This chart is a conservative one, for safety purposes. But, amazingly, notice that the BNi-family of BFMs is head-and-shoulders above the rest when it comes to allowable service-temps. Thus its popularity for use in many high-temp aerospace applications.
Which specific BFM to use? Even when you have narrowed your BFM selection to one family, such as the BNi- family of BFMs for your specific end-use application, which one of the many BNi alloys available to you should you use? In reality, it now comes down to personal preference, since a number of the alloys in that BNi-family could, in fact, meet your end-use needs. It is not uncommon to find that different companies brazing almost identical parts for use in very similar end-use conditions will use a different BNi-alloy than the other company. There is nothing wrong with this. As mentioned, it comes down to personal experience and thus, personal preference. I’ve worked with major aero companies in which one of the aero companies used a BNi-5 BFM for an application, whereas the other aero company used the BNi-2 BFM for their somewhat similar application. When each was asked why they did this, they explained that their own personal experience with that particular alloy over the years led them to select that BFM for their needs. That’s okay! You will generally find that you have a number of very acceptable BFMs in each of the BFM families that will work quite well for your application.
Next Month: We will look at BFM powder and paste, with some interesting guidelines about how to specify, and use them, properly.