A New Method Of Controlling Welding Hot Crack And Distortion Based On Electromagnetic Force During Welding

Based on electromagnetic (EM) induction law, a new method of controlling welding stress and strain during welding with trailing impact EM force (WTIEF) is put forward to control the welding hot crack and distortion. The mothod avoids surface flaws which reduce mechanical performances, especially fatigue strength, as the coil do not contact with weldment applying the electromagnetic force . Moreover, the following characteristics are also very attractive, such as easily controlled energy and flexibly operated equipment. In this thesis, the shape of EM coil, feature of EM force, plastic deformation history of workpiece and effect of processing parameters on EM force are studied in detail by means of ANSYS software. A EM impact equipment is developed. Welding hot crack is suppressed during welding with the actuator consisting of flat coil and coaxial concentrator (FCC),while control welding buckling distortion after welding is performed with EM force. A new concept, dynamic yielding stress, is first introduced. And the optimal applying force location is also determined while controlling welding distortion during welding.In the thesis, the key technology of simulation by finite element method is discussed; Electromagnetic-Circuit coupled finite element model is verified with the measurement results of magnetic field of coil and current in discharge circuit.The single EM impact experiment of metal sheet is carried out. And Electromagnetic-Circuit-Structure coupled finite element model is verified. The characteristic of EM force of plat coil is studied in detail. The results show the direction of axial (perpendicular to workpiece) EM force is always downward almost and is greater than radial (parallel to workpiece) EM force. These all improve ductility deformation of metal in weld and near weld. In addition, distribution of the radial EM force also facilitates controlling of welding hot crack and distortion when the position of the coil is appropriate. The effect of temperature on EM force and plastic deformation in workpiece under EM force is studied. When temperature increases, axial EM force is down and radial EM force is up. When temperature distribution agrees with Gauss distribution, axial EM force decreases and the location of maximum EM force moves outward along coil radius direction, while radial EM force increases and the change location of radial EM force direction moves inward along coil radius direction.The controlling welding hot crack with EM force during welding is completed. The FCC actuator concentrates magnetic field into small region and is able to apply EM force in high temperature region. But its drawback is loss of energy. EM force of FCC with right dimensions is capable of squeezing high temperature metal in BTR, so it can control welding hot crack. And the crack rate decreases with the increasing voltage and impactive frequency.As the tension plastic deformation occurs near location of maximum EM force of flat spiral coil, the flat coil is good to control welding distortion. To control welding distortion, EM force should equal to dynamic yield stress, i.e., difference between metal yield stress and welding yield stress. Thus, the optimal location of applying EM force is minimum location of dynamic yield stress. During welding of metal sheet, the location of the smaller dynamic yield stress is the area of short distance from weld pool and large distance from weld pool, and there is a region with the large dynamic yield stress between these. Although the axial EM force decreases a little for WTIEF because of weldment with temperature distribution, the optimal location of applying EM force does not change.The effect of processing parameters on EM force is analyzed. The results indicate that the maximum EM force density is decreased with increasing the circuit resistance, the inductance, the gap distance between coil and workpiece and the workpiece thickness. The effect of number of turns and dimension of coil on EM force is related to the circuit resistance and inductance. The bearing force for well magnetic material is larger than well electric material. In addition, it is difficult to increase EM force because of limitation of coil size and magnetic saturation when the magnetic core is inserted into coil.