サンリック［最新技術・企業情報］: （1４）高融点金属の溶融溶接について

2009年11月 6日

（1４）高融点金属の溶融溶接について

タングステン、モリブデン、タンタル及びこれらの合金など高融金属耐熱金属を扱っている当社にはよく溶接に関する質問・相談を受ける。中でも固相接合のほかＴＩＧ溶接、電子ビーム溶接、アーク溶接、レーザー溶接など溶融溶接に関する質問が多い。高融点純金属の溶融溶接に関しては古くから研究がなされており、一つには材料面から溶接性の改善、二つ目には技術面からの溶接性の改善がなされている。この結果、材料面からは材料自身に含有する不純物（特にガス成分）により溶接時に気孔を形成する、このため継ぎ手の性能を劣化させる。気孔の形成原因は溶接時の雰囲気及び材料自身の純度であるとされている。特に材料の純度の点では酸素量との関係に依存し、モリブデンの場合はＭｏＯ３などが気孔形成に関与していることが知られている。一方、技術面からは材料の脆化を防ぐために、ビーム振動を用いるなど溶融溶接後の凝固組織の制御を工夫することにより溶接性の改善の取り組が行なわれている。これらの研究結果から、高融点純金属の溶融溶接に関しては素材自身の純度（特にガス不純物量の低減）を高めることにより溶接性の改善はなされるが、タングステン、モリブデンなど溶接後の凝固粒が粗大化するために、結晶粒界が脆弱になりやすいばかりでなく、急冷凝固による熱応力の結果、時として粒界亀裂が生じる。したがって、タングステン・モリブデンなど高融点純金属の溶融溶接の根本的な溶接改善はなされていないのが現状である。一方、ＴＺＭ合金及び酸化物分散強化型モリブデン合金は溶融溶接中にモリブデンの添加元素が酸素と結合して粒界及び粒内に析出することにより気孔を形成し難いため溶接が可能であるとされている。
上記述べたように、高融点耐熱金属の溶融溶接にとって溶接性の改善には素材の純度、とりわけ、酸素量の低減化が必要であるとされている。このため、当社では酸素量を制御したモリブデン素材を用いてＴＩＧ溶接を行い、溶接性の挙動を調べたので以下にその一例を報告する。

EFFECTS OF AMOUNTS OF OXYGEN ON TIG-WELDABILITY OF MOLYBDENUM

Tadayuki FUJII 1) and Gary A.ROZAK 2)

1) Sunric Co, Ltd., 2-8-0 Keihinjima, Ota-ku ,Tokyo 143-0003 Japan
2) H.C. Starck Inc., 21801 Tungsten Road, Cleveland, OH 44117 USA

The purpose of this study is to investigate the effects of amounts of oxygen on TIG-weldabilitiy of the rolled molybdenum sheets. In this experiment, the molybdenum sheets having the amounts of oxygen from16 mass ppm to 78 mass ppm were used. The TIG-welding were carried out by melt-run technique.
Many large weld pores were formed mostly within weld bead and also bead cracking along weld bead was recognized along weld bead. However, few weld pores and no bead cracks for the molybdenum sheets having the amounts of oxygen less than 16 mass ppm were observed. It is confirmed that TIG-weldability is strongly affected by very small amounts of oxygen contained in molybdenum.

1.Introduction

Refractory metals such as molybdenum and tungsten are widely utilized in the fields of electric and electrical industries, high-temperature vacuum industry and chemical industry. For such a wider utilization of refractory metals, welding and joining is one of key technologies. It is well known that the weldability of metallic materials is generally affected by a small amount of gaseous and metallic impurities in original materials. Especially, it is considered that the formation of voids, pores and bead crack after welding is mainly due to the amounts of oxygen and/or carbon contained in molybdenum, tungsten and their alloys. In this paper we report a brief works on the effects of amounts of oxygen on TIG-weldabiltiy of the rolled molybdenum.

2. Experimental procedures

The materials used in this work were the rolled molybdenum sheets of about 1mm thickness made by H.C. Starck Inc. for welding. The oxygen concentration in molybdenum sheets was analyzed by the infrared absorption spectrometric method after melting in the current of inert gas ( based on Japan Industrial Standard for chemical analysis method in non- ferrous metals). The analytical level of about 1 ppm was obtained. Tungsten-inert-gas (TIG) welding was conducted by a melt-run technique. The TIG-welding was performed as shown in Photo.1. Welding conditions were as follows. First chamber was evacuated down to 2 x 10-3 Pa. Then valve was closed and the chamber was back-filled with argon to a static pressure of 1.01 x 105 Pa. Welding voltage was 10 V, current was about 90 A and welding speed was 1.6 mm/s. Top and Bottom-surfaces and cross-section of welded molybdenum sheet were examined by using an optical microscope.

Photo.1 Typical macrostructures after TIG-Weld Mo sheets made by H.C.Starck Inc., and Allied materials Co., Ltd.

3. Results and Discussion

Table 1 shows the amounts of oxygen and hardness of molybdenum sheets used in TIG-Welding. The amounts of oxygen contained in six samples of molybdenum sheets are detected by LECO Technique by H.C Starck Inc and determined to be from 16 mass ppm to 78 mass ppm, while by Sunric Co., Ltd those are determined to be from 26 mass ppm to 31 mass ppm. They show a large difference in oxygen concentration between analyzers because there exist many technical difficulties on the analysis of oxygen.

Table 1

Macrostructures of the Top-and bottom-surface and cross-section of TIG-welded molybdenum sheets are shown in Photo. 2 (a)-(d). Morphologies of no bead cracking and bead cracking are shown in Photo.3 and Photo.4, respectively. A number of pores within weld bead and bead cracking were observed along the weld bead. It was suggested that these pores are due to oxides such as MoOx. However, few pores were observed in the molybdenum sheets having the amounts of oxygen less than 16 mass ppm, such as those of ID number of F4104B or 100389A1( 26 mass ppm detected by Sunric Co., Ltd) and of Allied materials-made. Table 2 gives the relationship between amounts of oxygen and bead cracking. No bead racks were recognized in the sheets containing oxygen of 16 ( ID number of F4104B)and 48 mass ppm ( ID number of 100389A1, 26 mass ppm detected by Sunric Co., Ltd).

Photo．2(a)　Optical　microphotographs　of　welded　bead．A：Top　surface　　　　B：Cross　section　　C：Bottom　surface (x50)

Photo．2(b)　Optical　microphotographs　of　welded　bead．A：Top　surface　　　　B：Cross　section　　C：Bottom　surface (x50)

Photo．2(c)　Optical　microphotographs　of　welded　bead．A：Top　surface　　　　B：Cross　section　　C：Bottom　surface (x50)

Photo．2(d)　Optical　microphotographs　of　welded　bead．A：Top　surface　　　　B：Cross　section　　C：Bottom　surface (x100)

Photo.3 Typical microstructures showing bead-view after TIG-Weld Mo sheet made by H.C. Starck Inc. (a) and Allied materials Co., Ltd.(b).

Photo.4 Typical microstructures showing bead cracking-view after TIG-Weld Mo sheet made by H.C.Starck Inc.

Judging from the results of Photo.2 and Table 2, it is suggested that very small amounts of oxygen contained in molybdenum is closely related not only to the formation of pores but also to the generation of bead cracking in the TIG-welding.

4. Conclusions

Effects of amounts of oxygen on TIG-weldability of the rolled molybdenum sheets were investigated using optical microscopy. It is concluded that the amounts of oxygen has a major affect on the generation of pore and of bead cracking of molybdenum in TIG- welding as follows.
(1) In the case where the amounts of oxygen was less than 16 mass ppm, after molybdenum sheets have been welded, few weld pores within weld bead and no bead cracking along weld bead were observed.
(2) In the case where the amounts of oxygen was more than 21 mass ppm, after molybdenum sheets have been welded, many large weld pores within weld bead and bead cracking along weld bead were recognized,,except for sample of 100389A1

・Acknowledgement

The authors wish to thank the staff of H.C.Starck Inc. for supplying the rolled molybdenum sheets used in this work and also to thank the staff of Sunric Co. Ltd for helping TIG-welding and chemical analysis. We are also greatly indebted to Ph.D Olaf Schmidt-Park, President and CEO of H.C.Starck Inc and Mr. Takashi Yoshioka, President and CEO of Sunric Co., Ltd. for their accepting this work.

・References

[1]Y. Hiraoka and T.Fujii, Proc. of 12th International. Plansee Seminar'89,
(Austria,1989), p.265-279. "Welding and joining of single crystals of BCC refractory
metals"
[2] Y.Hiraoka and T.Fujii, J.Japan Inst. Metals, Vol.51.No.5 (1987), pp.407-412.
[3] For example, Weldanability of molybdenum, Reports of National Research Institute
for Metals in1977.

投稿者 sunric : 2009年11月 6日 11:17