Dynamic Behavior And Influence Of Arc Cathode Spots In Hyperbaric GMAW

At present,there is still a lot of uncertainty about the supply of fuel in the international markets,the competition for oil resources is very hot.Therefore,the development of petroleum resources in the ocean is an inevitable demand to ensure the sustained and rapid development of our country's economy.With the increase of working years,marine equipment is constantly corroded by sea water,and the equipment needs deep-sea repair.The common repair method is high-pressure dry MIG welding,but under high pressure,the arc shrinks,the welding process is extremely unstable,and the welding quality is poor.The study found that the cathode spot is one of the factors that affect the stability of the arc.By studying the dynamic behavior of the cathode spot,the influence mechanism of the cathode spot on the arc is obtained,which provides a reference for improving the stability of the high-pressure dry MIG welding process.The thesis first conducts a theoretical analysis of arc cathode spots,and it is clear that the MIG welding cathode is a cold cathode,and its electron emission is mainly field emission,supplemented by other electron emission methods.Cathode spots follow the principle of minimum voltage and are adjusted with the minimum energy consumption of the arc;surface charge density of the workpiece is high or the cathode spot is prone to appear in the position with a small work function.As a result,research on the cathode spots of high-pressure dry MIG welding was carried out for the application of activator on the surface of the workpiece and the change of the surface condition of the workpiece(increased bumps).The simulation study of cathode spots is carried out by the software COMSOL Multiphysics.Two models of surface condition change and surface activation are carried out for MIG welding simulation,and the MIG simulation work was carried out when the ambient pressure was between 0.1Mpa and 0.9Mpa and the welding current was 150 A,200A and 250 A.The simulation results show that the bump on the workpiece surface has an obvious effect on the cathode spots.The position with bump has a higher current density,and it is easier to form cathode spots.The welding current and the position of the bulge remain unchanged.With the increase of the pressure,the current density near the bulge decreases,and the ability of cathode spots decreases.Under the same environmental pressure and welding current,the closer the bulge position is to the welding wire,the higher the current density will be,and the more likely cathode spots will be formed.The surface activation also has a great effect on the cathode spots.The current density at the surface activation site increases sharply,and the cathode spots are more likely to appear.At the same pressure,as the welding current increases,surface activation will increase the current density,making it more prone to cathodic spots.When the welding current remains unchanged,the higher the environmental pressure,the lower the effect of surface activation on current density,but it is still higher than the normal value,and cathode spots are easy to appear.According to the simulation results,experiments were carried out to verify the conditions of the cathode surface and the activation of the cathode surface.Dry MIG welding experiments were carried out under different environmental pressures.The experimental results of changing the surface condition show that the bumps are prone to cathodic spots;as the welding current increases,the volume of the cathode spots increases;as the ambient pressure increases,the arc is unstable,and the cathode spots will lead the arc to the bulge The position is tilted.Cathode surface activation experiments show that the activation area is more likely to produce cathode spots,and the welding arc inclines to the active zone;the larger the welding current is,the larger the cathode spot volume is;as the pressure increases,the arc shrinks,and it is difficult to maintain the arc and activate Cathode spots in the area help maintain the arc and make the welding process more stable.The accuracy of the numerical simulation was verified.