Selection of process parameters for the hottest fl

2022-09-23
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Selection of process parameters for flash welding

high quality flash welding joints are guaranteed by reasonable welding process parameters. Reasonable welding process parameters can not only ensure that low-carbon steel workpieces get high-quality joints, but also make some metal workpieces with poor weldability get satisfactory joint quality

1. Welding voltage and current

proper welding current is a necessary condition to excite flash and maintain the stability of flash process. Usually, the strength of welding specification is expressed by current density (current through unit butt surface). The welding current density of flash welding is usually ampere/mm2

the current density is too small, the heating speed is low, and the flash cannot be excited quickly or the flash process is very unstable. If the current density is too high, the flash process will be too intense, which will leave too deep fire pits on the welding end face. When upsetting, it is often difficult to close them and produce defects. If the current density is too high and the flash process is too intense, the temperature field in the welding area will be too low. When upsetting, it is not easy to produce plastic deformation and the joint quality will be reduced. In mass production of low carbon steel and low alloy steel workpiece joints, the upper limit value of current density is usually selected

after the butt welding machine is determined, the connector current is determined by the voltage. The welding voltage and welding current can be adjusted by changing the grade of the primary coil tap of the welding transformer (the welding voltage of some welding machines is stepless)

2, burning speed, feeding speed

the burning rate of flash welding is determined by welding current density, preheating degree and chemical composition of workpiece. During flash welding, heat is generated on the contact surface of two workpieces, the temperature of workpieces increases with the increase of flash time, and the burning speed increases with the increase of workpiece temperature. In order to maintain the continuity and stability of the flash process, the feeding speed of the workpiece must be compatible with the burning speed. When the feeding speed is greater than the burning speed, the two contact surfaces of the workpiece will be short circuited. Since the burning speed increases with the temperature rise of the workpiece, the workpiece should be accelerated during feeding, and its motion curve should be close to the parabola

3. Burn retention

burning allowance is the length lost when the workpiece is flashed. This burning loss must be considered during flash butt welding of workpieces to ensure that the length of workpieces after welding meets the design requirements. The basis for determining the amount of burning allowance is that this allowance must be sufficient to ensure that uniform heating can be generated on all end faces of the two weldments at the end of the burning (flashing) process, and the degree of heating should be that plastic deformation is easy to occur during upsetting, so as to obtain high-quality joints. The larger the interface area of the weldment is, the slower the temperature homogenization speed on the contact section is, so the longer the flash (burning) time is, and the larger the burning allowance is. When the preheating method is adopted, the temperature of the contact surface is higher and diffuses evenly to the whole contact surface. During the flashing process, the time of homogenization of the section temperature can be shortened, so the amount of burning can be reduced. For steel parts with circular cross-section, when continuous flash welding is adopted, the burning allowance (the sum of the burning loss of two workpieces) is generally (0.60.7) d according to the different load measurement methods. When preheating flash welding is adopted, the burning allowance is (0.40.5) d, (D is the diameter of the workpiece)

4. Allowance for upsetting

upsetting allowance refers to the shortening of workpiece during upsetting

(1) all molten metals and metal oxides must be pushed out to the surface of the workpiece

(2 used to place the test piece) the two contact surfaces must be in close contact with metal on the whole section

that is, the contact part of the two workpieces should produce sufficient plastic deformation. Too small upsetting amount is the main reason for shrinkage cavity, casting structure, gray spots, oxidized inclusions, incomplete penetration and other defects in the joint. Excessive upsetting will extrude too much plastic metal and increase the difficulty of removing joint burrs. In order to ensure that the high-temperature metal (liquid) on the mating surface will not be oxidized at the beginning of the upsetting, and to supplement the insufficient heating in the flash process, the power supply cannot be cut off before the upsetting starts, but during the upsetting process. The upsetting process before cutting off the power supply is called live upsetting, and the subsequent process is called dead upsetting. Therefore, the allowance for upsetting is actually the sum of the electric upsetting and the non electric upsetting. Generally, the electric upsetting amount accounts for 30% and 40% of the total upsetting allowance

5. Upsetting speed

in flash butt welding, it is generally expected that the upsetting speed should be as high as possible, so as to fully squeeze out the molten metal before solidification, and the best upsetting of the notch when the metal has good plasticity. High upsetting speed will greatly reduce the risk of oxidation of high-temperature metal in the interface, so as to obtain high-quality welded joints. Generally, the upsetting speed shall not be less than 20 mm/s. Hydraulic drive welding machines generally have a large upsetting speed, which can reach 60mm/s

6. Extension length

extension length refers to the length of the workpiece protruding from the jaw (conductor) of the fixture. After the workpiece is clamped, the distance between the two conductors is the total extension length, and its value is the sum of the extension lengths of the two workpieces. When the cross-sectional area of the two workpieces is the same as the material brand, the plastic in the mixer with the total extension length becomes half of the powdery material. When welding workpieces of different materials, due to their different conductivity, conductivity

heat rate, or melting point, their melting speed is different when they flash. At this time, when clamping the workpiece, the extension length of the workpiece with fast melting should be greater than that of another workpiece. When welding metal workpiece with circular section, the total extension length is generally (11.4) d (D is the diameter of workpiece section)

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