Induction Brazing Guidelines

Induction heating is a sophisticated brazing method, especially well-suited for high production. Process guidelines are presented here, along with a guide to alternative brazing methods.

Brazing is a joining process in which a bond is formed by a molten filler metal that flows by capillary action into a joint formed by two materials. The process is performed at temperatures above 840°F, but below the melting temperature of the base materials. It is widely accepted that the strong adhesion between the metal components is caused by a sharing of electrons between the filler and the base metal that creates a bond that is permanent.

Brazing systems and technologies have advanced rapidly in recent years. Using modern laboratory equipment, companies are routinely discovering new products and techniques to enhance the performance, versatility and economy of this ancient metallurgical joining technique. In the last 100 years, a host of brazing products and systems have been developed, including oxygen-acetylene torches, controlled environment brazing furnaces and vacuum furnace brazing. Today, brazing is used in a broad range of manufacturing systems including refrigeration and air-conditioning, household utensils, tubular structures, automotive parts, tools and machinery, electrical components, nautical and aerospace equipment, farm implants, and business machines.

Brazing Processes

Effective brazing heats the proper area of the base material to the optimal brazing temperature at the lowest possible cost. This involves not only the method of supplying heat, but the proper heating technique to ensure the best flow of filler metal. In this article, the emphasis will be on induction brazing applications and techniques. The unique characteristics of this brazing procedure have rapidly advanced its use in current metal fabricating applications, particularly because of its ready adaptation to production line methods.

Induction Method

Induction brazing is similar to resistance brazing in that heat is generated from resistance to the flow of electricity. It differs, however, in several ways:

• Most fluxes are acceptable with induction processing because heat is derived from electricity passing through coils rather than through base metals, which may be partially insulated from the flux;
• Induction heat can be applied selectively, allowing economical use with rather large assemblies, or when heating the entire unit is undesirable;
• Induction brazing temperatures are usually reached in a matter of seconds for higher manufacturing productivity.

The induction process uses magnetism to induce electrical resistance to heat the base metals. Ferrous metals, which are magnetic and have high electrical resistance, are heated more rapidly and easily than copper. All metals, however, can be successfully induction brazed. Recent years have seen the re-design of complex forgings and stampings to allow fabrication of parts produced by mass production techniques. The result of these new designs has been major cost reductions. In other instances, brazing has permitted the construction of assemblies too costly or complex to be produced by other techniques.

Induction heating has proven a valuable aid in these joining processes. It allows for rapid localized heating, joining high-strength components with minimum loss of strength. Precise heat control allows sequential brazing to be performed effectively. Induction's adaptability to production line methods allow strategic arrangement of the equipment in a assembly line and, if necessary, heating by remote electrical command and control. An example of this is the foot pedal control.

Process Elements

Brazing is more than merely melting a filler metal in a joint. The joint must be properly designed, and manufactured to tolerances that will permit a proper flow of the filler metal. The base metals must be thoroughly cleaned of oxides to provide adequate wetting, and then protected from additional oxidation. An appropriate filler metal must be selected and properly melted to ensure capillary flow. Induction brazing is an operation wherein the heat is obtained from resistance of the work to the flow of induced electric currents. For example, the work area is located in an electromagnetic field set up by an induction work coil through which high-frequency alternating currents flow during the heating cycle. These currents induce opposing currents in the work, which develop the heat by electrical resistance. Each job being brazed requires a source of high-frequency current and suitable induction work coils.

When the operation is set up, the induction machine controls the heating cycle and brazing becomes a push button operation. The heating operation usually requires only a few seconds, and the parts are brazed about as fast as they can become assembled. For this reason, induction brazing is well-suited to large production items, although it is also useful for special applications because of the local nature of its heating. Brazing alloys melt at high temperatures and provide high strength joints that can resist reasonably elevated temperatures without failure. The metals to be joined include carbon and alloy steels, stainless steel, cast iron, copper and copper alloys, nickel and nickel alloys and, to a limited extent, aluminum alloys.

Many of the alloys used for brazing are available in the form of wire, strip and powder. In addition, preforms of ductile alloys are available in the shape of washers or rings. Such preforms permit pre-assembly for automated operations and control the amount of alloy used, thus conserving alloy and producing uniform joints of good appearance. A preform should make good contact with the work to ensure melting as the proper temperature is reached. The alloy should be melted by contact with the work rather than by heat generated within itself.

Importance of Coil Design

The design of the induction coil must be adapted to the properties of the metals and the geometry of the parts being brazed. In making brazed joints by induction heating, special consideration must also be given to the heating pattern; the method of pre-placing the joining alloy; the tolerances between mating parts; the thermal conductivity; and the expansion characteristics of the material to be joined. Tests of base material tensile strength will typically show failure near the joint, rather than at or in the joint. The base material should, therefore, be selected considering its tensile strength, as well as weight, thermal or electrical conductivity, resistance to corrosive chemicals, and other properties dictated by its eventual use.

The geometry of the joint also plays a significant role in both strength and economy. All other factors being equal, a larger brazing surface area provides greater shear strength than a smaller surface, but at the expense of increased use of base and filler materials. The engineer's objective is to provide the smallest possible brazing surface to meet the strength requirements of the specific unit. As for the heating pattern, proper coil design is, again, of critical importance, ensuring that all areas adjacent to the joint are above the melting point, with the whole joint area preferably being at a uniform temperature. It is also desirable to confine the heat in a such a way that the joint area arrives at the joining temperature first to avoid improper flow of alloy to higher temperature areas away from the joint.

For best results in pre-placement of the joining alloy, preforms of the brazing alloy should not form closed loops when subject to inductive coupling from the work coil. It is also desirable, whenever possible, to electromagnetically shield the pre-placed alloy with the components to be joined to avoid melting the alloy before joint surfaces are at joining temperature. This can be done by placing the preforms inside the assembly or by recessing a component. Clearance between members of the joint determines the thickness of the alloy film that will be formed between the parts during the brazing, and has an important influence on the joint strength. If maximum strength is to be obtained, clearance in the joint must be large enough to permit the entry of the molten alloy into this space and the escape from the space of molten flux and gases developed during the heating.

Ideal clearances for production work are .002 to .005 inches, with clearances up to .006 or .008 inches being acceptable in many cases. Joints with clearances below .001 inches and above .008 inches should be avoided if possible, not only because of poorer joint strength, but because of the expense of making such joints. When determining clearances to use for brazing parts of dissimilar metals, the thermal expansion of the parts must be considered and sufficient clearance allowed, so that at brazing temperatures there will be a clearance in the joint for entrance of the alloy.

In general, brazing designs should consider a combination of strengths of the components being joined, along with the strength of the joining metal alone. To achieve maximum strength, brazed joints should also be designed with large shear areas rather than as butt joints. Finally, simple inspection of the joint is recommended in the pre-placement of the joining alloy. This can be accomplished, in many instances, by pre-placing the joining alloy in such a way that it will flow into the joint by gravity and capillary attraction, showing uniform and complete penetration by appearance of the alloy at the opposite end of the joint.