As described in my article in April, induction brazing is an excellent way to quickly heat up a localized area of a large assembly in order to permanently join them together. We previously took a brief look at the induction-brazing process to see what it is and how it can be effectively used by brazing shops today to meet some of their production needs.
This article will examine the six variables that need to be controlled for effective induction brazing.
These six variables are coil spacing, coupling distance, frequency setting, induction-machine power, coil design and flux concentrators.
This refers to the vertical distance between each loop of the induction coil. If the vertical distance between each loop is quite small, as shown in the drawing on the right side of Fig. 1, then the steel bar lowered down inside the coils will heat uniformly. If the vertical spacing between each loop of the coil were large, however (as shown in the drawing on the left side of Fig. 1), it would be possible to create a “barber-pole” type of heating pattern on that steel bar. So, if you want uniform, even heating, keep the coil-spacing distance small.
This is the width of the gap between the outside diameter (OD) of the bar shown in Fig. 2 and the inside-diameter (ID) of the copper coil itself.
Notice the heating pattern (dark semicircular patterns in the bar) shown in Fig. 2 as well as the horizontal line in the middle of the bar representing the joint that is to be brazed. When the coupling distance is small, the heating pattern is concentrated on the surface of the bar. By the time it penetrates all the way through the bar, it might overheat the outside surface. Thus, the outside of the braze joint might get very hot, but the inside of the joint might remain much cooler. It may be necessary to “pulse” the inductor (turning the power on and off) to allow the heat to penetrate all the way through the joint without hurting the outside of the joint.
As the coupling distance gets wider and wider (looser and looser), the inductive heating of the bar becomes less efficient and the surface of the bar doesn’t get as quickly overheated, allowing heat to sink deeper into the braze joint when the induction cycle is held a bit longer at brazing temperature. This is helpful for brazing since we want the heat to steadily go deeper into the bar to adequately heat the entire braze joint up to brazing temperature rather than heating just the outside surface of the bar.
Examples of close coupling distance versus large coupling distance are shown in Fig. 3a and 3b.
It is very important that the coupling distance is kept as even as possible around the braze joint. It is essential that the coupling distance be carefully controlled during induction heating/brazing since it will strongly affect the heating of the metal placed inside the induction coil. If the part being brazed is held too close to one side of the coil (tight coupling distance) and thus quite far from the other side of the coil (loose coupling distance), the heating of the metal part will not be uniform. This can easily and quickly result in overheating of the metal surfaces that have the tight coupling distance, leaving the surface with the loose coupling distance cool by comparison. The result could be uneven melting of any applied brazing filler metal (BFM) as well as uneven flow of the BFM through the joint.
RECOMMENDATION: It is probably best to use a semiautomated setup when induction brazing so that the part to be brazed can be mechanically raised and lowered into and out of the coil, thus keeping the part accurately centered in the induction coil with as even a coupling distance around the part as possible.
The frequency setting of the induction machine tells you how frequently the direction of the electrical flow is reversed. Whereas the alternating current (AC current) we use in our homes in the U.S. may operate at a 60-cycle frequency, that of an induction-brazing machine is much higher – on the order of tens of thousands of times per second or higher (hundreds of thousands per second and even millions of times per second).
It is interesting to note that the higher the frequency, the more intense will be the surface heating rate. This may or may not be good for brazing, depending on the thermal conductivity of the metal being joined. The frequency level chosen is best determined by experimentation to optimize the frequency range best suited for your particular design and base metals being used.
The larger the part to be brazed, the greater the available power in the induction machine should be in order to be able to heat that greater mass in a reasonably short period of time. For small parts, a 1-kW or a 5-kW desktop machine may be quite adequate, allowing parts to come up to temperature in a matter of seconds. Typical induction-brazing cycles might range from about 15 seconds all the way up to a few minutes. As you can see, the induction-brazing cycle time is much shorter than would be the case if the same parts were being furnace brazed.
For induction brazing of large parts, large power units that sit on the floor may be needed. Such units may have power ratings from about 10 kW up to 50 kW or higher in order to be able to supply sufficient electrical power for large brazing needs.
Designing an appropriate coil for the part you wish to braze is not a simple matter. In my opinion, you should always have the induction-machine manufacturer (from whom you purchased the equipment) design and build the induction-brazing coils for you. That is their job! Do NOT merely buy the equipment from the manufacturer and then try to make your own induction-brazing coil. That is truly being “penny wise and pound foolish” as the British phrase goes.
Study some of the coil designs shown in Figure 4, which illustrates just a small sampling of the many possible coil designs available to the induction brazer today. Note the complex design of the induction coil at the bottom left of the diagram used for brazing a bicycle frame. Next to that on the bottom row is a special induction coil used to heat treat the inside surfaces of a hinge assembly.
Figure 5 shows a seven-loop induction-coil design used to braze a low-conductivity metal shaft into a high-conductivity copper fitting containing a silver-based BFM ring. Note that the poorer-conductivity metal shaft at the top uses a larger coupling distance and only two coil loops, whereas the more massive lower fitting made from copper (high conductivity) required a closer coupling distance (and more coil loops). This was needed in order to adequately heat the copper fitting to melt the BFM ring and then allow the molten BFM to flow up by capillary action along the surface of the shaft until a small fillet of BFM is visible on the outside of the joint.
One induction-equipment manufacturer I visited showed me one of their research lab’s coil storage cabinets, which contained several hundred different coils, every one different in design from each of the others. So, unless you need just a very simple circular coil, I strongly recommend that you work with your induction-equipment supplier to have them design any type of complex coil that you might need.
REMEMBER: You might, as an example, need a coil that can apply lots of heat to one part of a braze joint or metal with a lot less heat being applied simultaneously to another portion of the joint. This can all be achieved by varying the coupling distance and coil-loop spacing of a complex induction coil so that it correctly fits around different parts of the joint area to be brazed for proper heating.
Electromagnetic fields around induction coils can extend outward a great distance from the coils, thus potentially causing concern about unnecessarily heating items positioned near the brazing coils that you don’t want to heat. A “flux concentrator” is a putty-like substance (sometimes containing a lot of iron powder) that can be molded around the outside of some of the induction coils (Fig. 6). The purpose is to pull in (i.e., focus) the electromagnetic field so that it is “short circuited” through the putty, thus keeping the electromagnetic field away from places that it shouldn’t go.
Induction brazing is a wonderful tool that many shops may wish to use for brazing certain parts that need to be brazed quickly, are too large to fit inside a brazing furnace or have areas that cannot tolerate high heat since damage might result to those areas if heated up to brazing temperature.
The principles of induction brazing are not that hard to understand and in quick summary might be listed as:
- Coil design is extremely important to induction brazing, and it is rightfully the responsibility of the induction-equipment manufacturer.
- Coupling distance should be uniform around the braze joint.
- Be sure the induction equipment has enough power to handle the mass of the parts being brazed.
- Use an induction frequency that will allow you to heat the entire braze-joint area effectively and efficiently and not just the outside surface of the braze joint.
Example 1: If I wanted to harden the surface of a steel automotive crankshaft by induction heat treating, what coupling distance and induction frequency should I use? Generally speaking, you’d probably opt for close coupling distance (to more intensely heat only the outside surface of the part) as well as high frequency since that would also tend to keep the heating intensity on the outside surface of the part.
Example 2: If I wanted to braze the ends of two round bars together (Fig. 2), what coupling distance and induction frequency should I use? Generally speaking, you’d probably opt for a wider coupling distance (to more slowly heat the part and let the heat penetrate into the joint) and a lower frequency (since that, too, will allow the heat to sink into the bar better than high-frequency heating).