Abstract
Electrical explosion spraying has certain advantages in material surface spraying fields as a new, efficient and low cost surface modification method. Existing spraying material used in electrical explosion method are mostly wire or foil and the majority of spraying material exist in the form of power , so it's a certain advantage in powder electrical-thermal explosion spraying. Based on gas discharging in current mechanism,put forward a electrical explosion spraying method of continue feeding powder in constraining tube. Powder is sent to the conveyor belt by feeding powder device, and into the constraining tube with conveyor belt. When reached a certain position,current is imported in powder by gas charging and it occurs explosion. The products of explosion emit from the constraining tube on substrate forming coating.
This paper developed continuously feeding powder electrical explosion spraying device in constraining tube. This device included high voltage generating device,capacitor, electrode, feeding powder device, conveyor belt and constraining tube. The amount of powder could be effectively controlled by feeding powder device. Through conveyor belt, power could be continuously, uniformly, accurately sent to the place between two electrodes. Electrode which had a tip could import high current into powder by tip discharging, thus it made sure to breakdown the air gap between powder and electrode under low voltage and occur explosion. Under the action of constraint of constraining tube, products of explosion emit out with stable jet to achieve directional spraying.
This paper developed powder electrical explosion spraying device in constraining tube, put WC powder as spraying material to conduct tests. The change law of coating area in different process parameters was analyzed by observing surface morphology of coating. Coating was consisted with the center of spraying layer and external spraying layer. Coating area was large at a small distance. With the spraying distance increased, area deceased. When spraying distance increased to a certain extent, area reached minimum. Thereafter as the spraying distance continuously increased, coating area continuously increased. As the spraying distance increased,area of the center of coating remained unchanged and its size was similar to constraining tube diameter. The thickness of coating was analyzed by SEM in different process parameters. When the spraying distance was small, due to the formation of jet instability, the average thickness of coating was thin and thickness of coating was not uniform. As spraying distance increased, jet of explosion products tent to stability, the average thickness of coating increased, thickness tent to uniform.When spraying distance increased to a certain extent, coating thickness reached a maximum. Thereafter, with spraying distance continuously increased, coating thickness continuously deceased.
To study coating character in different process parameters, surface coating and morphology of cross section were observed, combined with EDS, and bonding of coating and substrate was analyzed. The results show that in a certain distance of spraying range, the liquid spraying coating is formed which high melting point WC particles are all in melted condition in the method of continuous feeding powder electrical explosion spraying in constraining tube. This kind of coating is dense and has a high bonding strength with substrate. When spraying distance is small, coating has vapor deposition characteristics. With the spraying distance increased, the partial melted particles of explosion products begin to solidify into solid phase particles. The density of this kind of coating deceases, and the bonding strength deceases with substrate. When the number of solid phase particles reach to a certain extent, the effective spraying coating is unable to be formed. In the high initial voltage, the range of spraying distance increases where liquid spraying coating and solid spraying coating are obtained.
Key words: tube constraint; WC; powder; electrical explosion spraying; spray distance; coating characteristics; Coating thickness
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