Induction Brazing and Induction Brazing Equipment

The application of high frequency induction heating for the Induction joining of metal parts with Induction solder and Induction brazing alloys offers a wide variety of applications. In many ways this process of joining metal parts enables the adoption of new manufacturing techniques, such as the design of fabricated parts to supercede those made from one piece, especially where silver Induction brazing alloys are used as the induction bonding medium. The outstanding features of induction brazing are the speed with which heat can be applied; the uniformity of heat transfer, once a cycle has been established; and economical heating cost when only a local surface is treated in contrast to an entire components or assembly.

Low-temperature solders, melting at temperatures of 250 to 500 ˚F, for the induction joining of metal parts can be bonded at exceptional speeds, because of the capacity of high-frequency current to cause metals to reach this relatively low temperature so quickly. With little difference in time, however, the harder solders, having a melting range of 500 to 700˚F, also are quickly melted into joints more effectively than by any other means. Induction soldering operations that ordinarily might require a half minute by irons or gas flames can be performed by induction heating equipment in a second or two.

High frequency induction brazing with silver alloys provides an excellent means of joining together ferrous, nonferrous, and dissimilar metals. An outstanding advantage of this is that the fluidity of silver alloys provides penetration into restricted openings, which other alloys might not reach. Capillary attraction results in their spreading evenly over the surfaces to be inductin brazed, thus forming a solid bond.

To some extent silver induction brazing is a form of hard soldering. However, the joint usually becomes stronger than the alloying material thus making exceedingly strong bonds possible. In many cases silver induction brazing, using high-frequency induction heat for the melting of the alloy and the heating of the parts to be joined, will produce joints with strengths equal to those obtained by copper brazing, and when considered as a substitute for copper brazing, the need of a controlled-atmosphere furnace can be eliminated.

Silver alloys melt at temperatures ranging from 1100 to 1500˚F, according to their composition. They have tensile strengths of 40,000 to 70,000 lb. per sq. in., but with some designs can provide joints with strengths in excess of 100,000 lb. per sq. in. To obtain the maximum strength, the spacing between the parts to be jointed must be closely held. The better the fit, the stronger the joint.

In the long run induction brazing with silver alloys is more economical than other methods. It is quick ,produces a uniform flow of the alloy, uses a relatively small amount of it, and produces a joint that requires practically no finishing. Briefly, a sliver induction brazing joint combines strength, smoothness, and ductility; it assures leak proof assemblies, and withstands temperatures that ordinarily would melt solder. High frequency heat for silver induction brazing of joints is suitable for more uses than other means of heating. The acetylene torch, for example, gives excessive heat which might have a tendency to distort parts adjacent to the brazed surfaces. The virtue of high frequency induction brazing heat lies in the closer control of heat to restricted surfaces which it provides.

Miscellaneous Induction Brazing Joints. In Fig. 93 are represented various types of silver-brazed joints which are adaptable to the induction brazing process. In the example at A, a preformed silver-alloy rings is brazed at the joint as indicated and when heat is applied around the bottom surface of the shaft the alloy flows in such a way that it fills the surface between the two parts.

At B shown a joint in which the induction brazing material is placed at the edge of the flange, so that when it melts it runs down through the jointed area. In the example at C the silver-alloy washer, as indicated at D, is placed between the shoulder of the shaft and the edge of the flange. With this type of induction brazing application, it is necessary to apply pressure on the shafts so that when the silver rung metals the excess metal will go down into the joint. At E is a similar application, although in this case the insert is made in the form of a bushing, which likewise requires a pressure applied to it, so that when the silver ring melts the flow of the material will go into the joint, and thus provide a metal-to-metal contact.

In applying bushings of this type, it also is possible to use a preformed ring of silver material placed under the shoulder, as shown at F. In the example at G, a ring is placed under the head of the bushing which, with an induction braze application, will flow into the joint. If an insert of this type has sufficient weight, it is likely that the part will settle into position as soon as the alloy is heated to its flowing point, and thus form a metal-to-metal contact. However, with light pieces it is preferable to apply a suitable pressure to insure a tight metal-to-metal joint.

Fig. 93 - A variety of brazed joints that lend themselves favorably to induction brazing.

Another group of induction braze joints is illustrated in Fig. 94. The example at A represents a shouldered flange induction brazed to an upset sheet-metal opening. For this application, a washer of silver alloy is inserted prior to the assembly of the two parts. At B is illustrated a method for mounting a tube to a flange, in which the silver-alloy disk is placed under the edge of the tube and then squeezed down into position after it is melted. The alloy then runs up through the joint and forms a solid induction brazing bond.

Another way to assemble a tube to a flange is shown at C. Here the performed silver-alloy ring is placed at the upper portion of the joint. When induction braze heat is applied the alloy will flow down the sides of the tubing to the bottom edge, thus covering the entire surface. Another way to perform this operation is to provide a counterbore at the upper portion of the flange and set the silver ring into it, as shown at D. One of the most satisfactory methods, however, is to machine a groove either into the flange or the member to be inserted into it, and to place a preformed ring of silver alloy into the groove prior to the assembly of the two parts. The example shown at E has a groove cut into the flange and, when the alloy reaches melting temperature from induction brazing, it will tend to flow up and down, so that the entire joining surface are covered. Still another way to braze such a joint is indicated at F, in which a preformed ring of silver alloy is placed within the tube. When the alloy melts it will run out along the edge of the tube and up the sides of the joint.

Fig. 94 - A miscellany of induction brazed joints showing various methods for applying silver alloy.

In joining together sheet-metal assemblies, such as tubing and covers, or drawn shells and caps, often required for containers and the like, there are many varieties of induction braze joints well adapted to the application of induction heat. In Fig. 95 are shown a variety of these joints. In the example at A both edges of the assembly are formed out, so that a preformed silver-alloy ring can be inserted between them, at assembly, as shown. For the joint at B, however, the silver-alloy ring is placed underneath the cover and, when induction brazing heat is applied, the alloy will flow up through the joint. One of the better type of joint, however, is illustrated at C. Here the inner body is preformed with a groove, into which a silver-alloy ring is placed before assembly of the cap. After induction brazing heat has been applied to melt the alloy, it will flow upward toward the top of the joint and also downward, so that both contacting surfaces become completely covered, thus forming an effective bond. For the joint at D, the preformed wire alloy is placed on the inside surface and, when the induction braze method melts it, flows downward through the joint.

Fig. 95 - Various ways in which silver alloy can be used in connection with the induction brazing of sheet-metal parts.

The reverse of this induction brazing is shown at E, where the alloy ring is placed on the outside edge and, when melted, flows through the inside joint. In the example at F the cover is inserted on the inside of the sleeve and, when melted, flows through the inside joint. In the example F the cover is inserted on the inside of the sleeve and a preformed ring of alloy is placed on the inside which, likewise, flows through the joint. The example at G also is arranged so that the cover has a groove rolled into it. Into this the alloy ring is placed prior to assembly, so that upon being heated the metal will flow up and down. Another form of induction braze, in a similar assembly, can be seen at H, where the ring is place on the inside and, when induction brazing melted, will flow upward. The example at J is similar, though here the cover is placed over the outside of the body.

Another type of assembly, comprising a cap to a sleeve, is shown at K, in which the preformed alloy ring is placed inside, as shown. As soon as the surrounding metal is induction brazing heated sufficiently to flow the alloy, it will run down through the joint. IN the example at L before induction braze, however, the ring is placed on the inside and must flow up into the joint. With this type of induction brazing it is important that the ring remain in position, and usually it would be better to turn the part upside down, to ensure a perfect bond, although because of the characteristics of silver alloys they will flow upward practically as well as downward.

The five examples of induction brazing shown at the lower position of the illustration are representative of the method used for applying shallow covers to tubular bodies. The preformed rings are mounted in various ways somewhat along the lines already discussed. With the induction braze joints at M and N, however, it is necessary to apply pressure to the outer sleeve in order to form a metal-to-metal contact, as soon as the preformed silver alloy ring begins to flow.

Production Induction Brazing Setups. In Fig. 96 are illustrated a variety of induction brazing operations for miscellaneous types of metal parts, which are representative of some of the joints that can be made by high frequency induction braze heating equipment generators. The assembly at A includes a tube to which a solid type insert is brazed. The silver induction brazing ring is placed within a groove, cut

Fig. 96 - Six representative types of joining operations performed by means of high frequency induction heating.

into the plug, prior to assembly, as shown. The tube is then placed over the plug, and induction braze heat is generated to the surface by the single turn inductor as shown. For example two stations are used, and one at the right of the fixture during the induction brazing operations. The fixture at the left however, is open and ready for loading. There are eight pieces induction brazed simultaneously, and each assembly is held in place firmly by means of individual spring plungers, located on the upper cross plate of the fixture. After the proper heat has been applied to the joint, the silver induction brazing alloy flows both upward and downward, so that a uniform induction brazing action results. Pressure on the tube against the shoulder of the insert assures correct alignment and eliminates the possibility of the tube's moving out of place while being induction braze heated.

An example of induction brazing setup for joining a nose cup and a spacer. Here six pieces are handled simultaneously. For this an internal type coil is used, details of which may be seen in Fig. 96 at B. The silver alloy ring is placed underneath the cup section and when heat is applied to the inside surface of the spacer, the molten alloy is drawn up through the joint, completely surfacing the contacting areas of both parts.

The operation illustrated at C in Fig. 96 represents the induction brazing of a tube to a body, in which the preformed silver induction brazing alloy ring is placed at the joint on top. This table is arranged with two fixtures and, four pieces are induction brazed at one time. The brazing coil is arranged so that heat is generated from the outside surface. When both parts attain a temperature slightly above the melting point of the alloy, the silver runs down through the joint, thus completing the induction braze.

For induction brazing an insert to a drawn cup, in which an internal type brazing coil is used. Details of this assembly are illustrated at D in Fig. 96. Each insert is held firmly in place during the induction brazing operation, so that after the ring of alloy has melted, the excess material will be squeezed out and distributed uniformly, thus assuring a perfect metal to metal joint.

For induction soldering eight Bourdon tubes, used in the manufacture of pressure gauges, to the body component. For this operation two parallel type inductors are used, arranged so that the current comes in on one bar and goes out on the other. At E in Fig. 96 may be seen details of the operations, showing the relation of the inductor to the parts to be induction brazed. The inductors when energized, generate heat to both sides of the body from where it conducts to the tube joints.

For induction brazing tubing to a flange and in this operation the fixture is arranged to handle four pieces at one time. Internal type brazing coils are used, as may be seen at F in Fig. 96. The silver alloy ring is located in the chambered section of the flange and, upon reaching melting temperature, flows down throughout the entire joint. The outer sleeve is used as a guide to centralize the work over the brazing coil.

Preparation of Surfaces. In any form of induction joining, whether an induction solder or a silver alloy is used, clean surfaces are essential. Even though a flux will have a tendency to dissolve oxide films on the surface of metal parts, it is better to clean the surfaces thoroughly before applying heat. This will assure a stronger bond. Also surfaces to be joined should be as smooth as can be commercially produced. This procedure assures a better flow of the alloy and a more equal distribution of it than would result with irregular surface contact.

In connection with silver alloy induction brazing, it is important that a suitable amount of flux be applied around the areas to be jointed, as a means of protection against oxidation. This flux, when heated, also provides a free flowing surface for the alloy, so that uniform bonding is made possible. Usually fluxes having a borax base are used and are applied in liquid form, which results in a bubbling action when subjected to heat. The fluidity of the flux must be sufficient for it to flow at a temperature below that required to melt the silver alloy. With a fluidity temperature differential of 50 to 100˚F, the flux will serve as a good temperature indicator during the induction brazing operation.

Induction heating equipment can be used effectively when a small temperature differential exists between the metal parts to be joined and the solder used for bonding. This is due to the fact that the heating coil can be placed far enough away to bring the heat up gradually to the melting temperature. Then, through the automatic timber, the heat can be cut off at the exact moment when the solder flows.