Papers by Author: Jin Seok Oh

Abstract: Welding tasks in shipbuilding create great problems for a manual welder since welding takes place in closed area with associated work environmental problems. This paper addresses the problems involved in the welding robot with control algorithm and system. The control system may similarly be modified as a tracking simulation test. The performance of the control system is assessed through the use of field data. The aim of this paper is to determine feasible parameters for a welding procedure with simulation for seam tracking of welding robot system. The main advantage of tracking simulation is its flexibility in that as the welding parameters are modified at a sufficiently high rate. Tracking simulation showed that the development of robot control algorithms should be performed by simulation, since it saves time, expenses and efforts. This paper will contribute to an increased use of automated welding technology with tracking simulation methods. Also, this paper’s results can be used for the optimization of welding process using simulation method with LabVIEW.

Abstract: The application of laser welding technology has been considered to shipbuilding structure. However, when this technology is applied to primer-coated steel, good quality weld beads are not easily obtained. Because the primer-coated layer caused the spatter, humping bead and porosity which are main part of the welding defect attributed to the powerful vaporizing pressure of zinc. So we performed experiment with objectives of understanding spatter and porosity formation mechanism and producing sound weld beads in 6
t primer coated steels by a CO2 CW laser. The effects of welding parameters; defocused distance, welding speed, coated thickness and coated position; were investigated in the bead shape and penetration depth on bead and lap welding. Alternative idea was suggested to suspend the welding defect by giving a reasonable gap clearance for primer coated thickness. The zinc of primer has a boiling point that is lower than melting point of steel. Zinc vapor builds up at the interface between the two sheets and this tends to deteriorate the quality of the weld by ejecting weld material from lap position or leaving porosity. Significant effects of primer coated position were lap side rather than surface. Therefore introducing a small gap clearance in the lap position, the zinc vapor could escape through it and sound weld beads can be acquired. In conclusion, formation and suspension mechanism of the welding defects was suggested by controlling the factors.

Abstract: The authors have observed directly and simultaneously the laser-induced plasma and keyhole behavior by high speed frame/streak camera using a special optical systems during the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. The dynamic behavior of Al-Mg alloys plasma was very unstable and this instability fluctuation period was about 440µs. After laser termination, abrupt collapse of keyhole within 600µs was observed and it was related to the formation of porosity. The authors also performed the spectroscopic analysis of laser-induced plasma, and clarified the structure and composition of evaporated particles during pulsed laser welding of Al-Mg alloys. In the air environment, the intensities of molecular spectrum of AlO and MgO were different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum was predominant in intensity, while the AlO spectrum became much stronger in higher power density. The same result was revealed in analyzing the composition of evaporated particles. These results were attributed to evaporation phenomena of metals with different boiling points and latent heats of vaporization.

Abstract: This paper describes the features and characteristics of plasma induced in the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. In the air environment, the identified spectra are atomic lines of Al, Mg, Cr, Mn, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectrum of AlO, MgO and AlH. It has been confirmed that the resonant lines of Al and Mg were strongly self-absorbed. These facts have shown that the laser-induced plasma is relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO are different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum is predominant in intensity, while the AlO spectrum became much stronger with the increase in high power density. This was attributed by the great difference in boiling point and vaporization energy of Al and Mg. In argon atmosphere the band spectra of MgO and AlO completely vanished, but AlH molecular spectra is detected clearly. The hydrogen source is presumably the hydrogen solved in the base metal, absorbed water on the surface oxide layer, or H₂ and H₂O in the shielding gas.

Abstract: Cathodic protection is a system of preventing corrosion by forcing all surfaces of a hull to be cathode by providing external anodes. A metal can be made cathodic by electrically connecting it to a more anodic metal within the electrolyte. Anodes of these metals corrodepreferentially, the corrosion current of the anode achieving cathodic protection of the underwater hull to which they are connected. This paper presents a new current control algorithm for ICCP (Impressed Current Cathodic Protection) system. The anode of ICCP system is controlled by an external DC source with converter. The function of anode is to conduct the protective current into seawater. The DC source is generally obtained from main power system that contains a transformer, converter and etc. The proposed algorithm can operate AC-DC converter with current link. This algorithm includes the harmonic suppression control strategy and the optimum protection strategy and has tried to test the requirement current density for protection, the influence of voltage, the protection potential. Also, the properties of ICCP protected hull of ship in seawater are evaluated.