Hot-wire Laser Welding Process Using Laser Diode

for Large-Diameter Pipe with Narrow Gap Joint

by Masatomo Todo**, Kenji Shinozaki***, Motomichi Yamamoto***, Kota Kadoi***, Masayuki Yamamoto****, Rittichai Phaonaim***** and Toshinari Okagaito******

We investigated a hot-wire laser welding method using a laser diode with a rectangular laser beam to weld a large-diameter pipe with a narrow gap. First, welding trials using plate specimens were performed to obtain the optimum welding conditions using a defocused beam. We achieved stable feeding of wire, a stable molten pool, and a well-formed weld bead, with few defects. Second, we welded a large-diameter pipe using optimum welding conditions discovered from the trials. Although some small lack of fusion occurrences were observed in the side bending test, we achieved a narrow gap, low dilution, narrow heat-affected zone weld without solidification cracks.

Key Words: Hot-wire, Laser welding, Narrow gap, Laser diode, Low heat input, Large diameter pipe

1. Introduction

Multi-pass narrow gap welding processes have successfully been applied to heavy thick sections to meet the demands of high-efficiency manufacturing. Examples of this include the use of hot-wire gas tungsten arc welding13) (GTAW) and laser narrow gap welding47) for thermal power plant components. We developed a novel narrow gap welding process8) which is combined with a hot-wire system and defocused laser beam. In this process, the reflected laser beam acts on the molten pool surface and contributes to base metal melting. This allows for a narrower gap, lower deformation, lower dilution, and higher deposition efficiency compared with conventional hot-wire GTAW. In our previous study9), the hot-wire laser welding process using laser scanning was applied in thick plate with a narrow gap but it was difficult to optimize welding conditions because there are too many welding parameters.

In this study, we propose a hot-wire laser welding method using a rectangular spot. The optimization of welding conditions was performed by visualizing welding phenomena. Thick plates with a narrow gap were used to observe welding phenomena and to obtain optimum welding conditions. The proposed welding process was applied to an actual pipe with a large-diameter and narrow gap.

2. Experimental procedure

The base metals used were KA-SCMV28 (plate) and ASME SA335 P91 (pipe). The chemical compositions of the materials are given in Table 1 and Figure 1 shows the types of specimens used. The plate and pipe sizes were 150 mm (l) 120 mm (w) 25 mm (t), and 390 mm (outer diameter) 282 mm (inner diameter) 200 mm (l) and 54 mm thickness, respectively. Both specimens had a U-shaped groove with a gap width of 4 mm and groove angle of 2.8 . A JIS Z3317 W62-9C1MV1 filler wire of 1.0 mm diameter was used.

Table 2 shows the welding conditions. A high-power 6 kW laser diode was used with a rectangular laser spot of 4 mm width and 11 mm length applied at the focus point. The beam width was adjusted to match the gap width and the defocus length was adjusted from 0 (just focus condition) to +40 mm to change energy distribution in the laser beam. Laser power was set to 5 or 6 kW to monitor the differences and the results are presented below. The welding speed was set to 0.3 m/min and wire feeding speeds of 3.1, 6.2, and 9.1 m/min were chosen to achieve target

*Received: 2014.11.28 **Student Member, Graduate, school of Engineering Hiroshima

University ***Member, Joining and Welding Research Institute Hiroshima

University ****Student Member, Graduate, school of Engineering

Hiroshima University (Currently at Nippon Steel & Sumitomo Metal Corporation)

*****Student Member, Graduate, school of Engineering Hiroshima University (Currently Rajamangala University of Technology Krungthep)

******Member, Mitsubishi Hitachi Power Systems, Ltd. Kure

Table 1 Chemical compositions of the materials

(a) Plate (b) Pipe Fig. 1 Two specimens used for welding.

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bead heights of 2, 4, and 6 mm, respectively. The wire current was set to between 46 and 90 A to heat a filler wire to near its melting point when the wire tip was inserted into a molten pool.

Figure 2 is a schematic illustration showing the position and feeding direction of the filler wire and laser beam. The filler wire was fed on the tail of the laser spot and molten pool. The laser irradiation angle of 5 and the wire feeding angle of 80 were fixed. Welding phenomenon was observed using a high-speed camera during welding. Using a combination of three filters (cold filter, hot mirror, ND3), in situ observation was conducted at a wavelength of 700 nm or less. Frame rate and shutter speed were set to 100 fps and 1/20000, respectively.

3. Results and discussion

Figure 3 shows images captured by the high-speed camera during welding when the defocus length was changed from 0 to +40 mm, and when the wire feeding position was changed between 5, 6, and 7 mm. Figure 4 shows schematic illustrations of energy distributions and wire feeding positions when the defocus length and wire feeding position were changed. The image shows that when the defocus was set to 0 mm, the filler wire feed into the molten pool was stable only when wire feeding position was 6 mm. At the wire feeding position of 5 mm, the filler wire tip melted down frequently because the laser beam with a high energy density was irradiated on the tip as (Fig. 4 (a)). The opposite occurred with a wire feeding position of 7 mm,

where the filler wire was fed on the solidifying region at the back end of the molten pool. With a focused laser, the steep energy distribution at the edge of the laser spot resulted in a narrow tolerance for acceptable wire feeding position.

When the defocus was set as +40 mm, the filler wire feed into a molten pool was stable at all wire feeding positions. The filler wire tip could melt smoothly because the defocus condition creates a gradual energy distribution at the edge of the laser spot (Fig. 4 (b)). Also, the defocus condition creates a physically longer laser spot and longer tail of molten pool (Fig. 4 (b)); therefore, the region where the filler wire can be stably fed into the molten pool becomes larger.

Our results showed that the gradual energy distribution at the edge of a laser spot under defocus condition results in a higher tolerance for filler wire feeding position, while the just focus condition, with its steep energy distribution, reduced the tolerance.

Figure 5 shows cross sections of the plate specimens welded under the defocus condition of +40 mm when laser power and bead height were varied. The red, blue, yellow, and pink arrows in Fig. 5 represent a solidification crack, lack of fusion, blow hole, and other defects, respectively. When the bead height was 2 mm and the laser power was 5 kW, we observed uniform penetration along a groove, indicated as a dashed line. Also, only a few small solidification crack and lack of fusion occurrences were observed under those conditions. In contrast, many lack of fusion occurrences were observed when laser power is 5 kW and the bead height is either 4 mm or 6 mm. Also, many large

Table 2 Welding conditions.

Fig. 2 Schematic illustration of narrow gap hot-wire laser welding.

Fig. 4 Schematic illustrations of energy distribution and wire feeding position.

(a) Just focus condition (b) Defocus condition

Fig. 3 Images captured by the high-speed camera during welding.

108s TODO et al.: Hot-wire Laser Welding Process Using Laser Diode for Large-Diameter Pipe with Narrow

solidification cracks were observed when 6 kW laser power is applied.

Figure 6 (a) and (b) shows the effects of bead height and laser power on solidification crack and lack of fusion occurrences. Three cross sections were observed for each welding condition. Solidification cracks increased with increases in laser power and bead height (Fig. 6 (a)). Also, lack of fusion occurrences increased with increasing bead height and decreasing laser power (Fig. 6 (b)). Figure 7 shows the relationship between W and solidification crack length in each layer. W represents the

difference between maximum penetration width and minimum penetration width. Solidification cracks occurred when W was more than 0.5. In short, solidification cracks occurred when W

increased. Our results showed that a lack of fusion can be prevented by

increasing the laser power and decreasing the bead height, while solidification cracks can be prevented by creating uniform penetration along a groove. With this in mind, a target bead height of 2 mm (wire feeding speed of 3.1 m/min) was selected for welding the large-diameter pipe.

A large-diameter pipe was welded using the optimum conditions obtained from the welding trials with plate specimens.

Figure 8 shows the welding setup for large-diameter pipe welding. The large-diameter pipe was turned, and the laser head was fixed for each layer.

Figure 9 shows the cross section of the welded pipe specimen. Solidification cracks or lack of fusion were not observed on the cross section and it was also clear that a narrow gap, low dilution, narrow heat affected zone (HAZ), which were features of the proposed hot-wire laser welding process, were achieved on the actual large-diameter pipe. However, a slightly wider and asymmetrical bead width formed on upper layers on the cross section, indicated by the black arrow in Fig. 9. The laser beam width was not adapted to groove width on the upper layer. This was caused by unstable molten pool formation because of inadequate laser radiation on the weld bead.

For the pipe specimen welded using the optimum conditions, a side bending test revealed no solidification cracks but some small lack of fusion occurrences were observed on the upper layers (Fig. 10). Solidification cracks could be prevented with a small W on the pipe specimen (Fig. 9) and we also found that

more precise control of the laser irradiation to adapt to groove width variation prevented lack of fusions.

Fig. 5 Cross sections when bead height and laser power were varied.

Fig. 6 Effects of bead height and laser power on the occurrence of weld defects.

(a) Solidification crack occurrences. (b) Lack of fusion occurrences.

Fig. 7 Relationship between W and solidification crack length.

Fig. 8 Welding setup image for large-diameter pipe welding.

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4. Conclusions

We used a hot-wire laser welding process using a laser diode with a rectangular spot to weld a large-diameter pipe with a narrow gap. The conclusions are as follows:

(1) The defocus condition with a gradual energy distribution at the edge of the laser spot resulted in a high tolerance for filler wire feeding position, while the just focus condition with a steep energy distribution decreased tolerance.

(2) A decrease in bead height and an increase in laser power prevented a lack of fusion, and uniform penetration along a groove with lower laser power suppressed solidification cracks.

(3) The hot-wire laser welding process can be used on large-diameter pipe and results in a sound joint with low dilution and narrow HAZ.

References

1) Katsuyoshi Hori, Toshiaki Takuwa, Toshiharu Nagashima and Nobuo Nakazawa: Development of Oscillation TIG-Hot Wire Equipment for Narrow Gap Welding - Study oh Hot wire Welding Processes (Report 11) -, Japan Welding Society, Japan, vol. 57 (1995), 80-81.

2) Hiroshi Watanabe, Yasuhiro Butsusaki, and Toshiharu Nagashima: Study of High Speed Welding Technology for Ultra-Narrow Gap Welding - Development of Ultra-Narrow Gap Hot Wire TIG welding Process (Report 2) -, Japan Welding Society, Japan, vol. 69 (2001), 316-317.

3) Hiroshi Watanabe, Yasuhiro Butsusaki, and Toshiharu Nagashima: Ultra-Narrow Gap Hot Wire TIG Welding Process, Japan Welding Society, Japan, vol. 70 (2002), 54-59.

4) Yosuke Yamazaki, Yohei Abe, Yukio Hioki, Mitsuyoshi Nakatani, Akikazu Kitagawa and Kazuhiro Nakata: Fundamental Study of Narrow Gap Welding with Oscillation Laser Beam, Japan Welding Society, Japan, vol. 32 (2014), 114-121.

5) Terumasa Ohnishi, Yousuke Kawahito, Masami Mizutani and Seiji Katayama: High-Power and High-Brightness Laser Butt Welding with Using Hot Wire for Thick High-Strength Steel Plate, Japan Welding Society, Japan, vol. 29 (2011), 41-47.

6) Takeshi Tsukamoto, Hirotsugu Kawanaka and Yoshihisa Maeda: Laser Narrow Gap Welding of Thick Carbon Steels using High Brightness Laser with Beam Oscillation, Congress proceedings of ICALEO, (2011), 141-146.

7) Miikka Karhu and Veli Kujanpaa: Experimental Test Set-up for Studying Hot Cracking in Multi Pass Laser Hybrid Welding of Thick Section Austenitic Stainless Steel, Congress proceeding of ICALEO, (2008), 535-544.

8) Kenji Shinozaki, Motomichi Yamamoto, Kota Kadoi, Shoko Tsuchiya, Hiroshi Watanabe and Toshiharu Nagashima: Investigation of Narrow Gap Hot-wire Laser Welding Phenomena, Conference proceedings of JWS, 87 (2010), 364-365.

9) Masayuki Yamamoto, Phaoniam Rittichai, Kenji Shinozaki, Motomichi Yamamoto, Kota Kadoi and Toshinari Okagaito: Hot-wire Laser Welding Process using Laser Diode for Narrow Gap Joint, Conference proceedings of JWS, 90 (2013), 8-9.

Fig. 9 Cross section of pipe specimen.

Fig. 10 Results of the side bending test on the pipe specimen.

110s TODO et al.: Hot-wire Laser Welding Process Using Laser Diode for Large-Diameter Pipe with Narrow