SMOOTHING THE SURFACE FINISH OF ROUGH METAL ARTICLES

Abstract

A process for smoothing the surface of a manufactured metallic workpiece, the workpiece having a region having an initial roughness (Ra) of greater than 2.0 m, the process involving (1) placing the metallic workpiece as the anode in an electrochemical cell, (2) arranging for the temperature of the electrolyte in the vicinity of the anode to be at least 50 C., (3) applying a voltage from 100V to 1000V across the electrochemical cell, thereby to generate a plasma membrane on the surface of the metallic workpiece which acts to remove material from the surface of the metallic workpiece, (4) maintaining the plasma membrane for a period effective to cause the roughness of the workpiece to be reduced.

Claims

1. A process for smoothing a surface of a manufactured metallic workpiece, the workpiece having a region having an initial roughness (Ra) of greater than 2.0 m, the process involving (1) placing the metallic workpiece as an anode in an electrochemical cell, (2) arranging for a temperature of an electrolyte in the vicinity of the anode to be at least 50 C., (3) applying a voltage from 100V to 1000V across the electrochemical cell, thereby to generate a plasma membrane on the surface of the metallic workpiece which acts to remove material from the surface of the metallic workpiece, and (4) maintaining the plasma membrane for a period effective to cause the roughness of the workpiece to be reduced.

2. The process according to claim 1, wherein the initial roughness (Ra) of the workpiece is greater than 4.0 m.

3. The process according to claim 1, wherein a cathode surface area is 2 to 20 times greater than that of an anode surface area.

4. The process according to claim 1, wherein the initial roughness (Ra) of the workpiece is less than 400 m.

5. The process according to claim 1, wherein the temperature of the electrolyte in the vicinity of the anode is from 50 to 100 C.

6. The process according to claim 1, wherein the voltage applied across the electrochemical cell is from 150 to 700V.

7. The process according to claim 1, carried out with a current density at the anode of 0.01 A/cm.sup.2 to 1.00 A/cm.sup.2.

8. The process according to claim 1, wherein a rate of material removal is from 1 to 12 m/min.

9. The process according to claim 1, wherein the plasma membrane is maintained for from 1 to 120 minutes.

10. The process according to claim 1, wherein a pH of the electrolyte is from 3.0 to 12.0.

11. The process according to claim 1, wherein an electrolyte solution concentration is from 1 wt/vol % to 25 wt/vol %.

12. The process according to claim 1, wherein the workpiece has an initial roughness as a result of being manufactured by any one of the following methods: sand casting, investment casting, additive manufacturing, metal cutting (sawing, shaping, drilling, milling, turning), hot rolling, forging, flame cutting, and additive manufacturing.

Description

[0043] The invention will now be illustrated, by reference to the following figures, in which:

[0044] FIGS. 1a to d are photomacrographs showing the surface appearance of AISI 316 s coupons: (a) initial surface texture (blasted surface) before the electrolytic plasma treatment; (b) after electrolytic plasma treatment for 5 minutes; (c) after electrolytic plasma treatment for 10 minutes and (d) after electrolytic plasma treatment for 15 minutes.

[0045] FIGS. 2a and b show material removal data for electrolytic plasma treatment of AISI 316 stainless steel. (a) Mass loss versus electrolytic plasma treatment (EPT) time and (b) Thickness loss versus electrolytic plasma treatment (EPT) time. Electrolytic solution: 2.5% (NH.sub.4).sub.2SO.sub.4 aqueous solution.

[0046] FIGS. 3a and b show 3D-roughness parameters obtained by 3D surface profilometry on coarsely blasted AISI 316 steel coupons before and after various electrolytic plasma treatment times. (a) S.sub.a and S.sub.q; (b) S.sub.z.

[0047] FIGS. 4a to d show 3D surface images and line profiles of AISI 316 steel coupons: (a) before electrolytic plasma treatments (i.e. initial blasted surface); (b) after electrolytic plasma treatment for 5 minutes; (c) after electrolytic plasma treatment for 10 minutes and (d) after electrolytic plasma treatment for 15 minutes.

[0048] FIG. 5 is a photomacrograph showing the surface appearance of a 3D-printed 15-5 PH steel component. Region (a): initial surface texture of 3D-printed surface. Region (b): after electrolytic plasma treatment (EPT) for 15 minutes.

[0049] FIGS. 6a and b are charts showing 3D-roughness paramenters obtained by 3D surface profilometry on 3D-printed 15-5 PH steel component before and after electrolytic plasma treatment (EPT) for 15 minutes. (a) S.sub.a and S.sub.q; (b) S.sub.z.

[0050] FIGS. 7a and b are 3D surface images and line profiles of 3D-printed 15-5 PH steel component before (a) and after (b) electrolytic plasma treatment (EPT) for 15 minutes.

EXAMPLES

[0051] The following examples will demonstrate how the electrolytic plasma treatment used in this invention can significantly improve the surface finish of very rough article surfaces (possessing initial surface roughness values of S.sub.a and S.sub.z2 m (i.e. in the macro-finish range)) by significantly reducing the initial S.sub.a- and S.sub.z-values.

Example 1

Electrolytic Plasma Treatment on Coarsely Blasted AISI 316 Stainless Steel Samples

[0052] AISI 316 stainless steel samples (3.0 cm2.5 cm0.2 cm) were coarsely blasted with 120-150 grit pink alumina to achieve initial average S.sub.a- and S.sub.z-values of 2.40.1 m and 525 m respectively.

[0053] The electrolytic plasma treatment set-up consisted of a 2 litre glass beaker in which a cathode made of an AISI 316 stainless steel sheet (37 cm10.5 cm0.1 cm) was rolled and wrapped around the inner wall of the beaker. The effective surface area of the cathode was 388.5 cm.sup.2. The 1.5 litre electrolytic solution used to plasma treat the coupons was an aqueous solution of 2.5% (NH.sub.4).sub.2SO.sub.4 (37.5 g of (NH.sub.4).sub.2SO.sub.4 in 1500 ml of water).The beaker arrangement was placed inside a water bath at a temperature of 181 C. The water bath volume was 17490 cm.sup.3. The solution temperature was monitored until it reached a value of T=601 C. before the electrolytic plasma treatment commenced. The solution pH at this temperature was 6.5.

[0054] An Advanced Energy MDX II 30 kW DC power supply was used in the voltage controlled mode. The AISI 316 coupon was the anode workpiece. The voltage was set to a value of 250V in the power supply, which was then switched on. The energised anode workpiece (AISI 316 steel coupon) was fully immersed in the electrolytic solution and treated for a specific time. The resulting voltage and current during the electrolytic plasma treatment were 160-250 V DC and 25-40 A respectively. Oscillations in voltage and current occurred due to micro-discharges that take place during the process (a plasma envelope formed around the coupon). This current range corresponded to a current density of 0.1 A/cm.sup.2.

[0055] AISI 316 steel coupons were electrolytically plasma treated for 5 minutes, 10 minutes and 15 minutes in the 2.5% (NH.sub.4).sub.2SO.sub.4 aqueous solution. Samples were weighed before and after each electrolytic plasma treatment to estimate volume loss (obtained by dividing mass loss by the density of the AISI 316 steel, 7.99 g/cm.sup.3). Thickness loss was also determined by dividing the volume loss by the total nominal surface area of the coupons (17.2 cm.sup.2). Finally the material removal rate was to estimated from the gradient of the plot between volume loss or thickness loss and electrolytic plasma treatment time. Surface roughness parameters (S.sub.a, S.sub.q and S.sub.z) were measured before and after electrolytic plasma treatments using a Zemetrics ZeScope 3D surface profiler. The sampled area was 449 m335 m for all 3D-surface roughness measurements.

[0056] Photomacrographs showing the initial surface texture of the coarsely blasted AISI 316 steel surface and after electrolytic plasma treatments for 5, 10 and 15 minutes are shown in FIG. 1. The dull and rough appearance of the blasted surface (FIG. 1a) progressively improves to a silver and shiny appearance from left to right after a total electrolytic plasma treatment of 15 minutes (FIG. 1d). Material removal data for the AISI 316 stainless steel for electrolytic plasma treatments in a 2.5% (NH.sub.4).sub.2SO.sub.4 aqueous solution are illustrated in FIG. 2. Under the experimental conditions herein reported, a small material removal rate of 3.2 m/minute resulted, suggesting that such electrolytic plasma treatments have great potential to be applied to high-precision parts where small tolerances must be retained.

[0057] 3D-surface roughness data for the coarsely blasted AISI 316 stainless steel is depicted in FIG. 3. Initial S.sub.a- and S.sub.z-values (higher than 2 m and 50 m respectively) were significantly reduced after an electrolytic plasma treatment (EPT) for 15 minutes. Three and five-fold reductions in S.sub.a- and S.sub.z-values were accomplished after a 15 minute-treatment. Moreover, 3D images and line profiles of these samples are shown in FIG. 4. A systematic reduction in profile peaks occurs as the electrolytic plasma treatment duration increases. After a 15 minute-treatment (FIG. 4d) the resulting surface texture is considerably more even and smooth compared to that of the untreated surface which was very rough and uneven (FIG. 4a).

Example 2

Electrolytic Plasma Treatment on 3D-printed 15-5 PH Steel Component

[0058] A 15-5PH component manufactured by 3D-printing was also trialled in this invention. The post-manufacturing surface roughness of this component was very high, characterised by S.sub.a=6.51.7 m and S.sub.z=57.18.3 m (very rough surface). High standard deviations in both S.sub.a- and S.sub.z-values reflect the uneven character of the component surface texture.

[0059] A similar experimental set-up to the one described in Example 1 was used. A 1.5 litre aqueous solution of 2.5% (NH.sub.4).sub.2SO.sub.4 (37.5 g of (NH.sub.4).sub.2SO.sub.4 in 1500 ml of water) was used to plasma treat the 3D-printed component. The beaker arrangement was placed inside a water bath at a temperature of 201 C. (water bath volume was also 17490 cm.sup.3). The solution temperature was monitored until it reached a value of T=621 C. before the electrolytic plasma treatment commenced. The solution pH at this temperature was 6.5. An Advanced Energy MDX II 30 kW power supply was used in the voltage controlled mode. The 3D-printed 15-5 PH steel component was the anode workpiece. The voltage was set to a value of 230V in the power supply. The power supply was switched on to energise the 3D-printed 15-5 PH steel component (anode workpiece). Then the bottom one-third of the component was immersed in the electrolytic solution and treated for 15 minutes. The remaining top two-thirds of the component were kept out of the electrolytic solution (i.e. left untreated). The resulting voltage and current during the electrolytic plasma treatment were 170-230 V DC and 25-35 A respectively. These conditions led to the formation of a plasma envelope around the immersed part of the component and micro-discharges (arcing) were present. Surface roughness parameters (S.sub.a, S.sub.q and S.sub.z) were measured before and after electrolytic plasma treatments using a Zemetrics ZeScope 3D surface profiler. The sampled area was 449 m335 m for all 3D-surface roughness measurements.

[0060] FIG. 5 depicts a photomacrograph of the 3D-printed component showing electrolytically plasma treated (Region a) and untreated (Region b) areas. The treated area (Region a) exhibits a shiny appearance compared to the rather dull and matt untreated area (Region b). This finding was confirmed by 3D surface profilometry measurements (FIG. 6). A considerable reduction in S.sub.a and S.sub.z was achieved after electrolytic plasma treatment (EPT) for 15 minutes (S.sub.a lowered from 6.51.7 m down to 1.70.1 m and S.sub.z lowered from 57.18.3 m down to 14.50.4 m, resulting in a significant smoother and shinier surface. 3D images and line profiles (FIG. 7) corroborate the benefit of the electrolytic plasma treatment on the very rough 3D-printed component. A much more even profile characterised by a considerable reduction in surface roughness resulted after the 15 minute electrolytic plasma treatment (FIG. 7b) compared to the untreated 3D-printed component part (FIG. 7a).