Koichi Masubuchi

Mechanisms of rapid cooling in underwater welding

Abstract Two inherent problems, rapid cooling and hydrogen embrittlement, associated with underwater ‘wet’ welding remain unsolved at the present time. Weld defects induced by rapid cooling usually appear in underwater welds. An understanding of the basic mechanisms is probably important and necessary in order to establish solutions to the problems of rapid cooling. Three types of weld defects related to rapid cooling during underwater welding are presented. A discussion on the thermal behaviour of underwater welding is then introduced. This discussion covers some important thermal-physical areas which control the cooling rate during underwater welding. It includes underwater bubble dynamics, underwater arc-heat distribution, Joule heating, heat input mechanism and boundary heat loss mechanism. Several important new concepts of reducing the effect of rapid cooling during underwater welding are finally recommended.

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.

GHTA welding phenomena of an aluminium alloy in a vacuum

Abstract The midair disintegration of the space shuttle ‘Colombia’, in which the total crew of 7 members was lost, occurred on 1st February 2003. The circumstances of the crash shown on TV screens reaffirmed the publics view of the risks of space development and the importance of security. NASA has stopped any further space shuttle launches until the causes are fully investigated – a postponement of the completion of the International Space Station is inevitable. The operational life of the ISS is considered to be 10 years but it could be quite possible to prolong this if required for various reasons. Accordingly, it is highly likely that there will be requests for remodelling in order to achieve high levels of functional performance and to repair damage caused by debris impact and for other reasons. In order to meet these requirements it is essential to establish space welding technology.1

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.

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.

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.

Models of Stresses and Deformation Due to Welding—A Review

This paper first discusses the use of computers in welding, then the development of studies on residual stresses and distortion during welding. Since the late 1960s, computer programs have been developed to analyze transient thermal stresses and metal movement during welding. Systematic research efforts have been carried out at the Massachusetts Institute of Technology on residual stresses and distortion in weldments. A series of computer programs has been developed to calculate residual stresses and distortion in various types of welds. Experimental data have been generated on weldments in various materials and processes; data on steel weldments are presented in this paper.

Arc initiation phenomena by space GHTA welding process using touch start technique in a vacuum

There is a pressing need to establish the technique to repair shuttle craft in space as illustrated by the accident that caused the aerial disintegration of ‘Columbia’ on 1 February 2003, and the accident relating to the heatresisting system, which occurred at the time of the ‘Discovery’ launch on 26 July 2005. Furthermore, the following requirements are expected for the International Space Station (ISS) that is currently under construction: damage repair caused by debris impact, repair against unforeseen events and establishment of space fabrication techniques, which can be employed for modifications to enhance the system’s functions. These techniques will be applied to the construction of space structures and lunar base construction in the future. The former Soviet Union recognised the necessity of space welding technology from around 1960 and research development, mainly of electron beam welding, was conducted. Studies into electron beam welding and Nd–YAG laser welding have been performed even in the USA. NASA programmed space welding experiments in which electron beam welding equipments from the former Soviet Union were to be installed in a shuttle; however, problems such as the shuttle schedules and the security of astronauts took precedence and the programme was postponed in haste indefinitely 6 months prior to the welding experiments planned for March 1997. The authors and others proposed, in 1993, the space GHTA (Gas Hollow Tungsten Arc) welding process, which employs hollow tungsten electrodes. 10 To date, aluminium alloy butt welding with the use of filler metal was carried out in a vacuum and it was demonstrated that adequate welded joint strengths could be obtained. GHTA starting experiments, employing the high frequency high voltage technique and the DC high voltage technique 13, 14 have been implemented to date. However, there are problems of electromagnetic interference in the case of the former and in the case of the latter, there are problems of transfer to glow discharge rather than arc discharge. Because of these issues, GHTA starting experiments using the touch start technique were carried out in this study and it was demonstrated that the touch start technique was feasible for application under the ISS orbital pressure and the effects of the starting conditions upon the arc initiation phenomena were investigated. Arc initiation phenomena by space GHTA welding process using touch start technique in a vacuum

Remotely Manipulated And Autonomous Robotic Welding Fabrication In Space

The results of a National Aeronautics and Space Administration (NASA) sponsored study, performed in order to establish the feasibility of remotely manipulated or unmanned welding fabrication systems for space construction, are first presented in this paper. Possible space welding fabrication tasks and operational modes are classified and the capabilities and limitations of human operators and machines are outlined. The human performance in remote welding tasks is experimentally tested under the sensing and actuation constraints imposed by remote manipulation in outer space environments. Proposals for the development of space welding technology are made and necessary future research and development (R&D) efforts are identified. The development of improved visual sensing strategies and computer encoding of the human welding engineering expertise are identified as essential, both for human operator assistance and for autonomous operation in all phases of welding fabrication. Results of a related follow-up study are then briefly presented. Novel uses of machine vision for the determination of the weld joint and bead geometry are proposed and implemented, and a first prototype of a rule-based expert system is developed for the interpretation of the visually detected weld features and defects.

The Need for Analytical/Experimental Orchestrated Approaches to Solve Residual Stress Problems in Real Structures

This paper discusses the need for analytical/experimental hybrid approaches to solve residual stress problems in real structures. Analytical models are used as the basis, and experimental data obtained on selected locations are used to calibrate the analytical models and to improve their accuracy. By properly combining recent analytical and experimental techniques, it is possible to significantly improve the state-of-the-art of studying residual stresses and distortion and their effects on service behaviors of welded structures.