METHOD FOR POLISHING AN OBJECT AND AN OBJECT PRODUCED BY IT

Abstract

Method (100) for polishing an object (10), comprising an aluminium alloy, the method comprising: the surface grinding (104) of an object's surface; the fine grinding (106) of the surface (12) of the object (10) with a plurality of first diamond chips (16) of a first diamond chip size; diamond polishing (108) of the surface (12) of the object (10) with a plurality of second diamond chips (22) having a second diamond chip size, wherein the second diamond chip size is smaller than the first diamond chip size; and fine polishing (110) of the surface (12) of the object (10) with a suspension (28) containing silica sol and an etching agent, wherein the etching agent has a higher pH value than the pH value of silica sol, characterized in that the method further comprises analyzing the surface (12) by means of metallographic image analysis in order to identify the contrast within silicon particles (32) and between silicon particles (32) and aluminium (34).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a mechanical and chemical polishing method for a surface of an aluminum alloy and an article produced by such a method. INTRODUCTION

[0002] Articles made from aluminum alloys, such as aluminum and silicon-containing alloys, typically undergo a multi-stage polishing process to treat the surfaces of the articles. The process may include mechanical surface grinding, as well as fine grinding and polishing. After the polishing process, the surfaces of an article are typically analyzed microscopically to determine the quality and reliability of the article. For example, the surfaces of the article are examined to detect defects and cracks.

[0003] Each sub-step in the polishing process typically helps correct the damage caused by the previous sub-step. However, for certain alloys, such as Al-Si alloys, plastic deformation of the aluminum occurs during the polishing process, resulting in smearing of α-aluminum matrix particles along the surface(s) of the object. The smeared α-aluminum matrix material mixes into cracks, creating low contrast within the primary silicon particles and between the silicon particles and aluminum, leading to difficulties in analyzing surface properties such as cracks and defects.

[0004] DE 10 2007 035 266 A1 discloses a method for polishing a substrate made of semiconductor material. SUMMARY

[0005] The present disclosure provides a metallographic chemical mechanical polishing process that improves the visual contrast resolution of alloy particles, such as primary silicon particles, and aluminum for increased accuracy of metallographic image analysis measurements of the surface of the aluminum alloy.

[0006] In one example, which may be combined or separated with the other examples and features included herein, a method of polishing an article comprising an aluminum alloy is provided. The method includes planar grinding a surface of the article and fine grinding the surface of the article with a plurality of first diamond chips having a first diamond chip size. The method further includes diamond polishing the surface of the article with a plurality of second diamond chips having a second diamond chip size. The second diamond chip size is smaller than the first diamond chip size. The method also includes fine polishing the surface of the article with a suspension containing silica sol and an etchant. The etchant has a higher pH than the pH of silica sol.The method further includes analyzing the surface using metallographic image analysis to identify the contrast within silicon particles and between silicon particles and aluminum.

[0007] In another example, which may be combined or separated with the other examples and features provided herein, a method for polishing an article comprising an aluminum alloy is provided. The method includes grinding a surface of the article and diamond polishing the surface of the article. The method also includes removing α-aluminum matrix material from the surface by finely polishing the surface with a suspension containing silica sol and an etchant. The etchant has a higher pH than the pH of silica sol.

[0008] Additional features may be provided, including, but not limited to, the following: etching substance present in the silica sol suspension in an amount of 0.5 wt.% to 3 wt.%; the etching substance containing potassium hydroxide and / or sodium hydroxide; the etching substance is present in the suspension in an amount of 2 wt.%; the method further comprising: providing the first plurality of diamond chips in a first diamond suspension lubricant and providing the second plurality of diamond chips in a second diamond suspension lubricant; wherein the first diamond chip size is 9 µm and the second diamond chip size is 3 µm; the planar grinding step is performed with an abrasive having a grit size in the range of 200 to 500 grit;wherein the method further comprises performing the step of planar grinding for 2 minutes, performing the step of fine grinding for 4 minutes, performing the step of diamond polishing for 4 minutes, and performing the step of fine polishing for 1 minute; wherein the method according to the present invention further comprises analyzing the surface to identify the contrast between aluminum and silicon particles and within silicon particles; and wherein the grinding step includes performing a substep of planar grinding the surface of the article and then performing a substep of fine grinding the surface of the article with diamond particles.

[0009] An article comprising an aluminum-silicon alloy is provided formed by any of the variations of the process disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The drawings are for illustrative purposes only and are not intended to limit this disclosure or the claims appended hereto. Fig. 1 is a schematic side view of an article formed from an aluminum alloy undergoing a surface grinding process in accordance with the principles of the present disclosure; Fig. 2 is a block diagram illustrating a method of polishing an article comprising an aluminum alloy in accordance with the principles of the present disclosure; Fig. 3 is a schematic side view of the subject matter of Fig. 1 subjected to a fine grinding process according to the principles of the present disclosure; Fig. 4 is a schematic side view of the subject of Fig. 1 and Fig. 3, which is subjected to a diamond polishing process according to the principles of the present disclosure; Fig. Figure 5 is a schematic side view of the subject matter of Fig. 1, Fig. 3 and Fig. 4, which is subjected to a fine grinding process according to the principles of the present disclosure; and Fig. 6 is a microscopic plan view of the subject of Fig. 1 and 3-5, after each step of the Fig. 2 was performed according to the principles of the present disclosure. DETAILED DESCRIPTION

[0011] Methods for polishing an article formed from an aluminum alloy are provided herein, as are articles produced from the disclosed methods. The methods include mechanical and chemical preparation steps to prepare the article for microscopic analysis and examination.

[0012] Referring to Fig. 1, an article 10 may initially be cast into a desired shape. The article 10 may be formed, for example, from an aluminum-silicon alloy, such as B390 Al-Si (according to ANSI / AA). In other examples, the article 10 may be an aluminum alloy containing at least one of the castability and strength-enhancing elements, such as silicon, copper, magnesium, manganese, iron, zinc, and nickel.

[0013] Referring to Fig. 2, a method 100 for polishing the article 10 is provided. The method includes one or more steps 102 of grinding the article to mechanically remove material from a surface 12 of the article 10 to impart a substantially uniform surface finish to the surface 12, at least as visible without a microscope.

[0014] The grinding step 102 may include a first sub-step 104 of surface grinding of the surface 12 of the object 10. For example, as in Fig. 1, the surface 12 may be planarized using a disk 14 or other tool having a grit in the range of No. 200 to No. 500. In some examples, a No. 220 magnetic disk is used, such as the 220 Grit MD Plano Disk available from Struer. The planarizing sub-step 104 may, for example, be performed on the surface 12 for two minutes. Water or another lubricant may be used to lubricate the surface 12 during the planarizing sub-step 104. The disk 14 may, for example, be rotated at a speed of 300 rpm and with a force of 25 N applied to the surface 12.

[0015] Referring to Fig. 3, with continued reference to Fig. 2, after the surface grinding sub-step 104, a fine grinding sub-step 106 may be performed on the surface 12 of the object 10. The fine grinding sub-step 106 may be used to remove the damage caused by the surface grinding sub-step 104 by using finer particles. The fine grinding sub-step 106 may involve fine grinding the surface 12 of the object 10 with a plurality of diamond chips 16. The diamond chips 16 may be contained in a diamond suspension lubricant 18, for example, DiaPro Allegro, which is provided by Struer, and which is continuously supplied to the surface 12 for the duration of the fine grinding sub-step 106. The diamond chips 16 may also be contained on a fine grinding wheel 20 or a tool. The diamond chips 16 could, for example, be provided with a chip size of 9 µm, 6 µm or 15 µm.The composite wheel 20 or tool can be rotated against the surface 12, the diamond chips 16 and the lubricant 18, for example at a speed of 150 rpm and with a force of 30 N applied to the surface 12. The fine grinding sub-step 106 can be carried out on the surface 12 for four minutes, for example.

[0016] Referring to Fig. 4 and with continued reference to Fig. 2, the method 100 includes a diamond polishing step 108 after the grinding step 102. The diamond polishing step 108 includes diamond polishing the surface 12 of the article 10 with a plurality of diamond chips 22, wherein the diamond chips 22 used for the diamond polishing step 108 are smaller than the diamond chips 16 used in the fine grinding sub-step 106. The diamond polishing step 108 can be used to remove the damage caused by the grinding step 102, particularly the fine grinding sub-step 106, by using finer particles 22.

[0017] The diamond polishing step 108 may involve polishing the surface 12 of the article 10 with the smaller diamond chips 22, which may be contained in a diamond suspension lubricant 24, such as DiaPro Mol, supplied by Struer. The diamond suspension lubricant 24 may be a water-free lubricant that is continuously supplied to the surface 12 for the duration of the diamond polishing step 108. The diamond chips 22 could, for example, be provided with a chip size of 3 µm or 1 µm. A disk 26 or tool may be rotated against the surface 12, the diamond chips 22, and the lubricant 24, for example, at a speed of 150 rpm and with a force of 25 N applied to the surface 12. The diamond polishing step 108 may be performed on the surface 12 for, for example, four minutes.

[0018] Referring to Fig. 5 and with continued reference to Fig. 2, the method 100 includes a fine polishing step 110 after the diamond polishing step 108. The fine polishing step 110 involves fine polishing the surface 12 of the article 10 with a suspension 28 containing silica sol and an etching substance. The fine polishing step 110 can be used to remove the damage caused by the previous step 108.

[0019] The fine polishing step 110 may involve polishing the surface 12 of the object 10 with the suspension 28 containing silica sol and the etching substance, which may be continuously supplied to the surface 12 for the duration of the fine polishing step 110. The silica sol could, for example, be provided with a grain size of 0.04 µm. A disk 30 or a tool may be rotated against the surface 12 and the suspension 28, for example, at a speed of 150 rpm and with a force of 15 N applied to the surface 12. The fine polishing step 110 may be performed on the surface 12 for, for example, one minute.

[0020] The suspension 28 can be prepared by starting with a silica sol, such as OP-U NonDry or OPS, available from Struer, and adding an etchant having a higher pH than the pH of the silica sol. The etchant can be or contain, for example, potassium hydroxide (KOH), sodium hydroxide (NaOH), lime, or another desired etchant. The etchant can be present in the suspension 28, for example, in an amount of 0.5 wt.% to 3 wt.%, 1 wt.% to 3 wt.%, or 2 wt.%.

[0021] Typically, during one or more of the grinding and polishing steps 102, 108, α-aluminum matrix material is generated by plastic deformation and distributed along the surface 12. The fine polishing step 110 using the suspension 28 containing the etching substance results in a substantial removal of the α-aluminum matrix material from the surface 12.

[0022] Referring to Fig. 6 shows the surface 12 of the article 10 after it has been subjected to the fine polishing step 110. Due to the addition of the etching substance to the silica sol suspension, most of the α-aluminum matrix material was removed from the surface 12 during the fine polishing step 110, resulting in a sharp contrast between the primary silicon particles 32 and the aluminum 34 and within the silicon particles 32 themselves. Cracks 36 or other defects in the silicon 32 or aluminum 34 are visible (under the microscope) because they are no longer covered and filled by α-aluminum matrix material.

[0023] The method 100 also includes analyzing the surface 12 of the article 10 by metallographic image analysis to identify a contrast between the silicon particles 32 and the aluminum 34 and within the silicon particles 32. For example, automated metallographic image analysis may be performed on the surface 12 of the article 10 to identify cracks 36, fractures, or other defects, particularly in the silicon particles 32.

[0024] Thus, the article 10 comprising an aluminum-silicon alloy formed by the process 100 described herein has a surface 12 that contains little or no plastic deformation caused by the process 100 in the form of α-aluminum matrix material located on the surface 12 of the article 10.

[0025] The detailed description and the drawings or figures support and describe many aspects of the present disclosure. While certain aspects have been described in detail, various alternative aspects exist in the appended claims. It is further understood that each of the concepts described above may be used alone or in combination with any or all of the other concepts described above.

Claims

[1] A method (100) for polishing an article (10) comprising an aluminum alloy, the method comprising: surface grinding (104) of a surface of the object; fine grinding (106) the surface (12) of the article (10) with a plurality of first diamond chips (16) having a first diamond chip size; diamond polishing (108) the surface (12) of the article (10) with a plurality of second diamond chips (22) having a second diamond chip size, wherein the second diamond chip size is smaller than the first diamond chip size; and fine polishing (110) the surface (12) of the object (10) with a suspension (28) containing silica sol and an etching substance, wherein the etching substance has a higher pH than the pH of silica sol, characterized bythat the method further comprises analyzing the surface (12) by metallographic image analysis to identify the contrast within silicon particles (32) and between silicon particles (32) and aluminum (34). [2] The method (100) of claim 1, wherein the etching substance is present in the suspension (28) in an amount of 0.5% to 3% by weight. [3] The method (100) of any preceding claim, wherein the etching substance comprises at least one of potassium hydroxide and sodium hydroxide. [4] The method (100) of any preceding claim, wherein the etching substance comprises potassium hydroxide. [5] A method (100) according to any one of the preceding claims, wherein the etching substance is present in the suspension in an amount of 2% by weight. [6] The method (100) of any preceding claim, further comprising providing the first plurality of diamond chips (16) in a first diamond suspension lubricant (18) and providing the second plurality of diamond chips (22) in a second diamond suspension lubricant (24), wherein the first diamond chip size is 9 µm and the second diamond chip size is 3 µm. [7] The method (100) of any preceding claim, wherein the surface grinding step (104) is performed with an abrasive having a grit in the range of 200 to 500 grit, the method (100) further comprising performing the surface grinding step (104) for 2 minutes, performing the fine grinding step (106) for 4 minutes, performing the diamond polishing step (108) for 4 minutes, and performing the fine polishing step (110) for 1 minute. [8] A method according to any one of the preceding claims, wherein the step of fine polishing (110) includes removing α-aluminum matrix material from the surface (12). [9] An article (10) comprising an aluminum-silicon alloy formed by the process (100) of claim 2.

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