The effect of alternate supply of shielding gases in austenite stainless steel GTA welding

Ar-MIG Arc Behavior in Local Addition of Small Amount of Oxygen into Shielding Gas

Purpose: Generally discrete alternate supply of shielding gas is a new technology which alternately supplies the different kinds of shielding gases in weld zone. In this study, welding characteristics under variation of alternate supply method of shielding gases in austenite stainless steel using a GTA welding process have been investigated and found the relationship between different types of shielding gas and shielding gas supply. Design/methodology/approach: 2-D axisymmetric heat and fluid mathematical model about weld pool has been developed for verifying the effect of alternate supply of shielding gas. The developed models were solved using a general thermofluid-mechanics computer program, PHOENICS code, which is based on SIMPLE algorithm. Findings: The computed results showed that the developed computational models are very adequate to predict in the weld pool and bead geometry, and the technique of alternate supply of shielding gas should be useful to increase higher productivity, cost savings and better quality in arc welding. Research limitations/implications: The effect of alternate supply of shielding gas should be useful to apply for a narrow-gap welding process. Originality/value: The range of molten metal at the top of weld pool for supply of He shielding gas became wider than that for supply of Ar shielding gas. The developed computational models are very adequate to predict in the weld pool and bead geometry.

Development of a mathematical model to study on variation of shielding gas in GTA welding

Purpose: Generally discrete alternate supply of shielding gas is a new technology which alternately supplies the different kinds of shielding gases in weld zone. In this study, welding characteristics under variation of alternate supply method of shielding gases in austenite stainless steel using a GTA welding process have been investigated and found the relationship between welding distortions and welding speed and different types of shielding gas and shielding gas supply. Design/methodology/approach: 2-D axisymmetric heat and fluid mathematical model about welding arc and weld pool has been developed for verifying the effect of alternate supply of shielding gas. The developed models were solved using a general thermofluid-mechanics computer program, PHOEINCS code, which is based on SIMPLE algorithm. Findings: The computed results showed that the developed computational models are very adequate to predict in the weld pool and bead geometry, and the technique of alternate supply of shielding gas should be useful to increase higher productivity, cost savings and better quality in arc welding.

A study on variation of shielding gas in GTA welding using finite element method

Welded joints used in some circumstances may generally face the problem of loss of joint performance due to welding heat input as well as welding deformation and welding residual stress. For example, ultra-fine grained steel welds may sustain softening in regions heated in a slightly lower temperature range than the Ac 1 transformation point in such a way that zones with a lower strength than the base metal strength may possibly arise. The heat-affected zone of recently developed ultrafine grained steels may also sustain softening due to grain coarsening, so that when such steels are used for fabrication of actual structures, they risk facing the problem of being satisfactorily able to maintain joint performance. Heat input reduction is an effective measure that may be taken to control these problems. Heat input reduction, however, implies loss of welding fabrication efficiency and it is therefore important to achieve lower heat input and higher efficiency as simultaneous objectives in welding fabrication. Ultra-narrow gap GMA welding has been developed from this perspective. Ultra-narrow gap GMA welding seeks to achieve higher welding speed and lower heat input through narrowing the welding groove gap to such an extent as to make weaving in the plate width direction unnecessary. Groove width narrowing generated problems such as arc instability phenomena or poor groove root penetration are satisfactorily overcome through the current, voltage and wire feed rate during welding being suitably controlled, and through the welding wire being oscillated in the plate thickness direction. The calorific value of the arc can be suitably dispersed through welding wire oscillation in such a way that the deposited metal throat thickness can be increased. During application of ultra-narrow gap GMA welding to fabrication of actual structures, however, it is necessary to clarify joint performance-related factors, such as the temperature profiles and strength properties of welds during consideration of the effects of welding wire oscillation. Ultra-narrow gap GMA welding is envisaged particularly for application to welding of steels, typified by ultra-fine grained steels that risk loss of weld properties due to the welding thermal cycle, and it is therefore imperative to undertake thorough evaluations of joint performance during consideration of welding thermal cycles and associated changes in material properties. Through application of numerical simulation techniques to solve the foregoing problems, it is expected to be able to make global joint performance evaluations that consider material properties and the temperature fields in which welds are formed. This study, as a first step towards joint performance evaluation of ultra-narrow gap GMA welded joints, uses detailed measurements of temperature profiles during welding alongside numerical simulations of welding thermal cycles and microstructural phase fractions to develop a technique for estimation of weld hardness distributions as an important feature of ultranarrow gap GMA welds.

Numerical simulations of weld hardness distributions in ultra-narrow gap GMA welding. Evaluation of weld hardness distributions by numerical simulations of welding thermal cycles and microstructural phase fractions (1st report)

In previous reports, an ultra-narrow gap CO2 gas metal arc welding process has been developed in order to produce excellent welded joints. In the ultra-narrow gap (less than 5 mm gap) welding process, as the welding wire melting behavior was controlled by low frequency pulsated arc current waveform, the arc generating at the wire tip was widely oscillated in the direction of thickness and a good weld bead with large throat thickness was formed. In such a higher efficient welding process, the strong arc force of CO2 arc is essential to obtain sound beads without lack of fusion at a groove bottom, however, the oxygen content of weld metal in CO2 arc welding has to be reduced to obtain high-toughness at weld metal. In this report, a newly welding process is proposed in which an arc is widely oscillated along groove walls and oxygen content of welded metal is reduced as periodically controlling composition of CO2 gas in shielding gas. As an arc length between the wire tip and groove wall is held constant in ultra-narrow I type gap joint, arc current at stable operating point of CO2 arc essentially decreases less than that at stable operating point of Ar arc in constant potential characteristics of power source. It was confirmed from the numerical simulations on both wire melting and electric welding circuit that pulsated current waveform was produced as periodically alternating CO2 arc with MIG arc. Using the developed welding torch locally introducing CO2 gas into Ar+2%O2 shielding gas, the welding was carried out under V-type groove with less than 30 degree in the bevel angle. It was shown that the arc was widely oscillated along V groove walls and sound bead was formed in one side welding. Furthermore, it was confirmed that the oxygen content of weld metal in the proposed process was reduced to the level of that in MIG arc welding, and absorbed energy of weld metal was the same level as the MIG arc welding.

誠高橋

Papers

シールドガス組成制御によるgma溶接プロセスの開発 : 超狭開先gma溶接プロセスの開発(第3報)

誠 高橋

Papers

シールドガス組成制御によるgma溶接プロセスの開発 : 超狭開先gma溶接プロセスの開発(第3報)

誠高橋