Thermal Spray Tips - Swinburne University of Technology

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Compiled by Jo Ann Gan, Edited and advised by Christopher C. Berndt Swinburne University of Technology Thermal Spray Group (SwinTS) Please contact Prof. Christopher Berndt at cberndt @swin.edu.au for further enquiries 3.12. Powder Feeding Systems: Rotating Wheel Devices Rotating wheel (a) and rotating disk (b) powder feeding systems A rotating wheel device delivers precise parcels of powder to the powder feed delivery tube. The wheel can be mounted either vertically or horizontally. When oriented in the vertical direction (in the geometry of a simple water wheel), the powder is dropped into the carrier gas stream. A gas tube is connected between both sides of the rotating wheel to equalized gas pressures on both sides of the wheel. Otherwise, the powder would be blown off the pockets that are machined into the outside rim of the wheel. The horizontally oriented wheel (or disk) has slots that pass over the gas delivery system. In this fashion, powder is taken out of the slots and enters the carrier gas stream. In both cases, the powder delivery rate is controlled by (a) the physical dimensions of the slots or pockets that hold the powder and (b) the rotational speed of the wheel. In either geometry, it is easy to overload the carrier gas with solids and either block the powder feed tube or cause the saltation effect by using too high a powder feed rate or an insufficient carrier gas flow rate. Of course, the dimensions of the powder feed tube and the powder injection port, as well as the pressure conditions into which the powder and carrier gas are injected, also influence the powder delivery. Source: Feedstock Material Considerations, Christopher C. Berndt, Stony Brook University (at the time of publish date), as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, p 139, 2005. Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 42 Back to TOC Back to Overview
Compiled by Jo Ann Gan, Edited and advised by Christopher C. Berndt Swinburne University of Technology Thermal Spray Group (SwinTS) Please contact Prof. Christopher Berndt at cberndt @swin.edu.au for further enquiries 4. Surface Preparation 4.1. Dry Abrasive Grit Blasting Mechanical bonding, showing a properly grit-blasted surface with a large number of reverse draft pits into which sprayed particles flow to achieve positively keyed mechanical interlocking Dry abrasive grit blasting is the most commonly used surface roughening technique. Dry abrasive particles are propelled toward the substrate at relatively high speeds. On impact with the substrate, the sharp angular particles act like chisels, cutting small irregularities into the surface. The amount of substrate (plastic) deformation is a function of the angularity, size, density, and hardness of the particles and the speed and angle at which the particles are directed toward the substrate. The action of the grit/shot causes irreversible plastic deformation of the surface material. The material beneath the deformed surface material remains elastic and tries to return the deformed surface material to its original (shorter) length. The resulting balance of forces places the outer layer of material in residual compressive stress, while the underlying material is in tension. To avoid reduction of fatigue life, titanium substrates are sometimes shot peened before dry abrasive grit blasting to reduce the underlying tensile stresses, effectively balancing the compressive/tensile forces. Irregularities on a perfectly grit-blasted surface will have a large number of reverse draft pits, providing a partially hidden space into which sprayed molten particles can flow. Coatings sprayed to this surface are said to be positively keyed, creating what is commonly referred to as a mechanical interlocking bond. Source: Coating Processing, Frank N. Longo, Longo Associates, as published in Handbook of Thermal Spray Technology, J.R. Davis (Ed.), ASM International, 2005, p 111. Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 43 Back to TOC Back to Overview
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Thermal Spray Tips - Swinburne University of Technology

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