8-1 Yoshima Kogyo Danchi Iwaki, Fukushima, JAPAN 1,5

Transcription

1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Development of Optical Device Housing Compacted Using SUS304L Granulated Powders Hironori Suzuki 1a, Toshihiro Hara 2b, Yukinobu Ogino 3c, Yasushi Sato 4d, Yo Tomota 5e Yoshima Kogyo Danchi Iwaki, Fukushima, JAPAN 1, Nakanarusawa, Hitach, Ibaraki, JAPAN a hnsuzuki@suruga-g.co.jp. b thara@suruga-g.co.jp c yogino@suruga-g.co.jp d yasushi.sato@suruga-g.co.jp. e tomota@mx.ibaraki.ac.jp. Keywords: Optical device housing, Laser welding, y, Granulated powder, Air-tightness, Welding sputters Abstract. In order to develop the SUS304L housing by powder metallurgy for an optical device useful for the FTTH communication system, the optimum compacting pressure and sintering temperature were investigated using granulated powder as the material to satisfy high air-tightness and high laser-weldability. Then the laser-welding test of specimen made under the optimum condition was carried out to observe welding sputters. Introduction Stainless steel SUS304L is applied for an optical device housing [1], which is a key component of FTTH (Fiber to the home) communication system, because of its high reliability. The housing is, for example, mm in outer size and has blind holes of 5 mm in diameter and 2mm in length on its upper and side faces. Devices are jointed on circumferences of the holes by a laser welding. Hence it is required to be high air-tightness of He-leakage less than 10-9 Pa m 3 /s and good laserweldability with no sputtering for the housing. From these demands, the housing was conventionally fabricated by complicated and costly machining for SUS304L wrought steel. It has been desired to fabricate the housing by powder metallurgy for a long time because of its easy netshaping. However, there were problems not to be capable of obtaining high air-tightness and to damage the optical device by welding sputters due to low density and rough surface in case of a product made by the conventional powder metallurgy [2]. This attributes to using coarse -100mesh powder (mean particle size 100μm) as the raw material. Therefore we have attempted to use granulated powder coalescing fine powder less than 10μm [3]. It is expected to make the sintered body high dense and its surface smooth. In this paper, the optimum compacting pressure and sintering temperature were investigated to develop the optical device housing with high density and high air-tightness using granulated powder. Then laser-welding test of specimen made under the optimum condition was carried out to confirm little welding sputters. Experimental procedures Preparing process of specimens. Preparing process of specimens is shown in Fig.1. Material powder is granulated powder prepared by coalescing SUS304L fine powder with less than 10μm in size. The apparent density of this granulated powder is 2.45 Mg/ m 3. And green density using this powder compacted at 500Mpa is 6.07 Mg/m 3. The fine powder is shown in Fig.2 (a) and the granulated powder in (b). The granulated powder is suitable for uniformly filling into a die because of its high fluidity. The powder was mixed with finely powdered 1 mass % wax and was compacted into a disk-like shape φ25 t3mm. The compacted specimens were de-waxed and then were sintered in hydrogen atmosphere using a pusher type Mo-heater furnace. Finally the specimens were ground to the thickness of 1mm with #400 wheel in the accuracy of ±0.01mm in order to remove the thickness difference among specimens. Specimens used for the laser-welding test were not ground to clarify the effect of the surface roughness on the welding sputters. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/04/16,00:15:17)

2 270 Progress in Powder Metallurgy Measurements. The sintered density was measured by the Archimedes method. He-leakage test was performed as follows; the disk-like specimen was put onto sucking port of the tester through rubber O-ring and was evacuated to a vacuum of 10-8 Pa. Atomic amount of He gas passed through the specimen was measured after spraying He gas onto it. The microstructure was examined using an optical microscope and a scanning electron microscope. The surface roughness was measured using surface texture measuring instrument (Tokyo Seimitsu Co.). Laser-welding test. Spot laser welding of specimen was carried out to observe sputters occurred during welding. The specimen was made under the optimum compacting pressure and sintering temperature applicable to practical housing. At the same time, specimen made under the same condition was spot welded using conventional 100mesh powder. Laser welding condition is shown in Table 1. Granulated powder Mixing Compacting De-waxing Sintering Machining Specimen φ25 t1 Lubricant(wax) 200~700MPa 1373K~1623K,3.6ks in H 2 t=1±0.01 Laser-test specimen φ25 t3 Results and Discussion Density. Fig.3 shows density of specimens compacted under the pressure from 200 to 700MPa and sintered at 1523K. The density increases rapidly up to 500MPa with elevating pressure and slowly in the range from 500 to 700MPa. In case of conventional 100mesh powder, its sintered density is 6.5 under condition of 500MPa and 1523K as will be mentioned later. Contrary to this, as shown Fig.3, in case of granulated powder its sintered density is 7.35 under the same condition. This suggests that the granulated powder has superior sinterability to the conventional powder. Fig.1 Preparing process of SUS304L specimen. (a) Powder before granulating (b) Powder after granulating Fig.2 SEM image of SUS304L powder. Type Power [W] Table1. Laser welding condition Frequency [Hz] Defocus [mm] Energy density [J/mm 2 ] CO Sintered density [Mg/m 3 ] Sintering temperature:1523k Compacting pressure [MPa] Fig.3 Relation between compacting pressure and sintered density of specimens. Sintered density [Mg/m 3 ] Compacting pressure:500mpa Sintering temperature [K] Fig.4 Relation between sintering temperature and sintered density of specimens.

3 Materials Science Forum Vols Fig.4 shows the density of specimens sintered in the range from 1373 to 1623K after compacting at 500MPa. The density increases rapidly with elevating temperature in the range from 1373 to 1473K and slowly in the range from 1473 to 1623K. It is considered that rapid increasing of density up to 1473K depends on re-arrangement and boundary diffusion among each fine powder particle composing granulated powder. And slow increasing of density in the range from 1473 to 1623K is considered to be related on solid diffusion where diffusion coefficient is low due to γ phase in the same temperature range [4,5]. Air-tightness. Fig.5 shows He-leakage of specimens sintered at 1523K after compacting in the range from 200 to 700MPa. As shown in the figure, the leakage decreases with increasing pressure and shows high air-tightness less than 10-9 Pa m 3 /s at the higher pressure than 400MPa. Fig.6 shows He-leakage of specimens sintered in the range from 1373 to 1623K after compacting at 500MPa. The leakage decreases rapidly in the range from 1373 to 1473K and approaches almost constant value Pa m 3 /s asymptotically in the range from 1523 to 1623K. It is clear that sintering temperature more than 1473K is appropriate for the optical device housing. From the above results, it is concluded that the optimum condition satisfying the air-tightness for the housing is compacting pressure more than 400MPa and sintering temperature more than 1473K. However, high compacting pressure and high sintering temperature are not preferable because of giving damage to a die and a furnace. Therefore, we have chosen the optimum condition to be compacting pressure of 500MPa and sintering temperature 1523K which are a little higher than their lower limits based on consideration of production allowance to some extent. Leakage [Pa m 3 /sec] Sintering temperature:1523k Compacting pressure MPa] Fig.5 Relation between compacting pressure and leakage of specimens. Leakage [Pa m 3 /sec] Compacting pressure:500mpa Sintering temperature [K] 10-9 Fig.6 Relation between sintering temperature and leakage of specimens. Laser-welding test. Fig.7 (a) and (b) show SEM micrographs of laser spot welded specimens sintered using granulated powder and similar specimen using conventional 100 mesh powder. As shown in the figures, sputters scattered on the circumferential area of spot welding are hardly observed in case of granulated powder, but considerably found in case of conventional powder. This attributes to the difference in surface roughness between the granulated specimen (Ra=0.5μm) and the conventional specimen (Ra=1.0μm). It is clear that the laser-welding of granulated specimen does not damage the device jointed on the housing. Microstructures. Fig.8 (a) shows the microstructure of granulated specimen sintered at 1523K after compacting at 500MPa. Fig.8 (b) shows the microstructure of conventional specimen made under the same condition. As seen, the pore size of granulated specimen is approximately less than 5μm and about 1/5 compared with that of conventional specimen. It was impossible to measure leakage of conventional specimen due to its low density 6.5. These results suggest that high airtightness of the granulated powder specimen depends on high density and small pores which are not inter-connected from internal area to surface.

4 272 Progress in Powder Metallurgy Application Fig.9 shows housing examples made under the condition using the granulated powder as material. The housings were compacted by CNC (Computer Numerical Controlled) press with a driving mechanism of a punch in perpendicular direction to the pressing. The housings have been applied for the optical device useful for FTTH communication system because of its high reliability and less expensive cost. Ra=0.5μm(not irradiated area) (a) Granulated powder Ra=1.0 μm(not irradiated area) (b) Conventional -100mesh Fig.7 SEM image of spot irradiated areas on specimens sintered using granulated powder and conventional 100mesh powder. 50μm 50μm Density: 7.35Mg/m 3 Leakage: Pa m 3 /s (a) Granulated powder Density: 6.50Mg/m 3 Leakage: Incapable measurement (b)conventional -100mesh powder Fig.9 Housing parts for FTTH device. Fig.8 Microstructure and leakage of specimens compacted at 500MPa and sintered at 1523K. Summary 1. It is found that the optimum compacting pressure is 500MPa and sintering temperature is 1523K to obtain the housing with high density and high air-tightness of He-leakage less than 10-9 Pa m 3 /s. 2 Laser-welding test revealed that there were no sputters in the granulated powder specimen made under the optimum condition and hence there is little risk of damaging the optical device. 3. The high air-tightness depends on high density and small pores which are not inter-connected from internal area to surface of sintered body. The good laser weldability depends on its smooth surface. 4. The housing has been successfully applied to the optical device useful for FTTH system. And its manufacturing cost is about 1/2 compared with the parts machined from wrought steel. References [1]Catalog for PON type optical device, edited by FUJITSU. [2]Y.Ishimaru: Foundation and application of powder metallurgy, Gijyutsu Syoin, (1993)236. [3] T.Kadomura, Ⅰ.Otsuka, Y. Maeta: Development of granulated powder using SUS316L fine powder, Abstract of autumn meeting of Japan Soc. of Powder and Powder metallurgy, (2005) 228. [4] K.Miura, H.Fukutomi, H.Onodera: Phase diagram, Ohm, (2003)157. [5] Jpn.Inst.Metals.:Metals data book,revised 3rd Ed.,(1993)

5 Progress in Powder Metallurgy / Development of Optical Device Housing Compacted Using SUS304L Granulated Powders /