A. Nishimura

K. Shinozaki

N. Takeshita

N.J. Heo

T. Muroga

T. Nagasaka

Heo,  N.J.

Nagasaka,  T.

Muroga,  T.

Nishimura,  A.

Shinozaki,  K.

Takeshita,  N.

NIFS-Repository (National Institute for Fusion Science)

§4. Development of Laser Welding Technology for Vanadium and Its Alloys Heo, N.J. (Dept. Fusion Sci., School of Math. and Physical Sci., Grad. Univ. Advanced Studies), Nagasaka, T., Muroga, T., Nishimura, A., Shinozaki, K., Takeshita, N. (Dept. Mechanical System Engineering, Hiroshima University) Two major remaining issues for applying the vanadium alloys for fusion reactors are the technology for production of large heat vanadium alloys and the development of welding method, which is the key requisite for fabricating structures. Some large heats were fabricated by US-DOE, and recently, high purity V -4Cr-4 Ti alloys (NIFS- HEAT-1 and 2) were fabricated by National Institute for Fusion Science (NIFS) in Japan. To establish the welding technology for obtaining the sound weldments, it is necessary to control the environment during the welding for preventing the contamination with the impurities. The purpose of this study is to develop the laser welding technology for vanadium and its alloys reducing the contaminations to the acceptable level. To prevent the contamination with interstitial impurities, such as hydrogen, oxygen, nitrogen, carbon, etc. an environmental control box capable of supplying high-purity argon gas (99.9999%), and ventilating the fume was designed and fabricated. The environmental control box has two nozzles for cleaning the specimen and two gas suppliers for substitution of the atmosphere. The laser beam came in the box thro- ugh the quartz glass window with the thickness of 2.3 mm. Bead-on-plate welds were produced on the 4 mm-thick plate, using 2.0 kW YAG laser in the welding condition of 1.8kW continuous wave and traveling speed of 0.5 mlmin. Focal point was the surface of the specimen, in which spot size of the laser light was 0.5 mm in diameter. To estimate the pickup of interstitial elements in the weldments, chemical analysis and hardness measurement were carried out. Table 1 indicates the result of chemical analysis before and after laser welding. The contamination of impurities, such as oxygen or nitrogen, was very small with the flow rate of 100 L/min and above. Fig. 1 shows the outward appearance of the beads for three welding environments. When welded in the air and with supplying the argon gas of 50 L/min each with two nozzles, the bead was colored due to contamination with impurities. However, use of the additional supply of the substitution gas of 300 L/min color due to the contamination was not detected. Fig. 2 shows the hardness of the specimen as-weld and annealed at 673 K and 1273 K for an hour in a vacuum «10-6 Torr) in the cases with and without the substitution 390 gas supply. In the both cases with and without the supply of substitution gas, annealing at 673 K did not change hardness, which implies that contamination with hydrogen was negligibly small in the present study. The decrease of hardness in the base metal of both specimens by annealing at 1273 K is the result of the recovery of the dislocations induced by the cold rolling. The hardness at and near the center of the weldment relative to that of the base metal after annealing at 1273 K is small and large in the cases with and without the substitution gas supply, respectively. The significant increase in hardness at and near the center of the weldment in the case without the substitution gas supply seems to be due to contamination. From this study, simple environmental control box with the nozzles for blowing out the fume and the supply of substitution gas made it possible to obtain the weld zone reducing the contamination of the impurities. Table 1 Oxygen and nitrogen levels before and after the laser weldin. (mass m) Flow rate (L/min) o N 200 180 = -. .., .., ~ 1:1 'E 120 «I ..c ~ 100 ~ ~ ~ 80 60 40 -6 Before 69 85 94 170 Substitution:500 LJmin Noules:l00 Umin 500 82 115 Fig. 1 The outward appearance of the weld beads for three cases of the welding environment. a) in the air b) nozzles: substitution:O Llmin c) nozzles: substitution:300 Llmin Submtution:O Umin NozzIes:l00 Umin . UnaU~ed vanadiuml . " -4 -2 Distance from beam center I mm Fig.2 Effect of tbe post-welding heat treatment on hardness. 

Development of Laser Welding Technology for Vanadium and Its Alloys

Development of Laser Welding Technology for Vanadium and Its Alloys

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