Fabrication Of Porous Iron Under Nitrogen Atmosphere By Zone Melting And Mechanism Of Pore Formation

Gasar is a directional solidification technique based on the large gap in gas absorption between the solid-liquid phase of the metal and the control of the solidification process parameters to obtain a regular pore arrangement structure.Compared to porous metals prepared by conventional methods(sintering,foaming,etc.),the porous materials produced by the Gasar process have excellent mechanical properties along the length of the pores besides being lightweight,sound absorbing and vibration damping.In order to obtain the desired structure of the porosity,the researchers conducted various studies,such as controlling the solidification process parameters,designing the alloy composition and adjusting the experimental equipment.However,conventional preparation processes(mould casting)are unable to prepare low thermal conductivity alloys,such as stainless steel.Although the Nakajima group in Japan has successfully prepared porous stainless steel using a zone melting process,the working gas during the experiments was hydrogen at a pressure of up to 25 atmospheres,which is dangerous.Therefore,in this paper,the dissolution and precipitation behavior of nitrogen in pure iron was investigated by preparing porous pure iron under a nitrogen atmosphere using a zone melting method with Fe,the main element in stainless steel,as the raw material.The details are as follows.Based on the group's own design,installation and testing of the zone melting equipment,the porous pure iron was successfully prepared under nitrogen atmosphere,which improved the safety of the porous metal material preparation process.The solubility of nitrogen in pure iron at different temperatures and air pressures was calculated,and the effect of solidification process parameters on the pore structure was investigated.The nucleation and growth process of nitrogen pores in pure iron was investigated based on the classical nucleation theory for the phenomenon of the first increase and then decrease of the porosity of cylindrical pores.The nitrogen pressure provides both the driving force for bubble nucleation—the difference in nitrogen solubility between the solid and liquid phases,and also acts as a resistance to bubble growth.In spite of the regularly shaped and oriented cylindrical pores,we also found more uniformly distributed spherical pores(0.15-12 ?m)in the specimens.As the nitrogen pressure increases,the nucleation position of the spherical pores slowly shifts from the grain boundaries to the inside of the grain.Bubble nucleation at low nitrogen pressures is dependent on the energy present at the grain boundaries and the high melting point heterogeneous nucleation core;at higher nitrogen pressures,the driving force caused by the difference in nitrogen solubility between the solid and liquid phases is sufficient for bubble nucleation,so there is no significant concentration of distribution.Based on the Sievert's law and the ideal gas equation,a simple porosity model for the precipitation of spherical pores in the solid phase was established and the model agrees well with the measured values.A model was established for the relationship between extraction speed and pore diameter and pore spacing based on the principle of melt thermodynamics.The model predictions are in good agreement with the experimental values and the overall trend is consistent.The model predictions are in good agreement with the experimental values and the overall trend is consistent.At lower specimen withdrawal speeds,the predicted values of pore spacing are slightly less than the experimental values due to melt convection at the solid-liquid interface and tumbling of the melt zone caused by the induction coil.