The principle of resistance welding is the Joule heating law where
the heat Q is
generated depending on three basic factors as expressed in the
following formula:
where I is the current
passing through the metal combination, R is the resistance of the base metals
and the contact interfaces, and t is the duration/time of the current
flow.
The principle seems simple. However, when it runs in an actual
welding process, there are numerous parameters, some
researchers had identified more than 100, to influence the results of a
resistance welding. In order to have a systematic understanding of
the resistance welding technology, we have carried out a lot
of experimental tests and summarized the most influential parameters
into the following eight types:
1) Welding current
The welding current is
the most important parameter in resistance welding which determines the
heat generation by a power of square as shown in the formula. The size of
the weld nugget increases rapidly with increasing welding current, but too
high current will result in expulsions and electrode deteriorations. The
figure below shows the typical types of the welding current applied
in resistance welding including the single phase alternating current (AC)
that is still the most used in production, the three phase direct current
(DC), the condensator discharge (CD), and the newly
developed middle frequency inverter DC. Usually the root mean square
(RMS) values of the welding current are used in the machine
parameter settings and the process controls. It is
often the tedious job of the welding engineers to find the
optimized welding current and time for each individual welding
application.
2) Welding time
The heat generation is directly
proportional to the welding time. Due to the heat
transfer from the weld zone to the base metals and to the electrodes,
as well as the heat loss from the free surfaces to the surroundings,
a minimum welding current as well as a minimum welding
time will be needed to make a weld. If the welding
current is too low, simply increasing the welding time alone will not
produce a weld. When the welding current is high enough, the size of
the weld nugget increases with increasing welding time until it reaches a
size similar to the electrode tip contact area. If the
welding time is prolonged, expulsion will occur or in the
worst cases the electrode may stick to the workpiece.
3) Welding force
The welding
force influences the resistance welding process by its effect on the
contact resistance at the interfaces and on the contact area due to
deformation of materials. The workpieces must be compressed with a certain
force at the weld zone to enable the passage of the current. If
the welding force is too low, expulsion may occur immediately after
starting the welding current due to fact that the contact resistance
is too high, resulting in rapid heat generation. If the welding
force is high, the contact area will be large resulting in low current
density and low contact resistance that will reduce heat
generation and the size of weld nugget. In projection welding, the
welding force causes the collapse of the projection in
the workpiece, which changes the contact area and
thereby the contact resistance and the current density. It
further influences the heat development and the welding results.
4) Contact resistance
The contact resistance at the
weld interface is the most influential parameter related to
materials. It however has highly dynamic interaction with
the process parameters. The figure below shows the measured contact
resistance of mild steel at different temperatures and different
pressures. It is noticed that the contact resistance generally decreases
with increasing temperature but has a local ridge around 300°C, and
it decreases almost proportionally with increasing pressure. All metals
have rough surfaces in micro scale. When the welding force increases, the
contact pressure increases thereby the real contact area at the interface
increases due to deformation of the rough surface asperities.
Therefore the contact resistance at the interface
decreases which reduces the heat generation and the size of weld
nugget. On the metal surfaces, there are also oxides, water vapour,
oil, dirt and other contaminants. When the temperature increases,
some of the surface contaminants (mainly water and oil based ones) will be
burned off in the first couple of cycles, and the metals will also
be softened at high temperatures. Thus the contact resistance
generally decreases with increasing temperature. Even though the contact
resistance has most significant influence only in the first
couple of cycles, it has a decisive influence on the heat
distribution due to the initial heat generation and distribution.
5) Materials properties
Nearly all material
properties change with temperature which add to the dynamics of the
resistance welding process. The resistivity of material influences
the heat generation. The thermal conductivity and the heat capacity
influence the heat transfer. In metals such as silver and copper with
low resistivity and high thermal conductivity, little heat is
generated even with high welding current and also quickly
transferred away. They are rather difficult to weld with
resistance welding. On the other hand, they can be good
materials for electrodes. When dissimilar metals are welded, more
heat will be generated in the metal with higher resistivity. This should
be considered when designing the weld parts in projection welding and
selecting the forms of the electrodes in spot welding. Hardness of
material also influences the contact resistance. Harder
metals (with higher yield stress) will result in higher contact
resistance at the same welding force due to the rough surface
asperities being more difficult to deform, resulting in a smaller
real contact area. Electrode materials have also been
used to influence the heat balance in resistance welding, especially
for joining light and non-ferrous metals.
6) Surface coatings
Most surface coatings are
applied for protection of corrosion or as a substrate for further
surface treatment. These surface coatings often complicate the welding
process. Special process parameter adjustments have to be made
according to individual types of the surface coatings. Some surface
coatings are introduced for facilitating the welding of
difficult material combinations. These surface coatings are strategically
selected to bring the heat balance to the weld interface. Most of the
surface coatings will be squeezed out during welding, some will
remain at the weld interface as a braze metal.
7) Geometry and dimensions
The geometry and
dimensions of the electrodes and workpieces are very important, since
they influence the current density distribution and thus the results
of resistance welding. The geometry of electrodes in spot
welding controls the current density and the resulting size of
the weld nugget. Different thicknesses of metal sheets need different
welding currents and other process parameter settings. The design of the
local projection geometry of the workpieces is critical in projection
welding, which should be considered together with the material properties
especially when joining dissimilar metals. In principle, the embossment or
projection should be placed on the material with the lower resistivity in
order to get a better heat balance at the weld interface.
8) Welding machine characteristics
The electrical
and mechanical characteristics of the welding machine have a significant
influence on resistance welding processes. The electrical characteristics
include the dynamic reaction time of welding current and the magnetic
/ inductive losses due to the size of the welding window and the amount of
magnetic materials in the throat. The up-slope time of a welding
machine can be very critical in micro resistance welding as the total
welding time is often extremely short. The magnetic loss in spot welding
is one of the important factors to consider in process
controls. The mechanical characteristics include the speed and
acceleration of the electrode follow-up as well as the stiffness
of the loading frame/arms. If the follow-up of the electrode is too slow,
expulsion may easily occur in projection welding. The figure below
shows measured process parameters in a projection welding process, which
include the dynamic curves of the welding current, the welding force and
the displacement of the electrode, where the sharp movement corresponds to
the collapse of the projection in the workpiece.
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