Question: I have heard the terms "isothermal solidification" in reference to brazing, but I don’t really understand what it means or how to use it. Can you clarify this for me so that I can determine if it is something that I could use in some of my brazing operations?
Answer: To better understand these terms, let’s first examine the component parts of the phrase “isothermal solidification.” “Iso” essentially means “equal, or the same” and “thermal” of course refers to temperature. So we’re looking at a brazing filler metal (BFM) solidification process that occurs while the BFM is still being held at brazing temperature! That’s right, the BFM actually solidifies while the molten BFM remains at brazing temperature! Thus, solidification of the BFM is actually occurring while the BFM is being held at the same (iso) temperature (thermal) to which it was heated to get the BFM to melt and flow into the joint.
Although it sounds illogical that solidification occurs at the same temperature used to get the BFM to melt and flow, there’s a good reason why it can, and does, occur. Isothermal solidification (we will refer to it simply as ITS) totally depends on certain components of the BFM diffusing away from the BFM while that BFM is being continuously held at the brazing temperature, the diffusion of which significantly changes the melting characteristics of the BFM. Let me explain further.
First, it is important to understand that this process takes time – usually a minimum of at least 30-45 minutes at brazing temperature to allow a sufficient amount of diffusion to occur to bring about ITS. This rules out most commonly used brazing methods except furnace brazing. Torch brazing and induction brazing are simply too fast to allow ITS to occur. Further, such long times at brazing temperature would also rule out continuous-belt furnace brazing (a type of furnace used for high-volume brazing). Thus, batch-type furnaces (e.g., a front-loading vacuum furnace) are those that can be used for ITS processes.
Secondly, ITS requires at least one of the metallurgical elements added into the chemistry of the BFM to be able to rapidly diffuse away from the BFM at brazing temperature. Thus, to be able to rapidly diffuse through the metallurgical structure of a BFM, the atoms of that diffusing-metal or semi-metallic element must be very small. That means it should be an interstitial type of atom (Fig. 1), since interstitial atoms can easily move into and through the atomic structure of most metals.
Interstitial atoms are not as strongly bonded into the matrix of atoms as substitutional atoms, which means that “interstitials” can enter or leave the atomic matrix much more easily than substitutional atoms can. As you know, as a metal gets hotter and hotter, it will expand. It does this because atoms vibrate in place more and more when heated, occupying more and more space to do so. Therefore, the overall dimensions of the metal get larger and larger as the metal gets hotter and hotter, since the spacing between each of the atoms in the alloy is increasing.
We will finish this discussion next time.