In the turbine airfoil refurbishment business, brazing cracks in investment cast parts made of expensive alloys is routinely required as hot section jet engine components are damaged due to oxidation, sulphidation, hot corrosion, fatigue, or foreign object damage.
However, proper brazing requires that all oxidation first be thoroughly removed from airfoil component surfaces, cooling passage, and cracks, which can be very narrow and deep.
Oxide scale in airfoil cracks
While a jet engine is in service, oxide scale forms on the mating faces of cracks that occur in the airfoils. These cracks become packed full of scale, all the way to the tips. It is the goal of the service shop to repair the airfoils by filling the cracks with a braze alloy, but braze alloy cannot flow into cracks that are full of oxide scale.
To complicate matters, the alloys used to make turbine airfoils are nickel (Ni) and cobalt (Co) based superalloys that usually contain aluminum (Al) and titanium (Ti) to improve strength. The presence of these elements causes the resulting scale to contain complex spinels that are extremely difficult to remove.
“At the narrow tip of a crack, scale forms during service. The scale occupies a larger volume than the metal from which it formed. This results in the narrow spaces at the tips of cracks being totally packed with scale,” says Donald Bell, chief engineer at P&WC Component Repair, a division of Pratt & Whitney Canada. “You cannot fill the crack with braze alloy if it is already filled with oxide scale.”
Traditionally, fluoride ion cleaning has been performed at atmospheric pressure to remove oxidants from components, but metallurgical studies have shown it only works well when cleaning wide cracks. Plus, it can add extra steps to the oxide cleaning process that result from chromium fluoride or chromium carbide buildup during the process.
More recently, however, an innovative dynamic fluoride ion cleaning (DFIC) process has offered turbine refurbishment professionals the ability to clean deep, narrow cracks of oxides by cycling between negative, atmospheric, and positive pressure for more ideal surface preparation prior to brazing.
Beyond fluoride ion cleaning
The DFIC process, also known as hydrogen fluoride (HF) ion cleaning, results from the reaction of fluorine with various oxides. HF gas can be toxic if it escapes into the atmosphere. However, improvements in gas monitoring sensors and digital electronics, resulting from its widespread use in the semiconductor industry, have made it safe and reliable for parts cleaning.
At temperatures greater than 1,750 F (950 C), the fluoride ion reacts with oxides that have formed on the crack faces in turbine airfoils, converting them to gaseous metal fluorides. This allows them to be easily removed. They depart through the off-gas stream of the reactor.
There are significant drawbacks to the early fluoride ion cleaning processes developed in the 1970s, which utilize fluoride compounds in powdered form and perform the work at normal atmospheric pressure. Besides having difficulty penetrating into deep, narrow cracks, the early processes were less flexible and not continuous. They relied on a single charge of powder to produce their HF gas. This often resulted in parts having to be processed through more than one cleaning cycle.
“When compounds are in powdered form, such as chromium-fluoride, aluminum-fluoride, or PTFE, there is a finite amount of reaction that can occur,” says Bell. “When they’re done, they’re done, and if the parts are not yet clean, the cleaning process often has to be repeated.”
Flexible and repeatable
Fortunately, the DFIC process has been proven to be more effective, flexible, and repeatable. What separates the DFIC process from first generation fluoride ion cleaning equipment is that the reaction temperature, fluorine concentration, pressure level, and duration are all independently controlled variables.
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