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Thermal Spraying.Pdf Mission: To strengthen the level of awareness in general industry and government on the increasing capabilities and advantages of thermal spray technology for surface engineering through business opportunities, technical support, and a social network that contributes to the growth and education of the thermal spray industry. What is Thermal Spray ? thermal spraying, a group of coating processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition to form a coating. The coating material may be in the form of powder, ceramic-rod, wire, or molten materials. (1) Thermal Spray Coating Process Graphic courtesy of Westaim Ambeon Introduction . .2 Methods . .2 Molton Metal FlameSpray . .2 Powder Flame Spraying . .3 Wire Flame Spraying . .3 Ceramic Rod Flame Spraying . .3 Detonation Flame Spraying . .4 High Velocity Oxy/Fuel Spraying (HVOF) . .4 Cold Spray . .5 Nontransferred Plasma Arc Spraying . .6 Electric Arc Spraying . .7 RF Plasma Spraying . .7 References . .7 Acknowledgements . .8 International Thermal Spray Association Information . .8 Thermal Spray Processes used by Various Industrial Segments Chart . .9 Thermal Spray Coating Applications According to Industry Served Chart . .10 Industrial Use of Gas Metallic Materials Chart . .11 International Thermal Spray Association 208 Third Street • Fairport Harbor, Ohio 44077 USA voice: 440.357.5400 • fax: 440.357.5430 [email protected] • www.thermalspray.org THERMAL SPRAYING: electrical energy is used to create a heat source into which What it Was and What it Has Become powder, and more recently wires, are fed, melted/plasticized by Frank J. Hermanek and conveyed onto the surface to be coated. Major, On the eve of celebrating the one hundred anniversary of commercially employed electrical methods, used to construct its discovery, thermal spraying looks back to its roots - early coatings include non-transferred arc plasma, RF plasma, and experiments in which liquids were broken up into fine wire arc. Based upon the two (2) heat sources a "family tree" particles by a stream of high-pressure gas. Efforts more of thermal spray methods can be constructed, Figure 1. directed at producing powders rather than constructing coatings. It fell to one Dr. Max Ulrick Schoop of Zurich who recognized the possibility that a stream of molten particles impinging upon themselves could create a coating. His work, THERMAL SPRAY METHODS and that of his collaborators, resulted in the establishment Molten Metal FlameSpray of the thermal spray process. This process has fostered a molten metal flame spraying, a thermal spraying process worldwide industry serving over thirty technology sectors variation in which the metallic material to be sprayed is in the and generating sales of over two billion dollars per year. molten condition(1) This article traces the history and development of the It has long been recognized that fluids may be broken up principal flame and electrical thermal spray processes. into very fine particles by a stream of high velocity gas thermal spraying, a group of coating processes in which emanating from a nozzle. Early experiments using this finely divided metallic or nonmetallic materials are deposited atomizing approach appear to have been directed at in a molten or semimolten condition to form a coating. The producing metallic powders rather than coatings. It was left coating material may be in the form of powder, ceramic-rod, to Schoop to appreciate the possibility that a stream of (1) wire, or molten materials. metallic particles, formed from a molten source, could produce In the early 1900s Dr. M. U. Schoop and his associates a coating. Myth has it that Schoop developed the concept developed equipment and techniques for producing coatings when playing "soldiers" with his son and observing the using molten and powder metals. Several years later, in deformation of lead pellets being fired from a toy cannon about 1912, their efforts produced the first instrument for against a brick wall. Whatever the rationale, it can be stated the spraying of solid metal in wire form. This simple device that the pioneer work of Schoop resulted in the discovery and was based on the principle that if a wire rod were fed into development of metal spraying and subsequently the "Thermal an intense, concentrated flame, (the burning of a fuel gas Spray Process". with oxygen), it would melt and, if the flame were The first spray technique developed by Schoop was the surrounded by a stream of compressed gas, the molten metal outcome of experiments in which molten metal was poured would become atomized and readily propelled onto a surface into a stream of high velocity gases. Schoop's apparatus to create a coating. This process was initially referred to as consisted of a compressor supplying air to a heated helical metallizing. Currently the technique is known as oxy-fuel or tube. The heated air was used to pressurize a crucible filled flame spraying. Other oxy-fuel methods include wire, powder with molten metal and eject it out as a fine spray that would (metallic and ceramic), molten metal, ceramic-rod, adhere to a suitable surface. This system was bulky, primitive detonation and high velocity oxy-fuel (HVOF). and inefficient; however, the concept did lead to the In addition to using chemical means to plasticize the input development of portable and user friendly equipment. consumables electrical currents are also used. Typically, There are no further accounts of molten metal spraying by Schoop, it appears that his efforts were directed at developing and improving Figure 1. Thermal spray powder and wire flame application methods by spraying. However, work by heat source schematic. others continued as a 1924 Dutch patent, describing equipment for spraying low melting point metals, was granted to Jung and Versteeg(2). Mellowes Ltd commercialized the process in the UK. Their system consisted of a gun, a furnace, an air compressor and a fuel supply. The gun had many air and gas valves, a heating chamber 2 (burner), nozzle, handle and a melting pot. The pot was criticality of flowmeters and pressure gauges are governing bulky having the ability to store 1.8 kg (4 lb) of molten factors. lead. The pot sat atop the heating chamber, which was similar in construction to a Bunsen burner. Compressed air, Wire Flame Spraying fed to the burner, intensified the flame. The handle jutted wire flame spraying, spray process in which the feed stock is out and downward from the pot; it was insulated using wood in wire or rod form.(1) and asbestos. Metal exited the pot through a front orifice where it was directed into a nozzle. Compressed air surrounded the nozzle, atomizing the molten metal and propelling it to the surface to be coated. The molten metal process has advantages and disadvantages. Advantages include: cheap raw materials; use of inexpensive gases; and, gun design is very basic. Noteworthy disadvantages are: gun is cumbersome to use in the manual mode, can only be held in a horizontal plane; high maintenance due to high temperature oxidation and molten metal corrosion; and, useful only with low melting temperature metals. Figure 3. Wire flame-spray. (Graphic from Flame Spray Handbook, Uses for the molten metal thermal spray process Vol. 1 Wire Process, “Wire Nozzle and Air Cap Cross Section - METCO include the fabrication of molds, masks and forms Metallizing Gun.”, page A-64, copyright 1969 located in International for the plastics industry, using low melting point Thermal Spray Association Historical Collection.) bismuth based alloys (the Cerro family of alloys); the deposition of solder alloys to joints that would be In about 1912 Schoop developed the first device for coalesced using torches or ovens; and, the production of spraying metal wires. The apparatus consisted of a nozzle in metal powders. which a fuel, probably acetylene or hydrogen, was mixed with oxygen and burned at the nozzle's face(3). A stream of compressed air surrounding the flame atomized and propelled the liquefied metal. Process continuation Powder Flame Spraying depended on feeding the wire at a controllable rate so it melted and was propelled in a continuous stream. Schoop powder flame spraying, a thermal spray process in which the approached this problem by using a turbine to actuated (1) material to be sprayed is in powder form. gears and drive rolls that pulled the wire into the nozzle. This apparatus appeared to him to be similar to a pistol or gun, and because of this, he and we, refer to thermal spray devices as "guns" or "pistols" and never "torches". A Flame Spray Process typical wire spray gun is shown in Figure 3. Schoop' concepts of Powder flame spraying is probably the simplest of all the spraying solid metals has given rise to the thermal spray spray processes to describe - feed a powder through the industry and for this reason it is sometimes referred to as center bore of a nozzle where it melts and is carried by the the "Schoop Process". Regardless, the wire flame spray gun escaping oxy-fuel gases to the work piece. Unfortunately, has not radically changed since the days of Schoop. While this approach yields coatings high in oxides and with void there have been changes in nozzle and air cap design, contents approaching 20 volume percent (v/o). However, replacement of the air turbine with an electrical motor and coating quality can be improved by feeding air to the nozzle even the use of barrel valves the basic principal, however, through a small jet, which reduces the pressure in a chamber remains the same "push or pull a wire into a flame, melt and behind the nozzle. This chamber is connected to the powder atomize it and deposit the molten droplets to form an feed hopper. In this way a gentle stream of gas is sucked adherent coating".
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