METHOD FOR POLISHING PARTS MADE OF ALUMINUM

Abstract

The present invention relates to a method for polishing parts made of aluminum, and more specifically parts made of aluminum with a high content of silicon and produced by additive manufacturing.

Claims

1. A method for polishing parts made of aluminum alloys comprising silicon, said method comprising the steps of: a) introducing the part in an alkaline aqueous solution with a concentration comprised between 0.1% and 15%; b) after step a), introducing the part in an alkaline aqueous solution with a concentration comprised between 1% and 70%; c) removing the smut generated in steps a) and b).

2. The method for polishing according to claim 1, characterized in that the alloy is AlSi10Mg.

3. The method for polishing according to claim 1, characterized in that the part has been manufactured by means of selective laser melting.

4. The method for polishing according to claim 1, characterized in that step a) is carried out in a time comprised between 15 seconds and 5 minutes.

5. The method for polishing according to claim 1, characterized in that step b) is carried out in a time comprised between 5 minutes and 15 minutes.

6. The method for polishing according to claim 1, characterized in that after step b), there is a step c) of introducing the part in a basic aqueous solution at a concentration comprised between 0.1% and 15%.

7. The method for polishing according to claim 6, characterized in that the part is in the basic aqueous solution for a period comprised between 30 seconds and 30 minutes.

8. The method for polishing according to claim 1, characterized in that step a) and/or b) and/or c) are carried out in the presence of a complexing compound and/or a thickening compound.

9. The method for polishing according to claim 1, characterized in that in step c) the part is introduced in an acidic solution in water at a concentration comprised between 0.1% and 10%.

10. The method for polishing according to claim 9, characterized in that step c) is carried out in a time comprised between 30 seconds and 5 minutes.

11. The method for polishing according to claim 1, characterized in that step c) is carried out by means of introducing the part under stirring in an aqueous solution of abrasive components selected from glass, corundum, and/or zirconium oxide.

12. Method according to claim 11, characterized in that the time of step e) is between 30 minutes and 8 hours.

13. The method for polishing according to claim 1, characterized in that in step c) the part is introduced in an acidic solution in water at a concentration comprised between 0.1% and 10% for a time comprised between 30 seconds and 5 minutes, and the part is subsequently introduced under stirring in an aqueous solution of abrasive components selected from glass, corundum, and/or zirconium oxide.

14. The method for polishing according to claim 1 characterized in that the step c) the part is introduced in an acidic aqueous solution at a concentration comprised between 0.1% and 10% with ultrasonic agitation.

15. The method according to claim 1, characterized in that step a) and/or b) and/or c) are carried out in the presence of surfactants.

16. The method according to any claim 1, characterized in that after step c) there is a step d) of introducing the part in an acid solution with a concentration comprised between 0.1% and 15%.

17. The method for polishing according to claim 1, characterized in that step c) is carried out in the presence of a corrosion inhibitor.

18. The method for polishing according to claim 1, characterized in that the steps a) and b), or steps a), b), and c) are repeated in the same number and order n times.

Description

DESCRIPTION OF THE INVENTION

[0017] The polishing described in the present invention improves the surface quality and dimensional precision of the polished parts. Parts which are polished with the method of the present invention are made of aluminum alloys comprising silicon. Due to the high silicon content, the known treatments for polishing aluminum are not effective.

[0018] The present method for polishing is especially useful in parts obtained by means of additive manufacturing, namely by means of SLM (selective laser melting), in that the characteristics of the surface of these parts are the high roughness and the high reactivity they exhibit in response to etching.

[0019] The method of the invention comprises a series of steps wherein the surface is activated and sensitized for the following steps of reducing surface roughness; chemical polishing is carried out wherein roughness is reduced in a controlled manner and selective etching of the roughness peaks is carried out; and finally, the smut that occurs is removed in order to continue polishing or to give it a final finish, given that the occurrence of the layer of smut means that the surface is passivated, preventing etching thereof. Activation of the surface is carried out by means of an alkaline solution; the following step of polishing is also performed by means of an alkaline solution, but with a higher concentration than the solution of the first step; finally, a step of removing depositions of metal oxides and depositions of other insoluble products generated in the alkaline steps, i.e., smut, is necessary.

[0020] The method of the invention is a method that can be readily implemented on an industrial level and is low-risk for operators.

[0021] Therefore, a first object of the invention is a method for polishing parts made of aluminum alloys comprising silicon, said method comprising the steps of: [0022] a) introducing the part in an alkaline aqueous solution with a concentration comprised between 0.1% and 15%; [0023] b) after step a), introducing the part in an alkaline aqueous solution with a concentration comprised between 1% and 70%; [0024] e) removing the smut generated in steps a) and b).

[0025] In the present invention the term “concentration” refers to weight concentration (w/w) when the solute is solid in the aqueous solution and volume concentration (v/v) when the solute is liquid in the aqueous solution.

[0026] When the term smut is used herein, it refers to depositions of mixed metal oxides and depositions of other insoluble products generated in steps a) and b).

[0027] As mentioned, polishing is preferably advantageous in elements having a complex geometry, as well as in cavities and internal conduits.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0028] As discussed above, the first aspect of the invention relates to a method for polishing parts made of aluminum alloys with a silicon content, said method comprising the steps of: [0029] a) introducing the part in an alkaline aqueous solution with a concentration comprised between 0.1% and 15%; [0030] b) after step a), introducing the part in an alkaline aqueous solution with a concentration comprised between 1% and 70%; [0031] e) removing the smut generated in steps a) and b).

[0032] The aluminum alloy preferably comprises silicon in a percentage by weight with respect to the total weight of 1% to 15%, more preferably 6% to 12%.

[0033] The alloy is preferably selected from: AlSiMgTi, AlSi7Mg, AlSi10Mg, AlSi, AlSi5Cu, AlSi9Cu, AlSiCuNiMg, AlSiSc. More preferably the alloy is selected from: AlSi7Mg, AlSi10Mg. In a particular manner it is AlSi10Mg.

[0034] The element to be polished has preferably been manufactured by additive manufacturing. In the present specification, the term “additive manufacturing” refers to the process of attaching materials together to create objects from a 3D model, where the attaching is usually layer after layer.

[0035] The element has preferably been manufactured by means of selective laser melting (SLM). Said method allows obtaining highly complex geometries. The method of the invention homogeneously polishes the complex geometries obtained by selective laser melting.

[0036] In step a), the alkaline solution is preferably selected from a solution of a hydroxide of alkali and/or alkaline earth metals or mixtures thereof, more preferably NaOH and/or KOH. The alkaline solution of step a) preferably has a concentration between 0.1% and 10%. Step a) is preferably carried out in a time comprised between 15 seconds and 5 minutes; more preferably between 15 seconds and 60 seconds.

[0037] Step a) is preferably carried out in the presence of a complexing compound.

[0038] Step a) is preferably carried out in the presence of a thickening compound. In the step a) the concentration of the complexing compound and/or the concentration of the thickening compound is between 1% and 5%.

[0039] In step b), the alkaline solution is preferably selected from a solution of a salt of oxoanions derived from weak acids or mixtures thereof, the oxoanions are more preferably selected from ClO.sup.−, CO.sub.3.sup.2−, HS.sup.−. The alkaline solution of step b) preferably has a concentration between 5% and 50%, more preferably between 20% and 50%. Step b) is preferably carried out in a time comprised between 5 minutes and 15 minutes. Step b) is preferably carried out in the presence of a complexing compound. Step b) is preferably carried out in the presence of a thickening compound. In the step b) the concentration of the complexing compound and/or the concentration of the thickening compound is between 1% and 5%.

[0040] Step b) is preferably carried out in the presence of a surfactant. Example of surfactants are: ethoxyl propoxyl terpene, dialkylthiourea, ethylene glycol monobutyl ether, nonylphenol ethoxylate, C.sub.12-C.sub.18 fatty acids, C.sub.12-C.sub.18 ethoxylated fatty acids, (C.sub.10-C.sub.13)alkyl benzene sulfonic acid, lauryl ether sulfate, cocamidopropyl betaine, cocoamine, alkyl dimethyl amine oxide, cocoyl bis-(2-hydroxyethyl) amine oxide, lauramine oxide, N,N-dimethyltetradecyl amine oxide, quaternary ammonium compounds, benzyl-C.sub.12-16-alkyldimethyl.

[0041] In a more preferred embodiment, the method comprises a step c) of introducing the part in a basic aqueous solution at a concentration comprised between 0.1% and 15%, preferably for a period comprised between 5 seconds and 30 minutes, or more preferably between 5 seconds and 300 seconds. The alkaline solution is more preferably selected from a solution of a hydroxide of alkali and/or alkaline earth metals or mixtures thereof, more preferably NaOH and/or KOH.

[0042] Step c) is preferably carried out in the presence of a complexing compound. Step c) is preferably carried out in the presence of a thickening compound.

[0043] The complexing agent is preferably selected from: citric acid, gluconic acid, sorbitol, iminodisuccinate acid, hydroxyethylethylendiaminetriacetic acid (trilon D), methylglycinediacetic acid (MGDA), diethylenetriamine penta(methylene phosphonic acid) (DTPMP), 2-phosphonobutane 1,2,4-tricarboxylic acid, iminosuccinic acid, polyaspartic acid, polyamino phosphonates, thiosulfuric acid, acrylic acid polymer, polyethylenimine, ethylenediaminetetraacetic acid, aminopolycarboxylic acid, diethylenetriaminepentaacetate, nitriloacetate, triphosphate acid, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), phosphonic acid, β-alaninediacetic acid, N-bis[2-(1,2-dicarboxy-ethoxy)ethyl]glycine (BCAS), N-bis[2-(1,2-dicarboxyethoxy)ethyl]-aspartic acid (BCA6), tetrakis(2-hydroxypropyl)ethylenediamine (THPED), N-(hydroxyethyl)-ethylenediaminetriacetic acid (HEDTA), and salts thereof. In a particular manner, the complexing compound is sorbitol.

[0044] The thickening compound is preferably selected from: polyamines, modified ureas, activated phyllosilicates, carboxymethylcellulose, and bentonite.

[0045] Preferably after the step c) there is a step d) of introducing the part in an acid solution with a concentration comprised between 0.1% and 15%. The step d) is preferably carried out in the presence of a surfactant.

[0046] Step e) of removing smut is preferably carried out by means of introducing the part in an acidic solution in water at a concentration comprised between 0.1% and 10%. The acids are more preferably selected from hydrochloric acid, hydrofluoric acid, nitric acid, sulfuric acid, or mixtures thereof. This step is more preferably carried out in a time comprised between 30 seconds and 30 minutes.

[0047] Step e) is preferably carried out by means of introducing the part under stirring in an aqueous solution of abrasive components selected from glass, corundum, and/or zirconium oxide. The time of this step d) is preferably between 30 minutes and 8 hours. Prior to introducing the part in this aqueous solution of abrasive components, the part is preferably introduced in the acidic solution in water at a concentration comprised between 0.1% and 10% described in the preceding paragraph.

[0048] This step e) is more preferably performed in the presence of a surfactant compound and/or a corrosion inhibitor additive.

[0049] The corrosion inhibitor additive is preferably a compound which belongs to the dialkylthiourea group. It is more preferably selected from citric acid, ascorbic acid, 3-aminooctanol, 2-amino-2-methyl-1-propanol, monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), monoisopropyl amine (MIPA), methoxypropylamine (MOPA), diglycolamine (DGA), N-alkylalkanolamine.

[0050] Particularly the step e) is performed in the presence of acetic acid and with ultrasonic agitation. More particularly acetic acid at a concentration from 0.1% to 10%. This step e) is preferably carried out in a time comprised between 5 minutes and 15 minutes. When the step e) is carried out in the presence of acetic acid and with ultrasound the process time is reduced, and the finish is much better, in terms of the visual aspect of the pieces.

[0051] Method steps a) and b), or a), b), and c) of the aspect of the invention or of any of the embodiments thereof are preferably repeated in the same number and order n times, preferably from two to ten times.

[0052] Method steps a), b), and d), or a), b), c), and e) of the aspect of the invention or of any of the embodiments thereof are preferably repeated in the same or different number n times, preferably from two to ten times.

[0053] After each step a), b), c), and/or e), the part is preferably introduced in a distilled water bath.

EXAMPLES

[0054] The following examples are merely illustrative of this invention and must not be interpreted in a sense that is limiting thereof.

Example 1

[0055] A part made of AlSi10Mg obtained by means of SLM (selective laser melting) is used.

[0056] The part is submerged consecutively in a bath of a 1.5% NaOH alkaline solution for 30 seconds followed by a bath of a 25% potassium carbonate alkaline solution for 5 minutes.

[0057] Once these first two steps were performed, a new step of immersing the part in a 1.5% NaOH alkaline solution for 5 minutes and finally introducing it in an acidic solution of 10% HCl by weight for 3 minutes to remove the smut is carried out.

[0058] In this example of the invention, a part with Ra=9.38 μm and Rz=93.0 μm was used to reach, after the described polishing treatment, a reduction in Ra of 67% and a reduction in Rz of 73%, since a value of Ra=3.24 μm and Rz=24.7 μm was reached.

Example 2

[0059] In the same manner as in Example 1, a part made of AlSi10Mg obtained by means of SLM (selective laser melting) is used.

[0060] The part is submerged in the following baths for a given time, in the order and in the number described in Table 1:

[0061] Bath 1: 0.5% NaOH alkaline solution;

[0062] Bath 2: 25% potassium carbonate alkaline solution;

[0063] Bath 3: acidic solution of 1% HCl w/w (0.37%)

[0064] Bath 4: aqueous solution, 200 ml. With 1% surfactants, 100 g of corundum with a particle size of 100 μm, 50 g of glass particles with a particle size of 200 μm.

TABLE-US-00001 TABLE 1 Description of the times, order, and number of rounds in which the part passes through the different baths Time in Time in Time in Time in Round number bath 1 bath 2 bath 3 bath 4 Round 1 1 min 15 min — — Rounds 2 to 6 15 sec 15 min — — Round 7 15 sec 15 min 5 min 1 h 30 min Round 8 15 sec 15 min — — Round 9 15 sec 15 min 5 min 2 h 30 min

[0065] Six rounds of passing through bath 1, step a) of the invention, and through bath 2, step b) of the invention, were carried out. In the first round, the time of step a) is 1 minute, and the time of step b) is 15 minutes. In the following rounds two to six, the time of step a) is 15 seconds and the time of step b) is 15 minutes.

[0066] In round 7, the part passes through steps a), b), and d) of the invention. Step d) is carried out by introducing the part in an acidic bath and subsequently in a bath made up of a solution of abrasive particles. In round 8, the part only passes through steps a) and b), and finally in round 9, the part passes through steps a), b), and d). The nine rounds are consecutive.

[0067] In this example of the invention, a part with Ra=8.97 μm and Rz=82.53 μm was used to reach, after the described polishing treatment, a reduction in Ra of 35% and a reduction in Rz of 47%, since a value of Ra=5.83 μm and Rz=43.5 μm was reached.

Example 3

[0068] In the same manner as the previous Examples, a part made of AlSi10Mg obtained by means of SLM (selective laser melting) is used.

[0069] The part is submerged in the following baths

[0070] First Round

[0071] Bath 1: 5% w/w NaOH, 30 seconds;

[0072] Bath 2: 35% w/w potassium carbonate and 2% w/w CMC (carboxymethyl cellulose); 5 min.

[0073] Bath 3: 5% w/w NaOH, 1% w/w sorbitol, 5 min.

[0074] Bath 4: acidic solution of 5% w/w HCl and 0.1% w/w surfactant, 1 min.

[0075] Bath 5: HAc 5% w/w and ultrasonic, 10 min.

[0076] Second Round

[0077] Bath 1: 1.5% w/w NaOH, 30 seconds;

[0078] Bath 2: 35% w/w potassium carbonate and 2% w/w CMC; 5 min.

[0079] Bath 3: 1.5% NaOH, 1% sorbitol, 5 min.

[0080] Bath 4: acidic solution of 5% HCl w/w(dissolution 1.85% w/w) and 0.1% w/w surfactant, 1 min.

[0081] Bath 5: HAc 5% v/v and ultrasound, 10 min.

[0082] These baths, first round and second round were repeated four times. The rugosities were measured in three different areas of the part. The results are showed in the Table 1

TABLE-US-00002 TABLE 1 Rugosities before and after the baths Before the bath After the bath Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 Ra (μm) 3.41 3.18 3.15 2.24 2.20 2.17 Rz (μm) 17.00 14.15 14.92 10.31 10.55 11.73 Rp (μm) 9.31 8.00 8.73 4.12 4.16 6.14 Ry (μm) 7.69 6.85 6.19 6.19 6.39 5.56

[0083] A significant reduction in roughness can be observed in all parameters (Ra, Rz, Rp and Rv).

[0084] Table 2 represents the percentage reduction in roughness for each area.

TABLE-US-00003 Area 1 Area 2 Area 3 Ra (%) 34.31% 30.82% 31.11% Rz (%) 39.35% 25.44% 21.38% Rp (%) 55.75% 48.00% 29.67% Ry (%) 19.51% 6.72% 10.18%