BFM Alloys for HCR Brazing
As mentioned earlier, the BFMs that are used to join HCR alloys should ideally meet or exceed the HCR characteristics of the alloys they are joining together. Otherwise, the BFM might corrode/erode or re-melt under the service conditions encountered, which severely limits the usefulness of the brazed HCR-alloy component. This would obviously be very undesirable.
A few of the HCR alloys commonly used in industry are shown in Table 1. Please note that there are many different commercially available HCR alloys that are not shown in this table. The few that are shown in Table 1 are merely there for brief illustrative use in this article. Similarly, the BFM alloys shown in Table 2 are only a small fraction of the many excellent commercially available alloys for joining HCR base metals.
Many of the metallic elements in traditional HCR alloys are easily brazeable. Nickel, chromium, tungsten, molybdenum, niobium and copper are all easily wetted by most of the readily available BFMs, such as the standard nickel-based, cobalt-based, gold-based and even the copper- and silver-based BFMs shown in Table 2.
As metals are heated, they have a greater and greater desire to react with oxygen and form an oxide. This is true for ALL metals, including gold. Of course, not all metal oxides are stable at room temperature under ambient conditions. Many oxides, such as those of nickel, gold, etc., are not stable (and will not form) under such conditions and, thus, are not of any real concern to us.
Other metals form oxides that are quite stable under ordinary room-temperature conditions. They will be readily observable and will have a negative effect on brazing unless dealt with appropriately. Many of these oxides are not stable when heated, however, and can easily be dissociated into pure metal with the release of the oxygen. Such is the case with iron, copper and silver at moderately low temperatures. Even tungsten and molybdenum oxides are easily dissociated by heat and should present no real issues in brazing.
The oxides of chromium, manganese, niobium, titanium and aluminum are quite stable, however, and require careful elevated-temperature controls in order to appropriately and effectively deal with and remove those oxides prior to the actual melting and flowing of the BFM. This is very important because BFMs do not like to bond to or flow over any oxides on the faying surfaces of the HCR alloys. It is very important, therefore, that the faying surfaces of the HCR alloys be thoroughly cleaned prior to assembling for brazing, thereby removing all oil, dirt, grease, fingerprints, oxides, etc. so that the molten BFM can flow over each of the faying surfaces and diffuse into those surfaces to form a strong metallurgical bond. Such BFM flow and diffusion cannot occur on uncleaned surfaces.
Because many of the HCR alloys that are brazed contain chromium – and some of them also contain small amounts of niobium, manganese, titanium and/or aluminum – the tenacious oxides of these metals may interfere strongly with brazing and, thus, require special procedures to ensure high-quality braze results (Fig. 4).