| With the development of welding technology, it has become the research focus how to improve the welding efficiency and quality and broaden its application scope. Traditional welding methods are limited in the application because of low deposition coefficient, so it is necessary to find a new way to distribute the energy between wires and base metal more properly. Twin-wire indirect arc welding is a new fusion welding method. In the process of twin-wire indirect arc welding, the welding arc burns between the twin wires instead of between the wire and the base metal. So the arc is called indirect welding arc. In this method, the twin wires are connected with the negative pole and the positive pole of the DC power source respectively, and the base metal is an independent unit. Most of the energy is used to melt the wires while a little for base metal, so the deposition coefficient is higher. What's more, because there is almost no current passing the base metal, the penetration ratio, residual stress and welding deformation is lower in this new method. So twin-wire indirect arc welding is a high efficiency method of good prospect.The equipment of indirect arc welding is different from conventional fusion welding equipment. In this study, the key component of the equipments-the welding torch was optimized firstly. On one hand, the welding arc shape in the indirect arc welding process affects the weld penetration and weld quality, on the other hand, the arc shape is affected by many factors. In this paper, the affecting laws of process parameters and magnetic field on the indirect arc shape were studied. Meanwhile, the penetration changes with different indirect arc shapes were discussed by experiments. The main devices in the experiments include FASTCAMSuper-10KC high-speed camera, Agilent54624A digital oscilloscope, LZ-630 teslameter, the self-designed external excitation equipment, twin-wire indirect arc welding equipment. The welding consumables include H08Mn2SiA wires ofΦ1.2, Q235 plate of 3 mm thick, shielding gases of pure Ar or compound of Ar and CO2. The welding experiments were carried out on plates by surfacing.While designing the welding torch, the torch structure, component connection and the boot cap shape were simulated and optimized. Many factors were considered, such as the centration of twin wire, insulativity of wire from the contacts, flexibility of the shape, the reliability and gases'protective effects. The new structure of twin severely bended contact matching insulated parts by vertical thread is replaced by twin slightly bended contacts matching insulated parts by horizontal thread, and the new structure is easy for operation and maintenance. The protective effects of three boot caps were contrasted by numerical method. And the varied cross-section boot cap was chosen finally for better protective effect, with its cylindrical surface length of 20mm and included angle of 45°.The experiments on indirect arc characteristics show that the melting rate of negative wire is highly faster than positive wire. It is the premise for arc burning stably that the wire melting rate is the same as the wire feeding rate for both poles. When the wire feeding rate is changed a little, the arc could be adjusted by itself and remain stable. However, the arc would burn unsteadily, even out if the wire feeding rate is severely higher or lower than the melting rate.In twin-wire indirect arc welding process, there are several parameters that could affect the appearance of the welds and the welding quality. The parameters include wire extension, included angles of twin wires, the distance from wires' crossing point to base metal, welding current, arc voltage, welding speed and percentage of CO2 in protective gas, called L,α, d, I, U, v, CO2%, respectively. Experiments show that the wires'melting rate increases with increasing of wire extension, which is mainly caused by increasing resistance heat, and the welding process would be unstable when the wire extension is too long. The proper value of L for good appearance of weld and welding quality is 10mm. The change of the included angle of twin wires would affect indirect arc shape and arc forces on base metal, and then the penetration depth of the joint. The arc shape turns thinner and longer and the arc forces on base metal turns bigger with decreasing of included angle, accordingly, the penetration depth increases. The distance of the two wires' crossing point to the base metal is a critical factor for penetration depth, when the distance becomes short, the indirect arc is compressed severely and the welding penetration would be increased. The proper value range of the distance is 6-8mm. The arc shape turns centralized and the penetration depth increases with the increasing of welding current. However, the arc shape turns dispersed and the penetration depth decreases when the current is too high. The arc shape turns large and bright and the weld width increases with the increasing of arc voltage while penetration depth changes little. The welding speed of 10mm/s-11mm/s is proper for higher penetration. Different composition of the protective gas affects the arc shape because of different physical and chemical properties, the arc turns thinner and the penetration depth increases with the increasing of CO2 percentage in Ar-CO2 mixed shielding gas. However, when CO2 percentage is higher than 50%, the amount of spatter increases, and weld appearance is bad. The proper composition of mixed shielding gas is 75%Ar+25%CO2.The change of magnetic field distribution around the twin wires would affect the motion of particles and the arc shape, and the arc force on the base metal and the penetration depth would alter too. By the self-designed external excitation equipment, external magnetic field including transverse magnetic field and longitudinal magnetic field was applied to change the magnetic field distribution around the twin wires in order to get ideal indirect arc shape. All the magnetic fields affect the arc shape and the process property. The arc shape turns thinner and the penetration depth increases with the increasing of forward direction transverse magnetic field intensity, inversely, the arc turns shorter and the penetration depth decreases. The penetration and the weld width affected by the longitudinal magnetic field were studied. The mechanism of the effect of applied magnetic fields on arc was discussed and the steady-state model of indirect arc was established, and indirect arc can be expressed by axial and radial component of current vector and external magnetic field vector.The modes of metal transfer in twin-wire indirect arc welding include globular transfer, short circuiting transfer, projected transfer and spray transfer. Usually, the transfer modes of twin wires are different because of different heat production of two poles, so there are totally seven different kinds of transfer mode considering different matching modes of two poles. The welding current and arc voltage recorded by the oscilloscope with different transfer modes show that the mode of spray transfer is the most stable mode, which is considered as the precondition for good weld appearance and stable welding process. The influence of the process parameters and the applied transverse magnetic field on the critical current value was investigated. The critical current value becomes lower with decreasing of included angle and increasing of CO2 percentage in the mixed shielding gas. Applied forward transverse magnetic field would refine the droplets and promote droplets transfer, and the reverse transverse magnetic field would hinder droplets transfer, inversely. |
