Takayoshi Ohji

Micro-arc discharge phenomena

Abstract Features of microseconds arc discharge with gap spacing of micrometers, between a cathode of thin tungsten wire with sharpened tip and an anode of stainless-steel plate, have been experimentally investigated. The experimental setup, which consists of a micro-arc torch, precise positioning mechanisms actuated by piezoelectric devices and pulse motors, and a power supply for discharge, was constructed. Time-resolution of micro-discharge appearance was observed through microscope with high-speed shutter CCD camera. It is shown that gap length is a dominant factor influencing on electrical breakdown voltage and melting spot formed at anode surface by micro-arc.

Mathematical modelling of metal active gas arc welding

Abstract A feasibility study has been conducted to determine whether mathematical models can be used for the numerical simulation of metal active gas (MAG) arc welding. In the present work, a three-dimensional, nonstationary thermal model for fillet welding is developed. The transient temperature distribution in the base metal is numerically analysed to estimate the molten pool size using a finite difference model based on the heat flow equation, and the theoretical configuration of the molten pool is calculated, taking account of the balance of gravity, surface tension, and arc pressure. The developed model can be applied to various welding processes such as multipass welding, lap welding, and welding with torch oscillation. To evaluate the validity of the model analysis, the calculated results are compared with experimental results for MAG welding. Good correspondence is demonstrated between experiment and calculation.

Characteristics of hollow cathode arc as welding heat source: arc characteristics and melting properties

Abstract A feasibility study has been conducted to determine whether a hollow cathode arc (HCA) can be used for a welding heat source in space, that is high vacuum and microgravity conditions. The HCA method enables the arc discharge to form under low pressure conditions by purging a small amount of gas through the centre of the hollow cathode. The characteristics of HCAs under low pressure conditions have been experimentally investigated, in particular the melting properties and voltage-current characteristics. Results show that the penetration profile and arc characteristics of the HCA method are sensitive to process parameters such as gas flowrate, arc length, and inner diameter of the electrode, and that the penetration in HCA melting is extremely deep under the conditions of low gas flowrate and long arc length.

Temperature measurement by UV thermal radiation

Summary Accurate measurement of temperature is of considerable importance in materials processing, such as welding, thermal cutting, and heat treatment of metals. Radiation thermometry has a distinct advantage over other temperature detection methods, since it requires no physical contact with the work. The reliability of thermometry, however, is limited if the emissivity of the work to be measured is not accurately known. Plancks law of radiation shows that the effect of temperature on the spectral radiance is greater at shorter wavelengths. The present paper experimentally describes the characteristics of UV (ultraviolet) thermometry. A temperature measurement test by the UV spot sensor (spectral range: 0.25 μm) shows that the melting point of Ni can be measured with an accuracy of ± 1 % regardless of any emissivity fluctuation. The CCD camera with UV filter used in this investigation makes it possible to detect the temperature image on the work in welding virtually independently of the emissivity of t...

Development of circumferential TIG welding process model: a simulation model for welding of pipe and plate

Abstract It is useful to develop a numerical model for various welding positions in order to understand welding phenomena. Welding simulation models for a particular application could help in offline robot programming and online robot control. The objective of this study was to develop a tungsten inert gas (TIG) welding simulation model for various workpieces of different shapes. The present model may be easily applied to several workpiece configurations. This article deals with a model for circumferential welding of a pipe to a plate of mild steel, and aluminium alloys. The calculated results clarify that a homogeneous weld is hardly obtained at a fixed pipe welding position due to gravity, and that weld bead shape varies sensitively with wall thickness in pipe to plate welding. It is concluded that the model proposed in this article is useful to simply simulate TIG welding and provides a powerful means to estimate the optimum process parameters in welding practice.

Modeling of Short-circuiting Transfer Process in GMA Welding-Numerical Model of Metal Transfer Phenomena in GMA Welding (Report 1)-

In this paper is described a 2D-numerical model of short-circuiting transfer process in GMAW. Time-dependent deformation and/or detachment of the molten drop at the tip of electrode wire were simulated with aids of the numerical solution technique based on finite-difference method, which was developed in order to investigate the effect of electromagnetic force and capillary force on the kinetics of metal transfer process. It is shown that the time required for breakup of liquid metal bridge, formed between the electrode and the weld pool, depends on the drop size before the contact and/or the short-circuiting current value.

Welding of Aluminum Alloy by Gas Hollow Tungsten Arc Welding Method in a Vacuum. A Study on Gas Tungsten Arc Welding in Space (Report 2).

The characteristics of the GHTA (Gas Hollow Tungsten Arc) welding of aluminum alloy in a vacuum are shown in the present paper. The results are summarized as follows, (1) There is a shifting phenomenon of arc discharge in which transient arc discharge shifts via the transitional arc discharge to the stationary arc discharge under GHTA welding in a vacuum, and the characteristics of current and voltage of these discharges are shown.(2) The period of transient arc discharge could be shortened by increasing the flow rate of argon, using the 2%La2O3-W electrode and/or by decreasing the initial current.(3) The surface of electrode and collet body is eroded by the vaporization of the material during the transient arc discharge. The amount of vaporized material increases linearly with increase of the period of transient arc discharge.(4) Taking account of the repair of the space station and space structures which may be made of an aluminum alloy with a thickness about 3-4 mm, the arc current at 50-70 A may be enough to weld aluminum alloy of plate thickness 3-4 mm by the GHTA method in a vacuum.(5) The characteristics of hardness distribution and strength of joints of aluminum alloy by GHTA welding in a vacuum are clarified.

Heat input properties of hollow cathode arc as a welding heat source

In order to clarify whether a hollow cathode arc (HCA) can be used as a welding heat source in space, investigations into the fundamental characteristics of HCA were experimentally performed under low pressure conditions. The HCA method enables an arc discharge to ignite and maintain under low pressure conditions; in contrast, low pressure conditions make it extremely difficult for the conventional gas tungsten arc method to form an arc discharge. In an earlier paper, it was shown that the melting process by HCA is very sensitive to process parameters such as the gas flow rate and arc length, and a deep penetration forms when the arc length is long and the gas flow rate is low. In this paper, the distribution of the arc current on the anode surface and the plasma properties of the HCA under low pressure conditions have been made clear and the total heat energy to the anode has been discussed in order to understand the heat input properties of the HCA. The result shows that the HCA in the case of a low gas flow rate is a high and concentrated energy source, and the high energy input to the anode contributes to the deep penetration.

Effect of current wave-form on AC TIG welding of aluminium alloys

Abstract Because of their superior properties (low specific gravity, corrosion resistance, recyclability etc.), aluminium alloys are currently employed in various sectors and their applications are still expanding. It is well-known that it is necessary to eliminate the oxide film formed on the aluminium alloy surface prior to welding, this characterises the welding of aluminium alloys.

Gas hollow tungsten arc characteristics under simulated space environment

Abstract A feasibility study has been conducted to determine whether gas hollow tungsten arc (GHTA) welding can be used for welding in space. As described in a previous paper by the present authors, the GHTA method has been tested in a simulated space environment using aircraft. The test result shows that the method is most promising as a welding process in space. In the present paper, some fundamental characteristics of the GHTA in a vacuum chamber, such as discharge characteristics and plasma properties, have been elucidated and the results of melting tests on stainless steel plate using the GHTA method have been compared with those obtained via a conventional gas tungsten arc method under atmospheric pressure.