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What is butt welding? (9)- New technology of flash butt welding

September 29, 2020

New technology of flash butt welding
1) Program-controlled reduced-voltage flash butt welding is characterized by the use of higher secondary no-load voltage at the beginning of the flash to facilitate the initiation of the flash. When the end surface temperature rises, the low-voltage flash is used and maintained The flash speed remains unchanged to improve thermal efficiency.

flash butt welding-schematic diagram

When approaching the upsetting, increase the secondary voltage to make the flash stronger to increase the self-protection effect.
Program-controlled reduced-voltage flash butt welding is compared with preheated flash butt welding. It has the advantages of short welding time, low power required, and uniform heating.

2) Pulse flash butt welding. The characteristic of this welding method is that in the stroke of the moving clamp, a reciprocating vibration stroke is superimposed through the hydraulic vibration device, the amplitude is 0.25-1.2mm, and the frequency is 3-35Hz. Tune. Due to the vibration, the end face of the weldment is alternately short-circuited and pulled apart, resulting in a pulse flash.

Compared with ordinary flash butt welding, pulse flash butt welding has no spontaneous blasting of the lintel, the splashed particles are small and the flame is shallow, so the thermal efficiency can be more than doubled, and the upsetting allowance can be reduced to 2/3-1/ 2.

The above two methods are mainly to meet the needs of flash butt welding of large section workpieces.

3) Rectangular wave flash butt welding Compared with power frequency AC sine wave flash butt welding, this welding method can significantly improve the stability of the flash. Because the sine wave power supply will instantly interrupt the flash when the voltage is close to zero, and the rectangular wave can evenly produce the flash in the full cycle. It has nothing to do with the voltage phase.

The number of flashes per unit time of the rectangular wave power supply is 30% higher than that of power frequency AC, the sprayed metal particles are fine, the flame is shallow and the thermal efficiency is high. The rectangular wave frequency can be adjusted within the range of 30-180Hz. This method is mostly used for continuous flash butt welding of thin plates and aluminum alloy wheels.

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What is butt welding? (8)- typical workpieces

September 29, 2020

Butt welding of typical workpieces
1, butt welding of small section workpieces

wire with diameter d≤5mm is mostly used for resistance butt welding,
Small diameter wires, wires of different materials, and wires and stamping parts (such as resistors and diode end caps) can be welded by capacitive energy storage type, which is characterized by very hard welding conditions and

resistance butt welding-Schematic diagram

extremely narrow heating range, which greatly reduces The influence of the thermal and physical properties of the welded metal on the formation of the joint.
2. Butt welding of rods
is mostly used in the butt welding of steel bars in the construction industry, usually resistance butt welding is used for diameter d<10mm; continuous flash butt welding for d>10mm; preheated flash butt welding for d>30mm. When using a manual butt welding machine, because the power of the welding machine is small (usually not more than 50KVA), when d=15-20mm, it is generally necessary to use preheated flash butt welding.

Semi-circular or V-shaped clamp electrodes can be used for butt welding of rods. The latter can be used in various diameters, so it is widely used. The rods are of solid cross-section, with greater rigidity, and longer extension lengths can be used.
3, pipe butt welding
Pipe butt welding is widely used in boiler manufacturing, pipeline engineering and petroleum equipment manufacturing. Choose continuous or preheated flash butt welding according to the section and material of the pipe. The clamp electrode can be semicircular or V-shaped. Usually when the ratio of pipe diameter to wall thickness is greater than 10, a semicircle can be selected to prevent the pipe from being crushed. V-shape can be used when the ratio is less than 10. To prevent the tube from slipping in the clamp electrode, the clamp electrode should have an appropriate working length. When the pipe diameter is 20-50mm, the length of the workpiece is 2-2.5 times the pipe diameter; when the pipe diameter is 200-300mm, it is 1-1.5 times.

Because the tube has an expanded cross-section, the heat dissipation is faster, the liquid metal on the end surface is easy to cool, and it is difficult to extrude during upsetting. The area is scattered, and the self-protection effect is weakened during the flashing process. Therefore, when the process parameters are not selected properly, non-metallic inclusions will remain in the interface to form gray spot defects. Maintain stable flash, increase flash and upsetting speed, and adopt gas protection to reduce or eliminate gray spots.

After the pipe is welded, the internal and external burrs need to be removed to ensure that the external surface of the pipe is smooth and there is a certain channel aperture inside. Deburring requires special tools.

4, thin plate butt welding

flash butt welding-schematic diagram

Thin plate butt welding is widely used in the continuous production line of rolled steel plate in the metallurgical industry. The width of the plate is from 300 to 1500mm or more, and the thickness is from less than 1mm to more than ten mm. The materials include carbon steel, alloy steel and non-ferrous metals and their alloys. After the plate is butt welded, the joint will undergo rolling and produce great plastic deformation, so it must not only have a certain strength, but also a high plasticity. For steel plates with a thickness of less than 5mm, continuous flash butt welding is generally used, and a plane electrode is used for single-sided conduction. When the plate is thick, preheated flash butt welding is used for double-sided conduction to ensure uniform heating along the entire end surface.

When welding thin plates, due to the relatively large cross-section length and width, the area is scattered, the joint cools quickly, and the self-protection effect is weak during the flashing process. At the same time, the liquid lintel is small and the liquid metal layer on the end surface is thin. Prone to oxidation and solidification. Therefore, the flashing and upsetting speed must be increased. After welding, the burr must be removed with a burr cutting device while it is hot.
5. Butt welding of ring parts
When welding ring parts (such as wheel rims, chain rings, bearing rings, jet engine mounting edges, etc.), in addition to considering the general rules of butt welding process, attention should be paid to the influence of shunting and ring deformation elasticity. Due to the shunt, the required power should be increased by 15-50%. Although the diameter of the ring decreases, the cross section increases, and the material resistivity decreases.

When the ring parts are butt welded, the upsetting pressure must consider the influence of the deformation rebound force, but because the shunt has the effect of heating the ring back, the increase in the upsetting pressure is not large.

Bicycles, motorcycle rims, and automobile rims all use continuous flash butt welding, and the front mouth of the clamp electrode must match the cross-section of the workpiece. During upsetting, in order to prevent the rebound force from affecting the quality of the joint, or even pull the joint apart, it is necessary to extend the time of no current upsetting.

Chain links such as anchor chains and drive chains are mostly used in the manufacture of low-carbon steel and low-alloy steel. Resistance butt welding can be used when diameter d<20mm, and preheating flash butt welding can be used when d>20mm. The purpose of preheating is to heat the interface. Uniform, easy to produce certain plastic deformation during upsetting.
6, tool butt welding
When cutting tool butt welding, one of the current process methods used to manufacture blanks in tool manufacturing is mainly the butt welding of high-speed steel (W8Cr4V, W-9Cr4V2) and medium carbon steel. Tool butt welding has the following characteristics:

1) The thermal conductivity and resistivity of high-speed steel and medium-carbon steel are quite different. At room temperature, medium carbon steel λ=0.42W/(cm℃), ρ0=18-22uΩcm; high-speed steel λ=0.23W/(cm℃), ρ0=48Ωcm. In order to make the temperature distribution on both sides of the joint surface basically the same , The extension length of high speed steel should be 30-50% smaller than that of medium carbon steel. Under normal circumstances, the extension length of high-speed steel is (0.5-1.0)d. In order to prevent excessive heat dissipation, the extension length is not less than 10mm.

2) High-speed steel has a high tendency to quench, the hardness after welding will be greatly increased, and quenching cracks may occur. In order to prevent cracks, preheating flash butt welding can be used. During preheating, heat the metal in the range of 5-10mm near the interface to 1100-1200℃. After welding, it is annealed in an electric furnace at 600-700℃ for 30 minutes.

3) When high-speed steel is heated to a high temperature, it will produce grain growth or the formation of ledeburite eutectic on the semi-melted grain boundary, making the joint brittle. The ledeburite eutectic cannot be eliminated by heat treatment. Therefore, it is necessary to use sufficient upsetting to eliminate this structure.

What is butt welding? (7)- Flash butt welding of common metals

September 29, 2020

The following describes the characteristics of flash butt welding of several commonly used metal materials:

1. Flash butt welding of carbon steel

This kind of material has the advantages of high resistivity, the oxidation of carbon element during heating provides a protective atmosphere for the interface CO and CO2, and does not contain elements that generate high melting point oxides. Therefore, they are all materials with better weldability.

flash butt welding-schematic diagram

As the carbon content in steel increases, the resistivity increases, the crystallization interval, the high temperature strength and the hardening tendency increase. Therefore, it is necessary to increase the upsetting pressure and the upsetting allowance accordingly. In order to reduce the effect of quenching. Preheating flash butt welding can be used, and post-weld heat treatment can be performed.

During flash butt welding of carbon steel, because carbon diffuses to the heated end surface and is strongly oxidized, and during upsetting, the molten metal with high carbon content in the semi-melting zone is squeezed out, so a low carbon content is formed at the joint. Carbon layer (white, also called bright band). The width of the carbon-depleted layer increases as the steel content increases and the preheating time increases; as the carbon content increases and the gas medium oxidation tendency decreases, it becomes narrower. Long-term heat treatment can eliminate the carbon-poor layer.

The most frequently used is carbon steel flash butt welding. As long as the welding conditions are selected appropriately, there will generally be no difficulties. The same is true even for cast iron, which is difficult to weld for melting.

Cast iron usually uses preheated flash butt welding, and continuous flash butt welding is easy to form a white hole. Due to the high carbon content, a large amount of CO and CO2 protective atmosphere is generated during flashing, and the self-protection effect is strong. Even when the process parameters fluctuate greatly, there are only a small amount of oxidized inclusions in the interface.

2, flash butt welding of alloy steel

The influence of alloying element content on steel properties and the process measures to be taken are as follows:

1) Aluminum, chromium, silicon, molybdenum and other elements in steel are prone to produce high melting point oxides. The flash and upsetting speed should be increased to reduce their oxidation.

2) As the content of alloying elements increases, the high temperature strength increases, and the upsetting pressure should be increased.

3) For pearlitic steel, the increase in alloying elements increases the tendency to quench, and measures should be taken to prevent quenching embrittlement.

The welding characteristics of low alloy steel are similar to that of medium carbon steel, and have a tendency to harden, so the corresponding heat treatment method should be adopted. This type of steel has high high-temperature strength and is easy to generate oxide inclusions. It requires higher upsetting pressure, higher flashing and upsetting speed.

In addition to the characteristics of high-carbon steel, high-carbon alloy steel also contains a certain amount of alloying elements. Due to the high carbon content and wide crystallization temperature range, the semi-melting zone at the interface is wider. If the upsetting pressure is insufficient and the plastic deformation is insufficient, the liquid metal remaining in the semi-melting zone will form a loose structure. It also contains high melting point oxide inclusions due to alloying elements. Therefore, higher flashing and upsetting speed, larger upsetting pressure and upsetting allowance are required.

The main alloying elements of austenitic steel are Cr and Ni. This kind of steel has high high-temperature strength, poor electrical and thermal conductivity, low melting point (compared to low carbon steel), and a large number of alloying elements that easily form high melting point oxides ( Such as Cr). Therefore, a large upsetting pressure, high flashing and upsetting speed are required. The high flashing speed can reduce the heating zone, which can effectively prevent the rapid growth of grains in the heat-affected zone and the reduction of corrosion resistance.

3, flash butt welding of aluminum and its alloys

This type of material has the characteristics of good electrical and thermal conductivity, low melting point, easy oxidation, high oxide melting point, and narrow plastic temperature zone, which bring difficulties to welding.

Aluminum alloy butt welding has poor weldability, and improper selection of process parameters can easily produce defects such as oxide inclusions and looseness, which will sharply reduce the strength and plasticity of the joint. In flash butt welding, high flash and upsetting speed, large upsetting allowance and forced upsetting mode must be used. The specific power required is much larger than that of steel parts.

4. Flash butt welding of copper and its alloys

Copper has better thermal conductivity than aluminum and has a higher melting point, so it is more difficult to solder than aluminum. In pure copper flash butt welding, it is difficult to form a liquid metal layer on the end surface and maintain a stable flash process, and it is also difficult to obtain a good plastic temperature zone. For this reason, high final flash speed, upsetting speed and upsetting pressure are required during welding.

Copper alloys (such as brass, bronze) are easier to butt welding than pure copper. During brass butt welding, the performance of the joint is reduced due to the evaporation of zinc. In order to reduce the evaporation of zinc, a high final flash speed, upsetting speed and upsetting pressure should also be used.

Aluminum and copper flash butt welding transition joints are widely used in the motor industry. Because their melting points are very different, aluminum melts 4-5 times faster than copper, so the extension length of aluminum should be increased accordingly. The process parameters of aluminum and copper flash butt welding can refer to the following table. When aluminum and copper are butt welded, the intermetallic compound CuAL2 may be formed, which increases the brittleness of the joint. Therefore, it is necessary to squeeze out CuAL2 from the interface as much as possible during upsetting.

5. Flash butt welding of titanium and its alloys

The main problem of flash butt welding of titanium and its alloys is the reduction of joint plasticity due to quenching and gas absorption (hydrogen, oxygen, helium, etc.). The quenching tendency of titanium alloys is related to the added alloying elements. If the stable β-phase element is added, the quenching tendency will increase and the plasticity will be further reduced. If continuous flash butt welding with strong flash is used, a satisfactory joint can be obtained without adding shielding gas. When using flash and preheated flash welding with a low upsetting speed, the welding should be performed in an Ar or He protective atmosphere. The preheating temperature is 1000-1200 degrees, and the process parameters are basically the same as when welding steel, but the amount of flash reserved is slightly increased. At this time, a higher plastic joint can be obtained.

What is butt welding? (6)- Flash butt welding of common metals

September 29, 2020

Flash butt welding of common metals
Almost all steel and non-ferrous metals can be flash butt welded, but to obtain high-quality joints, it is necessary to take necessary process measures according to the relevant characteristics of the metal. The analysis is as follows:

flash butt welding-schematic diagram

(1) Electrical and thermal conductivity For metals with good electrical and thermal conductivity, higher specific power and flashing speed, and shorter welding time should be used.

(2) High temperature strength For metals with high high temperature strength, the width of the temperature plastic zone should be increased and a larger upsetting force should be used.

(3) Crystallization temperature range The larger the crystallization temperature range, the wider the semi-melting zone. A larger upsetting pressure and upsetting allowance should be used to squeeze all the molten metal in the semi-melting zone so as not to stay in the joint Causes defects such as shrinkage, porosity and cracks.

(4) Thermal sensitivity There are two common situations. The first is quenched steel. After welding, the joint is prone to quenching structure, which increases the hardness and reduces the plasticity. In severe cases, quenching cracks will occur. Quenched steel usually adopts preheating flash butt welding with a wide heating zone, and adopts measures such as slow cooling and tempering after welding. The second type is metals strengthened by cold work (such as austenitic stainless steel). The joint and heat-affected zone soften during welding, which reduces the strength of the joint. Welding such metals usually uses a larger flash speed and upsetting pressure to minimize the softening zone and reduce the degree of softening.

(5) Oxidation The oxide inclusions in the joint have a serious hazard to the quality of the joint. Therefore, preventing and eliminating oxidation is the key to improving the quality of the joint. The composition of the metal is different, and its oxidative generation is also different. If the melting point of the formed oxide is lower than that of the metal to be welded, the oxide will have better fluidity and will be easily squeezed out during upsetting. If the melting point of the formed oxides is higher than that of the welded metal, such as SiO2, Al2O3, Cr2O3, etc., they must be discharged when the welded metal is still in a molten state. Therefore, when welding alloy steels containing more silicon, aluminum, chromium, and a class of elements, strict process measures should be taken to completely eliminate oxides.

What is butt welding? (5)- flash butt welding

September 29, 2020

(4) Flash speed vf A sufficiently large flash speed can ensure the strong and stable flash. However, if vf is too large, the heating zone will be too narrow, which will increase the difficulty of plastic deformation. At the same time, due to the increase in welding current required, it will increase the depth of the fire hole after the lintel blasting, which will reduce the joint quality. The following factors should also be considered when choosing vf:

flash butt welding-schematic diagram

1) The composition and performance of the material being welded. For materials with a lot of easily oxidizable elements or good electrical and thermal conductivity, vf should be larger. For example, it is larger when welding austenitic stainless steel and aluminum alloy than when welding low carbon steel;

2) Whether there is preheating. When there is preheating, it is easy to excite the flash, so vf can be improved.

3) There should be a strong flash before upsetting. vf should be large to ensure a uniform metal layer on the end face.

(5) Upsetting flow rate δu δu affects the removal of liquid metal and the magnitude of plastic deformation. If δu is too small, liquid metal will remain in the interface, which will easily form defects such as looseness, shrinkage, cracks, etc.; when δu is too large, the crystal lines will bend severely and reduce the impact toughness of the joint. δu is selected according to the cross-sectional area of the workpiece and increases with the increase of the cross-sectional area.

During upsetting, in order to prevent the interface from oxidizing, the current should not be cut off immediately before the end face interface is closed. Therefore, the upsetting flow should include two parts-current upsetting allowance and non-current upsetting allowance. The former is the latter. 0.5-1 times.

(6) Upsetting speed vu In order to avoid the difficulty of liquid metal removal and plastic metal deformation due to metal cooling in the interface area, and to prevent the end surface metal from oxidizing, the faster the upsetting speed, the better. The minimum upsetting speed depends on the properties of the metal. The minimum upsetting speed for welding austenitic steel is twice that of welding pearlitic steel. The welding of metals with good thermal conductivity (such as aluminum alloy) requires a high upsetting speed (150-200mm/s). For the same metal, if the temperature gradient in the interface area is large, the upsetting speed needs to be increased due to the fast cooling rate of the joint.

(7) The upsetting pressure Fu Fu is usually expressed by the pressure per unit area, that is, the upsetting pressure. The size of the upsetting pressure should ensure that the liquid metal in the joint can be extruded and a certain degree of plastic deformation will be produced at the joint. If the upsetting pressure is too small, the deformation will be insufficient and the strength of the joint will decrease; if the upsetting pressure is too high, the deformation will be too large, the crystal lines will bend seriously, and the impact toughness of the joint will be reduced.

The size of the upsetting pressure depends on the metal properties, temperature distribution characteristics, upsetting allowance and speed, and the shape of the workpiece section. High-temperature and strong metals require large upsetting pressure. Increasing the temperature gradient will increase the upsetting pressure. Because the high flash speed will increase the temperature gradient, when welding metals with good thermal conductivity (copper, aluminum alloy), a large upsetting pressure (150-400Mpa) is required.

(8) Preheating flash butt welding parameters In addition to the above process parameters, preheating temperature and preheating time should also be considered.

The preheating temperature is selected according to the cross-section of the workpiece and the material properties. When welding low carbon steel, it generally does not exceed 700-900 degrees. As the cross-sectional area of the workpiece increases, the preheating temperature should be increased accordingly.

The preheating time is related to the power of the welding machine, the size of the workpiece section and the performance of the metal, and it can be changed in a relatively large range. The preheating time depends on the required preheating temperature.

In the preheating process, the amount of shortening caused by preheating is very small, and it is not specified as a process parameter.

(9) The clamping force Fc of the clamp must ensure that the workpiece does not slip during the upsetting. Fc is related to the upsetting pressure Fu and the friction coefficient f between the workpiece and the clamp. Their relationship is: Fc≥Fu/2f. Usually F0=(1.5-4.0) Fu, the lower limit is taken for low carbon steel with compact section, and the upper limit is taken for cold-rolled stainless steel plate. When the clamp is equipped with a top support device, the tightening force can be greatly reduced, and Fc=0.5Fu is sufficient at this time.

3. Workpiece preparation

The preparation of the workpiece for flash butt welding includes: the geometry of the end face, the processing of the blank end and the surface cleaning.

When flash butt welding, the geometry and size of the butt surface of the two workpieces should be basically the same. Otherwise, the heating and plastic deformation of the two workpieces will not be guaranteed to be consistent, which will affect the quality of the joint. In production, the difference in diameter of round workpieces should not exceed 15%, and the difference between square workpieces and tubular workpieces should not exceed 10%.

When flashing butt welding large-section workpieces, it is best to chamfer the end of a workpiece to increase the current density to facilitate the laser flash. In this way, the secondary voltage can be increased without preheating or initial flashing.

The butt welding blank end can be processed on a shear, punch, lathe, or plasma or gas flame cutting, and then the end face can be removed.

During flash butt welding, the end metal is burned out during flash, so the end face cleaning is not very strict. However, the cleaning requirements for the contact surface between the clamp and the workpiece should be the same as for resistance butt welding.

What is butt welding? (4)- flash butt welding

September 29, 2020

2. Resistance and heating of flash butt welding

The contact resistance Rc during flash butt welding is the total resistance of the liquid metal lintel between the end faces of the two workpieces, and its size depends on the number of lintels and their cross-sectional area at the same time. The latter two are related to the cross-sectional area of the workpiece, the current density and the

flash butt welding-schematic diagram

approach speed of the two workpieces. With the increase of these three, the number of lintels and their cross-sectional area simultaneously existing increases, and Rc will decrease.

The Rc of flash butt welding is much larger than that of resistance butt welding, and it exists throughout the flashing stage. Although its resistance value gradually decreases, it is always greater than the internal resistance of the workpiece. Rc does not disappear completely until the upsetting starts. Figure 14-5 shows the general law of Rc, 2Rω and R changes during flash butt welding. The gradual decrease in Rc is due to the increase in the approach speed of the workpiece as the end surface temperature increases during the flashing process, and the number and size of the lintels increase accordingly.

Because Rc is large and there is the entire flashing stage, the heating of the joint during flash butt welding mainly depends on Rc.

Three, flash butt welding welding cycle, process parameters and workpiece preparation

1, welding cycle

The welding cycle of flash butt welding is shown in 14-7, and the reset time in the figure refers to the time from loosening the workpiece to returning to the original position. There are two preheating methods: resistance preheating and flashing preheating. The figure (b) uses resistance preheating.

2, process parameters

The main parameters of flash butt welding are: extension length, flash current, flash flow, flash speed, upsetting flow, upsetting speed, upsetting pressure, upsetting current, clamping force, etc. Figure 14-8 is a schematic diagram of each flow rate and extension length of continuous flash butt welding. The following describes the influence of various process parameters on welding quality and the principles of selection:

(1) Elongation length l0 is the same as resistance butt welding, l0 affects the temperature distribution along the axial direction of the workpiece and the plastic deformation of the joint. In addition, with the increase of l0, the impedance of the welding circuit increases, and the required power also increases. Under normal circumstances, bar and thick arm tube l0=(0.7-1.0)d, d is the diameter of the round bar or the side length of the square bar.

For thin plates (δ=1-4mm), in order not to lose stability during upsetting, generally l0=(4-5)δ.

When different metals are butt welded, in order to make the temperature distribution on the two workpieces consistent, usually the metal with poor electrical and thermal conductivity should be smaller. Table 1 is the l0 reference value for flash butt welding of different metals.

(2) The flash current If and the upsetting current Iu If depend on the cross-sectional area of the workpiece and the current density jf required for the flash. The size of jf is related to the physical properties of the welded metal, flashing speed, the area and shape of the workpiece section, and the heating state of the end surface. In the flashing process, as vf gradually increases and contact resistance Rc gradually decreases, jf will increase. During the upsetting, Rc disappears quickly, and the current will increase sharply to the upsetting current Iu. When welding large-section steel parts, in order to increase the heating depth of the workpiece, a smaller flash speed should be used, and the average jf used generally does not exceed 5A/mm2. Table 2 shows the reference values of jf and ju for flash butt welding of workpieces with a cross-sectional area of 200-1000mm2.

The magnitude of the current depends on the no-load voltage U20 of the welding transformer. Therefore, in actual production, the secondary no-load voltage is generally given. When selecting U20, in addition to considering the impedance of the welder circuit, when the impedance is large, U20 should be increased accordingly. When welding large-section workpieces, the method of adjusting the secondary voltage in stages is sometimes used. At the beginning, a higher U20 is used to excite the flash, and then it is reduced to an adaptive value.

(3) Flash flow rate δf The flash flow rate should be selected so that there is a molten metal layer on the end of the workpiece at the end of the flash, and the plastic deformation temperature is reached at a certain depth. If δf is too small, the above requirements cannot be met, which will affect the welding quality. If δf is too large, it will waste metal materials and reduce productivity. When choosing δf, you should also consider whether there is preheating, because the δf of preheating flash butt welding can be 30-50% smaller than continuous flash butt welding.

What is butt welding? (3)- flash butt welding

September 29, 2020

Flash butt welding

Flash butt welding can be divided into continuous flash butt welding and preheated flash butt welding. Continuous flash butt welding consists of two main stages: the flash stage and the upsetting stage. Preheating flash butt welding only adds a preheating stage before the flashing stage.

flash butt welding-schematic diagram

One, two stages of flash butt welding

1, flash stage

The main function of flash is to heat the workpiece. In this stage, the power is turned on first, and the end faces of the two workpieces are slightly contacted, forming many contact points. When the current passes, the contact points melt and become liquid metal lintels connecting the two ends. Due to the extremely high current density in the liquid lintel, the liquid metal in the lintel evaporates and the lintel blasts. With the slow advancement of the movable clamp, the lintels are also continuously produced and blasted. Under the action of vapor pressure and electromagnetic force, liquid metal particles are continuously ejected from the interface. The formation of a rapid stream of sparks-flash.

In the flashing process, the workpiece gradually shortens and the temperature of the tip gradually rises. As the end temperature increases, the blasting speed of the lintel will increase, and the advancing speed of the movable clamp must be gradually increased. Before the flashing process ends, a layer of liquid metal must be formed on the entire end surface of the workpiece, and the metal must reach the plastic deformation temperature at a certain depth.

Due to the strong oxidation of metal vapor and metal particles generated during lintel blasting, the oxygen content of the gas medium in the interface gap is reduced, and its oxidation capacity can be reduced, thereby improving the quality of the joint. But the flash must be stable and strong. The so-called stability means that no open circuit or short circuit occurs during the flashing process. The open circuit will weaken the self-protection effect of the weld, and the joint will be easily oxidized. A short circuit will overburn the workpiece and cause the workpiece to be scrapped. The so-called strong means that there are quite a lot of lintel blasts per unit time. The stronger the flash, the better the self-protection of the weld, which is especially important in the later flash.

2, upsetting stage

At the end of the flashing stage, immediately apply sufficient top pressure to the workpiece, and the interface gap is rapidly reduced, and the lintel stops blasting, that is, the upsetting stage is entered. The function of upsetting is to seal the gap between the end face of the workpiece and the fire hole left by the liquid metal lintel after blasting, while extruding the liquid metal and oxide inclusions on the end face, so that the clean plastic metal is in close contact, and the joint area produces a certain degree of plasticity Deformation to promote the progress of recrystallization, form common crystal grains, and obtain a strong joint. Although there is molten metal during the heating process in flash butt welding, it is actually welding in a plastic state.

Preheating flash butt welding is to heat the workpiece with intermittent current pulses before the flash phase, and then enter the flash and upsetting phase. The purpose of preheating is as follows:

(1) Reduce the required power to weld workpieces with larger cross-sectional area on a small capacity welder, because when the capacity of the welder is insufficient, it is impossible to stimulate continuous flashing without preheating the workpiece to a certain temperature. process. At this time, preheating is a last resort.

(2) Reduce the cooling rate after welding. This will help prevent the quenched structure and cracks of the quenched steel joint during cooling.

(3) Shortening the flash time can reduce the flash margin and save precious metals.

The shortcomings of preheating are:

(1) Extend the welding cycle and reduce the productivity;

(2) Make the process automation more complicated;

(3) Preheating control is difficult. If the preheating degree is inconsistent, the stability of the joint quality will be reduced.

What is butt welding? (1)

September 29, 2020

Butt resistance welding (hereinafter referred to as butt welding) is a type of resistance welding method that uses resistance heat to weld two workpieces along the entire end surface at the same time. Butt welding has high

welding

productivity and easy automation, so it is widely used.
Application of butt welding:
Butt welding has high productivity and easy automation, so it is widely used. Its application range can be summarized as follows:
(1) The length of the work piece, such as the butt welding of strip steel, profile, wire, steel bar, rail, boiler steel pipe, oil and natural gas transportation pipeline.
(2) Butt welding of annular workpieces, such as car rims and bicycles, motorcycle rims, butt welding of various chain links, etc.
(3) Assembling welding of components The simple rolling, forging, stamping or machining parts are butt-welded into complex parts to reduce costs. For example, butt welding of automobile steering shaft shell and rear axle housing, butt welding of various connecting rods and tie rods, and butt welding of special parts.
(4) Butt welding of dissimilar metals can save precious metals and improve product performance. For example, the butt welding of the working part of the tool (high-speed steel) and the tail (medium carbon steel), the butt welding of the head (heat-resistant steel) and the tail (structural steel) of the exhaust valve of an internal combustion engine, the butt welding of aluminum and copper conductive joints, etc. .
Classification of butt welding:
Butt welding is divided into resistance butt welding and flash butt welding.
Resistance butt welding
Resistance butt welding is a method in which the end faces of the two workpieces are always pressed tightly, heated to a plastic state by resistance heat, and then upset pressure is quickly applied (or no upset pressure is applied and only the pressure during welding is maintained) to complete the welding.
One, resistance and heating of resistance butt welding
The resistance distribution during butt welding is shown in the figure. The total resistance can be expressed by the following formula:
R=2Rω+RC+2Reω
where Rω-the internal resistance of the conductive part of a workpiece (Ω);
Rc–the contact resistance between two workpieces (Ω);
Rω–The contact resistance between the workpiece and the electrode (Ω);
The contact resistance between the workpiece and the electrode is usually ignored due to its small resistance and far from the joint surface.
The internal resistance of the workpiece is proportional to the resistivity ρ of the welded metal and the length l0 of the workpiece extending from the electrode, and inversely proportional to the cross-sectional area s of the workpiece.
Same as spot welding, the contact resistance of resistance butt welding depends on the surface condition, temperature and pressure of the contact surface. When the contact resistance has obvious oxides or other stolen goods, the contact resistance is large. The increase in temperature or pressure will reduce the contact resistance due to the increase in the actual contact area. At the beginning of welding, the current density at the contact point is very large; after the end surface temperature rises rapidly, the contact resistance decreases sharply. When heated to a certain temperature (steel 600 degrees, aluminum alloy 350 degrees), the contact resistance disappears completely.
Like spot welding, the heat source during butt welding is also the resistance heat generated by the resistance of the welding zone. In resistance butt welding, the contact resistance exists for a very short time, and the heat generated is less than 10-15% of the total heat. But because this part of the heat is generated in a very narrow area near the contact surface. Therefore, the temperature in this area will increase rapidly, and the internal resistance will increase rapidly. Even if the contact resistance disappears completely, the heat generation intensity in this area is still higher than other places.
The harder the welding rod used (that is, the larger the current and the shorter the energization time), the smaller the pressing force of the workpiece, and the more obvious the influence of contact resistance on heating.

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