Principle analysis and application guide of spot welding machine, professional knowledge

Professional knowledge of the principle analysis and application guide of spot welding machines. Our factory specializes in producing butt welding machines, spot welding machines, seam welding machines, special-shaped welding machines, etc.
Spot welding is the process of welding individual points on the contact surface of a welded component at a joint. Spot welding requires metals to have good plasticity. When welding, first clean the surface of the welded part, then assemble the welded sheet metal by overlapping and pressing it between two cylindrical copper electrodes, and apply force to press it tightly. When a sufficiently large current is passed, a large amount of resistance heat is generated at the contact of the plate, quickly heating the metal in the hottest central area to a high plastic or molten state, forming a lens shaped liquid melt pool. Continue to maintain pressure, disconnect the current, and after the metal cools down, a solder joint is formed.
Spot welding is only used for welding thin plate lap structures, metal mesh, cross steel structural components, etc. without sealing requirements due to the certain spacing between welding points. If the columnar electrode is replaced with a disc-shaped electrode, the electrode is tightly pressed against the welding piece and rotated, and the welding piece is continuously fed between the disc-shaped electrodes, and then pulsed with electricity. It can form a continuous and overlapping welding point, forming a weld seam, which is called seam welding. It is mainly used for welding thin plate overlapping structural components with sealing requirements or high joint strength requirements, such as oil tanks, water tanks, etc.
Spot welding machines can be classified into universal (universal) and specialized types according to their purposes. According to the number of solder joints welded simultaneously, there are single point, double point, and multi-point types. According to the transmission mode of the pressurization mechanism, there are pedal type, electric motor cam type, pneumatic type, hydraulic type, composite type (gas-liquid compression type), etc.
Principle of spot welding machine
The method of welding by applying pressure through electrodes and utilizing the resistance heat generated by current passing through the contact surface and adjacent areas of the joint after welding is called resistance welding. Resistance welding has the characteristics of high production efficiency, low cost, material saving, and easy automation. Therefore, it is widely used in various industrial sectors such as aviation, aerospace, energy, electronics, automobiles, and light industry, and is one of the important welding processes.
1, Production and influencing factors of welding heat
The heat generated during spot welding is determined by the following equation: Q=IIRt (J) -- (1)
In the formula: Q - heat generated (J), I - welding current (A), R - electrode resistance (ohms), t - welding time (s)
1. Resistance R and factors affecting R
The inter electrode resistance includes the resistance Rw of the workpiece itself, the contact resistance Rc between two workpieces, and the contact resistance Rew between the electrode and the workpiece. That is, R=2Rw+Rc+2Rew - (2) as shown in the figure
When the workpiece and electrode are constant, the resistance of the workpiece depends on its resistivity. Therefore, resistivity is an important performance of the welded material. Metals with high resistivity have poor conductivity (such as stainless steel), while metals with low resistivity have good conductivity (such as aluminum alloy). Therefore, when spot welding stainless steel, heat generation is easy but heat dissipation is difficult, and when spot welding aluminum alloy, heat generation is difficult but heat dissipation is easy. When spot welding, the former can use a small current (several thousand amperes), while the latter must use a large current (tens of thousands of amperes). The resistivity not only depends on the type of metal, but also on the heat treatment state, processing method, and temperature of the metal.
The duration of contact resistance is brief and generally exists in the early stages of welding, due to two reasons:
(1) The presence of high resistance oxide or dirt layers on the surface of workpieces and electrodes can significantly impede current flow. Excessive oxide and dirt layers can even prevent current from conducting.
(2) Under very clean surface conditions, due to the micro
Unevenness is observed, causing the workpiece to only form contact points locally on rough surfaces. The convergence of current lines at the contact point. Due to the reduction of the current path, the resistance at the contact point has increased.
Compared with Rc and Rw, the resistance Rew between the electrode and the workpiece is generally lower in copper alloys due to their lower resistivity and hardness, which has a smaller impact on the formation of fusion nuclei. Therefore, we consider its influence less.
2. The influence of welding current
From formula (1), it can be seen that current has a greater impact on heat generation than both resistance and time. Therefore, during the welding process, it is a parameter that must be strictly controlled. The main causes of current changes are voltage fluctuations in the power grid and impedance changes in the secondary circuit of the AC welding machine. Impedance changes are due to changes in the geometric shape of the circuit or the introduction of different amounts of magnetic metal in the secondary circuit. For DC welding machines, changes in secondary circuit impedance have no significant impact on current.
3. The impact of welding time
In order to ensure the size of the fusion core and the strength of the welding point, the welding time and welding current can complement each other within a certain range. In order to obtain a certain strength of solder joints, high current and short time (strong conditions, also known as hard specifications) can be used, or low current and long time (weak conditions, also known as soft specifications) can be used. The choice between hard and soft specifications depends on the performance, thickness, and power of the welding machine used for the metal. There is an upper and lower limit for the current and time required for metals with different properties and thicknesses, which should be used as the standard.
4. Effects of electrode pressure
The electrode pressure has a significant impact on the total resistance R between the two electrodes. As the electrode pressure increases, R decreases significantly, while the increase in welding current is not significant and cannot affect the reduction in heat generation caused by the decrease in R. Therefore, the strength of the solder joint always decreases with the increase of welding pressure. The solution is to increase the welding current while increasing the welding pressure.
5. The influence of electrode shape and material properties
Due to the fact that the contact area of the electrode determines the current density, the electrical 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 impact on the formation of fusion nuclei. As the electrode tip deforms and wears, the contact area increases and the strength of the solder joint decreases.
6. Influence of workpiece surface condition
The oxides, dirt, oil, and other impurities on the surface of the workpiece increase the contact resistance. An excessively thick oxide layer can even prevent current from passing through. Local conduction, due to excessive current density, can result in splashing and surface burning. The presence of oxide layer can also affect the uneven heating of various solder joints, causing fluctuations in welding quality. Therefore, thoroughly cleaning the surface of the workpiece is a necessary condition to ensure obtaining high-quality joints.
2, Thermal balance and heat dissipation
During spot welding, only a small portion of the heat generated is used to form the weld, and a larger portion is lost due to conduction or radiation to adjacent materials. The thermal equilibrium equation is:
Q=Q1+Q2- (3) where: Q1- heat of nucleation formation, Q2- heat loss
The effective heat Q1 depends on the thermophysical properties of the metal and the amount of molten metal, and is independent of the welding conditions used. Q1=10%-30%Q, Lower limit for metals with good thermal conductivity (such as aluminum, copper alloys, etc.); Metals with high electrical resistivity and poor thermal conductivity (such as stainless steel, high-temperature alloys, etc.) are subject to an upper limit. The heat loss Q2 mainly includes the heat conducted through the electrode (30% -50% Q) and the heat conducted through the workpiece (about 20% Q). Approximately 5% of the heat radiated into the atmosphere.
3, Welding cycle
The welding cycle of spot welding and projection welding consists of four basic stages (as shown in the spot welding process):
1) Pre pressing stage - the electrode is lowered to the current connection stage to ensure that the electrode presses the workpiece tightly, so that there is appropriate pressure between the workpieces.
2) Welding time - Welding current passes through the workpiece, generating heat to form a weld nugget.
3) Maintenance time - Cut off the welding current, and continue to maintain the electrode pressure until the molten core solidifies to sufficient strength.
4) Rest time - from the start of electrode lifting to the start of electrode descent again, the next welding cycle begins.
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 gaps between thick workpieces and ensure a tight fit.
2) Using preheating pulses to improve the plasticity of metals, making it easier for workpieces to fit tightly and prevent splashing; When performing convex welding, this can ensure that multiple convex points are evenly in contact with the flat plate before welding, to ensure consistent heating at each point.
3) Increase forging pressure to compact the melt nucleus and prevent cracking or shrinkage.
4) Using tempering or slow cooling pulses to eliminate the quenched structure of alloy steel, improve the mechanical properties of joints, or prevent cracks and shrinkage without increasing forging pressure.
4, Types and Applicable Range of Welding Current
1. Alternating current can be modulated to gradually increase and decrease the current, achieving the purpose of preheating and cooling, which is very beneficial for aluminum alloy welding. AC power can also be used for multi pulse spot welding, which leaves a cooling time between two or more pulses to control the heating rate. This method is mainly used for welding thick steel plates.
2. DC power is mainly used in situations that require high current. As most DC welding machines are powered by three-phase power sources, it avoids three-phase load imbalance when single-phase power is supplied.
5, Weldability during metal resistance welding
The following are the main indicators for evaluating the weldability of resistance welding:
1. Materials with low electrical and thermal conductivity resistivity and high thermal conductivity require high-power welding machines, which have poor weldability.
2. High temperature strength of materials. Metals with high yield strength at high temperatures (0.5-0.7Tm) are prone to defects such as spatter, shrinkage, and cracks during spot welding, requiring the use of high electrode pressure. When necessary, it is necessary to apply large forging pressure after power outage, which results in poor weldability.
3. The plastic temperature range of materials is very sensitive to fluctuations in welding process parameters for metals with narrow plastic temperature ranges (such as aluminum alloys), requiring the use of welding machines that can accurately control process parameters and good electrode tracking. Poor weldability.
4. The sensitivity of materials to thermal cycling. Under the influence of welding thermal cycling, metals with a tendency towards quenching are prone to develop hardened structures and cold cracks; Alloys with low melting points that are prone to melting impurities are prone to thermal cracking; Metals strengthened by cooling are prone to softening zones. Corresponding process measures should be taken to prevent these defects. Therefore, metals with high thermal cycling sensitivity also have poor weldability.