Enhancing Surface Finish of 3D Printed Metal Microchannels via Electrolytic Polishing

The study focused on the electrolytic polishing of the inner surfaces of stainless steel pipe fittings, specifically for additively manufactured slender inner flow channel structures. It involved designing the tool cathode, creating an insulating fixture, constructing an electrolytic polishing device, and selecting and adjusting process parameters. This research ultimately enhanced the quality of the inner surface of the product.

1 Introduction

Additive manufacturing technology operates on the principle of layer-by-layer melting and accumulation. This method allows for the creation of workpieces with highly complex geometric shapes, offering solutions for the production of intricate, lightweight, and integrated components. Its application in aerospace engines can significantly reduce both the production cycle and costs associated with engine manufacturing. However, additively manufactured workpieces often exhibit relatively poor surface quality, making them unsuitable for direct engineering applications.

To achieve a flat and smooth surface, enhance physical and mechanical properties, and improve the durability of these workpieces, various surface treatment methods have been developed.

Electrolytic polishing has gained popularity in surface polishing, particularly for the inner flow channels of additively manufactured components. This technique is favored for its high processing efficiency, absence of tool wear, lack of surface hardening layers, elimination of internal stress, corrosion resistance, and compatibility with various material hardness levels. Currently, electrolytic polishing is crucial in industries such as aerospace, medical equipment, and automotive, where strict surface quality control is essential to ensure product reliability.

2 Current status of research on electrolytic polishing technology

LASSELL et al. demonstrated that electrolytic polishing of electron beam melted titanium alloy workpieces 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 Ti-6Al-4V alloy formed through additive manufacturing in an electrolyte containing ethylene glycol. Their findings indicated that the best surface results were achieved in a 0.4 mol/L chloride electrolyte, resulting in a 75.04% reduction in surface roughness.

Chang Shuai developed a new polishing method that combines overpotential electrochemical polishing (OECP) with electrochemical polishing (ECP), effectively achieving high-quality smoothing of the internal surfaces of complex, specially shaped pipes produced by selective laser melting (SLM).

LOHSER J. electrolytically polished two different 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 workpiece surfaces became smooth and bright. Notably, there was no difference in compressive strength compared to the workpieces before polishing.

3 Electrolytic polishing technology theory

Electrolytic machining is a process that utilizes the principle of electrochemical reactions to dissolve metal in an electrolyte, allowing for the removal of material and the shaping of a workpiece into a specific form and size.

Electrolytic polishing, also known as electrochemical polishing, operates on the same principle as electrolytic machining. In this process, the workpiece is connected as the anode and the tool as the cathode, both submerged in an electrolyte solution. A direct current (DC) voltage is applied between the anode and cathode. Due to the higher current density at microscopic protrusions on the surface of the workpiece, these areas dissolve more quickly than the depressions, effectively leveling the surface of the workpiece.

The key differences between electrolytic polishing and electrolytic machining are that the voltage and current values used in polishing are lower, and the distance between the tool cathode and the workpiece anode is greater in polishing. Typically, electrolytic polishing can employ acidic solutions, such as composites of sulfuric acid, phosphoric acid, and chromic anhydride, or neutral solutions like sodium chloride or sodium nitrate. While acidic solutions are effective, they are also expensive and can be highly corrosive to equipment, which is why neutral solutions are commonly used in actual production.

3 Electrolytic polishing technology theory

4 Experiment and research

4.1 Research purpose

The electrolytic polishing of metal workpieces with slender inner flow channels in additive manufacturing presents several challenges. During the polishing process, it is crucial to ensure that the electrolyte circulates smoothly within these narrow channels, effectively removing electrolytic byproducts and the heat generated by the process. Additionally, it is important to maintain proper insulation and fixation between the tool cathode and the workpiece anode to prevent direct contact, which could lead to an electrical short circuit.

This study aims to develop a method for the electrolytic polishing process of metal workpieces with slender inner flow channels in additive manufacturing. It includes the design of the tool cathode and insulating fixture, the construction of the electrolytic polishing device, and the selection and adaptation of process parameters. The goal is to provide technical support for further exploring the application of electrolytic polishing technology to improve the inner surface quality of additive manufacturing products.

4.2 Experimental preparation

(1) Preparation of test pieces

Stainless steel pipe fittings with slender inner flow channel structure were selected on site (see Figure 1), with an inner diameter of 8 mm and a length of about 600 mm, made of 1Cr18Ni9Ti.

(2) Tool cathode preparation
Based on the structural characteristics of the workpiece, the inner core of the copper core wire was chosen to serve as the tool cathode. Copper core wire offers excellent conductivity and is made from a pliable material, allowing it to be 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 depicted 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 matching it with the inner hole of the workpiece, a copper core wire with a specification of 2.5 mm² was selected to create 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 moved and cannot come into contact with the workpiece during the electrolytic polishing of slender inner flow channels. Therefore, it is essential to design an insulating fixture that allows for the uninterrupted flow of the electrolyte. In this experiment, an insulating fixing gasket made of nylon was used to cover the outer surface of the copper core wire, providing physical isolation between the cathode and the anode.

The insulating fixing gasket and its installation method are illustrated in Figure 3. The central hole accommodates the tool cathode (copper core wire), 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 ensure that the cathode does not contact the workpiece during processing, which could lead to short circuit burns.

Figure 4 shows a simulation of the processing area after the tool cathode and the workpiece have been installed and secured.

(4) Preparation of electrolyte circulation system

A neutral sodium nitrate electrolyte is selected for the process. To accelerate ion exchange in the reaction zone, a circulation pump is utilized. This pump helps reduce concentration polarization, improves reaction efficiency, and ensures the effective discharge of gas and electrolysis byproducts during processing. Based on the electrolyte medium and the dimensions of the inner flow channel, we have chosen the anti-corrosion magnetic drive circulation pump MP15R.

We designed the connection tooling 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 becomes filled with electrolyte. The connection of the electrolyte circulation pump is illustrated in Figure 5.

4.3 Test process

(1) Parameter Settings for Processing
For the current equipment, the processing voltage is set to approximately 20V. The electrolyte concentration is established at 10% to 16%, and the temperature is maintained at room temperature. During trial processing, the length of the workpiece is set to about 450mm, and the processing time is around 10 seconds. The specific parameters for formal processing will be determined based on the results of the trial processing.

(2) To adjust the insulation fixture, first secure the tool as the cathode and the workpiece as the anode, as illustrated in Figure 6. Next, connect the electrolyte pump between the processing area and the electrolyte tank. Turn on the electrolyte pump to ensure that the electrolyte flows properly, and then turn off the pump once the flow is established.

(3) Electrolytic polishing test
After installing the cathode and CNC machined aluminium parts, and connecting them to the positive and negative terminals of the power supply, turn on the electrolyte pump to ensure that electrolyte flows out from the anode outlet of the workpiece, as illustrated 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.

When machining commenced, the current reached a peak of 229A, far exceeding the copper core wire’s operating range of 33A. Simultaneously, smoke appeared at the electrolyte outlet corresponding to the workpiece. After approximately five seconds of machining, the machine was stopped immediately. The cathode of the tool was then removed for inspection, revealing that the insulation layer had burned, as shown in Figure 8.

The length of the workpiece was reduced to approximately 300 mm, while the other processing parameters remained unchanged. The processing current reached 153 A, and there was a pause in processing for about 10 seconds. Upon inspection after removing the tool cathode, it was discovered that the insulating pad was still burned.

(4) Optimize the tool cathode and process parameters.
Preliminary tests revealed that the electrolytic polishing of the flow channel in the φ8mm hole was unstable. The instability was primarily exhibited through two issues: ① The tool cathode overheated, causing the insulating layer to burn or even melt, accompanied by the vaporization of the electrolyte. ② The cathode and anode frequently short-circuited, resulting in burns. To address these issues, improvements to the tool cathode structure and material, as well as adjustments to the process parameters, will be necessary in the future.

The overheating of the tool cathode can be attributed to several factors, including high processing current density, inadequate heat dissipation in the processing area, and poor conductivity. To mitigate these effects, the current density can be reduced by lowering the processing voltage, promoting electrolyte circulation, enhancing heat dissipation in the processing area, and using more heat-resistant insulating gaskets to prevent the insulating layer from burning.

The melting point of polyvinyl chloride (used in the copper conductor sheath) ranges from 100-200°C, while nylon has a melting point of around 250°C, and polytetrafluoroethylene has a melting point of 327°C. In the experiment, a more heat-resistant polytetrafluoroethylene insulating gasket was directly placed over the copper core wire to create the tool cathode. Additionally, the number of electrolyte flow holes in the gasket was increased from four φ1.2mm holes to five φ1.2mm holes to enhance electrolyte circulation and accelerate heat exchange. To reduce the occurrence of short circuits between the cathode and anode, a 1.5mm² copper core wire (with a diameter of 1.38mm) was utilized. This not only increased the gap between the cathode and anode, thereby reducing the current density, but also enhanced the flexibility of the tool cathode. A depiction of the tool cathode, made with the insulating gasket and the 1.5mm² copper core wire, is shown in Figure 9.

The processing voltage was set at 10V, and the workpiece processing length was approximately 300mm. All other parameters remained the same as before. The processing current stabilized between 50-60A, and no abnormal phenomena were observed during processing. After 1 minute of operation, the CNC manufacturing process was halted, and the tool cathode was found to be intact with no signs of burning. An endoscope examination of the inner cavity of the workpiece revealed distinct polishing marks compared to the unprocessed tube wall. The surface roughness improved by more than one level, indicating a good polishing effect. A comparison of the endoscope images before and after electrolytic polishing is shown in Figure 10.

5 Conclusion

This paper examines an electrolytic polishing process for additively manufactured slender inner flow channel metal workpieces. Through testing stainless steel pipe fittings and optimizing the process, we achieved improvements in the inner surface quality of the products and arrived at the following conclusions:

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

2) When the inner flow channel has a complex shape—such as multi-pass configurations, elbows, and reducers—and is small in size, manufacturing the tool cathodes and insulating fixtures becomes challenging, with an associated risk of short circuits, which presents certain limitations.

3) For workpieces that require higher polishing accuracy, it is essential to precisely control the electrolyte composition, current density, and the original surface quality of the workpiece, which also imposes greater design demands on the tool cathode.

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