Spectroscopic investigation of the vapor plume during laser processing of AISI 52100 steel using a high brightness diode-pumped Nd:YAG laser.

Laser atomic absorption spectroscopy was employed to investigate the plasma created during processing of AISI 52100 steel with a diode-pumped Nd:YAG slab laser. A tunable ring-dye laser was focused into various points throughout the plume and the dye laser frequency detuned across several low-lying transitions of Fe I. Absorption profiles were collected at time intervals from 3--12mus, average powers from 80--260W and a speed of 0.8mm/s. The absorption profiles were used to calculate atomic densities using an Abel inversion technique and the electronic temperatures via atomic Boltzmann plots. The resulting atomic densities and temperatures of the plume were mapped graphically. Broadening mechanisms were considered, and emission spectroscopy and time-of-flight measurements were also obtained. Atomic densities were determined for the lower energy levels of three Fe I transitions. The peak densities were on the order of 1016, 1014 and 1013cm-3 for the lower levels of the 446.29, 446.03 and 446.78nm transitions, respectively. Line broadening was determined to be a combination Doppler and Stark broadening. At early delays, as the power increased, the densities of the transitions studied increased at the top portions of the plume. The plume also widened with power. The lifetime of plume atoms in excited states was no more than 12mus. The peak electronic temperatures within the plume ranged from 4800--7500K. If time averaged, these temperatures were in agreement with time-averaged temperatures from emission spectroscopy. The highest temperatures were at 3mus with cooling rates on the order of 108K/s. The average gas temperature was determined from Doppler widths and found to be 6000K, thus indicating similar electronic and gas temperatures. The kinetic velocity of the iron atoms was on the order of 103m/s and in agreement with time-of-flight velocity measurements. The depth of penetration in the coupons was 1.3--5.1 mm. The remelt layer was composed of martensite, and the microstructure in the heat affected zone consisted of a mix of spheroidal chromium carbides within a ferrite matrix. This work represents the first comprehensive characterization of atomic species within a plasma created during processing of a ferrous alloy using a high brightness, diode-pumped Nd:YAG slab laser.PhDApplied SciencesMechanical engineeringPlasma physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125156/2/3016836.pd