Salt Bath Dip Brazing
Frequently asked questions about material and material affects
What materials are suitable to braze?
All aluminium alloys are not suitable for brazing. Too high amount of some
alloy materials will limit the brazability. The alloy constitution shall be of the
AlMgSi type, e g EN-AW 6082 and EN-AW 6063, but also pure aluminium is
excellent to braze.
How is the material affected by the brazing process?
The strength of the base metal is affected by the braze process. Cold
worked and hardened materials will immediately after brazing assume a
strength corresponding to annealed material, due to heating up to
temperatures near the melting point of the alloy. Certain harden able
alloys will, however regain part of their original strength after a few days
due to natural ageing.
In order to improve the strength of hardened alloys a renewed heat
treatment can be made. The material must then be solution treated and
aged.
Frequently asked questions about the brazing technique
What is in the salt bath?
The salt bath consists of potassium-, sodium- and lithium- chloride and a
small amount fluoride.
The salt is heated to reach a temperature of 595?C during brazing. The
melted salt is used both as heat source and as flux in the brazing process.
What dimensions can be brazed?
The dimension of the furnace limits the size of parts that can be brazed.
Alumbra has two furnaces, with dimensions 1000x1000x800 mm
and 1200x800x800 mm respectively. The brazing basket needs some
space which reduces the active braze area a little bit.
Are tools or fixtures needed?
If the parts are designed for self-fixing it is possible to avoid special tools
and fixtures.
This means a lot of advantages, for example;
• Lower cost for design and manufacturing of fixtures.
• Less service and redesign of the fixtures when the work pieces are
changed.
• Reduced heating and cooling time because it’s not necessary to heat up
or cool down fixtures.
• More parts can be brazed in one batch.
• Lower weight and due to this a smoother and faster way of work.
However, sometimes fixtures are necessary. These are made from
aluminium or stainless steel. To avoid that aluminium fixtures braze
together with the work pieces they are anodised.
What is the filler material made of?
The filler metal is made of aluminium with a high amount of Silicon,
approximately 12%.
Silicon is used to lower the melting point of the filler metal. Filler metal is
available in form of tread and foil in various dimensions and as pulverous.
It is also common to use clad sheets in various thickness.
When the filler metal melts it is drawn into the joint by capillary forces and
the salt is displaced. Some material diffusion between work piece and filler
material will take place. An exchange of material between the brazed parts
is a base for a strong joint.
The filler material is not possible to harden and it is not affected by heat
treatment. The breaking strength, Rm, of the filler metal is 100 - 110
N/mm2.
Does the salt affect the materials corrosion resistance?
The salt is very corrosive but totally soluble in water. To avoid corrosion
problems all salt rests are removed by careful cleaning with hot water in
several steps. The method therefore doesn’t affect corrosion resistance.
The filler metal has a chemical constitution near the AlMgSi alloy.
Therefore the risk of galvanic corrosion is limited.
What accuracy and tolerances can be achieved?
The parts that shall be brazed together must be cleaned from grease and
dirt and the aluminium oxide film. Degreasing is carried out in washing
machines and the oxide film is removed in pickling baths that contain
sulphuric and nitric acids.
This pickling operation and the flux in the salt bath affects the dimensions
of the work pieces, but the affect is marginal, at the maximum 0,02 mm.
After brazing the part is cooled in room tempered air. Thin parts will cool off
faster than thicker ones. This means that in some cases a construction
assembled by thin and thick parts might be slightly deformed. In most
cases this deformation can be taken care of by straightening the part.
The achievable accuracy to size is dependent upon the design of the part
and by the number of joints affecting the measurement.
The following example gives an indication of the possibilities concerning
measurement accuracy.
Assume that we braze a box with four walls and one bottom. The
measurements are 100x100x100 mm. We can then expect a result with
accuracy around 0,2 mm on width and height.
Generally accuracy within 0,2 to 0,3 mm is achievable in designs based on
sheet. Designs based upon cutting machining can reach accuracy within
0,05mm.
Are the possibilities of surface treatment limited?
The possibilities to surface treat are not affected at all. By decoration
anodising, however some speckled areas can be seen around the filler
metal. The high amount of silicon in the filler metal gives the joint a darker
surface than the rest of the work piece.