Study On Hydrogen Solubility In Molten Alloys And Directional Solidification Of Porous Cu-Mn Alloy

Directional solidification of metal-gas eutectic, which is based on the gap of gas (mainly hydrogen) solubility between liquid and solid metals, is a novel process for fabricating regular porous metals with pores aligned in solidification direction. Because of its special property characteristics to that of conventional porous metals, this kind of porous metal has important potential applications. It's well known that alloys have more application potentials than pure metals. Howerver, it was found in many researches that pure metals with oriented pore structure could be produced successfully by this process, but alloys with similar pore structure could not be made. Comparing with pure metals, two critical difficulties exist in fabrication of directional solidification of porous alloys. One is that there exist neither abundantly documented results nor reliable calculation models for hydrogen solubility of alloys. Therefore, it is difficult to estimate whether an alloy is suitable for this process. The other is that the influence of solidification mode of alloys on growth of oriented pores has not been researched extensively. This thesis will be concentrated on these two aspects.Variation of hydrogen solubility in different molten metals was summarized based on documented results. It was found that the electron interaction between hydrogen and the molten metal is the most major factor in determining hydrogen solubility in molten metals. According to the nearly-free-electron theory, a model was proposed for calculating hydrogen solubility in molten metals.Based on an improved method for estimating activity coefficient of multi-component molten alloys, a thermodynamic model was proposed for calculating hydrogen solubility in molten alloys. Calculated results show good agreements with documented experimental results.Directionally solidified porous Cu-Mn alloy was fabricated successfully by common directional solidification method. It was found that with the increasing of solidification height, oriented pores gradually become irregularly aligned and finally interrupted, which was determined by the solidification mode of the Cu-Mn alloy. Furthermore, it was found that under cellular and columnar dendritic solidification mode, oriented pore structure could also be formed if the cellular and primary dendritic arm spacings are much smaller than the pore diameter. In contrast, when the solidification mode transforms to equiaxed dendritic, no oriented pore structure could be formed.By numerical simulation, it was found that along with the increasing of solidification height, both solidification velocity and temperature gradient at the solid-liquid interface decrease quickly but their ratio changes little. Furthermore, it was noted that it is a rapid cellular solidification at the beginning of solidification stage, and along with the solidification, the solidification mode transforms from rapid cellular solidification to columnar dendritic solidification and finally equiaxed dendritic solidification. Through increasing the mold preheating temperature and melt pouring temperature, the range of equiaxed dendrite could be decreased and the region of oriented pore structure could be extended, which coincides with experimental results.