A DEVICE AND METHOD FOR ELECTROCHEMICALLY MACHINING A WORKPIECE

Abstract

A device for electrochemically machining a workpiece (10), the device comprising: (i) an elongate device body having a first end (3) and a second end (4) and an electrically insulating sidewall (8) with one or more electrolyte delivery apertures defined in the elongate device body; (ii) an electrode (7) within the device body; (iii) an electrolyte supply arrangement that confines electrolyte within the device body so that the electrolyte can only be delivered through the aperture(s) for electrochemically machining the workpiece (10). Also described is a method for electrochemically machining the workpiece.

Claims

1. A device for electrochemically machining a workpiece, the device comprising: (a) an elongate device body having a first end and a second end and an electrically insulating sidewall with one or more electrolyte delivery apertures defined in the elongate device body; (b) an electrode within the device body; (c) an electrolyte supply arrangement that confines electrolyte within the device body so that the electrolyte can only be delivered through the aperture(s) for electrochemically machining the workpiece.

2. A device according to claim 1 wherein the device is resiliently deformable, so that in use, it can flex without permanently deforming/kinking.

3. A device according to claim 1, wherein at least one electrolyte delivery aperture of the one or more electrolyte delivery apertures is defined in the electrically insulating sidewall so that electrolyte exits to the side of the elongate device body at a position intermediate the first end and the second end.

4. A device according to claim 1, wherein the electrically insulating sidewall of the elongate device body is formed by an electrically insulating outer tubular sheath.

5. A device according to claim 1, wherein the elongate body is configured for machining an internal surface of a bore defined in a workpiece.

6. A device according to claim 1, wherein the electrode within the device body is a tubular electrode defined by an annular sidewall.

7. A device according to claim 1, wherein the electrode is reticulated in a mesh form.

8. A device according to claim 1, further comprising an electrolyte outlet for allowing an electrolyte within the workpiece to be drawn out.

9. A device according to claim 1, wherein the electrolyte supply arrangement is for supplying positive or negative pressure to the electrolyte to allow the electrolyte to be supplied to and then drawn out of the device so as to provide a continuous supply of electrolyte.

10. A device according to claim 1, further comprising an electrically insulating inner sidewall, wherein the electrode is located between the inner sidewall and the outer sidewall; and/or wherein the electrode is a negative electrode; and/or wherein the device further comprises a control system for controlling the rate of delivery of the electrolyte; and/or wherein the device further comprises a control system for controlling the rate of movement of the device within a workpiece.

11. A device according to claim 1, wherein the workpiece is made by additive manufacturing, such as 3D printing.

12. A device for electrochemically machining the internal surface of a bore defined in a tubular workpiece, the device comprising: (i) an electrically conductive tubular electrode having an annular sidewall defining a hollow conduit; (ii) an electrically insulating outer tubular sheath having an annular sidewall defining a hollow conduit, the sidewall of the outer tubular sheath annular overfitted to the annular sidewall of the electrically conductive tubular electrode so that the tubular electrode is within the hollow conduit of the outer tubular sheath; (iii) an electrolyte intake for supplying electrolyte to the hollow conduit of the tubular electrode; and (iv) one or more apertures defined in the annular sidewall of the outer tubular sheath to allow passage of electrolyte from within the hollow conduit of the tubular electrode through the annular sidewall of the outer tubular sheath, so that the electrolyte is available for electrochemically machining the internal surface of the bore defined in the tubular workpiece.

13. A device according to claim 12, wherein at least of the first end and second end is steerable, optionally by a steering mechanism such as an anchored cable.

14. A method of electrochemically machining an internal surface of a workpiece, the method comprising the steps of: (a) providing a device according to any preceding claim; (b) inserting the device into the workpiece; (c) delivering electrolyte only through the aperture(s) for electrochemically machining the workpiece; and (d) electrochemically machining the workpiece.

15. A method according to claim 14 further comprising the step of moving the device within the workpiece to electrochemically machine the internal surface at different positions; optionally wherein the electrochemical machining is continuous while moving or the electrochemical machining is intermittent; and/or wherein the workpiece is negatively charged.

16. A device according to claim 1, wherein the electrode is in a reticulated tubular form.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

[0090] FIG. 1(a) is a schematic representation of a device of the invention in use within a bore formed within the workpiece;

[0091] FIG. 1(b) is an image of the device of the invention in use ejecting electrolyte onto a workpiece;

[0092] FIG. 2 is an image of a workpiece that is machined according to the schematic representation of FIG. 1;

[0093] FIG. 3 is a schematic representation of a layer structure within a device of the invention;

[0094] FIG. 4 is a schematic representation of a device of the invention with the same layer structure as FIG. 3 but with apertures in the externally insulating sidewall;

[0095] FIG. 5 is a schematic representation of a device FIG. 4 with arrows indicating electrolyte flow;

[0096] FIG. 6 is a schematic representation of a device FIG. 4 inserted within a workpiece and with arrows indicating electrolyte flow;

[0097] FIG. 7 is a schematic representation of a device FIG. 4 inserted within a workpiece and with arrows indicating electrolyte flow and the workpiece having being machined by Jet ECM;

[0098] FIG. 8 shows a pre-machined channel surface viewed on Keyence VHX-5000, (a) 100× magnification, (b) 500× magnification as described in more detail below;

[0099] FIG. 9 shows a tilted SEM micrograph of pre-machined channel surface, ×55 magnification as described in more detail below;

[0100] FIG. 10 is an image of a device of the invention in use to machine the internal surface of bores of a stainless steel substrate as described in more detail below;

[0101] FIGS. 11(a) to (f) shows Keyence VHX-5000 images of machined sections from trials as described in more detail below: (a) Trial 1 ×500 magnification, (b) Trial 2 ×500 magnification, (c) Trial 2 ×100 magnification, (d) Trial 3 ×100 magnification, (e) Trial 4 ×100 magnification, (f) Trial 4 ×500 magnification;

[0102] FIG. 12 is a device of the invention similar to that shown in FIGS. 4 to 7 but with a closed end and with electrolyte removal through an aperture in the closed end;

[0103] FIG. 13 is a device of the invention similar to that shown in FIGS. 12 but with electrolyte removal through an aperture in the side wall; and

[0104] FIG. 14 is an image of a tubular material suitable for use in forming a device of the invention. FIG. 14(a) shows the tubular material while FIGS. 14(b) and (c) show enlarged views of part thereof.

DETAILED DESCRIPTION OF THE DRAWINGS

[0105] The present invention proposes a novel device and technique for using Jet-ECM to post-process an internal channel within a workpiece. This is demonstrated using a workpiece formed from Inconel 625 (Inconel 625 is a nickel based superalloy) and that was produced by SLM.

[0106] The process used a device of the invention which is a newly designed jet nozzle system to alter the surface roughness of a 2 mm diameter through hole in the workpiece while analysing the effects of current and translational speed on the surface properties.

[0107] The results show the suitability of the device and method of the invention in a Jet-ECM process to post-process AM parts with internal features.

Experimental Equipment

[0108] This investigation is performed using a Jet-ECM system called R500 developed by Blueacre Technology (the present applicant).

[0109] The system comprises of a 3-axis gantry system with a device of the invention mounted on the z-axis. The working envelope of the system is 400 mm×400 mm×150 mm (X×Y×Z). The electrolyte is circulated from a reservoir to the device of the invention via a near pulseless, variable flow Micropump capable of providing flows up to 1180 ml/minute and 10 bar (1 MPa of pressure. An electric current is provided to the device and the workpiece, via an aluminium workbed, using a 1.5kW programmable power supply. A sensor array monitors pH, electrolyte temperature in the reservoir, flow rate and pressure. The axis movement, voltage and current output are also controlled in this case by being programmed and controlled via a local PC.

[0110] The device of the invention can be utilised within small spaces. As Jet-ECM relies on a small working gap (for example <1mm) between the negatively charged electrode (cathode) and the workpiece (anode) conventional Jet ECM devices used for machining external surfaces would be unable to fit into and machine holes in the workpiece.

[0111] With the present invention the device provides an entirely new nozzle arrangement that was designed and developed to allow machining on the internal features of parts.

[0112] The device 1 of the invention, as made in one configuration for the purposes of demonstrating the invention, has an elongate device body 2 having a first end 3 and a second end 4 and an electrically insulating sidewall 8 with one or more electrolyte delivery apertures 6 defined in the elongate device body 2. In the embodiment an electrode 7 is positioned within the device body 2. The electrically insulating sidewall 8 has an external surface 8a and an internal surface 8b. In this case the outer perimeter of the device body 2 is defined by the electrically insulating sidewall 8 or more particularly external surface 8a thereof.

[0113] The electrode 7 is elongate also and has an outside cross-sectional area C.sub.1 that is a little smaller than the inside cross-sectional area C.sub.2 of the electrically insulating sidewall 8. The electrically insulating sidewall 8 has an outside cross-sectional area C.sub.3.

[0114] This provides an electrolyte delivery conduit 9 which is defined between the electrode 7 and the internal surface 8b of the electrically insulating sidewall 8. In particular the conduit has a cross-sectional area defined by C.sub.2-C.sub.1.

[0115] The device 1 is resiliently deformable, so that in use, it can flex without permanently deforming/kinking. For example the electrically insulating sidewall may be made of a material which can flex such as silicone. And also the electrode 7 can flex also.

[0116] In FIG. 1(a) the device 1 is shown inserted within a conduit in the form of a bore 10a in a workpiece 10. The electrode supply arrangement is for supplying positive pressure to the electrolyte 11 to allow the electrolyte to be supplied to the device so as to provide a continuous supply of electrolyte 11 through aperture 6. An electrolyte supply arrangement delivers electrolyte 11 to, and confines electrolyte 11, within the device body 2 so that the electrolyte 11 can only be delivered through the aperture 6 for electrochemically machining the workpiece 10 in particular for electrochemically machining the internal surface of the bore 10a. Because of the pressure imparted to the electrolyte 11, and the confinement of the electrolyte 11 to the space (represented by C.sub.2-C.sub.1) between the electrode 7 and the internal surface 8b of the electrically insulating sidewall 8, the electrolyte 11 is ejected/delivered from the aperture 6 of the device 1 in the form of an expanding jet 12. The expanding jet 12 may be considered to be frustoconical in shape. The ejected electrolyte 11 impinges on the internal surface of the bore 10a. As the workpiece 10 is oppositely charged to the electrode 7 the arrangement forms an electrochemical cell where the internal surface of the bore 10a can be electrochemically machined.

[0117] It will be appreciated that the device 1 is a close fit within the bore 10a. Should the bore 10a be provided with one or more bends, the device 1 of the invention will be able to follow those bends as it can flex. Also as the external surface of the device of the invention is electrically insulating, there is no risk of shorting between the electrode 7 and the workpiece 10.

[0118] FIG. 1(b) is a photograph of an actual device 1 of the invention with an electrically insulating sidewall 8 in use to machine a workpiece 10 by ejecting electrolyte 11 in the form of an expanding, for example frustoconically shaped, jet 12. This will be described in more detail below.

[0119] In FIG. 1(b) the outer insulating sidewall 8 is provided in the form of an outer insulating silicon tube of length 20 mm which has an internal and external diameter of 1 mm and 1.8 mm respectively. The side wall thickness/width is approximately 0.4 mm.

[0120] An electrode 7 within the device body is provided in the form of a 0.8 mm diameter copper coated wire. The electrode 7 runs through the middle of the tube, with the bottom end located in line with the end of the tube (as shown in the schematic representation of FIG. 1a). The distance between the outer diameter of the wire and the internal wall of the tube allows sufficient electrolyte to flow to the workpiece. The tube is connected to a nozzle holder via a 1/16″ (1.5875 mm) ID hose barb adaptor.

[0121] The electrode (wire) is connected to the nozzle holder which is in turn connected to a power supply to provide a negative current to the electrode 7 so it acts as a cathode.

[0122] The flow out of the nozzle system is pressure dependent, with increasing pressure increasing the diameter of the jet shape.

[0123] Desirably the electrode/wire is held against movement within the tube and thus the flow characteristics and in turn the jet shape is kept constant and is not affected by any such movement.

[0124] A balance between pressure and flow shape should be selected. High pressure can cause an ejection velocity of the electrolyte that is too high to transmit current effectively. On the other hand if the pressure was too low the jet would not expand enough to make contact with the workpiece near to the cathode. A desirable representative flow shape can be seen in FIGS. 1(a) and 1(b).

[0125] FIG. 2 is a photograph of a workpiece 10 with a bore 10a defined therein. This is a real workpiece used in the examples below and it can be considered to be the workpiece 10 represented schematically in FIG. 1(a).

[0126] FIG. 3 is a schematic representation of a layer structure within a device 1 of the invention. In this configuration the device comprises an outer electrically insulating sidewall 8. The electrode 7 is in a reticulated form being a metal braid/mesh comprising a series of interconnected strands 7a of wire defining an array of apertures 7b. An electrically insulting inner sidewall 13 is also provided. This is a multilayer structure with the electrode 7 in a layer between the outer electrically insulating sidewall 8 and the electrically insulating inner sidewall 13. In this arrangement each of the electrode 7 the outer electrically insulating sidewall 8 and the electrically insulating inner sidewall 13 are circular in cross-section. The schematic representation has been shown in a cut-away view to show each of the electrode 7 the outer electrically insulating sidewall 8 and the electrically insulating inner sidewall 13. It will be appreciated that in a device of the invention the electrode 7 will not be exposed as in FIG. 3 but instead covered by the outer electrically insulating sidewall 8. Exposure of the electrode 7 through the outer electrically insulating sidewall 8 will only be through apertures where electrolyte 11 is to be delivered.

[0127] FIG. 4 is a schematic representation of a device of the invention with the same layer structure as FIG. 3 but with apertures 14 in the outer externally insulating sidewall 8. The apertures are in the form of rectangular slots 14a and circular holes 14b. Matching apertures 14 are provided on the opposite side of device 1 as best seen in FIGS. 5 to 7 below.

[0128] Also an exposed area 15 is provided. The exposed area 15 is to allow connection to one or more wires to the electrode 7 for providing a charge/voltage to the electrode. Alternatively an area 15 may also be provided to allow machining through 360 degrees.

[0129] FIG. 5 is a schematic representation of a device 1 of FIG. 4 with arrows 16 indicating electrolyte 11 flow. It will be appreciated that the end 4 of the device 1 is closed and that exit of electrolyte occurs only through apertures 14. The electrolyte 11 will exit as a jet. The shape of the jet is determined by the shape of apertures 14. The electrode 7 is exposed through the apertures 14 and Jet ECM can thus take place. As in this case the electrode 7 is reticulated with apertures 7a it will not interfere with delivery of electrolyte 11 through the outer electrically insulating sidewall 8. However the electrically insulating side wall 8 may have corresponding apertures defined therein to match the apertures in the outer electrically insulating sidewall 8.

[0130] FIG. 6 is the same schematic representation of a device 1 as in FIG. 5 but inserted within a bore 10a of a workpiece 10. The ejected electrolyte impinges on the internal surface at the bore 10 of the workpiece only through apertures 14. In this way machining can be controlled. As described above the workpiece 10 is connected to a positive potential so that the device 1 and the workpiece 10 form a JET-ECM arrangement.

[0131] FIG. 7 shows a similar arrangement to FIG. 6 but with the internal bore 10a of the workpiece 10 having being machined by Jet ECM from the device 1.

[0132] Anodic dissolution of the workpiece has taken place at the areas 17 of the workpiece.

[0133] FIG. 12 shows an alternative arrangement where the device 1 has a closed end 19. So the device body 2 has a closed end. The main difference is in the recirculation/drawing off of electrolyte. In the device 1 in FIG. 12 electrolyte 11 for machining exits the device 1 as indicated by arrow 16. The workpiece 10 is anodically dissolved at area 17. This is similar to earlier embodiments. However, as mentioned herein it is desirable to provide an electrolyte outlet for allowing electrolyte to be drawn out of the workpiece so that a continuous pressurised supply of electrolyte can be provided. In order to remove electrolyte as indicated by arrow 20 an aperture 21 is provided in the closed end 19. A lumen 18 having a tubular sidewall 18a is provided surrounding the aperture 21 and mating with the closed end 19. This means that a pump or other suitable means can be used to draw electrolyte back though aperture 21 into lumen 18 and through the device body 1 to a suitable discharge or reservoir as indicated by arrow 20.

[0134] FIG. 13 shows an alternative arrangement where the device 1 also has a closed end 19. So again the device body 2 has a closed end. Again there is recirculation/drawing off of electrolyte 11. In the device 1 in FIG. 13 electrolyte 11 for machining exits the device 1 as indicated by arrow 16. The workpiece 10 is anodically dissolved at area 17. However, as discussed herein it is desirable to provide an electrolyte outlet for allowing electrolyte within the workpiece to exit, for example be drawn out, through the outlet. In order to remove electrolyte from a workpiece as indicated by arrow 20 an aperture 21 is provided in the externally insulating sidewall 8. A lumen 18 having a tubular sidewall 18a is provided surrounding the aperture 21 and mating with the side wall 8 (and closed end 19). This means that a pump or other suitable means can be used to draw electrolyte back though aperture 21 into lumen 18 and through the device body 1 to a suitable discharge or reservoir as indicated by arrow 20.

[0135] FIG. 14 is an image of a tubular material in the form of a braided or reticulated metal tube within a plastics, in particular polyimide plastic tube. It is suitable for use in forming a device of the invention for example for forming device body 2. For example it shows the desired flexibility. FIG. 14(a) shows the tubular material while FIG. 14(b) shows an enlarged view thereof in which the exposed braided metal which forms the electrode is exposed such as in area 15 above. This may be done by cutting away the plastics (polyimide) material. Also as shown in FIG. 14(c) the plastics tube has been cut away to form slots similar to slots 14a of FIG. 4. These slots 14a will determine the machining pattern on the workpiece 10. It will be appreciated that the nature of the tubular material in FIG. 14 allows the plastics material to be cut away so that desired discrete areas of the electrode formed by the metal braid may be exposed.

[0136] A study was performed where series of tests were carried out using a device as shown in FIG. 2. Each workpiece 10 (FIG. 2) used for this study is an Inconel 625 part that is produced using AM in particular a machine called an EOS M290 which uses direct laser sintering. A series of such workpieces were made.

[0137] A bore 10a of 2 mm nominal diameter runs through the centre of the cross section in each workpiece.

[0138] One sample workpiece 10 was taken and machined into two sections along the length of the bore using wire EDM (electrode discharge machining). The chemical compositions of the workpieces were studied along the length of the bore using SEM imaging in particular an SEM device known as an EDX EVO-50. A total of 13 SEM spectrums were analysed, with a uniform chemical composition found across the samples taken. The SEM scans are labelled “Series A”. The results are seen in Table 1.

TABLE-US-00001 TABLE 1 Inconel 625 Sample material composition across internal surface of bore (%) Spectrum Al Ti Cr Fe Co Ni Mo Series A Mean 3.85 0.91 24.42 1.19 0.2 59.25 10.2 Std. 0.28 0.17 0.28 0.28 0.23 0.73 0.69 Deviation Max. 4.14 1.17 24.73 1.51 0.50 60.51 11.16 Min. 3.46 0.62 23.92 0.67 0 58.13 8.91

[0139] When viewed on an optical microscope (model Keyence VHX-5000) and an SEM (model Hitachi S5000-N), the surface showed an uneven profile. Firstly, there were visible ridges present thought to have been at the interface between two layers during the AM manufacturing process. The images taken with the Keyence machine at 100 times and 500 times magnification are shown respectively in FIG. 8(a) and FIG. 8(b).

[0140] Balling can also be easily seen on the material surface, and one example of balling is highlighted by being circled in FIG. 8(b).

[0141] The surface roughness value of the pre-machined sample was calculated using a NanoFocus pscan non-contact optical profilometer with a sampling rate of 5 μm along the channel (y-direction) and 25 μm across the channel (x-direction). A roughness value of 4.986 μm Ra was found by averaging the values calculated along three separate lengths across the area using a cut-off length of 0.8 mm over a minimum of 5 cut-off lengths. FIG. 9 shows an SEM of the pre-machined channel surface (it is a tilted SEM at 55 times magnification).

[0142] Once stabilised, the set current was supplied to the electrodes, and instantaneously the nozzle began travelling at a set velocity in an upward direction, away from the direction of flow for 4 mm. After this point the power supply stopped providing a current to the electrodes and the nozzle was allowed to travel a further 3 mm to exit the workpiece. At this point the flow of electrolyte was turned off, and the sample removed from the workbed for analysis.

[0143] Prior to the main trials, a series of preliminary trials were performed to test the system (FIG. 10). These trials were performed on a stainless steel plate with a 4×4 matrix of 2 mm diameter holes. Trials were run at 4 different translational velocities, 0.05, 0.1, 0.15 and 0.2 mm/s. At each velocity, currents of 0.2, 0.4, 0.6 and 0.8 A were attempted.

[0144] From these preliminary trials, and the pre-experimental jet shape analysis, the parameters for the main trials were selected and can be seen in Table 2.

TABLE-US-00002 TABLE 2 Parameters for machining trials Parameter Trial 1 Trial 2 Trial 3 Trial 4 Current (A) 0.2 0.4 0.2 0.4 Translational 0.05 0.05 0.15 0.15 Velocity (mm/s) Pressure (bar) 1.4 (+−0.1) 1.4 (+−0.1) 1.4 (+−0.1) 1.4 (+−0.1) Flow Rate (l/min) 0.145 (+−0.1) 0.145 (+−0.1) 0.145 (+−0.1) 0.145 (+−0.1)

[0145] It can be seen that in all the machined workpieces, the surface roughness value was improved over the original sample, with Trial 1 producing the best surface roughness value. It was expected that as the current increases, increasing the amount of material removed, the surface roughness would improve. Alongside this, as the speed decreases, the jet remains over a specific section for a longer period of time after initially breaking down the oxide layer, thus increasing the material removal over a specific section. It is expected that increased material removal will produce the best surface roughness values as the ridges and balling effect found on the workpiece would be most likely removed.

[0146] These characteristics would suggest that Trial 2, with the highest current (0.4 A) and lowest translational speed (0.05 mm/s) would produce the most material removal, and therefore the best surface finish. Although Trial 2 did produce the best surface finish, it was almost identical to the finish produced in trial 1, with trial 1 producing the lowest individual roughness value (2.466 μm Ra). This suggests that at this translational speed, an increase in current provides little benefit above 0.2 A, a suggestion that is repeated at in trials 3 and 4. In trial 3 and 4 an increase in current of 0.2 A provided an improvement of 0.2 μm Ra, a difference of 4% in comparison to the original value.

[0147] These results suggest that the translational velocity has a more significant impact on the surface roughness achieved during the Jet-ECM trials. At an applied current of 0.2 A, the average roughness value produced at 0.05 mm/s translational velocity is 1.19 μm Ra, and at 0.4 A is 0.975 μm Ra. This could be due to the increased time in which machining occurs, with more time to remove the ridges and ‘balling’ after breaking down the oxide layer. However, this improvement comes at a cost of increasing the machining time by three.

[0148] Each machined workpiece was viewed on a Keyence VHX-5000 optical microscope and images were taken at 100 times and 500 times magnification as shown in FIGS. 11(a) to (f).

[0149] In comparison to FIG. 8(b), it is seen in FIGS. 11(a) and 11(b) that the ridges and balling has been greatly reduced. FIG. 11(a) has a slightly less uniform profile from a visual standard, but both images show a reduction in the surface abnormalities. This reduction can again be seen in comparing FIG. 8(a) and FIG. 6c, where the trial at 0.4 A and 0.05 mm/s has greatly improved the surface profile. FIG. 11(d), showing the trial at 0.2 A and 0.15 mm/s shows little visible difference to the original sample workpiece. When looking at the process parameters, it was found that the machining current was not reached until halfway into the machining process, with a much smaller current being passed prior to this. This reduced current can explain the lack of visual improvement, with reduced material removal. However, as the surface roughness value measured indicates an improvement over the original, some machining has been performed but unlike Trials 1 and 2, the surface abnormalities have not been completely removed. FIGS. 11(e) and 11(f) again show that the process parameters used in Trial 4 were not sufficient to remove all the surface abnormalities. There is a visual improvement on the surface, especially when analysing the transitional area between the machined and non-machined areas on the workpiece. This level of material removal does correspond with the roughness values achieved, lying in between the non-machined workpiece and the workpieces produced in trials 1 and 2.

[0150] In all the machined workpieces, oxidisation can be seen on the surface. This layer could affect the roughness values achieved. A further form of chemical post-processing could be used to remove this oxide layer. An electrical discharge, or ‘spark’ occurred in each of the machining trials at the start of the machined section, possibly due to a metallic debris particles in the electrolyte flow forming an electrical bridge, or due to air gaps in the flow. These areas where material was removed by sparking were visible when analysing the workpieces, and were avoided when calculating the surface roughness values.

[0151] Results from this investigation show that in all the trials, the surface roughness of the Inconel 625 internal holes were improved by the Jet-ECM process. The best surface roughness improvement was found when using a working current of 0.4 A and 0.05 mm/s translational speed, reducing the surface roughness from 4.986 μm Ra to 2.662 μm Ra. The results suggest that the translation velocity has a much stronger influence on the surface roughness values achieved than the current used, within the parameters studied in this investigation. Experiments performed at a translational velocity of 0.15 mm/s were unable to entirely remove the surface abnormalities, but did show improvement over the original workpiece, however one of these trials was hampered by a reduced working current.

[0152] This study suggests that Jet-ECM has the ability to alter, and improve the surface profile on the internal features of an Inconel part produced by SLM, and also other metal workpieces produced by other additive manufacturing methods.

[0153] The words “comprises/comprising” and the words “having/including” when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0154] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.