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 2.4. Thermal Spray Processing Characteristics Typical thermal spray process parameters and variables From a simplistic point of view, the powder spray process can be viewed as high-speed heat treating (dealing with time, temperature, and mass) where the objective is to bring the powder mass to the required temperature in a given time period. The time a particle spends in the process jet is called dwell time, which is governed by gas velocity and powder particle characteristics. Gas velocity, in turn, is determined by the total gas flow through the nozzle, gas characteristics, and energy acting on or resulting from the process. Particle velocity is a function of jet velocity coupled with particle characteristics; i.e., particle size, morphology, and mass. Particle temperature is a function of enthalpy, velocity, trajectory, and its own physical and thermal properties. Gas temperatures in the spray stream vary greatly with the process. Source: J.R. Davis (Ed.), Handbook of Thermal Spray Technology, ASM International, 2004, p 45 Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ 10 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 2.5. Thermal Spray Process Variations Heat energy input and particle velocity for common thermal spray processes Process Input heat energy to particle Information and data acquired from ASM International Thermal Spray Society website at http://asmcommunity.asminternational.org/portal/site/tss/SprayTips/ Output particle velocity (a) High Low Highest High Medium high Medium Low Combustion wire X X Combustion powder X X Standard plasma X X High-velocity plasma X X Vacuum plasma X X Standard wire arc X X Vacuum arc X X High-velocity oxyfuel X X Detonation gun X X (a) Particle speed ranges from a high of approximately 1000 m/s (3000 ft/s) to a low of 25 m/s (80 ft/s). Further variations within each process depends on the particle size, material type and gas velocity Source: ASM Handbook: Metallography and Microstructures, Vol.9, p.1038, 2004 The controllable parameters for each of the major thermal spray types are different, but all share some common fundamentals. Converting feedstock material (whether powder, wire, or rod) into a coating requires heat energy for melting and a gas flow for atomization and/or particle acceleration to propel the particles toward the substrate. Input heat energy and gas flow, in turn, directly influence process variables including flame/plasma/jet temperature and velocity; particle temperature, speed, and trajectory; and deposit temperature. Each of these variables affects spray-pattern formation and particle heating differently. In general, combustion-flame, combustion-wire, and twin-wire arc spray coatings are generated from high heat input plus low particle velocity, producing coatings having high oxide content, a large number of rounded particles, and relatively high porosity. Due to higher particle velocity, superheated particles in plasma spray coatings are flattened more than in combustion spray coatings, producing denser coatings with finer porosity. Very high-velocity systems such as detonation gun and HVOF, combined with lower heat inputs, typically produce very dense coatings not achievable by the other processes. 11 Back to TOC Back to Overview
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