Study On Copper Microtubes Electroforming Process And Properties

Copper（Cu）microtubes（the characteristic size of microtubes is usually<1 mm and wall thickness is usually<350μm）are in demand for their use in miniaturized equipment because of their excellent thermal conductivity and electrical conductivity.However,no matter from the technical point of view or from the perspective of manufacturing costs,microtubes cannot be produced on a large scale as conventional-sized tubes.Thus,new microtube-fabrication methods are necessary to explore.In this dissertation,a Cu-microtube electroforming process based on a plastic mandrel was proposed and explores the feasibility of electroforming microtubes;Based on the finite element method,the electroforming parameters,such as cathode current density and conductivity,which affect the uniformity of Cu microtubes,are analyzed numerically,and different electroformed is obtained The current density distribution near the cathode and the growth and uniformity of the electroforming layer were predicted.The effects of electroforming process parameters on the microstructure evolution and crystallization mechanism of Cu microtubes were studied.The effects of additives and power sources on the microstructures and mechanical properties of the microtubes were also investigated.The main conclusions are as follows:（1）The square Cu-microtubes with a wall thickness of 35-350μm and a cross-section length of less than 2mm was successfully fabricated via electroforming.With the help of centrifugal separation technology,the rapid and nondestructive separation of electroforming layer and mandrel is realized at room temperature;by changing the roughness of silver paint on the surface of mandrel,the roughness of the inner wall of microtubule is adjusted and controlled,and the roughness Ra of the inner wall is as low as～0.8μm,which is in line with the standard of waveguide level miniature pipe fittings.（2）The results of simulation were consistent with those of electroforming experiments.When the conductivity of Cu²⁺in electrolyte was low,the section of electroforming layer is easy to form the saddle structure of"high on edge sides,low on the middle";When the conductivity is high,the section of electroforming layer is easy to form the cap structure of"low on edge sides,high on the middle";When the current density is increased,the growth of the corner area of electroforming layer will be accelerated,thus forming the saddle structure;the distance between the anode and the cathode has little effect on the thickness uniformity of electroforming layer.（3）The thickness uniformity of Cu-microtubes can be further improved by applying pulse power and electroforming additives under appropriate conditions.Under higher conductivity conditions of the 60S/m,the thickness uniformity of the Cu-microtubes can be improved by adding gelatin and commercial additives cupracid;When the concentration of Cu²⁺was low,the microtubes obtained by pulse electroforming has excellent thickness uniformity.Considering the time cost,the complexity of the process and the uniformity of the thickness of the electroformed layer,under this experimental condition,the optimal electroforming process is a pulse duty cycle of 30%,a pulse frequency of 1000Hz,a Cu²⁺concentration of 0.1M,and a cathode current.The density is 2A/dm²,the distance between the cathode and anode is 2cm,and the electroforming solution does not contain any additives.In this process,the thickness of the micro-copper tube is good,and its CV is 3.8%.（4）The microstructure of Cu-microtubes varies with the current waveform,the type of additives and the current waveform.When there is no additive in the electrolyte,the columnar grain structure with<110>and<111>texture can be obtained by changing the current waveform.After adding gelatin,the DC and PC samples showed the preferred orientation of<111>and there were fine twins perpendicular to the growth direction in the columnar crystal.After adding commercial additives,the original coarse columnar grains were replaced by equiaxed grains with random orientation.When the concentration of copper ion is low,the microtubes with columnar nanotwinned can be prepared by high-frequency pulse power.The content of the nanotwinned was increased with the pulse frequency.（5）Based on the experimental results,the surface of columnar nanotwinned Cu has the typical morphology of the screw dislocation growth mechanism and periodic（111）twin structure.It is speculated that the growth mechanism of columnar nanotwinned Cu was through screw dislocation growth.Cu atoms are deposited on the most closely arranged surface（111）in the form of spiral stacking around the screw dislocation center.Under the joint influence of the principle of the lowest surface energy and the low stacking fault energy,the stacking fault then evolves into periodic twins in the direction of<111>to reduce the total energy of the system.Among them,the twin boundary formed during the growth of screw dislocation can provide re-entrant edge or 2D nucleation position,which is very important for the periodic continuity of twin structure.（6）The interaction among additives,the concentration of Cu²⁺and current waveform type affects the mechanical properties of Cu-microtubes.Compared with the samples without additives,the addition of commercial additives can greatly refine the grain size of the samples and improve the strength of electroformed microtubes.The addition of gelatin did not play a role in refining grains,but a large number of twin boundaries were introduced into the columnar crystals.In addition,the strength of the microtube was closely related to the texture and twin content.Among them,the samples with columnar nanotwinned structures show good strong plastic matching ability.The introduction of nanotwinned can improve the strength of the material without affecting the elongation as much as possible.However,the work hardening ability of the columnar nanotwinned Cu is closely related to the size of the columnar grain.The size of the columnar grain is smaller,and the work hardening ability of the material is weaker,which shows the decrease of the uniform elongation.