Masahiro Ohara

Welding experiments of aluminium pipe by space GHTA welding in aircraft-borne simulated space environment

The gas hollow tungsten arc (GHTA) welding experiments on aluminum pipe were carried out in a simulated space environment using an aircraft. A vacuum chamber and welding machine for GHTA welding test were placed in the cabin of the aircraft and the 10− 2 G gravity environment was produced by a parabolic flight of the aircraft. The square butt welding joints with non root gap on aluminum pipe were made by orbital welding in the vacuum chamber without wire filler metal using DC or DC-pulsed power supply under the 10− 2 and 1 G gravity conditions. The welding phenomenon during the aluminum GHTA welding recorded in the high-speed video image was analysed and also the macrostructure and mechanical properties of butt weld joints were investigated. The welding experiments under simulated space environment showed that the DC-pulsed GHTA process could make the welding joints without the weld defects such as a lack of fusion, oxide film inclusion and spattering, though throat thickness decreased by the impulsive arc pressure of pulsed current. It was also clarified that the arc discharge phenomenon and melting characteristic at the molten pool surface during the DC-pulsed GHTA welding were insensitive to the gravity condition. However, the sagging weld metal made at 1 G gravity condition increases a little more than that welded under the 10− 2 G gravity condition.

Effect of applying electrical potential to a CO2 laser welding of different thickness plates

Abstract This study was aimed at developing laser welding with an applied voltage potential to increase the bead root width in laser welding. Also, in order to enhance the welding speed and the butt joint gap tolerance, the influences of the experimental conditions: supplied voltage between plate and backside electrode, welding speed, plasma operate gaseous species, and the butt joint gap, on the bead root width were investigated. Although it is necessary to avoid over heating and melting the plates, it is applicable for higher speed and wider gap butt joint welding than a conventional laser welding. In the case of butt joint welding with a thickness of 2.0 and 0.8 mm steel sheets by using 5 kW CO2 laser system, it is concluded that this method is effective for increasing of the welding speed from 5 to 8 m/min. Knowledge of optimum conditions and configurations has guided to extend this process to more challenging structural materials such as a tailored blank steel sheet.

Interfacial structure of multi-layered thin-films produced by pulsed laser deposition for use in small-scale ceramic capacitors

The aim of this study was to develop thin film capacitors with superior properties that could provide an alternative to materials currently used in conventional multi-layer ceramic capacitors fabricated by sintering. To this end, an artificial dielectric super lattice technique, incorporating pulsed laser deposition, was applied to improving the dielectric properties of thin film capacitors. This method permits the A-site atoms of a perovskite ABO3 structure to be selected layer by layer at a nanoscopic scale; consequently, multi-layer BaTiO3- SrTiO3 thin films were produced on Pt(111)/Ti/SiO2/Si(100) and SrTiO3(111) substrates. Hetero-epitaxial grain growth was observed between BaTiO3 and SrTiO3, with the lattice mismatch between them introducing a compressive residual strain at the interface. The dielectric properties of these multi-layer thin-film capacitors were found to be superior to those of conventional solid-solution thin films once the thickness of the layers and the ratio of the two oxides were optimized.

Porosity formation in CO2 laser welding of steel sheets

In laser welding there are weld metal internal defects, such as porosity and cracking, and shape inferiorities like undercut and droop. The mechanism of porosity formation alone is thought to vary with the plate thickness, material and the groove conditions. For CO2 laser welding of steel sheet, porosity formation is empirically probable with thinner plate and inferior butt conditions, such as a widened gap. The objective of this study was to clarify the mechanism of porosity formation in the CO2 laser welding of steel sheet. Ultra-low-carbon cold rolled steel sheet (thickness 0.7 mm) was mainly employed as the specimen material; in addition, in order to avoid externally disturbing factors, such as contamination at the groove face and the variation in the butt condition, melt-run welding was carried out. Under these circumstances Ar coaxial shielding gas was employed; welding experiments were carried out where Ar + N2 mixed gas with varied nitrogen content was used as either coaxial shielding gas or for the backside atmosphere, in addition to normal welding with a steel sheet backside air atmosphere; the mechanism for porosity formation was also studied.