Abstract: In view of the characteristics of the influence of different rotational speeds on the flow field distribution of electropolishing nitinol cardiovascular stents, ANSYS software is used to simulate the flow rate distribution of electrolyte during electropolishing cardiovascular stents. The influence of electrolyte velocity field and eddy current field on the removal mechanism of electropolishing cardiovascular stents is analyzed, and the correctness of the simulation results is verified through experiments. The results show that with the increase of the rotational speed, the velocity field is evenly distributed, the eddy current changes are small, the electrolyte is updated in a timely manner, the electrolytic products can be discharged in a timely manner, the machining accuracy is improved, and the surface integrity of the polished cardiovascular stent is good; however, when the rotational speed is too high, the eddy current distribution area is significantly increased, and more gas precipitates, resulting in uneven and discontinuous velocity field distribution, low machining accuracy, and poor surface integrity of the polished cardiovascular stent.
With the development of the economy and society, the incidence of cardiovascular disease is getting higher and higher worldwide, so that its mortality rate has long ranked in the top three in disease mortality. With the continuous increase in the number of deaths from cardiovascular disease, the implantation of cardiovascular stents as the main treatment method has been more and more widely used. Nitinol alloy stands out among many metal materials because of its unique shape memory function, good biocompatibility and superelasticity, as well as its low density and corrosion resistance. It has become the material of choice for the manufacture of cardiovascular stents. The surface modification methods mainly include surface coating, surface oxidation, surface mechanical treatment, surface laser modification, chemical polishing, electrolytic polishing, etc.
As an implant, the surface integrity and accuracy of cardiovascular stents have an important impact on the quality of the stents. The smooth surface of the stents is not easy to scratch the human endothelium, and can effectively prevent the activation of platelets and affect the formation of blood clots. At present, electrolytic polishing has become the most effective means of surface modification of stents due to its unique advantages of short processing time, high production efficiency and good repeatability. However, during the polishing process of nitinol cardiovascular stents, the electrolytes in the processing gap are discharged only by the scouring of the electrolyte, so the flow rate of the electrolyte has a direct impact on the discharge of the electrolytes. In addition, the distribution of the flow field has an important influence on the stability of the electropolishing process, and there are many factors that affect the distribution of the flow field, such as electrolyte pressure, eddy current phenomenon, and cathode structure. Therefore, how to improve the stability and processing efficiency of electrolytic polishing, as well as to study the influence of the velocity field and the distribution of the eddy current field on the surface integrity of cardiovascular stents, has important theoretical and practical significance.
In this paper, the nitinol alloy cardiovascular stent after laser cutting is used as the research object. Solidworks is used to establish a three-dimensional model of the electrolytic polishing flow channel. Based on ANSYS software, the flow rate distribution of the electrolyte at different rotational speeds is simulated, and the influence of the electrolyte velocity field and eddy current field on the surface integrity of the cardiovascular stent is analyzed. Law. On this basis, the perchloric acid-glacial acetic acid system is selected as the basic system of the cardiovascular stent polishing solution, a self-designed electrolytic polishing device is used, and a single-factor test method is used to evaluate the surface roughness value of the polished nitinol cardiovascular stent. The optimal speed value of the electrolytic polishing cardiovascular stent is explored and the correctness of its simulation results is verified.
The geometric model of the flow channel of the electropolished nitinol alloy cardiovascular stent is simplified, and the flow rate distribution characteristics of the electropolished stent at different rotational speeds are simulated. The distribution characteristics of the electrolyte flow field at different rotational speeds are studied, and its influence on the surface integrity of the cardiovascular stent is further explored.
A model diagram of the processing principle of electropolishing cardiovascular stents, as shown in Figure 1：
During the electrolytic polishing process, the three-dimensional flow channel model of the electrolyte is composed of the outer contour of the cathode of the tool, the anode contour of the workpiece, the flow domain of the electrolyte inside the anode and the processing gap. Using the solving software ANSYS, and using the meshing mesh module that comes with its Workbench component system, the channel model is meshed. The number of mesh nodes is 48,626 and the number of units is 41,083. The standard k-e turbulence model is used. When applying this model, since it is only applicable to turbulent areas at a certain distance from the wall, the density of the mesh near the wall is larger. If the mesh density value is too small, the significant impact of the changing gradient on the flow cannot be captured during the solution process. The influence of the gradient on the flow is significant.
With the help of Solidworks modeling software and ANSYS solving software, the specific steps are as follows：
Through simulation analysis, a cloud map of the velocity distribution of the vertical cross-section of the polished area of the outer surface of the stents at different rotational speeds is obtained. The results are shown in Figure 2. Comparing Figure 2 (a)〜(d), it can be seen that the gradient value of the velocity field increases continuously with the increase of the rotational speed. The reason is that the electrolytic polishing product increases with the speed of rotation.
Nitinol alloy tube is selected as the workpiece material, with an outer diameter of 2.6mm and an inner diameter of 2.4mm. After laser processing, a cardiovascular stent with a length of 13mm, a porosity of 24%, and a rib width of 0.52mm is obtained. Since laser processing is thermal processing, it will cause thermal damage to the surface of the stent, causing a small amount of slag and oxide film to be attached to the surface of the stent, so the stent needs to be pretreated. First, use a configured mixed solution (HF:HCL: HNO₃:HOO=5:10:35:10) to ultrasonic clean the stent for 30s, then clean each with deionized water and acetone for 2 minutes, remove and blow dry and set aside.
The polishing device of the electropolishing cardiovascular stent includes a DC stabilized power supply, a polishing tank, an electropolishing clamping system and an electrolyte circulation system. The electrolyte circulation system adopts a miniature diaphragm self-priming pump, and a filter is installed to facilitate the circulation and filtration of the polishing liquid. During the test, the nitinol cardiovascular stent was installed in a polytetrafluoroethylene tube sleeve as the positive electrode of the workpiece connected to the power supply, and the self-designed ring-shaped 304 stainless steel tube was used as the negative electrode of the tool connected to the power supply. It was fixed on a base with a ring groove to ensure that the yin and yang poles are the same in all directions during operation, which is conducive to the uniform distribution of the flow field.
Using the self-developed electrolytic polishing device, a DC stabilized power supply with model number MS1003D and an electrolyte with a ratio of perchloric acid to glacial acetic acid were used for testing. The rotational speed n was selected as the variable parameter, and 20 sets of comparative tests were carried out. Among them, 4 different rotational speeds were mainly selected for comparative analysis. In order to reduce the test error, 5 sets of parallel tests were carried out for each rotational speed, and the average value of the results was taken, and the electrolyte was guaranteed to flow smoothly in the flow field area. , In order to complete the electrolytic polishing process of nitinol cardiovascular stent, the specific test parameters are shown in Table 1.
Stable processing is a requirement to ensure the surface quality and shape accuracy of the processed surface. Surface integrity is an important index to test the quality of electrolytic processing. The test uses the surface roughness value as the evaluation index of surface integrity. A white light interferometer was used to test the surface roughness of nitinol cardiovascular stents, of which 4 sets of test results are shown in Figure 4.
According to the measured 20 sets of test data, the specific surface roughness Ra values calculated are shown in Table 2.
Combining the analysis of the data in Figure 4 and Table 2, it can be seen that the surface roughness value of nitinol cardiovascular stents tends to decrease first and then increase as the speed increases. This is due to the electrolytic products produced during electrolytic polishing, which are discharged only by the scouring of the electrolyte. When the rotational speed is low, the flow rate of the electrolyte is low, and the electrolytic products generated during the polishing process are not easy to discharge, and will remain in the processing gap and have an adverse impact on the continuous polishing process. As the electrolytic products continue to gather on the surface of the stent, the surface roughness value of the stent after polishing will become larger and larger, resulting in poor surface integrity；
When the rotational speed is increased to 40r/min, the electrolyte update speed is fast, the electrolytes can be eliminated in time, the machining accuracy is high, and the polished stent surface roughness value is small and the surface integrity is good; and when the rotational speed continues to increase, due to the acceleration of the electrolyte flow rate, although the electrolytes can be discharged from the processing gap in time, but too fast liquid scouring, will cause ripples on the surface of the stent, the faster the flow rate, the more obvious the ripples, and the greater the surface roughness value, the worse the surface integrity of the stent.
The electrolyte flow velocity distribution of electropolished nitinol alloy cardiovascular stents at different rotational speeds is simulated. The influence laws of electrolyte velocity field and eddy current field on the surface integrity of the stent are studied, and the correctness of the simulation results is verified through experiments.