Research On Optimization Of Fabrication Process And Three-Dimensional Reconstruction Of The Lotus-Type Porous Copper

Lotus-type porous copper is a porous metal structure prepared by directional solidification method.During the preparation process,the porous copper pores are cylindrical and along the direction of solidification in copper matrix because of the combination of directional solidification and metal-gas eutectic reaction.The pore structure is straight and uniform,the inner wall is smooth and so on.The unique structure of lotus-type porous copper makes it possible to prepare various special microchannel heat sink,which has an important application prospect in the field of heat dissipation.In this thesis,lotus-type porous copper was prepared by Gasar process.The effects of process parameters on pore morphology and structure parameters of lotus-type porous copper were studied theoretically and experimentally,and the process parameters were optimized.The drawing speed has an important influence on the pore morphology,pore growth direction and pore structure parameters of lotus-type porous copper,which is mainly related to the difference of solidification interface morphology at different drawing speeds.The solidification interface can be pushed forward in a flat interface only under proper drawing speed conditions,and thus the high quality lotus-type porous copper with flat porosity can be obtained.In this experiment,0.7mm/s is the appropriate drawing speed.When the drawing speed increases,the average pore diameter,through hole rate and average pore length of the lotus-type porous copper ingot decrease gradually,but the porosity remains basically unchanged.The non-destructive nano-CT scanning technique was used for CT scanning and three-dimensional reconstruction of lotus-type porous copper structure.The three-dimensional structure of lotus-type porous copper was analyzed,especially the change of pore nucleation was observed and studied,and the effect of drawing speed on the morphology and distribution of pore and nucleation was clarified.It is found that the nucleation of lotus-type porous copper does not start from the initial solidification of liquid metal,but only when the hydrogen in the melt reaches a certain supersaturation at the solidification interface.Hydrogen bubble nucleation is mainly affected by hydrogen supersaturation in melt at the solidification interface and solidification velocity.In the early stage of solidification,the solidification rate is large,a large number of bubbles nucleate in ashort period of time as long as the hydrogen concentration reaches the critical value.With the progress of solidification,the distance between the solid-liquid interface and the water-cooled copper crystallizer increases,the cooling effect weakens,the solidification rate decreases,the gas diffusion distance increases,and the number of pore nucleation decreases.For the prepared porous copper structure,the number of pore nucleation per unit volume increases with the increase of drawing speed,and the porous copper ingot's nucleation number tends to be stable faster in the speed of 0.55 mm/s and 0.7 mm/s.The pore structure of porous copper tends to be more uniform.With the increase of drawing speed,the number of new pores nucleation per unit volume increases,the mean diameter of pores decreases and the uniformity of stomatal distribution decreases.