Method for providing stop-off on a workpiece

A method for providing a stop-off material on a workpiece is provided, as is an associated assembly. A maskant is provided on the workpiece such that a first portion of the workpiece is exposed and a second portion of the workpiece is covered by the maskant. The stop-off is then disposed, with the maskant on the workpiece preventing the stop-off from being applied to the second portion of the workpiece. The maskant is removed from the workpiece leaving the stop-off disposed on a select portion of the workpiece. The removal of the maskant can leave the workpiece substantially free of debris. Thereafter, the workpiece can be further processed, e.g., by diffusion bonding, with the stop-off preventing diffusion bonding of part of the workpiece.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to selective processing of a workpiece and, more particularly, to a method of providing stop-off on select portions of the workpiece so that the workpiece can be selectively diffusion bonded or otherwise processed.

2) Description of Related Art

Stop-off generally refers to a material disposed on a workpiece for facilitating subsequent processing of select portions of the workpiece. For example, according to one conventional diffusion bonding (DB) process, stop-off is provided in a predetermined pattern between workpieces to prevent bonding of corresponding portions of the workpieces. In particular, the stop-off is typically disposed in a select pattern on a surface of a first workpiece on one or more portions of the workpiece where bonding is not desired. Thereafter, a second workpiece is positioned against the surface of the first workpiece so that the stop-off is between the two workpieces. The workpieces are pressed together and heated to a diffusion bonding temperature so that diffusion bonds are formed where no stop-off is provided between the workpieces. In those portions where stop-off is provided, the stop-off prevents bonding from occurring. The unbonded portions of the resulting assembly can define any pattern, such as a pattern of cells that are to be expanded subsequently by a gas pressure during superplastic forming (SPF) of the workpieces.

One conventional method for applying the stop-off to the workpiece includes the use of a silk-screening device. The silk-screening device includes a flat screen or template for disposing the stop-off onto the workpiece. That is, the screen defines a perforated portion that corresponds in shape to the areas of the workpiece where the stop-off is to be applied. The other portions of the screen are impervious to the stop-off. Thus, with a first side of the screen directed toward the workpiece, stop-off is spread onto the opposite side of the screen and pushed therethrough and onto the workpiece in the desired pattern.

The conventional silk-screening method is effective for disposing the stop-off but requires the use of the silk-screening machine as well as careful operation and handling of the workpiece, e.g., to prevent the stop-off from being spread onto the clean portions of the workpiece during or after the silk-screening operation. In addition, the precision and accuracy of the conventional silk-screening method is limited. Further, periodic cleaning of the screen is generally required, e.g., between each successive operation of the device. The cleaning operation slows the overall operating speed of the device and typically requires special disposal facilities, as the stop-off can include hazardous chemicals.

According to another conventional process, a strippable mask coating can be provided on the workpiece to prevent the stop-off from being applied to select portions of the workpiece. For example, U.S. Pat. No. 5,513,791 to Rowe, et al. describes a strippable mask coating that is applied to the surface of a sheet. The mask coating is selectively removed from the surface, i.e., from the areas where bonding is not desired, and the stop-off is then applied and dried on the sheet. The remainder of the flexible mask material is then removed. Any stop-off disposed onto the flexible mask material is removed with the mask material. Thus, the method does not require precise placement of the stop-off material. However, the stripping of the strippable mask material from the sheet requires cleaning of the sheet prior to diffusion bonding in order to remove the oily residue of the mask material, e.g., using an aqueous detergent solution. If not cleaned from the sheet, the residue from the mask material can prevent proper diffusion bonding.

BRIEF SUMMARY OF THE INVENTION

An improved method of the present invention for applying stop-off on a workpiece, especially in preparation for SPF-DB or other hot sizing operations on metal sheets, creates a predetermined pattern with masking tape eliminating the need for silk screening tooling or chemical cleaners. That is, the present invention provides a method for applying a stop-off material on a workpiece using a maskant such as masking tape. The maskant can be cut to a predetermined pattern to prevent stop-off from being applied to portions of the workpiece where bonding is desired. After the stop-off is disposed, the maskant can be removed. Further, the maskant can be peeled or otherwise removed from the workpiece so that a select portion of the workpiece is exposed substantially free of debris without cleaning.

According to one embodiment of the present invention, the maskant is disposed on a first workpiece. The maskant includes a substrate and an adhesive for securing the substrate to the workpiece. For example, the maskant can include a sheet of paper with an adhesive such as latex thereon. The maskant is cut to define first and second portions, e.g., using a laser that is automatically controlled through a predetermined motion to cut the maskant according to a predetermined configuration. With the maskant disposed on the workpiece, a first portion of the workpiece is exposed and a second portion of the workpiece is covered by the maskant. The maskant can be disposed only on the second portion, or the maskant can be disposed on both portions and selectively removed from the first portion. In either case, a stop-off material such as yttrium-oxide can thereafter be disposed on the first portion of the workpiece, such as by spraying. The second portion of the maskant is then removed from the workpiece, i.e., by peeling or otherwise removing the substrate with the adhesive, so that the second portion of the workpiece is exposed substantially free of debris.

The second portion of the workpiece can be diffusion bonded, with the stop-off material on the first portion of the workpiece preventing diffusion bonding of the first portion of the workpiece. For example, the workpiece, which can be formed at least partially of titanium, can be heated, pressed against a second workpiece, and diffusion bonded thereto. Such diffusion bonding can be performed without cleaning the second portion of the workpiece after the maskant is removed. In some cases, the workpiece can also be superplastically formed. For example, the workpieces can be diffusion bonded to define one or more cells therebetween, the cells being coincident with the first portion of the first workpiece. The cells can be expanded by superplastically forming either or both workpieces.

According to another embodiment of the present invention, there is provided an assembly for diffusion bonding. The assembly includes a first workpiece defining first and second portions. The workpiece can be formed at least partially of titanium or other materials. A maskant is disposed on the workpiece such that a first portion of the workpiece is exposed and a second portion of the workpiece is covered by the maskant. For example, the maskant can be disposed only on the second portion of the workpiece, or the maskant can be disposed on the first and second portions of the workpiece with corresponding first and second portions of the maskant being adapted to be removed separately from the workpiece. The maskant can be removed from the workpiece by peeling the substrate of the maskant from the workpiece so that the adhesive of the maskant is removed from the workpiece with the substrate and the workpiece is exposed substantially free of debris. According to one aspect of the invention, the substrate of the maskant can be a sheet of paper with the adhesive disposed thereon. Further, the maskant can define a cut having a predetermined configuration to define the first and second portions of the maskant.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures and in particular toFIGS. 1 and 2, there is shown an assembly10according to one embodiment of the present invention. The assembly10includes a workpiece12with a maskant20disposed on a first surface14thereof. As shown inFIG. 1, the workpiece12can be a metallic sheet such as a sheet formed of titanium or titanium alloys. In other embodiments of the present invention, the workpiece12can be formed of one or more sheets, plates, blocks, tubes, angles, channels or other shapes, and the workpiece12can be formed of other materials. As shown inFIG. 1, the maskant20can be disposed over the entire first surface14of the workpiece12, though in other embodiments of the present invention, the maskant20can be disposed on only a portion of the surface14or on portions of multiple surfaces of one or more workpieces of the assembly.

The maskant20is generally a tape that includes a substrate22and an adhesive24. The substrate22is a sheet-like member that can be applied to and removed from the workpiece12. Typically, the substrate22is formed of a material that is sufficiently durable and can be relatively inexpensive. For example, the substrate22can be a flexible sheet of material such as paper, plastic film, and the like. The adhesive24can be any of various substances that adhere the substrate22to the workpiece12and which can be removed from the workpiece12with the substrate22, as described further below. The substrate22and the adhesive24can be provided together, i.e., the substrate22with the adhesive24disposed thereon can be applied together onto the workpiece12. For example, the maskant20can be a sheet of latex saturated paper with an adhesive disposed on one side thereof, such as latex saturated paper marketed under the tradename PROTEX®, available from Mask-off Company, Inc. of Monrovia, Calif. Alternatively, the maskant20can be various other conventional tape materials, such as Scotch® masking tapes, a registered trademark of Minnesota Mining and Manufacturing Company. Further, in other embodiments of the present invention, the adhesive24and substrate22can be disposed separately onto the workpiece12, e.g., by first disposing the adhesive24onto the workpiece12and then applying the substrate22to the workpiece12. In any case, the substrate22can be provided with text, graphics, or other indicia30, e.g., regarding the type of maskant20, the recommended use of the maskant20or the workpiece12, a pattern along which the maskant20is to be cut, other instructions or warnings, and the like.

The maskant20is configured to be disposed on part of the workpiece12and subsequently removed therefrom, e.g., as two or more portions that can be removed separately from the workpiece12. Typically, the maskant20is disposed onto the workpiece12and subsequently cut to define the multiple portions. The maskant20can be cut using a variety of devices. For example, as illustrated inFIG. 1, a laser40or another source of radiation can be configured to direct radiation toward the maskant20to cut the maskant20. A controller42can adjust the position and/or orientation of the laser40or the assembly10so that the laser40progressively cuts the maskant20along one or more predetermined paths corresponding to the predetermined dimensions of the portions. Alternatively, the maskant20can be cut using a knife or other instrument, which can be controlled automatically or manually. Typically, the laser40or other cutting device is controlled to prevent scoring or scratching of the workpiece12while the substrate22is being cut.

As shown inFIG. 2, the maskant20has been cut to define five first portions26and a two second portions28, though in other embodiments of the invention, any number of first and second portions26,28can be defined. The first portions26are removed from the workpiece12before a stop-off material50(FIG. 3) is applied to the workpiece12, and the second portions28of the maskant20are removed after the stop-off50is applied. Thus, the stop-off50is disposed on first portions16of the workpiece12, which are initially covered by the first maskant portions26. Second portions18of the workpiece12are covered by the second portions28of the maskant20while the stop-off50is applied so that the second portions18of the workpiece12do not receive the stop-off50. In other embodiments of the present invention, the maskant20can be disposed only on the second portions28of the workpiece12so that removal of the first portions26from the workpiece12is unnecessary.

Various types of stop-off50can be used, and the particular stop-off50material can be chosen according to the workpiece12and the type of processing to be performed with the workpiece12. For example, where the workpiece12is to be diffusion bonded, materials such as yttrium-oxide (or yttria) or boron nitride can be used as the stop-off50to prevent the first portions16of the workpiece12from bonding. In particular, the stop-off material can be Dag® MS-401 SPF/DB Stop Off, a registered trademark of Acheson Industries, Inc. In addition, the stop-off50can be disposed by spray, brush, squeegee or other manners of application, and the stop-off50can be disposed as a powder, paste, slurry, film, or the like. For example, as illustrated inFIG. 3, the first portions26of the maskant20have been removed from the workpiece12, and the stop-off50is being sprayed onto the workpiece12before removal of the second portions28. A sprayer52is controlled by a controller56to dispose the stop-off50as a spray of material provided from a source54of the stop-off50. The sprayer52can be configured to spray the stop-off50on select portions of the assembly10, or the sprayer52can be configured to substantially cover an entire surface of the assembly10.

After the stop-off50has been applied, the second portions28of the maskant20are removed from the assembly10. Any stop-off50that has been disposed on the second portion28of the maskant20is removed from the workpiece12with the second maskant20. The first and second portions26,28of the maskant20can be removed by peeling or otherwise separating the maskant20from the workpiece12. For example, a person or machine can grip the substrate22at a periphery of one of the portions26,28and peel the respective portion26,28away from the workpiece12to reveal the workpiece12. Thus, as shown inFIG. 4, the resulting assembly10includes a predetermined pattern of the stop-off50on the surface14, i.e., with the stop-off50disposed only on the first portions16of the workpiece12corresponding to the first portions26of the maskant20.

Preferably, the adhesive24is adapted to be removed from the workpiece12with the substrate22. That is, as the substrate22of the maskant20is removed from the workpiece12, the adhesive24is also removed from the workpiece12. Thus, when the maskant20is removed from the workpiece12, the respective underlying portion16,18of the workpiece12is substantially free of debris, such as oil from the adhesive24or substrate22. By the term “substantially free of debris” it is meant that the workpiece12is sufficiently clean for a subsequent operation such as diffusion bonding. Therefore, while various maskants can be used according to the present invention, the maskant20should be adapted to be removed from the substrate22to leave the workpiece12substantially free of debris. For example, if the workpiece12is to be diffusion bonded, the adhesive24of the maskant20should be adapted to be removed from the workpiece12with the substrate22as the substrate22is peeled from the workpiece12so that the diffusion bonding is not prevented by adhesive24, or components thereof, deposited from the maskant20. In particular, the maskant20can be a sheet of PROTEX® latex saturated paper, and the maskant20can be removed so that at least the second portions18of the workpiece12are ready for diffusion bonding without subsequent cleaning. For example, as shown inFIG. 4, the second portions28of the maskant20have been removed from the workpiece12so that the workpiece12defines the first portions16, which are covered with the stop-off50, and the second portions18, which are substantially free of debris.

After the stop-off50is applied to the workpiece12, the workpiece12can be selectively processed. According to one embodiment of the present invention, the workpiece12is diffusion bonded and/or superplastically formed, with the stop-off50preventing diffusion bonding of select portions of the workpiece12. In this regard,FIG. 5illustrates an apparatus60that can be used to diffusion bond and/or superplastically form the workpiece12to a desired shape. The apparatus60includes two dies62,64that cooperably define a die cavity66in which the workpiece12can be supported. The apparatus60also includes one or more heating devices68that are configured to heat the workpiece12to a processing temperature. The heating device68can be a gas heater, an electric resistance heater, or an electromagnetic induction heater that generates an electromagnetic field for heating the workpiece12, e.g., via a susceptor material provided proximate to the workpiece12. In some embodiments, the dies62,64can remain relatively unheated while the workpiece12is heated by a susceptor in which an electrical current is induced by an induction coil, as described in U.S. Pat. No. 5,683,607 to Gillespie, et al, the entirety of which is incorporated herein by reference.

For example, the workpiece12can be positioned in the die cavity66with a second workpiece32so that the workpieces12,32form successive layers of a stacked pack that can be selectively diffusion bonded and/or superplastically formed. The workpieces12,32can be urged together, e.g., by pressurizing the die cavity66with a nonreactive gas such as argon from one or more fluid sources70via first ports72and valves74. The heater68can heat the workpieces12,32to a diffusion bonding temperature at which bonds34form between the first portions16of the workpiece12and the second workpiece32. For example, for diffusion bonding of workpieces12,32formed of Ti-6-4, the workpieces12,32can be pressurized to about 400 psi and heated to a temperature of about 1650° F. for a period of about 120 minutes so that the bonds34form. In other embodiments, the workpieces can be bonded at other temperatures and pressures, e.g., at about 1400° F. or greater, and/or at about 250 psi.

The workpieces12,32can optionally be superplastically formed, and superplastic forming can be performed in the same apparatus60as the diffusion bonding operation or in a separate apparatus. Superplastic forming generally refers to a process in which a material is subjected to conditions under which the material exhibits superplastic properties such as the capability of the material to develop unusually high tensile elongations and plastic deformation at elevated temperatures, with a reduced tendency toward necking or thinning. The characteristics of superplastic forming are further described in U.S. Pat. No. 3,927,817 to Hamilton, U.S. Pat. No. 4,361,262 to Israeli, and U.S. Pat. No. 5,214,948 to Sanders, each of which is incorporated in its entirety herein by reference, and which generally describe apparatuses in which the methods of the present invention can be performed.

As described in the foregoing patents, superplastic forming of a workpiece12can be performed by applying a pressure differential on opposite sides of the workpiece12so that the pressure exerts a sufficient force for forming the workpiece12against a die that defines the desired configuration of the workpiece12. For example, as shown inFIG. 5, the dies64,66define contour surfaces that correspond to a desired configuration of the workpieces12,32. A second port82fluidly connects a fluid source80to an area between the workpieces12,32, i.e., generally coincident with the first portions16of the workpiece12. Thus, the fluid source80can provide a pressurized fluid, such as argon gas, through the port82and valve84and between the workpieces12,32for superplastically forming the workpieces12,32to form a space86(FIG. 6). For example, gas pressurized to about 300 psi can be delivered by the fluid source80to the area between the workpiece12,32. Gas between the workpieces12,32and the dies62,64can be vented through the first ports72so that the workpieces12,32are urged outward by the pressure of the fluid between the workpieces12,32. The workpieces12,32and/or the dies62,64are also heated to a superplastic forming temperature, i.e., a temperature within the superplastic forming temperature range of the workpieces12,32, such as about 1400° F. or greater for superplastic forming titanium. When subjected to a sufficient pressure differential and heated to the superplastic forming temperature, the workpieces12,32deform superplastically such that the space86expands and the workpieces12,32are urged against the contour surface of the dies62,64, as shown inFIG. 6. As shown inFIGS. 5 and 6, both of the workpieces12,32are superplastically formed in the apparatus60, though a single one of the workpieces12,32can instead be formed according to other embodiments of the present invention.

Thus, the workpieces12,32can be diffusion bonded to define internal spaces or cells that are inflated, such as in the formation of an expanded honeycomb structure. Further, in other embodiments of the present invention, any number of the workpieces12,32can be diffusion bonded and/or superplastically formed, e.g., to form multi-layer stacks of sheets that are bonded and formed to define multiple layers of inflated cells. Alternatively, in some cases, the superplastic forming can be performed prior to diffusion bonding. While exemplary temperatures and pressures for the diffusion bonding and superplastic forming operations are provided above, it is noted that the temperatures and pressures can be selected according to the material and size of the workpieces, the extent of diffusion bonding and/or superplastic forming to be performed, and the like. Also, it is noted that while pressurized fluids are used in the foregoing examples for urging the workpieces12,32together for diffusion bonding and for inflating the workpieces12,32during superplastic forming, a die can alternatively be used to exert the forces necessary for forming or bonding. For example, U.S. Pat. No. 6,612,020 to Sanders, the entirety of which is incorporated herein by reference, describes such a superplastic forming operation in which a die is used to exert the forming force.