Keyhole/Plasma Behavior And Detection During Keyhole Mode Laser Welding

In keyhole mode laser welding,because of the high-power density,the weld material is rapidly heated to boiling temperature and a keyhole filled with metal vapor and plasma is formed.Because of the complex thermodynamic inside the keyhole,the state of the keyhole keeps changing during the whole welding process.As a result,the plasma,both inside and outside the keyhole,fluctuates with the fluctuation of the keyhole.In this paper,the electrical detection mechanism and method of the plasma plume were studied.The keyhole and plasma behavior was deduced by the analysis of the characteristics of electric signal waveform.In addition,a numerical mode was built to compute the shape of the keyhole and molten pool,and the selective vaporization of alloy elements.The major work and results are as follows:(1)Two sets of electrical signal detection devices were set up,based on the plasma sheath theory and the plasma effect theory.By comparing with the synchronous spectral diagnosis results,the rationality of the passive probe signal detection device based on the plasma sheath theory was verified.By comparing the results with the synchronous gap method,the plasma power supply model and the rationality of the plasma electric feature detection device were verified.(2)The characteristics of the electrical signals under different welding parameters were analyzed.The detected electrical signals were compared with the synchronous high-speed photos.A good agreement between the electrical signal waveform and shape of the plasma plume was obtained.It is proved that the electrical signal can be used to reflect the behaviour of the plasma plume.By comparing with the electrical signals of conduction mode laser welding,it is proved that the movement of keyhole is the main reason for the fluctuation of plasma electrical signals.In addition,the behaviors of keyhole and plasma was indirectly predicted by analyzing the waveform of electric signals.(3)Various methods were proposed to distinguish the keyhole mode and conduction mode laser welding through analyzing the electrical signals characteristics.The frequency of the detected electrical signals during keyhole mode laser welding was studied.The results showed that,the range of the frequency was about 300 Hz-600 Hz,and the fluctuation frequency of detection signals decreased with the increase of penetration depth.(4)A three-dimensional steady heat transfer and fluid flow model was constructed to compute the shape of the keyhole and the temperature field and the fluid flow of the molten pool.A selective vaporization model was built to calculate the vaporization of the alloy elements and the composition change of the welds during stainless steel keyhole mode laser welding.The results show that,the vaporization occured mainly inside the keyhole.The difference of the equilibrium vapor pressure between different alloying elements leaded to the difference of the vaporization rates.The concentration of Mn in the welds decreased significantly.In addition,the composition change decreased with the increase of laser power,however,the composition change was not sensitive to the welding speed.