Original Date: 03/08/1999
Revision Date: 01/18/2007
Best Practice : Abrasive Flow Machining
The Naval Aviation Depot at Cherry Point, North Carolina always prepared its turbine blades for re-coating by using an iterative, time-consuming nitric/hydrofluoric acid etch and manual media blast process. This expensive process generated hazardous wastewater due to the pH concentration and the presence of chromium. The Navy approached the Applied Research Laboratory at the Pennsylvania State University (ARL Penn State) to evaluate Abrasive Flow Machining (AFM) as an alternative to this process. The laboratory developed analytical techniques to optimize AFM’s setup for removing thin coatings (two to four mils).
AFM removes very small amounts of surface material by forcing a highly viscous fluid or carrier, containing an abrasive, back and forth across the surface. The carrier behaves like a non-Newtonian fluid, making its flow difficult to predict and causing uneven material removal. A typical solution is to install spacers which limit the flow area and create an environment for consistent material removal. However, the success of this approach is by trial and error, and optimum material removal is not always produced. To resolve this issue, the ARL Penn State developed a computational fluid dynamics flow model to predict the insert size and shape needed to produce optimum removal. The laboratory is also evaluating AFM as an alternative method for surface coating removal.
Approximately 100,000 to 200,000 turbines are stripped annually. By implementing AFM, the ARL Penn State estimates that facilities could decrease cost by $30 per turbine ($3 to $6 million a year) and reduce hazardous wastewater by 500,000 gallons per year. The process is several orders of magnitude faster than current techniques of grinding, sandblasting or chemical etching and generates no environmentally hazardous waste stream.
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