Patent Publication Number: US-9888557-B2

Title: Plasma spraying apparatus

Description:
The entire disclosure of Japanese Patent Application No. 2010-276141 filed on Dec. 10, 2010 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a plasma spraying apparatus which transfers plasma-arc to an electrically conductive wire to thereby generate plasma flame, melts the wire into droplets, and sprays the droplets onto a target. 
     Description of the Related Art 
       FIG. 1  is a cross-sectional view of a conventional plasma spraying apparatus. 
     As illustrated in  FIG. 1 , the conventional plasma spraying apparatus  90  includes a first gas nozzle  91  defining a first gas path  91   a , a second gas nozzle  92  disposed outside of the first gas nozzle  91  to define a second gas path  92   a , a cathode  93  disposed substantially on central axes of both a nozzle opening  91   b  of the first gas nozzle  91  and a nozzle opening  92   a  of the second gas nozzle  92 , a battery unit  94 , and a wire guide hole  95  for introducing an electrically conductive wire W to be sprayed, into a vicinity of the nozzle opening  92   a  of the second gas nozzle  92 . 
     The wire W is supplied obliquely of a central axis of the nozzle opening  92   a  and in front of the nozzle opening  92   a  through the wire guide hole  95 . A first gas sprayed through the first gas path  91   a  is turned into plasma flame F by means of arc generated between the wire W indirectly electrically connected to an anode of the battery unit  94  through the second gas nozzle  92 , and the cathode  93  electrically connected to a cathode of the battery unit  94 . The thus generated plasma flame F melts the wire W into droplets D, and sprays the droplets D. The droplets D are further reduced in size and further accelerated by a second gas sprayed forwardly of the second gas nozzle  92  through the second gas path  92   a , and sprayed onto a target T to thereby form a sprayed coating S on the target T. 
     In a plasma spraying apparatus in which the wire W is molten into the droplets D by means of the plasma flame F and a second gas flow, and the droplets D are sprayed onto the target T, it is necessary to stably generate plasma flame F, and it is also necessary for a tip end of the wire W to be always disposed within the plasma flame F in order to uniformly spray the droplets D. 
     However, in the conventional plasma spraying apparatus, the wire guide hole  95  through which the wire W is supplied has a circular cross-section, and is designed to have a greater diameter than that of the wire W in order to prevent the wire W from being hooked or clogged in the wire guide hole  95  due to deformation the wire W originally has. Accordingly, it is difficult to supply the wire W with the distortion of the wire W being reformed, and thus, the wire W repeats going out of and going back to a center of the plasma flame F due to the original deformation of the wire W. Thus, the conventional plasma spraying apparatus is accompanied with a problem that it is not possible to stably supply the wire W to a center of the plasma flame F. 
     In order to solve the above-mentioned problem, for instance, Japanese Patent Application Publication No. H9 (1997)-308970 has suggested a plasma spraying apparatus including a first guide for reforming original deformation of a wire inserted into a support plate formed integral with a plasma arc torch, and a second guide for guiding the wire from the first guide, and causing the wire W to bend beyond elastic limit thereof. The wire is supplied after the original deformation of the wire was removed, and a tip end of the wire is kept at a center of plasma gas flow to thereby stably generate plasma flame. 
     However, as suggested in the above-mentioned Publication, if the second guide were integrated with the plasma arc torch for causing a wire to bend beyond elastic limit, a force for feeding a wire would become excessive, because the second guide causes a wire to bend elastic limit thereof. Accordingly, a wire feeding unit is inevitably big-sized, and the torch would be big-sized at its entirety. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problem in the conventional plasma spraying apparatus, it is an object of the present invention to provide a plasma spraying apparatus which is capable of stably feeding a wire without including a second guide for causing a wire to bend beyond elastic limit, in spite of deformation which a wire originally has, in the case that a wire is to be supplied into a vicinity of a nozzle opening of a second gas nozzle. 
     In one aspect of the present invention, there is provided a plasma spraying apparatus including a cathode, a first gas nozzle surrounding a head of the cathode therewith to form a first gas path between the cathode and the first gas nozzle, and a second gas nozzle surrounding the first gas nozzle therewith to form a second gas path between the first gas nozzle and the second gas nozzle, wherein the second gas nozzle is formed with a wire path through which a wire is inserted such that a distal end of the wire is disposed in front of a nozzle opening of the second gas nozzle, a first gas sprayed through the first gas nozzle is turned into plasma flame by arc generated between the cathode and the distal end of the wire, the distal end of the wire is molten into droplets by the plasma flame, and the droplets are sprayed onto a target by means of both the plasma flame and a second gas sprayed through the second gas nozzle, and the wire path has a substantially rectangular cross-section having a longer side extending in a direction in which the plasma flame extends, the wire path causing the wire to bend within elastic limit of the wire. 
     In the plasma spraying apparatus in accordance with the present invention, a wire is caused to bend within elastic limit thereof to thereby allow the original deformation of a wire to release in a direction in which plasma flame extends. Thus, it is possible to prevent a wire from moving in a direction perpendicular to a direction in which plasma flame extends. It should be noted that even if a wire moves at a tip end thereof in a direction in which plasma flame extends, since the tip end is disposed on a central axis of the plasma flame, the plasma flame is prevented from being instable. Thus, the plasma spraying apparatus in accordance with the present invention makes it possible to stably supply a wire to a center of plasma flame. 
     It is preferable that the cross-section of the wire path has a shorter side having a length greater than a diameter of the wire by 3 to 10% only 3 inclusive. 
     By designing the wire to have such a shorter side, it is possible to allow the original deformation of a wire to release only in a direction in which plasma flame extends, and to prevent a wire from moving in a direction perpendicular to a direction in which plasma flame extends. 
     If the shorter side of the wire were greater than a diameter of the wire path by X % (X&lt;3), there would be just an insufficient space for allowing the original deformation of the wire to release in a direction in which plasma flame extends, the wire might be hooked or clogged in the wire path. If the shorter side of the wire path were greater than a diameter of the wire by 10% or more, the space would be too large, resulting in that the original deformation of the wire would be released not only in a direction in which plasma flame extends, but also in a direction perpendicular to a direction in which plasma flame extends. 
     It is preferable that the wire path includes a first wire path having an exit disposed in the vicinity of a nozzle opening of the second gas nozzle, and a second wire path inclining relative to the first wire path by a predetermined angle. 
     When a wire is fed into the first wire path from the second wire path, the wire is caused to bend within elastic limit due to the predetermined angle formed between the first and second wire paths, resulting in that it is possible to allow the original deformation of a wire to release only in a direction in which plasma flame extends, and to prevent a wire from moving in a direction perpendicular to a direction in which plasma flame extends. 
     It is preferable that the predetermined angle is in the range of 1 to 5 degrees both inclusive. 
     The predetermined angle in this range makes it possible to cause a wire to bend within elastic limit, and stably supply a wire to a center of plasma flame. 
     If the predetermined angle were smaller than 1 degree, the angle could not cause a wire to bend in a desired degree with the result that a wire is instably fed. If the predetermined angle were greater than 5 degrees, a wire might be caused to bend beyond elastic limit. 
     It is preferable that the first and second wire paths are spaced away from each other, preferably by 3 to 10 millimeters both inclusive. 
     The first wire path, the second wire path, and a space between them artificially form a curved wire path to thereby cause a wire to bend within elastic limit. 
     If the space between the first and second wire paths is smaller than 3 mm, the wire path would be considered to substantially comprise only the first wire path. If the space would be greater than 10 mm, the second wire path could not provide effective bending to a wire, in which case, the wire path is considered to substantially comprise only the first wire path. 
     The plasma spraying apparatus may be designed to further include a third gas nozzle disposed between the first gas nozzle and the second gas nozzle to form a third gas path between the first gas nozzle and the third gas nozzle. 
     It is preferable that the cross-section of the wire path is chamfered at corners thereof such that the wire does not make contact with the corners. 
     It is preferable that the cross-section of the wire path is rounded at corners thereof such that the wire does not make contact with the corners. 
     It is preferable that at least one of the first wire path and the second wire path is linear or curved. 
     It is preferable that the first wire path has a substantially rectangular cross-section. 
     It is preferable that the second wire path has a substantially rectangular cross-section. 
     The advantages obtained by the aforementioned present invention will be described hereinbelow. 
     Firstly, since the wire path has a substantially rectangular cross-section having a longer side extending in a direction in which plasma flame extends, and causes a wire to bend within elastic limit thereof, it is possible to release the original deformation of a wire in a direction in which plasma flame extends, prevent a wire from moving in a direction perpendicular to a direction in which plasma flame extends, and stably feed a wire into a center of plasma flame, without designing the plasma spraying apparatus to include a second guide for causing a wire to bend beyond elastic limit. 
     Secondly, the wire path has a substantially rectangular cross-section having a shorter side having a length greater than a diameter of a wire by 3 to 10% only 3 inclusive. Thus, it is possible to allow the original deformation of a wire to release only in a direction in which plasma flame extends, and to prevent a wire from moving in a direction perpendicular to a direction in which plasma flame extends. As a result, it is possible to stably supply a wire into a center of plasma flame. 
     Thirdly, the wire path comprises a first wire path having an exit disposed in the vicinity of a nozzle opening of the second gas nozzle, and a second wire path inclining relative to the first wire path by a predetermined angle, for instance, by 1 to 5 degrees both inclusive. Thus, it is possible to cause a wire to bend within elastic limit, and hence, it is possible to stably supply a wire into a center of plasma flame. 
     Fourthly, the first and second wire paths are spaced away from each other, for instance, by 3 to 10 millimeters both inclusive. Thus, there can be artificially formed a curved wire path bigger than the first and second wire paths, ensuring a wire to bend within elastic limit thereof. 
     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a conventional plasma spraying apparatus. 
         FIG. 2  is a schematic view of the plasma spraying apparatus in accordance with the preferred embodiment of the present invention. 
         FIG. 3  is a longitudinal cross-sectional view of a main part of a plasma spraying torch illustrated in  FIG. 2 . 
         FIG. 4  is an enlarged view seen from an arrow A shown in  FIG. 3 . 
         FIG. 5  is a view showing the action of the plasma spraying torch illustrated in  FIG. 3 . 
         FIG. 6  shows a relation between a cross-section of the wire path and a direction in which a force acts on the wire. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  is schematic view of the plasma spraying apparatus in accordance with the preferred embodiment of the present invention,  FIG. 3  is a longitudinal cross-sectional view of a main part of a plasma spraying torch illustrated in  FIG. 2 ,  FIG. 4  is an enlarged view seen from an arrow A shown in  FIG. 3 , and  FIG. 5  is a view showing the action of the plasma spraying torch illustrated in  FIG. 3 . 
     As illustrated in  FIG. 2 , the plasma spraying apparatus  1  in accordance with the preferred embodiment of the present invention includes a plasma spraying torch  2  for spraying droplets generated by melting a wire W by means of plasma flame, onto a target, a gas source  3  for supplying a first gas and a second gas to the plasma spraying torch  2 , a battery  4  for supplying electric power to the plasma spraying torch  2 , a wire reel  5  around which a wire W is wound, a wire straightener  6  for straightening the wire W unwound from the wire reel  5 , and a wire feeder  7  for feeding the wire W to the plasma spraying torch  2  through a wire-feeding tube  8 . 
     As illustrated in  FIG. 3 , the plasma spraying torch  2  includes a first gas nozzle  10  defining a first gas path  11 , a second gas nozzle  20  disposed outside of the first gas nozzle  10  and defining a second gas path  21 , a third gas nozzle  30  disposed between the first gas nozzle  10  and the second gas nozzle  20  and defining a third gas path  31 , a cathode  40  disposed substantially on central axes of both a nozzle opening  12  of the first gas nozzle  10  and a nozzle opening  22  of the second gas nozzle  20 , and a wire path  50  for feeding a wire W to be sprayed, into a vicinity of the nozzle opening  22  of the second gas nozzle  20 . 
     The first gas nozzle  10  surrounds a head of the cathode  40  such that the first gas path  11  is defined between the first gas nozzle  10  and the cathode  40 . A first gas comprising an inert gas such as a nitrogen gas or an argon gas is supplied into the first gas path  11 . As an alternative, compressed air may be used as the first gas. The first gas supplied through the first gas path  11  is sprayed through the nozzle opening  12  of the first gas nozzle  10  towards the nozzle opening  22  of the second gas nozzle  20 . 
     The third gas nozzle  30  surrounds the first gas nozzle  10  such that the third gas path  31  is defined between the first gas nozzle  10  and the third gas nozzle  30 . A third gas to be supplied into the third gas path  31  comprises compressed air or a carbon dioxide gas, for instance. 
     The second gas nozzle  20  surrounds the third gas nozzle  30  such that the second gas path  21  is defined between the third gas nozzle  30  and the second gas nozzle  20 . A second gas to be supplied into the second gas path  21  comprises compressed air or a carbon dioxide gas, for instance. 
     The wire path  50  includes a first wire path  51   a  having a wire exit  51   b  formed in the vicinity of the nozzle opening  22  of the second gas nozzle  20 , and a second wire path  52   a  through which the wire W is supplied at a predetermined angle relative to the first wire path  51   a.    
     The wire path  50  causes the wire W to bend within elastic limit thereof by means of the first wire path  51   a  and the second wire path  52   a.    
     As illustrated in  FIG. 4 , the first wire path  51   a  has a substantially rectangular cross-section extending in a direction in which the plasma flame extends, and is formed by linearly passing through a first wire guide  51  disposed outside of the second gas nozzle  20 . Similarly, the second wire path  52   a  has a substantially rectangular cross-section extending in a direction in which the plasma flame extends, and is formed by linearly passing through a second wire guide  52  disposed away from the first wire path  51   a.    
     A length “a” of a longer side of the first wire path  51   a  is designed to be longer than a diameter “d” of the wire W by 10% to 95% both inclusive. A length “b” of a shorter side of the first wire path  51   a  is designed to be longer than a diameter “d” of the wire W by 3% to 10% only 3% inclusive. 
     In the current embodiment, the wire W has a diameter of 1.6 mm, a longer side of the first wire path  51   a  has a length “a” longer than a diameter “d” of the wire W by about 0.2 to about 1.5 mm, and a shorter side of the first wire path  51   a  has a length “b” longer than a diameter “d” of the wire W by about 0.05 to about 0.15 mm. The longer and shorter sides of the second wire path  52   a  are designed to have the same lengths as those of the first wire path  51   a.    
     It should be noted that the substantially rectangular cross-sections of both the first wire path  51   a  and the second wire path  52   a  may be chamfered or rounded at corners unless the corners make contact with the wire W. Accordingly, only a force oriented perpendicular to the longer or shorter side of both the first wire path  51   a  and the second wire path  52   a  acts on the wire W in the current embodiment in the first wire path  51   a  and the second wire path  52   a.    
     An inclination angle θ formed between the first wire path  51   a  and the second wire path  52   a  is defined as an angle formed between a central axis of the first wire path  51   a  and a central axis of the second wire path  52   a . In the current embodiment, the inclination angle θ is set in the range of about 1 to about 5 degrees both inclusive. 
     The second wire guide  52  through which the second wire path  52   a  passes is disposed away from the first wire path  51   a  by a space “c”. In the current embodiment, the space “c” is set in the range of about 3 to about 10 mm both inclusive. 
     In the plasma spraying torch  2  in the current embodiment, as mentioned above, since the first wire path  51   a  and the second wire path  52   a  are spaced away from each other by a space “c”, the first wire path  51   a  and the second wire path  52   a , both of which are linear, cooperate with each other to artificially define the curved wire path  50  to thereby cause the wire W to bend within elastic limit. 
     Though the first wire path  51   a  and the second wire path  52   a  are designed linear in the current embodiment, they may be designed curved. 
     The battery  4  is electrically connected at an anode thereof with the first wire guide  51 , and hence, is indirectly electrically connected with the wire W inserted into the first wire path  51   a  formed through the first wire guide  51 . The battery  4  is electrically connected at a cathode thereof with the cathode  40 . The battery  4  may be directly electrically connected at an anode thereof with the wire W. 
     In the plasma spraying apparatus  1  having the above-mentioned structure, when the wire W wound around the wire reel  5  is fed to the plasma spraying torch  2  through the wire feeder  7 , the original deformation of the wire W, that is, the intensive characteristic by which the wire W tends to be curled, is removed by means of the wire straightener  6 , and thus, the wire W is straightened to a slightly curled condition. 
     Then, the wire W is fed to the wire path  50  through the wire-feeding tube  8 . In the wire path  50 , only a force oriented perpendicular to a longer side or a shorter side of both the first wire path  51   a  and the second wire path  52   a  acts on the wire W, and thus, as illustrated in  FIG. 5 , the wire W is caused to bend within elastic limit thereof in a direction in which the plasma flame F extends. 
     Since both the first wire path  51   a  and the second wire path  52   a  are designed to have a rectangular cross-section having a longer side extending in a direction in which the plasma flame F extends, the original deformation of the wire W is released in a direction in which the plasma flame F extends. In particular, in the current embodiment, since the shorter side of the first wire path  51   a  and the second wire path  52   a  is designed to have a length “b” greater than a diameter “d” of the wire W by X % (3≦X&lt;10), the original deformation of the wire W is not released in a direction perpendicular to a direction in which the plasma flame F extends. Accordingly, even if a tip end of the wire W were slightly shifted in a direction in which the plasma flame F extends, the tip end is prohibited from shifting in a direction perpendicular to a direction in which the plasma flame F extends, and thus, it is ensured that the tip end of the wire W is disposed on an axis of the plasma flame F. 
       FIG. 6  shows a relation between a cross-section of the wire path  50  and a direction in which a force acts on the wire W. 
     In  FIG. 6 , the cross-section A indicates a rectangular cross-section, the cross-section B indicates a rectangular cross-section which is chamfered at corners such that the wire W does not make contact with the chamfered corners, and the cross-section C indicates a rectangular cross-section which is rounded at corners such that the wire W does not make contact with the rounded corners. 
     In these cross-sections A, B and C, when the wire W makes contact with not only a longer side, but also a shorter side, only a force oriented perpendicular to the longer side and shorter side acts on the wire W. 
     Since it is not possible to completely straighten the wire W even by the wire straightener  7 , the wire W unavoidably has the original deformation, specifically, a characteristic of curling. Furthermore, the wire-feeding tube  8  is varied into various shapes in dependence on a position of the plasma spraying torch  2  in assembling the plasma spraying apparatus  1 , and hence, cannot keep a uniform shape. Thus, when the wire W having the original deformation is being fed through the wire-feeding tube  8  which is not capable of keeping a uniform shape, a bending force and/or a torsion force act on the wire W in dependence on a shape of the wire-feeding tube  8 . The wire W randomly bends like a spring in elastic limit thereof by such forces, and is fed in meandering condition through the wire-feeding tube  8  in a route at which the forces are stabilized. 
     While the wire W is being fed in the wire path  50 , when the wire W makes contact with a shorter side of the above-mentioned cross-section A, B or C, a force oriented perpendicular to a shorter side, that is, a force oriented in parallel with a direction (hereinafter, referred to as “direction X”) in which the plasma flame F extends acts on the wire W, and hence, the original deformation is released in the direction X. If a force oriented in a direction (hereinafter, referred to as “direction Y”) perpendicular to the direction X acts on the wire W while the wire W makes contact only with a shorter side, the wire W randomly moves by spaces formed in the length “b”, and makes contact with a longer side, however, in which case, since a force oriented in a direction perpendicular to a longer side, that is, in the direction Y acts on the wire W, the wire W is able to stably keep its position. 
     In contrast, when the wire W makes contact with a curved surface of a circular cross-section or an elongated circular cross-section, since a force oriented perpendicular to the curved surface, the wire W can freely move along the curved surface. In particular, when a torsion force acts on the wire W, the wire W freely rotates along a curved surface, and hence, the wire W is not prevented from being distorted. Thus, a direction in which a torsion force acts on the wire W is not fixed, and hence, a position of the wire W is not fixed. 
     As mentioned above, the plasma spraying apparatus  1  in accordance with the present embodiment makes it possible to stably supply the wire W at its tip end to a center of the plasma flame F. The first gas sprayed through the first gas path  11  is turned into the plasma flame F by both the wire W indirectly electrically connected to an anode of the battery  4  through the first wire guide  51 , and the cathode  40  electrically connected to a cathode of the battery  4 . The plasma flame F melts the wire W into droplets D, and sprays the droplets D. The droplets D are reduced in size and further accelerated by the second gas sprayed through the second gas path  21  and leaving the second gas nozzle  20 , and sprayed onto the target T to thereby form the sprayed coating S. 
     In the plasma spraying apparatus  1  in accordance with the present embodiment, a third gas flow sprayed through the third gas path  31  defined between the first gas path  11  and the second gas path  21  absorbs heat from the plasma flame F to thereby generate a high-temperature gas jet G. The high-temperature gas jet G drastically restricts the second gas sprayed outside of the gas jet G to thereby weaken turbulence generated externally of the plasma flame F, resulting in that a gas of the plasma flame F is prevented from dispersing, and surfaces of the droplets D are reduced in being oxidized. Thus, it is possible to form the sprayed coating S which is difficult to be oxidized, onto the target T. 
     In the case that the third gas comprises an inert gas such as a nitrogen gas or an argon gas, as mentioned above, the third gas drastically restricts the second gas to thereby avoid turbulence generated externally of the plasma flame F, and further generates a high-temperature inert gas jet which absorbed heat from the plasma flame F, externally of the plasma flame F. Thus, particles comprising the droplet D are reduced in size with components of the particles being prevented from varying by virtue of the high-temperature inert gas jet, and further, accelerated, resulting in that the particles are protected from being oxidized by the second gas. Thus, it is possible to form the sprayed coating S which is further difficult to be oxidized. 
     Though both the first wire path  51   a  and the second wire path  52   a  in the present embodiment are designed to have a substantially rectangular cross-section extending in a direction in which the plasma flame F extends, one of them may be designed to have such a cross-section, in which case, the original deformation of the wire W can be released in a direction in which the plasma flame F extends, by means of the first wire path  51   a  or the second wire path  52   a  having a substantially rectangular cross-section extending in a direction in which the plasma flame F extends, to thereby supply a tip end of the wire W to a center of the plasma flame F. 
     INDUSTRIAL APPLICABILITY 
     The plasma spraying apparatus in accordance with the present invention is useful for forming an anti-corrosive sprayed coating on a surface of a steel structure. 
     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.