超狭開先ＧＭＡ溶接プロセスの開発 １ 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

Abstract

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.