Physical Mechanism And Welding Process Of Low Power Pulsed Laser Induced Enhanced Twin Arc

With the proposal of the national“Double Carbon”goal(Carbon Peak,Carbon Neutralization)and the development concept of“Green Manufacturing”,energy-saving,high-quality and efficient modern welding technology has become a research hotspot in the welding field.Laser induced twin arc hybrid welding technology makes full use of the advantages of laser welding and twin arc welding.It has attracted extensive attention of scholars because of its low welding energy consumption,high welding efficiency and good welding quality.The excellent welding characteristics of this hybrid heat source come from the mutual promotion and enhancement between laser and twin arc plasma.However,the conditions of laser induced enhanced twin arc discharge,particle migration,composition change and physical state between plasmas,and the influence mechanism of coupling discharge delay characteristics are not clear.The study of the physical mechanism of the interaction between laser and twin arc plasma in hybrid welding is conducive to the further development and application of this technology.In this paper,low power pulsed laser induced twin arc hybrid heat source welding of magnesium alloy is taken as the research object.This is because magnesium alloy has the characteristics of low melting point,low boiling point and low ionization energy,which can enlarge the welding experimental phenomenon and is conducive to the observation of experimental results.The coupling discharge enhancement and delay phenomena of laser induced twin arc are studied.Through the comprehensive analysis of the behavior and spectral characteristics of twin arc plasma under the action of laser,the dynamic behavior characteristics of laser keyhole and the welding effect of hybrid heat source,the physical mechanism and conditions of pulsed laser induced enhanced twin arc discharge are revealed.Finally,under the guidance of the above theoretical research results,a new low power pulsed laser induced twin arc hybrid welding process for magnesium alloy and titanium alloy medium and thick plates is developed to investigate the applicability of this method in medium and thick plate welding.The main research contents and conclusions of this paper are as follows:(1)Study on the coupling discharge enhancement of pulsed laser induced twin arc.In this part,by studying the spectral characteristics and behavior of twin arc plasma under the action of laser pulse,the coupling discharge enhancement phenomenon of pulsed laser induced twin arc is found.The results show that the enhancement effect of laser on twin arc plasma can occur in laser induced parallel twin arc hybrid welding(LITTW).When the coupling discharge enhanced phenomenon occurs,the discharge position of the independent twin arc of the circuit moves to the keyhole to form a common conductive channel with concentrated energy.At this time,the twin arc is in the form of coupling plasma,the plasma electron temperature decreases and the electron density increases,the welding penetration and melting efficiency increase;when the laser pulse energy is 15 J,the pulse width is 3 ms,and the twin arc current is 100+100A,the distance between the laser and the tip of the tungsten electrode is 4 mm,the distance between the two tungsten electrodes is 1 mm and welding speed is 800 mm/min,the average energy density of the hybrid heat source is increased from 2.51×10~3 W/cm~2 of the twin arc heat source to 2.4×10~5 W/cm~2,which is about 95.6 times that of the twin arc heat source.Laser induced tandem twin arc hybrid welding(TLTHW)is investigated.In TLTHW hybrid welding,Dla(the distance between the laser and the tip of the tungsten electrode)and Daa(the distance between the two tungsten electrodes)are adjusted synchronously,they cannot meet the conditions of“laser induced arc discharge”and“twin arc forming common conductive channel”at the same time.Therefore,the coupling discharge enhanced phenomenon cannot be realized in TLTHW welding.(2)Study on the coupling discharge delay of pulsed laser induced twin arc.By analyzing the delayed recovery behavior of twin arc plasma under the action of laser pulse,combined with the dynamic behavior characteristics of keyhole under the action of laser pulse,the coupling discharge delay phenomenon of pulsed laser induced twin arc is found.The results show that when the laser pulse energy is 15 J,the pulse width is 2.4 ms,the total arc current is 200 A,the distance between the laser and the tip of the tungsten electrode is 4 mm,the distance between the two tungsten electrodes is 1 mm and the welding speed is 800 mm/min,the keyhole existence time of laser induced twin arc is 9.2 ms,which is 4 ms longer than that of laser induced single arc,and the welding penetration is increased by 29.2%.After the end of the laser pulse,the time that the laser keyhole continuly supplies Mg atoms to the twin arc plasma is the main factor affecting the plasma recovery delay.Increasing the electron density and arc voltage of plasma and reducing the diameter of arc conductive channel can inhibit the backfilling behavior of laser keyhole and prolong the delay time of laser induced arc coupling discharge.(3)Study on the physical mechanism and conditions of pulsed laser induced enhanced twin arc heat source.According to the physical phenomenon and theoretical basis of hybrid welding,the physical mechanism of low power pulsed laser induced enhanced twin arc discharge is proposed:When the laser pulse acts,the twin arc plasma and the laser keyhole plasma are coupling to form a common conductive channel with concentrated energy,which is“once enhancement”;At the same time,the coupling electromagnetic field of twin arc is formed,which further compresses the conductive channel of twin arc,which is“secondary enhancement”.When the sum of electromagnetic force between keyhole current and twin arc current can overcome the arc stiffness,the induced enhanced phenomenon of laser on twin arc occurs.The mass and energy transfer between twin arc plasma and keyhole plasma is realized by the migration and violent collision of charged particles.The physical state of laser induced enhanced twin arc plasma is analyzed.The results show that the electron kinetic energy of twin arc plasma increases by 37.8%after laser pulse action.The increase of electronic kinetic energy promotes the rapid transfer of arc energy,which is conducive to reducing welding energy consumption and improving welding efficiency.(4)Study on welding processes of medium and heavy plate with laser induced twin arc hybrid heat source.Based on the secondary enhancement mechanism,the single side welding and double side forming butt welding process of medium and heavy plate magnesium/titanium alloy with low power pulsed laser induced twin TIG arc hybrid heat source was developed.The results show that on the premise of realizing single-sided welding,double-sided forming and uniform penetration of 6mm magnesium alloy plate,the energy consumption of laser induced twin arc hybrid heat source welding is only 44.3%of that of single arc welding and 71.7%of that of laser induced single arc hybrid heat source welding,That is,the energy utilization rate of laser induced twin arc hybrid heat source is 2.26 times higher than that of single arc welding and 1.40 times higher than that of laser induced single arc hybrid heat source,while the welding efficiency is 5.25 times that of single arc welding,which is 32%higher than that of laser induced single arc hybrid heat source.At the welding speed of 680 mm/min,the low energy consumption,low heat input,high efficiency and high quality jointing of 6mm thick TA2 pure titanium plate are realized.The welding seam is formed continuously and uniformly without defects such as oxidation,porosity,cracks,etc.;the welding efficiency is 2.3 times and 3.3 times that of the laser-single TIG arc and twin TIG arc respectively,while the welding energy consumption is only 50.9%and 67.6%of both,the welding heat input is 59.2%and 35.6%of both;The grains in the weld zone are refined,and the tensile strength of the joint is equivalent to that of the base material.Therefore,the secondary enhancement mechanism of low power pulsed laser induced twin arc discharge is suitable for high-efficiency welding of medium and thick plates.