The relation between fluid flows, solidification mechanisms and welding parameters during GTA weld­ ing of a Cu30Ni plate is investigated. An experimental study, based on in-situ observations of the trailing edge of a fully penetrated weld pool is carried out. The experimental setup includes: (i) a high-speed camera used to observe the solidification front and the fluid flow at microscale, (ii) two cameras placed over and below the plate used to observe the entire weld pool on the top side and back side respectively (iii) and an infrared camera used to monitor the tempe rature field on the back side. Ali these observation devices are employed to do measurements that allow us to explore and better understand the influences of the welding parameters on the solidification and the fluid flow behaviours, as well as between the different physical phenomena being involved.