ELECTROPOLISHING METHOD

Abstract

A method for electropolishing a manufactured metallic article, the method comprising: contacting the metallic article with an electropolishing electrolyte; and electropolishing the metallic article in the electropolishing electrolyte through the application of an applied current regime comprising: at least one electropolishing regime, each electropolishing regime comprising a current density of at least 2 A/cm.sup.2 and a voltage comprising a shaped waveform having a frequency from 2 Hz to 300 kHz, a minimum voltage of at least 0 V and a maximum voltage of between 0.5 to 500 V.

Claims

1. A method for electropolishing a manufactured metallic article, the method comprising: contacting the metallic article with an electropolishing electrolyte; and electropolishing the metallic article in the electropolishing electrolyte through the application of an applied current regime comprising: at least one electropolishing regime, each electropolishing regime comprising a current density of at least 2 A/cm.sup.2 and a voltage comprising a shaped waveform having a frequency from 2 Hz to 300 kHz, a minimum voltage of at least 0 V and a maximum voltage of between 0.5 to 500 V.

2. A method according to claim 1, wherein each electropolishing regime is carried out with a current density of 2 A/cm.sup.2 to 200 A/cm.sup.2, preferably 20 to 50 A/cm.sup.2, more preferably greater than 25 A/cm.sup.2.

3. A method according to claim 1, comprising at least two, preferably at least three electropolishing regimes, wherein at least one of: the frequency, current density, or maximum voltage of each electropolishing regime is changed compared to the preceding electropolishing regime.

4. A method according to claim 3, wherein each successive electropolishing regime has at least one of: generally lower maximum voltage or generally lower current density than the preceding electropolishing regimes.

5. (canceled)

6. A method according to claim 3, wherein each electropolishing regime is applied for a duration of 1 to 30 s, preferably 2 to 20 s, more preferably 2 to 15 s.

7. A method according to claim 3, wherein the electropolishing step includes at least one cooling regime comprising lowering the current density following at least one electropolishing regime, the current density being preferably lowered to be 0.5 or less the current density of the preceding electropolishing regime.

8. (canceled)

9. A method according to claim 3, wherein the electropolishing step includes application of an applied current regime comprising: an initial pulse comprising a current density of at least 2 A/cm.sup.2 and a voltage comprising a shaped waveform having a frequency from 20 to 300 kHz, a minimum voltage of at least 0 V and a maximum voltage of between 50 to 500 V, and applied for a duration of at least 1 s; followed by at least two electropolishing regimes comprising a current density of at least 2 A/cm.sup.2 and a voltage comprising a shaped waveform current having a frequency from 2 Hz to 300 kHz, a minimum voltage of at least 0 V and a maximum voltage of between 0.5 to 500 V and wherein the frequency and/or maximum voltage of each electropolishing regime is changed compared to the preceding electropolishing regime.

10. A method according to claim 9, wherein the initial pulse has at least the following characteristics: a current density of 2 A/cm.sup.2 to 200 A/cm.sup.2, preferably 20 to 50 A/cm.sup.2, more preferably greater than 25 A/cm.sup.2; the voltage of the initial pulse is greater than the voltage of each of the successive electropolishing regimes; the current density of the initial pulse is greater than the current density of each of the successive electropolishing regimes; the applied frequency of the alternating voltage of the initial pulse is different to the applied frequency of each of the successive electropolishing regimes; or the initial pules is applied for a duration of 2 to 10 s, preferably 2 to 7 s, more preferably 2 to 5 s.

11.-14. (canceled)

15. A method according to claim 3, wherein each successive electropolishing regime has a different frequency.

16. A method according to claim 1, wherein the shaped waveform comprises at least one of: one of square wave, sinusoidal, pulsed, or a combination thereof; a pulsed width modulation (PWM) waveform, preferably a square wave pulse, preferably having a variable dead time; or a current having a frequency from 10 to 300 kHz, preferably 10 to 200 kHz, more preferably 20 to 100 kHz.

17.-18. (canceled)

19. A method according to claim 1, wherein the waveform comprises a change between the maximum voltage and the minimum voltage of at least one of: greater than 80% of the maximum voltage; or greater than 1 V; or greater than 5 V.

20. (canceled)

21. A method according to claim 1, wherein the metallic article comprises one of: a chromium containing metal alloy, titanium, a titanium alloy, nickel alloys, aluminium or an aluminium alloy; iron-chromium alloys, nickle-chromium (nickel.chrome), nickel-chromium alloys, cobalt-chromium alloys, or cobalt.chromium.molybdenum alloys; or a stainless steel or Inconel.

22.-24. (canceled)

25. A method according to claim 1, wherein the electropolishing electrolyte includes at least one of: H.sub.3PO.sub.4 phosphoric acid (H.sub.3PO.sub.4) in combination with sulfuric acid (H.sub.2SO.sub.4), hydrochloric acid (HCl) or combinations thereof, and one of water or a C.sub.1-C.sub.4 alcohol; or phosphoric acid (H.sub.3PO.sub.4) in combination with one of water or a C.sub.1-C.sub.4 alcohol.

26.-27. (canceled)

28. A method according to claim 1, wherein the pH of the electrolyte is from 1.0 to 7.0, preferably from 1.0 to 3.0.

29. A method according to claim 1, wherein the electropolishing electrolyte is held at a temperature of −25 to 200° C., preferably from 0 to 150° C., more preferably 50° C. to 100° C., and yet more preferably 60° C. to 90° C.

30. A method according to claim 1, wherein the initial average surface roughness (Ra) of the metallic article is at least one of: greater than 2 μm, preferably 5 μm, more preferably greater than 10 μm, and more preferably between 10 and 20 μm: or less than 400 μm, more preferably less than 300 μm, most preferably less than 200 μm.

31. (canceled)

32. A method according to claim 1, wherein the final average surface roughness (Ra) of the metallic article is less than 2 μm, more preferably less than 1.5 μm.

33. A method according to claim 1, wherein the rate of material removal is from 1 to 50 μm/min, more preferably from 2 to 30 μm/min.

34. A method according to claim 1, wherein the electropolishing electrolyte is contained in an electropolishing cell.

35. A method according to claim 1, wherein the electropolishing electrolyte is applied as a fluid flow onto the surface of the metallic article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

[0089] FIG. 1 provides the schematic of an electropolishing apparatus used to run an electropolishing method according to a first embodiment of the present invention.

[0090] FIG. 2 provides the schematic of an electropolishing apparatus used to run an electropolishing method according to a second embodiment of the present invention.

[0091] FIG. 3 shows microscope images (magnified 10× of the surface of a cobalt-chromium alloy coupon (A) formed from a 3D printed process; and (B) after application of an electropolishing run according to one embodiment of the present invention.

[0092] FIG. 4 shows microscope images (magnified 15× of the surface of a stainless steel alloy coupon (A) as formed from a 3D printed process (prior to electropolishing); and (B) after application of an electropolishing run according to one embodiment of the present invention.

[0093] FIG. 5 shows microscope images (magnified 10× of the surface of a laser cut aluminium coupon (A) as formed (prior to electropolishing); and (B) after application of an electropolishing run according to one embodiment of the present invention.

[0094] FIG. 6 shows microscope images (magnified 10× of the surface of an Inconel coupon (A) as formed from a 3D printed process (prior to electropolishing); (B) after application of an electropolishing run according to one embodiment of the present invention; and (C) a photograph comparing the Inconel coupons before (bottom) and after (top) electropolishing.

[0095] FIG. 7 illustrates a non-submerged electropolishing apparatus having a carbon fibre electrode brush used to run an electropolishing method according to a third embodiment of the present invention.

[0096] FIG. 8 shows microscope images (magnified 20×) of the surface of a stainless steel coupon (A) as formed from a 3D printed process (prior to electropolishing); and (B) after application of an electropolishing run according to one embodiment of the present invention.

[0097] FIG. 9 shows microscope images (magnified 20×) of the surface of a titanium coupon (A) as formed from a 3D printed process (prior to electropolishing); and (B) after application of an electropolishing run according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0098] The present invention provides an electropolishing polishing method that provides a rapid, quality, safe and quantitative finishing method for use with manufactured metallic articles particularly those formed from metals and metal alloys that have a protective oxide coating. Examples of metals and metal alloys that the electropolishing method can be used on include chromium based metallic alloys, such as stainless steel, nickel-chromium (nickel-chrome), nickel-chrome alloys, cobalt-chromium alloys, cobalt-chromium-molybdenum alloys, and also titanium, titanium alloys, nickel alloys such as nitinol, aluminium or aluminium alloys.

[0099] Electropolishing of metals and metal alloys that have a protective oxide coating can be difficult as the oxide coating can inhibit normal electropolishing processes. For example, chromium based metallic alloys have a protective chromium oxide layer, aluminium and aluminium based alloys have a protective aluminium oxide outer layer, and titanium and titanium based alloys have a protective titanium oxide outer layer. The method of the present invention assists in overcoming the deleterious effect of that layer when electropolishing these materials.

[0100] The present invention is also concerned with improving the surface finish of manufactured metallic articles produced by engineering production methods such as 3D-printing (additive manufacturing), flame cutting, snagging, coarse blasting or the like. The present invention is particularly suitable for additive manufacturing as this produces a rough finish and the present invention can cope with the wide range of geometries that can be produced by this manufacturing method.

[0101] When 3D printing metals, the surface is always rough, usually 8-20 roughness average (Ra). For most industrial applications, including jet engines, and medical applications such as implants, the printed item must be polished before it can be used. The conventional finishing method of polishing is time consuming and manual, which makes it prone to errors and very costly. Current polish times typically exceed an hour using manual labour.

[0102] The electropolishing method of the present invention can be used produce a smooth finish from a typical roughness 8 to 20 μm Ra to a roughness of 2 μm Ra in under 10 minutes, in some cases in under 3 minutes. The electropolishing method of the present invention is able to rapidly polish and reduce the average surface roughness of these types and other types of metallic articles to produce a surface finish preferably free of a heterogeneous texture and/or exhibiting a lower average surface roughness value.

[0103] A schematic of a typical electropolishing apparatus 100 suitable for practicing the electropolishing method of the present invention is illustrated in FIG. 1. The illustrated electropolishing apparatus 100 includes an electrolytic cell 110 having an electrolyte reservoir 120 that is configured to hold an electropolishing electrolyte 140. The electropolishing apparatus 100 also includes an inverter power supply 130 capable of delivering a desired current waveform (DC, DC pulses or variable frequency AC) in short pulses. The inverter power supply 130 is controlled by a computer controller 135.

[0104] A metallic article 180 is electrically connected to the positive terminal of the inverter power supply 130, while the negative terminal of the inverter power supply 130 is connected to a cathode 190 which also comprises the container containing the electrolyte 140. It should be appreciated that in other embodiments the cathode 190 could be a separate conductive article. Suitable conductive materials include carbon-based materials such as graphite, graphene, carbon fibre or the like, metallic/metal materials for the cathode 190 include stainless steel, lead, copper or any other material which exhibits good conductivity and corrosion resistance. The metallic article 180 is suspended in the reservoir 120 in the electrolyte 140 forming a complete electrical circuit with the electropolishing electrolyte 140.

[0105] Whilst not shown, the electropolishing apparatus 110 may also include a mixing device, for example a mixing rotor for stirring/mixing the electropolishing electrolyte 140 and ensuring even distribution of the electrolyte 140 around the metallic article 180 and the cathode 190.

[0106] The computer-controlled inverter power supply 130 is used to apply current and a voltage difference between the metallic article 180 and the cathode 190. The computer 135 runs a program that steps the inverter 130 (power source) through an applied current regime comprising a range of voltages/currents and frequencies that have been pre-determined to be optimum for the particular metallic article 180 and the comprising material to be polished.

[0107] An alternative schematic of an electropolishing apparatus 200 suitable for practicing the electropolishing method of the present invention is illustrated in FIG. 2. The illustrated electropolishing apparatus 200 also includes an electrolytic cell 210 having an electrolyte reservoir 220 comprising a glass container (or other suitable material) that is configured to hold an electropolishing electrolyte 240 and an inverter power supply 230 capable of delivering a desired current waveform (DC, DC pulses or variable frequency AC) in short pulses. The inverter power supply 230 is controlled by a computer controller 235.

[0108] A metallic article 280 is electrically connected to the positive terminal of the inverter power supply 230, while the negative terminal of the inverter power supply 230 is connected to a cathode 290 which in this case comprises a selected metallic article also immersed in the electrolyte 240. Again, suitable metal materials for the cathode 290 include stainless steel, lead, copper or any other metal or conductive material which exhibits good conductivity and corrosion resistance. The metallic article 280 is suspended in the reservoir 220 in the electrolyte 240 forming a complete electrical circuit with the electropolishing electrolyte 240. This form of the electropolishing apparatus 200 includes an optional mixing rotor 295 for stirring/mixing the electropolishing electrolyte 240 and ensuring even distribution of the electrolyte 240 around the metallic article 280 and the cathode 290.

[0109] Electropolishing is carried out with the electropolishing electrolyte 140, 240 of the electropolishing apparatus 100 or 200 at a temperature in a range of −25° C. to 200° C., and preferably 0 to 150° C. In embodiments, the electropolishing electrolyte 140, 240 is held at a temperature of about 50° C. to 100° C., preferably 60° C. to 90° C. The electropolishing apparatus 100, 200 may also include a combined temperature probe/heating and cooling unit (not illustrated), which can be attached to a computer controller 135, 235 or a separate controller (not illustrated) to monitor and control the temperature of the electropolishing electrolyte 140, 240.

[0110] In each embodiment, the computer-controlled power inverter 130, 230 is used to apply current and a voltage difference between the metallic article 180, 280 and the cathode 190, 290. The computer 135, 235 runs a program that steps the inverter power supply 130, 230 (power source) through an applied current regime comprising a range of voltages/currents and frequencies that have been pre-determined to be optimum for the particular metallic article 180, 280 and the comprising material (metal or metal alloy) to be polished.

[0111] The applied current regime broadly comprises the following steps:

(A) an optional initial pulse comprising a current density of at least 2 A/cm.sup.2 and a voltage having a shaped waveform having a frequency from 20 to 300 kHz, a minimum voltage of at least 0 V (preferably close to 0 V) and a maximum voltage of between 50 to 500 V, applied for a duration of at least 1 second, followed by:
(B) at least one electropolishing regime comprising a current density of at least 2 A/cm.sup.2 and a voltage having a shaped waveform having a frequency from 2 Hz to 300 kHz, a minimum voltage of at least 0 V (preferably close to 0 V) and a maximum voltage of between 0.5 to 500 V. Each electropolishing regime is typically applied for a duration of at least 1 s, preferably at least 2 s. Where two or more electropolishing regimes are used, the frequency and/or maximum voltage of each electropolishing regime is changed compared to the preceding electropolishing regime.

[0112] Whilst not wishing to be limited to any one theory, the inventor considers that at the start of a regime, the material removal rate is high. Within less than a minute, a diffusion layer quickly forms on the surface of the material being treated. The diffusion layer appears to act in a similar way to an insulator or resistive load. This diffusion layer significantly reduces the speed of material removal and, if it is not removed, the process speed degrades significantly. A change of at least one of the frequency, the current density (current), or voltage disrupts the diffusion layer and re-establishes the high speed of material removal. A high rate of current density (current) change, voltage change or frequency change sustains a high rate of diffusion layer disruption and therefore allows the process to sustain a very high rate of material removal.

[0113] The current (current density) change will usually, but not necessarily, be a reduction in current for the reasons stated above. The frequency change can be either an increase or a decrease. The change in frequency of the waveform is critical to disrupting the diffusion layer in that it need only change frequency between regimes. It may increase or decrease.

[0114] Therefore, a high rate of currency (current density) changes and frequency changes, either increasing or decreasing in frequency, allow a much faster surface finishing process. The increase in overall surface finishing speed is in the order of ten times faster when there is a high rate of current or waveform frequency changes.

[0115] The optional initial pulse can be used to provide a large maximum voltage and current density capable of removing any loosely attached and/or partially bonded material on the metallic article 180. The pulse is intended to be applied for a short time frame. Accordingly, the maximum voltage and current density of the initial pulse is preferably greater than the maximum voltage and current density of each of the at least one electropolishing regimes.

[0116] It is noted that it is possible to use a single electropolishing regime, however, multiple electropolishing regimes will achieve an improved surface finish. The electropolishing regimes are preferably designed to progressively electropolish the metallic article 180, 280 with decreasing intensity. Whilst a single electropolishing regime could be used, it is typical to have at least two electropolishing regimes, with each successive electropolishing regime preferably having a generally lower maximum voltage and current density that the preceding electropolishing regimes. In this respect, the maximum voltage and current density are generally lower in relation to being overall lower in a downwards trending pattern. It should be appreciated that individual electropolishing regimes may vary from that pattern and have a higher component. Where multiple electropolishing regimes are used, the initial electropolishing regimes preferably have high maximum voltage/current to remove high amounts of material quickly. This reduces the surface roughness in very rough materials to a moderately smooth roughness very quickly. Maximum voltage/current can be reduced in subsequent regimes to obtain increasingly finer results. Hence, the optimal surface finishing process is a series of regimes with initially high, but then decreasing voltage/current for increasingly smoother and finer finishes.

[0117] Each successive electropolishing regime is preferably applied with a different frequency to the preceding electropolishing regime. However, it should be appreciated that in some cases, one or more successive electropolishing regime could be applied with the same frequency as the preceding electropolishing regime.

[0118] Each electropolishing regime could be optionally followed by a cooling regime in which the current density is lowered compared to the current density of the preceding electropolishing regime. The lowered current density can be 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less or even 0.1 or less the current density of the preceding electropolishing regime. In some embodiments, the current density is lowered to zero (or close to zero) current density/applied current. The cooling regime may follow only one electropolishing regime, a number of electropolishing regimes, or in some cases each electropolishing regime. The cooling regime can be applied for any suitable timeframe, for example 0.5 to 5 s. The cooling regime has a reduced current density, and thus a reduced current applied compared to the preceding electropolishing regime to allow the metallic article and cathode to cool.

[0119] The shaped waveform of the voltage can be one of square wave, sinusoidal, pulsed, or a combination thereof. In some embodiments, the shaped waveform current comprises a pulsed width modulation (PWM) waveform, preferably a square wave pulse, preferably having a variable dead time. It should be appreciated that whilst a square waveform is used in the preceding examples, other waveforms could equally be used.

[0120] The total duration all electropolishing regimes is preferably less than 10 minutes, more preferably less than 5 minutes, and if possible less than 2 minutes. However, the total time is generally dependent on the type of material and other conditions. To achieve this, each electropolishing regime may be applied for a duration of 10 to 60 s, preferably 10 to 30 s, more preferably 10 to 20 s, yet more preferably 10 to 15 s.

[0121] The exact applied current regime is tailored for each particular metal or alloy composition and configuration.

[0122] As noted above, the Inventor considers that when used, the initial high pulse current/current density removes the partially bonded material. This is advantageous for electropolishing as asperities (peaks) on the work surface are dissolved much faster than the material in “micro-valleys”. Such selective dissolution is a result of different values of the electrical potential of the peaks and valleys. The positive charge of the anodically connected metallic article is concentrated in the peaks where the current density is higher than average which causes a selective dissolution of the peaks and smoothening the surface. Accordingly, the removal of any partially bonded material aids in the production of a smooth surface.

[0123] The sequence of frequency, current (current density) and voltage changes (steps) of the electropolishing regime or regimes are selected to electropolish the part to leave an even, smooth and lustrous polished surface. It should be appreciated that that electropolishing method of the present invention can leave some craters due to the nature of the initial surface. However, the electropolishing regimes are selected to improve processing speed, to bring a coarse surface (˜10 Ra) to below 2 Ra surface roughness in 60 s or less. Expected removal rates (current) fluctuate with feedback data but it is expected that removal rates will be up to 1.0 μm/min to 50 μm/min, preferably from 2 to 30 μm/min.

[0124] For a given electropolishing electrolyte, the quantity of metal removed from the metallic article is proportional to the amount of current applied and the time. Other factors, such as the geometry of the metallic article, affect the distribution of the current and, consequently, have an important bearing upon the amount of metal removed in local areas.

[0125] The electropolishing electrolyte 140, 240 preferably comprises a phosphoric acid (H.sub.3PO.sub.4) based solution, typically of 85% concentration diluted with water of a C1 to C4 alcohol. However, the electropolishing electrolyte 140, 240 may include other components. For example, in some embodiments the electropolishing electrolyte 140, 240 includes phosphoric acid (H.sub.3PO.sub.4) in combination with sulfuric acid (H.sub.2SO.sub.4), hydrochloric acid (HCl) or combinations thereof, and one of water or a C.sub.1-C.sub.4 alcohol. Other electropolishing electrolyte compositions are also possible.

[0126] Multiple metallic articles 180, 280 (part) can be placed in the same electrolytic cell 110, 210 allowing the method of the present invention to be scalable and cost effective. The part size (size of the metallic article 180) is also scalable as the control system can run multiple inverter power banks in parallel to achieve the desired output current. The method and system of the present invention can equally be configured to electropolish a part the size of a golf ball from a starting surface roughness of 10 Ra to less than 2 Ra and also a part the size of a car by scaling up the electrolyte reservoir 120, 220 (bath) and the inverter power supply 130, 230.

[0127] The above electropolishing method is taught as being conducted in a conventional electropolishing cell where the metallic article is immersed in the electropolishing electrolyte, or in non-immersed techniques, for example brush applied electropolishing techniques.

[0128] In other embodiments (not illustrated), the electropolishing is applied as a fluid flow onto the surface of the metallic article. Such electropolishing techniques are known as non-submerged electropolishing techniques, and generally involve a flow of electropolishing electrolyte being applied to the surface of the metallic article, and a conducting electrode being immersed in the electropolishing electrolyte and moving across the surface to electropolish the surface surrounding the conducting electrode.

[0129] In this non-submerged method, the metallic article is connected to the positive terminal of a power supply thereby becoming an anode. A cathode comprising a suitable conducting electrode is connected to the negative terminal of the power supply. The conducting electrode is configured to be engaged with a selected portion of the surface of the metallic article. In some embodiments, the conducting electrode comprises a carbon fibre brush. However, any suitable conductor (for example metal such as copper or the like) can be used. In use, electrolyte is pumped from a reservoir to the selected portion of the surface of the metallic article to immerse part of the cathode and surface of the metallic article and therefore form an electropolishing cell on the surface of the metallic article. Coolant can be supplied to cool the electropolishing area. Again, examples of this electropolishing technique are taught in patent publications No. WO2009/105802, AU2013242795A1 and AU2017204328A1 the contents of which should be considered to be incorporated into this specification by this reference.

[0130] The electropolishing method of the present invention has been developed to electropolish metallic articles, particularly those metals and metal alloys that have a protective oxide coating. Examples of metals and metal alloys that the electropolishing method can be used on include chromium based metallic alloys, such as stainless steel, nickel-chromium (nickel-chrome), nickel-chrome alloys, cobalt-chromium alloys, cobalt-chromium-molybdenum alloys, and also titanium, titanium alloys, nickel alloys such as nitinol, aluminium or aluminium alloys.

[0131] One example of a chromium containing metal alloy that can be electropolishing using the method of the present invention are cobalt-chromium alloys. Like stainless steels, the presence of sufficient chromium content on the outside surface passivates the surface. Co—Cr alloys show excellent mechanical properties such as strength and toughness, castability, corrosion resistance, and wear resistance. In particular, Co—Cr alloys have excellent wear resistance, so they are used for sliding parts of artificial joints

[0132] Co—Cr alloys are nominally equal in chromium and cobalt content, yielding alloys in the middle of the Co—Cr phase diagram. Co—Cr and alloys of Co—Cr typically have hexagonal close-packed (HCP) crystal structures with both chromium and cobalt taking positions as substitutional defects in the other crystal. In some embodiments, Co—Cr alloys further include molybdenum and nickel alloying elements. Co—Cr—Mo alloy also is a good choice for permanent implants, due to its high corrosion resistance. These further alloying elements create other substitutional defects that strengthen the alloy and reduce the machining capacity of cast alloy forms. The Co—Cr alloy may include further minor alloying elements (less than 1 wt %) such as Mn, Ni, Fe, C, Ti, S, P, N and W. Table 1 provides non-limiting examples of biologically compatible Co—Cr composites that could be electropolished using the method of the present invention:

TABLE-US-00001 TABLE 1 Examples of Co—Cr compositions F75 F799 F90 F562 Co 59.9%-69.5% 58%-59% .sup. 45%-56.2% 29-38.8 Cr 27%-30% 26%-30% 19%-21% 19-21% Mo 5%-7% 5%-7% —  9%-10.5% Mn 1% max 1% max 1%-2% 0.15% max Ni 1% max 1% max  9%-11% 33%-37%.sup.  Fe 0.75% max 1.5% max 3% max 1% max Si 1% max 1% max 0.4% max 0.15% max C 0.25% max 0.35% max 0.1% — nominally Other N: 0/ P: max Ti: 1% max 25% max 0.04% S: 0.01% max W — — 14%-16% —

[0133] In other embodiments, the electropolishing method of the present invention can be used to electropolish metallic articles formed from a nickel-chromium alloy, for example Inconel. Inconel is a family of austenitic nickel-chromium-based superalloys. Inconel alloys are oxidation-corrosion-resistant materials well suited for service in extreme environments subjected to pressure and heat. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack.

[0134] Inconel alloys vary widely in their compositions, but all are predominantly nickel, with chromium as the second element.

TABLE-US-00002 TABLE 2 Examples of Inconel compositions Element, proportion by mass (%) Type Ni Cr Fe Mo Nb & Ta Co Mn Cu 600 ≥72.0 14.0-17.0 6.0-10.0 ≤1.0 ≤0.5 617 44.2-61.0 20.0-24.0 ≤3.0 8.0-10.0 10.0-15.0 ≤0.5 <0.5 625 ≥58.0 20.0-23.0 ≤5.0 8.0-10.0 3.15-4.15 ≤1.0 ≤0.5 690 ≥58   27-31 7-11 ≤0.50 ≤0.50 Nuc ≥58   28-31 7-11 ≤0.10 ≤0.50 ≤0.50 grade 690 718 50.0-55.0 17.0-21.0 Remndr 2.8-3.3  4.75-5.5  ≤1.0 ≤0.35 ≤0.3 X-750 ≥70.0 14.0-17.0 5.0-9.0  0.7-1.2 ≤1.0 ≤1.0 ≤0.5 Element, proportion by mass (%) Type Al Ti Si C S P B 600 ≤0.5 ≤0.15 ≤0.015 617 0.8-1.5 ≤0.6 ≤0.5 0.05-0.15 ≤0.015 ≤0.015 ≤0.006 625 ≤0.4 ≤0.4 ≤0.5 ≤0.1  ≤0.015 ≤0.015 690 ≤0.50 ≤0.05 ≤0.015 Nuc ≤0.50 ≤0.04 ≤0.015 grade 690 718 0.2-0.8 0.65-1.15 ≤0.35 ≤0.08 ≤0.015 ≤0.015 ≤0.006 X-750 0.4-1.0 2.25-2.75 ≤0.5 ≤0.08 ≤0.01

[0135] The 3D printed articles polished using the electropolishing method of the present invention play an important role in industry. Stainless Steel and Aluminium are involved in general purpose manufacturing. Inconel is used for jet engines. Cobalt-chrome is used in dentistry and titanium for biomedical applications.

[0136] In addition to the above advantages, it should be appreciated that electropolishing according to the present invention produces a number of favourable changes in a metallic article including, but are not limited to, one or more of: [0137] Brightening [0138] Burr removal [0139] Oxide and tarnish removal [0140] Reduction in surface profile [0141] Removal of surface occlusions [0142] Increased corrosion resistance [0143] Improved adhesion in subsequent plating [0144] Removal of directional (draw) lines [0145] Radiusing of sharp edges, sharp bends, and corners [0146] Reduced surface friction [0147] Stress relieved surface.

EXAMPLES

Example 1—Cobalt-Chromium Alloy

[0148] A rough surfaced (8-13 μm Ra) 3D printed cobalt chromium alloy coupon (cobalt chromium “MP1” with a surface area of 3.2 cm.sup.2 with an average Ra of 9.973 μm) was placed in a stainless steel bowl forming the electrolytic reservoir and providing the cathode. The bowl contained an electrolyte bath comprising 85% H.sub.3PO.sub.4 aqueous solution. The overall electropolishing apparatus follows the general schematic shown in FIG. 1. The cobalt chromium coupon was placed in the centre of the bath equal distance from the sides and the bottom of the bowl. The positive terminal of a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the cobalt chromium coupon immersed in the electrolyte bath and the stainless steel bowl cathode. A cooling fan was aimed at the electrolyte bath to effect cooling of the electrolyte to maintain at a temperature of between 60 to 90° C. therein.

[0149] A power regime (as specified below in Table 3) was then applied to the bath using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00003 TABLE 3 Pulse current regime Pulsed DC Time Frequency Voltage Current Regime (s) (kHz) (V) (A) 1 (pulse) 5 40 65 75 2 15 63 50 52 3 10 63 40 39 4 15 63 28 29 5 15 63 18 19

[0150] As shown in Table 3, the current is started at maximum, manageable limits to remove the partially bonded material from the surface of the coupon. Frequency and voltage and current are then stepped via the computer programme and this is to disrupt the diffusion layer around the coupon. The current is stepped down via the computer programme towards the end of the process to achieve the best, final finish. The target surface roughness average was 2 microns or less so programmes were developed for a length of time where this target could be met.

[0151] For each programme time, frequency, voltage and current are recorded.

[0152] For each material the surface roughness average (Ra) was measured in microns and calculated pre and post submersion. The average Ra is taken via 10 individual line scans of the surface to be treated. The pre process average is then compared to the post process average surface roughness (Ra) which is measured in microns.

[0153] The improved speed still maintains results that bring a coarse surface (˜10 μm Ra) to below 2 μm Ra surface roughness in 60 s. Expected removal rates (current) fluctuates with feedback data.

[0154] The results of an electropolishing run is shown in FIG. 3 which show the before (FIG. 3A) and after (FIG. 3B) microscope images. The relative change in surface finish was determined using ten line scans taken using a Starrett sr100 surface roughness meter from random locations of the samples front and back and an average derived from that data. The results of these measurements are provided in Table 4:

TABLE-US-00004 TABLE 4 Roughness measurements pre and post electropolish Sample Prepolish Ra Post-polish Ra No. (μm) (μm) 1 8.65 1.45 2 10.35 2.52 3 11.27 0.90 4 9.43 2.79 5 10.23 1.87 6 8.92 1.42 7 8.59 0.66 8 10.75 1.22 9 12.23 1.40 10 9.26 0.80 Average Ra 9.97 1.50 (μm)

[0155] These provided the following results: [0156] Pre finish: Ra=9.97 μm. [0157] Post finish: Ra=1.50 μm—based on 10 line scans. Best result 0.656 μm
Worst result 2.793 μm (crater).

[0158] These show that average surface roughness decreases from an initial coarse surface (˜10 μm Ra) to below 2 μm Ra surface roughness in 60 s. As can be seen, the sharp regions illustrated in FIG. 3A are eroded away leaving a substantially flat, defect free surface in the sample shown in FIG. 3B. Furthermore, FIG. 3B shows that the electropolishing process left some craters in the surface of the material. The surface finish produced is therefore not medical grade, but it can still be used for many other applications, for example jet engine parts, aerospace applications or similar.

Example 2—Stainless Steel

[0159] A rough surfaced (12-18 μm Ra) stainless steel coupon (stainless steel “SS17” with a surface area of 3.2 cm.sup.2 with an average Ra of 15.62 μm) was placed in a glass bowl forming the electrolytic reservoir. The bowl contained an electrolyte bath comprising 85% H.sub.3PO.sub.4 aqueous solution. The overall electropolishing apparatus follows the general schematic shown in FIG. 2, using a stainless steel based cathode. The stainless steel coupon was placed in the centre of the bath spaced away from the cathode. The positive terminal of a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the stainless steel coupon immersed in the electrolyte bath. A cooling fan was aimed at the electrolyte bath to effect cooling of the electrolyte to maintain at a temperature of between 60 to 90° C. therein.

[0160] A power regime (as specified below in Table 5) was then applied to the bath using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. The frequency ranged from 100 to 100000 Hertz (Hz); voltage from 0 to 100 volts (v) and current 0 to 50 amps. The program ran for 296 s total and included 53 different regimes. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in Table 5.

TABLE-US-00005 TABLE 5 Pulse current regime Pulsed DC Time Frequency Voltage Current Regime (s) (Hz) (V) (A) 1 (Pulse) 10 100  50 50 2 5  22K 15 15 3 2 100K 27 28 4 5  60K 0 0 5 2 100K 20 23 6 5  22K 18 24 7 2 100K 26 24 8 5  60K 30 35 9 2 100K 100 0 10 5  84K 26 29 11 2 100K 25 30 12 5  22K 24 27 13 2 100K 23 25 14 5  22K 16 22 15 2 100K 23 29 16 5  84K 19 26 17 2 100K 18 24 18 5  22K 15 19 19 2 100K 20 29 20 5  60K 15 23 21 2 100K 20 28 22 5  84K 15 24 23 2 100K 19 26 24 5  22K 15 24 25 2 100K 18 23 26 5  84K 10 15 27 2 100K 15 22 28 5 100K 12 17 29 10  22K 16 16 30 5 100K 26 24 31 10  60K 16 19 32 5 100K 22 25 33 10  84K 20 23 34 5 100K 19 22 35 10  22K 17 24 36 5 100K 12 17 37 5  60K 11 13 38 10 100K 12 14 39 10  84K 12 13 40 10 100K 12 15 41 10  84K 16 16 42 5 100K 26 24 44 10  22K 16 19 45 5 100K 22 25 46 10  60K 20 23 47 5 100K 19 22 48 10  84K 17 24 49 5 100K 12 17 50 5  22K 11 13 51 10 100K 12 14 52 10  60K 12 13 53 10 100K 12 15

[0161] The changes in frequency, voltage and current are key to the speed and quality of finish. The improved speed still maintains results that bring a coarse surface (˜10 μm Ra) to below 2 μm Ra surface roughness in less than 300 s.

[0162] The results of an electropolishing run is shown in FIG. 4 which show the before (FIG. 4A) and after (FIG. 4B) microscope images. The relative change in surface finish was determined using ten line scans taken using a Time RTD-300 surface roughness meter from random locations of the samples front and back and an average derived from that data. The results of these measurements are provided in Table 6:

TABLE-US-00006 TABLE 6 Roughness measurements pre and post electropolish Sample Prepolish Ra Post-polish Ra No. (μm) (μm) 1 17.17 1.80 2 17.31 1.74 3 13.63 1.78 4 12.40 2.17 5 17.74 2.15 6 16.58 2.17 7 15.29 1.80 8 15.14 2.16 9 14.71 2.17 10 16.18 1.67 Average Ra 15.62 1.96 (μm)

[0163] These provided the following results: [0164] Pre finish: Ra=15.62 μm. [0165] Post finish: Ra=1.96 μm—based on 10 line scans. Best result 1.67 μm Worst result 2.17 μm.

[0166] These show that average surface roughness decreases from an initial coarse surface (˜10 μm Ra) to below 2 μm Ra surface roughness in less than 300 s. Electropolishing SS17 created a very smooth and fine finish to the sample. The high, rough parts were taken away and the holes were not.

Example 3—Laser Cut Aluminium

[0167] A rough surfaced (5 to 8 μm Ra) laser cut aluminium coupon (5005 Grade aluminium “Al 5” with a surface area of 3.2 cm.sup.2 with an average Ra of 6.40 μm) was placed in a glass bowl forming the electrolytic reservoir. The bowl contained an electrolyte bath comprising 85% H.sub.3PO.sub.4 aqueous solution. The overall electropolishing apparatus follows the general schematic shown in FIG. 2, using a stainless steel cathode. The aluminium coupon was placed in the centre of the bath spaced away from the cathode. The positive terminal of a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the Aluminium coupon immersed in the electrolyte bath. The electrolyte started at 40° C. A cooling fan was aimed at the electrolyte bath to effect cooling of the electrolyte to maintain at a temperature of between 60 to 90° C. therein.

[0168] A power regime (as specified below in Table 7) was then applied to the bath using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. There were 12 regimes of total time 130 s (s). The frequency ranged from 22000 to 100000 Hz; voltage from 26 to 49 V and current 6 to 24 A. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00007 TABLE 7 Current regime Pulsed DC Time Frequency Voltage Current Regime (s) (Hz) V) (A) 1 15 60K 49 24 2 15 84K 46 20 3 10 60K 44 20 4 10 22K 27 14 5 10 84K 40 14 6 10 22K 27 13 7 10 60K 35 14 8 10 22K 32 13 9 10 100K  34 9 10 10 60K 26 8 11 10 22K 26 10 12 10 100K  29 6

[0169] Again, the changes in frequency, voltage and current are key to the speed and quality of finish.

[0170] The results of an electropolishing run is shown in FIG. 5 which show the before (FIG. 5A) and after (FIG. 5B) microscope images. The relative change in surface finish was determined using ten line scans taken using a Time RTD-300 surface roughness meter from random locations of the samples front and back and an average derived from that data. The results of these measurements are provided in Table 8:

TABLE-US-00008 TABLE 8 Roughness measurements pre and post electropolish Sample Prepolish Ra Post-polish Ra No. (μm) (μm) 1 5.77 1.38 2 7.83 1.85 3 6.90 1.17 4 7.47 1.56 5 5.58 1.45 6 6.04 1.81 7 6.00 1.72 8 5.68 1.83 9 5.58 2.54 10 7.19 1.68 Average Ra 6.40 1.70 (μm)

[0171] These provided the following results: [0172] Pre finish: Ra=6.40 μm. [0173] Post finish: Ra=1.70 μm— based on 10 line scans.

[0174] Again, electropolishing created a very smooth and fine finish to the sample.

Example 4—Inconel

[0175] A rough surfaced (4 to 7 μm Ra) 3D printed Inconel coupon (Inconel “Inconel 2” with a surface area of 3.2 cm.sup.2 with an average Ra of 5.65 μm) was placed in a glass bowl forming the electrolytic reservoir. The bowl contained an electrolyte bath comprising 85% H.sub.3PO.sub.4 aqueous solution. The overall electropolishing apparatus follows the general schematic shown in FIG. 2, using a stainless steel cathode. The Inconel coupon was placed in the centre of the bath spaced away from the cathode. The positive terminal of a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the Inconel coupon immersed in the electrolyte bath. A cooling fan was aimed at the electrolyte bath to effect cooling of the electrolyte to maintain at a temperature of between 60 to 90° C. therein.

[0176] A power regime (as specified below in Table 9) was then applied to the bath using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. There were 16 programmes of total time 235 s. The frequency ranged from 22 to 100 kHz; voltage from 8 to 31 V and current 6 to 35 A. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00009 TABLE 9 Pulse current regime Pulsed DC Time Frequency Voltage Current Regime (s) (Hz) (V) (A) 1 10 60 31 35 2 15 84 30 30 3 15 60 30 30 4 15 84 26 28 5 15 100 23 26 6 15 60 26 24 7 15 84 20 23 8 15 22 19 22 9 15 22 17 20 10 15 100 14 13 11 15 22 17 15 12 15 100 8 7 13 15 22 15 14 14 15 22 14 12 15 15 60 10 8 16 15 100 8 6

[0177] Again, the changes in frequency, voltage and current are key to the speed and quality of finish.

[0178] The results of an electropolishing run is shown in FIG. 6 which show the before (FIG. 6A) and after (FIG. 6B) microscope images. The relative change in surface finish was determined using ten line scans taken using a Time RTD-300 surface roughness meter from random locations of the samples front and back and an average derived from that data. The results of these measurements are provided in Table 10:

TABLE-US-00010 TABLE 10 Roughness measurements pre and post electropolish Sample Prepolish Ra Post-polish Ra No. (μm) (μm) 1 4.56 2.29 2 5.76 1.78 3 4.16 1.92 4 7.27 1.76 5 5.79 1.72 6 6.71 1.77 7 4.74 1.82 8 4.64 1.94 9 6.38 2.1 10 6.48 1.64 Average Ra 5.65 1.87 (μm)

[0179] These provided the following results: [0180] Pre finish: Ra=5.65 μm. [0181] Post finish: Ra=1.87 μm—based on 10 line scans.

[0182] Again, electropolishing created a very smooth and fine finish to the sample.

Example 5—Inconel Coupon—Non-Submerged Electropolishing

[0183] A rough surfaced (4 to 7 μm Ra) 3D printed Inconel coupon (Inconel “Inconel Coupon 3” with a surface area of 3.2 cm.sup.2 with an average Ra of 5.65 μm) was held in an electrically connected clamp. It is noted that the actual area polished in the example was 1 cm.sup.2 as only one large side and no edges were polished by the brush (in non-submerged examples the polished area is only that area that is contacted with the brush). The Inconel coupon 3 was then electropolished using a modified non-submerged electropolishing apparatus, model EASYkleen Easy Feeder (300 in FIG. 7) available from EASYKleen Pty Ltd, 43 Shelley Road, Moruya, NSW, 2537, Australia using a carbon fibre brush cathode for electropolishing. The Inconel coupon was connected to the power source through the clamp to form the anode of the electropolishing circuit.

[0184] An 85% H.sub.3PO.sub.4 aqueous solution electrolyte was supplied through the brush and applied to the Inconel coupon for electropolishing. As in the previous examples, the power source was modified (not illustrated) to be connected to a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the Inconel coupon.

[0185] A power regime (as specified below in Table 11) was then applied to the carbon fibre brush cathode using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. There were 16 programmes of total time 235 s. The frequency ranged from 22 to 100 kHz; voltage from 8 to 31 V and current 6 to 35 A. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00011 TABLE 11 Pulse current regime Pulsed DC Time Frequency Voltage Current Regime (s) (Hz) (V) (A) 1 5 60 12 44 2 10 60 9 40 3 10 84 8 38 4 10 22 8 37 5 10 60 8 39 6 10 84 6 38 7 10 22 7 39 8 10 100 6 28 9 10 22 7 30 10 10 60 6 32 11 10 84 4 18 12 10 100 4 18 TOTAL 115

[0186] Again, the changes in frequency, voltage and current are key to the speed and quality of finish.

[0187] The results of an electropolishing run are provided in Table 12:

TABLE-US-00012 TABLE 12 Roughness measurements pre and post electropolish Sample Prepolish Ra Post-polish Ra No. (μm) (μm) 1 5.48 1.62 2 4.46 1.45 3 5.26 1.95 4 6.49 2.03 5 4.47 1.28 6 5.01 2.01 7 5.73 1.34 8 3.97 0.96 9 5.67 1.8 10 3.72 2.11 Average Ra 5.65 1.87 (μm)

[0188] These provided the following results: [0189] Pre finish: Ra=5.65 μm. [0190] Post finish: Ra=1.87 μm—based on 10 line scans.

[0191] Again, electropolishing created a very smooth and fine finish to the sample. Moreover, the results are similar to immersed method as detailed in Example 4.

Example 6—Stainless Steel—Non-Submerged Electropolishing

[0192] A rough surfaced (13.95 μm Ra) 3D printed Stainless Steel coupon (“SS 5” with a surface area of 3.2 cm.sup.2 with an average Ra of 13.95 μm) was held in an electrically connected clamp. It is noted that the actual area polished in the example was 1 cm.sup.2 as only one large side and no edges were polished by the brush (in non-submerged examples the polished area is only that area that is contacted with the brush). The stainless steel coupon was then electropolished using a modified non-submerged electropolishing apparatus, model EASYkleen Easy Feeder (300 in FIG. 7) available from EASYKleen Pty Ltd, 43 Shelley Road, Moruya, NSW, 2537, Australia using a carbon fibre brush cathode for electropolishing. The stainless steel coupon was connected to the power source through the clamp to form the anode of the electropolishing circuit.

[0193] An 85% H.sub.3PO.sub.4 aqueous solution electrolyte was supplied through the brush and applied to the stainless steel coupon for electropolishing. As in the previous examples, the power source was modified (not illustrated) to be connected to a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the stainless steel coupon.

[0194] A power regime (as specified below in Table 12) was then applied to the carbon fibre brush cathode using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. There were 29 programmes of total time 74 s (60 s of electropolishing time). The frequency ranged from 20 to 200 kHz; voltage from 4 to 20 V and current 30 to 80 A. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00013 TABLE 12 Pulse current regime Pulsed DC Time Frequency Voltage Current Duty Regime (s) (kHz) (V) (A) Cycle % 1 4 20 20   80 20 2 1 20 Cooling 0 20 3 4 60 12   40 30 4 1 60 Cooling 0 30 5 4 40 7.1 50 20 6 1 40 Cooling 0 20 7 4 100 5.6 57 20 8 1 10 Cooling 0 20 9 4 80 6   40 20 10 1 80 Cooling 0 20 11 4 100 5   35 20 12 1 100 Cooling 0 20 13 4 120 5.5 50 20 14 1 120 Cooling 0 20 15 4 160 5.4 40 20 16 1 160 Cooling 0 20 17 4 80 3.6 32 10 18 1 80 Cooling 0 10 19 4 140 4.5 45 20 20 1 140 Cooling 0 20 21 4 120 4   35 20 22 1 120 Cooling 0 20 23 4 180 4.3 37 20 24 1 180 Cooling 0 20 25 4 200 4.5 30 20 26 1 200 Cooling 0 20 27 4 120 4   35 20 28 1 120 Cooling 0 20 29 4 160 4   30 20

[0195] The changes in frequency, voltage and current are key to the speed and quality of finish. In this particular carbon fibre brush cathode run, cooling steps where zero current is applied is used between each electropolishing regime to allow the metallic piece to cool. The cooling time is aimed at cooling the metallic work piece, although there is also a minor benefit from cooling the brush electrode. This off-time assists in the effectiveness of this form of the electropolishing apparatus. As shown in the results, the power regime in Table 12 was applied in FOUR successive runs to achieve the final surface roughness.

[0196] The relative change in surface finish was determined using ten line scans taken using a Time RTD-300 surface roughness meter from random locations of the samples polished side and an average derived from that data. The results of these measurements are provided in Table 13:

TABLE-US-00014 TABLE 13 Roughness measurements pre and post electropolish Regime 4a SS Coupon 5 (60 sec) (Ra μm) Pre polish 13.95 First run 7.56 Second run 4.35 Third run 2.88 Fourth run 1.22

[0197] These show that average surface roughness decreases from an initial coarse surface (13.95 μm Ra) to below 2 μm Ra surface. Electropolishing of coupon SS5 created a very smooth and fine finish to the sample.

Example 7—Titanium—Non-Submerged Electropolishing

[0198] A rough surfaced (˜10.28 μm Ra) 3D printed titanium coupon (“Ti AMS” with a surface area of 3.2 cm.sup.2 with an average Ra of 10.28 μm) was held in an electrically connected clamp. It is noted that the actual area polished in the example was 1 cm.sup.2 as only one large side and no edges were polished by the brush (in non-submerged examples the polished area is only that area that is contacted with the brush). The titanium coupon was then electropolished using a modified non-submerged electropolishing apparatus, model EASYkleen Easy Feeder (300 in FIG. 7) available from EASYKleen Pty Ltd, 43 Shelley Road, Moruya, NSW, 2537, Australia using a carbon fibre brush cathode for electropolishing. The titanium coupon was connected to the power source through the clamp to form the anode of the electropolishing circuit.

[0199] An 85% H.sub.3PO.sub.4 aqueous solution electrolyte was supplied through the brush and applied to the titanium coupon for electropolishing. As in the previous examples, the power source was modified (not illustrated) to be connected to a computer-controlled power inverter purpose built to vary peak voltage, peak current, voltage frequency, current frequency, voltage waveform, current waveform was then connected to the titanium coupon.

[0200] A power regime (as specified below in Table 14) was then applied to the carbon fibre brush cathode using a computer-controlled power inverter. The computer runs a program that steps the inverter (power source) through a range of voltages/currents (current densities) and frequencies that have been pre-determined to be optimum for the part and the material to be polished. There were 23 programmes (regimes 2 to 24) of total time 93 s (of which 60 s is electropolishing). The frequency ranged from 20 to 200 kHz; voltage from 8 to 11 V and current 30 to 60 A. The voltage waveform is a square wave having a minimum voltage close to zero and a maximum voltage as outlined in the table.

TABLE-US-00015 TABLE 14 Pulse current regime Pulsed DC Time Frequency Voltage Duty Regime (s) (kHz) (V) (A) Cycle (%) 2 5 100   9.5 60 25 3 3 60 Cooling 0 30 4 5 60 9 58 30 5 3 20 Cooling 0 20 6 5 20 10  55 20 7 3 120 Cooling 0 25 8 5 120 11  45 25 9 3 20 Cooling 0 10 10 5 20 10  50 10 11 3 80 Cooling 0 20 12 5 80   9.5 53 20 13 3 60 Cooling 0 15 14 5 60 8 50 15 15 3 140 Cooling 0 25 16 5 140 11  38 25 17 3 200 Cooling 0 25 18 5 200 9 30 25 19 3 160 Cooling 0 25 20 5 160 9 37 25 21 3 180 Cooling 0 25 22 5 180 9 30 25 23 3 40 Cooling 0 25 24 5 40   8.5 45 25

[0201] The changes in frequency, voltage and current are key to the speed and quality of finish. In this particular carbon fibre brush cathode run, cooling steps where zero current is applied is used between each electropolishing regime to cool the metallic work piece, although there is also a minor benefit from cooling the brush electrode. This off-time assists in the effectiveness of this form of the electropolishing apparatus. As shown in the results, the power regime in Table 14 is applied in FOUR successive runs to achieve the final surface roughness.

[0202] The relative change in surface finish was determined using ten line scans taken using a Time RTD-300 surface roughness meter from random locations of the samples polished side and an average derived from that data. The results of these measurements are provided in Table 15:

TABLE-US-00016 TABLE 15 Roughness measurements pre and post electropolish Regime 8a Ti AMS Coupon (60 sec) (Ra μm) Pre polish 10.28 First run 8.39 Second run 7.15 Third run 5.91 Fourth run 4.15

[0203] These show that average surface roughness decreases from an initial coarse surface (10.28 μm Ra) to 4.15 μm Ra surface. Electropolishing Ti AMS created a smooth finish to the sample.

CONCLUSION

[0204] The results of each of the examples above show that rapidly electropolishing 3D printed articles is generally enhanced by high current, high frequency and stepping the power system. This leaves the 3D printed article with a smooth finish. Further research is required to refine the parameters.

[0205] Overall, the results demonstrate that where applicable, an ultra-high current can be used remove partially bonded material, in some cases by melting or otherwise severing the attachment point between that partially bonded material and the base material. High voltage/current can be used to rapidly remove material but prolonged application can leave indents and streaks in the surface. The ideal programme runs ultra-high current then high current density. The current density then decreases to enable the fine finish. The diffusion layer around the part is disturbed by stepping (up or down) frequency and voltage. One voltage works well for a short timeframe in seconds (generally less than 15 s) and then the process slows.

[0206] Materials with strong oxide layers such as titanium and aluminium break down with more speed and at lower voltages by using higher frequency.

[0207] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[0208] Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.