The Study On Working Head For Titanium Laser Welding

Titanium alloys have been widely applied in aviation, aerospace, shipbuilding and other industries because of their excellent properties such as low density, high strength to weight ratio and corrosion resistance. Compared to the conventional welding processes, laser welding has advantages such as high energy density, low heat input, high weld speed, good microstructural and mechanical characteristics.Because of the chemical activity of titanium alloys, harmful gases such as oxygen, hydrogen and nitrogen would easily be absorbed during welding, and poor properties of the joint would occur. So it is important to protect the high temperature zone of the weld seam as well as the weld pool from atmosphere. In the present study, the protection apparatus for titanium laser welding are complicated, and have only a restricted application. Further more, there are spatter and smoke in laser penetration welding, polluting the focus lens and damaging the optical system. In order to protect the optical system, the common method is to put a crossjet under the laser working head. However, the airflow kinetic energy is not great enough to effectively blow off the spatter from the beam path and the airflow also disturbs the weld shielding gas flow when an ordinary crossjet made of a simple nozzle is used. In this thesis an integrated laser welding head, consisting of an ultrasonic crossjet and a new type of protecting nozzle, was studied.First of all, the size and speed of the spatter were researched with the help of a high-speed video camera and other assistant equipments during laser welding. The necessary airflow speed was calculated to blow off the spatter from the beam path. It is found that the airflow speed is close to the velocity of sound. An ultrasonic crossjet with a non-symmetric laval cavity was designed based on the aerodynamic theory. Schlieren trials show that a rigid airflow with a small divergence was obtained. Welding tests prove that the focusing mirror is well protected with the designed ultrasonic crossjet.Secondly, a new type of protecting nozzle for titanium laser welding was designed, consisting of a main protecting nozzle and a subsidiary protecting nozzle. The main...