A method of electrochemical polishing of 316L stainless steel by additive manufacturing using a solid medium with current feedback control

Abstract

The application discloses a kind of solid medium electrolytic polishing methods of current feedback control of additive manufacturing 316L stainless steel, the method uses porous particle of adsorbed sulfuric acid electrolyte as solid medium, and is electrolytically polished to workpiece under the action of pulse power;By real-time monitoring polishing current, and it is compared with the preset current threshold based on polishing area and preset current density threshold, when current is below the preset current threshold for two times and more than once, trigger feedback mechanism intermittent replenishment deionized water, to realize the dynamic control of polishing process.The method can compensate the water loss of solid medium caused by heating in the polishing process, improve the surface quality and polishing effect consistency of additive manufacturing 316L stainless steel after continuous polishing.

Description

Technical Field

[0001] This invention relates to the field of metal surface treatment, and in particular to a current feedback controlled additive manufacturing method for electrolytic polishing of 316L stainless steel solid dielectric. Background Technology

[0002] 316L stainless steel, a typical low-carbon austenitic stainless steel, possesses excellent corrosion resistance, good processing properties, and biocompatibility. Due to its stable performance even under complex service environments, 316L stainless steel is widely used in aerospace, medical devices, and chemical industries.

[0003] In recent years, metal additive manufacturing technology has become an important development direction in the field of advanced manufacturing, and a key means to achieve integrated manufacturing of complex structural components. This technology uses energy beams to melt powder or filament materials and stack them layer by layer to form a shape, overcoming the limitations of traditional subtractive manufacturing in polishing complex curved surfaces, internal channels, and lightweight structures. However, due to the inherent process characteristics of additive manufacturing, the surface of additively manufactured parts generally exhibits a "step effect" and unmelted powder adhesion, resulting in high initial surface roughness, which in turn restricts their service performance. Therefore, it is necessary to polish their surfaces.

[0004] Common polishing methods mainly include mechanical polishing, laser polishing, chemical and electrochemical polishing, and solid-medium electrolytic polishing. Among them, mechanical polishing easily introduces microcracks, residual stress, and work hardening on the workpiece surface; laser polishing technology has poor geometric adaptability to complex morphologies and has high equipment and operating costs; chemical and electrochemical polishing usually uses strong acidic electrolytes, which easily generate harmful gases and industrial wastewater, facing severe environmental pressure. In contrast, solid-medium electrolytic polishing uses porous solid particles to adsorb the electrolyte as a conductive medium, which has advantages such as being environmentally friendly and suitable for polishing complex internal cavities, thus becoming an emerging direction in metal surface treatment.

[0005] Existing technologies, such as CN111032929A, CN115029768A, CN112534088A, and CN109415839A, disclose a series of electrolytic polishing methods and media formulations based on solid media. These methods primarily utilize porous resin or polymer particles as carriers, adsorbing a certain concentration of acidic or salt electrolyte to form a conductive solid media. Polishing of the metal surface is achieved through the contact and ion migration between the solid media and the workpiece surface. However, existing technologies mainly focus on the chemical formulation screening and material modification of the solid media in its initial state, with insufficient consideration given to the changes in the internal composition and conductivity of the media caused by electrothermal coupling during the actual polishing process. For additively manufactured 316L stainless steel with complex surface morphology, the actual polishing process often lasts for several hours. During prolonged continuous polishing, Joule heating is inevitable, and friction between particles and the workpiece also generates heat. These heat generation events accelerate the evaporation of moisture from the solid media. As moisture is continuously lost, the concentration and conductivity of the electrolyte in the solid medium will change dynamically, causing fluctuations in the polishing current and affecting the uniformity of the polished surface and the stability of the process.

[0006] To address the aforementioned water loss problem, some existing technologies attempt to use changes in the humidity of the polishing environment as a basis for replenishing electrolyte or water. However, different polishing parameters and workpiece structures lead to varying degrees of temperature rise, and porous particulate media experience heat accumulation during continuous operation, resulting in a non-constant water evaporation rate that exhibits non-linear characteristics. In this situation, changes in ambient humidity often lag behind changes in the medium's bulk state and are relatively unstable, making it difficult to accurately and in real-time reflect the actual ion transport state of the electrolyte in the solid medium. Consequently, it is challenging to guarantee the consistency and repeatability of polishing effects during continuous or large-scale polishing processes.

[0007] Therefore, there is an urgent need to develop a solid dielectric electropolishing method suitable for the complex surface morphology of additively manufactured 316L stainless steel. This method needs to be able to accurately and timely control the moisture loss of the solid dielectric caused by heat generation during the polishing process in order to maintain the dynamic stability of the polishing current, thereby improving the surface quality and polishing effect consistency of additively manufactured 316L stainless steel after polishing. Summary of the Invention

[0008] To address the problems existing in existing solid-dielectric electropolishing technology when processing additively manufactured workpieces, the present invention aims to provide a current feedback-controlled solid-dielectric electropolishing method for 316L stainless steel in additive manufacturing. This invention aims to solve the technical problems of poor polishing uniformity and insufficient consistency of polishing effect caused by the evaporation of moisture from the solid dielectric during the solid-dielectric electropolishing process of 316L stainless steel in additive manufacturing.

[0009] The technical solution adopted by this invention to solve the technical problem is as follows: a current feedback controlled additive manufacturing method for electrolytic polishing of 316L stainless steel solid dielectric, the specific steps of which are as follows:

[0010] Step 1: Provide a porous particulate medium with liquid storage capacity in a polishing container, add sulfuric acid electrolyte to the porous particulate medium, and allow the porous particulate medium to adsorb the sulfuric acid electrolyte to form a solid medium;

[0011] Step 2: Connect the additively manufactured 316L stainless steel workpiece to the positive terminal of the pulse power supply, and connect the cathode plate inside the polishing container to the negative terminal of the pulse power supply.

[0012] Step 3: Make the solid medium contact the surface of the additively manufactured 316L stainless steel workpiece, and drive at least one of the additively manufactured 316L stainless steel workpiece and the polishing container to move.

[0013] Step 4: Start the pulse power supply and set its output unidirectional rectangular wave pulse voltage to 20V~50V, duty cycle to 20%~70%, and pulse width to 50μs~5ms;

[0014] Step 5: During the polishing process, the current value in the polishing circuit is collected in real time;

[0015] Step 6: Compare the current value with a preset current threshold. When the current value is lower than the preset current threshold twice or more consecutively, start intermittently replenishing water to the solid medium until the current value rises back to be greater than or equal to the preset current threshold, then stop replenishing water.

[0016] Step 7: Continuously repeat steps 3 to 6 until the cumulative polishing time reaches 260 min to 360 min.

[0017] Preferably, in step 1, the mass percentage of the sulfuric acid electrolyte in the solid medium is 40% to 80%.

[0018] Preferably, in step 3, the relative frictional motion speed between the solid medium and the workpiece is 0.5 m / s to 1.2 m / s.

[0019] Preferably, in step 6, the preset current threshold is obtained by multiplying the polishing area by a current density threshold, wherein the current density threshold is 10 mA / cm². 2 ~35mA / cm 2 .

[0020] Preferably, in step 6, the water added to the solid medium is deionized water, and the amount added in a single replenishment is 0.5 mL to 5 mL.

[0021] Preferably, in step 6, the interval between intermittently replenishing the solid medium with water is 1s to 15s.

[0022] Specifically, compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention uses current feedback to dynamically control the polishing process. Compared with existing methods based on humidity or experience, it can more accurately and in real time reflect the electrochemical state of the interface, thereby maintaining the water content of the solid medium in the optimal range, avoiding electrolytic failure caused by blind water replenishment, achieving uniform removal of complex surface morphology, improving the surface quality and polishing effect consistency of 316L stainless steel after continuous polishing in additive manufacturing, and has good engineering application value. Attached Figure Description

[0024] Figure 1 This is a scanning electron microscope image of the workpiece before polishing in this embodiment;

[0025] Figure 2 This is a scanning electron microscope image of the workpiece after polishing in this embodiment. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] In this embodiment, the workpiece to be polished is an additively manufactured 316L stainless steel sample formed using selective laser melting technology, and its surface morphology after forming is as follows: Figure 1 As shown. The sample was not pretreated by grinding or sandblasting, and its initial surface roughness Ra was measured to be 8.552 μm using a portable surface roughness measuring instrument.

[0028] The electrolytic polishing hardware system used in this embodiment includes a polishing container with an insulating liner. Acid-resistant stainless steel cathode plates, tightly fitted to the container wall, are continuously arranged circumferentially on the bottom and inner wall of the polishing container. The workpiece to be polished is fixed to the upper end of an insulating rotating spindle by a clamp. After clamping, the distance between the workpiece and the cathode plate is maintained at 5 cm, and the distance between the workpiece and the axis of the rotating spindle is also 5 cm. The clamp is connected to the positive terminal of a pulse power supply via a conductive slip ring, and the cathode plate is connected to the negative terminal of the pulse power supply via a conductive metal roller. The pulse power supply can output single-pulse or double-pulse signals, and its pulse voltage, duty cycle, frequency, and pulse width can be independently adjusted. A constant voltage control mode is used during the polishing process. The system's power unit can drive at least one of the workpiece and the polishing container to move, thereby achieving controllable relative motion between the workpiece and the solid medium. In addition, a current sensor connected in series in the anode polishing circuit is also provided in the system for real-time acquisition of the circuit current.

[0029] The porous particulate medium used in this embodiment is a commercially available macroporous strong acid cation exchange resin. The matrix of the resin is polystyrene-divinylbenzene copolymer, the functional group is sulfonic acid group, and the particle size is 0.4mm~0.6mm. In order to ensure the consistency of liquid absorption rate of different batches of medium, before adsorbing electrolyte, the ion exchange resin is placed in a constant temperature drying oven and dried at 80°C for 8 hours. After cooling to room temperature, it is sealed for later use.

[0030] The sulfuric acid electrolyte used in this embodiment is a 1% sulfuric acid aqueous solution prepared by diluting 98% concentrated sulfuric acid with deionized water.

[0031] A current feedback controlled additive manufacturing method for electropolishing of 316L stainless steel using solid dielectric is described below:

[0032] Step 1: Add the macroporous strong acid cation exchange resin, which has been pretreated by constant temperature drying at 80℃ for 8 hours, to the polishing container; atomize a 1% sulfuric acid aqueous solution and spray it into the ion exchange resin intermittently, while continuously stirring during the spraying process, so that the ion exchange resin uniformly adsorbs the sulfuric acid aqueous solution to form a solid medium; control the mass percentage of the sulfuric acid aqueous solution in the solid medium to be 45% by weighing;

[0033] Step 2: Clamp the additively manufactured 316L stainless steel workpiece to be polished in the fixture and use it as the anode, wherein the polishing area is 25cm². 2 The fixture is connected to the positive terminal of the pulse power supply via a conductive slip ring, while the acid-resistant stainless steel cathode plate inside the polishing container is connected to the negative terminal of the pulse power supply via a metal roller.

[0034] Step 3: Make the solid medium completely cover the workpiece, and make the top surface of the solid medium 3cm higher than the highest point of the workpiece surface to be polished; start the power device to drive the workpiece's insulated rotating spindle to rotate clockwise, and at the same time drive the polishing container to rotate counterclockwise, so that the solid medium and the workpiece surface to be polished form a relative frictional motion of 1m / s.

[0035] Step 4: After the relative friction motion reaches a stable state, start the pulse power supply, set the output unidirectional rectangular wave pulse voltage to 50V, the duty cycle to 20%, and the pulse width to 65μs, and start the electrolytic polishing operation.

[0036] Step 5: During the polishing process, the instantaneous current value in the polishing circuit is collected in real time using a current sensor connected in series in the anode polishing circuit.

[0037] Step 6: Pre-select a current density threshold of 16 mA / cm² and multiply it by the polishing area to obtain a preset current threshold of 0.4 A; continuously compare the real-time collected current value with the preset current threshold. When the current value collected twice or more consecutively is lower than 0.4 A, it is determined that local moisture loss has occurred in the solid medium; then, intermittently spray deionized water onto the solid medium at 6-second intervals, with each spray being 3 mL, until the real-time collected current value rises back to greater than or equal to 0.4 A, at which point the spraying of deionized water is stopped.

[0038] Step 7: While maintaining the relative motion state and electropolishing parameters described in Steps 3 and 4, continue to execute Steps 5 and 6 until the cumulative polishing time reaches 340 min; then turn off the pulse power supply and stop the power drive system; remove the workpiece from the fixture, place it in deionized water for ultrasonic cleaning for 5 min, and after cleaning, dry it at a constant temperature and seal it for storage.

[0039] To evaluate the polishing effect of the workpiece surface, scanning electron microscopy tests were performed on the workpiece before and after polishing. Figure 1 This is a scanning electron microscope image of the workpiece before polishing in this embodiment. Figure 2 This is a scanning electron microscope (SEM) image of the workpiece after polishing in this embodiment. Figure 1 It is evident that the surface of the additively manufactured 316L stainless steel before polishing has unmelted or semi-melted spherical metal powder adhering to it, the underlying layer exhibits a wavy molten pool texture, and the surface has obvious undulations; Figure 2 As can be seen, after processing by the method of this embodiment, the spherical metal powder and wavy protrusions attached to the surface of the workpiece are uniformly removed, and the surface tends to be flat.

[0040] Furthermore, the surface roughness of the additively manufactured 316L stainless steel workpiece after solid dielectric electropolishing was tested using a surface roughness measuring instrument. After treatment by the method of this embodiment, the surface roughness Ra of the additively manufactured 316L stainless steel workpiece was reduced from 8.552μm to 0.901μm.

[0041] The results show that the method of the present invention can adapt to the complex surface morphology of additive manufacturing 316L stainless steel. By maintaining the dynamic stability of the polishing current, it can achieve high-quality polishing of the surface of additive manufacturing 316L stainless steel workpieces and ensure the consistency of the polishing effect.

Claims

1. A current feedback controlled additive manufacturing method for electrolytic polishing of 316L stainless steel solid dielectric, characterized in that, Includes the following steps: Step 1: Provide a porous particulate medium with liquid storage capacity in a polishing container, add sulfuric acid electrolyte to the porous particulate medium, and allow the porous particulate medium to adsorb the sulfuric acid electrolyte to form a solid medium; Step 2: Connect the additively manufactured 316L stainless steel workpiece to the positive terminal of the pulse power supply, and connect the cathode plate inside the polishing container to the negative terminal of the pulse power supply. Step 3: Make the solid medium contact the surface of the additively manufactured 316L stainless steel workpiece, and drive at least one of the additively manufactured 316L stainless steel workpiece and the polishing container to move. Step 4: Start the pulse power supply and set its output unidirectional rectangular wave pulse voltage to 20V~50V, duty cycle to 20%~70%, and pulse width to 50μs~5ms; Step 5: During the polishing process, the current value in the polishing circuit is collected in real time; Step 6: Compare the current value with a preset current threshold. When the current value is lower than the preset current threshold twice or more consecutively, start intermittently replenishing water to the solid medium until the current value rises back to be greater than or equal to the preset current threshold, then stop replenishing water. Step 7: Continuously repeat steps 3 to 6 until the cumulative polishing time reaches 260 min to 360 min.

2. The current feedback controlled additive manufacturing method for electrolytic polishing of 316L stainless steel solid dielectric according to claim 1, characterized in that, In step 6, the preset current threshold is obtained by multiplying the polishing area by the current density threshold, where the current density threshold is 10 mA / cm². 2 ~35mA / cm 2 The water is deionized water, and the interval between intermittent replenishment of water to the solid medium is 1s to 15s, with a single replenishment volume of 0.5mL to 5mL.

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