Terumi Nakamura

狭開先化に伴うアーク溶接現象の基本的な特性把握と溶接安定化の提案 : 超狭開先GMA溶接プロセスの開発(第1報)

Narrow gap welding (NGW) joints offers many advantages over conventional welding methods, such as good mechanical properties of joints, high welding efficiency and low residual stress. As the groove gap width becomes narrower, the arc heat input can be reduced and the merits in narrow gap welding increases more. Generally, GMA welding method has been never applied to less than 5 mm groove gap, because it is guessed that it is arc instability and lack of fusion at the groove bottom area occur. In this paper, first of all, arc behavior under narrower gap joints is discussed, and it was concluded that the arc in MIG arc welding irregularly perturbates up-to-downwards along the groove wall under less than 5 mm gap, but CO2 arc was stable under narrower gap. Next, penetrations at the groove bottom area in CO2 arc welding were discussed. Characteristics of bead formation phenomena in CO2 buried arc welding of bead-on-plate were analyzed. From the results, the relationship between hydrostatic potential of molten metal and arc force corresponding with welding current was estimated. Furthermore, the width of gauging region of penetration by arc force was measured and the relationship between the melting width at groove bottom and welding conditions (welding current and welding speed) can be suggested. With these results, numerical simulation model was proposed and the optimum welding conditions to melt the groove bottom area sufficiently and to minimize heat input were searched by numerical simulation. And then narrow gap welding with 5 mm groove gap was carried out using these simulated welding conditions. In the experimental results, the weld bead was obtained without lack of fusion at groove bottom, but the convex surface bead was formed which is disagreeable in multi-pass welding. The new welding process was proposed from numerical simulations in order to prevent this convex bead and to obtain sufficient melting at bottom area. In the new process, the wire extension can be controlled by welding current waveform and then arc regularly oscillated up-to-downwards along the groove wall. In this arc oscillation, arc heating distribution along groove wall led to both sufficient penetration at groove bottom and concave surface bead shape.

GMA Welding Process with Periodically Controlling Shielding Gas Composition. Development of Ultra-Narrow Gap GMA Welding Process. Report 3.

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.

Application of Numerical Simulation Considering the Effect of Phase Transformation to the Estimation of Hardness Distribution in Welds

This study investigated a method for estimating hardness distribution in welds, considering the effect of phase transformation and weld thermal cycles. Hardness distribution in welds was estimated from fractions and hardness of each microstructure by using rule of mixture. Finite element heat conduction analysis was performed to calculate weld thermal cycles. Microstructures formed corresponding to the thermal cycle were also calculated based on the continuous cooling transformation (CCT) diagram. The method mentioned above was applied to welds of Ultra-Narrow Gap welding process, which was developed for welding of ultrafine-grained steels. The calculated thermal cycles in the welds corresponded with measured results. Moreover, the estimated hardness distribution in the welds, which were estimated by using calculated thermal cycles and the phase fraction of each microstructure, was also in good agreement with measured values.

Modelling of Wire Melting Behaviour of Coaxial Hybrid Solid Wire in Gas Metal Arc Welding in a Pure Argon Shielding Gas

Coaxial multi layer solid wires (coaxial hybrid solid wires) have been developed in order to stabilize welding in pure Ar shielding gas. This coaxial hybrid solid wire has a double structure, the composition of inner part and outer part is different. The wire melting behaviour of coaxial hybrid solid wire is different from that of conventional solid wires because the electric current path and the heat transfer behaviour depend on the characteristics of inner and outer materials. Therefore, in order to obtain stable welding, to combine appropriate materials for inner part and outer part is necessary. To understand the influence of the material properties of the inner part and outer part on the wire melting behaviour, a simulation model for wire melting has been developed. The influences of the melting temperature, the specific heat, and the thermal conductivity on wire melting behaviour were examined. Simulations show that the shape of the solid wire tip is affected by changing melting temperature and specific heat of the inner part. Furthermore, these changes have a large effect on the length of the wire extension. The thermal conductivity effects on wire tip shape and temperature distribution, but has little effect on the wire extension.

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)

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.

Reconstitution of Charpy Impact Specimens by Surface Activated Joining

The method of surface activated joining (SAJ) was applied to reconstitution of Charpy impact specimens. SAJ makes use of friction at material surfaces in a vacuum to achieve joining without melting. This paper describes verification to apply SAJ to reconstitution of Charpy impact specimens with unirradiated reactor pressure vessel plate materials, JRQ and HSST-03. By optimizing joining parameters, heat affected zones induced by friction had a width of about 1 mm to either side of the joint. The original Charpy impact properties in the transition region were reproduced from reconstituted impact specimens with the insert length of 10 mm. The maximum temperature during joining was appreciably low at a given distance from the joining interface, compared with other conventional welding techniques.

Numerical investigation of the influence of heat source modeling on simulated residual stress distribution in weaving welds

The influence of the modeling of the weaving of a heat source on the simulated residual stress distribution was examined. Two types of heat source models, “weaving” and “quasi-weaving,” were used. The former modeled the weaving directly, and the latter simplified the weaving heat source to a wide and straight-moving one. When the weaving heat source was used, fluctuating temperature histories, wave-like fusion lines, and serrated residual stress distributions were obtained. Three different quasi-weaving heat source models have different combinations of the heat flux value per unit volume and the elements to which the heat source was introduced. The temperature distributions around the welds were different depending on quasi-weaving models used. However, when the temperature distributions of the weaving welds could be reproduced precisely by a quasi-weaving model, smoothed temperature histories and residual stress distributions were simulated, and the values obtained were comparable to those obtained with the weaving model. On the other hand, when the temperature distribution was not well reproduced, the residual stress distribution did not agree with that of the weaving model. The results suggest that the modeling of a weaving heat source using a quasi-weaving model requires adjustment of the temperature distribution.

Influence of surface asperities on interfacial extension during solid state pressure welding

The interfacial contact and extension of two joining surfaces are numerically studied using a finite element method. The joining surface is assumed to have two-dimensional triangular asperities. Surface-asperity-induced voids are formed at the initial contact. The interfacial extension of the initial contact area is very different from that of void surface area. The initial void surface area shrank once before full contact was attained. The large asperity angle facilitates interfacial extension and void-surface shrinkage, resulting in the formation of numerous metallic bonds. The pressure-welding-induced interfacial extension and metallic bond formation are largely influenced by the asperity angle, whose effects are experimentally confirmed.

GMA welding of 9% Ni steel in pure argon shielding gas using coaxial multilayer solid wire

A coaxial multilayer solid wire was developed to perform stable gas metal arc welding of 9% Ni steel in pure argon shielding gas. This wire has a double structure consisting of an inner and outer part with different compositions. The average composition of this wire is similar to that of the wire for solid welding wire of 9% Ni steel. A potassium compound was added to the area between the inner part (center wire) and the outer part (hoop) of the wire to stabilize the welding in pure argon shielding gas. First, the wire melting behavior was observed with a high-speed video camera system. Then, the effect of potassium was confirmed experimentally. Next, the relation between the amount of potassium and the length of the column of liquid molten metal (CLM) was analyzed. The coaxial multilayer solid wire which contained more than 0.001 wt% potassium was able to shorten the CLM. Lastly, the welding was carried out in the V-groove to assess the ductility and strength of joints. The optimized coaxial multilayer solid wire that was developed and a constant current characteristics power source were used to obtain acceptable weld joints in pure Ar shielding gas.