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.
