Aluminium brazing

Aluminium brazing
Applications
Brazed aluminium heat exchangers are
largely utilised in modern automobiles
for all major engine cooling and climate
control systems – such as condensers,
evaporators, radiators, oil coolers and
charge air coolers. In addition, various heat
exchangers are brazed for home appliances
and industrial applications.
Basics about brazing
Brazing is a thermal process for joining two
pieces of metal or metal alloys.
Brazing by definition takes place above
450 °C.
Industrial aluminium brazing allows joining
of alloys having a melting temperature
well above 600 °C with a low melting AlSi
filler metal (7 to 13 wt.% Si), as shown
schematically in fig. 1.
• The filler metal starts to melt at about the
(AlSi) eutectic temperature of 577 °C
• The melting point of the base metal of
the components is at least 30 to 40 °C
higher than that of the filler metal, in
order to assure that only the filler metal
melts during the brazing process
• The filler metal is normally placed
between or adjacent to the component
surfaces to be joined
• The assembly is heated to a process temperature
(e.g. 600 °C) at which the filler
metal melts while the base metal of the
components remains unmelted
• The molten metal fills the gap by capillary
action
• Upon subsequent cooling, the filler alloy
solidifies and forms a fillet / joint
Fig. 2 illustrates a cross-section of a brazed
aluminium joint.
Brazing techniques
The most common brazing methods for
aluminium heat exchangers are:
• Controlled atmosphere brazing (CAB)
with non-corrosive flux
• Vacuum brazing (VB)
• Salt bath brazing
The CAB process became the most important
of these during the nineties.
Brazing cycle
• The time-temperature cycle has to be
carefully adjusted, because of its direct
influence on the final products
• The temperature in the furnace must be
homogeneous to ensure that melting of
the braze filler metal starts everywhere at
the same time
Fig. 3 shows a typical brazing cycle, and its
steps, for aluminium brazing.
Fig. 1
Fig. 2
Fig. 3
Base metal
Base metal
Temperature
Brazing temperature
Heat penetration
temperature
Heat up time
Filler
metal
(cladding)
Heat
penetration
time
Brazing time
Resolidified
filler metal
(cladding)
Brazing
400 µm
Dwell
time
Total time
Fillet
or
joint
Base metal
Base metal
Working temperature
Time
Cooling
time

Contact:
Günter K. Rybasch
Hydro Aluminium
Precision Tubing Tønder a.s
Hydrovej 6
DK-6270 Tønder
Denmark
Tel: +45 7472 0304
Mobile: +45 4017 2219
Fax: +45 7472 3313
E-mail: gunter.k.rybasch@hydro.com
Kevin Hartley
Hydro Ellay Enfield Ltd.
Joseph Noble Rd.
Workington
Cumbria
CA14 4JX UK
United Kingdom
Tel: +44 1900 601166
Mobile: +44 07711 723078
E-mail: kevin.hartley@hydro.com
www.hydro.com/HVACR-service-center
Bernd Gerdautzki
Hydro Aluminium Rolled
Products GmbH
Aluminiumstrasse
21129 Hamburg
Germany
Tel: +49 40 740-11747
Fax: +49 40 740-2989
Mobile: +49 170 2247885
E-mail: bernd.gerdautzki@hydro.com