Resistance welding
Resistance welding is to press the workpiece to be pressed between the two electrodes, and apply a current, which is heated to a molten or plastic state by a resistance heat generated by the current flowing through the contact surface of the workpiece and the adjacent region, so as to form A method of metal bonding. The basic definition of resistance welding is to press the workpiece to be pressed between the two electrodes, and apply a current, which is heated to a molten or plastic state by the resistance heat generated by the current flowing through the contact surface of the workpiece and the adjacent region. A method of forming a metal bond. 3-Way Screwdriver,Magnetic Bits Screwdriver,Phillips Screw Driver,Precision Hand Tools Yucheng Weisite Measuring Tools Co., Ltd , https://www.wsttool.com
There are four main types of resistance welding methods, namely spot welding, seam welding, projection welding, and butt welding.
Spot Welding
Spot welding is a method of electric resistance welding in which a weldment is assembled into a lap joint and pressed between two columnar electrodes to melt the base metal by electric resistance to form a solder joint. Spot welding is mainly used for thin plate welding.
Spot welding process:
1. Preloading to ensure good contact with the workpiece.
2. Power on, so that the weld is formed into a nugget and a plastic ring.
3. Power-off forging, so that the nugget cools and crystallizes under the pressure, and forms a welded joint with dense structure, no shrinkage hole and crack.
Seam Welding
The seam welding process is similar to spot welding, except that the cylindrical electrode is replaced by a rotating disc-shaped roller electrode, and the weldment is assembled into a lap joint or a butt joint, and placed between the two roller electrodes, and the roller pressurizes the weldment and rotates. A method of resistance welding that continuously or intermittently delivers electricity to form a continuous weld.
Seam welding is mainly used for welding welds that are relatively regular and require sealing. The thickness of the joint is generally less than 3 mm.
Butt welding
Butt welding is a method of electric resistance welding in which the weldment is welded along the entire contact surface.
Projection Welding
Projection welding is a variant of spot welding; there are prefabricated bumps on a workpiece, and one or more nuggets can be formed at the joint at a time.
1, resistance butt welding (Resistance Butt Welding)
Resistance butt welding is a method in which the weldment is assembled into a butt joint, the end faces are in close contact, heated by electric resistance to a plastic state, and then the power is cut off and the forging force is quickly applied to complete the welding. The resistance butt welding is mainly used for the simple section and the diameter. Or weldments with a side length of less than 20 mm and less strength requirements.
2. Flash Butt Welding
Flash butt welding is to assemble the weldment into a butt joint, turn on the power supply, gradually close the end face to reach the local contact, heat the contact points with resistance heat, generate a flash under the action of large current, and melt the end face metal until the end When the part reaches the predetermined temperature within a certain depth range, the method of powering off and rapidly applying the upsetting force to complete the welding is performed.
The joint quality of flash welding is better than that of electric resistance welding. The mechanical properties of the weld are equivalent to that of the base metal, and it is not necessary to clean the pre-welded surface of the joint before welding. Flash butt welding is often used for the welding of important weldments. It can weld the same kind of metal or weld dissimilar metals; it can weld 0.01mm wire or weld 20000mm metal bar and profile.
The quality of resistance welding is determined by the following four factors: 1. current, 2. energization time, 3. pressure, 4. resistance tip diameter.
Advantages of resistance welding 1. When the nugget is formed, it is always surrounded by the plastic ring, and 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. Therefore, the heat affected zone is small, and the deformation and stress are also small. Usually, it is not necessary to arrange correction and heat treatment processes after welding.
3. There is no need for filler metal such as welding wire and welding rod, as well as welding materials such as oxygen, acetylene and hydrogen, and the welding cost is low.
4, the operation is simple, easy to achieve mechanization and automation, and improve working conditions.
5, high productivity, and no noise and harmful gases, in mass production, can be combined with other manufacturing processes to the assembly line. However, flash butt welding requires spark isolation and needs to be isolated.
Disadvantages of resistance welding 1. At present, there is still a lack of reliable non-destructive testing methods. The quality of welding can only be checked by the destructive test of process samples and workpieces, and by various monitoring techniques.
2. The lap joint of the point and seam welding not only increases the weight of the component, but also forms an angle around the weld nugget of the two plates, resulting in low tensile strength and fatigue strength of the joint.
3. The equipment has high power, high mechanization and high degree of automation, which makes the equipment cost high and maintenance difficult, and the commonly used high-power single-phase AC welding machine is not conducive to the balanced operation of the power grid.
The application status of resistance welding in China has become more and more important with the development of aerospace, electronics, automobiles, household appliances and other industries. At the same time, higher requirements are placed on the quality of resistance welding. Fortunately, the development of microelectronics technology in China and the development of high-power thyristors and rectifiers have provided conditions for the improvement of resistance welding technology. At present, China has produced a secondary rectifying welder with excellent performance. Control boxes consisting of integrated circuits and microcomputers have been used for the matching of new welders and the modification of old welders. Advanced closed-loop monitoring technologies such as constant current, dynamic resistance, and thermal expansion have begun to be popularized in production. All of this will help improve the quality of resistance welding and expand its application.
The basic principle of resistance welding The generation of welding heat and the factors affecting the heat generation The heat generated during spot welding is determined by the following formula Q = I" Rt (6-1) where Q - the heat generated (J) I" - welding Square of current (A) R - resistance between electrodes (Ω) t - welding time (s)
The resistance R and the factor affecting R The inter-electrode resistance in the formula (6-1) includes the resistance R of the workpiece itself. Contact resistance R} between two workpieces, resistance between electrode and working room contact point welding R = 2Rw, -l-Rc-I-2Rm (6-2) distribution and current line When the workpiece and electrode have been timed, the resistance of the workpiece It depends on its resistivity. Therefore, resistivity is an important property of the material to be welded. Metals with high resistivity have poor thermal conductivity (such as stainless steel "target=_blank> stainless steel), and metals with low resistivity have good thermal conductivity (such as aluminum alloy). Therefore, spot welding stainless steel "target=_blank> stainless steel produces heat easily. It is difficult to dissipate heat, and it is difficult to heat and heat when spot welding aluminum alloy. For spot welding, the former can use a small current (several thousand amps), and the latter must use a large current (tens of thousands of amps).
The influence of main parameters on welding The influence of welding current
It can be seen from the formula that the effect of current on heat production is greater than both resistance and time. Therefore, it is a parameter that must be strictly controlled during the spot welding process. The main causes of current changes are grid voltage fluctuations and AC welder secondary loop impedance changes. 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 welders, the secondary loop impedance changes without significant effect on the current.
In addition to the total amount of welding current, the current density also has a significant effect on heating. By shunting the solder joints and increasing the contact area of ​​the electrodes or the bump size during the projection welding, the current density and the heat of soldering are reduced, resulting in a significant drop in joint strength.
Effect 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 strong specification) can be used, and small current and long time (weak condition, also called weak specification) can be used. The choice of strong or weak conditions depends on the properties of the metal, the thickness and the power of the welder used. However, there is still an upper and lower limit for the current and time required for metals of different properties and thicknesses. If this limit is exceeded, a qualified nugget will not be formed.
Effect of electrode pressure
The electrode pressure has a significant effect on the total resistance R between the two electrodes, and R decreases significantly as the electrode pressure increases. At this time, although the welding current is slightly increased, it cannot affect the decrease in heat generation due to the decrease in R. Therefore, the solder joint strength always decreases as the electrode pressure increases. While increasing the electrode pressure, increase the welding current or extend the welding time to compensate for the effect of the resistance reduction, and the solder joint strength can be kept constant. The use of such soldering conditions is advantageous for improving the stability of the solder joint strength. If the electrode pressure is too small, it will cause splashing and will also reduce the strength of the solder joint.
Effect of electrode shape and material properties
Since the contact area of ​​the electrode determines the current density, the resistivity and thermal conductivity of the electrode material are related to the generation and dissipation of heat, and thus 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 will increase and the solder joint strength will decrease.
Effect 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 unevenness of the oxide layer also affects the inconsistency in the heating of the individual solder joints, causing fluctuations in the quality of the solder. Therefore, thorough cleaning of the surface of the workpiece is necessary to ensure the quality of the joint.
Common equipment for resistance welding
The spot welder consists of a base, a pressurizing mechanism, a welding circuit, an electrode, a transmission mechanism, and a switch and adjustment device. The main part is a pressurizing mechanism, a welding circuit and a control device.
The pressurizing mechanism is a mechanism in which electric resistance welding is responsible for pressurization.
The welding circuit welding circuit refers to the conductive path composed of all the parts involved in the welding current conduction except for the welding.
The control device control device is composed of two parts: switch and synchronous control. In spot welding, the function of the switch is to control the on and off of the current. The function of the synchronous control is to adjust the welding current, accurately control the welding procedure, when the network voltage fluctuates. When it is time, it can be automatically compensated.
Butt welder
The butt welding machine is composed of a frame, a guide rail, a fixed seat plate and a moving plate, a feeding mechanism, a clamping mechanism, a fulcrum (top seat), a transformer, and a control system.
The main part is the frame and guide rails, the feeding mechanism and the clamping mechanism.
All basic components of the butt welder are attached to the frame and rail frame. The guide rail is used to ensure reliable movement of the moving plate for feeding the weldment.
The feeding mechanism of the feeding mechanism is to move the weldment together with the moving plate and to ensure the required forging force.
The clamping mechanism clamping mechanism consists of two clamps, one fixed, called the fixed clamp, and the other movable, called the movable clamp. The fixing fixture is directly mounted on the frame, and the moving fixture is mounted on the moving plate, and the moving plate can be moved to the left and right.
Resistance welding power supply
Resistance welding often uses power frequency transformers as power sources. The external characteristics of resistance welding transformers use reduced external characteristics. Compared with common transformers and arc welding transformers, resistance welding transformers have the following characteristics.
(1) High current and low voltage
The commonly used current is 2~40KA. It can even reach 150~200KA in spot welding or rail butt welding of aluminum alloy. Since the welding circuit resistance of welding parts is usually only a few micro-ohms, the power supply voltage is low, and the fixed welding machine is usually within 10V. The hanging spot welder can reach 24V.
(2) Power can be adjusted
Because the welding current is very large, although the voltage is not high, the dry machine can still reach relatively large power. The high-power power supply is even higher than 1000KW. In order to meet the needs of various weldments, the power of the welder should be easily adjusted.
(3) Intermittent working state without no-load operation
Resistance welding usually turns on the power after the weldment is assembled. Once the power supply is turned on, the transformer runs under load. Generally, no no-load operation occurs. Other processes, such as loading, clamping, etc., generally do not need to be performed. The power is turned on, so the transformer is in an intermittent state.
Spot welding laps the workpiece between the upper and lower electrodes and presses it. After energization, the workpiece partially melts and solidifies to form a solder joint after cooling (Fig. 1). The diameter of the solder joint is usually 2 times the thickness of a single workpiece plus 3 mm, and the height of the solder joint is 30 to 70% of the total thickness of the workpiece. The number of solder joints and the amount of current are selected according to the required strength of the joint. Spot welding is often used for the joining of thin-walled components such as airplanes, automobiles, railway vehicles and electrical appliances, as well as cross-connections of steel bars, bars or wire mesh. The maximum thickness suitable for spot welding: low carbon steel is generally 3 mm, and steel bars and bars are up to 25 mm in diameter. The thickness ratio should be less than 1:3 when welding two workpieces of different thicknesses. The productivity of single spot welding is generally up to 100 points per minute. Dedicated multi-spot welders are often used in mass production.
The projection welding will punch or extrude one of the welded workpieces before the welding or the bump or the convex ring, and solder with a flat electrode. The welding process is the same as for spot welding. The bumps are flattened during soldering to form joints that can weld many points or a ring at the same time. The projection welding is suitable for mass production and welding of workpieces with large differences in thickness, such as the hole cover of the aircraft, the reinforcing plate, and the envelope of the transistor.
Seam welding, also known as rolling, uses a rotating disc-shaped electrode. It can pressurize, energize and drive the workpiece forward to form a series of solder joints (Fig. 3). The welding current can be continuous or intermittent. When the weld is required to be sealed, the weld points overlap by more than 30%. Seam welding is mainly used for welding of straight, circular or circular welds, such as fuel tanks, gas cylinders, flame tubes for jet engines, and housings and mounting edges. The thickness is generally less than 2 mm and the welding speed is about 0.5~. 3 m / min.
The entire workpiece is brought into contact with the butt weld. The strength of the low carbon steel joint can reach the strength of the base metal. Butt welding includes flash butt welding and resistance butt welding. 1 flash butt welding: the two workpieces are connected to the power supply, and the contact surface is moved closer to the contact. The generated resistance heat causes the metal to be heated and burned, and is emitted from the interface in the form of a spark. When heated to a certain extent, Apply pressure quickly to complete the weld. Flash butt welding extrudes molten metal, slag and oxide from the interface. Therefore, the workpiece does not require pre-weld cleaning. Flash butt welding is widely used in the industry for welding bars, plates, pipes, rails, chains and knives, as well as automotive and bicycle rims. 2 Resistance butt welding: When the contact surfaces of the two workpieces are pressed and the electric heating reaches the thermoplastic state, the forging force is quickly applied to complete the welding. The shape of the joint is relatively uniform and there is no burr, but the front end of the welding is required to be cleaned. It is only suitable for welding small-section workpieces, such as bars or tubes with a diameter of 20 mm or less.
Resistance welding In the production of automobiles, current resistance welding machines use a large number of AC single-phase AC power supplies of 50 Hz, with large capacity and low power factor. Development of three-phase low-frequency resistance welding machine, three-phase secondary rectification contact welding machine (already used in ordinary spot welding machine, seam welding machine, projection welding machine) and IGBT inverter resistance welding machine can solve grid imbalance and improve The problem of power factor. At the same time, it can further save energy and facilitate the control of the parameters of the microcomputer. It can be better applied to the welding of aluminum alloy and stainless steel "target=_blank> stainless steel and other difficult-to-weld metals. Southwest Jiaotong University is aimed at a factory aluminum alloy rim pair. The welding was developed into a rim welding PLC (programmable controller) intelligent controller, which modified the original machine, solved the welding quality problem of the aluminum alloy rim and improved the welding productivity. Later, the same factory developed the PLC seam welding control. The problem of seam welding for the general cleaning requirements is solved. Through the development of these two controllers, it is proved that the PLC has stronger anti-interference ability and higher reliability than the single-chip microcomputer controller; Low cost, using a common single-phase power frequency AC resistance welding machine to complete the difficult butt welding and seam welding work.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn