Plasma control in deep penetration laser welding

Abstract

During high power CO2 laser beam welding, the plasma above the keyhole has a shielding effect that it not only absorbs part of the laser energy but also defocuses the laser beam. As a result, the welding efficiency and the aspect ratio of the welds are influenced. In order to reduce the effect of plasma, Helium as a plasma control gas has been used successfully and effectively. However, the cost of Helium in South East Asia is extremely high and therefore the production cost is significantly increased when Helium is used as a continuous bleeding plasma control gas. Furthermore, due to the directional property of the side jet, profile welding is not possible by laser welding with a gas jet. To search for an alternative plasma control technique, feasibility in using magnetic and electric field effects for plasma control was explored in this project. The individual and synergistic influences of magnetic field, electric field and both electric and magnetic fields on the penetration depth and the width of bead on stainless steel were studied. In addition, the effect of electric and magnetic field strengths, field direction, laser power, welding speed, shielding gases on the penetration depth and the width of bead were also investigated. Experimental results indicated that the fields can influence the shielding effect of the plasma. It was found that at suitable magnetic field strength, the magnetic field can increase the penetration depth by about 3%. On the other hand, using electric field can enhance the penetration depth by 8%. Using the effect of both electric and magnetic fields can not only increase the penetration depth by 13%, but also reduce the bead width significantly. In addition, the results were explained in terms of directional flow of ionic species in the laser plasma. These new plasma control methods, especially both electric and magnetic fields, are found applicable to profile welding. Moreover, the cost of production of laser welding can be reduced.