imagesIt is erroneously believed by many that brazing filler metal (BFM) pastes need to be refrigerated prior to use, or for long-term storage. This has led to misunderstandings and difficulties in the handling and use of BFM pastes in many shops, and to incorrect, misleading product labels.

Having been involved in the manufacture and testing of brazing pastes for many years, I will categorically state that there is nothing inherent in the chemistry of brazing pastes that requires their refrigeration! Some industry specs (and some cartridge labels) still state: “This paste must be refrigerated prior to use”. This very misleading statement has caused some people to believe that the paste must be cold when used in order to perform properly. This is completely wrong! Cold brazing-paste is much stiffer and harder to extrude (which can also be dangerous), and serves no useful purpose. by Dan kay

figure-1-wcMost brazing shops today will occasionally be given “rush” jobs by their good customers, and these brazing shops will usually do their best to respond as fast as they can, so that the customer will remain one of their best customers! No-one wants to do anything that would turn a good customer into an unhappy customer! by Dan kay

danger-sign wsArgon is a favorite gas used in many vacuum brazing shops, since it is an inert gas that will not react with any of the metals being heat-treated or brazed in those vacuum furnaces. Thus, dry argon (as measured by a dewpoint meter right at the furnace) is often used for partial-pressure brazing applications, or for rapid-cooling needs, or merely as a gaseous atmosphere to allow better conduction of heat between components inside the furnace. But argon can also be dangerous, and even lethal!

Argon is an odorless, colorless, tasteless gas, and because it is heavier than air it will flow to the lowest spot in your shop floor, often down into holes or pits built into your shop floor. Many companies build those pits in their shop floors so that equipment can be lowered down into them, thus eliminating the need to add height to the ceilings of the buildings. By Dan Kay

fig 4 wsJoint clearances must be tight for effective Ni-brazing. 1. Nickel-based brazing filler metals (BFM) can leave a hard, non-ductile eutectic phase in the middle of a brazed joint.

The hard, non-ductile metallurgical phase-structures that form upon solidification of Ni-brazed joints must be carefully controlled, or else they can, and will, result in cracks inside the joint in stressful mechanical or thermal-cycling service. By Dan Kay

bonding-zone ws-2Brazing is a highly versatile joining technique that can be used to join many different types of metals, and can even be used to permanently bond engineered-ceramic materials, such as alumina, to a variety of metals. It is being done everyday in industry.

Alumina, which consists of aluminum-oxide powder granules imbedded in a glassy matrix binder system of calcium-oxide and silicon-dioxide (among others), can be joined to ceramic or metal structures primarily by two different methods, as shown in Fig. 1. By Dan Kay

repairing-cracks-comp lgWide Gap Brazing when Parts don't Fit Together well for Brazing - A common occurrence (unfortunately) in the brazing world is the need to join two parts together by brazing in which the brazing gap is too large, i.e., in the range of 0.010-inches (0.25 mm) or larger. Ideal brazing clearances should be in the area of 0.000-inches to 0.005-inches (0.00mm to 0.125mm) maximum for most brazing filler metals (BFMs).
Brazing depends on capillary action to draw the liquid BFM into the brazing joint, and tight clearances are needed for best brazing to occur. If the BFM is pre-placed in the brazing joint prior to assembly of the parts then capillary action is not a major factor since the BFM will melt in-situ and join the two members without the need for flowing any distance through tight capillary spaces. By Dan Kay

Image_6-wsWhy does Brazing require Temperatures above 450C (840F)? Brazing, when performed correctly, is a joining process that produces a permanent bond between two or more materials by heating them to a temperature above 450C (840F), but lower than the melting-temperature of any of the materials being joined, and a permanent, metallurgical bond between these materials is produced when capillary action draws a molten brazing filler metal (BFM) through the clean, closely fitted faying surfaces of the joint.

The filler metal is not supposed to become fully liquid (i.e., have a "liquidus") until the brazing temperature reaches at least 450C (840F). If the liquidus of the filler metal is below 450C (840F) then that filler metal would commonly be called a "soldering alloy". People often wonder about the temperatures used to differentiate brazing from soldering. Why 450C (840F)? Is there some significance to these "exact" numbers? By Dan Kay

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