Download hot joining of metals

HOT JOINING OF METALS
•SOFT SOLDERING
•HARD SOLDERING
•BRAZING
•WELDING OXY-ACETYLENE (S5HS
•WELDING ELECTRIC ARC (S4HS)
– Flux coated filler rod
– MIG (Metal Inert Gas)
– TIG (Tungsten Inert Gas)
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Soft soldering
• Soft soldering is a quick method of joining most
metals such as copper, brass, tinplate and steel. An
exception is aluminium.
• The process is best confined to light fabrication
where joints are not subjected to heat and vibration
and so do not need to be very strong.
• Soft solder is an alloy made from varying proportions
of tin and lead with antimony.
• The melting point varies according to composition,
ranging from 183 to 250 degrees Centigrade.
• The solder used for electronics contains more tin than
lead, making it flow more easily at a lower
temperature.
• The solder for tinplate or plumbing copper water
pipes contains more lead than tin. It melts at a higher
temperature and sets harder.
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Soft soldering fluxes
Fluxes, available as liquids,
powders or pastes, have been
specially developed to protect
the cleaned surfaces from
oxidisation during heating;
solders only stick to clean metal.
The flux also helps the molten
solder to flow freely by
breaking down surface tension.
Active fluxes (e.g. bakers’ flux)
contain zinc chloride which
chemically clean the surfaces.
However, it is highly corrosive
and must be washed off
immediately upon completion.
Passive flux are non-corrosive,
but they only protect and do not
actually clean. an example is the
multi-core solder used in
electronics, which has cores of
resin flux running throughout its
length.
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Joining Process
In preparation for soldering the joint surfaces must
be clean. Use a suitable abrasive (e.g. emery cloth or
steel wool) and avoid touching the area. In the joining
process close-fitting joints are essential to ensure
that the capillary action unites the surfaces.
• A good solder joint depends on
– A clean surface,
– the correct flux
– appropriate heat
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Joining Process
There are several ways
of applying the
necessary heat and
solder.
An electric soldering
iron is cleaned, while
hot, using a wet sponge
and then ‘tinned’ with a
thin film of solder. Or
the joint is sweated, by
tinning both parts of
the joint first.
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Hard soldering
• Hard soldering is much stronger than soft
soldering and requires higher temperatures.
• Soft solders melt at around 200 degrees C,
whereas the lowest melting point of hard
solder is 625 degrees C.
• The principle of local alloying and using a flux
remains the same.
• The extra heat requirement is supplied by
using a gas/air torch.
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Silver soldering
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Silver soldering is so called
because hard solder contains
silver alloyed with copper and
zinc, giving melting points
ranging from 625 to 800
degrees Centigrade.
It enables work to be joined in
several stages,
– first using solder with a high
melting point,
– working through lower
melting points
– to finally the lowest, called
‘easy-flo’. This avoids the
risk of earlier joints coming
apart when applying heat for
the later ones.
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Joint Preparation
• Proper joint preparation is important and thorough cleaning
is necessary with the application of an active flux cramping
the work with soft binding wire.
• A special easy-flo’ flux is used for the lowest melting silver
solder, whilst medium and hard grade solders use a borax
flux (‘Tenacity’).
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Soldering the joint
• Pre-heat the joint with a gentle flame, then concentrate to a
small hot flame to achieve a dull red heat. Solder flows to the
hottest part, following the flame along the line of the joint.
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Brazing
• Brazing is a technique similar to soldering, except that
considerably higher temperatures are needed.
• Brazing spelter is an alloy of copper and zinc (brass) and melts
in the range 870—880 degrees Centigrade. This results in a
much stronger joint, since brass is stronger than solder.
•An air blown (brazing) gas torch is needed to maintain a hot flame.
This does create a limiting factor, making it too hot to use with brass
and copper, but it is ideal for mild steel.
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Joint Preparation
• Joints do benefit from interlocking, but
they should all be wired or held securely
to allow for expansion during heating.
Use a flux with borax or a proprietary
brand like ‘Sift bronze.’
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Hints for hard soldering
• The joint area must be thoroughly clean and fluxed.
• Allow time for spelter or solder to flow, melting on
the hot metal (not in the flame).
• Pre-heat gently, avoid too fierce a flame which might
blow away flux and spelter or solder.
• Surround with fire-bricks to reflect all possible
applied heat.
• Heavy sections will require the most heat, at least to
dull red.
• Maintain the heat on the join, until the spelter or
solder flows throughout the joint.
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Welding
• Welding offers a permanent
method of fastening and
fabricating products from a
wide range of materials.
Welding is the joining of two
materials (usually metal) in
their liquid form which
solidifies and fuses together
to form a joint that is as
strong as the parent metal.
• Industrially there are many
ways of achieving this fusion.
• Within school workshops two
basic methods of welding
metal are possible: oxyacetylene and electric arc.
Health and safety
references
• COSHH Regs
-1.021
-1.022
-1.023
-1.024
• BS4163 2000
-Pages 40- 41
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Types of welding processes
Oxy-acetylene welding uses a mixture of the two gasses to
produce a very hot flame (35000C) that is used to melt the
metal. The molten pool is moved along the joint line and
additional material is introduced to it from a filler rod of the
same metal.
Electric arc welding makes use of a flux coated filler rod that
acts as an electrode. A low voltage, high electrical current is
struck between the electrode rod and the workpiece. The heat
produced by the resulting electric arc melts both the rod and
the material to be joined. The rod acts as a filler for the joint
and so is consumed in the welding process.
Metal Inert Gas (MIG) welding is another electric arc welding
process. In this instance a continuous wire electrode is fed from
a coil through the welding torch. The process is shielded by an
inert gas enabling it to be used for aluminium welding. MIG
welding has developed into a very controllable process and is now
one of the most popular applications for robots.
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Oxy-acetylene welding
•In oxy-acetylene welding a
heat source of around 3500
degrees C is produced by burning
acetylene gas in oxygen.
•Fine adjustment to the ratio of
gases is made on the hand-held
blowpipe.
•Excess oxygen gives the hottest
flame, but a neutral flame, with
equal volumes of gas, is the most
widely used.
•Adjustments are made to suit
the thickness and type of metal
being welded.
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The Process
• During the welding
process a pool of molten
metal is created.
• A filler rod, of the
same metal as that
being joined, is dipped
into this and melts,
filling the joint.
• Fluxes are used with
some materials, but not
steels.
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Electric arc welding
In electric arc welding an electric arc,
of low voltage but high current of 10—
120 amps, is struck between a metal
electrode and the material to be
joined.
The electrode, as well as carrying
current, is a flux-coated filler rod.
Very intense heat is produced at the
end of the arc, melting the electrode
and the metals to be joined to form
the weld bead.
Protection from oxidation is given by
the special flux. This generates a
gaseous shield, forming a molten
blanket over the weld pool. As it
solidifies a brittle glassy slag is
formed, which can be easily chipped
away when cold.
Different metal thicknesses require
different diameters of electrode and
different currents. This process is
widely used because of its low capital
and running costs.
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PREPARATION OF JOINTS
To prepare the joints,
paint, rust and any
galvanised (zinc)
coating must be
removed. Thicker
metal requires edge
treatment such as
bevelling, so that the
weld achieves strength
by penetration into the
metal.
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The electric arc welding process
• SPECIAL EYE SHIELDING
OR VISORS ARE
COMPULSORY
• LEATHER APRON SHOULD
BE WORN
• LEATHERGLOVES SHOULD
BE WORN
• WORK SHOULD BE
CARRIED OUT IN A
SHIELDED BOOTH AWAY
FROMANY OBSERVERS
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INDUSTRIAL APPLICATION
• Spot welding is used
commercially to give
intermittent welds and
some pre-tacking may be
necessary with long runs.
• Aluminium is an important
material but is difficult to
work because of its oxide
film. MIG (metal inert
gas) and TIG (tungsten’
inert ~gas) are processes
used for welding
aluminium.
© Ideas In2 Action 2006