ELSEVIER Journal of Materials Processing Technology 6~ (1997) 241 245

Journal of

' f~ e i recess ag Techuology

Commercial appficafions of superplasfic forming Kuniaki Osada *

3-13-5 Nih,nbashi, Chu-o-Ku, Tok.v~J, Japtm 103

Abstract

Japanese commercial superplastic forming of Ti, AI alloys and duplex stainless steels started in 1989 by a joint venture. This was several years behind Europe and USA. The joint venture, however, found some unique applications which could not be seen in other countries, owing greatly to information available nowadays and also to excellent superplastic alloys having been developed in Japan. From statistical data, emphasis is placed on features of Japanese new, practical superplastic materials. As unique applications, a series of cookware for magnetic induction cooking al:d a mold for pulp molding are introduced. A key technology that enables SPF with a high material yield, called modified driving-sheet SPF, is employed in many aspects of SPF operation. To assist this driving-sheet SPF, a new duplex stainless steel suitable tbr Ti alloys was developed, the steel has the nominal composition of Fe-23.5Cr-5.7Ni-l.4Mo and other materials. In this paper, unique applications and materials are focused on and discussed. ~ 1997 Published by Elsevier Science S.A.

Keywords: Superplastic forming: Aluminum alloy: Titanium: Stainless steel

1. Characteristics of Japanese market

The Japanese market tbr SPF products had not been widely open until the establishment of a company called NAS/Murdock in 1989. Before this time, how- ever, a few companies had engaged in commercial SPF business using Zn-AI and AI alloys, and three aircraft body manuthctures and their associated companies had conducted SPF on AI and Ti alloys for their own requirements for many years.

One of the excellent achievements done by Mitsubishi Heavy Industries (MHI) was the success of SPF in making T i -6AI -4V hemispherical shapes with uniform thickness distribution. To accomplish this work, MHI computed the thickness profile of the sheet beforehand so that the bulged hemisphere had an unitbrm thickness from edge-to-edge. Two hemispheres were integrated into a round fuel tank by electron beam welding [1].

According to a newspaper report published in 1993, a very thin metallic balloon with a flat section had been successfully blown by Kawasaki Heavy Industries

* Corresponding author. Nippon Yakin Kogyo Co., Ltd., Research and Development Division, Products and Development Department. 9F Sauti Bldg., 1-5-8 Kyobashu, Chu-o-ku, Tokyo, 104 Japan.

(KHI). The balloon was made of SP700 and reported to be used as a part for a nuclear fusion reactor. SP700 is the alloy developed by Nippon Kokan (NKK). and it features a low forming temperature as a Ti alloy and aging sensitivity. In the SPF operation of SP700, the temperature can be kept at 775C or thereabouts, which will reduce die cost and ease the operational working conditions [2-6].

Nippon Hikouki is another company active in SPF, having built their own SPF press. A7475, A5083 and SP700 have been tested and used practically to form parts for aircraft.

Some of the material manufacturers promote the marketing of superplastic alloys as well as NKK. Nip- pon Yakin Kogyo (NYK) and Sky Almninum are suppliers of NAS SD65(AISI 329 grade) and GX- 42(A5083). These two alloys have been developed and modified whilst reflecting the results obtained in practi- cal SPFs.

In this paper, the Japanese characteristic SPF market is categorized and summarized, together with some of the unique superplastic materials. Further, new indus- trial fields where SPF plays an important role are introduced.

0924-0136/97/$17.00 1997 Published by Elsevier Science S.A. All rights reserved. PII S0924-0136(96J00108-2

242 K. Osada / Journal of Materit,'/s Pn~cessing Technology 68 (1997~ 241-245

2. Market trend

2.1. Market for typical superplastic 77 alloys

In Fig. 1, the number of SPF components produced by NM from 1991 to 1993 are classified by industrial catego~. A large area is occupied by sports, this SPORTS field including mainly golf-dub-head shells made of SP700. Because of the small size of the parts, a multiple-forming technique, i.e. a technique forming a multiple number of parts during one forming cycle, has been used for their production. Typically, a golf club head consists of three or four shaped shells and other parts. This means that the number of parts increases three or four times faster than that of the other parts, which are formed by one-by-one operation.

The first SPF head example is believed to be the EOS Ti-22 No. 1 wood club by Yamaha, shown in Fig. 1. The club utilizes Ti alloy's features such as superplastic property, and high hardness and strength, so that a large volume of cavity is achieved. It is advertised that by using the club one can gain 20 to 30% longer distance, together with good control of direction.

EOS Ti-22 was the first commercial application of SP700 Ti alloy in large quantities, SP700 has the nomi- nal composition: T i -4.5AI-3V-2Fe-2Mo. The alloy has a lower temperature range compared to that of T i -6AI-4V and shows its maximum m-value at 775C. Moreover, because of the fl-rich alloy design, SP700 can be hardened by heat treatment.

The most popular superplastic Ti alloy throughout the world is Ti -6AI-4V as the material for aircrafts, submarines and so on. However, in Japan, since the size of aerospace and military business stays very small, Ti-6AI--4V can not be expected to be used in these businesses. Nevertheless, this alloy occupies about one third of aerospace application in the graph of Fig. 2 and also has a big potential for golf-club-head shells.

A new application of Ti alloy has appeared, being cookware such as a cooking skillet and a sauce pan. Cookware made of Ti alloy is sensitive to an electro-

OTHERS AEOSPACE 1 ~ 20%

ARCH'TECTURE 1~,~ t /~

coL.\.z , / 9% %""~'~- '~S PORTS

45%

Fig. 2. SPF components classified by induslrial category.

magnetic field and generates a sufficient amount of heat for cooking.

Amongst smooth-top cooking products, an induction unit creates a magnetic field by copper coils under a ceramic plate. For effective cooking, an appropriate cookware is necessary. As can be seen in every cata- logue provided with a cooking unit by appliance manu- factures, it is explained that the unit requires cookware that is ferro-magnetic. However, according to the basics of induction heating, materials which possesses high resistivity also can be used as cookware for induction heating, if their thickness is adjusted carefully to the physical conditions.

A particular type of Ti alloy shows very high resistiv- ity and generates heat effectively under a high-fre- quency magnetic induction field such as 22 kHz. Additionally, Ti alloys offer light weight, low heat capacity and high strength. These features render Ti alloy cookware attractive even though Ti alloy is not a good heat conductive metal. One of the new items of cookware is the skillet shown in Fig. 3. When forming Ti alloy cookware by SPF~ a modified driving sheet method has been applied, an example of the driving sheet being shown in Fig. 4, in this case NAS SD65 being used as the driving sheet.

Fig, i. EOS Ti-22 No. I golf-club head by Yamaha. Material: SP700. Fig. 3. A 28-cm cooking skillet made of Ti-6AI-4V.

K. Osa&t Journal ol Materials Prm'essi~lg Teclmoh~Kv 68 (1997) 241 2~5 243

Fig. 4. View of a driving sheet tiscd in the SPF of cookwares.

2.2. Market for superpktstic duplex stainless steel

The usage of superplastic material in the same period is arranged in Fig. 5. The amount of the usage of duplex stainless steel exceeds 30% in this graph, which is unusual in the manufacture of other SPF parts.

Sink decks installed in the lavatory of the Boeing 737 passenger aircraft are pressed by the SPF technique. In a passenger airplane, the lavatories are designed so that many functions work effectively in small spaces. Be- cause of this, the shapes of the sink decks are com- pressed of three-dimensional surfaces without welding seams. Sor,ae of the sink decks were formed by SPF using the daplex stainless steel, NAS SuperDux64 [7-- 9]. Nominally, the steel contains Fe-25Cr-6.5Ni-3.2 Mo and N with other specific elements. The MID-type sink deck which is polished and completely integrated with a dust box lid is shown in Fig. 6. The MID type sink deck meastlres 1100 mm width, 350 mm depth and 270 mm height [10].

A5083SPF 13% Duplex Sta=nless

/ Steel 3O%

SP700 Ti-6AI-4V 44% 13%

Fig. 5. Material usage in SPF operations.

Fig. 6. A sink deck for passenger aircraft. Material: NAS Super- Dux64.

The duplex stainless steel used in the sink deck, however, needs to be pressed at around 980C or higher to avoid precipitation of brittle sigma phase. Also, an ordinary heat-resistant alloy can not be used satisfac- tory as a tooling material under hydrostatic pressure at that temperature. A new duplex stainless steel, which is superplastic without any brittle phase at 900C, has been developed [11]. This new steel shows a tensile elongation of up to 2500'7,, and a low flc~ stress at 900C, its superpiastic properties bei~,~g presented in Table 1.

2.3. Market/or SUlWqdastic ,45083

Many trial SPFs have conducted by Sky Aluminum, a superplastic A5083 Al-alloy manufacturer. The trade name of the AI alloy is GX-42. Case covers for electric devices and ticket vending machines, window frames for trains, and gate panels, have been formed success- fully by SPF. The market size, hc, wever, remains small.

2.4. New aspect e.[stq~e~7~ktstic ,naterial

Superplastic metallic alloys are l-ecognized as being expensive. Specifically, the price of Ti alloys is 10--30

TaHet St~perplastic properties of the new duplex stainless steel

Temperature (CI m-Value Flow stress ,MPa)

Elongation C";,}

876 (i.69 28.0 > 2500 900 0.76 21.2 224(.I 95() 0.89 1(}.7 1985

244 K. Osada /Journal of Materials Process#tg Technology 68 (1997)241-245

Conventional Type JNPMold

Copper or Aluminum Alloy Casling ~l~fflre~

Fig. 7. Configuration of a conventional mold (left); and a new mold (right).

times that of typical stainless steels. Therefore, increas- ing the material yield is very important in practical SPF operations. The driving-sheet technique was introduced some years ago, and its modified formiag techniques have been proposed.

To save Ti alloy, Ti alloy sheets which do not meet their specification have been used. There was the possi- bility to use a duplex stainless steel sheet as a driving sheet. If a duplex stainless is superplastic at 900C and has an m-value greater than 0.5, the steel is suitable for driving Ti alloy sheets, NAS SD65 has been developed for driving-sheet forming and has been confirmed as a driving sheet material. The sheet is discarded as a sacrifice material and recycled.

3. New application concept

As one of the new applications of Ti alloy, cookware was introduced in the previous section. Another inter- esting application of SPF is as a mold for pulp-molding technology, Pulp molding is a means of forming three- dimensional objects from fibers by a paper-making process. In this method, fibers are deposited onto a shaped mesh wire similar in size to the desired object. The mesh wire is usually combined with a support made of heavy cast bronze. Disadvantageous points of classical molds are their heavy weight, daily mainte- nance work of failed mesh wire, long lead time, and high cost.

The combination of SPF and electron-beam perfora- tion has changed the old configuration to a newer design (Fig. 7, right). The new shell-type mold, called the JNP mold, has resolved the most of the disadvanta- geous points seen in the old mold and added merits. The JNP mold can provide smooth surface on shaped fibers, lighter weight, short lead time and no clogging- up of fibers in the holes.

The JNP mold has realized by forming superplastic AI, Ti alloy or duplex stainless sheet with a huge

number of tiny holes of 0.3 mm diameter and 0.6 mm pitch. These holes are perforated by an electron beam. The forming operation is assisted by a driving sheet. One of the JNP mold examples is shown in Fig. 8, Although it is impossible to recognize tiny holes on the surface in the photograph, this is for a recycled paper contained for a set of 12 apples.

4. Conclusions

Japanese commercial SPF activity has been outlined. The market scale for SPF parts has been estimated to be small. The size of the market, however, is increasing steadily. Moreover, unique applications such as the Ti cooking skillet and the JNP mold for pulp molding, have been developed, these applications supporting the advanced industries.

In developing superplastic material or in making existing material superplastic, information feed-back from the forming side to the material R&D section is very important, development of the driving sheet made

Fig. 8. A JNP mold for 12-apples containers.

K. Osada ,Journal qf Malerials Pro'e.vs#tg Technoh~gy 68 (1997) 241 245 245

of duplex stainless steel being an example. Fortunately, for the alloys described in this paper, there are some practical superplastic materials linking closely to SPF operation, and these materials are supported by basic research in Japan.

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