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Bellman Melcor

How does the tensile strength of a brazed joint vary with the amount of clearance between the parts being joined?

Because brazing employs the principle of capillary action, the distance between the base metals must be appropriate for capillary action to effectively distribute the molten brazing alloy filler metal between their surfaces. Remember – the strength of a properly brazed joint should exceed that of the base metals being joined. Almost without exception, this means a brazed joint must be designed with a close clearance. To create the strongest brazed joint, clearance should be .0015″. If the joint clearance is tighter, the brazing alloy filler metal may not flow through well enough to form a strong brazed joint. Conversely, if the joint is too wide, the brazing alloy filler metal will not completely fill the joint, and the strength will likely be no greater than that of the brazing alloy filler metal itself. However, satisfactory capillary action can occur over a range of clearances, so all designs need not be this precise; clearances from .001″ to .005″ produce joints with 100,000 psi tensile strength. In practical application, an easy slip fit on tubular parts or, in the case of a “lap” joint, simply resting one flat part on top of another is generally enough to achieve good clearance for adequate brazed joint strength.


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