Question:We are nickel-brazing Inconel 718 base metals in a vacuum furnace and are having difficulties with the brazing. The base metals come out of the furnace dark and discolored, and the brazing filler metal doesn't wet the surfaces well. What's happening, and what can we do about it?

Answer:Traditionally, as mentioned in the first part of this article last week, we get concerned whenever the percent of titanium and/or aluminum is about 0.4% or greater in any base metal that is to be brazed. Inconel 718 contains approximately 0.9% titanium and 0.6% aluminum.

To handle this concern in brazing, I mentioned two good ways to deal with this. One is by nickel plating the Inconel surfaces that will be brazed, and the second method involves the use of fluoride-ion cleaning procedures to achieve a readily brazeable surface. Let's look at this in more detail.

Fluoride-Ion Cleaning

Also known as "FIC," "FIC cleaning" (a double use of the word "cleaning") or simply as "F-Clean," this is a relative newcomer to the world of cleaning processes for braze preparation, having really come into its own in the late 1980s. The process utilizes HF (anhydrous hydrogen fluoride gas) to effectively remove Cr, Ti and Al from the surfaces of nickel superalloy materials down to a depth typically ranging from 0.0005-0.0015 inch (0.015-0.05mm), i.e. about 8-30 microns.

Depletion of the Cr, Ti and Al is necessary so that the readily formed oxides of these metals cannot interfere with the wetability of the base metals by the brazing filler metals involved in the joining process.

The FIC process is performed in a special retort chamber that looks very much like a typical vertical-loading vacuum-furnace setup. Actual operating temperatures for this process are about 2000°F (1100°C), and the pressure in the chamber can vary from about 100 Torr (about 1/8 atmosphere) up to full atmospheric pressure.

This cleaning process has become very popular for use with braze repair of gas-turbine components for both aerospace and ground-power turbine applications and represents, in my opinion, the current state-of-the-art in cleaning surfaces and deep cracks in most superalloys today. As a repair technique, it is highly effective at removing oxides and other foreign matter from deep within cracks on the surface of these metals. Thus, it is an excellent method to "deep clean" surfaces of aero components coming back from service for overhaul and repair. The nice thing about this process is that, when used properly, it does not attack nickel or cobalt and, thus, does not have a negative impact on the dimensional control of such surfaces.

Metal components coming out of the FIC furnaces are very bright and shiny, and I have personally never seen metal look so shiny, bright, clean, etc. as metal parts do when exiting from this process. And, since the surfaces are depleted of Cr, Ti and Al, they can remain in this cleaned state at room temperature for long periods of time with no significant change in surface coloration or cleanliness since the surface of such FIC superalloys is essentially pure nickel and/or cobalt. Thus, they behave almost as if they had been nickel plated!

Two companies in Michigan specialize in manufacturing FIC furnace equipment for this vapor-phase FIC process, and companies using their equipment are located around the world. A number of these shops offer FIC services on a job-shop basis.