Precision Electropolishing Techniques for 3D-Printed Metal Microchannels

The electrolytic polishing test was conducted on the inner surface of stainless steel pipe fittings to examine a process for electrolytic polishing additively manufactured slender inner flow channel structures in metal workpieces. This study focused on the design of the tool cathode, the insulating fixture, and the construction of the electrolytic polishing device. It also involved selecting and adapting process parameters, which ultimately enhanced the inner surface quality of the products.

 

01 Introduction

Additive manufacturing technology utilizes a layer-by-layer melting and forming process to create workpieces with highly complex geometric shapes. This technology offers a solution for producing integrated, lightweight components. In the aerospace engine sector, it can significantly reduce both production cycles and costs. However, the surface quality of additively manufactured workpieces often falls short and is unsuitable for direct engineering applications. To achieve a flat and smooth surface, excellent physical and mechanical properties, and enhanced durability, various surface treatment methods have been developed.

One prominent method is electrolytic polishing, which is extensively used for surface polishing, particularly within the inner flow channels of additively manufactured components. This method is favored due to its high processing efficiency, lack of tool wear, absence of a surface hardening layer, no residual stress, corrosion resistance, and compatibility with all material hardness levels. Currently, in industries such as aerospace, medical equipment, and automotive manufacturing, where precise surface quality is crucial, electrolytic polishing has become a critical process to ensure product quality.

 

02 Research status of electrolytic polishing technology

LASSELL et al. demonstrated that electrolytic polishing of titanium alloy workpieces, which were melted using an electron beam laser and immersed in an anhydrous alcohol-based solution, significantly reduced surface roughness and improved fatigue strength. ZHANG Y.F. and colleagues investigated the electrolytic polishing behavior of the Ti-6Al-4V alloy, created through additive manufacturing, using an electrolyte that contained ethylene glycol. Their findings indicated that the optimum polishing surface was achieved with a 0.4 mol/L chloride electrolyte, resulting in a 75.04% reduction in surface roughness.

Chang Shuai developed a novel combined polishing method that incorporates overpotential electrochemical polishing (OECP) alongside traditional electrochemical polishing (ECP). This approach effectively produced a high-quality, smooth finish on the internal surfaces of complex, specially-shaped pipes created through selective laser melting (SLM). LOHSER J electrolytically polished two distinct 316L stainless steel thin disc lattice structure workpieces formed by SLM. The results showed that after 40 minutes of electrolytic polishing in a mixed electrolyte of phosphoric acid and sulfuric acid, the workpieces exhibited a smooth and shiny surface. Notably, comparisons of the workpieces before and after polishing revealed no differences in compressive strength.

 

03 Electrolytic polishing technology theory

Electrolytic machining is a process that utilizes the electrochemical reaction principle of metal dissolution in an electrolyte to remove metal materials and shape the workpiece to specific dimensions.

Electrolytic polishing, also known as electrochemical polishing, operates on the same principle as electrolytic machining. In this process, the workpiece acts as the anode and the tool serves as the cathode, both of which are immersed in an electrolyte. A direct current (DC) voltage is applied between the two electrodes. Due to the geometry of the surface, the current density at microscopic protrusions on the workpiece is higher than that at the depressions. As a result, the dissolution rate is faster in the protruded areas, effectively leveling the surface of the workpiece.

The key differences between electrolytic polishing and electrolytic machining are the voltage and current values used, which are lower in electrolytic polishing, and the greater distance between the tool cathode and the workpiece anode compared to electrolytic machining. In practice, electrolytic polishing can employ acidic solutions, such as composite solutions made of sulfuric acid, phosphoric acid, and chromic anhydride, or neutral solutions like sodium chloride and sodium nitrate solutions. While acidic solutions are effective, they are often expensive and highly corrosive to equipment, so neutral solutions are typically preferred in actual production scenarios.

 

04 Experiment and research

4.1 Research purpose

Electrolytic polishing of metal workpieces with slender inner flow channels in additive manufacturing presents significant challenges. To achieve an effective polishing process, it is essential to ensure smooth circulation of the electrolyte within the narrow flow channels and to quickly remove electrolytic by-products and heat generated during processing. Additionally, proper insulation and fixation between the tool cathode and the workpiece anode are necessary to prevent accidental contact between the two electrodes, which could lead to electrical short circuits.

The aim of this study is to develop a process for electrolytic polishing of metal workpieces featuring slender inner flow channels in additive manufacturing. This includes the design of the tool cathode and insulating fixtures, the construction of electrolytic polishing equipment, and the selection and adaptation of process parameters. The goal is to provide technical support for enhancing the inner surface quality of products made through additive manufacturing using electrolytic polishing technology.

 

4.2 Experimental preparation

(1) Stainless steel pipe fittings with slender inner flow channel structures were selected on-site (see Figure 1). The inner diameter is 8 mm, the length is approximately 600 mm, and the material is 1Cr18Ni9Ti.

 

(2) Tool Cathode Preparation

Based on the structural characteristics of the workpiece, we selected the inner core of copper core wire to create the tool cathode. The copper core wire is known for its excellent conductivity and softness, allowing it to be easily adjusted to fit the curvature of the flow channel.

For this test, three types of copper core wires were prepared: 4 mm², 2.5 mm², and 1.5 mm². Some actual samples are illustrated in Figure 2, and the current-carrying capacities of the copper core wires are listed in Table 1.

After measuring the outer diameter of the copper core and ensuring a proper fit with the inner hole of the workpiece, we selected a copper core wire with a specification of 2.5 mm² to manufacture the tool electrode. The diameter of the copper core is 1.78 mm, and its current-carrying capacity is approximately 33 A.

 

 

(3) Preparation of insulating fixtures

The tool cathode must be repositioned and cannot come into contact with the workpiece during the electrolytic polishing of the slender inner flow channel. Therefore, it is essential to design an insulating fixture that ensures the smooth flow of the electrolyte. In the experiment, an insulating gasket made of nylon was utilized to cover the outer surface of the copper core wire, creating physical isolation between the cathode and the anode. The insulating gasket and its installation method are illustrated in Figure 3. The central hole allows the tool cathode (copper core wire) to pass through, while the four surrounding holes facilitate the flow and circulation of the electrolyte. An insulating fixing gasket is installed on the copper core wire every 60 mm to prevent the cathode from making contact with the workpiece during processing, which could lead to short circuit burns. The simulation of the processing area, showing the setup of the tool cathode and the workpiece, is depicted in Figure 4.

 

(4) Preparation of electrolyte circulation system

A neutral sodium nitrate electrolyte is selected for the process, and a circulation pump is utilized to enhance ion exchange in the reaction zone. This helps to reduce concentration polarization, improve reaction efficiency, and ensure effective discharge of gases and electrolysis products during processing. Based on factors such as the electrolyte medium and the dimensions of the inner flow channel, an anti-corrosion magnetic drive circulation pump, specifically the MP15R model, is chosen.

The connection tooling is designed according to the pump’s interface size. The water inlet of the electrolyte pump is connected to the electrolyte tank using a PVC hose, while the water outlet is connected to the workpiece. Once the electrolyte pump is activated, the inner flow channel of the workpiece will be filled with the electrolyte solution. The configuration of the electrolyte circulation pump connection is illustrated in Figure 5.

 

4.3 Test process

(1) Processing parameter preset According to the existing equipment, the processing voltage is set to about 20V, the electrolyte concentration is set to 10%~16%, and the temperature is room temperature. During the trial processing, the workpiece processing length is set to about 450mm, and the processing time is set to about 10s. The specific parameters of the formal processing are determined based on the actual situation of the trial processing.

(2) Insulation fixture adjustment Fix the tool cathode and the workpiece anode, as shown in Figure 6, connect the electrolyte pump between the processing area and the electrolyte tank, turn on the electrolyte pump, ensure that the electrolyte flows well, and then turn off the electrolyte pump.

 

(3) Electrolytic polishing test

After installing the cathode and workpiece and connecting them to the positive and negative terminals of the power supply, activate the electrolyte pump to ensure that the electrolyte flows out of the anode outlet of the workpiece, as shown in Figure 7. Use a multimeter to check for any short circuit between the cathode and anode. If a short circuit is detected, readjust the cathode, anode, and tooling before proceeding with the processing.

 

During the machining process, the current surged to a high of 229A, which far exceeded the 33A working range of the copper core wire. Simultaneously, smoke was produced at the electrolyte outlet corresponding to the workpiece. The machine was stopped immediately after approximately 5 seconds of operation. Upon removing the cathode of the tool for inspection, it was discovered that the insulation layer had burned, as shown in Figure 8.

 

The processing length of the workpiece was reduced to approximately 300 mm, while all other parameters remained the same. The processing current reached 153 A, and the operation was halted for about 10 seconds. During this pause, the tool cathode was removed for inspection, and it was discovered that the insulating pad was still burned.

 

(4) Optimize the tool cathode and process parameters.

Through preliminary tests, it was observed that electrolytic polishing of the flow channel in the 8 mm diameter hole was unstable. This instability manifested in two primary ways: 1) The tool cathode overheated, causing the insulating layer to burn or even melt while simultaneously vaporizing the electrolyte. 2) There were occurrences of short circuits between the cathode and anode, resulting in burns. To address these issues, future improvements are needed in the tool cathode’s structure and materials, as well as adjustments to the process parameters.

The overheating of the tool cathode can be attributed to several factors, including high processing current density, inadequate heat dissipation in the CNC milling processing area, and poor conductivity. Current density can be decreased by reducing the processing voltage, enhancing electrolyte circulation, strengthening heat dissipation in the processing area, and utilizing more heat-resistant insulating gaskets to prevent the burning of the insulating layer.

The melting point of polyvinyl chloride (used in the copper conductor sheath) is 100-200°C, while nylon has a melting point of approximately 250°C, and polytetrafluoroethylene (PTFE) has a melting point of 327°C. In the experiments, a more heat-resistant polytetrafluoroethylene insulating gasket was placed directly on the copper core wire to create the tool cathode. An electrolyte flow hole was added to the gasket, increasing the number of holes from four (1.2 mm diameter) to five (1.2 mm diameter), which improved electrolyte circulation and enhanced heat exchange.

To mitigate the short circuit occurrences between the cathode and anode, a copper core wire with a cross-sectional area of 1.5 mm² (with a diameter of 1.38 mm) was used. This modification not only increased the gap between the cathode and anode, reducing the current density, but also enhanced the flexibility of the tool cathode. The design of the tool cathode, which incorporates the insulating gasket and the 1.5 mm² copper core wire, is shown in Figure 9.

 

The processing voltage was set at 10V, and the processing length of the workpiece was approximately 300mm, with all other parameters remaining the same as before. The processing current was consistently stable, ranging from 50-60A, and no abnormalities were observed during the process. Processing was paused for 1 minute, during which the tool cathode was examined and found to be intact, showing no signs of burning.

An endoscopic examination of the workpiece’s inner cavity revealed noticeable polishing marks when compared to the unprocessed tube wall. The surface roughness level was observed to have improved by more than one level, indicating effective polishing. The comparison of endoscopic images taken before and after the electrolytic polishing process is shown in Figure 10.

 

05 Conclusion

This paper examines an electrolytic polishing process for additively manufactured slender inner flow channel metal workpieces. Through testing of stainless steel pipe fittings and process optimization, we have improved the inner surface quality of the products and drawn the following conclusions:

1. The electrolytic polishing of additively manufactured slender inner flow channel metal workpieces can be successfully achieved using specially designed tool cathodes, insulating fixtures, and electrolytic polishing devices.

2. Polishing inner flow channels with complex shapes (such as multi-pass designs, elbows, and reducers) and small sizes presents challenges. The manufacturing of tool cathodes, insulation, and circulation fixtures can be difficult, increasing the risk of short circuits and imposing certain limitations.

3. For CNC metal parts requiring higher polishing accuracy, it is essential to precisely control the electrolyte, current density, and the original surface quality of the workpiece. This also demands higher standards in the design of the tool cathode.

 

 

 

If you want to know more or inquiry, please feel free to contact info@anebon.com

Anebon has numerous great personnel members who are good at advertising, QC, and working with troublesome dilemmas from the creation course of action for China Supplier. LST-106MY High Cost-Effectiveness aluminum die casting parts, milling aluminum parts with high-speed 5 axis machining.