| Laser-arc hybrid welding technology has the advantages of high welding efficiency,good joint quality,good process adaptability,small welding deformation and residual stress,which is expected to replace the traditional arc welding technology.Laser-arc paraxial hybrid welding has been the focus of research and application,but because of the high requirements of welding direction for the "one front and one back" arrangement of the two heat sources and the poor accessibility of welding torch,the welding work can not be completed usually in large and complex structures.By contrast,laser-arc coaxial hybrid welding has the advantages of better symmetry of heat source,lower sensitivity to welding direction,smaller volume of welding torch and better accessibility of space than paraxial hybrid welding.However because of the complex structure and high integration difficulty of coaxial hybrid welding torch,there are relatively few researches on it at home and abroad.In this paper,the coupling mechanism of laser and hollow tungsten electrode arc in coaxial hybrid welding was studied.A numerical model of hollow tungsten arc assisted by fiber laser(HTAAL)was established,the physical field distribution of the arc was simulated numerically,and the influence of laser power and welding current on the physical field were studied.A welding torch of HTAAL was designed,and the influences of inner diameter,cone angle,shielding gas and central airflow on the arc discharge characteristics and heat source melting characteristics were studied.The optimum structure parameters of HTAAL welding torch were obtained.It was found that there were double critical effects in HTAAL welding.The formation mechanism of double critical effects were explored by studying the characteristic curve of current and voltage,plasma spectrum,current density,conductivity,plasma interaction and keyhole behavior.The results showed that the peak temperature of HTAAL was significantly higher than that of hollow tungsten arc(HTA),the anode surface HTAAL pressure and the electromagnetic field intensity was decreased,and the discharge mode of HTAAL on the anode surface was changed obviously.The current density of spot center first increased and then decreased with an increase in the laser power.The larger the welding current,the wider the arc width,the larger the area of the high temperature zone,the higher the arc peak temperature,the greater the arc pressure.The key parameters of laser-hollow tungsten arc coaxial hybrid welding torch have important influence on the stability and penetration ability of arc discharge.When the inner diameter of hollow tungsten electrode increased from 1mm to 4mm,the high temperature area(>15000K)of the arc decreased,the peak temperature first increased and then decreased,the arc pressure first increased and then remained constant,and the weld depth first increased and then decreased.With the increase of cone angle from 20° to 100°,the peak arc temperature remained unchanged,the pressure first increased and then decreased,the voltage increased,and the arc penetration ability weakened.When the protective gas flow rate increased from 6.6L/min to 19.2L/min,the peak arc temperature first increased and then stayed the same,the arc pressure increased gradually and the arc penetration ability was enhanced.With the increase of central gas flow,the peak temperature and pressure of arc increased first and then decreased,and the arc symmetry decreases.When the central gas flow rate was less than 160 m L/min,vortex appeared in the plasma flow.The recommended internal diameter size of the laser-hollow tungsten arc coaxial hybrid welding torch was between 2.5mm and 3mm,the cone Angle was between 20° and 33°,the protective gas flow was between 15L/min and 20L/min,and the tungsten center was not allowed to enter the gas.There were obvious melting enhancement and double critical effect in laser-hollow tungsten arc coaxial hybrid welding.With the increase of laser power,the penetration ability of laser-hollow tungsten electrode coaxial composite welding arc increased first and then decreased,and there were upper and lower critical values of laser power.When the laser power exceeded the lower critical value,the increase rate of penetration depth was accelerated obviously.when the laser power exceeded the upper critical value,the weld penetration depth decreased.Under the condition that the welding current was 70 A,the upper and lower critical values of laser power were 600 W and 60 W respectively.The higher the current,the smaller the upper and lower critical values of laser power.The formation mechanism of the lower critical point of the double critical effect was that the discharge mode of heat source on the anode surface changed from ring discharge to speckle discharge,and the formation mechanism of the upper critical point was that the arc plasma expanded and blocked the laser energy transmission,resulting in the reduction of keyhole depth.With the increase of the laser,the electron density increased,the electric arc conductivity increased,the power density increased,the penetration ability increased,and the weld penetration depth increased.Compared with traditional TIG welding,the line energy of laser-hollow tungsten coaxial hybrid welding of 2mm stainless steel was reduced by 50%,the welding efficiency was increased by 80%,the weld width was reduced by 26.5%,the weld grain was fine and uniform,the joint tensile strength was increased by 5%,and the welding Angle deformation was reduced by 34%. |
