Brazing-joint-design

SOLUTIONS with Effective, Practical Advice

Good Brazing-joint-design is essential to successful process application.One should not attempt to braze joints that were conceived for different processes like welding or soldering, unless they are redesigned with brazing requirements in mind.

Brazing can be the perfect and most economic process for joining suitable applications, if all factors are duly accounted for in design and planning.

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In a previous page on Brazing this class of processes was introduced outlining the main characteristics that make it so important and presenting both advantages and limitations.

See also our new page on Brazing Heating.

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Here we intend to explain thoroughly the principles of Brazing-joint-design with the help of useful easily accessible References. May we suggest to our interested readers to create in their computer a special folder on Design for Brazing, where they will collect all pertinent information for easy consulting.

But first let us review in short the main requirement.
The brazing joint must present, at brazing temperature, a capillary gap capable of drawing or of holding the liquid, molten filler metal which has to wet the surfaces being brazed.

Successful Brazing-joint-design is usually obtained by overlapping the faying surfaces of the two elements for a substantial amount while leaving between them the all important capillary gap of controlled size.

Therefore butt joints are ruled out because the thickness is generally too thin to allow sufficient surface and because the stress direction may be unfavorable to the joint resistance.

The Brazing-joint-design purpose is to make sure that, upon stressing the assembly beyond its load capacity, the failure be in the base metal, not in the joint.

If the strength of the brazed joint is an issue, one should remember that the graph of Tensile Strength of a simple lap joint as a function of the Brazed Joint Thickness shows a characteristic shape.

The strength rises rapidly from very thin joints up to a maximum at the optimal gap and then falls substantially at increasing thickness. A graph illustrating this behavior is shown in one of the pages of the second reference down this page.

Designers are responsible for establishing suitable tolerances for the elements, so that the required capillary gap will be present at brazing temperature.

The problem of establishing correct capillary gap may become critical when designing a brazed joint for two concentric elements of dissimilar materials with different coefficients of thermal expansion.

If the element with higher thermal expansion is on the outside, to get proper clearance at brazing temperature one would need a force fit at room temperature. The joint will be in compression upon cooling, a favorable situation.

Conversely, if the higher thermal expansion material were on the inside, the gap would tend to close upon heating, but the worse would happen when cooling, as the braze material in the joint, pulled by the shrinking bushing will tend to crack, an inadmissible situation.

The articles on Brazing-joint-design, reachable from References 4), 5) below discuss in deeper detail the influence of unlike coefficients of thermal expansion. Be aware of the unusual shrinking upon heating in certain materials.

Reference 6) reports how concentricity in assembly was obtained, in a certain case, by resorting to knurling.

The following References propose examples of good Brazing-joint-design.

1) - Design of Brazed Joints.

2) - The Brazing Book Online.
From the Introductory page above click on the right arrow in the page to reach
The Brazing Book Index
The third line of Section 1 is titled:
The Principles of Joint Design
Click on this link in the page just opened and see hyperlinked titles of six new pages:
Click on each one of them and save for further study.
The thickness/strength graph mentioned above is in the first page reached by clicking on "The six basic steps in brazing" in the Index page above.

3) - Joint Design Considerations.

4) - Differential Metal Expansion, Part 1.

5) - Differential Metal Expansion, Part 2

6) - Knurling.

7) - Principles of Brazing Technology.

A short Section on
Joint Design (page 532) is included in Chapter 12 on Brazing in the

AWS WHB-2.9
WELDING HANDBOOK VOLUME 2 -
PART 1: WELDING PROCESSES (AWS WHB V2)
American Welding Society, 28-Apr-2004, 720 pages
Click to Order.

See also:

ANSI/AWS C3.3:2002
Recommended Practices for Design, Manufacture, and Inspection of Critical Brazed Components
American Welding Society, 01-Jan-2002, 32 pages
Click to Order.

ANSI/AWS C3.2M/C3.2:2001
Standard Methods for Evaluating the Strength of Brazed Joints In Shear
American Welding Society, 17-Dec-2001, 32 pages
Click to Order.

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Successful Brazing applications can be only as good as the Brazing-joint-design was in the first place. Therefore whoever is involved with Brazing should be familiar with the essential principles of good joint design for brazing.