Abstract:
A thermal spray system is provided. The thermal spray system includes a spray assembly for applying a coating to a workpiece. The thermal spray system also includes a fixturing assembly couplable to the workpiece for positioning the workpiece relative to the thermal spray system, wherein the fixturing assembly permits the workpiece to distort during application of the coating to minimize residual strain buildup within the workpiece and reduce coating stresses.

Description:
BACKGROUND 
       [0001]    The invention relates generally to fixturing methods, and more specifically to fixturing methods and apparatus for thermal spray processes. 
         [0002]    Conventional thermal spray processing typically involves mechanically constraining or clamping a workpiece to a backing plate or other surface or device. Once the workpiece is constrained and positioned, a thermal spray gun is translated along one or more axes of the workpiece and coats the workpiece with a high temperature thermal spray. 
         [0003]    Workpieces that are coated by these thermal spray processes often warp due to residual stresses caused by large thermal gradients within the workpieces. The residual stress occurs because of the imposed mechanical constraints applied upon the workpiece that prevent free expansion or contraction of the part during the thermal spray process. When the region directly under a spray gun heats up excessively it will naturally expand. When the heated portions of the workpiece begin to expand naturally in response to the heat, the mechanical constraints limit this and when stresses exceed material yield strength, residual stresses are accumulated upon cool down. Upon releasing it from the backing plate or other surface, the built up residual stress within the workpiece causes the structure to distort by bending, curling or otherwise warping. Such distortions are highly undesirable. Such distortions are especially aggravated in thinner lightweight components. In addition to causing warpage, the residual stress translates into high residual stresses in the coating applied upon the substrate. Developing spray techniques with minimal warpage and reduced stresses on coating of the workpiece is critical in many applications. 
         [0004]    To alleviate effects of the warpage as discussed above, certain conventional techniques apply thermal management to the thermal spray system to minimize the temperature gradients in the workpiece during the spray process. Accordingly, the expansion of the particular region under the plasma gun is limited to that due to the smaller temperature gradient. Even in an ideally isothermal state, however, workpieces will warp if the mechanical constraints cause thermally induced stresses to exceed material yield stress or plastic limit. In terms of coating stresses, being held in an isothermal state will not adequately alleviate the high residual stresses developed upon cool down. Typically, if a coating with coefficient of thermal expansion (CTE) different than the substrate is deposited on a mechanically constrained substrate (i.e. the substrate is heated to the operating temperature of deposition under constraints) it generally develops tensile stresses on cooldown. 
         [0005]    Accordingly, there is a need for a new thermal spray process, and associated apparatus to allow efficient thermal spray applications, while limiting the warpage of the underlying workpiece. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with an embodiment of the invention, a thermal spray system is provided. The thermal spray system includes a spray assembly for applying a coating to a workpiece. The thermal spray system also includes a fixturing assembly couplable to the workpiece for positioning the workpiece relative to the thermal spray system. The fixturing assembly permits the workpiece to distort during application of the coating to minimize residual strain buildup within the workpiece and also minimize coating stresses upon cool down of the workpiece. 
         [0007]    In accordance with another embodiment of the invention, a thermal spray apparatus is provided. The thermal spray apparatus includes a fixturing device configured to physically hold the part over the entire non-sprayed surface while permitting the sprayed and unsprayed part to flex during application of a thermal spray. 
         [0008]    In accordance with another embodiment of the invention, a method of thermal spraying including fixturing a workpiece is provided. The method also includes applying a thermal spray to the workpiece. The method also includes allowing the workpiece to expand or contract due to the application of the thermal spray with minimal interference from said fixturing. The method further includes cooling the workpiece down to room temperature to allow the workpiece to recover from during spray expansion or contraction. 
     
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a diagrammatic illustration of a typical workpiece under a thermal spray system; 
           [0011]      FIG. 2  is a diagrammatic illustration of the workpiece in  FIG. 1  under mechanical constraints; 
           [0012]      FIG. 3  is a diagrammatic illustration of a thermal spray system forming a coating on the workpiece in  FIG. 1 ; 
           [0013]      FIG. 4  is a diagrammatic illustration of warping in a workpiece after process of thermal spraying is completed; 
           [0014]      FIG. 5  is a diagrammatic illustration of a fixturing assembly including a clamp for the workpiece in  FIG. 1  in accordance with an embodiment of the invention; 
           [0015]      FIG. 6  is a diagrammatic illustration of a fixturing assembly including a bed of springs for the workpiece in  FIG. 1  in accordance with an embodiment of the invention; 
           [0016]      FIG. 7  is a diagrammatic illustration of a fixturing assembly including multiple tethers for the workpiece in  FIG. 1  in accordance with an embodiment of the invention; 
           [0017]      FIG. 8  is a diagrammatic illustration of a fixturing assembly including a thermally resistant cushion for the workpiece in  FIG. 1  in accordance with an embodiment of the invention; 
           [0018]      FIG. 9  is a flow chart representing exemplary steps in a method of thermal spraying the workpiece in  FIG. 1  in accordance with an embodiment of the invention; 
           [0019]      FIG. 10  is a diagrammatic illustration of an experimental set up of a supported strip of stainless steel used in an electrolyzer electrode; 
           [0020]      FIG. 11  is a tabular representation of the results of measurements made for detection of warpage on the strip of stainless steel in  FIG. 10 ; 
           [0021]      FIG. 12  is a graphical comparison of coating stresses on a constrained sample and an unconstrained sample as a function of ratio of thickness of a coating to thickness of a substrate; 
           [0022]      FIG. 13  is a magnified view of a coating stress curve of an unconstrained sample in  FIG. 12  as a function of ratio of thickness of a coating to thickness of a substrate; 
           [0023]      FIG. 14  is a graphical comparison of coating stresses on a constrained sample and an unconstrained sample as a function of ratio of coating modulus to substrate modulus; 
           [0024]      FIG. 15  is a magnified view of a coating stress curve of an unconstrained sample in  FIG. 14  as a function of ratio of coating modulus to substrate modulus; 
           [0025]      FIG. 16  is a graphical comparison of coating stresses on a constrained sample and an unconstrained sample as a function of difference in coefficient of thermal expansion between substrate and coating; and 
           [0026]      FIG. 17  is a magnified view of a coating stress curve of an unconstrained sample in  FIG. 16  as a function of difference in coefficient of thermal expansion between substrate and coating. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    As discussed in detail below, embodiments of the present invention include a fixturing assembly for thermal spray systems and processes and a method for the same. Thermal spraying is a commonly used engineering coating process that offers a wide choice of materials and techniques. During thermal spraying, particles of about 1 to about 90 microns are partially or fully melted and accelerated to high velocities by various techniques. These particles then strike a substrate or a workpiece wherein they get deformed and are bonded onto the workpiece. A coating is formed as the particles are deposited on top of each other. Some non-limiting examples of ‘thermal spray systems’ used herein may include a thermal spray system involving a DC plasma spray, wire-arc spray, flame spray, or high-velocity oxygen fuel thermal spray process (HVOF). 
         [0028]    In an illustrated embodiment of the invention as shown in  FIG. 1 , a thermal spray system  10  is depicted. The thermal spray system  10  includes a spray assembly or a spray gun  12  for applying a coating  14  to a workpiece  16 . Components such as the workpiece  16  when coated by the thermal spray system  10  commonly tend to warp due to residual stresses caused by thermal gradients within the workpiece  16 . Residual stresses generally occur due to imposed mechanical constraints that prevent natural tendency of free expansion or contraction of the workpiece  16  during a thermal spray  20 . Some non-limiting examples of the workpiece  16  may include a fuel cell component, an electrolyzer component, and a gas turbine hot gas path component. In another embodiment, the workpiece  16  may be a solid oxide fuel cell component. In a particular embodiment, operating temperature of the thermal spray  20  may be in the range between about 1000° C. to about 7000° C. When the thermal spray  20  that includes highly accelerated particles strikes a portion  22  of the workpiece  16 , the portion  22  gets heated excessively resulting in a natural tendency of thermal expansion. Commonly found constraints that prevent free expansion or contraction of the portion  22  are externally applied mechanical constraints such as clamps and backing plates and thermal gradients between the portion  22  and surrounding portions and through thickness on the underside of portion  22 . The portion  22  may also tend to expand or contract due to transformation of its material state. Some non-limiting examples of transformation in material state may include melting, resolidification and recrystallization. 
         [0029]      FIG. 2  is a diagrammatical illustration of a system  30  including a workpiece  16  as referenced in  FIG. 1  that may be physically constrained by mechanical constraints  34 . The mechanical constraints  34  exert a force in a direction  32  on the workpiece  16 . In a particular embodiment, the workpiece  16  may include strips of stainless steel. Some non-limiting examples of mechanical constraints include clamps and backing plates. The workpiece  16  may be attached to a base  36 . 
         [0030]      FIG. 3  is a diagrammatical illustration of a workpiece system  40  undergoing heating by a thermal spray assembly  12  as referenced in  FIG. 1 . The workpiece system  40  includes the workpiece  16  as referenced in  FIG. 1  that may be physically constrained by mechanical constraints  34  as referenced in  FIG. 2 . The workpiece  16  may also be attached to a base  36  as referenced in  FIG. 2 . The thermal assembly  12  forms a coating  42  on the workpiece  16  as referenced in  FIG. 1 . In a particular embodiment, the workpiece  16  may include strips of stainless steel. A portion or hot spot  22  as referenced in  FIG. 1  that is directly under the spray assembly  12  heats up excessively leading to expansion of the portion  22 . As the portion  22  tends to expand, the mechanical constraints  34  as referenced in  FIG. 2  prevent the expansion. In addition, thermal gradients across thickness of the workpiece  16  may constrain expansion of the portion  22 . 
         [0031]      FIG. 4  is a diagrammatical illustration of a workpiece system  50  without a mechanical constraint after a process of thermal spraying is completed. The portion  22  as referenced in  FIG. 1  of the workpiece  16  as referenced in  FIG. 1  being directly under the spray assembly  12  as referenced in  FIG. 1  tends to warp as it gets heated excessively. As shown, the workpiece  16  bends about a direction  52  once heating process is completed and upon release of any mechanical constraint. Depending on the degree of the constraints (bolting being the worst and suspending it in air being the best case) this residual stress in the  16  can translate to an unfavorable stress state within  42  or coating. 
         [0032]    In an illustrated embodiment of the invention as shown in  FIG. 5 , a fixturing assembly  60  for a part of the workpiece  16  as referenced in  FIG. 1  that is being sprayed is depicted. The fixturing assembly  60  tends to minimize residual stresses in the workpiece  16  leading to minimal warpage and coating stress. A spray gun  12  as referenced in  FIG. 1  travels in a direction  62  in a plane of the workpiece  16  and sprays a front face  70  of the workpiece  16 . The fixturing assembly  60  includes a clamp  64  fixed at a location  66  on the workpiece  16 . The clamp  64  is necessary only to hold the work piece  16  in place and constrain it from moving in the out-of-plane direction  68 . In order to prevent free expansion of the work piece to minimize residual stress build-up, the clamp  64  constrains the work piece only at location  66 . The work piece  16  may be free without any constraint at other locations. During a process of thermal spraying, the workpiece  16  has more degrees of freedom to expand or contract thus resulting in lesser distortion upon cooling down to room temperature. In this embodiment, the clamp  64  at location  66  exerts minimal constraint on expansion or contraction of the workpiece  16  resulting in reduction in a build up of permanent strains and lower coating stresses built up on surface  70  once the process of thermal spraying is completed. 
         [0033]    When a substantially planar component such as the workpiece  16  is thermally sprayed on the front face  70 , temperature gradients develop through thickness of the workpiece  16  with a surface being sprayed such as the front face  70  being hotter than an opposite surface or a back face  72 . Hence, the front face  70  has a tendency to expand relative to the back face  72  leading to a natural tendency for the workpiece  16  to curl out of plane in the direction  68 . This leads to the front face  70  being convex. In a case where the workpiece  16  may be mechanically constrained at many locations to prevent the expansion, the front face  70  will get compressed further due to the natural constraint of a cooler underside. Consequently, the compression may lead to plastic yielding of the front face  70  thus building up a residual strain on the workpiece  16 . Clamping the workpiece  16  at only one location  66  allows the workpiece  16  to curl out of plane relatively freely according to natural tendency due to differential heating of the front face  70  and the back face  72 . This leads to minimal distortion of the workpiece  16  after the process of thermal spraying is completed. By relieving the compressive force or constraints on the front face  70  will also alleviate or reduce the tensile stresses developed in the coating. 
         [0034]    In another illustrated embodiment of the invention as shown in  FIG. 6 , a fixturing assembly  80  for a part of the workpiece  16  as referenced in  FIG. 1  is depicted. A spray gun or assembly  12  as referenced in  FIG. 1  sprays a front face  70  as referenced in  FIG. 5  of part of the workpiece  16 . The fixturing assembly  80  includes a bed or a foundation of independent springs  82  on a mounting plate  84  supporting a back face  72  as referenced in  FIG. 5  of the workpiece  16  being sprayed in an out of plane direction  68  as referenced in  FIG. 5 . The independent springs  82  may include any material that may withstand operating temperature range resulting from exposure to the spray assembly  12 . In a particular embodiment, the springs  82  may be shielded or insulated from direct heating of the thermal spray assembly  12 . When the workpiece  16  is thermally sprayed on the front face  70 , temperature gradients develop through the thickness of the workpiece  16 . Thus, the front face  70  tends to get hotter than the back face  72  resulting in a natural tendency to expand relative to the back face  72 . The front face  70  expands by becoming convex resulting in curling of the workpiece  16  in an out of plane direction  68  as referenced in  FIG. 5 . The bed of independent springs  82  supports the backface  72  such that it provides minimal constraint on the curling of the workpiece  16  in the out of plane direction  68 . In addition, the flexibility of the springs  82  allows relatively free expansion or contraction and out-of-plane flexing of workpiece  16 . This consequently reduces residual stress and build up of permanent strains leading to lesser distortions and coating stresses upon cool down and release of the workpiece  16  after the process of thermal spraying. 
         [0035]      FIG. 7  is a diagrammatical illustration of another embodiment of a fixturing assembly  90  for a part of the workpiece  16  as referenced in  FIG. 1 . A spray gun  12  as referenced in  FIG. 1  sprays a front face  70  as referenced in  FIG. 5  of part of the workpiece  16 . In the particular embodiment, the fixturing assembly  90  includes spring supports  92  tethered in-plane of the workpiece  16  at peripheral locations of the part of the workpiece  16  being sprayed. The spring supports  92  may be attached to a frame  94 . During a thermal pray process, the front face  70  tends to become convex resulting in curling of the workpiece  16  in an out of plane direction  68  as referenced in  FIG. 5 . The spring supports  92  tethered in-plane provide limited out-of-plane stiffness leading to relatively free expansion and contraction of the part of the workpiece  16  in the out-of-plane direction  68  during a spray process. Further, the flexible nature of the spring supports  92  facilitates curling of the workpiece  16 . This consequently reduces residual stress and build up of permanent strains leading to lesser distortions and lower coating stresses upon cool down and release of the workpiece  16  after the process of thermal spraying. In fact, this method can also act as a pre-tensioner to  16  and allow this elastic stress to be relieved upon cool down to further reduce coating stresses. 
         [0036]    In another illustrated embodiment of the invention as shown in  FIG. 8 , a fixturing assembly  100  including a cushion of air  102  is depicted. A spray gun  12  as referenced in  FIG. 1  travels in a direction  62  as referenced in  FIG. 5  in plane of the workpiece  16  and sprays a front face  70  of the part of the workpiece  16 . The cushion of air  102  supports a backface  72  as referenced in  FIG. 5  of the part of the workpiece  16  so that the part can bend freely in an out-of-plane direction  68  as referenced in  FIG. 5  without any build up of strain. The cushion of air  102  serves a purpose of a solid wall without mechanical stiffness. In a particular embodiment, the fixturing assembly  100  may also include a foam backing material. Lines  104  indicate a spring like quality of the fixturing assembly introduced in this embodiment. During a thermal spray process, the front face  70  tends to become convex due to a temperature differential between the front face  70  and the back face  72 . This results in a natural tendency of curling of the workpiece  16  in the out-of-plane direction  68 . The cushion of air  102  facilitates in such a movement due to spring like quality in it. Consequently, this reduces residual stress and build up of permanent strains leading to lesser distortions and coating stresses upon cool down and release of the workpiece  16  after the process of thermal spraying. 
         [0037]      FIG. 9  is a flow chart representing exemplary steps in a method  110  of thermal spraying a workpiece  16  as referenced in  FIG. 1 . The method  110  includes fixturing the workpiece in step  112 . In one embodiment, the fixturing may include clamping at least one location of the workpiece. In another embodiment, the fixturing may include coupling series of springs to the workpiece. In another embodiment, the fixturing may include tethering spring supports to said workpiece. In another particular embodiment, the fixturing may include providing a cushion of air or a foam of backing material to the workpiece. Once the fixturing has taken place, a thermal spray is applied on the workpiece in step  114 . During the thermal spray process, a portion of the workpiece directly under the spray heats up consequently tending to expand or contract. To minimize residual stress in the component and coating stress, the method  110  also includes allowing the workpiece to expand or contract with minimal interference from the fixturing in step  116 . Once the workpiece expands or contracts freely, the method  100  further includes cooling the workpiece in step  118  to allow the workpiece to recover from a warped position. 
       EXAMPLES 
       [0038]    The examples that follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention. 
         [0039]    A series of experiments using strips of stainless steel as a sample were performed to detect warpage in a hydrogen electrolyzer electrode. The strips of stainless steel were thermally sprayed using wire-arc spraying technique. Measurements were made on strips of varying thicknesses that had a supported or an unsupported backface.  FIG. 10  is a diagrammatic illustration of an experimental set up of a strip of stainless steel with a fixturing assembly  130 . The fixturing assembly  130  included a clamp  132  to a backing plate  134  on a location at the backface  136  of a stainless steel strip  138  being sprayed. The backing plate  134  prevented backward curl of the sample during spraying process. Length of the sample was about 18 inches and breadth of the strip was about 2 inches. Varying thicknesses of the sample were 0.032 inch, 0.062 inch and 0.125 inch. The speeds of the spray gun used were 700 mm/sec and 1100 mm/sec. The spray gun was placed at a distance of about 3 inches from the sample and length of a spray window was about 26 inches. 
         [0040]      FIG. 11  is a tabular representation  150  of the results of the measurements made on the sample at varying thicknesses and with a supported and an unsupported backface. The measurements include measuring deflections or warpage in the sample during the thermal spraying process. As seen from the table, samples with an unsupported backface show no deflection or warpage while the samples with a supported backface indicate a small amount of deflection. In addition, samples of thickness 0.032 inch and a supported backface showed no change in measure of deflection with increase in speed of the spray gun. Hence, the results indicate that an unsupported backface that allows for free movement of a part of the workpiece during spraying process leads to no residual stresses or warpage of the workpiece. Further, warping of a sample is independent of the spray gun speed. 
         [0041]      FIG. 12  is a graphical comparison  152  of stresses induced in a coating deposited on a mechanically constrained substrate, also referred to as ‘constrained sample’ versus stresses induced in a coating deposited on a mechanically unconstrained substrate, also referred to as ‘unconstrained sample’, as a function of the coating thickness. X-axis  154  represents ratio of coating thickness to substrate thickness. Y-axis  156  represents stress due to coating and is measured in N/m 2  units. Curve  158  represents the stress in a constrained sample while curve  160  represents stress in an unconstrained sample. As seen, there is a significant difference in stress levels in a sample that is constrained as compared to a sample that is unconstrained. There are high levels of coating stress in a constrained sample and much lower levels of coating stress in an unconstrained sample. 
         [0042]      FIG. 13  is a magnified version  170  of the curve  160  in  FIG. 12 . Curve  160  seems to be a straight line in  FIG. 12  due to a large difference in stress levels in a constrained sample as compared to an unconstrained sample. As seen in  FIG. 13 , curve  160  is relatively sensitive to the thickness of coating and there seem to be compressive stresses, as indicated by negative values, associated with a sample under no constraints during a thermal spraying process. 
         [0043]      FIG. 14  is a graphical comparison  180  of coating stress on a sample under constraints versus an unconstrained sample as a function of coating modulus. X-axis  182  represents ratio of coating modulus to substrate modulus. Y-axis  184  represents stress due to coating and is measured in N/m 2  units. Curve  186  represents the stress in a constrained sample while curve  188  represents stress in an unconstrained sample. As seen by comparison of the two curves  186  and  188 , there is a significant difference in stress levels in a sample that is constrained as compared to a sample that is unconstrained. There are high levels of coating stress in a constrained sample and much lower levels of coating stress in an unconstrained sample. 
         [0044]      FIG. 15  is a magnified view  200  of the curve  188  in  FIG. 14  drawn to an expanded scale. Curve  188  seems to be a straight line in  FIG. 14  due to a large difference in stress levels in a constrained sample as compared to an unconstrained sample. As seen, curve  188  has limited sensitivity to the coating modulus and there seem to be compressive stresses associated with a sample of specific thickness and moduli system, as indicated by negative values, under no constraints during a thermal spraying process. 
         [0045]      FIG. 16  is a graphical comparison  210  of coating stress on a sample under constraints versus an unconstrained sample as a function of thickness of coating. X-axis  212  represents difference of coefficient of thermal expansion (CTE) between coating and the substrate. Y-axis  214  represents stress due to coating and is measured in N/m 2  units. Curve  216  represents the stress in a constrained sample while curve  218  represents stress in an unconstrained sample. As seen, there is a significant difference in stress levels in a sample that is constrained as compared to a sample that is unconstrained. There are high levels of coating stress in a constrained sample and much lower levels of coating stress in an unconstrained sample. 
         [0046]      FIG. 17  is a magnified version  230  of the curve  218  in  FIG. 16  drawn to an expanded scale. Curve  218  seems to be a straight line in  FIG. 12  due to a large difference in stress levels in a constrained sample as compared to an unconstrained sample. As seen, curve  218  is very sensitive to the thickness of coating and there seem to be compressive stresses associated with a sample, as indicated by negative values, under no constraints. 
         [0047]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.