Question: When vacuum brazing a metal such as Inconel 738 (which contains a significant amount of titanium and aluminum in its chemistry as residual elements) to another superalloy that does not contain any such titanium and/or aluminum in its chemistry (such as Hastelloy), I understand that electrolytic nickel plating is recommended for that process. Which surface in the joint should be plated? Should it be the Inconel 738, the other superalloy (Hastelloy) or both?
Answer: Both titanium and aluminum will easily oxidize when being heated up to brazing temperature, and once those oxides are formed they cannot be easily removed in a standard vacuum-furnace atmosphere. Therefore, it is important to nickel plate the surface of the base metal that contains the titanium and/or aluminum in its chemistry. If the mating surface of the other superalloy (Hastelloy) being brazed does not contain any titanium and/or aluminum, then that superalloy (Hastelloy) surface does NOT need to be nickel plated.
You should always electrolytically nickel plate the surface of the base metal containing the titanium and/or aluminum because you want to keep any titanium and/or aluminum “buried” down inside the part (under the nickel plating) so that those very-sensitive-to-oxidation elements are not exposed to any oxygen on the surface of the parts or in the brazing atmosphere during the brazing process.
I encountered a situation recently where a company knew they were supposed to nickel plate one of the surfaces inside the brazed joint, but they plated the wrong metal (namely, the one that did not contain any of the titanium and aluminum in it). As a result, the brazed joint was weak and did not hold up in service well at all. It had to be scrapped because of poor brazing and leaks in the joint.
The base metal that needs to be plated is the one that has the titanium and/or aluminum in its composition. It is NOT normally necessary to nickel plate the surface of any metals being brazed that do not contain titanium and/or aluminum or where the total amount of titanium and/or aluminum is less than 0.4%.