Yoshinori Hirata

Numerical analysis of the heat source characteristics of a two-electrode TIG arc

Various kinds of multi-electrode welding processes are used to ensure high productivity in industrial fields such as shipbuilding, automotive manufacturing and pipe fabrication. However, it is difficult to obtain the optimum welding conditions for a specific product, because there are many operating parameters, and because welding phenomena are very complicated. In the present research, the heat source characteristics of a two-electrode TIG arc were numerically investigated using a 3D arc plasma model with a focus on the distance between the two electrodes. The arc plasma shape changed significantly, depending on the electrode spacing. The heat source characteristics, such as the heat input density and the arc pressure distribution, changed significantly when the electrode separation was varied. The maximum arc pressure of the two-electrode TIG arc was much lower than that of a single-electrode TIG. However, the total heat input of the two-electrode TIG arc was nearly constant and was independent of the electrode spacing. These heat source characteristics of the two-electrode TIG arc are useful for controlling the heat input distribution at a low arc pressure. Therefore, these results indicate the possibility of a heat source based on a two-electrode TIG arc that is capable of high heat input at low pressures.

Development of numerical simulation model for FSW employing particle method

Abstract The friction stir welding (FSW) has been used widely in the field of industry. Numerical analysis models for FSW also have been developed and there are many papers about it. In these papers, the most frequently used method is finite element method or finite difference method. However, by employing these methods, it is difficult or troublesome to calculate the advective term both for momentum and temperature. It is also difficult to calculate the big deformation of the material. Moreover, complex process is required to analyse the dissimilar joining with respect to dealing with substance transfer. In this paper, to avoid these difficulties or troublesome processes, particle method is adopted for dissimilar FSW simulation. In particle method, advective term, substance transfer and surface deformation are calculated automatically mainly because that Lagrangian approach is used. Performing some of analysis for FSW, the effectivity of this method is verified.

Fabrication of SiO2 hybrid microlens structures using femtosecond laser nonlinear lithography

We fabricated three-dimensional SiO2 surfaces using femtoseond laser nonlinear optical lithography. By pattern transfers of polymerized resist patterns, which were written via nonlinear absorption, into underlying substrates, SiO2 hybrid microlens structures were created. Fresnel zone plate patterns were written on curved surfaces of microlenses whose diameters and curvature radii were 240 μm and 380 μm, respectively. The surface observation revealed that the surface roughness of the transferred SiO2 was improved with O2 concentration during plasma etching. When He-Ne laser light of 632.8 nm wavelength was coupled to the hybrid lenses, the focal lengths became shorter than those of the original refractive lenses. This shift amount of 216 μm was consistent with theoretical value of 213 μm, indicating nonlinear lithography was useful for the precise microfabrication of photonic devices with three-dimensional surfaces.

Spatter reduction in gas metal arc welding of stainless steel sheets using controlled bridge transfer process

Abstract In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by controlling the short circuit current to a low level just before the re-arcing. The controlled bridge transfer (CBT) process, which optimises the accuracy of predicting the re-arcing in real time in response to the metal transfer, realises stable, low spatter level. In this research, the methods for controlling short circuit transfers to minimise spatter and realise stable arcs in GMAW of stainless sheet using argon rich shielded gases are investigated. The new CBT process has been developed by applying the specific arc length estimation method that is not affected by abnormal rise in arc voltage. This process can suppress the spatter generation caused by a fluctuation in the vibratory motion of the weld pool or inaccurate prediction of the re-arcing in the succeeding short circuit/re-arcing cycle, and thereby spatter free GMAW in the short circuit transfer mode can be carried out even on stainless steels.

Controlled bridge transfer (CBT) gas metal arc process for steel sheets joining

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short-circuit current to a low level just before the re-arcing. The reduction in spatter requires an improvement in the accuracy of predicting the re-arcing by stabilizing the metal transfer and improving the robustness of the accuracy against disturbances. The controlled bridge transfer (CBT) process optimizes the accuracy of predicting the re-arcing in real time in response to the metal transfer, realizes spatter reduction and stable arc in non-pulsed GMAW. Traditionally, GMAW is carried out using electrode positive polarity. However, this polarity is not sufficient for welding extra-thin steel sheets, specifically those thinner than 1.0 mm. With electrode negative (EN) CBT process, although slight arc voltage fluctuation occurs caused by the behaviour of cathode spots on the tip of the wire during EN polarity GMAW, instantaneous voltage uses command computation to improve the transient response against the disturbance. Consequently, a stable arc can be obtained without increasing the number of short circuits in a unit time to obtain spatter-free welds.

Spatter reduction in GMAW of stainless steel sheets using CBT process

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short-circuit current to a low level just before the re-arcing. The controlled bridge transfer (CBT) process, which optimizes accuracy of predicting the re-arcing in real time in response to the molten metal transfer, realizes stable, low spatter level. In this paper, the methods for controlling short-circuit transfers to minimize spatter and to realize stable arcs in GMAW of stainless sheet using argon-rich shielded gases are investigated. The new CBT process has been developed by applying the specific arc length control that is not affected by abnormal rise in arc voltage in argon-rich shielded gas welding. This process can suppress the spatter generation caused by fluctuation in the vibratory motion of the weld pool or inaccurate prediction of the re-arcing in the succeeding short-circuit/re-arcing cycle, and thereby spatter-free GMAW in the short-circuit transfer mode can be carried out even on stainless steels.

Off-Axis Diffractive Focusing Reflectors for Refractive Index Sensing in Microfluidic Devices

Microfluidic devices with off-axis diffractive focusing reflectors (DFRs) were fabricated for refractive index sensing in biological and chemical fields. Such devices can detect refractive index changes of dielectric liquid samples just above the DFRs by measuring the changes of diffracted power from the fluidic devices. The focusing reflectors consist of curved SiO2 lines with periods which gradually varied from 1.23 to 3.73 µm. When a probe laser with 632.8 nm wavelength was coupled to the DFRs at the off-axis angle of 20°, a focal spot of 3.5 µm was observed at the distance of 1000 µm from the lens surface, which was close to theoretical value.

Femtosecond Laser Nonlinear Lithography

Micro-optical elements with precisely controlled three-dimensional surfaces are expected to play important roles in the fields of functional photonic devices, micro-opto-electromechanical systems, lab-on-a-chip devices, and so forth. For example, blazed gratings exhibit more than two times higher diffraction efficiencies than those of their standard binary counterparts. Aspheric lenses are effective for precise control of light refraction for compensation of optical spherical aberration. Sub-wavelength periodic structures can markedly suppress Fresnel-reflection losses−generated at the interface between lens materials and air−over a wide spectral bandwidth and a large field of view. Most micro-optical elements are manufactured using semiconductor fabrication technology including lithography and various etching methods. Although such a technology is an effective means for arbitrary nano/microstructures with high quality on a wafer, the crosssectional profiles of the structures are limited to binary shapes. Consequently, the fabrication of three-dimensional surfaces such as slopes and hemispherical shapes remains a challenging task. For obtaining slope structures, a gray-scale mask method (Suleski & O’Shea, 1995), a multiple mask technique (Herzig, 1997), and a moving mask process (Hirai et al., 2007) are often used. Electron-beam direct writing with dose control can also be utilized (Fujita et al., 1982). However, these techniques entail strong restrictions in spatial resolution and cross-sectional profiles of the resist. Alternatively, highly precise control of exposure conditions and stage alignments are required. In addition to this limitation, the semiconductor fabrication technology has another problem: this technology typically cannot be applied to the microfabrication on non-flat substrates because of its planar nature. When photoresists are coated onto non-flat substrates, the resist thickness varies from area to area because of the surface tension. The focal position and the light power are modulated by the resist thickness. Consequently, the pattern sizes are strongly affected by the depth profiles of underlying substrates. The spraycoating technique was reported to obtain uniform distribution of the resist thickness even on non-flat substrates (Singh et al., 2005). However, its long-focal-depth optics is unsuitable for achieving high spatial resolution. Some research groups have demonstrated the lithographic fabrication onto convex or concave lenses using specially modified stage systems with ultraviolet laser sources (Radtke & Zeitner, 2007). Although this technique is useful, complex exposure systems are necessary. To overcome these difficulties of current semiconductor fabrication technology, we have proposed a combined process of nonlinear lithography and plasma etching, and have