The present invention relates generally to thermal spraying of workpieces and more particularly to focusing the spray plume of a spray stream generated from a thermal spray device onto a workpiece for deposit optimization and coating consistency.
Turbine parts or components are one example of workpieces that typically undergo a thermal spraying process in order to inhibit corrosion or oxidation. Typically, the turbine components are sprayed with a metallic coating to inhibit the corrosion or oxidation that arises due to the temperatures in which they operate. The metallic coating acts as a sacrificial layer that oxidizes before the turbine component and, thus slows the oxidation process. In addition to the metallic coating, turbine components may be coated with a ceramic layer that functions to slow the heating of the component.
In a typical thermal spraying process of a turbine component, a metallic powder is heated up to a softened point where its particles can be sprayed onto the component by a spray device to make a coating that has a thickness that can range from about 5 mils—thousandths of an inch—to about 20 mils; although the thickness will vary depending upon the ultimate application of the component. The spray stream generated from the spray device is generally divergent which makes it difficult to coat the complicated geometries associated with many turbine components such as an airfoil (e.g., fillet radiuses) with uniform, high coating quality. In particular, the divergent spray stream is characterized by a spray plume (i.e., the hot molten center) and a cooler overspray (i.e., the periphery of the stream) with particles that perhaps may not be molten at all. It is the particles from the cooler periphery that collect and build up in the coating of areas having complex geometries and ultimately reduce the quality of the coating of the component. For example, a platform in a turbine bucket where an airfoil transitions from a dovetail will have build up of particles from the periphery. Because the particles associated with the periphery are inferior in comparison to the particles with the spray plume, the quality of the coating at the platform will be reduced, which ultimately affects the life and performance of the component because this area will be susceptible to corrosion and oxidation.
One approach that has been utilized to address the issues associated with a divergent spray stream is to use a masking process in conjunction with the thermal spraying process. Using a masking process in a thermal spraying process of a turbine component generally involves placing a hard metal mask over a particular part of the turbine component to protect it from being coated with the spray stream. In the case of a turbine bucket, there are a multitude of cooling holes (e.g., hundreds) that have to be laboriously filled with the masking material before applying the coating and then laboriously removed after the thermal spraying process has concluded. Even so, a thermal spraying process that utilizes a mask is unable to control focusing of the spray stream in and around the cooling holes and other fine areas such that only the spray plume is used to provide coating and not both the spray plume and periphery.