The spot welding process is a metal connection that forms a
bond. During welding, the weldment is heated locally by welding
current, and pressure is applied to the contact heating point
of the weldment to form a solder joint. Spot welding is a
high-speed, economical connection method, it is suitable for
the manufacture of lap joints, joints do not need to be
airtight, thickness of less than 5mm stamping rolled sheet
components. Spot welding process is currently used in various
industrial sectors, not only can weld low carbon steel and low
alloy steel, but also can weld high carbon steel, high
manganese steel and stainless steel, aluminum alloy, titanium
alloy and other materials composed of parts.
The selection of spot welding process parameters: the process
parameters that affect spot welding include the structural
diameter of the welding electrode, welding energy, welding time
and welding pressure. According to the welding speed and
welding effect can be divided into fast welding, medium speed
welding, ordinary welding three conditions, for the workpiece
requirements of high welding strength, welding deformation of
small occasions, the use of high-power, short time strong
specification fast welding. For the workpiece with less
stringent requirements, the ordinary welding method of low
power and long time can be used, so that a relatively small
welding equipment can be selected, and the impact on the power
grid is also relatively small. It is usually selected according
to the material and thickness of the workpiece, referring to
the welding condition table of the material, first determine
the end face shape and size of the electrode, then
preliminarily select the electrode pressure and welding time,
and then adjust the welding current, weld the sample with
different currents, and then adjust the electrode pressure,
welding time and current within the appropriate range after
checking that the diameter of the molten core meets the
requirements. The sample is welded and inspected until the
quality of the solder joint fully meets the requirements
specified in the technical conditions. The most commonly used
method for testing samples is the tear method, and the mark of
a good quality solder joint is that there is a round hole in
one piece of the tear sample and a round boss in the other
piece. Thick plates or quenched materials sometimes cannot tear
out round holes and convex tables, but the diameter of the
molten core can be judged by the shear fracture. If necessary,
it is also necessary to carry out low-power measurement,
tensile test and X-ray inspection to determine the penetration
rate, shear strength and whether there are shrinkage holes and
cracks. When selecting the process parameters with the sample,
the differences between the sample and the workpiece in the
shunt, the influence of ferromagnetic substances, and the
assembly gap should be fully considered, and the appropriate
adjustment should be made.
The main factors affecting the welding quality of spot welding
joints are welding current, electrode pressure, welding time,
prepressure and rest time, welding electrode diameter and so
on.
1. Welding current
The source of heat required for the fusion core formed by spot
welding is the heat generated by the current passing through
the resistance of the welding area. In the case of other
conditions given, the size of the welding current determines
the welding penetration of the molten core. When welding low
carbon steel, the average penetration rate of molten core is *0
~ *0% of the thickness of the steel plate, and the welding
strength is higher when the penetration rate of molten core is
*5 ~ *0%. When the welding current exceeds a certain standard
value, continuing to increase the current can only increase the
rate of molten core, but not improve the joint strength.
Therefore, it is very uneconomical in terms of manufacturing
costs. If the current is too large, it will also produce
defects such as too deep indentation and welding burn through.
2, electrode pressure
The influence of electrode pressure on the size of molten core
is also relatively large in spot welding. Too high electrode
pressure will make the indentation too deep, and will
accelerate the deformation and loss of the welding electrode.
The lack of pressure is easy to produce shrinkage holes, and
the welding electrode will burn out due to the increase of
contact resistance and shorten its service life.
3. Welding time
In spot welding, the core size is mainly controlled by welding
time. When other welding parameters are unchanged, the longer
the welding time, the larger the core size. In the case that
the welding strength is relatively high, the welding energy and
the welding time should be relatively large. Special attention
should be paid to the lengthening welding time will increase
the energy consumption of the welder, but also increase the
wear of the electrode and reduce the service life of the
equipment.
