The sophisticated digital control systems that come with today’s equipment can be programmed 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 are introduced into the system through precision metering, so time and gas concentrations can be precisely controlled. For example, a typical cleaning cycle may begin as 94 to 96 percent hydrogen. But within that cycle, it may be changed to a 92:8 or 86:14 H2 to HF ratio, depending on the substrate material.
Some DFIC systems, such as those available from Hi-Tech Furnace Systems, are designed to perform the cleaning process at sub-atmospheric pressures from 100 to 650 Torr (133 to 867 Millibar) while at processing temperature.
By varying the pressure between positive, negative, and atmospheric levels, the DFIC 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 DFIC furnace from Hi-Tech Furnace Systems. The DFIC furnace is used to prepare hot section engine parts, such as LPT/HPT vane and combustor parts, for brazing.
One of only a few DFIC manufacturers in the world, Hi-Tech Furnace’s customers include General Electric, Pratt & Whitney, Snecma Services, Lufthansa Technik, Chromalloy, Goodrich, and others.
“The DFIC 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.
Pratt & Whitney Canada Component Repair’s Donald Bell concurs. “We use the DFIC 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,” explains Bell. “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 surface oxidation removed, but aluminum and titanium are depleted from the substrate, creating a denuded zone approximately 0.0005 inch deep.
“This gives us a buffer. During furnace brazing, residual oxygen in the vacuum chamber can re-oxidize a clean part. The denuded zone gives us time to get the braze filler to flow and wick into the cracks before re-oxidation occurs,” explains Bell.
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 be moved to a vacuum furnace where the part is subjected 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, which should be kept very clean and are not designed to handle acidic gases.
According to Bell, at pressures of about 150 Torr absolute, chrome fluoride will remain gaseous, “so we’re able to clean without depositing a residue on the joint.” If any chrome fluoride is created 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 DFIC equipment, we are able to clean components in one shot, instead of the multiple cleanings typically required with more traditional fluoride ion cleaning,” adds Bell.
Another benefit of the dual vacuum process is that it uses significantly less HF, because oxides are volatilized at a lower temp 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. AMT
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