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APPLICATION-ORIENTED TECHNOLOGIES
Soldering and brazing <strong>of</strong> <strong>aluminium</strong> and its alloys
Christian Eisenbeis, SLV Duisburg
The joining <strong>of</strong> <strong>aluminium</strong> always requires
<strong>the</strong> use <strong>of</strong> special technologies. In particular,
soldering and brazing techniques
need special experience in order to obtain
joints <strong>of</strong> good quality and sufficient
strength. In addition, <strong>the</strong>re is <strong>of</strong>ten a
good chance to use <strong>the</strong> brazing technologies
for an efficient and highly mechanized
fabrication process. Compared with
welding, soldering and brazing allow
appropriate higher or lower process-temperatures,
depending on <strong>the</strong> type <strong>of</strong> <strong>the</strong>
<strong>aluminium</strong> base metal. The market <strong>of</strong>fers
new and improved filler metals and fluxes,
and – at <strong>the</strong> same time – <strong>the</strong> demands
on quality and strength <strong>of</strong> <strong>the</strong> brazed <strong>aluminium</strong>
joints have increased. Traditional
and highly developed procedures like
flame brazing, resistant brazing, or controlled-arc-brazing
are available ei<strong>the</strong>r
for simple or sophisticated brazed components
<strong>of</strong> <strong>aluminium</strong>. In this presentation
<strong>the</strong> basics and <strong>the</strong> technical solutions will
be explained and illustrated by figures
and practical examples.
1. The brazing process
For soldering and brazing unlike welding an
additional brazing filler metal is used whose
melting temperature (or melting range) is
below <strong>the</strong> melting temperature <strong>of</strong> <strong>the</strong> parent
metal. The joining surfaces <strong>of</strong> <strong>the</strong> parent metals
are wetted by <strong>the</strong> liquid braze without being
molten <strong>the</strong>mselves.
As a result, <strong>the</strong> following aspects are <strong>of</strong>
importance:
• It is possible to join dissimilar metals and
metal alloys, respectively
• Due to <strong>the</strong> relatively low process tempera-
ture <strong>the</strong> component is less <strong>the</strong>rmally
influenced
• The amount <strong>of</strong> filler metal is relatively
small
• The rapid sequence <strong>of</strong> <strong>the</strong> brazing process
• Low distortion compared with welding
• Brazing/soldering <strong>of</strong> a variety <strong>of</strong> areas to
be joined at <strong>the</strong> same time.
The joint is built up by <strong>the</strong> exchange <strong>of</strong> diffusion
<strong>of</strong> <strong>the</strong> metals within a small zone (Fig.
1). To enable this diffusion-mechanism, <strong>the</strong>
surface <strong>of</strong> <strong>the</strong> joining area must be free from
<strong>oxide</strong>s and be protected against <strong>the</strong> renewed
formation <strong>of</strong> <strong>oxide</strong>s during soldering/brazing.
Therefore, <strong>the</strong>re are two preconditions to be
fulfilled when brazing, using a flux:
• The melting temperature <strong>of</strong> <strong>the</strong> braze
must be below <strong>the</strong> melting temperature
<strong>of</strong> <strong>the</strong> parent metal
• The surface <strong>of</strong> <strong>the</strong> material must be
completely free from <strong>oxide</strong>s
• <strong>the</strong> flux must be intensive enough having
<strong>the</strong> adequate effective temperature.
2. The process ranges
Brazing <strong>of</strong> <strong>aluminium</strong> is classified into brazing
and soldering. At brazing, filler metals with
melting temperatures above 450°C are used.
Solders, having a melting point below 450°C,
are called solder filler metals and <strong>the</strong> process
is called soldering or s<strong>of</strong>t-soldering.
The process temperature can be freely
chosen, but it must be within <strong>the</strong> solidus temperature
<strong>of</strong> <strong>the</strong> filler metal and <strong>the</strong> melting
temperature <strong>of</strong> <strong>the</strong> parent metal. The flux will
be chosen matching <strong>the</strong> soldering or brazing
temperature; <strong>the</strong> effective temperature and
<strong>the</strong> chemical intensity must be coordinated
with <strong>the</strong> soldering/brazing process.
More generally, <strong>the</strong> higher <strong>the</strong> brazing
temperature, <strong>the</strong> higher is <strong>the</strong> mechanical
strength.
3. Structural requirements on brazing
During brazing usually a brazing gap is produced
by a braze drawn into <strong>the</strong> gap due to
<strong>the</strong> capillary forces (capillary brazing). The
braze displaces <strong>the</strong> flux as soon as <strong>the</strong> <strong>oxide</strong>s
are dissolved on <strong>the</strong> joining surface. A relatively
large-surface joint is produced and <strong>the</strong>
braze <strong>layer</strong> located in <strong>the</strong> soldering gap will
be mechanically streng<strong>the</strong>ned if a load is exerted
on it. By this means considerable forces
may be exerted that are able to exceed <strong>the</strong>
strength <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> parent metal.
4. Properties <strong>of</strong> <strong>aluminium</strong> brazed joints
4.1 Brazed joints: The strength <strong>of</strong> <strong>the</strong> <strong>aluminium</strong>
brazed joint <strong>of</strong>ten is <strong>the</strong> same as a
welded joint if <strong>the</strong> ‘joint is suitable for brazing’.
During brazing <strong>of</strong> Al-materials <strong>of</strong> higher
strength as well as <strong>of</strong> Al-cast alloys <strong>the</strong>re is
<strong>the</strong> risk <strong>of</strong> a melt down <strong>of</strong> <strong>the</strong> surfaces to be
joined since <strong>the</strong> melting temperature <strong>of</strong> such
type <strong>of</strong> alloys can be near <strong>the</strong> melting point <strong>of</strong>
<strong>the</strong> brazing filler metal. The chemical resistance
<strong>of</strong> a brazed joint <strong>of</strong> <strong>the</strong> commonly used
braze material Al88Si is not worse than that
<strong>of</strong> a welded joint. It is true that brazed joints
can be anodized, but in <strong>the</strong> joining area (braze
<strong>layer</strong>) <strong>the</strong>y will get a darker colour.
4.2 Soldered joints: Both, strength and corrosion
resistance <strong>of</strong> soldered joints are much
lower. A dry environment or corrosion protection
(lacquering, greasing) will <strong>the</strong>n become
necessary. It is nei<strong>the</strong>r possible to
perform anodic oxidation nor to have oper-
ating temperatures considerably above
100°C.
Fig. 1: Built-up <strong>of</strong> a brazed joint by diffusion (Source: Hydro Aluminium)
54 ALUMINIUM · EAC CONGRESS 2011