Induction Heating Consultant

Induction Heating

Stanley Zinn

Stanley Zinn • Induction Consultants • Tel: 585-737-8824
15307 Strathearn Drive, Unit 11202, Delray Beach FL 33446 •
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Stanley Zinn

Induction Brazing and Soldering    View as PDF

by STANLEY ZINN, Induction Heating Consultants • June 2010


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Differential Brazing

In many applications, adjacent components must be assembled. Yet, physical location may preclude simultaneous brazing of these parts. In addition, close proximity requires that the heating of one part does not influence the quality of the braze on adjacent parts. This is generally handled by using alloys with different melting points at each joint. Starting with the highest melting point alloy, succeeding brazes are made with lower melting point alloys.

Fixturing
At this point, fixturing of the assembly should be considered. The result of the joining operation is to produce a part whose characteristics (length, angularity, size, etc.) match a specific requirement. The purpose of the fixturing therefore is to hold these parts in a manner that will assure those characteristics until the brazing is accomplished.

Parts can be designed to be self-fixtured. Dependent on the components and their tolerances, some assemblies will hold relative position easily. If the shape and weight of the parts permit, the simplest way to hold parts together is by gravity. Another technique would be the possible use of spot welding to hold the parts in place. However, it should be noted that the welded area will not get alloy flow and therefore, this should not be included when considering required shear area.

Where self-fixturing is not feasible, consideration should be given to ease of assembly in the fixture, as well as determining if the completed assembly may be easily removed after joining.

Parts near the coil should be made of insulating materials compatible with the temperatures to which they will be exposed. Materials used in the fixture should be poor heat conductors, such as stainless steel, Inconel or ceramics. Poor conductors will pull less heat away from the joint area. In some cases, however, generally due to tolerances, metal must be used. It is important that the metal components be made of alloys that do not “wet” to the brazing alloy resulting in the part being brazed to the fixture. Titanium is often used for this type of locator. Where stainless steel must be used, 300 series non-magnetic stainless would be the material of choice.

Clamps of various types can be used to locate the assembly components and hold them in place during the brazing cycle. One concern is the heat transfer from the assembly being brazed to metal components of the fixture. Where these must be in contact, heat transfer can be minimized by using point or knife-edge contact with the part being heated. The comparative expansion rates of the components being joined and the metal holding fixture should also be considered so that misalignment can be minimized and further, allow parts to be easily removed from the fixture. Finally, attention should be paid to placement of the coil relative to the fixture. The coil should be rigidly mounted to the fixture to assure that it is always in the same position from assembly to assembly. Threaded studs or pads on the coil, mounted to the insulating components of the fixture, will assure repetitive joints from part to part.

In soldering and brazing operations, because of the splatter caused by boiling flux, the fixture components may become coated with the flux. This can prevent the easy and accurate assembly of subsequent parts. So long as the area being coated is not directly in contact with the joint itself, a coating of Vaseline or similar material should be applied to the fixture. When flux build-up appears to be a problem, the area can be easily wiped down and a new coating of Vaseline applied.

In some cases, it will not be possible to assemble the components while the joint area is within the coil. Accordingly, provision must be made to raise or lower the part, after assembly in the fixture, so that the area to be heated is within the coil. In some instances, it is simpler to raise or lower the coil itself into the proper location.

Jigs can be used to fulfill more than a brazing function. For example, it can serve as a heat sink or as a heat source. Graphite is often favored as a fixture material. It is inexpensive, easy to machine, a good thermal conductor and absorber of heat, and is not wetted by the majority of molten filler metals. Where parts must be slow cooled subsequent to brazing, as with ceramics, the graphite acts as a heat sink to modify the cooling time. Graphite also has the merit of absorbing trace oxygen in an oxidizing atmosphere to form CO and CO2.

Dependent upon the fixturing required, brazing can be accomplished with rotary tables or in-line conveyor systems. These systems can be either continuous or indexing types.

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