Study On The Stability Of Rotating Transfer Under The Control Of Longitudinal Magnetic Field

ABSTRACT With the application of the MAG welding process in the industry, in order tostrengthen their competition, manufacturers scream for the less cost and higherefficiency welding process. So it has become the researching focus in the MAGwelding process. As we know, the continuous and high current welding is the key tothe high performance arc welding. But, as for the specific wire, with the increasing ofthe current, the process will become instable and the rotating transfer will beobserved which makes it impossible to be applied in the industry because of thelarger spatter and bad bead profile. In this thesis, the mechanism of the instability ofthe rotating transfer is demonstrated, and the factors which affect the metal transferare discussed. In this case, a new MAG welding process which is high performanceand less cost is proposed under the control of longitudinal magnetic field with thecommon shielding gas of helium-free. In this thesis, the forces acting on the liquid metal jet (LMJ) and the arc arediscussed during the process of rotating transfer, and the concept of the stability ofthe LMJ is introduced. The author thinks that the forces acting on the LMJ which iscaused by the special arc behaviour of the rotating transfer is the key to the stabilityof the LMJ. The pinch instability and the twisting instability of LMJ is demonstratedunder high current. It is the twisting instability of the LMJ that causes the rotatingtransfer of the LMJ. At the same time, the dynamic characteristics of the LMJ and therelationship between the motion of the arc and the LMJ is demonstrated. Based on it,we can draw a conclusion, it is the forces acting on the LMJ that make the LMJrotate, then make the arc rotate. This process is instable and irregular. At last, theexperimental results show the factors which affect the stability of the rotatingtransfer process, such as the shielding gas, the arc voltage and the wire stick-out. In this thesis, a new MAG welding process with continuous and high current isproposed under the control of longitudinal magnetic field. The acting mechanism ofthe longitudinal magnetic field on the LMJ during the rotating transfer is - III -北京工业大学工学博士学位论文demonstrated in this thesis. The forces acting on the LMJ and the dynamic motionbehaviour caused by the additional longitudinal magnetic field are analysed. Afterthe action of the additional longitudinal magnetic field, the forces acting on the LMJand the motion behaviour are changed. The motion behaviour of the LMJ is stable,regular and better-controlled. Base on the finite element software of ANSYS, the distribution of the magneticfield is simulated. The factors which affect the distribution of the magnetic field arediscussed, such as the shape of the excitation coil, the location of the excitation coil,the materials to be welded, and the shape of the groove, etc. Based on the simulation,we can select the scientific and suitable additional magnetic field. Based on the theoretic analyse and simulation, the high performance MAGwelding system is established, including the good performance power source, thestable and reliable wire feed system, the less cost common shielding gas and thesimply, practical and portable excitation coil. Based on the high performance weldingsystem, the interaction of the LMJ and the additional magnetic field is testified by theexperimental results. Under the shielding gas of 80%Ar+20%CO2, a stable andbetter-controlled rotating transfer is realized with the wire feed speed of 45m/min.Thus the melting efficiency of the welding materials is increased obviously. In thiscase, a new MAG welding process which is higher performance and less cost isrealized under the continuous and high current with the common shielding gas ofhelium-free.