Abstract:
A thermal spray mixed with a substrate using a non-consumable cylindrical rotating tool. The process may be repeated to create a composite-like coating or material. The coating or material may be machine to improve surface quality.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a system and method for processing materials, and more particularly to a system and method for densification of a thermal spray coating deposited on a substrate material.  
           [0003]    2. Discussion of Related Art  
           [0004]    Friction stir welding (FSW) is a welding technique invented by The Welding Institute (TWI) in 1991. FSW is a solid-state joining process that is a combination of extruding and forging. FSW works at a temperature below the melting point of the work piece material.  
           [0005]    Referring to FIGS. 1A and 1B, FSW uses a cylindrical, shouldered tool  101  that is rotated and plunged into a joint line  102  between two pieces of sheet or plate material  103 - 104 , which are butted together. The tool  101  comprises a nib  105  and is constructed from a suitable material, e.g., having suitable thermochemical stability, hot hardness, and abrasion resistance. Frictional heat is generated between the tool  101  and the pieces of work material  103 - 104 . This heat causes the work piece material to soften and allows the tool  101  to traverse the joint line  102 , wherein the plasticized work material is transferred from a leading edge  106  of the tool  101  to the trailing edge  107  of the tool shoulder and nib. The tool  101  creates a solid phase bond  108  between the two pieces of work material  103 - 104 .  
           [0006]    During welding, three flow fields may exist: a primary rotational flow field due to the rotating sliver of material surrounding the nib; a secondary translational flow field created as a sliver of metal entrained in the threaded nib traverses the substrate; and a tertiary vortex field forming smoke rings adjacent to the sliver of entrained metal as a result of the primary and secondary flow fields. This model helps to explain the intercalated, lamellar, or onion-like structures of a friction stir weld. The normal force (Force plunge =6πRτ(T); where R is the radius of the shoulder and τ is the temperature dependent shear flow stress) and the moment (Power=2πR2tτ(T)ω; where R is the shoulder radius, t is the plate thickness, τ is the temperature dependent shear flow stress, and ω is the angular velocity of the weld tool=2π(rpm)/60) needed for welding and translational forces (Force Translational =12Rtτ(T), where R is the shoulder radius, t is the plate thickness, and τ is the temperature dependent shear flow stress) produced during welding can be estimated.  
           [0007]    Since its invention, FSW has emerged as an important solid state joining process for applications in industries including aerospace, shipping, railway, as well as military.  
           [0008]    Thermal spray is a directed spray process in which material in molten form, semi-molten, or solid form, is accelerated to high velocities, and impinged upon a substrate, where a dense and strongly adhered deposit is built. Material may be injected in the form of a powder, wire or rod into a high velocity combustion or thermal plasma flame, which imparts thermal and momentum transfer to the particles. By carefully controlling the plume characteristics and material state, it is possible to deposit a vast range of materials (e.g., metals, ceramics, polymers and combinations thereof) onto virtually any substrate in various conformal shapes. For metals, the particles can be deposited in solid or semi-solid state. For ceramic deposits, the particles may need to be well above the melting point, which is achieved by either a combustion flame or a thermal plasma arc. The deposit is built-up by successive impingement of droplets, which yield flattened, solidified platelets, referred to as ‘splats’. The deposited microstructure and, thus, properties, aside from being dependent on the spray material, depend on the processing parameters, which can be numerous and complex.  
           [0009]    However, no know system or method exists for processing a thermal spray coating using friction stir welding. Therefore, a need exists for a system and method for processing a thermal spray coating and a substrate by friction stir welding.  
         SUMMARY OF THE INVENTION  
         [0010]    In thermal spray, metallic or nonmetallic surfacing materials are deposited in a molten or semi-molten condition on a substrate. In this process the feedstock, in the form of a wire, rod, cord or powder, is heated to its plastic or molten state by an oxy-fuel flame, electric arc, plasma or by detonation of an explosive gas mixture. The hot material is propelled from a spray gun to the substrate in a gas stream. The bond (e.g., mechanical, chemical, metallurgical, or a combination of these) between the substrate and the coating material is stirred using a consumable or non-consumable rotating tool, creating a composite material.  
           [0011]    According to an embodiment of the present disclosure, a method for densification of a thermal spray coating comprises depositing a thermal spray coating on a substrate, and mixing the thermal spray coating and the substrate by friction stir welding.  
           [0012]    The mixing causes metal flow of the thermal spray coating to a depth controlled by a nib of a weld tool into the substrate.  
           [0013]    The thermal spray coating is deposited by as a plasma spray. The thermal spray coating is deposited by oxy-fuel combustion acceleration of a powder feedstock. The thermal spray coating is deposited by two-wire electric arc spray.  
           [0014]    The substrate is a ferrous alloy. The substrate is a non-ferrous alloy.  
           [0015]    The thermal spray coating is a ceramic, a carbide, a metal, a composite, or a plastic.  
           [0016]    The method comprises determining a time between depositing the thermal spray coating and the friction stir welding according to a distance between a spray gun of a thermal spray system and a tool of a friction stir welding system and a speed of the substrate relative to the spray gun and tool.  
           [0017]    According to an embodiment of the present disclosure, a system for densification of a thermal spray coating comprises a first thermal spray gun for depositing a first coating on a substrate, and a densification tool for mixing the coating and the substrate.  
           [0018]    The densification tool is housed in a protective tube. The tube is ceramic.  
           [0019]    The system comprises a second thermal spray gun. The first thermal spray gun applies the first coating before mixing and the second thermal spray gun applies a second coating after mixing.  
           [0020]    The first thermal spray gun and the densification tool are fixed relative to one another, and are moveable relative to the substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings:  
         [0022]    [0022]FIG. 1A is a diagram of a cut-away view of a friction stir weld and tool;  
         [0023]    [0023]FIG. 1B is a top-down view of a friction stir weld and tool;  
         [0024]    [0024]FIG. 2 is a diagram of an apparatus according to an embodiment of the present disclosure;  
         [0025]    [0025]FIG. 3 is a diagram of an apparatus according to an embodiment of the present disclosure;  
         [0026]    [0026]FIG. 4 is a diagram of a thermal spray apparatus according to an embodiment of the present disclosure;  
         [0027]    [0027]FIG. 5 is a cross-section of a substrate and surfactant processed according to an embodiment of the present disclosure; and  
         [0028]    [0028]FIGS. 6A and 6B are diagrams of apparatus according to an embodiment of the present disclosure. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    Composite materials may be created by friction stir welding a thermal spray coating and a substrate. Referring to FIG. 2, a thermal spray is deposited on a substrate  201 . The thermal spray and substrate are processed by a friction stir welding tool  202 . Process variations are shown in  203 , wherein the thermal spray coating may be deposited by oxy-fuel  204 , plasma  205 , or an electric arc  206 . Process variations for the densification include rotational speed  207 , plunge force  208  of the tool into the substrate and thermal spray, and travel speed  209  of the tool relative to the substrate, or vise versa. The process may repeat, wherein an end point  210  or  211  may be determined after an initial thermal spray  201  and densification process  202 .  
         [0030]    A metallic or nonmetallic surfacing material is deposited in a molten or semi-molten condition on a substrate  301 . A thermal spray gun  302  deposits the surfacing material as a coating  303  on the substrate  201 . A thermal spray feedstock, in the form of a wire, rod, cord or powder, is heated to its plastic or molten state by an oxy-fuel flame, electric arc, plasma, or by detonation of an explosive gas mixture. The hot surfacing material is propelled at a velocity, e.g., about less than 3000 ft/sec, from the thermal spray gun  302  to the substrate  303  in a gas stream. The initial bond between the substrate  301  and the coating material  303  may be mechanical, chemical, metallurgical, or a combination of these. The thickness of the coating material  303  may be between about one tenth and several tens of millimeters thick. The coating material is stirred using a non-consumable rotating tool  304 , creating a composite material  305 . The process may improve wear resistance, corrosion resistance, electrical, physical, and mechanical properties of the substrate material  301  depending on the characteristics of the thermal spray coating  303 .  
         [0031]    A time between depositing the thermal spray coating and mixing the coating and substrate may be controlled by a distance, d, between the thermal spray gun  302  and the tool  304  of the friction stir welding device. Another variable for controlled the time between depositing the thermal spray coating and mixing the coating and substrate is the speed of the substrate relative to the spray gun and tool, or vise versa. The speed may need to be set according to the properties, e.g., hardness, of the substrate material or tool.  
         [0032]    A process for densification of thermal spray coatings (DTSC) can be employed to admix a thermal sprayed coating into a substrate, producing a monolithic or surface-based composite material having two or more inter-dispersed phases, comprising the sprayed material and a substrate. The depth of the tool  304 , including a nib  306 , controls whether the composite is monolithic or surface-based. Terms such as surface alloying, friction stir welding/surfacing filler material addition, coating, cladding, or buttering may be used to further describe DTSC.  
         [0033]    DTSC conveys thermal energy to a metal surface to the extent that metal flow occurs and mixes the thermal sprayed coating  303  to a depth controlled by the nib  306  of the tool  304  inserted into the substrate  301  producing a dispersion-strengthened microstructure. The DTSC may affect an improvement of a variety of technological properties, including mechanical properties.  
         [0034]    Thermal spray technology, employing plasma spray (e.g., using a non-transferred arc as the source of heat to ionize gas, wherein a plasma flame melts and propels a powder feedstock to the substrate), oxy-fuel combustion (e.g., feedstock powder is melted in an oxy-fuel flame to form a fine spray) or two-wire electric arc spray technology (e.g., two wires arc two consumable electrodes and a compressed gas to atomize and propel a metallic wire feedstock surfacing material to the substrate) are employed to apply a metallic coating. Thermal spray techniques are well known in the art. DTSC then energetically/mechanically mixes the sprayed coating into the substrate to a depth controlled by the depth of the nib  306 .  
         [0035]    Friction stir processing (welding/surfacing) techniques are well known. For DTSC, the tool  304  rotates at about several hundred to over several hundred thousand revolutions per minute (RPMs). The travel speed of the tool relative to the substrate is similar to a conventional welding process and is dependent on the RPMs and loading. The normal loading, or downward force, is dependant on the temperature dependant shear stress and the geometry of the tool. The tool material may depend on the application and can be a variety of heat-treated steels, ceramic, and composite materials.  
         [0036]    The surfacing or coating material  303  may be a ferrous or non-ferrous alloy, ceramic, composite material, or polymer.  
         [0037]    The substrate material  301  can be a variety of polymers, composite materials, ferrous and non-ferrous alloys, and aluminum alloys. DTSC can improve the surface strength of commercial aluminum alloys, increasing the utility of aluminum alloys.  
         [0038]    Referring to FIG. 4, depicting an example of a thermal spray system, a gas line  401  provides pressurized gas to a material feeder  402  and an accelerant supply  403 . The accelerant supply  403  provides an accelerant such as Helium, Argon, Air, Hydrogen, and Nitrogen. The material from the feeder  402  and the accelerant from the heater  403  are combined in at a mixing head  404  comprising a tube  302 . The material is ejected from the tube and deposited on a substrate  301 . The mixing head  404  and tube  302  can be connected to a programmable robotic arm. In addition, the friction stir welding tool may be connected to the arm such that the tool  304  and spray gun (e.g.,  404  and  302 ) move in unison. Similarly, the substrate  301  can be positioned on a programmable stage. Thus, the substrate and or system can be moved in relation to one another to produce a feature such as a line.  
         [0039]    According to an embodiment of the present invention, the initial bond (mechanical, chemical, metallurgical, or a combination of these) between the substrate and the coating is stirred (e.g., churned briskly, agitated) using a non-consumable rotating tool  304 / 306 . The process may improve wear resistance, corrosion resistance, and electrical, physical, and mechanical properties of material depending on a characteristic of the coating. For example, a stainless steel coating may increase a substrate&#39;s resistance to corrosion.  
         [0040]    During experimentation, in an effort to improve the wear and corrosion resistance of heat-treated aluminum alloys (e.g., 6061-T6), stainless steel (e.g., type  304 ) was two-wire electric arc sprayed (with atmospheric atomization) onto the aluminum substrate of flat geometry. Specialized surface preparation may not be needed, e.g., smooth as-rolled or as-cast, and was not employed during this experiment. Immediately following spraying, the non-consumable densification tool, was rotated in contacted with the coating. The tool cuts into the coating, transforming the porous coating into one with significantly improved density and bonding. This transformation occurs due to a combination of friction heating, mechanical wrinkling, rearrangement and microstructure kneading, leading to nano-microstructuring with the attendant enhancement of mechanical properties. DTSC processing can be repeated to improve bonding and coating density. FIG. 5 shows the preliminary results of Austenitic Stainless Steel on 6061 Aluminum.  
         [0041]    Referring to FIG. 6A, the friction stir welding tool  304  may be incorporated with the thermal spray gun  302  into an integrated device. The tool  304 / 306  passes through a mixing head and spray nozzle  601  of the thermal spray system. A portion of the thermal spray coating deposited before mixing  303  is worked into the substrate as a composite material  305 . A portion of the thermal spray coating deposited after mixing  602  coating the composite material  305 .  
         [0042]    As shown in FIG. 6, the substrate  301  may be fixed to a stage  603  using bolts  604  or other suitable means such as a clamp or pins. The stage  603  may move under the thermal spray gun and friction stir welding tool, which may remain stationary.  
         [0043]    [0043]FIG. 6B is a view of the mixing head and spray nozzle  601  of FIG. 6A. The mixing head and spray nozzle  601  comprise multiple two-wire arc spray guns  605 - 606  surrounding a DTSC tool  304 . The apparatus is flexible in that multiple plasma arc guns, multiple detonation guns, multiple two-wire arc sprayers or any combination thereof can be designed to spray a variety of coatings onto the substrate  301 . A tube  607  protects the densification tool  304  from heat generated by the guns  605 - 606 . The tubing  607  is, for example, a ceramic tube.  
         [0044]    In FIG. 6B each two-wire arc spray gun  605 - 606  comprises a primary atomizing air inlet  608  and secondary air inlet  609 . Two contact tubes or electrodes are provided, a first contact tube  610  has a positive charge, +VE, and a second tube  611  has a negative charge, −VE. A wire feedstock, e.g.,  612 , is provided through each contact tube. The contact tubes impart electric charges to the wire feedstock, which arc in front of the primary air inlet  608 . Compressed gas is supplied through the atomizing air inlets to atomize and propel the wire feedstock surfacing material to the substrate.  
         [0045]    Having described embodiments for a system and method for improving an interfacial bond between a thermal spray coating and a substrate, it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention.