Energy source properties of electric arcs strongly depend on physical properties of the arc plasma. In a welding process, it has been experimentally confirmed that an admixture of metal vapor diffused from a high temperature weld pool drastically changes the properties of arc plasma and lowers its temperature. However, the effect of the admixture on heat input characteristics to a base metal is not clear due to the difficulty of the experimental study. In this paper, energy source properties of He GTA with the admixture of metal vapor were numerically analyzed. Consequently, it was found that the intense radiation generated by the dense metal vapor constricts the current density distribution especially near the arc axis and decreases the heat input to the base metal.
With the objective of interpreting penetration mechanisms and porosity formation mechanisms during laser or hybrid welding, Type304 stainless steel plates containing low S content were subjected to YAG laser only or YAG laser-TIG arc hybrid welding, and surface of the molten pool, keyhole behavior and melt flows during welding were observed through a high-speed camera and the X-ray real-time transmission apparatus. It was confirmed that melt flows near the molten pool surface were different between in air and in Ar atmosphere: the ones approached to the keyhole inlet and spread from the keyhole inlet to the surrounding pool, respectively. It is therefore considered in Ar atmosphere that the melt flows attributed to surface tension produce the "nail head" and the resulting weld bead displays a wine glass shape. In addition, it was revealed that the melt flows and keyhole behavior were greatly affected by arc current. In the hybrid welding at about 100A, there was a strong melt flow from the keyhole tip toward the rear part near the bottom part of the molten pool, resulting in deepened welds. It was observed that porosity reduction in hybrid welding at 200 A was due to reduced bubbles formation.
Objecting to the rolled H section steel joined by using a newly developed flash welding system, a temperature of a specimen at joining was measured. Moreover, residual stress generated in the specimen was also measured. On the other hand, flash welding on the rolled H section steel was simulated by three-dimensional thermal elastic-plastic analysis based on FEM. The obtained results are as follows. In the developed flash welding system, joints are heated by an electric current from the electrodes attached on the flange. From the results of temperature measurement, the same magnitude of heat input per unit volume could be recognized in the web and the flange. From the results of the non-steady heat conduction analysis for the developed model, a temperature history of the specimen joined by the flash welding could be simulated with a good accuracy. Residual stress obtained by the thermal elastic-plastic analysis agreed accurately with the measured one. From this, it was elucidated that residual stress generated by flash welding on the rolled H section steel could be obtained with a high accuracy by the analysis. In welds, σx of the flange and σz of the web were large tensile respectively. That is, the stress components in the direction of welding line became large tensile like as in arc welding. σy became compressive at the edge of the flange and became tensile at the center of the flange and at the web. It was elucidated that a stress resultant of σy is in a state of equilibration at the whole of the cross section. It was elucidated that an influence of generation of heat by arc at the weld interface was larger than that by a resistance on the generation of residual stress.
Finite element method is a powerful tool for predicting welding distortion. However, the mechanical phenomena in welding are strong non-linear transient problems and thermal-elastic-plastic FE analysis requires very long computational times. To overcome this problem, an interactive substructure method is developed as a method to reduce the computational time in three-dimensional analysis. In this paper, in order to confirm calculation efficiency for a large scale problem, calculation of the pipe model with 538,200 degree of freedoms is performed. Further, calculation of the residual distortion in engine component is performed and its applicability for practical problems is demonstrated.
Mechanical properties of friction stir welded joints of austenitic stainless steels, SUS301L-1/4H and SUS304, under different heat input conditions were studied. The tool rotation speed was set at 600rpm and the welding speed was changed from 60 to 540mm/min. The tensile strength of the SUS301L-1/4H welded joints is equivalent to that of the base material at the welding speeds of 120 to 300 mm/min. The proper welding condition range is smaller than that for SUS304 which is a typical austenitic stainless steel. This result should be related with the difference in the strengths between SUS301L and SUS304 at a high temperature.
The shielding controlled plasma spraying process was investigated to improve corrosion resistance of metal surfaces. In this process, a shielding nozzle that covered just only spraying area was attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet was controlled by nitrogen flow. As the oxygen concentration in the shielding nozzle was maintained as low as 0.5%, the metal oxide contents in volume of CoNiCrAlY coating and the porosity of the coating reduced to 0.2% and 0.3% respectively under optimal spray particle size. The corrosion potential of CoNiCrAlY coating sprayed by this process in an acid solution including chloride ions stayed about -150 mV for 1000 hours, and no rust was observed during this test. On the other hand, that of the coating sprayed by atmospheric plasma spraying process changed from about -300 mV to about -500mV for 1000 hours, and the rust came to the surface of the coating after 10 hours. Therefore the developed shielding controlled plasma spraying process is concluded to improve the corrosion of the metal.