Study On Synchronous Control Of Short-circuit Metal Transfer In CO₂ Welding Under A Magnetic Field

CO₂ welding has been widely used in industrial production due to its two advantages,high efficiency and lowcost.However,the further development of CO₂ welding is limited by its two shortcomings,high welding spatter and bad welding formation.In this thesis,the method of magnetic control is used and the external longitudinal magnetic field is added to the CO₂ welding process.Droplet's transfer frequency,droplet's shape and the welding spatter rate will be studied on different magnetic field conditions.The base metal used in the experiment is low-carbon steel Q235,and magnetic field parameters will be changed in condition that welding current equals 200A.During different phase of short-circuit transfer,the shape of the droplet will be photographed by high speed camera system.The arcing time,short-circuit time and droplets'transfer frequency will be confirmed by calculating the high speed camera pictures.According to the high speed camera picture,the spherical droplet's radius and arc angle at arcing stage were measured.Besides of these,contacting radius at early short circuit stage,equatorial plane minimum radius of necking liquid bridge at end short circuit stage,liquid bridge and molten metal's contacting radius at end short circuit stage were also measured.In order to compare these parameters'changing rules under different magnetic fields,we use mathematical calculation to analysis the force of droplets formed at each short circuiting transfer stages under different magnetic fields.Finally,the rules how external longitudinal magnetic field influences droplets'transfer frequency are obtained.Based on these researches,a principle using synchronous magnetic pulse to control droplets'transfer is proposed.Using this principle a synchronous magnetic field generator is designed,and this equipment was also used in the experiment how synchronous magnetic field influences droplets'transfer frequency.At last the spatter rate of CO₂ welding under the condition of different external magnetic fields was measured.The experiment results show that:?1?The droplets'transfer frequency can be increased by introducing external longitudinal magnetic field.When the parameter of the external high-frequency longitudinal magnetic field is 2A2kHz,the droplets'transfer frequency comes to the highest,which is 97Hz;When the parameter of the external low-frequency longitudinal magnetic field is 2A60Hz,the droplets'transfer frequency comes to the highest,which is 101Hz.?2?At the early short circuit stage,comparing with no magnetic field,external low-frequency longitudinal magnetic field can increase the droplet and molten metal's contacting radius size,which can promote droplet's transverse-spread on the molten pool.This shows that introducing low-frequency magnetic field during the early short circuit stage can decrease the transient electromagnetic resistance which added on the droplet.Besides,the droplets'transfer frequency can also be increased;At the end stage of short circuit,comparing with no magnetic field,the external high-frequency longitudinal magnetic field can be beneficial to pull off the necking liquid bridge formed at the end short circuit stage,the droplets'transfer frequency can be increased as well.?3?The low-frequency longitudinal magnetic field can reduce the welding spatter rate in a certain welding range.When the welding current is 200A and the parameter of the low-frequency longitudinal magnetic field is 2.5A70Hz,the minimum welding spatter rate comes to 2.68%.