SELECTIVE PLATING SYSTEM

Field of the Invention

[0001] The present invention relates generally to plating, and more particularly, to selective plating of intricate surfaces on a work piece.

Background of the Invention

[0002] The present invention relates to a method and apparatus for selectively plating intricate surfaces on a work piece. By way of example and not limitation, the present invention finds advantageous application in selectively plating threaded surfaces on the end of a shaft, such as threads on the end shaft for a jet engine. Heretofore, it was known to plate nickel onto the threaded ends of a jet engine shaft by masking certain portions of the shaft and suspending the shaft over a large plating tank, and then dipping the end of the shaft in an electroless nickel solution. Such a plating process requires large facilities, handling equipment and plating tanks. With such large handling equipment and tanks, it is difficult to accurately control the plating conditions and the properties of the metal plated on the surfaces of the shaft. [0003] The present invention provides a system for encapsulating and plating the end surfaces of a large, elongated work piece.

Summary of the Invention

[0004] In accordance with the present invention, there is provided an apparatus for plating a portion of a work piece. The apparatus comprises a fixture having a cavity formed therein. The cavity is dimensioned to receive the portion of the work piece. An attaching means is provided for attaching the fixture to the work piece whereby a sealed chamber is formed between the fixture and the work piece. An inlet port in the fixture fluidly communicates with the sealed chamber. An outlet port in the fixture fluidly communicates with the sealed chamber. A circulation system fluidly connects to the inlet port and the outlet port and includes a source of a plating solution. The circulation system is operable to circulate a plating solution through the sealed chamber.

[0005] ^• In accordance with another aspect of the present invention, there is provided a method of plating an end portion of an elongated work piece comprising the steps of:

(a) attaching a fixture to the end portion of a work piece to create a sealed, fluid-tight chamber between the work piece and the fixture, the fixture having an inner surface with a profile conforming to an outer surface profile of the work piece, and further having fluid connection means for fluidly connecting the fixture to a source of a plating solution; and

(b) circulating the plating solution through the sealed chamber. [0006] An advantage of the present invention is a method and apparatus for selectively plating sections of a work piece.

[0007] An advantage of the present invention is a system having a plating fixture for encapsulating the sections of the work piece to be plated.

[0008] An advantage of the present invention is a system as described above wherein the encapsulating fixture includes means to facilitate cleaning of the areas to be plated.

[0009] Another advantage of the present invention is a method for encapsulating and selectively plating select portions of a work piece wherein a test coupon is simultaneously plated with the work piece.

[0010] Another advantage of the present invention is a system for selectively plating large, unwieldy parts that are difficult to plate in a conventional tank plating process.

[0011] A still further advantage of the present invention is a selective plating system that finds advantageous application in plating a small number of parts.

[0012] A still further advantage of the present invention is a selective plating system wherein the plating parameters are more easily controlled as compared to a conventional tank plating process.

[0013] These and other advantages will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.

Brief Description of the Drawings

[0014] The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein:

[0015] FIG. 1 is a perspective view showing a work piece and a plating fixture illustrating a preferred embodiment of the invention;

[0016] FIG. 2 is a schematic view of a fluid circulation system mounted to the plating fixture shown in FIG. 1 ;

[0017] FIG. 3 is a microscopic image showing an electroless nickel (EN) coating thickness in the valley of an internal thread;

[0018] FIG. 4 is a microscopic view showing an electroless nickel (EN) coating thickness at the peak of a thread; and

[0019] FIG. 5 is a microscopic view showing an electroless nickel (EN) coating thickness along the side of a thread.

Detailed Description of Preferred Embodiment

[0020] Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting same, the drawings show a plating system 10 for selectively plating select areas of a large work piece 20. The present invention is particularly applicable to plating screw threads on the ends of a long shaft, such as, by way of example and not limitation, the internal and external threads on the ends of a jet engine shaft, and shall be described with particular reference thereto. It shall be understood, however, that the present invention finds advantageous application in plating other types of work pieces. System 10 is generally comprised of a plating fixture 100, a fluid circulation system 200 and a test strip container 300. [0021] Referring now to FIG. 1, work piece 20 is a shaft having a tubular end portion 22. Tubular end portion 22 has a threaded outer section 24 and a threaded inner section 26. Tubular end portion 22 defines a cylindrical cavity 28 at the end of

work piece 20. In another embodiment (not shown), work piece 20 is shaft wherein an opening extends a length of the shaft. In this embodiment, a second seal element (not shown) is disposed in the opening to seal a portion of the opening to be plated from a remaining portion of the opening. In this respect, the seal element and a portion of the opening in work piece 20 define cylindrical cavity 28.

[0022] Plating fixture 100 is comprised of a housing 110 that is generally cylindrical in shape and has an end wall 112 and a cylindrical side wall 114 extending to one side thereof. Housing 110 is formed of a non-conductive material, such as, by way of example and not limitation, a polymer. End wall 1 12 defines a closed end of plating fixture 100. Side wall 114 defines a cylindrical opening 116 that is dimensioned to receive end portion 22 of work piece 20. A cylindrical, center column 122 is disposed within opening 116 defined by side wall 114. Center column 122 is coaxial with the axis of side wall 114. Center column 122 has an outer diameter that is smaller than cavity 28 in tubular end portion 22 of work piece 20. In this respect, an annular opening is defined between side wall 114 and center column 122 of plating fixture 100. The annular opening is dimensioned to receive end portion 22 of work piece 20, as illustrated in FIG. 2. A slot or groove 132, best seen in FIG. 2, is formed along the inner surface of side wall 114 near the end thereof. Slot 132 is dimensioned to receive an inflatable seal 134 therein. An air line 136 extends through the side wall and communicates with seal 134. Air line 136 is connectable to a source of air (not shown) to inflate seal 134.

[0023] As best seen in FIG. 2, an axially aligned opening 142 extends through end wall 112 and through center column 122.

[0024] An electrode 144 is disposed along the outer surface of center column

122. An electrode 146 is also provided along the inner surface of side wall 114, as best seen in FIG. 2. In the embodiment shown, electrodes 144, 146 are in the form of a metal screen. Leads 148 connected to electrodes 144, 146 extend through fixture 100, and are connectable to a power source (not shown), to provide current to electrodes 144, 146. A lead 149 is connected to work piece 20. Lead 148 and lead 149 are shown in FIG. 2 connected to a positive (+) and a negative (-) terminal of the

power source, respectively, to illustrate opposite polarities and is not intended as a limitation of the invention disclosed herein.

[0025] Referring now to FIG. 2, fluid circulation system 200 is shown. In the embodiment shown, fluid circulation system 200 includes a water reservoir 212, a cleaning fluid reservoir 214 and a plating solution reservoir 216. As illustrated in the drawings, a heater 218 is associated with plating solution reservoir 216 to heat the plating solution therein. In accordance with one aspect of the present invention, each reservoir 212, 214, 216 is dimensioned to hold ten (10) gallons or less of fluid, and more preferably, five (5) gallons or less of fluid, and more preferably, four (4) gallons or less of fluid. Water reservoir 212 is connected to an input valve 222 by line 232. A pump 234 is disposed in line 232 to convey water to input valve 222. Similarly, cleaning fluid reservoir 214 is connected to input valve 222 by a line 242. A pump 244 is disposed in line 242 to convey cleaning fluid from reservoir 214 to input valve 222. In accordance with one aspect of the present invention, reservoir 214 may be comprised of one or more isolated chambers (not shown) that selectively fluidly communicate with line 242. It is contemplated that the isolated chambers may contain a de-smut solution, an etching solution, an activating solution or a pre-plating solution. Plating solution reservoir 216 is connected to input valve 222 by a line 252. A pump 254 in line 252 is provided to convey the plating solution to input valve 222. Input valve 222 is connected to a fixture feed line 262 that is connected to fixture 100. In the embodiment shown, fixture feed line 262 has a first branch feed line 262a connected to opening 142 through center column 122, and a second branch feed line 262b connected to a fitting 264 through side wall 114. A second fitting 266 in side wall 114 is connected to a return line 272 that connects to an output valve 274. Output valve 274 has three (3) return lines 282, 284, 286 connected respectively to water reservoir 212, cleaning fluid reservoir 214 and plating solution reservoir 216. In accordance with one aspect of the present invention, reservoir 214 may be composed of one or more isolated chambers (not shown) that selectively fluidly communicate with return line 284. In accordance with another aspect of the present invention, output valve 274 has a fourth return line (not shown) connected to a waste container

(not shown). In this respect, the fourth return line fluidly connects fluid circulation system 200 to the waste container to allow for the removal of waste generated therein. [0026] Container 300 is disposed within fixture feed line 262. Container 300 defines a test chamber 312 dimensioned to receive test strips 314. [0027] System 10 shall now be further described with respect to a process for electroless plating of nickel (Ni) on the threaded sections of work piece 20. Portions of work piece 20 that are not to be plated are masked by conventionally known materials to prevent metal from plating thereon. The areas to be plated, i.e., the threaded sections, are then cleaned with a conventionally known cleaning material, such as acetone, to remove grease or organic materials therefrom. It is also contemplated that cleaning of work piece 20 may involve the use of electrodes 144, 146 and other solutions to aid in the cleaning process. Plating fixture 100 is then attached to the end of work piece 20. Fixture 100 is dimensioned such that a predetermined gap is formed between the inner surface of side wall 114 and work piece 20 and between central column 122 and work piece 20, as shown in FIG. 2. Air is provided through air line 136 to inflate seal 134, thereby forming an enclosed chamber around tubular end portion 22 of work piece 20. Work piece 20 is thus encapsulated within fixture 100.

[0028] In one embodiment of the invention, a test strip 314 (or strips) is inserted into test chamber 312 of container 300, prior to initiating a plating process. In this embodiment, test strip 314 is attached to lead 149 and test container 300 is attached to lead 148. In another embodiment (not shown), test strip 314 is inserted into cavity 28 of work piece 20. In this embodiment, test strip 314 is electrically grounded to work piece 20. Test strip(s) 314 are formed of the same material that forms the threaded surfaces of work piece 20 to be plated.

[0029] With plating fixture 100 in place and securely sealed onto the end of work piece 20, work piece 20 undergoes preparatory procedures prior to plating. Such procedures are generally conventionally known. Cleaning fluid may also be applied to the encapsulated region to clean and prepare the threaded surfaces of work piece 20 for plating. Following any cleaning process, work piece 20 is preferably rinsed with water.

[0030] Once the preparatory procedures have been completed, input valve 222 and output valve 274 are moved to positions to allow the nickel plating solution to be circulated through the chamber defined in fixture 100. The nickel plating solution has preferably been heated by heater 218 to a desired temperature to facilitate plating. The nickel plating solution is continuously conveyed through the plating chamber for a predetermined period of time. During this predetermined period of time, the nickel plating solution is constantly replenished to maintain a desired chemistry for plating the internal and external threads of work piece 20. After the plating has been completed, input valve 222 and output valve 274 move to a position to allow rinse water to flow through the plating chamber so as to remove any residual plating solution therefrom. Air seal 134 is then deflated and plating fixture 100 is removed from work piece 20.

[0031] As will be appreciated from a description of the foregoing, while various solutions are being conveyed through plating fixture 100, such solutions also flow through test container 300, thereby exposing test strips 314 to the same processes and chemistries experienced by the surface of work piece 20. Since test strips 314 are formed of the same material as the surfaces of work piece 20 to be plated, test strips 314 provide a method of verifying the plating process. Test strips 314 can thus be examined without requiring that the actual surface of work piece 20 be tested in any way. Test strips 314 in test container 300 are analyzed to determine an adhesion characteristics and a thickness of the electroless nickel plating.

[0032] The foregoing process is used to apply electroless nickel onto select sections of work piece 20. The foregoing procedure may find advantageous application in numerous types of electroless plating processes.

[0033] In one application of the present invention, the foregoing process and apparatus are used to apply an electroless nickel onto a shaft formed of Maraging 250. Maraging 250 is an 18% nickel, cobalt strengthened steel (C-type), with excellent mechanical properties, workability and heat treatment characteristics. In the application described below, Atotech 2810-1 NICHEM EN is used as the electroless nickel solution. The electroless nickel solution is heated to about 200° F by heater 218 in tank 216. While the electroless solution is heating, portions of work piece 20 that

are not to be plated are masked using aluminum tape, and other conventionally known masking materials. Test strips 314 are mounted in cavity 28 of work piece 20 and electrically grounded to work piece 20. Work piece 20 is then cleaned using acetone and plating fixture 100 is attached to work piece 20 as described above. [0034] Following the attachment of fixture 100 to work piece 20, the following preparatory procedure is performed on work piece 20 and test strips 314. A cleaning solution, is circulated through the chamber defined in plating fixture 100 for about 30 seconds while simultaneously applying about 10 volts to electrodes 144, 146 and work piece 20. Cleaning solution is formulated to clean portions of work piece 20 and test strips 314 that are to be plated. In present application, cleaning solution is SIFCO ASC ElectroCleaning solution Code 1010/4100. A de-ionized water solution is then circulated through the chamber defined in plating fixture 100. An etching solution is circulated through the chamber for about 15 seconds while simultaneously apply a reversed voltage of between about 10 volts to about 13 volts to electrodes 144, 146 and work piece 20. The etching solution etches portions of work piece 20 and test strips 314 that are to be plated. In the present application, the etching solution is SIFCO Etching Solution #1024. De-ionized water is then circulated through the chamber to rinse work piece 20 and test strips 314. Next, a de-smut solution is circulated through the chamber in fixture 100 for about 30 seconds while simultaneously applying between about 15 volts to about 20 volts to electrodes 144, 146 and work piece 20. The de-smut solution de-smuts, i.e. removes smut, from portion of work piece 20 and test strips 314 to be plated. In the present application, the de-smut solution is SIFCO Desmut Solution #1023. Work piece 20 and test strips 314 are then rinsed again with de-ionized water. Next, an activation solution is then circulated through the chamber in fixture 100 for about one minute while applying a reversed voltage of about 15 volts to electrodes 144, 146 and work piece 20. The activation solution activates portions of work piece 20 and test strips 314 to be plated. In the present application, the activation solution is SIFCO Solution 1024. Work piece 20 and test strips 314 are then wetted with a pre-plating solution. In the present application, the pre-plating solution is SIFCO Solution 5630. After work piece 20 and test strip 314 are rinsed with the pre-plating solution, a voltage of about 6 volts is

applied to electrodes 144, 146 and work piece 20 for about one minute while the pre- plating solution continues to circulate through the chamber in fixture 100. The pre- plating solution pre-plates portions of work piece 20 and test strips 314 that are to be plated. Work piece 20 and test strips 314 are then rinsed with de-ionized water. [0035] Following the preparatory procedure described above, the electroless nickel solution is circulated through the chamber in fixture 100 for about 65 minutes to plate work piece 20 and test strips 314. After the plating process, fixture 100 is removed from work piece 20 and work piece 20 and test strips 314 are rinsed and air dried. Test strips 314 are then examined to characterize the electroless nickel plating relative to adhesion, coating thickness, composition, internal stress, corrosion resistance and hydrogen embrittlement, as described below. The test strips 314 are made of various materials and have various features formed therein, depending on the characteristic of the electroless nickel plating to be examined. For example, in one embodiment, a test strip 314 made of carbon steel and a test strip 314 made of Maraging 250 are plated to examine adhesion. In another embodiment, test strips 314 are formed to have grooves and notches along the surface thereof to simulate internal and external threads, wherein a coating thickness on the groves or notches is indicative of the coating on the internal and external threads of work piece 20. [0036] To determine the adhesion characteristics of electroless nickel plated on test strips 314, an adhesion test is conducted using a 180° bend method specified by ASTM E 290 - 97a (Standard test methods for bending testing of material for ductility). Two test strips 314 are used in the adhesion test. One test strip 314 is made of carbon steel and the other test strip 314 is made of Maraging 250, the same material as work piece 20. Both test strips 314 are bent rapidly at room temperature through about 180° around a diameter equal to a nominal thickness of test strip 314. The test strips 314 are then observed under a microscope using 8X magnification. Flaking or peel-off of the electroless nickel plating is not observed for test strip 314 made of steel, although the electroless nickel plating is cracked. Flaking and peel-off are occasionally observed on test strip 314 made of Maraging 250. [0037] After a bending test, described above, bent test strips 314 are mounted and polished following a standard metallographic procedure. The adhesion of the

electroless nickel plating on an outer/tension side and on an inner/compression side of bent test strip 314 are analyzed. It is determined that some portions of test strip 314 have separated. Although occasional flaking/peel-off occurs when the substrate separates under severe bending, the adhesion of the electroless nickel plating is considered acceptable.

[0038] An adhesion referee test is conducted per 3.4.3.1 of AMS 2405c. A steel test strip 314 is plated with work piece 20 in plating fixture 100. Test strip 314 is then heated to about 700 ⁰ F for about 24 hours and then to about 1000 ⁰ F for about one (1) hour in air using a Blue M ^® furnace. A color change of the electroless nickel plating on test strip 314 is observed after heating, but no blistering or cracking is observed.

[0039] Based on the results above, it is concluded that the electroless nickel plating shows good adhesion to test strip 314 and thereby passes the adhesion referee test.

[0040] A coating thickness of the electroless nickel plating is determined by using a test strip 314 having internal and external threads. The test strip 314 is mounted and polished. The electroless nickel plating thickness is measured using a Nikon® microscope with Buehler Omnimet ^® image analyzer software. The microscope and analyzer software are used to measure the electroless nickel plating thickness in a valley of a thread, and on a peak of a thread, best seen in FIGS. 3 and 4 respectively. The microscope and analyzer software are also used to measure the electroless nickel plating thickness distribution over a thread, best seen in FIG. 5. [0041] As seen in FIGS. 3-5, the electroless nickel plating thickness is uniform over the threads and measures no more than about 0.0008" at the valley of a thread. [0042] A composition of the electroless nickel plating is measured using two electroless nickel plated test strips 314. The test strips are analyzed using an Energy Dispersive Spectroscopy following ASTM E 1508 — 98. The results are shown in Table 1.

Table 1. Results of composition analysis

[0043] The composition of the electroless nickel plating is mainly determined by the electroless nickel solution. In a preferred embodiment, the electroless nickel solution is a low phosphorus solution, which gives from about 3% to about 5% phosphorous content in the coating.

[0044] As required by AMS 2405c (Electroless Nickel Plating Low

Phosphorus), the phosphorus content in the electroless nickel plating is less than 8%. [0045] An internal stress test is conducted on the electroless nickel plating to perform an internal stress test, a test strip 314, made of a piece of copper foil with thickness of about 0.0032" is used. The electroless nickel plating having a thickness of about 0.0012," as measured by a Mitutoyo ^® digital micrometer, is applied to copper foil test strip 314 The copper foil test strip 314 does not show any irregular warp after electroless nickel plating. During one subsequent internal stress test, test strip 314 is a stainless steel foil. The test strip 314 is plated with electroless nickel and then the electroless nickel plating is removed from the stainless steel test strip 314. The electroless nickel plating shows no irregular warp.

[0046] Based on the internal stress test above, the electroless nickel plating has a very low internal stress or is free of internal stresses.

[0047] A corrosion test is performed on a test strip 314 made of Maraging

250. The Maraging 250 test strip 314 is plated with electroless nickel then heat treated to about 375°F for about eight (8). The Maraging 250 test strip 314 is then put into a salt fog chamber for about 48 hours. The corrosion test follows ASTM B 117 — 03. At a conclusion of the corrosion test, the Maraging 250 test strip 314 shows no sign of coupon corrosion.

[0048] Based on the corrosion test above, the electroless nickel plating meets a corrosion resistance requirement of AMS 2405c.

[0049] A hydrogen embrittlement test is next performed on four (4) test strips

314. Test strips 314 are notched bars supplied by a manufacturer of work piece 20. The four test strips 314 are electroless nickel plated and then stored for about twelve (12) hours. The test strips 314 are then heated to about 375 ⁰ F for about eight (8) hours. The test strips 314 are tested for hydrogen embrittlement per ASTM F 519-97 (Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating Processes and Services Environments). The test strips 314 are subjected to about 200 hours of sustained tensile load at about 75% (7442 LBS) an ultimate tensile strength (about 9923 LBS). No hydrogen embrittlement failures of test strip 314 are recorded. The test strips are visually inspected after the test and no embrittlement cracks are noted in the test strips 314.

[0050] Based on the hydrogen embrittlement test above, the electroless nickel plating produced by the present invention conforms ASTM F 519. [0051] The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. [0052] The present invention thus provides a method and apparatus for the selective plating of surfaces. By encapsulating only a small portion of the work piece having the surfaces to be plated therein, the present method and apparatus allows for use of smaller volumes of solution. The volume of solution necessary is generally related to the part size and the thickness of the plating. One benefit of using smaller volumes of solution is the ability to more easily and quickly adjust and maintain desired chemical and pH levels. In addition, by moving a smaller volume of solution over the work piece surface, the temperature of the work piece can be more easily maintained and depletion of the ions from the solution, particularly in the vicinity of the work piece, can be regulated.

[0053] Another aspect of the present invention is the provision of electrodes along the inner surface of plating fixture 100. While the foregoing embodiment has

been described with respect to an electroless nickel plating process, as will be appreciated by those skilled in the art, an electro-deposition process can be performed using plating fixture 100.

[0054] It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.