A Spectra Study On Keyhole Plasma Emission During Deep Penetration Laser Welding Of Aluminum Alloy

The mechanism of deep penetration laser welding was introduced at first in this paper, and then the status and the problem exist in the research on keyhole effect both domestic and overseas were analyzed from such as aspect as keyhole shapes and sizes, temperature of keyhole wall and keyhole plasma characteristics. Finally, studies at home and abroad on alloy element loss of the material during laser welding were presented. To make up the limitation in some domestic and overseas studies on keyhole effect and alloy element loss, a novel experimental method was applied on the study of the keyhole’s shape, size and the plasma inside synthetically.First of all, the real keyhole shape during laser deep penetration welding of aluminum alloy 6016 was recorded by a digital video.Based on a so-obtained keyhole picture, combined analysis method with curve fitting technique, the mathematical keyhole shape was reconstructed.Second, the spectral signal emitted from keyhole plasma and how it changes with such processing parameters as laser mode, laser power, welding speed and defocus were studied synthetically by spectroscopy observation method.Then, classical theory of heat transfer model of keyhole plasma at the depth and radial directions are established. Based on an approximate calculation on the thermal diffusion coefficient, the thermal diffusion coefficient at radial direction was far larger than that at depth direction. To simplify calculation, the heat transfer at depth direction can be neglected. A simplified analysis formula is unilised to calculate the plasma temperature inside the keyhole and FDM method was used to calculate the temperature distribution of melt pool around the keyhole plasma. On the basis of the above experimentally-obtained spectral data, relative strength method of spectrum is used to obtain the keyhole plasma temperature and the temperature distribution in depth and radial directions both under continuous and pause laser mode. The effects of such processing parameters as laser mode, laser power, welding speed, and defoc -us on the keyhole plasma temperature is also analyzed in this paper.Finally, alloy element loss model during laser welding was established with Knudsen-Langmuir theory, and loss rate of alloy element during laser welding is calculated, which shows that the calculation result is in good agreement with the experimentally-measured result.