Fluoride Ion Cleaning (FIC) at Sub-Atmospheric Pressure

Abstract

The fluoride Ion cleaning (FIC) process is used to assist in the successful braze repair of nickel-based superalloy components. This process is especially effective in removing embedded oxides in cracks typically found in aircraft parts such as in combustors and turbine blades and vanes. Where, Al and Ti are present in the base metal, the FIC process depletes these elements from the surface, thereby improving the braze flow and repair of the cracked components. The objective of this paper is to show that mechanical properties of the braze repaired region can be adequate when the FIC process at sub-atmospheric pressure is utilized, prior to brazing. To achieve this, firstly entailed designing and producing specimens suitable to study the fatigue crack behavior of braze repaired cracks/joints under mechanical and thermal loading. The cracks/joints prior to brazing were either in an unclean form i.e. had an oxide layer on, or were fluoride ion cleaned at sub-atmospheric pressure. Fatigue crack propagation tests under constant load as well as under constant stress intensity factor range were conducted in order to study fatigue crack growth characteristics in the parent and braze repaired area. In addition, the resistance to thermal cycling was investigated using single-edge wedge specimens containing braze repaired joints. For the unclean crack specimens, abnormally high crack growth rates were obtained from the brazed repaired area as compared to parent Ni-based material; whereas for the fluoride ion cleaned crack specimens, lower crack growth rates were experienced. Similarly for the unclean crack specimens, the thermal fatigue crack initiation life was significantly shorter compared with the fluoride ion cleaned crack specimens.