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Forming
Hot-dip galvanized sheet can be formed almost as readily
as uncoated cold rolled sheet. It can be formed in a variety of
ways, from simple rollforming and brake pressing to complex and
intricate stamping.
 As with uncoated steel sheet, factors other than
metallurgical designation (CS, FS, DDS, etc.) of the base sheet
enter into the successful forming of galvanized sheet. The
galvanized coating exhibits frictional characteristics that are
different from those of uncoated steel. Thus, lubricants, die
materials and other elements of fabrication must be compatible with
the coating to optimize productivity. In stamping and forming
operations, it is advantageous to use dies that are harder and
smoother than would be used for the same uncoated steel sheet. When
proper attention has been given to forming parameters, hot-dip
galvanized sheet is essentially as formable as cold rolled sheet
that possesses similar ductility.
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Soldering
Soldering is an established method for joining galvanized sheet or
attaching solderable items to galvanized coatings. Galvanized sheet
can be readily soldered with many types of conventional solders and fluxes. Lead/tin solder alloys are typically used. Flux
residues should be removed after soldering to prevent corrosive
damage to the zinc coating.
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Welding
Galvanized sheet can be readily welded by the same methods
used to weld cold rolled sheet. For example, spot welding is
readily performed, but caution must be applied to avoid more rapid
deterioration of the spot-welding electrodes. For metal-arc welding
and other high temperature welding methods, precautions must be
taken to avoid porosity and cracking of the weld that can be caused
by penetration of zinc into the weld pool.
Zinc has a much lower melting point (787oF) than steel and a
lower boiling point (about 1600oF). During welding,
zinc vapor burns in the air to produce dense, white zinc oxide
fumes. These fumes need to be adequately vented. Specific
precautions are given in ANSI Publication Z49.1 Safety in Welding
and Cutting. The thicker the zinc coating, the more fumes are
generated. In general, resistance welding is preferred for joining
galvanized sheet products because it results in less fuming than
other types of welding.
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Electric Resistance Welding
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Spot Welding
Because the galvanized coating has a lower surface contact
resistance than bare steel, welding conditions for galvanized sheet
will differ somewhat from those used for welding hot rolled or cold
rolled sheet. Generally, higher electrode forces and higher welding
currents and/or longer weld-cycle times are required to produce the
same fused-zone diameter in galvanized sheet as achieved with
uncoated sheet.
Zinc pickup on the
electrodes during spot welding can cause fairly rapid electrode
wear, so that only several thousand welds can be made before
electrode dressing. To minimize zinc pickup, the electrodes should
be kept as cool as possible by utilizing water cooling and
controlling the rate of welding. The use of copper alloy,
truncated-cone-shaped electrodes is preferred for spot welding
galvanized sheet. Dome-type electrodes also may be used when
electrode alignment is a problem.
For all galvanized
products, extremely high welding current is to be avoided, because
excessive heating tends to cause expulsion of the zinc coating
under the electrodes. The optimum setting of the welding parameters
must be determined by trial and are dependent on the specific
application and sheet characteristics such as thickness or coating
weight.
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Seam Welding
Conventional seam welding equipment can be used for welding
lap joints made with galvanized steel sheet. Seam welding of
galvanized steel sheet can be considered to be continuous spot
welding; the guidelines for spot welding should be followed for
seam welding. To produce acceptable welds, the procedures used for
cold rolled sheet should be modified toward slightly narrower
electrode width, higher electrode force, higher welding current and
slightly lower welding speed.
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Restoration of Weld Areas
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When
good appearance and maximum corrosion resistance are desired in the
weld area, oxides and fluxes from the welding operation should be
removed by sand blasting, wire brushing or grinding. Satisfactory
recoating methods are soldering, metal spraying or application of
metallic-pigmented paints.
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Fastening
From a mechanical standpoint, any style of fastener suitable
for use with sheet metal can be used to join galvanized sheet to
itself or to other materials, provided the fastener design is
appropriate for the structural requirements of the application. The
list of acceptable fasteners includes common ones like nuts and
bolts, screws and rivets, and special types like clamp fasteners,
clips and blind screws.
The fastener
material should be selected carefully. First, the fastener should
possess the same or greater corrosion resistance as the galvanized
sheet to ensure that the fastener life does not contribute to
premature failure of the fabricated part.
Second, the fastener
material should be compatible with the coating; that is, it should
be selected to avoid accelerated corrosion that, at times, can
occur between certain types of dissimilar metals. Suitable
materials include nylon or other plastics, stainless steels and
carbon steel fasteners with thick zinc alloy heads. Galvanized or
other plated fasteners also are suitable, but the coating must be
thick enough so that the corrosion resistance is at least equal to
the corrosion resistance of the galvanized sheet.
Fasteners made of
lead or copper dramatically accelerate the corrosion of the zinc
when an electrolyte is present. These materials set up an
electrolytic cell, and the zinc coating sacrifices itself to
protect the fasteners. Therefore, these fasteners must be
avoided.
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Adhesive Bonding
Adhesive bonding is a viable technique for joining coated
steel sheet to either other coated or uncoated sheet as well as to
other materials. The growing use of adhesive bonding in industries
such as the automotive industry clearly demonstrates the
reliability and durability of this fastening method.
Adhesive bonding is
an excellent method of joining dissimilar steel sheet products. It
does not alter the properties of the steel or its coating. It
provides uniform stress distribution and can be applied to reduce
vibration and noise. Adhesive bonding can serve to enhance product
design, so that mechanical fasteners and/or welding can be
avoided. It can function as a moisture sealant. Also,
it increases the range of material choices and material
combinations available to the product designer.
Two important
conditions are necessary for proper use of adhesive bonding: (1)
the load on the bonded area should be evenly distributed; (2) the
joint should be stressed mainly in a shear mode; peel and cleavage
forces should be minimal.
Two types of
adhesives, thermosets and thermoplastics, are widely used to join
steel sheet. Thermoset adhesives generally possess high shear
strength, rigidity and durability, and are capable of supporting a
wide range of specified design loads. They include acrylics,
epoxies, and urethanes. Thermoplastic adhesives generally are tough
and ductile, and are excellent for energy-absorbing
applications.
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Sealants
Sealants are often used between overlapping areas of
steel sheet to form watertight joints and to achieve superior
environmental durability. Neutral-cure silicone rubber
sealants are recommended for use with galvanized sheet. These
materials need no primer. They are flexible and noncorrosive as
well as resistant to heat, cold, water and ultraviolet rays.
Other types of sealants, like butyl rubber and styrene butadiene
rubber, may also be used successfully. For more information about
sealants for specific applications, consult a sealant
manufacturer.
Painting
U. S. Steel Galvanized Sheet can be readily painted
either before or after fabrication. When painted in coil form, the
product is called prepainted sheet. Paint provides obvious
aesthetic effects as well as increased corrosion protection.
Increased corrosion protection is a major attribute of painted
galvanized sheet. The paint film provides substantial improvement
by shielding the zinc coating from acid rain and other
corrosion-accelerating contaminants in the environment.
Selection of a pretreatment that provides an adherent bond
between the zinc coating and the paint is vitally important for
correct application of painted galvanized sheet. One of the keys to
good performance of painted galvanized sheet is the selection and
use of a primer designed for galvanized surfaces.
Items made from galvanized sheet should be thoroughly cleaned
and dried prior to application of the pretreatment. The most
common type of pretreatment is zinc phosphate. Other pretreatments
can be used, but they should have a proven performance record for
use on galvanized sheet.
Many types of paint can be used on galvanized sheet. The choice
depends on the ultimate service requirements of the product. For
example, some types are formulated to maximize the resistance to
fading and chalking. Others provide good flexibility for use with prepainted sheets. To maximize the performance
of painted galvanized sheet, it is highly recommended that users
consult with paint suppliers to obtain maximum performance.
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