ABSTRACT
A compact and low-cost two-dimensional (2D) thermal imager was developed for real-time temperature mapping of a melt pool during coaxial laser cladding (the additive manufacturing technique). The device combines a color CMOS camera and a compact spectrometer. The spectrometer was utilized for internal calibration and validation of a 2D temperature map that was acquired by the CMOS camera. The remarkable feature of the thermal imager is that camera images are calibrated directly during the measurement process utilizing spectral pyrometry. Additionally, the spectrometer decreased a potential systematic error due to a spectrum contribution from atomic/ionic lines emission. The spectral pyrometry provided temperature accuracy measurements of ±23K while two-dimensional (2D) temperature mapping by two-color pyrometry accuracy was estimated as ±38K in the range of 1700-3500 K. The system has been installed at a coaxial laser cladding head for real-time temperature mapping. The system was developed for measuring a melt pool during laser cladding production of a high wear-resistant coating (tungsten carbide particles in a nickel matrix). Tungsten carbides powder flow rate has been changed sequentially from 5 to 27 g/min by powder feeder programming, which resulted in a variation of melt pool dimensions and temperature maps during the cladding process.
ABSTRACT
Laser-induced breakdown spectroscopy (LIBS) has been utilized for in situ diagnostics of the laser welding process. The influence of different weld spot areas (melt pool, solid weld) on LIBS signals and plasma properties has been studied in detail. Liquid metal sampling and high target surface temperature of the melt enhance LIBS plasma intensity and increase plasma temperature. The influence of laser welding process parameters on LIBS measurements has been studied in order to differentiate optimal and defective laser welding. In case of defective laser welding, the melt pool was intensively boiling, so we have observed greater LIBS signals but poor reproducibility. For the first time, the LIBS technique was demonstrated to detect defective laser welding during in situ measurements utilizing atomic and ionic line comparison by paired sample t-test hypotheses testing.
ABSTRACT
The comparison of laser ablation and plasma evolution has been carried out for a molten steel sample in the absence and in the presence of surface plasma. A continuous wave (cw) laser beam was utilized for local melting of a steel (Fe>99 wt.%) sample, but it also induced a surface plasma according to optical emission spectroscopy. The cw laser was switched off for a few milliseconds to dissipate the surface plasma, but the surface temperature did not change according to optical pyrometer measurements. Molten metal was ablated by a nanosecond Nd:YAG laser pulse during cw laser operation and when it was switched off for 5 milliseconds. Comparison of laser ablation and plasma evolution in the presence and in the absence of the near-surface plasma induced by the cw laser beam has been carried out. Time-integrated plasma imaging detected slightly greater emissivity of the plasma induced during cw laser operation. The cw laser operation resulted in a twofold enhancement of the intensity of atomic lines in the spectra as well as slower decay of plasma emission. Plume temperature and electron density were slightly greater at early stages of plume expansion in surface plasma.
