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Voids in Brazed Joints

tube braze2-smBrazers commonly encounter voids in brazed joints and often wonder where they come from and how to avoid them in future brazements. Some common sources of voids in braze joints are: surface contamination, base metal and brazing filler-metal (BFM) constituents, brazing methods/temperatures used and poor joint fitup. The first three items listed above can often result in gas bubbles being formed in brazed joints. Such gas-bubble voids will usually try to form in spherical shape as they move through a joint. The "rounded" edges of such bubble-voids can often be clearly seen in cross-section photomicrographs of brazed joints, especially under high magnification. The inside surfaces of a bubble-void will often appear "clean" or "shiny" as well. By Dan Kay

Last Updated on Sunday, 02 February 2014 22:32

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Braze Joint Design Considerations - Part 2

joint-magnified-smFor successful brazing to occur, the joints to be brazed have to be designed properly, and then properly manufactured to attain and maintain those shapes and dimensions. This second article looks at joint clearance considerations in more detail. Joint clearances must be close together and parallel. The amount of clearance between the faying surfaces (the mating surfaces inside a joint being brazed) should ideally be kept small, on the order of about 0.000"-- 0.002" (0.000-0.050 mm) total, so that capillary action can most effectively pull the molten brazing filler metal (BFM) completely into and throughout a braze-joint. By Dan Kay

Next Month: In next month's article we'll address some additional factors in joint design, specifically the topic of "differential metal expansion". All metals expand at different rates when heated, and since braze-joint clearances are calculated based on expected clearances at brazing temperature, we need to know how to properly optimize brazing of different metals in the same assembly.

Last Updated on Sunday, 02 February 2014 21:59

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Braze Joint Design Considerations

butt_lap_joints_xsmThere are basically two types of joint designs used in brazing:  butt-joints and lap-joints.  All other joint designs are modifications of these two.  There are a number of important considerations when designing such joints in order to insure proper service life.  This article looks at just a few of those considerations. By Dan Kay

Next Month: Next month I'll discuss the issue of braze gap clearance for some different base-metal and brazing filler metal (BFM) combinations, and how that affects joint strength and hermeticity.  In succeeding months we'll address issues such as dissimilar metal brazing and how that affects joint design.

Last Updated on Sunday, 02 February 2014 21:57

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How to Determine Braze Gap-Clearances for Your Furnace

vcts-after_xsmGood brazing depends on the ability of capillary action to draw the molten brazing filler metal (BFM) in all directions throughout the joint being brazed, either vertically or horizontally.  A unique Vertical Capillary Test Specimen (VCTS) was developed to help brazers find out the maximum gap-thicknesses that capillary action can fill in their particular brazing furnaces. By Dan Kay

Next Month: Next month I'll discuss some important design issues regarding braze gap clearances and configurations, and in succeeding months we'll address issues of dissimilar metal brazing, and joint strength and its optimization in more detail.

Last Updated on Sunday, 02 February 2014 22:18

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Are the use of dead-weights for fixturing a recommended practice?

fixture_fig_2_smmThe effective use of "metallurgical fixturing", instead of a lot of dead-weights, to effectively "load" parts with enough pressure to keep braze joints close together for effective brazing is described in detail.

The use of heavy weights on top of parts being brazed is a common practice. Its purpose is to load the top of the assembly with enough weight so as to insure that the components of the assembly will be pressed together sufficiently to keep the joints from opening up during furnace brazing. This should then insure that good capillary action of the brazing filler metal (BFM) into those joints will occur during the furnace cycle.  But in real-life brazing, such use of dead-weights can lead to extended cycle times, and fail to so what it was supposed to do.  This article explains why. By Dan Kay

Last Updated on Sunday, 02 February 2014 21:58

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Partial Pressure Brazing

mass-furnace-door-openAs mentioned in previous articles, more and more brazing shops are using vacuum furnaces.  These furnaces are quite complex, offering more options for heating, cooling, partial pressure, or multi-bar pressure (pressure capabilities above one atmosphere) for high-speed cooling.

The overall effectiveness of the equipment, however, still lies with the people who program and run the furnaces. Figure 1 (below) shows a series of vapor pressure curves for a number of common metallic elements.  Each curve shows the melting point of the element (indicated by a small circle along the curve) and, where the curve crosses the dotted line across the top of the chart, its approximate boiling point (where it wants to turn to a gas) at one standard atmosphere. By Dan Kay

Last Updated on Sunday, 02 February 2014 22:20

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Selecting Brazing Fixture Materials

select_brz_fix_mat_smWhich base-metal should I use for braze fixturing so that it will last the longest?

This question is not an uncommon one. Although I have never personally seen any kind of chart showing an "expected life" for fixture materials, it is important that people understand that there are a number of factors that will control the "life expectancy" of any fixturing material used in brazing, and all of these factors relate to the service conditions that the fixtures will encounter during the brazing process. By Dan Kay.

In August's article, I'll address the commonly used method of adding a lot of "dead weight" onto parts in an attempt to keep them flat during brazing!

Last Updated on Sunday, 02 February 2014 22:23

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