Research On Plasma Characteristics Of Laser Deep Penetration Welding Keyhole Based On Layered Spectral Observation

The essential characteristic of laser deep penetration welding is the existence of the keyhole effect,which improves the energy coupling efficiency between the laser beam and the material.In the process of laser deep penetration welding,a large amount of photo-induced plasma will be generated,and these photo-induced plasmas are divided into plasma outside the keyhole and plasma in the keyhole.From these plasma spectra,useful plasma physical parameters such as plasma temperature and density and its inverse bremsstrahlung absorption coefficient can be extracted,which are beneficial to systematically study the energy distribution on the keyhole wall.Due to the existence of the keyhole wall and the plasma plume outside the keyhole,it is difficult to measure the plasma inside the keyhole wrapped inside the opaque keyhole,which is also a hot spot and difficulty in the field of laser welding.In this regard,this paper mainly proposes a method to directly observe the plasma radiation spectrum in the keyhole in a layer-by-layer section in all directions.Plasma radiation spectra in the cross section of the keyhole on the joint surface of 304 stainless steel and JGS2 quartz glass specimens at different depths.Through this scheme,the direct observation of the plasma in the keyhole can be realized in a global domain and without dead angle,which will ensure to provide real and reliable plasma physical property parameters for the study of the keyhole effect and the energy distribution on the keyhole wall of laser deep penetration welding.The research shows that the temperature of the plasma in the keyhole will decrease with the depth of the keyhole in the range of about 0mm～3mm in the depth of the keyhole,and the plasma temperature in the keyhole is roughly in the range of8600K～11000K.The plasma density in the keyhole shows an opposite trend to its temperature law.The plasma density is about 1.125×10¹⁶ cm^-3～1.36×10¹⁶cm^-3.This result is also verified by the single-section method of 304 stainless steel sheets with equal thickness of 2 mm and 1.5 mm to verify the temperature and density of the plasma in the keyhole,and the results are consistent.In addition,the study of the plasma spectrum outside the keyhole was also carried out,and the temperature of the plasma inside and outside the keyhole was compared,and it was found that the temperature of the plasma inside the keyhole was higher than that of the plasma outside the keyhole.