Abstract Scope |
1. Introduction
Laser wire-feed additive manufacturing (LWAM) is efficient in producing large expensive metal components with complex geometry in many industries especially for aerospace and automotive industries. Researchers have found that increasing deposition velocity is an important way to reduce additive manufacturing production time. However, humping defect always occurs when the deposition velocity is increased to a certain level. The humping defect limit the increase of deposition velocity and decrease the quality of laser wire-feed additive manufacturing production. So it is necessary to take some measures to suppress the humping bead while also ensuring the sufficiently high deposition velocity. Magnetic controlled welding technique has been widely applied to control the welding process and improve the weld quality because of its low cost, simple set-up and easy operation. However, in LWAM process, the thermoelectric current in molten pool was too small to produce an adequate Lorentz force in magnetic field to control the flow of the molten pool. In this paper, the authors propose a new electromagnetic controlled molten pool method which produces the forward Lorentz force by inserting a filler wire into the molten pool and passing a unidirectional current. The results indicate that the humping was disappeared when the external magnetic field was applied.
2. Experimental Procedures
The magnetically controlled high speed LWAW system is consisted of a high-speed digital camera, a solid-state Ytterbium fiber laser, a six-axis high precision industrial robot, and a wire feeder, a DC power source for generating unidirectional current and a permanent magnet. The NdFeB permanent magnet with the dimensions of 100 mm × 50 mm × 25 mm were used to provide steady magnetic field for the molten pool. The magnet can afford the magnetic flux density of 0.3 T. The permanent magnet is mounted under the work-piece to produce a longitudinal magnetic field in weld pool. In LWAM process, the wire is fed into the molten pool at 45 degrees perpendicular to the welding direction, while the tungsten needle is fed into the other side. The feed wire and a tungsten needle were connected to the positive and negative electrode respectively. The external magnetic field B interacts with the unidirectional current to produce the forward electromagnetic force (Lorentz force) in the weld pool.
3. Results and Discussion
3.1 Hump formation in LWAM Beads
The humping bead formation undergoes three stages: the initial stage, the growth stage and the solidification stage. In initial stage, the laser beam heated and melted the wire to form liquid metal, which transitions to the base material under the action of surface tension and metal vapor recoil. The action of metal vapor recoil caused fluctuations in the molten pool. Then the filler metal and the molten base metal with high momentum moving towards the tail of the weld pool and form a slight swelling. In growth stage, the tail of the molten pool is continuously filled with filler metal and molten base metal and grows up. In solidification stage, as the laser beam moved forward rapidly, the tail of the molten pool solidified with the decrease of heat.
3.2 The influence of external magnetic field and directional current on the flow of molten pool
The unidirectional conduct current which flows through the wire into the weld pool is the main source of current density existed in molten pool to induce electromagnetic force. The electromagnetic force was produced by the interaction between the external magnetic field and strong unidirectional conduct current density. Therefore, the combination of the direction of the conduction current in the molten pool and the direction of the external magnetic field have different effects on the molten pool. The interaction between the current perpendicular to the welding direction and the longitudinal magnetic field produces forward or backward electromagnetic force. The interaction of the current parallel to the welding direction and the longitudinal magnetic field produces a left or right electromagnetic force. The interaction of the current parallel to the welding direction and the transverse magnetic field produces an upward or downward electromagnetic force.
3.3 Suppression mechanism of humping
As mentioned, high-speed liquid metal flow is the main reason for the formation of hump defects. Therefore, reducing the flow velocity of the molten pool is an effective method to suppress the hump defect. When the longitudinal magnetic field with the unidirectional current perpendicular to the welding direction method was used, the flow velocity of the molten pool was significantly reduced and the hump defect disappears.
4. Conclusions
The influence of magnetic fields in different directions on the formation of welds is analyzed. The additional forward Lorentz force generated by the interaction of the longitudinal magnetic field with the unidirectional current could suppress the backward metal flow in the weld pool. The influences of unidirectional current level and the external magnetic field strength B on the additional electromagnetic force in weld pool are quantitatively analyzed. The additional Lorentz force mainly acted on the forepart of the melt pool as the lateral current was mainly distributed in the forepart of the melt pool. It could significantly reduce the initial momentum of the backward molten metal flow in high speed LWAM.
Keywords: laser wire additive manufacturing; external magnetic-field; high deposition velocity; humping defect |