Procedure for Finishing Stainless Steel Parts

Abstract

The present invention relates to a procedure for finishing stainless steel parts. The finishing process of the invention is particularly useful for stainless steel parts of complex structure such as those produced by additive manufacturing of metals.

Claims

1. A procedure for finishing a stainless steel part manufactured by additive manufacturing comprising the steps of: (a) placing the stainless steel part into an acidic aqueous solution with a pH of 0 to 4 comprising a non-oxidizing acid, and an oxidizing acid and a surfactant (b) after step (a) placing the stainless steel part into water; (c) after step (b) placing the stainless steel part into an acid solution with a pH between 0 and 4; and (d) electropolishing the stainless steel part, wherein the stainless steel part is connected to a positive pole so that the stainless steel acts as an anode and a cathode is connected to a negative pole.

2. The procedure according to claim 1 characterized in that the part is manufactured by SLM additive manufacturing.

3. The procedure according to claim 1 characterized in that the acidic aqueous solution comprises 1% to 50% phosphoric acid, 1% to 20% hydrochloric acid, and 1% to 30% nitric acid.

4. The procedure according to claim 1 characterized in that the temperature of the step a) is between 20° C. and 80° C.

5. The procedure according to claim 1 characterized in that in step c) the acid solution comprises between 0% and 85% phosphoric acid and 10% to 80% sulphuric acid.

6. The procedure according to claim 1 characterized in that in step c) the acid solution does not present alcohol.

7. The procedure according to claim 1 characterized in that the cathode is partially covered with a protective element.

8. The procedure according to claim 7 characterized in that the protective element is made of silicone.

9. The procedure according to claim 1 characterized in that the step c) is carried out at a voltage between 2V and 10V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 shows a comparative picture on the left showing an unpolished part, in the middle the electrochemically polished sample with a 12% loss of matter and on the right a part polished with the procedure of the invention with a 12% loss of matter.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0028] As discussed above, the first aspect of the invention relates to a process of finishing stainless steel parts manufactured by additive manufacturing comprising the steps of [0029] (a) place the part in an acidic aqueous solution with a pH of 0 to 4 comprising non-oxidising acids and oxidising acids [0030] (b) after step (a) place the part into the water; [0031] (c) after step (b) place the part into an acid solution with a pH of 0 to 4; and polish the part by electropolishing, where the part is connected to a positive pole so that it is the anode and the cathode is connected to the negative pole.

[0032] The element is preferably manufactured by means of selective laser melting (SLM). With this method, highly complex geometries can be obtained. The procedure of the invention polishes homogeneously the complex geometries obtained by selective laser melting.

[0033] With the procedure of the invention, complex parts are polished both on their inner and outer surfaces.

[0034] In particular the starting product is 316L stainless steel parts manufactured by SLM additive manufacturing.

[0035] Preferably step a) is performed in the presence of additives.

[0036] Examples of additives are surfactants such as: ethoxyl propoxyl terpene, dialkylthiourea, ethylene glycol monobutyl ether, nonylphenol ethoxylate, C12-C18 fatty acids, ethoxylated C12-C18 fatty acids, alkyl(C10-C13) benzene sulfonic acid, lauryl ether sulfate, cocoamidopropyl betaine cocosamine, alkyl dimethyl amine oxide, cocoyl bis-(2-hydroxyethyl) amine oxide, lauramine oxide, N,N-dimethyltetradecyl amine oxide, quaternary ammonium compounds, benzyl-C12-16-alkyl dimethyl, non-ionic surfactants such as triton-X-405.

[0037] The additives can be of nitrogenous additives such as: mono ethanol amine, diethanol amine, triethanol amine, methoxypropylamine, monoisopropylamine, ethalamine, dimethylamine, butylamine, diethylamine, butyl diethylamine, polyNmethylaniline and polyaniline. triazole, tretazole and derivatives, imidazole and derivatives, pyrazole, pyrazine derivatives such as 2-aminopyrazine, 2-methylpyrazine, and 2-amino 5-bromopyrazine, nicotinic acid, thiourea, thiosemicarbazide and thioacetamide, sulfonamides and semicarbazones, thiazole derivatives such as 2-mercaptothiazoline and 2-amino-5-mercapto-1,3,4-thiadiazole, benzisothiozole-3-piperizine hydrochloride

[0038] Other examples of additives are: benzyl triphenylphosphonium bromide; organic polyhydroxides such as propylene glycol, ethylene glycol and glycerol; natural chemical compounds such as tannins, alkaloids such as berberine, organic molecules such as xanthene and xanthosine, 1,3-dicetone malonates, bis (benzimidazole-2-yl) disulfide, ketoconazole, amino acids such as L-leucine, glycine, ascorbic acid, citric acid, acrylic terpolymer, butyl acrylate, methyl methacrylate, acrylic acid.

[0039] Preferably non-oxidizing acid is selected from phosphoric acid, sulfuric acid and hydrochloric acid, and preferably oxidizing acid is selected from nitric acid and perchloric acid. Most preferably in step (a) the acidic aqueous solution is 1% to 50% phosphoric acid, 1% to 20% hydrochloric acid and 1% to 30% nitric acid.

[0040] Preferably the percentage of additives in the solution is 1% to 20%. Preferably the duration of step a) is 10 minutes to 8 hours. Preferably step a) is carried out in a temperature range between 20° C. and 100° C. Preferably between 25° C. and 80° C.

[0041] Preferably the water in step b) is deionised water.

[0042] Preferably the acid solution of step c) comprises sulphuric acid. Most preferably the acid solution comprises 0% to 80% phosphoric acid and 10% to 80% sulphuric acid. Preferably the solution is alcohol-free.

[0043] Preferably the temperature at which step c) is carried out is between 20° C. and 100° C. More preferably between 25° C. and 80° C., particularly at room temperature.

[0044] Preferably the cathode is partially covered with a protective element to avoid direct contact of the cathode with the part surface. Preferably the material of the protective element is a polymer. More preferably the polymer is selected between a polyamide or a silicone. The present invention by having a previous step of chemical polishing protects this polymer by shortening the time of electropolishing and therefore the time the polymer is in contact with the acid solution, as this deteriorates with time. In particular, it is preferred that the cathode protector be made of silicone. Silicone is an inert material that can withstand the acid bath with less deterioration than other materials. The silicone shield is net-shaped or has a complementary shape to the cathode with openings to allow the acid solution to act as an electrolyte.

[0045] The cathode is preferably shaped in a negative way with respect to the polished part.

[0046] Preferably the voltage at which it is worked in the electropolishing step c) is between 2V and 10V. Preferably the duration of the step c) is between 1 min and 60 min. More preferably the time at which this voltage is applied is between 5 min and 15 min.

EXAMPLES

[0047] The following examples are only illustrative of this invention, and should not be interpreted as limiting it.

Example 1

[0048] It was based on a part obtained by selective laser fusion, the part was made of 316L stainless steel.

[0049] The procedure of the invention consisting of a chemical polishing step, a water washing and an electrochemical polishing in this order was compared with a procedure consisting of only the electrochemical polishing step.

[0050] In the first step, step a), the part is placed into a bath of an acid solution of 30% H.sub.3PO.sub.4, 6% HCl and 2% HNO.sub.3; for two hours at a temperature of 70° C.

[0051] This was then followed by a deionised water washing bath, step (b) of the procedure.

[0052] After this bath, it was proceeded to a bath with an acid solution of 60% H.sub.3PO.sub.4 and 40% H.sub.2SO.sub.4, for 10 minutes at a temperature of 70° C. where an electropolishing was performed, step c) of the procedure. The anodic current density was 1.5 A/cm2.

[0053] The 316L stainless steel starting part had a Ra=6.6 μm and a Rz=56.3 μm. The reduction obtained was the following.

TABLE-US-00001 TABLE 1 Roughness and material loss after each step Initial Step (a) Step (c) Ra (μm) 6.6 4.0 2.4 Rz (μm) 56.3 34.5 18.6 Material loss (%) 7 5

TABLE-US-00002 TABLE 2 Percentage of reduction of roughness and loss of material with the procedure of the invention and only with the electropolishing step Example 1 Electropolishing only % reduction of Ra 63 39 % reduction of Rz 67 46 Material loss (%) 12 12

[0054] This procedure saves energy and material by combining both technologies. Table 2 shows that for the same material loss, the roughness reduction is much greater when chemical polishing and electropolishing are combined. When polishing with electropolishing alone, the conditions were the same as when chemical polishing and electropolishing were combined. If only chemical or electro-polishing was used, these percentages of reduction for 12% material loss would not be achieved, the material loss would be higher.

[0055] As can be seen in FIG. 1 the finish of the part subjected to the procedure of the present invention has a mirror finish.

Example 2

[0056] It started from a part made of stainless steel by additive manufacturing through selective laser sintering

[0057] The procedure of the invention consisting of a chemical polishing step, a water washing and an electrochemical polishing in this order was compared with a procedure consisting of only the electrochemical polishing steps.

[0058] In both cases the conditions of the electrochemical polishing are the same, the same bath is used and in the same conditions.

[0059] In the first step, step a), the part is placed into a bath of an acid solution of 30% H.sub.3PO.sub.4, 6% HCl and 2% HNO.sub.3; for four hours at a temperature of 70° C.

[0060] This was then followed by a deionised water washing bath, step (b) of the procedure.

[0061] After this bath, it was proceeded to a bath with an acid solution of 60% H.sub.3PO.sub.4 and 40% H.sub.2SO.sub.4, for 10 minutes at a temperature of 70° C. where an electropolishing was performed, step c) of the procedure. The voltage was 5V.

TABLE-US-00003 TABLE 3 Comparison between the process of the invention and an electropolishing process Procedure of the invention Electropolishing Initial Ra (μm) 5.98 5.95 Initial mass (g) 6.1296 6.1947 Final mass (g) 5.4463 5.2693 Electropolishing time 10 30 (min) Ra final (μm) 2.1 2.1 Loss of material (%) 11 15

[0062] As shown in Table 3, to achieve the required roughness, 10 minutes of electropolishing are required thanks to the process described; whereas, for a conventional electropolishing process, 30 minutes are needed. This figure represents a very important energy saving. Moreover, it can be seen how the described process reaches the same final roughness with a loss of 11% of the starting material and, therefore, it also implies a saving in material compared to the conventional electropolishing process.

[0063] In summary, the process described in the invention reduces the electropolishing steps by up to 20 min, providing the same finish and consuming less material.