Brazing-appropriate design

The design of a component is dependent on many factors. If a component is manufactured using a brazing process, brazing-specific factors should also be considered which are primarily triggered by the selection of the filler metal to be used and the brazing process itself.

In this regard, the following considerations should be taken in the order listed below: [1, 2]:

  1. Clarification of operational demands (stress types, stress dimensions, stress direction, media, temperatures)
  2. Determination of base materials
  3. Selection of filler metal and brazing process (please also refer to “The foundations of brazing” and “Brazing processes”)
  4. Determination of the brazing gap widths in dependency on the filler metal and brazing process

Brazing gap widths

The gap widths listed in Table 1 are recommended for different brazing methods [1]. Depending on the selected filler metal these can also be determined separately in the listed ranges. It must be noted that these gap widths must be upheld when at brazing temperature. The brazing gap when at brazing temperature is different to the brazing gap at room temperature when using different base materials for a brazing joint. This is attributable to the different thermal expansion coefficients of the base materials.

Brazing process Gap width [mm]
Vacuum brazing 0 – 0,1
Protective gas brazing 0 – 0,2
Flux brazing,
automatic
0,05 – 0,2
Flux brazing, manual 0,05 – 0,5
Table 1: Gap widths in dependency on the brazing process

Dimensioning of brazing joints

Brazed joints should be as durable as the base material itself. The calculation of the brazed joint is done, as a rule, according to the strength of materials. Generally speaking, the following strengths for appropriately designed and executed brazed joints with a gap filling level of 80% [3, 4, 5] can be expected:

Tensile strength: 200 MPa (N/mm2)
Shear strength: 100 MPa (N/mm2)

Detailed information on strengths are contained in part in the delivery programs and technical data sheets of filler metal suppliers. Separate inspections are to be carried out as required.

In order to achieve the base material strength with lap joints, the lap length should equal three- to six times the thinnest sheet thickness of the work piece to be brazed. The minimum value, three times the wall thickness, is selected for work pieces with relatively low residual strength (e.g. copper). The maximum value, six times the wall thickness, applies to high-strength steels [6]. Moreover, nomograms are used for determining the optimal lap length [1].

 

Bild: Beispiele von Überlappverbindungen
Image 1: Examples of lap joints

Construction rules

The following construction rules apply in particular for brazing:

  • Provide parallel-walled abutting edges (please see image 2)
  • the filler metal must retain contact with the brazing gap when melting
  • filler metal flow paths are to be kept as short as possible
  • to the greatest extent possible, work pieces are to be designed as self-positioning
  • changes to the dimensions caused by heat expansion (gap widths according to table 1 must be upheld when at brazing temperature) should be taken into consideration
  • enable the switching of position of filler metal and flux (avoid the flux inclusions)
  • avoid abrupt cross-sectional changes at brazing joints which are subject to high levels of stress
  • keep the deburring chamfer small and flat, so they can be filled up with the filling metal

The design rules listed here are explained in extensive detail in the literature sections 1, 2, 4 and 6, and on the basis of construction examples. Furthermore, German Industry Standard DIN 65169 [7] and European Standard EN 14324 [8] also focus on the design of brazing joints.

 

Bild: Beispiele für schlechte (links) und verbesserte (rechts) Stoßkantenausführung [2]
Image 2: Examples of bad (left) and improved (right) abutting edge execution [2]

Literature

[1] Design of brazed joints – 

Publication of Umicore AG & Co. KG
[2] Hartlöten – Eine Einführung

Erarbeitet vom Arbeitskreis „Schulungsunterlagen“ und der Arbeitsgruppe V6.1 „Hartlöten“ im Ausschuss für Technik des DVS

Herausgegeben von der Fachgesellschaft „Löten“ im DVS

ISBN 978-3-87155-839-9, DVS Media GmbH, Düsseldorf
[3] Selection Rules of Brazing Alloys and Fluxes 

Publication of Umicore AG & Co. KG
[4] Hart- und Hochtemperaturlöten

Die Schweißtechnische Praxis, Band 20

Paul Zaremba

DVS Verlag GmbH Düsseldorf, 1988
[5] Technik die verbindet Nr. 36

Veröffentlichung der Degussa AG
[6] Technik die verbindet Nr. 9

Veröffentlichung der Degussa AG
[7] DIN 65169

Luft- und Raumfahrt

- Hart- und hochtemperaturgelötete Bauteile -

Konstruktionsrichtlinien
[8] EN 14324
 Brazing -

Guidance on the application of brazed joints