A high-brigthness fiber laser can produce an ultra-high peak power density of MW/mm2 level corresponding to a focused electron beam, and is promising as one of the desirable heat sources for deep-penetration welding. The objectives of this research are to elucidate the factors affecting weld penetration and defects formation mechanisms, to obtain a fundamental knowledge of interaction between a fiber laser beam and the laser-induced plume, and to assess laser absorption with water-calorimetric method in bead-on-plate welding of Type 304 austenitic stainless steel plates with a 10 kW fiber laser beam. Concerning the weldablity and defects, the penetration depth reached 18 mm at the maximum. At 50 mm/s or lower welding speeds, porosity was generated under the conventionally-focused and tightly-focused conditions. X-ray transmission in-site observation images demonstrated that pores were formed not only at the tip of the keyhole but also near the upper part. The keyhole behavior was stabilized by using nitrogen shielding gas, which led to the porosity prevention. As for the interaction under the normal Ar shielding gas conditions, the temperature and ionization degree of the laser-induced plumes were calculated to be 6,000 K and 0.02, respectively, by the Bolzman plots and Saha's equation. It was also found that the attenuation and the refraction between the 10-kW fiber laser beam and the short weakly-ionized plume were too small to exert the reduction in weld penetration. The laser absorption of the stainless steel plate was approximately 85 % high at 10 kW laser power and 50 mm/s welding speed. Compared X-ray transmission observation images of the keyhole with the focusing feature of the fiber laser beam, most of the incident laser passed through the keyhole inlet, and the center part of the beam was delivered directly to the tip of the deep keyhole. Consequently, as far as the adquate welding procedures were utilized on the basis of eclucidation of the welding phenomena, 10 kW high-brightness fiber laser welding, which can produce sound welds, was confirmed to be one of the highest-quality, high-efficiency processes owing to a small effect of weakly-ionized plume and deep keyhole with a sufficient inlet for the incident laser beam absorption.
Outer Surface Irradiated Laser Stress Improvement Process (L-SIP) was developed as a counter measure for stress corrosion cracking at the tube and nozzle stub weld in the nuclear power plant and the thermal power plant. L-SIP is a technique for improving the inside tensile residual stress to the compressive stress. Rapidly heating by the laser beam irradiation to outer surface causes the temperature difference between outer surface and inner surface, and it can reduce the residual stress. In this paper, the developed system, the verification test results and the practical use situation are described.
This research aims at building a turbulent diffusion combustion model based on chemical equilibrium and kinetics for simplifying complex chemical mechanisms. This paper presents the combustion model based on chemical equilibrium combined with an eddy dissipation concept model (CE-EDC); the model is validated by simulating a CO-H2-air turbulent diffusion flame. In the CE-EDC model, the reaction rate of fuels and intermediate species are estimated by using the equations of the EDC model. Then, the reacted fuels and intermediate species are assumed to be in chemical equilibrium; the amounts of the other species are determined by the Gibbs free energy minimization method by using the amounts of the reacted fuels, intermediate species, and air as reactants. An advantage of the CE-EDC model is that the amounts of the combustion products can be determined without using detailed chemical mechanisms. Moreover, it can also predict the amounts of the intermediate species. The obtained results are compared with Correa's experimental data and Gran's computational data by using the EDC model, which uses the complex chemical mechanisms. The mole fractions of CO, H2, H2O, OH, temperature, and mixture fraction obtained by using our CE-EDC model were in good agreement with these reference data. Using the present CE-EDC model, amounts of combustion products can be calculated by using a reduced chemical mechanism and the Gibbs free energy minimization theory. The accuracy of this model is in the same order as that of the EDC model.
The present investigation was undertaken to examine the effect of Ti undercoating on the corrosion fatigue properties of stainless steel bar coated with plasma-sprayed Al2O3. Corrosion fatigue tests were carried out under push-pull loading at a stress ratio R= -1 and at a frequency of 2 Hz. A physiological saline solution was used as corrosive environment. The fatigue strength of Ti undercoated specimen increased over the whole regime of S-N curve in comparison with that of the annealed substrate metal. Through the longitudinal section observation with a scanning electron microscope, it was revealed that Ti undercoating controlled the infiltration of a physiological saline solution to the substrate surface until 107 cycles. From the measurement of residual stress distribution by X-ray diffractometry along radial axis on the longitudinal section, it was found that the residual stress in the subsurface layer of substrate was changed from the tension to compression along radial axis and undercoat layer had compressive residual stresses in Ti undercoated specimen. In addition, it was clarified from the electrochemical experiments that Ti deposit was slightly nobler than annealed substrate metal. Consequently , it was concluded that the improvement of corrosion fatigue strength of Ti undercoated specimen was chiefly caused by both insulation effect by Ti undercoating and compressive residual stress in the subsurface layer of substrate and undercoat layer.