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ISO 14917-2017 pdf free download
ISO 14917-2017 pdf free download.Thermal spraying Terminology classification
Projection thermique — Terminologie, classification.
5.6.3 Plasma transferred wire arc spraying
The plasma transferred wire arc (PTWA) process presents another special plasma spraying process. The function is shown schematically in Figure 13.
The nozzle unit contains a tungsten cathode, an air cooled pilot nozzle of copper and an electrically conductive wire type spray material, which is fed perpendicular to the pilot nozzle. The plasma gas, usually a mixture of argon and hydrogen, is fed through drilled holes which are located tangentially to the circumference of the cathode fixture (not shown in the figure). The process is started by a high voltage discharge, which ionizes the plasma gas between the pilot nozzle and the cathode. The created plasma streams with hyper sonic speed through the nozzle’s muzzle and expands along the axis of the nozzle. By that way, the plasma jet runs through the nozzle, where it heats the spray wire, which is continuously fed perpendicular to the nozzle, where the electric circuit is closed. The tip of the wire will be melted due to the high temperatures of the plasma. The melted material is atomized bya pressurized air jet and projected onto the substrate. Usually, a coating produced by this process contains a porosity of less than 2 %.
5.7 Laser cladding
Laser cladding can be carried out by the one step method or the two step method. The main difference is given by the different melting location of the powder. In the case of the two step method, the powder is melted in the laser beam. In the case of the one step method, the melting of the powder takes place in the melt pool. The two step method is more sensitive and the coating thickness is typically 0,3 mm to 0,8 mm.
In the case of the one step method, the powder starts melting in the laser spot and reduces the heat available. This effect lowers the heat input to the substrate and the dilution.
Different types of laser can be used; however, diode laser is mainly used due to transportation of the beam via a glass fibre cable. Thus, working can be nearly independent from the location of the power source and laser head.
The process of laser cladding is characterized by the injection of a powder into the laser beam using a suitable powder nozzle. The powder particles are projected to a melt pool on the substrate by the carrier gas and/or by gravity. They are partially or entirely melted by laser radiation. The result is a fully dense, metallurgically bonded coating, usually 0,5 mm to 3 mm thick. Due to a relatively low heat input into the substrate material and a controlled motion of the laser spot, the substrate will be melted at the surface only. The dilution between the spray material and substrate material will typically be less than 5 %.
Usually, a shielding gas serves for the protection of the melt pool.
In special cases, metal matrix composites with carbides can be fed simultaneously to the melt pool generated by the laser beam, The aim is to keep the carbides intact with minimal dissolution and to melt the matrix material only. If the difference between the melting temperature of the matrix and the carbides is high enough, the aim can be achieved due to the low heat input of laser cladding.
Laser cladding is defined as a (hard) surfacing process and therefore, it typically belongs to the area of welding processes. Nevertheless, due to its application, in terms of this specification, laser cladding shall be included within the area of thermal spraying.
For details, see Figure 14.ISO 14917-2017 pdf free download.
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