Welding Phenomena during Vertical Welding on Thick Steel Plate using Hot-wire Laser Welding Method *

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1 [ 溶 接 学 会 論 文 集 第 33 巻 第 号 p. 000s-000s(2015)] Welding Phenomena during Vertical Welding on Thick Steel Plate using Hot-wire Laser Welding Method * by Eakkachai Warinsiriruk**, Koei Hashida**, Motomichi Yamamoto**, Kenji Shinozaki**, Kota Kadoi**, Tadakazu Tanino***, Hiroshi Yajima***, Tsutomu Fukui****, Shin Nakayama*****, Tetsuro Nose ******, Syoko Tsuchiya*******, Hiroshi Watanabe********, Tatsunori Kanazawa******** The objective of this research was to develop a single-pass vertical welding process for thick steel plates using hot-wire laser welding to reduce the heat input. A laser diode with a large rectangular beam spot and twin hot-wire feeding system were used. Welding phenomena were investigated using stationary and weaving laser with small and large groove widths. Single-pass vertical welding for thick steel plates with a low heat input and low dilution could be achieved. A critical energy density existed for each welding speed at which fusion of the groove surface could commence. The energy density affected fusion of the groove surface compared with welding speed. Weaving laser made it possible to maintain a high energy density and accommodate the large gap variation. Key Words: Hot-wire, Laser welding, Single-pass vertical welding, Thick steel plate, Low heat input 1. Introduction In recent years, larger container ships have been built for more efficient maritime transportation, and consequently thicker steel plates have been used in their structure 1). Electrogas arc welding (EGW) is usually used to join vertical heavy-thick steel plates during ship building, especially for a hatch side coaming 2-4) in the part of the container ship structure. However, EGW results in a high heat input and reduces the welded joint toughness significantly because of grain coarsening and widening in a heat-affected zone (HAZ) 5-6). Therefore, there is strong demand for a novel lower heat input vertical welding process to join heavy-thick steel plates with high strength and toughness. Hot-wire laser welding has been developed for various types of joints and materials, and it can achieve a simultaneous low heat input, low dilution, high efficiency and high quality joint 7-8). The hot-wire method allows for a high deposition rate independent from the main heat source since a filler wire is heated to its melting point by Joule heating. The main heat source can thus be used only for base metal melting. In this research we propose novel vertical welding for thick steel plates using a laser diode as a heat source in combination Received: ** Graduate School of Engineering, Hiroshima University *** Department of Engineering, Nagasaki Institute of Applied Science **** Nippon Kaiji Kyokai ***** Mitsubishi Heavy Industries, Ltd. ****** Nippon Steel & Sumitomo Metal Corporation. (Present: Nippon Steel & Sumikin Welding Co., Ltd.) ******* Nippon Steel & Sumitomo Metal Corporation. ******** Mitsubishi Hitachi Power Systems Engineering Co., Ltd. with the hot-wire method to achieve a much lower heat input compared with the EGW process. This welding process with a low heat input should prevent grain coarsening and widening HAZ width and maintain the toughness of a welded joint. In addition, this process requires a high efficiency by single-pass welding for thick steel plates in the vertical direction such as with EGW. Figure 1 shows a schematic illustration of the proposed welding process. A high-power laser diode is used as main heat source and the hot-wire method is used for efficient deposition. A large rectangular spot-shaped laser beam, which fits a groove width (gap) and plate thickness, is used. The laser is irradiated continuously from above the joint into the groove to create and maintain a molten pool during welding. A reflected laser on the molten pool surface is used to melt groove surfaces in front of the molten pool efficiently. Filler wires fed from both sides of the groove are heated to their melting point by Joule heating using a hot-wire system before entering the molten pool. Jigs and the motion are similar to those of an EGW process. It is expected from the aforementioned features that the proposed hot-wire laser welding method allows for single-pass vertical welding of a heavy joint with a low heat input, low dilution, high efficiency and reduction in a laser power. The welding phenomena and the effects of primary parameters on the welding phenomena of this process were investigated. A stationary laser beam without weaving and a specimen with a relatively small gap were used in the basic investigations. A weaving laser beam and specimen with a relatively large gap were used to investigate the effect of laser irradiating condition on the welding phenomenon. The potential of the proposed process for practical use was demonstrated in this investigation.

2 2 研 究 論 文 他 : 2. Experimental procedure 2.1 Materials and specimen KE47 steel plates and filler wires of JIS Z 3325 YGL2-6A (AP) with a 1.6 mm diameter were used. The specimen dimensions (50 mm 100 mm 26 mm) are shown in Fig Experimental setup for 5 mm gap with stationary laser Stationary laser without weaving and a specimen with a relatively small gap (5 mm) were used. Table 1 shows the welding conditions used. The laser power and welding speed were varied from 4 6 kw and set at 1.7 and 3.3 cm/min, respectively, to determine the effect of primary parameters on welding phenomena. The rectangular laser spot size of 3.5 mm 26 mm and the conditions were fixed. The hot-wire feeding speed was adjusted to the welding speed, and the wire current was set to heat a filler wire tip to below its melting point. Two hot wires were inserted from both sides of the groove. Argon gas was used for shielding. A high-speed camera was used to observe molten pool formation and stability, and filler wire feeding during welding. 2.3 Experimental setup for 10 mm gap with weaving laser Weaving laser and a specimen with a relatively large gap (10 mm) were used in the second experiment. The weaving system can sweep a large rectangular laser spot with a long length fitting a plate thickness and a narrower width than the gap parallel to the gap width direction. The weaving amplitude can be adjusted to a variable gap and the weaving frequency can be also changed. This weaving system provides gap flexibility and maintains a high power density. Table 2 shows the welding conditions when using weaving laser for a specimen with 10 mm gap. The laser spot (2 mm wide and 26 mm long) at a focus position was used with a 6 kw laser power. The weaving amplitude was controlled to fit to a 10 mm gap, and the weaving frequency was set at 15 Hz. The welding speed was set to 1.7 and 3.3 cm/min. The other welding conditions are the same as those when using stationary laser. 3. Results and discussion 3.1 Results from welding on 5 mm gap with stationary laser In-situ observations using a high-speed camera indicated the stable formation of a molten pool and stable feed of filler wires during welding. The proposed hot-wire laser welding method can form a stable large molten pool, which fills a groove constantly using a laser diode with a large rectangular spot as a heat source. Figure 3 shows the macro cross-sections of the welded joints at the thickness center in the vertical direction, and Fig. 4 shows the Fig. 1 Schematic illustration of proposed vertical welding process using hot-wire laser welding method. Fig. 2 Specimen dimensions used for hot-wire laser welding. Table 1 Welding conditions for stationary laser. Gap, mm 5 Laser method Stationary Laser power, kw 4 6 Spot size, mm 3.5 w x 26 l Defocus amount, mm 50 Welding speed, cm/min 1.7, 3.3 Wire feed speed, m/min 0.54, 1.12 Wire current, A Wire feeding angle, degree 45 Wire feeding position, mm 1 4 Shielding gas (Argon), l/min 1 Table 2 Welding conditions for weaving laser. Gap, mm 10 Laser type Weaving (15 Hz) Laser power, kw 6 Spot size, mm 2 w x 26 l Defocus, mm 80 Welding speed, cm/min 1.7, 3.3 Wire feed speed, m/min 1.24, 2.48 Wire current, A 91, 124 Wire feeding angle, degree 45 Wire feeding position, mm 0 Shielding gas (Argon), l/min 20

3 溶 接 学 会 論 文 集 第 33 巻 (2015) 第 号 3 lack of fusion ratio and HAZ width measured on vertical sections for various welding conditions and for a 5 mm gap. With increasing laser power to 6 kw, the weld bead width and HAZ increase, however, the HAZ width is narrower than that of conventional EGW (15 20 mm). Single-pass vertical welding for thick steel plates with a low heat input and low dilution could be achieved using the proposed process. When a relatively low laser power of 4 or 4.5 kw was applied, only some of the upper region on the groove surface melted, and a lack of fusion occurred under some conditions. When a relatively high laser power of 5 or 6 kw was applied, the groove surface melted sufficiently, then the lack of fusion decreased and a sound weld bead was obtained. In contrast with the effect of laser power, that of the welding speed on the melting of base metal and the lack of fusion ratio is small, as shown in Figs 3 and 4. It appears that the laser power density affects fusion of the groove surface and sound weld bead formation compared with the welding speed. Figure 5 shows the relationship between the energy density (laser power / laser irradiating area) and penetration width of the groove at each welding speed using stationary laser and the 5 mm gap. A critical energy density exists at 25 and 35 W/mm 2 for each welding speed of 1.7 and 3.3 cm/min, at which fusion of the groove surface commences. The penetration width increases linearly with increase in energy density for both welding speeds. In hot-wire laser welding, a reflected laser on a molten pool surface and heat conduction from a molten pool around a groove surface are used to melt groove surfaces in front of the molten pool. The energy density is a primary factor to obtain sound fusion since the critical energy density does not double even when the welding speed doubles. 3.2 Results from welding on 10 mm gap with weaving laser It was found from in-situ observation using a high-speed camera that weaving laser resulted in the stable formation of a molten pool and stable feed of filler wires during welding when the gap was 10 mm at 6 kw laser power. Figure 6 shows the cutting plan and macro cross-sections of welded joints in the vertical direction at the thickness center and horizontal direction under the welding speed at a 1.7 and 3.3 cm/min for the 10 mm gap using weaving laser. Fig. 4 Lack of fusion ratio on vertical sections and HAZ width in case of 5 mm gap with stationary laser. Fig. 3 Macro cross-sections of welded specimens for 5 mm gap and stationary laser. Fig. 5 Relationship between energy density, welding speed and penetration width of weld metal.

4 4 研 究 論 文 他 : Observation of the cross-section in the vertical direction shows stable fusion of the groove surface and sound beads for both welding speeds. It can be seen from the cross-section in the horizontal direction that although a lack of fusion existed at both groove edges, a small and uniform penetration width was achieved along the groove. Figure 7 shows the lack of fusion ratio and the bead width on the horizontal sections. The bead width created at both welding speeds is slightly larger than 10 mm, which is the initial gap. Therefore, the dilution ratio is small. The lack of fusion exists only at the groove edges and its percentage of the full thickness is ~20 % at both welding speeds. Perfect fusion is achieved at ~80 % of the middle region of the groove in the thickness direction. The optimum energy distribution at the groove edge must be considered to obtain adequate fusion and the sound bead in this region. The welding speed does not affect the bead width and lack of fusion ratio significantly. These results indicate that weaving laser with a narrow rectangular laser spot enables formation of a stable molten pool and creates suitable fusion at the groove surface on a large gap since this method maintains a high laser power density. Acknowledgements This research was supported by Nippon Kaiji Kyokai in the Joint R&D with Industries and Academic Partners research program. (a) Cutting plan 4. Conclusions A novel single-pass vertical welding process for thick steel plates using the hot-wire laser welding method was proposed and its melting phenomena were investigated. The conclusions are as follows. (1) Stable formation of a molten pool and stable feed of filler wires during single-pass vertical welding on the joint of 26 mm thick steel plates was achieved using a laser diode with a large rectangular beam spot and the hot-wire system. (2) The energy density is the principal parameter to obtain adequate fusion and a sound weld bead, and it affects the melting phenomena compared with the welding speed. (3) Stable formation of a molten pool and sound fusion were achieved by using the laser weaving method with a narrow rectangular laser spot even when a large gap was used. The combination of a long and narrow rectangle laser spot fitting to a plate thickness and laser weaving achieves a large gap tolerance. (4) Although the energy distribution in the thickness direction must be optimized to obtain a sound bead at bead surfaces, the proposed hot-wire laser welding process could be used as a single-pass vertical welding process for thick steel plates with low heat input and low dilution. (b) Cross-sections Fig. 6 Cutting plan and macro cross-sections of 10 mm gap using weaving laser beam. Fig. 7 Lack of fusion ratio on horizontal sections and bead width for a 10 mm gap.

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