Spectroscopic Characterization Of Keyhole Plasma During Deep Penetration Laser Welding

Keyhole effect is known as the essential characteristic of deep-penetration laser welding, the research focused on keyhole mechanism is the hot and difficult point in laser welding fields. The laser induced plasma resides both outside and inside the keyhole, so spectroscopic measurements of laser induced plasma is very helpful for keyhole mechanism research, and moreover, can contribute theoretical support towards practical application of laser welding. Temperature and electron density are considered to be two of the most important parameters of plasma, as other parameters are highly dependent on temperature and electron density. This article develops an Optical Multichannel Analyzer (OMA) system to research the characters of laser induced plasma, basing on measurements of temperature distribution of keyhole plasma, electron density and absorption coefficient, thus provides necessary parameters for building better laser welding models.A specially designed OMA system, combined with multi-channel optical fiber detecting assembly and an area array CCD, is employed to measures line integral curve of plasma intensity of plasma. Then the 2-D temperature distribution of the plasma inside the keyhole is calculated by Abel inverted transform method. The specially designed jig of optical detector and diaphragm can eliminate the effect of the plasma plume covering the keyhole on the spectroscopic signals of the keyhole plasma. A uniform experimental design method is used to arrange the experiments to investigation the effect of process parameters during laser welding on the temperature of keyhole plasma. The electron density and absorption coefficient are also estimated to research the absorption mechanism of keyhole plasma quantitatively.The experimental results suggest that the temperature of keyhole plasma is above 150000K. The temperature distribution is almost cylindrically symmetrical and temperatures are higher in the centre of keyhole than those around. These maxima temperatures in temperature distribution curved surfaces range from 15000K to 20000K. Our results indicate that the process parameters during laser welding have hardly any influence on temperature distribution, but have considerable influence on temperatures. Different welding parameters have different effect on the temperature distribution of plasma. Laser power, as one these process parameters, has the most intense effect on temperature of keyhole plasma. In contras, welding speed has the least intense effect on temperature of keyhole plasma. Experimental results show that microscopic parameters of keyhole plasma are dissimilar to the plasma plume outside the keyhole. For our welding conditions, the electron density is about 1.0×10cm-3, electron temperature is 15000K, thus the absorption coefficient of keyhole plasma is about 1.2cm-1. These results indicate that the inverse bremsstrahlung (IB) absorption mechanism of keyhole plasma is significant during the process of laser energy coupling into materials.