Advantages of fluoride ion cleaning at sub-atmospheric pressure

Abstract

The fluoride ion cleaning (FIC) process is used to assist in the successful braze repair of nickel-based super alloy components. This process is especially effective in removing deeply embedded oxides in wide and narrow 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 highlight the benefits of using the FIC process at sub-atmospheric pressure. To achieve this, firstly entailed designing and producing specimens suitable to study the fatigue crack behaviour of brazed 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 brazed repaired joints. Two different braze materials were also under investigation. 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 were significantly shorter compared with the fluoride ion cleaned crack specimens.