|Innovative Dynamic Flouride 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.|
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 damaged due to oxidation, sulphidation, hot corrosion, fatigue, or foreign object damage. However, proper brazing requires thorough removal of all oxidation from airfoil component surfaces, cooling passages, 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 then pack 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 super alloys 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, 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 occurs at atmospheric pressure to remove oxidants from components, but metallurgical studies have shown it only works well when cleaning wide cracks. In addition, it can add extra steps to the oxide cleaning process that result from chromium fluoride or chromium carbide build-up during the process.
More recently, however, a 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.
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.
|With the Dynamic FIC, process the reaction temperature, fluorine concentration, pressure level, and duration are all independently controlled variables.|
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, allowing easy removal. 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, often resulting in parts requiring more than one cleaning cycle.
“When compounds in powdered form, such as chromium-fluoride, aluminum-fluoride, or PTFE, there is a finite amount of reaction that can occur,” Bell says. “When they are done, they are done, and if the parts are not yet clean, the cleaning process often has to be repeated.”
Fortunately, the Dynamic FIC process shows proof to be more effective, flexible, and repeatable. What separates the Dynamic FIC process from first generation Fluoride Ion Cleaning equipment is that the reaction temperature, fluorine concentration, pressure level, and duration are all independently controlled variables.
The sophisticated digital control systems that come with today’s equipment are programmable with hundreds of recipes for cleaning specific alloy types, widths of cracks, and levels of scale and oxidation.
During the cleaning process, HF and H2 gas introduces into the system through precision metering, for precise control of time and gas concentrations. For example, a typical cleaning cycle may begin as 94% to 96% hydrogen. However, within that cycle, it may change to a 92:8 or 86:14 H2 to HF ratio, depending on the substrate material.
Some Dynamic FIC systems, such as those available from Hi-Tech Furnace Systems, are designed to perform the cleaning process at sub-atmospheric pressures from 100 Torr to 650 Torr (133 Millibar to 867 Millibar) while at processing temperature.
By varying the pressure between positive, negative, and atmospheric levels, the Dynamic FIC system pulses HF in and out of cooling channels, deep cracks and small holes to more effectively clean oxidized, hard to reach areas.
In recent years, the Lufthansa Technik Turbine repair facility in Shannon, Ireland, added a Dynamic FIC furnace from Hi-Tech Furnace Systems. The Dynamic FIC furnace is for preparing hot section engine parts, such as LPT/HPT vane and combustor parts, for brazing.
|During the cleaning process, HF and H2 gas introduce into the system through precision metering, for precise control of time and gas concentrations.|
One of only a few Dynamic FIC manufacturers in the world, Hi-Tech Furnace’s customers include General Electric, Pratt & Whitney, Snecma Services, Lufthansa Technik, Chromalloy, Goodrich, and others.
“The Dynamic FIC works equally well on a variety of alloys, and allows us to cycle between positive and negative pressure to get component surfaces as well as deep cracks and crevices extremely clean,” says Philip Kelly, a process engineer at Lufthansa’s Technik Turbine Shannon repair facility.
“We use the Dynamic FIC process to modulate atmosphere from low to high to pneumatically push the fluoride ions down into the tips of the cracks and hold them there for a while,” Bell explains. “We can cycle back and forth as needed for the best cleaning results.”
Bell adds that by performing the process under vacuum, not only is there removal of surface oxidation, but depleted from the substrate are aluminum and titanium, creating a denuded zone approximately 0.0005" deep.
“This gives us a buffer. During furnace brazing, residual oxygen in the vacuum chamber can re-oxidize a clean part. The denuded zone give us time to get the braze filler to flow and wick into the cracks before re-oxidation occurs,” Bell explains.
As an added benefit, the use of HF at sub-atmospheric pressure often eliminates extra steps in the brazing preparation process.
Cobalt-based alloys, used to make jet engine turbine airfoils, contain a significant amount of chromium. This can react with fluorine during the process to create a chromium fluoride film on the surface of the parts. Chromium fluoride is the most refractory (temperature resistant) compound of all the metal fluorides. As a result, it does not volatize at the usual temperatures used in FIC.
Without the vacuum capability in the cleaning process, the part must then move to a vacuum furnace where the part subjects to the higher temperature and lower pressure required until the chrome fluoride volatilizes.
However, the resulting fluorides can contaminate the brazing furnace or the vacuum pump, as their design is not to handle acidic gases and require being very clean.
According to Bell, at pressures of about 150 Torr absolute, chrome fluoride will remain gaseous, “so we are able to clean without depositing a residue on the joint.” If there is the creation of any chrome fluoride during the process, the control system can be set to subject the part to the higher temperature and appropriate pressure to remove it.
“With the Dynamic FIC equipment, we are able to clean components in one shot, instead of the multiple cleanings typically required with more traditional fluoride ion cleaning,” Bell adds.
Another benefit of the dual vacuum process is that it uses significantly less HF, because oxides volatilize at a lower temperature and concentration of HF when performed sub-atmospherically. Using less HF also reduces the risk of inter granular attack (IGA), which could otherwise chemically alter the microstructure of the metal being cleaned.
Hi-Tech Furnace Systems Inc.
Shelby Township, MI