| Vacuum environment is an ideal way of molten- pool protection in metal melting welding process. Vacuum melt ing welding technique is of significant technical advantage. C ompared with o ther vacuum melting welding methods, hollow cathode vacuum arc(HC VA) welding is a kind of important melting welding method in vacuum and has many advantages, such as low welding cost,simple operation process and etc. Despite the benefits of the HC VA weld ing technique, there are some technique problems retarding its wide applications such as the low melting efficiency and shallow penetration. In this paper, magnetic- field enhanced hollow cathode vacuum arc is focused on. Electron kinetic energy, with high utilization, is used as welding heat source. Confined magnetic field is installed to prevent discharging in the gap. A kind of electron beam welding based on magnetic confinement of hollow cathode vacuum arc put forward. The arc characteristics, acceleration characteristics of electron beam and practical welding are investigated.Basing on the hollow cathode vacuum arc discharge and plasma cathode electron emission principle, hollow cathode vacuum arc electron beam welding system has been developed. It is featured by high current density and higher electron energy of hollow cathode vacuum arc. In small hole extraction system, the movement of charged particles is constrained by magnetic field. This may suspress free expansion of collis ion plasma. Consequently insulation, the strength of the discharge gap is improved. In this way, due to magnetic confine ment of hollow cathode arc electron energy may be increased by applying high accelerating voltage. The power supplies for hollow cathode vacuum arc discharge and electronic accelerate have been designed and developed based on IGBT inverter in present work.High frequency arc ignition has been investigated and the result show that the aging phenomenon of hollow cathode electrode exists. After aging, the roughness of the inner surface of electrode may be larger, which is benefit for fie ld emission and arc ignition. O n the precondition of the good insulation of the hollow cathode system, the ignition rates may reach 97 % and the igniting time lasts less than 2 s.The results on HC VA plasma physics characteristics show that the plasma impedance increases and V-I characteristic curve wholy shifts up with the smaller gas flow. Compared with the HC VA, the novel HC VA discharge voltage increases substantially with increasing confined magnetic field strength. It is attributed to that the higher magnetic fie ld suspresses the plasma expansion. The lower the gas flow rate the stronger the influence of the confined magnetic field. The voltage is increased by 161 % when the confined magnetic field current increased from 10 mT to 110 mT. The confined magnetic field can confines the arc and enhance the collision rate of the electrons in turn to improve the plasma density. Compare d to the cases without the magnetic fie ld, the intensity of Ar spectral line increase by four times. The radial spectrum intensity in the middle section is nonuniform, and it is larger than that in the middle edge zone. The radius of hollow cathode vacuum arc is reduced obviously by confined magnetic field. When the discharge current is lower than 30 A and the gas flow is lower than 30 sccm, The radius of arc spot is smaller than 4.0 mm.Particle- in-Cell(PIC) mode has been used to simulate the electron beam behavior in hollow cathode vacuum arc electron beam system. The accelerating fie ld is permeated to the plasma zone through extraction electrode hole. Electro n extraction barrier is reduced due to the introduction of the accelerating field, leading to the acceleration of electrons in hollow cathode vacuum arc discharge. Because of the focus effect of plasma surface near the extraction exit, the electrons from the plasma gather together first, and then quickly defined.The accelerating characteristics of hollow cathode vacuum arc electron beam are also investigated. The results show that the extracted current is dependent on the voltage as three stages increasing rapidly, increasing slowly and keeping saturated state as the accelerating voltage increasing. This is beneficial for controlling the welding parameters. O f HC VA the extracted current increases with discharge current and the confined magnetic field. The extraction current increases by about 56 % with increasing the arc discharge current from 15 A to 25 A. When the electrons are confined and accelerated, the collision between the electrons and the neutral gas effectively increase the plasma density. C urrent consists of plasma emission electrons and electrons produced in the collision plasma in the accelerating gap. This leads to current density can reach to 150 A/cm2.Hollow cathode vacuum arc electron beam system with magnetic confinement(HC VA- EB) is applied to implement the welding experiment in this paper. Compared with conventional cathode vacuum arc welding, HC VA- EB improves the electron energy effectively, consequently gives rise to larger melting depth the higher heat effic iency. Due to the high energy utilization rate of the electron kinetic energy, the welding melting efficiency in HC VA-EB is much more than that in HCVA with the same welding heat input. |
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