Direct observation of keyhole plasma characteristics in deep penetration laser welding of aluminum alloy 6016

Deep penetration laser welding is associated with violent plasma generation which consists of metal vapour, ionized ions and electrons. The plasma resides both outside and inside the keyhole, known as the plasma plume and keyhole plasma, respectively. Plasma plumes have been studied extensively due to the convenience of observing them. However, very little work has been carried out on the investigation of keyhole plasmas. In this paper, a novel experimental set-up is designed to observe the keyhole plasma directly in CW and PW deep penetration laser welding of aluminum alloy 6016. Then on the basis of the experimentally obtained spectra, the electron temperature distribution of the keyhole plasma both in the radial and depth directions of the keyhole is calculated, and the effects of processing parameters such as laser power, welding velocity and defocus on the keyhole plasma temperature are studied. The results show that the electron temperature of the keyhole plasma both in the radial and depth directions is not uniformly distributed. The temperature increases as the laser power increases, decreases as the welding velocity increases and decreases as the location of the laser beam focal point is moved from within to above the keyhole.