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Volume 07 - Powder Metal Technologies and Applications Part 7

Tham khảo tài liệu 'volume 07 - powder metal technologies and applications part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 4 Major thermal spray processes Combustion spray bums mixtures of fuels such as acetylene and oxidizing agents air or oxygen that are used to subsequently heat and accelerate particles of material injected into the hot expanding gas jets. Combustion spraying has during the last 15 years been subdivided into two distinct categories a Flame spray which uses low pressure 70 to 140 kPa or 10 to 20 psi fuel oxygen mixtures burning unrestricted and external to the spray gun itself in ambient air and b HVOF spraying which utilizes internal combustion at higher gas flow rates and pressures up to 1.5 MPa or several hundred psi generating supersonic jet exit velocities. The powder particles are also injected internally in HVOF such that the heat and momentum transfer to the particles and hence particle velocity are substantially higher 1000 m s than in conventional flame spray. Wire arc spray utilizes two electrically conducting wires as feedstock rather than powders. A continuous direct current de arc melts the tips of the advancing wires and a high flow typically 1500 slm gas jet atomizes the molten material and accelerates the resulting small droplets toward the substrate being coated. A typical wire arc spray gun consists of a nozzle that directs a cool high-velocity air nitrogen or in special cases argon gas jet at the wire tips and a feedback-controlled wire-feed mechanism that continuously positions and advances the wires. Melt rate and interelectrode voltage determine the arc gap and size distribution of the atomized droplets leaving the tips of the wires and hence affect the final coating microstructure. Because cool gas jets are used to atomize the molten tips of feed wires in wire arc spray the components being coated are not heated as much as in other thermal spray processes an advantage when thermally sensitive substrate materials are being coated. Even thin sheets of paper can be coated by the wire arc process. Wire arc spraying is however limited to .