Resistance seam welding
by Steve Westgate
Description
The seam welding process is an adaptation of resistance spot welding and
involves making a series of overlapping spot welds by means of rotating
copper alloy wheel electrodes to form a continuous leak tight joint. The
electrodes are not opened between spots. The electrode wheels apply a
constant force to the workpieces and rotate at a controlled speed. The
welding current is normally pulsed to give a series of discrete spots, but
may be continuous for certain high speed applications where gaps could
otherwise occur between individual spots. Seam welding equipment is normally
fixed and the components being welded are manipulated between the wheels.
The process may be automated; it is illustrated in Fig.1.
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Fig.1. Conventional wide wheel seam welding
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Current status
There are a number of process variants for specific applications:
Wide wheel seam
- Wheel contact width normally 5t
mm flat (where t is single sheet thickness in mm).
- General purpose welding (domestic radiators up to about 6 m/min).
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Fig.2a) Wide wheel seam
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Narrow wheel seam
- Wheel contact shape typically 6mm radius.
- Knurl drive on wheel edge with contact surface continuously planished.
- Controls electrode contamination when welding coated steels, such as
for vehicle fuel tanks.
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Fig.2b) Narrow weld seam
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Consumable wire seam welding
- Shaped, consumable copper wire fed between the wheels and sheets to be
joined to give consistent clean contact.
- Used for welding coated steels such as tin cans and vehicle fuel
tanks.
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Fig.2c) Consumable wire seam
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Mash seam welding
- Narrow overlap of sheet edges, which are partly crushed together
during welding.
- High speed welding of tin cans and drums (0.2mm tinplate up to
100m/min).
- Wide wheel or consumable wire processes used.
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Fig.2d) Mash seam welding
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Foil butt seam welding
- Foil welded on to each side of the butted edges of the sheets to be
joined.
- Typically 4mm wide stainless steel foil used to preserve corrosion
resistance on coated steel.
- Virtually flush finish with no crevice and used to produce wide
panels.
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Fig.2e) Foil butt seam welding
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Important Issues
The main issues concerning seam welding are in weld quality control and
welding speeds.
- High speed welding of coated steels is of particular importance in
manufacture of tin cans. Specialist consumable wire seam welding machines
are used. Quality monitors have been developed for these applications and
welding speeds up to about 100m/min are possible. Steels with alternative
coatings to tin, for canning applications, present difficulties due to
their high surface resistance.
- Weld quality is controlled mainly by tight process control together
with periodic testing of samples. Factors such as material and pressing
quality, and alignment of the electrodes to the material, are critical to
achieve high speed, quality welding, for example in coated steels for fuel
tanks.
Benefits
Seam welding enables high welding speeds to be obtained compared with
many other techniques, but can be limited by component shape and wheel
access.
Risks
There are some limitations on material weldability but attention to
correct setting up and good process control can solve most production
problems. The main hazards are: (i) the risk of crushing fingers or hands;
and (ii) burns or eye damage from splash metal. Little fume is produced but
may need attention when welding coated steels or when oils or organic
materials are present.
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