Which BFM should I use?
This is a commonly asked question, and it has many possible answers. The most metallurgically sound response is to use the BFM that as much as possible matches the chemistry and corrosion resistance of the HCR alloy being joined.
Let’s look at copper BFM. Unfortunately, the heat- and corrosion-resistance characteristics of copper prevent its use in many brazing applications for HCR alloys, but a very important exception stands out – copper BFM in the automotive industry. For many years the automotive industry has preferred pure-copper BFM (BCu-1 and BCu-1a) for brazing carbon-steel and 304L stainless components, which are used to carry/transmit fuel and for oil-cooler assemblies. From fuel-sender components inside the fuel tanks to the fuel rails injecting fuel into the engine cylinders, copper-brazed stainless components are very common today in automotive fuel-transmission lines.
Figure 5 shows a portion of a new, high-pressure fuel-rail assembly. Higher pressures and higher temperatures are keys to better fuel economy. Even so, copper is the preferred BFM. According to experts in the automotive industry, the temperatures and corrosive conditions in field use are such that pure copper is still quite acceptable for use as a BFM for joining their HCR alloys. One of the keys for this success is the need to keep the brazed-joint clearances as tight as possible to minimize the size of the exposed edges of any copper-brazed joint.
Silver brazing has been used for many years to join a wide variety of stainless steel components in the medical, food-handling and electrical industries and even in some aerospace applications. Temperature limitations must be observed, however, so as not to remelt any of the silver-based BFMs used in those parts. Thus, the use of silver-based BFMs is usually limited to applications that do not see any significant temperature excursions.
Gold-based BFMs are still used to braze a number of HCR alloys for aerospace components due to their ease of use, corrosion/oxidation resistance, forgiving nature when joint gap fit-ups are not ideal, and higher-temperature capabilities (as compared to any silver-based BFM). These benefits are obviously offset by the extremely high cost of such BFMs.
Figure 6 shows an example of an Inconel 625 JT9D fuel nozzle that is gold-brazed using the BAu-4 BFM. Three threaded connectors on the right and the nozzle tip on left were brazed into an Inconel 625 forged nozzle used in the combustion chamber of a JT9D jet engine. Fuel flows through the center fitting of the three on the right side and compressed air flows through the two outer fittings.
Nickel-based BFMs exhibit some of the finest characteristics for joining HCR alloys used at elevated temperatures in service (sometimes exceeding 2000°F/1100°C). These BFMs have chemistries that often closely match those of the HCR alloys they are joining, and their remelt temperatures (when properly brazed) often exceed the initial brazing temperature used by several hundred degrees. It is not uncommon to have an aerospace component brazed at about 2000°F (1100°C) and see service temperatures in excess of 2100°F (1150°C) without remelting. Some of the boron-containing nickel-based BFMs, when brazed such that isothermal solidification occurs during brazing, are capable of handling temperature over-runs significantly higher than their original brazing temperature. This is rarely possible to achieve with any other type of BFM (in the author’s experience).
Cobalt-based BFMs (BCo-1) can be used on HCR alloys where nickel cannot be used because of health concerns, such as in dental braces and similar mouth appliances. Some governments do not allow nickel-based BFMs to be used in such applications, and BCo-1 is a suitable alternative. Cobalt has shown itself to be less erosive than nickel in a number of high-temperature brazing applications, particularly where HCR alloys are used as thin sheet metal.