Suppression Mechanism Of Humping Bead In High Speed GMAW With The External Transverse Magnetic Field

Gas metal arc welding(GMAW)is the most widely used welding process in the industry,because of its versatility and specific advantages,such as high deposition rate,low cost,easy operation and automation.There is an increasing demand on the welding efficiency in modern manufacturing with the development of economy and more severe market competition.It is an important way to raise productivity and lower production cost by increasing the welding speed.However,if the welding speed exceeds a critical value,severe geometric defects such as humping bead will occur in GMAW,and this becomes a barrier for the improvement of weld productivity,since the welds with humping bead are unacceptable in the manufacturing industry.Consequently,it is of great theoretical value and practical significance to take low-cost,highly adaptable and easy-operation measures to suppress the humping bead and achieve high speed GMAW without weld bead defects.In this study,a novel magnetically controlled high speed GMAW method is proposed.An external transverse magnetic field is exerted to the high speed gas metal arc weld pool.The additional electromagnetic force,produced by the interaction of welding current and the external magnetic field,can be used to control the weld pool behavior and molten metal flow so that the humping bead can be suppressed or eliminated.The high speed GMAW system with the transverse external magnetic field generator is established to conduct the welding experiments.The needed excitation current(and associated magnetic flux density)under different welding conditions is found,and the matching relation between the welding parameters and the exciting current is determined.A three-dimensional numerical model is established to calculate the distribution of external magnetic field and current density in workpiece based on the electric magnet arrangement applied in welding experiment.And finally,the additional electromagnetic force in the weld pool is calculated.A deflection model of liquid metal stream at the wire end under the action of external magnetic field is improved to calculate the deflection distance.The visual sensing system for molten metal flow at weld pool surface,arc shape and droplet transfer is developed.By combination with the tracer particle technique and vision sensors,the sequence images of weld pool are captured under different welding conditions and excitation current levels.By image processing,camera calibration and the tracing the flow of tracer particles,the fluid flow information at weld pool surface is obtained indirectly.The characteristic parameters of the backward molten metal flow momentum inside weld pool is extracted,and then the length and the velocity of the backward molten metal flow is calculated.In this study case,there exists critical values for the maximum flow velocity of tracer particle and the length of backward molten metal flow in weld pool.When the maximum flow velocity of tracer particle and the length of backward molten metal flow are beyond the critical values,the humping bead is formed.It is found that the backward molten metal flow with very high momentum inside weld pool and the pinching&rapid solidification of the backward molten flow channel cause the humping bead formation in high speed GMAW.The forward additional electromagnetic force due to the external magnetic field can suppress the length and velocity of backward molten metal flow effectively and improve the transverse filling of molten metal so that the pinching and solidification of thin and narrow backward molten metal flow is avoided.Meanwhile,the external magnetic field results in a forward deflection of the arc column and liquid metal stream on wire tip,which can help to increase the thickness of molten metal layer in the front of the weld pool to reduce the momentum of backward molten metal flow.The length and maximum velocity of backward molten metal flow can fall below the critical values so that the humping bead is suppressed.Based on the above research results,the high speed GMAW process is optimized by combining the external magnetic field generator with a suitable backward inclination angle of torch to suppress the humping bead and spatter.Under the conditions of this study,the welding speed can reach 2.01 m/min when the mild steel plates of 3 mm thickness is welded.