CHEMICAL POLISHING BATH FOR ALUMINUM AND ALUMINUM ALLOYS, AND METHOD USING SUCH A BATH

Abstract

A chemical polishing bath for polishing a part made of aluminum or of aluminum alloy, or a portion of such a part, which bath includes an oxidizing agent capable of attacking the aluminum, chosen from nitric acid, hydrogen peroxide, permanganate, or a mixture thereof, at a concentration between 1.2 and 3.0 mol/L. The polishing bath also includes a fluoride complexing agent capable of forming a complex with the oxidized aluminum, at a concentration between 0.3 and 1.6 mol/L; a catalyst (Cu); and phosphoric acid at a concentration between 10.10 and 14.30 mol/L and also sulfuric acid at a concentration between 1.50 and 3.60 mol/L. The bath is particularly suitable for polishing parts resulting from additive manufacturing (3D printing).

Claims

1. A chemical polishing bath for polishing an aluminum or aluminum alloy component, or a part thereof, said chemical polishing bath comprising: an oxidizing agent capable of attacking aluminum selected from nitric acid, hydrogen peroxide, permanganate, or one of the mixtures thereof, at a concentration of between 1.2 and 3.0 mol/L; a fluoride complexing agent capable of forming a complex with oxidized aluminum at a concentration of between 0.3 and 1.6 mol/L; a catalyst; and phosphoric acid at a concentration of between 10.10 and 14.30 mol/L together with sulfuric acid at a concentration of between 1.50 and 3.60 mol/L.

2. The chemical polishing bath as claimed in claim 1, wherein the fluoride complexing agent is added to the bath in the form of NH.sub.4F.Math.HF, NaF HF, KF HF, SiF.sub.6 or mixtures thereof.

3. The chemical polishing bath as claimed in claim 1, wherein the fluoride complexing agent is present at a concentration of between 0.6 and 1.2 mol/L.

4. The chemical polishing bath as claimed in claim 1, wherein the oxidizing agent is present at a concentration of between 1.3 and 2 mol/L.

5. The chemical polishing bath as claimed in claim 1, wherein the phosphoric acid is present at a concentration of between 11 and 13 mol/L, and/or the sulfuric acid at a concentration of between 1.6 and 3 mol/L.

6. The chemical polishing bath as claimed in claim 1, wherein the catalyst is copper and copper is present in an amount of 0.10 to 0.20 mol/L.

7. The chemical polishing bath as claimed in claim 1 comprising aluminum, present at a concentration ranging from 1 to 50 g/L, namely between 0.037 and 1.85 mol/L.

8. The chemical polishing bath as claimed in claim 1, said bath having a density of from 1.6 to 1.8 g/cm.sup.3.

9. A chemical polishing method for polishing an aluminum or aluminum alloy component, or a part thereof, the method including the following steps: (i) providing a chemical polishing bath as claimed in claim 1, (ii) immersing an aluminum or aluminum alloy component to be polished, or a part thereof in the chemical polishing bath, and; (iii)removing the component after a predetermined immersion time.

10. The chemical polishing method as claimed in claim 9, wherein the immersion time is between 5 and 15 minutes.

11. The chemical polishing method as claimed in claim 9, wherein the bath is agitatedduring the immersion.

12. The chemical polishing method as claimed in claim 9, wherein the aluminum or aluminum alloy component, or the part thereof, to be polished is subjected to a chemical degreasing and/or pickling step before being immersed in the chemical polishing bath.

13. The chemical polishing method as claimed in claims 9, wherein the component to be polished, or the part thereof, is made of aluminum alloy belonging to the 10000 to 70000 series.

14. The chemical polishing method as claimed in claim 9, wherein the aluminum or aluminum alloy component to be polished is obtained by an additive manufacturing method.

15. The chemical polishing bath as claimed in claim 3, wherein the fluoride complexing agent is present at a concentration of between 0.8 and 1.1 mol/L.

16. The chemical polishing bath as claimed in claim 4, wherein the oxidizing agent is present at a concentration of between 1.4 and 1.6 mol/L.

17. The chemical polishing bath as claimed in claim 5, wherein the phosphoric acid is present at a concentration of between 11.5 and 12 mol/L, and/or the sulfuric acid at a concentration of between 1.7 and 2.2 mol/L.

18. The chemical polishing bath as claimed in claim 6, wherein copper is added to the bath in the form of a copper salt, the copper salt being selected from a group composed of copper sulfate, copper nitrate or mixtures thereof.

19. The chemical polishing bath as claimed in claim 7, wherein aluminum is present at a concentration ranging from 1 to 30 g/L, namely between 0.037 and 1.1 mol/L.

20. The chemical polishing method as claimed in claim 9, wherein the chemical polishing bath is maintained at a temperature of between 70° C. and 100° C.

21. The chemical polishing method as claimed in claim 10, wherein the immersion time is between 7 and 12 minutes.

22. The chemical polishing method as claimed in claim 11, wherein the bath is agitated at an agitation rate of between 5 and 10 L/min, during the immersion.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0053] Other details and features of the disclosure will emerge from the following detailed description of at least one advantageous embodiment, provided below by way of illustration with reference to the appended drawings, in which:

[0054] [FIG. 1] shows diagrams representing (points 1 to 6 of) the principle of operation of the present method;

[0055] [FIG. 2] shows confocal micrographs comparing the surface state of an AS7G06 aluminum alloy component before (a) and after (b) chemical polishing using a first preferred embodiment of a chemical polishing bath according to the disclosure; and

[0056] [FIG. 3] shows confocal micrographs comparing the surface state of an AS7G06 aluminum alloy component before (a) and after (b) chemical polishing using a second preferred embodiment of a chemical polishing bath according to the disclosure.

DETAILED DESCRIPTION OF THE FIGURES WITH REFERENCE TO EXAMPLES

[0057] The principle of operation of the polishing bath according to the disclosure will firstly be explained with reference to FIG. 1. As explained previously, the present disclosure surprisingly proposes a polishing bath which allows roughness to be significantly diminished, without increasing size reduction, whereas these two effects generally entail antagonistic parameters.

[0058] The increased viscosity of the bath, which is controlled by the sulfuric and phosphoric acid, makes it possible to slow the diffusion phenomena of the chemical species of the solution toward the surface of the component. This depletion of active species on the surface, due to their being consumed in the vicinity of the surface, is responsible for a surface polishing effect by selective dissolution of the surface relief.

[0059] The principle is shown in FIG. 1: [0060] 1): attack and oxidation of the material by the oxidizer and formation of a first passivation layer (metal oxides from aluminum and alloy elements); [0061] 2, 3): complexation of the aluminum and alloy oxides by the complexing agents to solubilize the oxides; [0062] 4, 5): slow diffusion of the oxidizing compounds from the solution toward the surface of the alloy to be polished, the sulfuric acid and phosphoric acid making it possible to slow the arrival of the oxidizing compounds; [0063] 6): attack of the upper parts of the relief, bringing about the reduction in roughness.

EXAMPLES

[0064] Two examples of polishing aluminum components in two different baths, baths 1 and 2, according to the disclosure, together with two examples of treating aluminum components in two different baths, baths 3 and 4, not in accordance with the disclosure (comparative examples).

[0065] Four AS7G06 aluminum alloy components denoted A to D resulting from the same additive manufacturing method take the form of a plate of dimensions 60 × 40 × 6 mm. The upper face is the face on which the layers of material are added one after the other during the manufacturing method, while the lower face is the opposite face. The plates were first subjected to a chemical degreasing and pickling step with nitric acid in order to prepare their surface using conventional methods familiar to a person skilled in the art. The four components A to D were then each partially covered with a self-adhesive masking tape.

[0066] The resultant partially masked aluminum alloy components A and B were then subjected to chemical polishing methods using polishing baths of different compositions, denoted 1 and 2, according to two preferred but non-limiting embodiments of the disclosure. The resultant partially masked aluminum alloy components C and D were then subjected to chemical polishing methods using comparative polishing baths of different compositions, denoted 3 and 4, which were not according to the disclosure. The four different baths 1 to 4 were prepared in similar tanks with solution volumes of 4 L; their compositions are set out in Table 1.

TABLE-US-00001 Sample H.sub.2SO.sub.4 g/L H.sub.3PO.sub.4 g/L HNO.sub.3 g/L Aluminum g/L NH.sub.4F.square-solid.HF g/L C.sub.USO.sub.4.square-solid.5H.sub.2O g/L T °C Duration min Bath 1 180 1140 100 10 50 25 85 10 Bath 2 300 1140 100 20 50 50 85 10 Bath 3 180 950 100 10 100 25 85 10 Bath 4 300 1140 100 10 100 50 85 10 Bath 1 1.88 11.6 1.55 0.37 0.88 0.10 85 10 Bath 2 3.06 11.6 1.55 0.74 0.88 0.20 85 10 Bath 3 1.88 10.0 1.55 0.37 1.76 0.10 85 10 Bath 4 3.06 11.6 1.55 0.37 1.76 0.20 85 10

[0067] Aluminum alloy component A was immersed in bath 1, component B in bath 2, component C in bath 3 and component D in bath 4. For each of the components, the zone covered by the self-adhesive masking tape is not in direct contact with the bath in which the component is immersed and does not undergo any change in its surface state.

[0068] For the purposes of the present disclosure, since the elevated viscosity of the polishing bath (typically a density of 1.6 to 1.8 g/cm.sup.3) and the local nonuniformities in concentration (due to the thickness of the diffusion layer) of oxidizing agent and complexing agent are crucial to controlling polishing and promoting chemical attack of the peaks, agitation of the bath when the component to be treated is immersed therein is strictly controlled. The bath is preferably agitated at a lower agitation rate when the component to be polished is immersed (treatment phase) than during preparation of the polishing bath (homogenization phase). This agitation rate is typically between 5 and 10 L/min during the treatment phase and around 100 L/min during homogenization phases. These agitation rate values are given solely by way of example and do not limit the present disclosure. In particular, since the agitation rate influences the renewal of chemical species present in the liquid layer, known as the diffusion layer, in the vicinity of the component to be treated, a lower agitation rate enables the formation of a thicker diffusion layer, which slows the diffusion of reactants and promotes polishing.

[0069] After immersion in the polishing bath, the treated component was rinsed and then observed under a confocal microscope in order to determine the decrease in its roughness and its size reduction. In particular, for each of the components, the self-adhesive masking tape was removed and a zone comprising a treated surface and an untreated surface (because it had previously been masked by the self-adhesive masking tape) was observed using an Olympus DSX510 microscope. The topography of each of the surfaces was recorded and analyzed in order to determine the size reduction due to material removal and the variations in roughness parameters due to the polishing effect of the different baths.

[0070] Table 2 shows the surface roughnesses obtained after manufacture (untreated surface) and resulting from the chemical polishing (treated surface) for aluminum alloy components A to D, together with the size reduction of the components which has taken place during this polishing. In said table, the expression before polishing refers to the part of the surface which was masked by the self-adhesive masking tape during immersion in the bath, while the expression after polishing refers to the portion of the surface which was not masked by the tape and was subjected to the chemical polishing treatment.

[0071] As is apparent from Table 2 together with FIG. 2a and FIG. 3a, components A to D obtained by the additive manufacturing method have a similar surface state before polishing.

[0072] After polishing, components A to D have all undergone a size reduction on each of their faces (Table 2). FIG. 2b and FIG. 3b together with Table 2 show a great decrease in the surface relief of the components treated with the two baths according to the disclosure, and therefore polishing of components A and B. It should be noted that, among the two components treated with a bath according to the disclosure, component B exhibits a greater size reduction than component A (Table 2), which may be linked to a doubling of catalyst concentration between bath 2 and bath 1. This is because the catalyst promotes chemical attack and therefore size reduction.

TABLE-US-00002 Components treated with a bath according to the disclosure Component A - upper face Component A - lower face Component B - upper face Component B - lower face Before polishing After polishing Before polishing After polishing Before polishing After polishing Before polishing After polishing Ra .Math.m 22.65 0.99 18.11 1.13 19.22 2.17 19.31 2.75 Rz .Math.m 151.47 5.9 137.74 6.83 124.88 15.35 117.12 20.69 Sa .Math.m 17.10 1.21 15.17 1.40 18.42 1.72 17.20 2.03 Size reduction .Math.m 187.4 174.9 283.1 234.7 Components treated with a comparative bath Component C - upper face omponent C - lower face Component D - upper face Component D - lower face Before polishing After polishing Before polishing After polishing Before polishing After polishing Before polishing After polishing Ra .Math.m 18.55 7.58 17.93 6.66 23.90 37.77 19.70 28.32 Rz .Math.m 179.37 43.73 144.93 48.70 156.80 220.42 170.56 186.74 Sa .Math.m 18.01 4.22 14.52 4.17 19.86 29.07 17.02 25.35 Size reduction .Math.m 397 314 70.6 9.3

[0073] A comparison of the results obtained with bath 2 (according to the disclosure) and with bath 4 (comparative example) reveals that increasing the concentration of complexing agent enables a significant decrease in size reduction. Said size reduction is indeed only 9.3 .Math.m for the lower face of component D as compared to 234.7 .Math.m for the lower face of component B. However, the surface roughness of component D has not decreased during processing in bath 4 and is on the contrary higher after treatment than it was before treatment (Table 2). There is no polishing effect.

[0074] A comparison of the results obtained with bath 1 (according to the disclosure) and with bath 3 (comparative example) reveals that increasing the concentration of complexing agent while decreasing the concentration of phosphoric acid enables a significant decrease in the surface roughness of the treated component (Table 2). However, the size reduction of each of the faces of component C treated with bath 3 (comparative example) is almost twice that of the faces of component A treated with bath 1 (according to the disclosure). Bath 3 is a chemical machining bath and not a chemical polishing bath.