Optical Diagnosis Of Laser Welding Characteristics

As an advanced joining technology,laser welding has advantages of narrow heat affected zone,less distortion,high-precision and good-flexibility.The laser welding technology is applied in many fields such as aerospace,auto motive and shipbuilding.Generally,two typical modes can occur during laser welding,namely conduction mode and keyhole mode.Conduction mode is characterized as shallow penetration,stable processing and high quality.Keyhole mode is characterized as hig h energy concentration,high aspect ratio,and high productivity.Recent researches have shown that the switch between conduction mode and keyhole mode in laser welding is not a sharp.There is a transition mode between the two welding modes.The heat tran sfer and fluid flow of different welding modes have a great influence on the temperature field and the shape of the weld pool,thereby affecting the microstructure of the welds,the quality and mechanical properties of the weldments.A deeper understanding of the welding mode transition from the conduction mode to the keyhole mode is essential to comprehensive understanding,improvement and development of laser welding technology.The optical emissions generated during the laser welding process are the central link of laser-metal interaction,which can reflect the energy transfer mechanisms of the welding process.The most important emissions are the thermal and the plasma emissions.Therefore,conducting research of the characteristics of different welding modes based on optical emissions is helpful to reveal the mechanism of laser welding and further understand physical characteristic of laser-induced plasma.Firstly,infrared thermography and high speed imaging are used to capture the global temperature evolution and melt pool dynamics in different welding modes during laser welding of 316L stainless steel.During laser welding,the welding modes can be classified as conduction,transition and keyhole mode based on the heat transfer in the weld pool.In conduction mode,heat transfer is mainly more due to conduction.Marangoni-driven flow can have a profound impact on heat transfer in the radially outward direction of weld pool as well as the melt pool shape in transition mode.The heat and flow characterizations of keyhole mode are dominated by recoil pressure.Recoil pressure acts as the driving force to push the molten metal flow backwards in the penetration direction.The heat transfer and fluid flow in the weld pool have a profound impact on the final weld pool boundary which determines the weld surface ripple patterns and the grain growth direction.In order to monitor weld metrics,such as weld bead width,depth of penetration and defects,a weld monitoring system for laser welding is designed using a coaxi al infrared pyrometer.A linear relationship exists between the signal and penetration depth and weld width in conduction mode and keyhole mode.The signal is less sensitive in the transition mode,which can be used to distinguish the welding modes.Moreover,coaxial infrared temperature signal can be used as reference for on-line monitoring of humping defects.In contrast,porosity could not be sensed.Laser induced plasma is one of the core parts of keyhole mode welding,which is of considerable impact on the welding productivity and welding quality.Space-dependent plasma characteristics including spectrum intensity,plasma temperature and electron density are investigated using optical emission spectroscopy technique.The evolution of plasma characteristics in different laser energy are studied.The mechanisms for laser power attenuation by the plasma are quantitatively analyzed.The results show that plasma temperature and electron density vary from 5300 K to 5600K and from 3×10¹⁶ cm^-3 to 4×10¹⁶ cm^-3 respectively.The hottest core of the plasma shifts toward the surface of the workpiece as the laser power increases.Unlike the plasma temperature,the electron density is found to be spatially homogeneous up to a certain distance（about 5 mm in this study）,and then decreases at longer distance.The inverse bremsstrahlung（IB）absorption is the main mechanism of the laser-plasma interaction under low laser radiation.Once above a certain threshold,the ultra-fine particles in plasma plume would cause a significant attenuation of laser beam.Finally,the keyhole characteristics of CW laser penetration welding are investigated based on“sandwich”method and streak imaging technique,including keyhole formation,keyhole melt fluid flow,keyhole emission and keyhole temperature.In keyhole welding,the fluid flow velocities down the keyhole front range from 6 m/s to 13 m/s,and the closer to the keyhole front,the higher the fluid flow velocity.The main conclusion of the study of keyhole characteristics is the fact that no plasma is occurring in keyhole for the CW fiber laser penetration welding process.The keyhole primarily consists of hot vapors and the plasma is generated and maintained outside the keyhole.The calculated keyhole vapor temperature is 200 K higher than vaporization temperatures of the 304 stainless steel,and the vapor temperature in the top and bottom of the keyhole are higher than the middle part.