What is the welding principle of common resistance welding?

The principle of resistance welding: pressure is applied through the electrodes after the weldment is assembled, and the current is used to connect through the joint; 1. The output of welding heat and influencing factors; the heat generated during spot welding is determined by the following formula: Q=IIRt(J)— ; In the formula: Q——heat generated(J), I——welding current (A; 1. Resistance R and factors affecting R; the resistance between electrodes includes the resistance of the workpiece itself Rw, the contact resistance between the two workpieces; when the workpiece and When the electrode is fixed,the resistance of the workpiece depends on its resistance welding principle.

After the weldment is assembled, pressure is applied through the electrodes, and the method of welding by the resistance heat generated by the current passing through the contact surface of the joint and the adjacent area 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 aviation, aerospace, energy, electronics, automobiles, light industry and other industrial sectors, and it is one of the important welding processes.

1. The output and influencing factors of welding heat

The heat generated during spot welding is determined by the following formula: Q=IIRt(J)————(1) where: Q——heat generated (J), I——welding current (A), R——electrode Resistance (ohm), t——welding time (s)

1. Resistance 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+2Rew——(2) When the workpiece and the electrode are constant, the resistance of the workpiece depends on it. The electrical resistivity. Therefore, electrical resistivity is an important property of the material to be welded. Metals with high electrical resistivity have poor electrical conductivity (such as stainless steel). Metals with low electrical resistivity have good electrical conductivity (such as aluminum alloy). Therefore, spot welding of stainless steel is easy to produce heat but difficult to dissipate, and spot welding of aluminum alloy is difficult to produce heat but easy to dissipate. When spot welding, the former can use a small current (several thousand amperes), while the latter must use a large current(several thousand amperes). Million amps). 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 contact resistance exists for a short period of time, usually in the early stage of welding, which is formed by two reasons:

1) The oxide or dirty material layer with high resistivity on the surface of the workpiece and the electrode will greatly hinder the current. Too thick oxide and dirty material layercan even make the current unable to conduct.

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. At the contact point, the current line is gathered. Due to the shrinking of the current path, the resistance at the contact is increased.

The resistance Rew between the electrode and the workpiece is compared with Rc and Rw. Because the resistivity and hardness of the copper alloy are generally lower than that of the workpiece, it is very small and has a smaller effect on the formation of the nugget. We less consider its influence.

2. The influence of welding current

It can be seen from the formula (1) that the influence 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 cause of the current change is the fluctuation of the grid voltage and the change of the impedance of the secondary circuit of the AC welding machine. The impedance changes are due to changes in the geometry of the loop or due to the introduction of different amounts of magnetic metal in the secondary loop. For DC welding machines, the impedance change of the secondary loop has no obvious 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 welding time and the welding current can complement each other within a certain range. In order to obtain a certain strength of the solder joints, high current and short time (strong condition, also called hard specification) can be used, or small current and long time (weak condition, also called soft specification) can be used. Whether to choose a hard specification or a soft specification depends on the properties and thickness of the metal and the power of the welding machine used. There are upper and lower limits for the current and time required for metals with different properties and thicknesses, which shall prevail when used.

4. The influence of electrode pressure

The electrode pressure has a significant effect on the total resistance R between the twoelectrodes. As the electrode pressure increases, R decreases significantly, but the increase in welding current is not large, which cannot affect the decrease in heat generation caused by the decrease in R. Therefore, the strength of the solder joint always decreases as the welding pressure increases. The solution is to increase the welding pressure at the same time

When, increase the welding current.

5. The influence of electrode shape and material properties

Since the contact area of the electrode determines the current density, the resistivity ad thermal conductivity of the electrode material are related to the generation and loss of heat. Therefore, the shape and material of the electrode have a significant influence on the formation of the nugget. With the deformation and wear of the electrode tip, the contact area increases, and the strength of the solder joint will decrease.

6. The influence of the surface condition of the workpiece

Oxides, dirt, oil and other impurities on the surface of the workpiece increase the contact resistance. An oxide layer that is too thick may even make it impossible for current to pass. Partial conduction, due to excessive current density, will produce splashes and surface burns. The presence of the oxide layer will also affect the uneven heating of each solder joint, causing fluctuations in soldering quality. Therefore, a thorough cleaning of the surface of the workpiece is a necessary condition to ensure high-quality joints.

2. Thermal balance and heat dissipation

During spot welding, only a small part of the heat generated is used to form a solder joint, and a larger part is lost due to conduction or radiation to nearby materials. The heat balance equation is:

Q=Q1+Q2————(3) Among them: Q1——The heat that forms the nugget, Q2——The heat lost

The effective heat Q1 depends on the thermophysical properties of the metal and the amount of molten metal, and has nothing to do with the welding conditions used. Q1=10%-30%Q, take the lower limit for metals with good thermal conductivity (aluminum, copper alloy, etc.); take the upper limit for metals with high resistivity and poor thermal conductivity (stainless steel, high temperature alloys, etc.). 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). About 5% of the heat radiated into the atmosphere.

Three, welding cycle

The welding cycle of spot welding and projection welding consists of four basic stages (as shown in the spot welding process):

1) Precompression stagethe electrode is lowered to the currenton stage to ensure that the electrode is pressed against the workpiece to ensure proper pressure between the

workpieces.

2) Welding time the welding current passes through the workpiece to generate heat to

form a nugget

3) Maintenance time cut off the welding current, the electrode pressure continues to

maintain until the nugget solidifies to sufficient strength.

4) Rest timethe electrode begins to be lifted to the electrode begins to fall again, start 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 items to the basic cycle:

1) Increase the prepressure to eliminate the gap between thick workpieces and make them fit closely.

2) Use the preheating pulse to improve the plasticity of the metal, so that the workpieceis easy to fit tightly and prevent splashing; doing so during projection welding can make multiple bumps evenly contact with the plate before energized welding, so as to ensure that the heating of each point is consistent.

3) Increase the forging pressure to compact the nugget and prevent cracks or shrinkage.4) Use tempering or slow cooling pulse to eliminate the quenched structure of alloy steel, improve the mechanical properties of the joint, or prevent cracks and shrinkage without increasing the forging pressure.

Fourth, the type and scope of application of welding current

1. Alternating current can slowly increase and decrease the current through 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 multipulse spot welding, that is, for 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 occasions that require large currents. Since DC welding machines are mostly powered by threephase power, it can avoid unbalanced three-phase

loads when single-phase power is supplied.

5. Weldability during metal resistance welding

The following items are the main indicators for evaluating the weldability of resistance welding:

1. The electrical conductivity and thermal conductivity of the material. Metals with low electrical resistivity and high thermal conductivity require high-power welding machines.

Its weldability is poor.

2. High temperature strength of the material Metal with high yield strength at high temperature (0.5-0.7Tm) is prone to spatter, shrinkage, cracks and other defects during

spot welding, which requires a large electrode pressure. If necessary, it is necessary to apply a large forging pressure after the power is cut off, and the weldability is poor.

3. The plastic temperature range of the material Metal with a narrow plastic temperature range (such as aluminum alloy) is very sensitive to the fluctuation of welding process parameters. It requires the use of a welding machine that can accurately control the process parameters and requires good electrode followability. Poor weldability.

4. The sensitivity of materials to thermal cycles. Under the influence of welding thermal cycles, metals with a tendency to quench are prone to hardened structures and cold cracks; alloys that are prone to form low melting points with fusible impurities are prone tohot cracks; Strengthened metals tend to produce softened areas. Corresponding process measures should be taken to prevent these defects. Therefore, the weldability of metals with high thermal cycle sensitivity is also poor.

Resistance welding (resistance welding) is to press the workpiece to be welded betweentwo electrodes, and apply an electric current, using the electric resistance heating effectgenerated by the current flowing through the contact surface of the workpiece and the adjacent area to heat it to a molten or plastic state, and make it form A method of metal bonding.

There are four main resistance welding methods, namely spot welding, seam welding, projection welding, butt welding

Overview of resistance welding

There are many types of resistance welding, three commonly used are spot welding, seam welding, butt welding and projection welding.

1. Spot welding

Spot welding is a resistance welding method in which weldments are assembled into lap joints and pressed between two cylindrical electrodes, and the base metal is melted by resistance heat to form a welding spot. Spot welding is mainly used for thin plate welding.

The process of spot welding:

1. Preload to ensure good contact of the workpiece.

2. Turn on electricity to form a nugget and plastic ring at the weld.

3. Poweroff forging, so that the nugget cools and crystallizes under the continued pressure to form a solder joint with dense structure, no shrinkage or cracks.

Second, seam welding

The process of seam welding is similar to spot welding, except that the cylindrical electrode is replaced by a rotating discshaped roller electrode, and the weldment is assembled into a lap or butt joint and placed between the two roller electrodes. The roller presses the weldment and rotates. The resistance welding method of continuous or intermittent power supply to form a continuous welding seam.

Seam welding is mainly used for welding structures with relatively regular welding seams and sealing requirements. The thickness of the plate is generally below 3mm.

Three, butt weldingButt welding is a resistance welding method that welds the weldment along the entire contact surface.

Four, projection welding

Projection welding is a modified form of spot welding; there are prefabricated bumps on a workpiece, and when projection welding i, one or more nuggets can be formed at the joint at a time.

1. Resistance butt welding

Resistance butt welding is a method of assembling the weldment into a butt joint so that its end faces are in close contact, using resistance heat to heat it to a plastic state, and then cutting off the power and quickly applying upsetting force to complete the welding.

Resistance butt welding is mainly used for weldments with a simple cross section, a

diameter or side length of less than 20mm, and a low strength requirement.

2. Flash butt welding

Flash butt welding is to assemble the weldment into a butt joint, turn on the power, andmake the end surface gradually move closer to achieve local contact. The contact pointsare heated by resistance heat. Under the action of large current, flash is generated and the end surface metal is melted until the end is reached. When the part reaches a predetermined temperature within a certain depth range, the power is turned off and the upsetting force is quickly applied to complete the welding. The joint quality of flash welding is better than that of resistance welding, and the mechanical properties of the weld are equivalent to that of the base metal, and there is no need to clean the prewelded surface of the joint before welding. Flash butt welding is often used for welding important weldments. It can weld the same kind of metal, and also can weld dissimilar metals; it can weld 0.01mm metal wire, and also can weld 20,000mm metal rod and profile.

The quality of resistance welding is determined by the following 4 factors:

1. Current, 2. Power-on time, 3. Pressure, 4. Diameter of the resistor tip

Advantages of resistance welding

1. When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.

2. The heating time is short and the heat is concentrated, so the heat affected zone is

small, and the deformation and stress are also small. Usually, it is not necessary to arrange the correction and heat treatment process after welding.

3. No filler metal such as welding wire and welding rod, and welding materials such as oxygen, acetylene, hydrogen, etc. are required, and the welding cost is low.

4. Simple operation, easy to realize mechanization and automation, and improve working conditions.

5. High productivity, no noise and no harmful gas. In mass production, it can be programmed into the assembly line together with other manufacturing processes. However, flash butt welding needs to be isolated due to sparks and splashes.

Disadvantages of resistance welding

1. At present, there is still a lack of reliable nondestructive testing methods. Welding quality can only be checked by destructive tests on process samples and workpieces, as well as by various monitoring technologies.

2. Point and seam welded lap joints not only increase the weight of the component, butalso because of the angle formed around the weld nugget of the two plates, the tensile strength and fatigue strength of the joint are low.

(3) The equipment has high power, high mechanization and automation, which makes the equipment cost higher and more difficult to maintain, and the commonly used high-power singlephase AC welding machine is not conducive to the balanced operation of the power grid.

Application status of resistance welding in my country

With the development of aerospace, electronics, automobiles, household appliances and other industries, resistance welding has received more and more attention. At the same time, higher requirements are put forward for the quality of resistance welding. Fortunately, the development of my country's microelectronics technology and the development of highpower thyristors and rectifiers have provided conditions for the improvement of resistance welding technology. At present, my country has produced secondary rectifier welding machines with excellent performance. The control box composed of integrated circuits and microcomputers has been used for the matching of new welding machines and the transformation of old welding machines. Advanced closedloop monitoring technologies such as constant current, dynamic resistance, and thermal expansion have begun to be popularized and applied in production. All this will help improve the quality of resistance welding and expand its application areas.

Basic Principles of Resistance Welding

The generation of welding heat and the factors affecting heat generation The heat generated during spot welding is determined by the following formula:

Q =I″Rt (6-1)

In the formula Q-the heat generated (J)

I″——The square of welding current (A)

R-resistance between electrodes (Ω)

t——welding time (s)

1. Resistance R and factors affecting R The resistance between electrodes in the formula(61) includes the resistance R of the workpiece itself. , The contact resistance between the two workpieces R}, the contact resistance between the electrode and the work Resistance during spot welding

R = 2Rw, -l-Rc-I-2Rm (6-2) distribution and current line

When the workpiece and the electrode are timed, the resistance of the workpiece depends on its resistivity. Therefore, electrical resistivity is an important property of the material being welded. Metals with high electrical resistivity have poor thermal conductivity (such as stainless steel), and metals with low electrical resistivity have good thermal conductivity (such as aluminum alloy). Therefore, it is easy to generate heat but difficult to dissipate heat when spot welding stainless steel, and it is difficult to generate heat but easy to dissipate heat when spot welding aluminum alloy. When spot welding, the former can use a small current (several thousand amperes), and the latter must use a large current (tens of thousands of amperes).