WO1998013143A1

WO1998013143A1 - A spray nozzle and system for coating piston rings

Info

Publication number: WO1998013143A1
Application number: PCT/BR1996/000043
Authority: WO
Inventors: José Roberto Neto ZALLI; Marcos Rolando Piccilli
Original assignee: Cofap - Companhia Fabricadora De Peças
Priority date: 1996-09-23
Filing date: 1996-09-23
Publication date: 1998-04-02

Abstract

A spray nozzle and system for coating piston rings by producing droplets of molybdenum and stainless steel from these materials in wire form which are sprayed, preferably in equal amounts to coat the outer surfaces of the rings. A plurality of rings are mounted on a body which are moved in a helical path during the spraying. A nozzle is used having a central passage (11) through which the wire is fed and two sets of passages (12, 13) one set for an oxyacetylene mixture and the other for compressed air, with the outlet orifices of the two sets of passages (12, 13) arranged in concentric rings around the central passage (11) outlet orifice.

Description

A SPRAY NOZZLE AND SYSTEM FOR COATING PISTON RINGS

Technical Field

The present invention relates to a thermal spraying lining process for piston rings, which uses different materials, presents an operational low cost and maintains stable, average lining properties . It also relates to a nozzle for effecting the spraying. Background Art A known thermal spraying lining process for piston rings employs 99.9% pure molybdenum feeding wires, which are melted by respective torches of oxygen and acetylene. In this process, droplets of melted molybdenum are sprayed against the external contact faces of piston rings, said faces presenting a relatively low temperature as compared to that of the droplets upon reaching said external contact face, compressed air being used as a propellant. The adhesion between the droplets and the external contact face of the piston rings is mechanically achieved, by the solidification of said droplets onto a somewhat roughened surface of the external contact face of the rings, due to the high contraction tension during solidification . Besides the high cost of this process, which uses only molybdenum, the resultant lining layer proves to be fragile, due to the fact that the welding of successive lining layers occurs through oxide-rich interfaces. Such fragility makes the lining susceptible to cracks and chips, when the piston ring is submitted to mechanical stresses. The integrity of the lining with molybdenum is affected by the operational high temperatures of said piston ring close to the combustion chamber, mainly due to the substantial difference between the thermal expansion coefficients of the molybdenum and the basic metal of iron alloy, giving rise to said cracks when under stress. To overcome such above-mentioned mechanical shortcomings, molybdenum has been associated with a second softer metal, preferably stainless steel.

In the state of the art, a process for coating an object with equal mass or volume amounts of two metals having substantially different melting points is only possible through the spraying of a blend of metal or metal powders, by means of an equipment adequate to spray powdered metals, at plasma temperatures. Plasma spraying of powders is usually effected under inert atmospheres, whereby to avoid the oxidation of molybdenum. Furthermore, it has been observed that a plasma arc spray process s much more costly than another one simply using stainless steel and molybdenum wires .

West German Patent no. 39 379, granted 08.05.66 discloses a process and a device for thermal spray coating a supply consisting of a molybdenum wire and a stainless steel wire molten in a single oxyacetylene torch, being the melt sprayed by a single spray nozzle. Considering the substantial difference between the melting points of molybdenum (2.600^*C) and that of stainless steel (1.480'C), the process being effected at the highest temperature, a very high consumption of stainless steel occurrs, resulting in high stainless steel assay alloys, i.e., not very different from the 5:1 in relation to molybdenum, which the document cites as an example. Furthermore, this process produces a very low quality coating: due to the differences between the densities of the droplets of the two metals, and the consequent difference of their output speeds from the spray nozzle, a very irregular surface is formed, due to the non-homogeneous distribution of the metals. In view of the evident cost advantage potentially attainable, several attempts have been made to thermal spray piston rings with molybdenum and stainless steel, using conventional spray nozzles and parting from wire supplies of said metals, molten by means of oxyacetylene torches .

These attempts confirmed the execellent processability of molybdenum wires. However, it was not possible to obtain a good quality 50% thereof/50% stainless steel coating, notwithstanding all the attempts made to adequate process conditions for stainless steel wires . Since the temperature provided by burning oxyacetylene is substantially invariable, between 2.900 and 3.000oC, the essays were directed to varying the conditions of supply of the stainless steel wire.

The first attempt consisted in reducing the speed of supply of the stainless steel wire, in order to reduce the amount supplied thereof, consequently approximating the amount of said supply to that of molybdenum. Thereby, an accumulation of molten stainless steel occurred at the output of the spray nozzle, clogging it. A further reduction of said speed provoked the destruction of the spray nozzle, which melted due to excessive accumulation of heat adjacent to the flame output zone. ^

As a second alternative, both the spray nozzle and the stainless steel wire were redimensioned downwards: the droplets thereby obtained were so much smaller that they never attained sufficient kinetic energy to adequately anchor onto the substrate, upon collision therewith. Furthermore, the coating thereby obtained was rough and excessively rich in molybdenum and, hence, too hard and subject to cracking, when submitted to efforts . Moreover, the spray coating process is not very versatile, in relation to the combination of various materials .

Objects of the Invention Hence, it is an object of the present invention to provide a spray nozzle for spraying droplets of a metal or metal alloy having an oxyacetylene flame- low melting point, molten from a wire supply of said metal or metal alloy by means of an oxyacetylene torch, which permits obtaining and uniformly spraying a flow, adjustable within a wide range of concentrations, of droplets of said metal or metal alloy.

It is also an object of the present invention to provide a system for metal coating piston rings through thermal spraying of droplets molten from a supply of stainless steel wire, through a spray nozzle as defined above, and from a metal wire supply of a hard metal selected from molybdenum and niobium, through a conventional spray nozzle, whereby to obtain a homogeneous metal coating having any desired assay of stainless steel.

Brief Description of the Invention

These and other objects and advantages of the present invention are attained through the provision of a spray nozzle, to produce and spray drojplets of a metal or metal alloy having an oxyacetylene flame-low melting point, from a wire supply of said metal or metal alloy molten through an oxyacetylene torch, said spray nozzle comprising a body provided with: an output end; a central axial wire supply passage; a compressed air passage assembly, having respective output ends forming a homogeneous oxyacetylene torch around the extension of wire that leaves the output end of the body. According to the invention, the output ends of the compressed air passages are disposed radially internally to the output ends of the oxyacetylene fuel passages, whereby to form, between said oxyacetylene torch and said metal or metal alloy, a thermal barrier retardant of the melting of the latter.

In a second aspect, the invention comprises a system for metal coating piston rings, through spraying droplets molten from supplies of wires of stainless steel and of a hard metal selected from molybdenum and niobium through, respectively, a spray nozzle of the invention and a conventional spray nozzle, onto at least one piston ring mounted on a carrier displacing said ring helically, whereby to deposit equal amounts of stainless steel and hard metal onto the peripheral surface thereof. Brief Description of Drawings The invention will be now described, according to the attached drawings, in which:

Fig. 1 shows a partial longitudinal section view of a piston operating inside a cylinder and provided with piston rings, the pressure of the gases being indicated by the arrow; and an enlarged detail of one of said piston rings, enhancing the lining of its external contact face;

Fig. 2 illustrates, schematically, the piston rings being sprayed by means of feeding wires, according to the present invention; ^

Fig. 3 shows an enlarged cross sectional view of part of a piston ring provided with lining on its external contact face; and Figs. 4A-4B are cross-sectional views offset by 45o, taken, respectively, lines I-I and II-II in Figure 5;

Fig.5 represents a plan view of a spray nozzle of the invention, used for forming and spraying stainless steel droplets; and Fig.6 is a view aimilar to that of Fig.5, however illustrating a coventionally built spray nozzle, for forming and spraying droplets of molybdenum. Fig. 5 represents a plan view of a spray nozzle according to the invention, used for forming and sraying

Detailed Description of the Invention According to the present invention and referring to Figs. 1-3, there are provided piston rings 1, to be mounted on a reciprocating piston P and which operate, during combustion, close to the internal wall of a cylinder C. Each piston ring 1 presents a somewhat roughened external contact face la, as shown in Fig. 3, which is to be provided with an anti-adhesive and anti- abrasion lining. The lining is to protect the piston rings 1 when they are submitted to high pressure and temperature during the combustion cycle. In one embodiment of the invention, the external contact face la of each piston ring 1 is initially produced by machining the piston ring 1, in order to make a superficial annular groove at the circular mid portion of said piston ring 1. The groove is to be later filled with the lining, as described below, until the diameter of the external contact face la of the piston ring 1 is built back up to the desired outer diameter dimension. In another way of carrying out the invention, the lining layer is externally provided on the external contact face la, thus increasing the diameter of the part at the region where the lining is applied, until there is obtained a lining layer with an annular width sufficient to have, after final machining, a lining thickness from 2 to 7% of the basic annular width of the ring.

As illustrated in Figure 2, a basic structure 2, such as a mandrel, carries and anchors, along its length, by using retaining means such as bolts and nuts or other suitable fasteners, a plurality of piston rings 1. The rings 1 are restrained from mutual relative displacements, and are arranged so as to define a cylindrical surface which has a rotational motion around a rotation shaft, which passes through the center of the ring set and which is coincident with the geometrical axis of the basic structure 2, and a translational motion, towards said geometrical axis. The rotational and translational motion is produced by any suitable mechanical arrangement.

At a certain distance from said basic structure 2, two gas nozzles 3 are provided, which are simultaneously fed through their respective inlets lb, with oxygen and acetylene, so as to form a melting flame capable of melting the chemical elements used in the present spraying process. Each gas nozzle 3 is capable of flame regulation, which is achieved by varying the proportions of oxygen and acetylene in the flame.

Besides the gases which are fed to the nozzles 3, in order to form the flame described above, each nozzle 3 receives, through third and fourth inlets c and d, respectively, chemical elements to be melted and used in the lining of said piston rings 1. The nozzles also receive a volume of compressed air which carries, through an outlet "s" of each said nozzle 3, the droplets of each chemical element as melted by the flame towards the external contact face of the cylindrical surface of the basic structure 2 as formed by the outer surfaces of the rings 1.

In a preferred way of carrying out the invention, the lining is obtained through the simultaneous thermal spraying of two chemical elements, which are different but have the same concentration, and which are presented in the form of wires from feeding rolls 4, each wire respectively passing through a nozzle 3. In the preferred embodiment, one of the chemical elements used in the lining is a hard metal selected from the group consisting of molybdenum and niobium, which is responsible for the anti-abrasive and anti- adhesive characteristics of the lining, and which is employed in a condition of 99.9% purity.

Niobium, mentioned above, was tested in parallel with and under similar conditions with molybdenum, presenting a very similar performance. However, due to its very high cost for commercial use, from this point on the invention will only be described in relation to molybdenum .

The other of said materials is a metal or metal alloy having an oxyacetylene flame- low melting point, preferably stainless steel. This element, which presents a lower hardness in relation to molybdenum, is responsible for the metallurgical bonding of molybdenum to the external contact face la of the piston rings 1, and for the cohesion of several lining layers applied over anterior layers.

In the description above and throughout the following disclosure and claims, the expression "metal or metal alloy having an oxyacetylene flame-low melting point" should be understood as a metal or metal alloy having a melting point substantially lower than that, from 2900 to 3000 'C, provided by oxyacetylene flame. For each feeding wire that is used, the oxyacetylene flame has a specific regulation, obtained through an adequate proportion of oxygen a d acetylene defined according to the melting point of each element for the composition of the feeding wires.

The stainless steel, which has a lower melting point, makes possible a better metallurgical bonding, increasing the strength against cracks in the lining under mechanical stresses, improving the adhesion thereof to the basic metal of the piston ring 1, as well as the cohesion of the sprayed droplets. The adhesion of the lining to the basic metal of the rings 1 is mainly metallurgical. In this adhesion, the molybdenum droplets, when sprayed onto the external contact face of the piston rings, reach this surface, which is at a lower temperature, thus being solidified in an anchored condition to the roughened surface la of the basic metal due to its solidification. According to the present invention, the lining is obtained by employing an alloy, in this case being a chemical metallurgical alloy . of molybdenum and stainless steel, in order to obtain a better adhesion of molybdenum to the external contact face la of the piston rings, thus guaranteeing its resistance against cracks and breakages during the high working temperatures to which the piston rings 1 are subjected close to the combustion chamber. The spraying takes place without interruption during a determined number of cycles of motions of the basic structure 2, until the external contact face la of each piston ring 1 of said basic structure 2 presents a homogeneous and continuous lining layer of the molybdenum/stainless steel mixture with an annular thickness, as previously described.

In the present solution, the lining is obtained by providing the simultaneous spraying of molybdenum and stainless steel, taking into account the points that are longitudinally displaced throughout the extension of the basic structure 2, so as^. to define, on the cylindrical surface of said basic structure 2, helical paths caused by the rotational and translational motions of said basic structure 2 in relation to gas nozzles Bl, B2 and, consequently, to the mass of droplets that are melted by spraying.

The stainless steel improves the adhesion of molybdenum to the basic metal of the external contact face la of each piston ring 1, and also allows the bonding of the molybdenum droplets to each other. Each feeding wire roll 4 has its own feeding speed for its respective nozzle Bl, B2, defined according to the concentration required for the mixture of each chemical element used in the lining. Details of a preferred form of nozzle are given below.

Moreover, the rotational speed of the basic structure 2 keeps a proportionality with the rotational speed of the feeding wire rolls 4,5 feeding gas nozzles Bl, B2, which is defined according to the number of layers and annular width of each lining layer to be metallurgically adhered to the external contact face la of each piston ring 1. The modulus of said rotational speed of the basic structure 2, in relation to the modulus of the rotational speed of each feeding wire roll 4,5 can be, for example, null. After finishing the spraying of the molybdenum stainless steel alloy onto the cylindrical surface, each piston ring 1 is submitted to machining at its external contact face la, in order to finish its working profile. Figs. 4A-4B and 5 show the details of a nozzle Bl to be used to produce the flame to transform the stainless steel wire into the droplets to produce an effective spraying pattern. Spray nozzle B2 adequate for forming and spraying droplets of molybdenum is a conventional spray nozzle, thus not having to be detailed. The constructive difference between_ Tf this and stainless steel nozzle Bl will be described further on. Referring to Figs. 4A-4B, nozzle Bl has an elongated body 10 with a reduced diameter end 14. A through axial passage 11 extends the length of the nozzle body and the stainless steel wire from the supply roll 4 is fed through the passage 11. The feeding direction is from the top as shown in Figs. 4A and 4B. Threads or ridges 15 are provided at the nozzle reduced diameter section 14 and below at 16 on the main body section 10 to connect the nozzle Bl to the supply means (not shown) for the air, oxygen and acetylene gas supply and the wire.

Through axial passage 11 presents end inlet portions 11a and an outlet portion lib open towards respective wire inlet 10a and outlet 10b ends of body 10, wherein the outlet end portion lib presents a larger diameter than that of the metal wire, to form therewith an annular space. In the embodiment illustrated, through axial passage 11 further presents, upstream from outlet end portion lib, a strangled portion lie, with a diameter substantially equal to that of the stainless steel wire.

As seen in Fig. 5, nozzle Bl has a plurality of passages 13 arranged in a circular ring for the flow of the oxyacetylene gas, the discharge orifices 13b of which are shown in Fig. 4B. The inlet to passages 13 is at the transition between the reduced diameter section 14 and the main body 10. Four such passages 13 are shown, with a spacing of approximately 90o between each. Of course, it should be understood that three or more passages can be used, if desired, but four and equally circumferentially spaced, their discharge orifices 13b open towards outlet end 10b of body 10, are preferred. There are also a plurality of c mpressed air supply passages 12 arranged concentrically around the nozzle central opening 11 and which are internal to oxyacetylene gas passages 13. The air passages 12 have discharge orifices 12, as shown in Fig. 4A, which are oriented around the central passage 11 and inward of the oxyacetylene discharge orifices 13b. As shown in Fig. 5, there are four air passages 12 spaced apart by approximately 90" which are offset by about 45" relative to the fuel gas passages 13. The number of air passages is preferably made equal to the number of fuel-gas passages. That is, as seen in Fig. 5, there is alternately a fuel gas passage and an air passage. The discharge of the compressed air from an orifice 12b between two of the fuel gas orifices 13b provides for efficient mixing of the air and fuel and the spray of the droplets of metal which are produced.

The compressed air discharge orifices 12 are open inwards to the outlet end portion lib of central opening 11, according to axes inclined lOo to 15o relative to the axis of central opening 11, producing in the annular space around the wire, a homogeneous flow of air axially directed outwards from outlet end 10b of body 10. Such a disposition of compressed air passages 10, internal to oxyacetylene gas passages 13, constitutes the main feature of the present invention, due to providing a compressed air "cover" , i.e., a thermal insulating barrier between the oxyacetylene torch and the stainless steel wire, delaying the melting of the stainless steel wire, so as to give time for the melting of the molybdenum wire, having a melting point substantially higher that of stainless steel. Through this artifice, and using adequate settings for spray nozzle Bl , it becomes possible to produce and spray, controlledly , any amount of droplets of stainless steel. Hence, the coating of piston rings with equal amounts of stainless ^ eel and molybdenum, parting from corresponding wire supplies, becomes feasible .

On conventional spray nozzle B2, for melting and spraying molybdenum, according to Fig.6, the oxyacetylene gas passages 13 are internal to the compressed air passages 12, whereby to permit the flame of the oxyacetylene torch to act directly on the molybdenum wire, thus speeding up its melting. In this nozzle, compressed air acts only as a propellant for the droplets of molybdenum. In operation, the pressure for the acetylene component of the fuel for nozzles for both molybdenum and the stainless steel is equal . It is preferred that the flow ratio for a nozzle using the stainless steel wire is between 70% and 80% of the flow for a nozzle used to form and spray molybdenum droplets, the latter varying between 990 and 1190 cu.m/hour. Thus, acetylene flow for a nozzle for stainless steel is from 693 to 952 cu .m/hour . In the operation of the nozzle for the molybdenum, oxygen is preferably applied on the basis of about 3.0 to 3.5 times the pressure of the acetylene. It is preferred that the flow ratio of the oxygen be between 90 and 105% of that of the acetylene. For stainless steel, the pressure is preferably 1.4 to 1.8 times that of oxygen for the molybdenum nozzle at a flow rate between 60 and 73% of that of the oxygen for the molybdenum nozzle. The pressures of the compressed air for both types of nozzles are preferably substantially equal. The flow ratio of compressed air for stainless steel should be about 90% to 100% of that for molybdenum, with the latter being in the range of from 70 to 80 cu.m/hr. The distance between the nozzles and the surfaces to be coated is preferably such that ^.the nozzle spraying stainless steel is between 70 and 80% of that of the nozzle spraying molybdenum.

The stainless steel and molybdenum wires are fed at the same rate and volume of material . The relationship between the exit area of the four fuel gas (oxygen and acetylene) mixture orifices 13b and the four cooling and compressed air spray orifices 12b is preferably between 0.95 and 1.45. Utilizing the described dimensions and parameters the compressed air orifices 12b permits melting of the stainless steel wire, a condition similar to that for molybdenum since an air channel is formed between the flame and the wire. This delays melting and considerably reduces the flame temperatures, thus permitting spraying of stainless steel droplets at the same rate as that of molybdenum droplets .

Using the above parameters, a nozzle produces droplets and sprays stainless steel (melting temperature of 1480 ^*C) at substantially the same rate and volume as molybdenum (melting temperature of 2600^*C). Of course, the materials for the nozzles themselves are such as to be able to withstand the temperatures produced. In the prior art, a process for coating an article with the same mass or volume proportions was only possible via a spray of a metallic powder mixture or of previously prepared alloys in powder form in equipment suitable for spraying materials in powder form.

Claims

1. A nozzle for forming droplets of molten metal from the metal in wire form and spraying the droplets of molten metal comprising: a nozzle body having an outlet end and a central passage through said bodv having an outlet orifice at said outlet end through which the wire is fed; a first set of passages in said nozzle body through which compressed air is fed and a second set of passages in said nozzle body through which an oxygen- acetylene fuel is fed, each passage having an outlet orifice at the nozzle body outlet end, the outlet orifices of said first and second sets of passages each arranged in a respective ring concentric with the central passage outlet orifice, the outlet orifices of the first set of passages for feeding compressed air being closer to the central passage outlet orifice than the outlet orifices of the second set of passages and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of said central passage and the wire passing therethrough.

2. A nozzle as in claim 1, wherein the outlet orifices of the first and second sets of passages alternate. ^

3. A nozzle as in claim 2, wherein the outlet orifices of the first and second sets of passages alternate .

4. A nozzle as in claim 1, wherein the outlet orifices of said first set of passages are in said central passage outlet orifice to discharge the compressed air into said central passage outlet orifice and directly onto the wire in the central passage outlet orifice. 5. A system for flame spraying an article with molten droplets formed from each of a wire of stainless steel and a wire molybdenum comprising: a supply of each of stainless steel wire and molybdenum wire; a separate nozzle for each of said wires, and means for supplying compressed air and oxyacetylene fuel to each of said nozzles in an amount and at a pressure to produce and spray the article being coated with substantially equal volumes of molten stainless steel and molten molybdenum droplets .

6. A system as in claim 5, wherein each said nozzle comprises a nozzle body having an outlet end and a central passage through said body having an outlet orifice at said outlet end through which the wire is fed; a first set of passages in said body through which compressed air is fed and a second set of passages in said body through which an oxygen-acetylene fuel is fed, each passage having an outlet orifice at the nozzle body outlet end, the outlet orifices of said first and second sets of passages each arranged in a respective ring concentric with the central passage outlet orifice, the outlet orifices of the first set of passages for feeding compressed air being closer to the central passage outlet orifice than the outlet orifices of the second set of passages and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of sa central passage and the wire passing therethrough.

7. A system as in claim 6, wherein said article being sprayed is a piston ring, and further comprising: a carrier on which at least one such piston ring is mounted, each said nozzle having its outlet end mounted to spray molten droplets of the metal toward the outer surface of said carrier, means for rotating and moving said carrier in a translational path to produce a generally helical motion while the article is being sprayed.

8. A system as in claim 7, wherein each said nozzle comprises a nozzle body having an outlet end and a central passage through said bodv having an outlet orifice at said outlet end through which the wire is fed, a first set of passages in said body through which compressed air is fed and a second set of passages in said body through which an oxygen-acetylene fuel is fed, each passage having an outlet orifice at the nozzle body out let end, the outlet orifices of said first and second sets of passages each arranged in a respective ring concentric with the central passage outlet orifice, the outlet orifices of the first set of passages for feeding compressed air being closer to the central passage outlet orifice than the outlet orifices of the second set of passages, and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of said central passage and the wire passing therethrough.

9. A system as in claim 5, wherein the means for supplying compressed air and oxyacetlylene fuel comprises : a. acetylene at an equal pressure for each of the nozzles for molybdenum and stainless steel wires and at a flow ratio for the nozzle for stainless steel wire which is 70%-80% of that for the gjozzle for molybdenum wire; b. oxygen at a pressure 3.0 to 3.5 times that of acetylene and a flow rate 90-105% that of acetylene for the nozzle for molybdenum wire, with the nozzle for stainless steel wire being supplied oxygen at a pressure 1.4 to 1.8 times that of the pressure for the nozzle for molybdenum wire and at a flow ratio of between 60% and 73% of the oxygen for the nozzle for molybdenum wire; c. compressed air at equal pressure for each of the nozzles for molybdenum and stainless steel wires and at a flow ratio for the nozzle for stainless steel wire which is in the range of 90%-100% of that for the nozzle for molybdenum wire.

11. A nozzle as in claim 6, wherein the outlet orifices of said first set of passages are in said central passage outlet orifice to discharge the compressed air into said central passage outlet orifice and directly onto the wire in the central passage outlet orifice.

AMENDED CLAIMS

[received by the International Bureau on 05 December 1997 (05.12.97) original claims 1-11 replaced by amended claims 1-8 (4 pages)]

1. A nozzle for forming droplets of molten metal from the metal in wire form and spraying the droplets of molten metal comprising: a nozzle body having an outlet end and a central passage through said body having an outlet orifice at said outlet end through which the wire is fed; a first set of passages in said nozzle body through which compressed air is fed and a second set of passages in said nozzle body through which an oxygen-acetylene fuel is fed, each passage having an outlet orifice at the nozzle body outlet end, the outlet orifices of said first and second sets of passages each arranged in a respective ring [concentric with] pattern around the central passage outlet orifice, characterized in that the outlet orifices of the first set of passages for feeding compressed air being closer to the central passage outlet orifice than the outlet orifices of the second set of passages and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of said central passage and the wire passing therethrough.

2. A nozzle as in claim 1, characterized in that the outlet orifices of the first and second sets of passages alternate. v-

3. A nozzle as in claim 2, characterized in that the outlet orifices of the first and second sets of passages alternate.

4. A nozzle as in claim 1, characterized in that the outlet orifices of said first set of passages are in said central passage outlet orifice to discharge the compressed air into said central passage outlet orifice and directly onto the wire in the central passage outlet orifice. 5. A system for flame spraying an article with molten droplets formed from each of a wire of stainless steel and a wire of molybdenum comprising: a supply of each of stainless steel wire and molybdenum wire; a [separate! first nozzle -for [each of said wires, 1 the stainless steel wire and a second nozzle for the molybdenum wire;fandl means for supplying compressed air and oxygen-acetylene fuel to each of said nozzles in fan] respective amounts. and at \ a , respective pressures to produce and spray the article being coated with [substantially equal volumes of] molten stainless steel and molten molybdenum droplets^.[.

6. A system as in claim 5, wherein] each [said! such nozzle compris fes 1 ing a nozzle body having an outlet end and a central passage through said body having an outlet orifice at said outlet end through which the wire is fed; a first set of passages in said body through which compressed air is fed and a second set of passages in said body through which an oxygen-acetylene fuel is fed, each passage having an outlet orifice at the nozzle body outlet end, the outlet orifices of said first and second sets of passages each arranged in a respective ring [concentric with] pattern around the central passage outlet orifice, characterized in that the outlet orifices of the first set of passages for feeding compressed air of said first nozzle rbeingl are closer to the central passage outlet orifice thereof

Tr than the outlet orifices of the second set of passages and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of said central passage and the stainless steel wire passing therethroughr .1 , thus delaying the melting of said stainless steel, and providing the melting and spraying of substantially equal volumes of molten stainless steel and molten molybdenum. [7] 6.. A system as in claim 5, characterized in that said article being sprayed is a piston ring, and further comprising: a carrier on which at least one such piston ring is mounted, [each] said first and second nozzles_, having [its] their outlet ends, mounted to spray molten droplets of the metals, toward the outer surface of said carrier, means for rotating and moving said carrier in a translational path to produce a generally helical motion while the article is being sprayed .

[[8] 7. A system as in claim 6, wherein each said nozzle comprises a nozzle body having an outlet end and a central passage through said body having an outlet orifice at said outlet end through which the wire is fed, a first set of passages in said body through which compressed air is fed and a second set of passages in said body through which an oxygen-acetylene fuel is fed, each passage having an outlet orifice at the nozzle body outlet end, the outlet orifices of said first and second sets of passages each arranged in a respective ring concentric with the central passage outlet orifice, the outlet orifices of the first set of passages for feeding compressed air being closer to the central passage outlet orifice than the outlet orifices of the second set of passages, and the outlet orifices of said first set of passages directing compressed air toward said outlet orifice of said central passage and the stainless steel wire passing therethrough.]

[9] - A system as in claim 5, characterized in that the means for supplying compressed air and oxygen- acetylene fuel comprises: a . acetylene at an equal pressure for each of the nozzles for molybdenum and stainless steel wires and at a flow ratio for the nozzle for stainless steel wire which is 70%-80% of that for the nozzle for molybdenum wire; b. oxygen at a pressure 3.0 to 3.5 times that of acetylene and a flow rate 90-105% that of acetylene for the nozzle for molybdenum wire, with the nozzle for stainless steel wire being supplied oxygen at a pressure 1.4 to 1.8 times that of the pressure for the nozzle for molybdenum • wire and at a flow ratio of between 60% and 73% of the oxygen for the nozzle for molybdenum wire; c . compressed air at equal pressure for each of the nozzles for molybdenum and stainless steel wires and at a flow ratio for the nozzle for stainless steel wire which is in the range of 90%-100% of that for the nozzle for molybdenum wire.

[11] .8. A nozzle as in claim 5, characterized in that the outlet orifices of said first set of passages are in said central passage outlet orifice to discharge the compressed air into said central passage outlet orifice and directly onto the wire in the central passage outlet orifice.