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.
What is the coefficient of expansion?
Also known as the “coefficient of thermal expansion”, this means that when designing a joint for brazing, one should consider the fact that when the metals being joined are heated to proper brazing temperatures, they can expand! Find out by how much your base metals expand and design your braze joint appropriately.
How much overlap should there be on a good lap joint?
As a general rule, design the lap joint to be three times as long as the thickness of the thinner joint member. Longer laps may waste brazing alloy filler metal (and money) that doesn’t translate into a stronger joint. A shorter lap will result in decreased joint strength. If you are unsure and brazed joint strength is critical, contact Bellman-Melcor and we can help you design a proper lap joint using a special formula that mathematically accounts for all factors.
How do I ensure against voids in brazed joints?
The key to any good braze joint is a tightly controlled process -follow the same good brazing practices on every brazed assembly every time. This means each joint should be properly designed with good clearance, all metals should be clean and free of contaminates, braze assemblies should be aligned or otherwise fixtured so that proper capillary action can occur, metals should be properly fluxed, compatible brazing alloy filler metal should be applied in the precise minimum volume required, the assembly should be heated in a way that draws the molten brazing alloy into the joint in order to achieve a proper seal (never direct heat onto the surface of a joint!), and always remove residue left behind by corrosive fluxes to prevent post-braze corrosion.
What are brazing fillets and how can they help my braze joints?
A fillet is nothing more than solidification of molten braze alloy filler metal on the external part of a braze joint. They are an expected outcome of the brazing process. Fillets do not add to the strength of brazed assemblies. However, a brazing fillet can tell a lot about the brazed joint itself. First and foremost, brazing fillets should be small and concave. If your brazing fillet is raised and otherwise large, it is likely an indication of one or more of the following factors: the brazing alloy filler metal lacks good compatibility with the base metals, braze joint contamination, inhibited flow of braze alloy filler metal due to incorrect temperature or clearance. A brazing fillet should simply indicate that your brazing alloy has flowed adequately around your properly designed assembly.