Alloying element vaporization and emission spectroscopy of plasma during laser welding of stainless steels

During laser welding, the use of a high power laser beam focussed to a very small area leads to relatively high weld pool temperatures and significant vaporization of volatile elements. A fraction of the atoms of the vaporized material can become excited or ionized leading to the formation of a plasma. The plasma absorbs the laser beam energy and, in turn, reduces the power density available at the metal surface. Therefore, a knowledge of the vaporization losses and the plasma properties under various welding conditions is required as these parameters affect not only the penetration but also the composition of the weldment.; The vaporization rates and the weld pool temperatures were determined under various welding conditions using a carbon dioxide laser. Since the weld pool is surrounded by plasma during laser welding, the role of the plasma in the vaporization of alloying elements was physically modeled by allowing molten copper drops to vaporize isothermally both in the presence and the absence of plasma. Emission spectroscopy (using an Optical Multi-Channel Analyzer) was utilized to monitor the alloying element loss and for the characterization of the plasma formed during pulsed laser welding of AISI 201 stainless steels under various welding conditions. Plasma diagnostic techniques were utilized to determine the electron temperatures and the number density of electrons in the plasma to gain an understanding of the laser-plasma-solid interactions.; The overall vaporization rates during laser welding were controlled by plasma-influenced intrinsic vaporization of alloying elements from the weld pool surface with the plasma suppressing the vaporization rate. The dominant species in the plasma was found to be iron, manganese and chromium, which were present predominantly in an excited neutral state and to a lesser extent in an ionized state. When welding was conducted at constant welding speed, shielding gas flow rate and composition, the intensities of various emissions could be used to monitor the vaporization rate and the weld pool shape. The electron temperatures were determined to be in a wide range of about 4,000 K to about 9,000 K. The electron temperatures, velocity profiles of helium gas jet and the concentration profiles of the various species in the plasma were determined by a combination of experimental and theoretical studies. This information, in turn, was used to determine the absorption of the laser beam by the plasma. The plasma enveloping the weld pool was found to absorb about 5.8% of the total laser power. The utility of emission spectroscopy in determining the composition of the base metal was also demonstrated.