Study On The Characteristics Of Plasma Plume In Pulsed Laser Welding On Stainless Steel

As an important physical phenomenon,laser welding plasma can seriously affect the laser energy density by work piece through absorbing,refracting and reflecting the CO2 laser energy,which will then greatly influence the welding process and weld quality.The dynamic behavior of laser welding plasma contains much transient information related to the welding process and welding quality.Therefore,it is of great importance to do some research on dynamic behavior of the plasma during laser welding for improving the stability and reliability of the laser welding process.Therefore,this thesis firstly designs a new data synchronous acquisition about acoustic emission signal,audible sound signal,electric signal and high-speed photograph of the plasma during laser welding.This acquisition could collect multi-dimensional information of laser welding plasma at the same time.The paper studies propagation characteristics of energy in pulsed laser welding plasma plume on stainless steel,effect of protective gas medium on laser welding plasma and the characteristics of the plasma energy coupling of laser welding.The experiment results show the evolution of the plasma process can be divided into arouse,maintain and attenuation three stages through the plasma charge signal under the action of single pulse laser energy.Then,base on laser welding plasma current model,calculate the electron temperature of laser welding plasma to study the effect of laser welding plasma body heat.The results show that laser welding plasma electron temperature and plasma charge voltage was a positive correlation,plasma electron temperature is maintained between 2000 K and 5500 K,and there is a great influence that laser welding parameters on plasma electron temperature.Moreover,results obtained from the analysis of the heat transfer characteristics of laser welding plasma show that the electron number density of laser welding plasma between nozzle and workpiece increase,the laser plasma shielding effect strengthen,so that the weld penetration decrease.On the contrary,the weld penetration increases.Then,base on the charge voltage signal and the electron temperature of laser welding plasma,the paper analyses the effect of protective gas on laser welding plasma dynamic behavior and thermal effect.The results show that,relative to Ar,the shielding gas use of He can produce stronger inhibition and cooling effect on laser welding plasma,the inhibition effect and cooling effect of shielding gas on laser welding plasma become more significant with shielding gas flow rate increased.Finally,based on the energy coupling characteristics of plasma acoustic wave,acoustic emission signal and electron temperature,the results shows that audible sound waves,acoustic emission and charge voltage signal of laser welding plasma can reflect the dynamic behavior and energy transfer characteristics of the plasma between workpiece and nozzle and the plasma in weld pool.Therefore,in the early stage of the laser plasma produced,thermal effect change and mechanical impact change on the weld pool or keyhole of the laser welding plasma are most significant,and reach the maximum at the maintenance stage of laser plasma,gradually decay in the late of laser pulse energy.