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Brazing-joint-designPlanned for Best Brazing Results.SOLUTIONS with Effective, Practical Advice
One should not attempt to adapt Brazing-joint-design 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.
An assembly joined by brazing is called a brazement. Brazement design comprises specification of base metals and their condition, design variables including stress distribution, filler metal, brazing method, process parameters, cleaning, heat treatments, testing and inspection. In principle, singling out the geometrical details of joint design, as if they were independent and unrelated (they are not) to the whole picture of the art of Brazing, seems to defeat the purpose. But this must be understood: it is only a didactical (for teaching purposes) useful trick, to break down the mass of knowledge in manageable chunks, to be addressed and assimilated in sequence. In this exposition on Brazing-joint-design, therefore, only the specific items referring to the joint shape and dimensions will be addressed in some detail, leaving out all other items listed above, to be treated elsewhere. Geometrical Brazing-joint-design is only one variable in the complex task of designing a physical assembly capable of performing its functions successfully for the required life time, while all the economic constraints are taken into account in order to maximize the value and minimize the cost. Therefore it is essential that the designer be acquainted with the characteristics that good design will guarantee, as well as with the failures that some avoidable design details may cause. Until the designer has such a developed experience that no additional input is needed, it is recommended to prepare a set of informative material arranged in logical sequence for fast retrieval, and to refer to it often. The purpose is to refresh the memory of suggested joint configurations as recommended by the collective wisdom of renowned experts. In a previous page on Brazing this class of processes was introduced outlining the main characteristics that make it so important while presenting both advantages and limitations.
Here we intend to show the principles of Brazing-joint-design and the main requirements. Any joint should be able to transmit without failure the forces operating in service. Brazed joints exhibit their maximum strength when the brazing filler material is stressed in shear, that is in a direction parallel to the surface it covers. Therefore one must avoid any Brazing-joint-design where tensile forces act in perpendicular direction to the brazed surfaces. As with any other design, one must be aware of the fact that the joint itself introduces sharp changes in material cross section, where dangerous stress concentration may initiate failures, especially under dynamic conditions. Several recommended designs introduce progressive thinning in the edges of the base metals elements, in order to provide gradual stressing of the joint, instead of stress concentration. While a rigid construction can be adequate for static stressing, one should remember that to prevent fatigue failure under dynamic conditions a certain flexibility should be not only tolerated but introduced on purpose. The properties of the base materials, assuming they are readily brazeable, can be affected by the temperature and by the heating method used for brazing. Therefore the designer must be alert to their consequences on the load carrying capacity of the brazed materials. The Brazing-joint-design must include, at brazing temperature, a capillary clearance 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 length while leaving between them the all important gap of controlled capillary size. The limitation of this design is the creation of stress concentration at both ends because of the abrupt cross section change. Introducing a tapered shape to the ends, instead of their blunt section, is highly recommended to reduce adverse effects. 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. To obtain this result even in the event of occasional voids or inclusions in the filler metal, the overlapping length can be increased, if necessary. The strength of the brazed joint rises rapidly from very thin joints up to a maximum at the optimal gap and then falls substantially at increasing thickness. As a first guideline in Brazing-joint-design one can assume that a suitable capillary clearance be between 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 clearance 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 tends to crack, an inadmissible situation. An Article on Advanced Microwave Brazing was published (2) in Issue 161 of Practical Welding Letter for January 2017. An Article on Sinter Brazing was published (3) in Issue 166 of Practical Welding Letter for June 2017. An Article on Resistance Brazing Filler Metal was published (4) in Issue 167 of Practical Welding Letter for July 2017.
Anyone having professional interest in Brazing in all its aspects and interested readers are directed, for reference and for comprehensive study of the matter, to the following authoritative publications, that should hold a honorable place on their bookshelf.
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