The invention relates to an electropolishing method, in particular for metal components produced in 3D printing, wherein at least one component to be machined functions as a first electrode and at least one second electrode is provided as a counter electrode, and at least a partial removal of a portion of the component surface occurs in an electrolyte bath by applying current to the component.

The smoothing of metal surfaces is effected in industrial production by different methods. The final choice of method depends on the initial roughness, and a basic distinction is made between mechanical, chemical and electrochemical methods.

In the electrochemical method of electropolishing the removal is effected by the electrolytic dissolution of metal ions from the workpiece surface, with the purpose of eliminating micro-roughnesses. Electropolishing also serves for deburring, polishing and passivation. A significant advantage of electropolishing is that the workpiece is subjected to virtually no mechanical stress. Hence, this method is particularly suitable for small and delicate parts of almost any size and any shape.

During electropolishing, the workpiece is immersed in an electrolyte, and by applying current, the material on the workpiece (anode) is removed. The metal ions in the electrolyte thereby dissolve, and an inert metal sheet usually serves as the counter electrode (cathode).

For electropolishing, a basic principle to be noted is that the base material is not attacked (etched or stained), and a significant attack on/removal of material does not occur. For conventional electropolishing, the anodic potential is chosen such that it is in the transpassive range of the current-voltage curve. In this range, a so-called “polishing film” forms at the surface of the part, which is responsible for a homogeneous removal of the metal and levels the surface.

During electropolishing, the parts are exposed neither to mechanical nor to high thermal stresses and do not bump against each other. The results can be reproduced with high precision.

In this regard, numerous methods in particular for milled or punched parts are known from the prior art. They are used mainly for health technology and in the food industry. In most cases, standard electrolytes are used for this, which are adapted to the respective material, usually stainless steel, more rarely aluminium or other light metals. In this case, the removal required for the smoothing is controlled only via the composition of the electrolyte or controlled in terms of potential using a direct current.

Furthermore, pulse methods for electropolishing have become known, wherein the desired removal is adjusted via rectangular pulses in repeating pulse sequences.

WO 2014/018171 A1 describes a method for the electrochemical polishing of superconducting high-frequency cavities using pulse methods. A similar method of deburring and electropolishing stents is disclosed in WO 2005/066395 A2. Other methods of electrochemical polishing using pulse methods can also be learnt from WO 2003/088316 A2, WO 2003/046262 A or U.S. Pat. No. 6,558,231 B1.

The methods, which have become known, have the disadvantage that they are not very suitable for workpieces with a heterogeneous formation of surface roughnesses, i.e., areas of high roughness and areas of low roughness on a single workpiece, since each of those individual methods is optimally suited only for a certain roughness range.

It is therefore the object of the invention to provide a method which allows the treatment of the machining of metal parts with different levels of surface roughness and the adjustment of a defined surface roughness.

According to the invention, this object is achieved in that at least one anodic pulse is provided, the current intensity of which displays a steady increase over the course of time up to a specifiable value, wherein the increase is preferably linear, non-linear or exponential. Hence, this anodic pulse does not display a rectangular shape, as it is conventional in the prior art, but its current intensity increases in the course of the pulse duration so that no sudden, but a steady increase in the current intensity in the form of a non-linear slope or a linear ramp over the pulse duration is obtained.

The pulse shape, in particular the speed of the increase and, if applicable, the drop of the current, are responsible for the time course of the formation and the degradation of the polishing film. By selectively controlling the time course of the pulse slope, this essential part of the polishing process can be performed optimally. By controlling the temporal structure of the polishing film, it becomes possible to pass through the areas etching—passivation—polishing during electropolishing in a planned and reproducible manner and, thus, to selectively control the material removal. For metal parts with high roughness, the etching phase can thus be extended by a slowly rising ramp or, respectively, a stepped pulse, thus shortening the process time and improving the result.

In a preferred embodiment of the invention, the pulse displays micropulses subsequent to the increase. In this case, higher-frequency micropulses are “superimposed” on the pulse, the so-called base pulse. The effect of those micropulses is that the polishing process is accelerated, since those micropulses have a disproportionately strong impact at tips and edges in those places due to the field line concentration. In this way, the local current distribution across the workpiece is furthermore influenced as a function of the frequency of the micropulses, which, in turn, is essential for the smoothing effect of the method according to the invention.

Since the electrolytic double-layer capacitance has locally different values on a rough and chemically inhomogeneous surface, as it is usually provided, for example, in a 3D-printed part, the distribution of the current at higher frequencies is different than at lower ones. The reason for this is that the effective resistance of a capacitance decreases as the frequency of the applied current increases so that the higher-frequency portions of the current pulse flow preferentially over places of high capacitance, while the low-frequency portion flows through areas having a low polarization resistance. Thus, the local current distribution on the workpiece can be influenced significantly by the pulse shape.

According to the invention, it is preferably provided that at least one second anodic pulse adjoins the at least one anodic pulse, wherein the at least second pulse is equal to or different from the first pulse, and preferably at least two consecutive anodic pulses and/or pulse sequences are interrupted by a pulse pause and/or by at least one cathodic pulse. Those pulse sequences may exhibit the same or different repeating pulses. In this connection, it is essential that at least one anodic pulse exhibits the steady increase according to the invention, wherein said anodic pulse particularly preferably initiates the pulse sequence, i.e., is located at the beginning thereof.

In a further embodiment of the invention, the pulse frequency may vary with the duration of the method according to the invention due to the progressive smoothing of the component. In this case, the control of the frequency is another influencing value for controlling the removal rate in case of an inhomogeneous initial roughness and usually ranges between 0.2 and 2000 Hz.

Overall, it should be noted that the control of the current or, respectively, voltage signal of the pulses as well as the control of pulse lengths, pulse slopes and pulse pause phases allow the polishing effect to be controlled. In this case, the effect of the pulse slopes is based, on the one hand, on the temporal structure of the effective polishing film, which is controlled by the ramp steepness, and, on the other hand, on the varying frequency spectrum of the pulses that are used.

In this connection, it is provided, depending on the application, that the pulses have an average current density of 0.5 A/dm2to 30 A/dm2and the pulses and/or the pulse pauses have a pulse length of 0.0005 s to 5 s.

Furthermore, the electrolyte may contain additional additives, in particular wetting agents, inhibitors and/or complexing agents. The electrolyte usually used in the method according to the invention has a temperature ranging from 5° C. to 90° C. and a pH ranging from 1 to 10. Similarly, it may be provided that the electrolyte is moved periodically or continuously. In the method according to the invention, it is provided in particular that inert cathodes, particularly titanium cathodes, lead cathodes, steel cathodes or stainless-steel cathodes, are used as counter electrodes.

In the method according to the invention, an electrolyte is preferably provided which is selected from the group comprising an electrolyte based on mineral acids or organic acids or a mixture thereof.

Alternatively, the electrolyte can be constructed on the basis of alkalis such as caustic soda, caustic potash or ammonia or, respectively, on the basis of basic organic compounds such as amines.

Upon completion of electropolishing, a post-treatment and/or a coating step preferably follow(s) subsequently. The post-treatment usually involves the removal of electrolyte residues and reaction products and, optionally, a drying of the machined component.

By adjusting the desired surface roughness by means of the electropolishing according to the invention, the machined component is prepared especially for a subsequent coating step. In particular, this coating step may include methods which comprise electroplating, PVD or CVD coating, polymer coating or plasma electrolytic oxidation.

The method according to the invention has proved its worth notably in the use for metal components produced in 3D printing.

In this case, a cleaning occurs in a first step, electrochemical polishing in a second step, and a post-treatment of the component in a third step.

In addition to the removal of contaminants, in particular oils, fats and oxides, the cleaning furthermore comprises a mechanical pre-polishing in case of higher roughnesses and inhomogeneous topographies and/or staining of the component in order to achieve a more homogeneous removal.

In a particularly preferred embodiment of the invention, a detection and digitization is initially provided before the component to be machined is cleaned. Subsequently, the component is treated by means of wet blasting, preferably with the aid of a jet system with rigid or movable nozzles, in order to remove any supporting structures. This is followed by a grinding step in particular by means of vibratory finishing or, respectively, drag finishing. Subsequently, electrochemical polishing of the prepared component is effected.

Upon completion of electropolishing, additional plasma polishing is required in some areas of application, which may be followed by further post-treatment steps and optionally coatings.

In this case, the above-described steps for machining, in particular, 3D-printed components may partly occur also simultaneously, that is, in one process step.

After detecting the geometry and surface finish of the component to be machined at a first station, the supporting structures, if present, are removed at a second station by wet blasting under potential, and/or the macro-roughness of the surface is reduced by vibratory finishing in order to achieve a suitable surface for the further step of electropolishing.

Prior to the electropolishing, the parts are cleaned in a stain and then rinsed with water. The pre-cleaned parts were then immersed in the electropolishing solution and anodically switched. In doing so, the operating parameters during electropolishing (electropolishing solution, temperature, current signal) are adapted to the respective substrate material.

The resulting surfaces meet the requirement in terms of surface roughness for the given application, further machining is not required therefor. However, depending on the application, a further functionalization of the surface in the form of a coating or an additional surface treatment may occur.

The method according to the invention is suitable in particular for components made of steel, nickel-based alloys, aluminium alloys, magnesium alloys or titanium alloys.

