Resistance welding spot welding machine technology

Resistance welding spot welding machine technology
The carbon content of low-carbon steel is less than 0.25%. Its electrical resistivity is moderate, and the required welding machine power is not high; Wide plastic temperature range, easy to obtain the required plastic deformation without the need for high electrode pressure; Low carbon and trace element content, no high melting point oxides, generally does not produce quenched structure or inclusions; The crystallization temperature range is narrow, the high-temperature strength is low, and the coefficient of thermal expansion is small, so the cracking tendency is small. This type of steel has good weldability, and its process parameters such as welding current, electrode pressure, and energization time have a large adjustment range.
Spot welding methods for aluminum alloys
The application of aluminum alloys is very extensive, divided into two categories: cold work strengthening and heat treatment strengthening. The weldability of aluminum alloy spot welding is poor, especially for aluminum alloys strengthened by heat treatment. The reasons and process measures to be taken are as follows:
(1) High conductivity and thermal conductivity require a larger current and shorter time to achieve sufficient heat to form a molten core; It can also reduce surface overheating, avoid electrode adhesion and diffusion of copper ions into the pure aluminum cladding layer, and reduce the corrosion resistance of the joint.
(2) A narrow range of plastic temperature and a large coefficient of linear expansion require a larger electrode pressure and good electrode follow-up to avoid cracks caused by excessive tensile stress during melt nucleation solidification. For aluminum alloys with a high tendency towards cracking, such as LF6, LY12, LC4, etc., it is also necessary to use the method of increasing forging pressure to ensure sufficient plastic deformation and reduce tensile stress during melt nucleation solidification, in order to avoid cracking. When the bent electrode is difficult to withstand large forging pressure, the method of adding slow cooling pulse after welding pulse can also be used to avoid cracks. For thick aluminum alloys, two methods can be used simultaneously.
(3) Before welding, it is necessary to strictly clean the surface to prevent splashing and poor formation of the melt nucleus (during tearing inspection, the shape of the melt nucleus is irregular, and the protrusions and holes are not circular), which can reduce the strength of the solder joint. Uneven cleaning will cause unstable solder joint strength.
Based on the above reasons, welding machines with the following characteristics should be selected for spot welding aluminum alloys:
1) Can provide high current in a short period of time;
2) The current waveform should have the characteristics of slow rise and slow decrease;
3) Can accurately control process parameters without being affected by voltage fluctuations in the power grid;
4) Can provide pressure for valence and saddle shaped electrodes;
5) The inertia and friction of the machine head are small, and the electrode follow-up is good.
Currently, most of the DC pulse, three-phase low-frequency, and secondary rectifier welding machines used in China are 300-600KVA, with some reaching 1000KVA, all of which have the above characteristics. There are also single-phase AC welding machines used, but only for unimportant workpieces.
The electrode for spot welding aluminum alloy should be made of Class 1 electrode alloy with a spherical end face to facilitate pressing and heat dissipation.
Due to the high current density and the presence of oxide film, electrode adhesion is easily generated during aluminum alloy spot welding. Electrode adhesion not only affects appearance quality, but also reduces joint strength due to reduced current. For this, it is necessary to regularly adjust the electrodes. The number of points that can be welded to the workpiece after each adjustment of the electrode is related to factors such as welding conditions, the type of metal being welded, cleaning conditions, the presence or absence of current waveform modulation, electrode materials, and their cooling conditions. Usually, spot welding of pure aluminum involves 5-10 points, while spot welding of LF6 and LY12 involves 25-30 points.
Anti penetration aluminum LF21 has low strength, good weldability after ductility, and does not produce cracks. Usually, fixed and constant electrode pressure is used. Hard aluminum (such as LY11, LY12) and ultra hard aluminum (such as LC4, LC5) have high strength, poor ductility, and are prone to cracking. Therefore, it is necessary to adopt the pressure of the valence curve. However, for thin parts, using high welding pressure or dual pulse heating with slow cooling pulses, cracks are not inevitable.
When using valence pressure, it is very important for the forging pressure to lag behind the moment of power outage, usually 0-2 weeks. If the forging pressure is applied too early (before power outage), it is equivalent to increasing the welding pressure, which will affect heating and lead to a decrease and fluctuation in the strength of the solder joint. If the forging pressure is applied too late, cracks have already formed during the cooling and crystallization of the melt nucleus, and the forging pressure is of no use. Sometimes it is necessary to apply forging pressure before the power outage time, because the electromagnetic valve action is delayed or the air circuit is not smooth, causing the forging pressure to increase slowly. Not applying it in advance is not enough to prevent cracks.
The method of copper alloy welding
Compared with aluminum alloys, copper alloys have slightly higher electrical resistivity and slightly lower thermal conductivity, so spot welding is not too difficult. Copper alloys with a thickness less than 1.5mm, especially those with low conductivity, are most widely used in production. Pure copper has extremely high conductivity, making spot welding difficult.
Usually, it is necessary to add a gasket between the electrode and the workpiece, or use a composite electrode with tungsten embedded at the electrode end to reduce heat dissipation to the electrode. The diameter of the tungsten electrode is usually 3-4mm.
When welding copper and high conductivity brass and bronze, Class 1 electrode alloys are generally used as electrodes. When welding low conductivity brass, bronze, and copper nickel alloys, Class 2 electrode alloys are used. Composite electrodes embedded with tungsten electrodes can also be used to weld copper alloys. Due to the poor thermal conductivity of tungsten, a much smaller welding current can be used for spot welding on commonly used medium power welding machines. However, tungsten electrodes are prone to sticking to the workpiece, affecting its appearance.
Spot welding of stainless steel
Stainless steel is generally divided into three types: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Due to the high electrical resistivity and poor thermal conductivity of stainless steel, smaller welding currents and shorter welding times can be used compared to low-carbon steel. This type of material has high high-temperature strength and must be subjected to high electrode pressure to prevent defects such as shrinkage and cracking. Stainless steel has strong thermal sensitivity, usually using short welding time, strong internal and external water cooling, and accurately controlling heating time, welding time, and welding current to prevent grain growth and intergranular corrosion in the heat affected zone.
It is recommended to use Class 2 or Class 3 electrode alloys for spot welding stainless steel electrodes to meet the needs of high electrode pressure.