Controlling Welding Distortion And Cold Cracks Based On External Electromagnetic Field

According to electromagnetic induction principle, varying magnetic field induces eddy current inside conduct. Electromagnetic force occurs under the interaction between the eddy current and magnetic field, meanwhile joule heat also develops due to resistance of the conduct. The stress field and thermal process of conductive workpiece can be adjusted by the electromagnetic force and heat. The electromagnetic induction, as a material processing way, has many advantages, such as noncontact loading, being free from pollution, rapid loading and easy automation. In the thesis, new methods for controlling welding distortion and cold crack based on electromagnetic induction were studied by theoretical and experimental way. The main contents are as follows:The electromagnetic force of flat spiral coil were studied by theoretical and simulative analysis. An electromagnetic force formula of the spiral coil was developed based on the electromagnetic theory and electromagnetic force features. The formula establishes the relationship between electromagnetic force and physical parameters (such as discharging parameters, coil parameters and conduct material parameters). The electromagnetic force at different times inside conduct can be calculated by this formula. The calculation results from the formula agree well with the data from the literatures.The revolution of welding stress and strain under electromagnetic force was systematically studied by numerical analysis. The interaction between electromagnetic force and welding stress was described in details. Electromagnetic loading results in obvious strain rate sensitivity of material, and resistance to deformation increases correspondingly. Welding stress promotes plastic strain change under electromagnetic force. The plastic variation primarily occurs in the weld metal and near weld zone where electromagnetic forces are high. The direction of plastic variation is longitudinal rather than transverse. In addition, it is found that using double coils can obviously increase utilization efficiency of electromagnetic energy. Compared to single coil, using double coils produces more plastic change and more obviously decreases welding stress. The numerical simulation was made to compare the difference of the stress and strain between multiple electromagnetic impacts and conventional welding. The numerical results show the multiple electromagnetic impacts during welding can distinctly reduce welding residual stress and distortion, and using double coils have more energy efficiency than single coil.New pulse electromagnetic impact equipment was developed. The experimental system of multiple electromagnetic impacts was built to control welding stress and distortion during welding. With the control program and hardware circuit, welding machine cooperated with electromagnetic power in time sequence, in order to load multiple electromagnetic impacts during welding. The experimental results also show that double coils have more efficiency than single coil.The hybrid electromagnetic induction method, which uses both electromagnetic force and joule heat, has been proposed to control welding cold crack of high-strength steel. As an auxiliary heating source, a flat coil was introduced behind welding heating source to adjust welding thermal cycle and improve the microstructures and toughness of the joint. Moreover, a flat spiral coil was used below the welded plate to reduce welding residual stress with multiple electromagnetic impacts along welding direction. Both actions control the welding cold crack of joint together. The results of Y groove cracking test show that the heating from flat coil obviously decreases the amount of brittle martensite and maximum microhardness of HAZ (Heat Affected Zone). The welding residual stresses are reduced due to the plastic extension under electromagnetic impacts. The resistance of the joint to cold crack is improved obviously.