and this will result in a "bamboo" like structure. For small gas pores, the pressure difference between two neighboring gas pores drives the hydrogen gas from the short one to the long one. This is the reason that a short pore more easily coalesce with a long pore.The compression property of porous Cu-xCr alloy depends on the Cr content, porosity, pore diameter and the loading directions. The compressive sthength increases with Cr content when the loading direction is parallel with pore diretion, and direcases with increasing porosity and pore diameter. The compression properties show obvious anisotropy, and depend on the effective loading area, the crystal orientation of the matrix.