Spot technology, fast, low-voltage –
Spot welding with tungsten shielding gas
by Heinz Lorenz, Mündersbach, Germany
Introduction
In mechanical engineering, switch cabinet
construction, car construction, as well as in various
other sectors, such as the manufacture of household
appliances, including refrigerators, washing machines
etc., there are many welding tasks in which thin
panels need to be welded on one side using a spot
welding technique. These welds can also be
performed using resistance spot welding. If the point
to be welded is only accessible from one side, the
electrodes are aligned with one another on one side of
the workpiece in this process, and the metal sheets lie
on a copper base, Figure 1 [1]. However, accessibility
is often so restricted that it is not even possible to
One-sided
spot welding
(RPE)
impressions in the heated surface of the metal sheet.
Resistance welding systems also need to be
available, which is not the case at every site. Arc spot
welding machines, on the other hand, can also be
used for seam welding and therefore provide universal
usage.
In comparison to MIG/MAG spot welding, tungsten
shielding gas welding offers the advantage of
improved spot surfaces. As the welding can be
performed without filler material and is only melted
onto the parent material, the welded points produced
are particularly flat and smooth. This is a significant
benefit, especially on visible areas. In addition, the
heat feeding is very low thanks to the short welding
times, and this results in very low thermal stress and
low distortion on the workpieces.
Systems for tungsten shielding gas spot welding
Standard inverter power sources (Figure 3) can be
used for TIG and spot welding, fitted with special
torches (Figure 4).
copper base
Figure 1 Principle of one-sided resistance spot welding [1]
insert the copper base underneath, and therefore
resistance welding cannot be performed. For many
years, variations of the arc welding process have
therefore been used for one-sided spot welding on
thin metal, such as MIG/MAG, TIG (Figure 2) and
Figure 3 Modern inverter power source Foto: EWM
Figure 2 Principle of TIG spot welding
more recently the plasma process. Especially for
tungsten shielding gas spot welding, there have been
recent developments in terms of the power sources,
controls and torches, which make this process easier
to use and which improve the quality of the spots.
Advantages of arc spot welding
The surface formation produced on the welding points
is one argument against the use of one-sided
resistance spot welding. To position and fix the metal
sheets sufficiently well on the copper base, a higher
pressure needs to be used that can cause
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A spot/pulse remote control (Figure 5) ensures high
quality, reproducible results thanks to the adjustable
spot welding time and the optimum preset pulse
parameters.
Welding with current pulses offers the advantage that
deeper fusion penetration is produced during the
pulse phase, and a certain level of cooling is achieved
early on in the longer lasting base current phase,
which permits good fusion penetration and low heat
loading of the material.
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Applications
The process can be used for tacking (Figure 7), and
Figure 4 spotArc-spot welding torch, water cooled
The torches are especially important here as well.
These have feet or specially shaped ends on the
shielding gas nozzle, which can be placed on the
surface of the metal sheet during welding. They permit
to maintain a constant distance between the torch and
Figure 7 Tack welding with spotArc-spot welding machine
for joint welding steel sheets and CrNi alloys up to a
thickness of around 2.5 mm. Metal sheets of different
thickness can also be welded on top of one another.
As this is a one-sided process, it is also possible to
weld on hollow components, such as round or square
pipes.
In arc spot welding, the arc melts through the upper
sheet and the lower sheet is melted onto it. This
produces flat, fine-textured welding points (Figure 8)
which require little or no finishing work, even in visible
areas.
Figure 5 Remote control for TIG spot welding Foto: EWM
workpiece, and thus also a constant arc length, which
contributes to reproducibility of the welding results. At
the same time, the gas nozzle attached can be used
to exert force onto the surface, which presses the
parts being joined against one another. This
contributes to the high quality of the welding points
and to the workpieces keeping their correct
dimensions. These special shielding gas nozzles are
available for butt welds, fillet welds and corner joints
(box welds). Figure 6 shows various gas nozzle
shapes for tungsten shielding gas spots.
butt weld
fillet weld
Figure 8 View of the surface using TIG-spot welding
Literature:
[1] Killing, R. and H. Lorenz: Kompendium Schweißtechnik
Band 1 – Verfahren der Schweißtechnik Fachbuchreihe
Schweißtechnik Band 128/1 (Compendium of Welding
Technology Volume I – Welding Processes, Specialist Book
Series, Volume 128/1), DVS-Verlag Düsseldorf 2002
edge weld
Figure 6 Spot welding nozzle for different groove shapes
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