Principles of brazing technology - technicalmaterials.umicore.com

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Principles of brazing technology Start of the wetting Intermediate state II = Workpiece 1 II = Workpiece 2 Both bare metal, and heated to working temperature • Liquid brazing alloy Capillary filling pressure Final state The brazing alloy is pressed into the gap by capillary forces. The narrower the brazing gap, the higher the capillary filling pressure. For a 0.1 mm parallel gap, the capillary filling pressure reaches ca. 100 mbar, corresponding to about 0.1 Atm. This in turn corresponds to about a 1 m column of water ( ρ = 1); assuming ρ = 10 g/cm 3 (the density of a brazing alloy), the capillary height for low melting point brazing alloys in a 0.1 mm wide gap can be calculated to be ca. 10 cm. This agrees reasonably well with experiences in practice. Gap too narrow Correct gap width (only applies for brazing with flux) Permissible gap width for manual brazing Gap too wide Different gap cross-sections give different filling pressures. An open fillet has a six times higher capillary filling pressure than a parallel flat gap. 3. Brazing alloy and flux groups 3.1 Brazing alloys According to DIN 8505, alloys with a liquidus temperature below 450°C are solders and those with a liquidus temperature above 450°C are brazing alloys. The upper and lower brazing temperature limits are determined by the following: lower limit - the working temperature upper limit - the flux (becomes saturated with oxides at too high temperatures), or - the brazing alloy (individual components of the alloy can evaporate), or - the economics of the process (unnecessarily high temperatures cost unnecessary time and energy), or - the base material (structural transformation; strength loss). Umicore AG & Co. KG - BrazeTec, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang Telefon: +49 (0) 6181-59-03 Telefax: +49 (0) 6181-59-3107 Email: info@BrazeTec.de Internet: www.BrazeTec.de
Principles of brazing technology 3.2 Fluxes Fluxes are solvents for metal oxides. They have an effective temperature range within which they are able to dissolve metal oxides. The solvating capacity of the flux is limited. About 5% of the weight of flux in metal oxides can be dissolved. If oxides are present in greater amounts, the flux saturates and it loses its functionality. Atmospheric oxygene Flux Oxide More than 5 min. The hygroscopic flux residues must be removed by scouring in water or by pickling in pickling baths suitable for the base materials. Ultrasound aids the removal of these flux residues. The non-hygroscopic flux residues do not have to be removed for fear of corrosion. If they need to be removed for other reasons (e.g. to paint the components), they are usually removed by mechanical means (e.g. sand-blasting). Too little Sufficient Oxide film remains Oxide film is dissolved The solvating capacity of modern brazing fluxes for use at low temperatures (i.e. between 600 and 800°C) for common heavy metal oxides is between 1 and 5%, meaning that it is limited. That means that relative to the oxide which is present a relatively large amount of flux must be available, and ultimately so in the molten state, otherwise sound brazing work is not achieved. Extremely narrow gaps, e.g. less than 0.02 mm, hence cause problems because there is an inadequate amount of flux in the gap. This naturally has a big effect on the soundness of the brazed joint. Wire of brazing alloy Umicore AG & Co. KG - BrazeTec, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang Telefon: +49 (0) 6181-59-03 Telefax: +49 (0) 6181-59-3107 Email: info@BrazeTec.de Internet: www.BrazeTec.de Flux For surface brazing, the high capillary filling pressure in the open fillets leads to running of the brazing alloy on the external sides. The supply of brazing alloy to the narrow surface gap is reduced; increased inclusion of flux hence occurs. Brazing alloy Flux Increased flux inclusion can be avoided by inserted brazing alloy sheet. Gaseous fluxes Joints (V-seams and fillets) can be brazed using gaseous fluxes. For brazing gaps – especially for gaps having larger depths and small widths – the use of gaseous fluxes is not recommended because the flame does not penetrate into the capillary gap. When using flux pastes, their working life can be significantly prolonged by the additional use of gaseous fluxes. The effective temperature of gaseous fluxes extends from about 750°C to 1100°C.
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