First, the output of welding heat and its influencing factors
The heat generated during spot welding is determined by the following formula: Q=IIRt(J)â€”â€”â€”â€”(1)
Where: Q - heat generated (J), I - welding current (A), R - resistance between electrodes (ohms), t - welding time (s)
1. Resistor R and factors affecting R
The resistance between the electrodes includes the resistance Rw of the workpiece itself, the contact resistance Rc between the two workpieces, and the contact resistance Rew between the electrode and the workpiece. That is, R=2Rw+Rc+2Rewâ€”â€”(2)
When the workpiece and the electrode are fixed, the resistance of the workpiece depends on its resistivity. Therefore, the resistivity is an important property of the material to be soldered. The metal with high resistivity has poor conductivity (such as stainless steel). The metal with low resistivity has good conductivity. (such as aluminum alloy). Therefore, when spot welding stainless steel, heat generation is easy and heat dissipation is difficult. When spot welding aluminum alloy, heat generation is difficult and heat dissipation is easy. When spot welding, the former can use a small current (several thousand amps), while the latter must use a large current (a few Wan Anpei). The electrical resistivity depends not only on the type of metal, but also on the heat treatment state of the metal, the processing method, and the temperature.
The contact resistance exists for a short period of time, generally in the early stages of soldering, and is formed by two reasons:
1) The surface of the workpiece and the electrode has a high resistivity oxide or dirt layer, which will greatly hinder the current. Excessive levels of oxide and dirt can make the current unconductive.
2) Under the condition that the surface is very clean, due to the microscopic unevenness of the surface, the workpiece can only form contact points on the rough surface. A convergence of current lines is formed at the contact points. The resistance at the contact is increased due to the reduction in the current path.
The resistance Rew between the electrode and the workpiece is lower than that of Rc and Rw. Since the resistivity and hardness of the copper alloy are generally lower than that of the workpiece, it is small and has less influence on the formation of nugget. We have less consideration of its influence.
2. The influence of welding current
It can be seen from equation (1) that the effect of current on heat generation is greater than both resistance and time. Therefore, it is a parameter that must be strictly controlled during the welding process. The main causes of current changes are grid voltage fluctuations and AC welder secondary loop impedance changes. The change in impedance is due to a change in the geometry of the loop or due to the introduction of different amounts of magnetic metal in the secondary loop. For DC welders, the secondary loop impedance changes without significant effect on the current.
3. The influence of welding time
In order to ensure the size of the nugget and the strength of the solder joint, the soldering time and the welding current can complement each other within a certain range. In order to obtain a certain strength of the solder joint, high current and short time (strong condition, also known as hard specification) can be used, and small current and long time (weak condition, also called soft specification) can be used. Whether hard or soft specifications are used depends on the properties of the metal, the thickness and the power of the welder used. There is an upper and lower limit for the current and time required for metals of different properties and thicknesses, whichever is used.
4. Effect of electrode pressure
The electrode pressure has a significant effect on the total resistance R between the two electrodes. As the electrode pressure increases, R decreases significantly, but the increase of the welding current is not large, and can not affect the heat production reduction caused by the decrease of R. Therefore, the solder joint strength always decreases as the welding pressure increases. The solution is to increase the welding current while increasing the welding current.
5. Effect of electrode shape and material properties
Since the contact area of â€‹â€‹the electrode determines the current density, the resistivity and thermal conductivity of the electrode material are related to the generation and dissipation of heat. Therefore, the shape and material of the electrode have a significant influence on the formation of the nugget. As the electrode tip deforms and wears, the contact area increases and the solder joint strength decreases.
6. Influence of surface condition of the workpiece
Oxides, dirt, oil and other impurities on the surface of the workpiece increase the contact resistance. Excessive oxide layers can even cause current to pass. Local conduction, due to excessive current density, can cause splash and surface burnout. The presence of the oxide layer also affects the unevenness of heating of the individual solder joints, causing fluctuations in solder quality. Therefore, thorough cleaning of the surface of the workpiece is necessary to ensure the quality of the joint.
Second, heat balance and heat dissipation
In spot welding, only a small portion of the heat generated is used to form the solder joint, and a larger portion is lost due to conduction or radiation to adjacent materials. The heat balance equation:
Q=Q1+Q2â€”â€”â€”â€”(3) where: Q1â€”â€”heat forming nugget, Q2â€”â€”heat lost
The effective heat Q1 depends on the thermophysical properties of the metal and the amount of molten metal, regardless of the welding conditions used. Q1=10%-30%Q, the lower limit of the metal (aluminum, copper alloy, etc.) with good thermal conductivity; the upper limit of the metal (stainless steel, superalloy, etc.) with high resistivity and poor thermal conductivity. The loss of heat Q2 mainly includes the heat conducted through the electrode (30% - 50% Q) and the heat conducted through the workpiece (about 20% Q). The amount of heat radiated into the atmosphere is about 5%.
Third, the welding cycle
The welding cycle for spot welding and projection welding consists of four basic stages (as in the spot welding process):
1) Pre-pressing phase - the electrode is lowered to the current-on phase to ensure that the electrode is pressed against the workpiece so that there is proper pressure between the workpieces.
2) Welding time - the welding current passes through the workpiece and heat is generated to form a nugget.
3) Maintenance time - the welding current is cut off and the electrode pressure is maintained until the nugget solidifies to a sufficient strength.
4) Rest time - the electrode begins to lift until the electrode begins to fall again, starting the next welding cycle.
In order to improve the performance of welded joints, it is sometimes necessary to add one or more of the following to the basic cycle:
1) Increase the pre-pressure to eliminate the gap between the thick workpieces and make them fit snugly.
2) Use the preheating pulse to improve the plasticity of the metal, so that the workpiece can be easily fitted tightly and prevent splashing; in the case of projection welding, multiple bumps can be evenly contacted with the flat plate before energization welding to ensure uniform heating at each point.
3) Increase the forging pressure to compact the nugget to prevent cracks or shrinkage.
4) Use tempering or slow cooling pulse to eliminate the quenching structure of the alloy steel, improve the mechanical properties of the joint, or prevent cracks and shrinkage holes without increasing the forging pressure.
Fourth, the type and scope of welding current
1. AC power The current can be ramped up and down by amplitude modulation to achieve the purpose of preheating and slow cooling, which is very beneficial for aluminum alloy welding. Alternating current can also be used for multi-pulse spot welding, where there is a cooling time between two or more pulses to control the heating rate. This method is mainly used for the welding of thick steel plates.
2. DC power is mainly used in applications where large current is required. Since DC welding machines are mostly supplied with three-phase power, the three-phase load is unbalanced when single-phase power supply is avoided.
5. Weldability during metal resistance welding
The following items are the main indicators for evaluating the weldability of resistance welding:
1. Conductivity and thermal conductivity of materials A metal with a small resistivity and a high thermal conductivity requires a high-power welder, which has poor weldability.
2. High-temperature strength of materials High-temperature (0.5-0.7Tm) metals with high yield strength are prone to spatters, shrinkage cavities, cracks, etc. during spot welding, requiring large electrode pressures. If necessary, a large forging pressure is required after the power is turned off, and the weldability is poor.
3. Plastic temperature range of materials Plastics with narrow plastic temperature range (such as aluminum alloy) are very sensitive to the fluctuation of welding process parameters. It is required to use a welding machine that can accurately control the process parameters, and requires the follow-up of the electrode to be good. Poor weldability.
4. Sensitivity of materials to thermal cycling Under the influence of welding thermal cycle, metals with quenching tendency tend to produce hardened structure and cold cracks; alloys with low melting point are easy to form hot cracks with fusible impurities; The strengthened metal is prone to softening zones. To prevent these defects, appropriate technological measures should be taken. Therefore, the weldability of the metal having high heat cycle sensitivity is also poor.
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