Abstract
This paper is to brie¯y outline the recent research and development activities in the ®eld of superplasticity and SPF/DB applications among academic and industrial community in Taiwan. Academic research activities can be roughly divided into material development and SPF/DB experiments. The material development emphasizes the development of ®ne-grained materials, resulting in high-temperature superplastic 7075/7475 Al-Zn-Mg alloys and low-temperature superplastic 2090/8090 Al-Li alloys, as well as several high-strain-rate superplastic aluminum matrix composites. The thermomechanical treatments involved include (1) conventional rolling and reheat method, (2) equal channel angular extrusion, (3) reciprocal extrusion, (4) asymmetrical rolling, (5) wedge-shape forging, and (6) directional solidi®cation plus extrusion. The SPF/DB places more weight on forming practices and applications. Titanium base alloys and stainless steel are of more interest. Major breakthrough includes the successful fabrication of high-pressure vessels, spherical coolant containers for infrared detectors, ¯oating balls for level control in chemical industry, internal gas blow forming using inorganic powders, small-scaled SPF/DB straight- or arc-rib reinforced hollow structures, SPF/DB golf heads, and certain special assemblies for electronic products. Limited applications on production lines are undergoing, e.g., the front-head covers for missiles by Military Institute and turbine engine blades by Shan-Tung Aerospace Company. Efforts in applying to titanium golf heads have once been popular, but become less intense due to the recent drop of sale price. The application in electronic industry seems to have promising potential. Finally, the production of extra ®ne-grained aluminum alloys has attracted the attention of local aluminum company recently. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Superplasticity; Superplastic forming; Diffusion bonding
1. Historical background
This paper is to brie¯y outline the recent research and development (R and D) activities in the ®eld of super-plasticity (SP) and superplastic forming and diffusion bond-ing (SPF/DB) applications among academic and industrial community in Taiwan, R.O.C.
The R and D work in this ®eld was initiated in Aerospace Industry Development Center in 1987, intended to fabricate several aluminum aircraft parts for the modi®ed version of F16 ®ghter, Jing-Gwo generation I. Intensive efforts were made in all aspects in order to reduce weight of the ®ghter. An international collaboration R and D program established with a German company was launched; one item was aimed on the SPF techniques using the 8090 Al-Li thin sheets. The ®rst SPF hot press machine introduced to Taiwan, made by Murduck company of USA, was installed soon after. It was a
scale machine that can produce sizable aircraft parts. Unfor-tunately, mass applications of SPF products did not continue due to the termination of fabrication plan for Jing-Gwo ®ghters generation II in 1994.
Since 1987, major superplasticity activities were centered on aircraft or military applications. In addition to the SPF hot press in Aerospace Industry Development Center, there are two others in commercial scales, both also made by Mur-duck company; one on Materials Development Center of Chung-Shan Institute of Military Science and Technology for making mainly titanium missile-related parts and the other in National Central University for general R and D purpose. Self-designed forming machines in smaller scales and self-written computer programs for blow-gas control are widely available in several universities, such as National Taiwan University, National Sun Yat-Sen University, and Tatung Institute of Technology.
Wide spread of R and D activities should be indebted to several stimulating seminar courses; half of them were
*Corresponding author.
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These include the academic workshops in 1992 and 1994, and industry-oriented promotional workshops in 1992 (by Metal Industries Development Center) and 1994 (by National Taiwan University).
2. Academic activities
Academic research activities can be roughly divided into material development, SPF/DB experiments, and superplas-ticity mechanisms. The group centered in National Sun-Yat University (Huang, Hwang, Kao, and Shih) [1±25] empha-sizes the former. The research group centered in National Taiwan University (Chuang, Koo, Wang, and Yang) [26±51] places more weight on SPF/DB and applications. Other academic studies include simulation work in National Cen-tral University (Lee) [31,52±54], superplasticity of Al-Li and Zn-Al alloys in Nation Tsing Hua University (Chang and Yeh) [55±58], superplasticity of Al/Al3Ni in National
Cheng Kung University (Chen and Lui) [59,60], and super-plasticity of Fe-Al intermetallics (Chu) in National Ocean University [61].
2.1. Materials development
The group centered in National Sun-Yat University (Huang, Hwang, Kao, and Shih) emphasizes the develop-ment of high-temperature superplastic 7075/7475 Al-Zn-Mg alloys [1,9] and low-temperature superplastic 2090/ 8090 Al-Li alloys [3,4,7,8,10] via patented thermomecha-nical treatments, in collaboration with China Steel Corp., Aerospace Industry Development Center, Metal Industries Development Center, and National Science Council of Taiwan, ROC.
The joint cooperation between National Sun Yat-Sen University led by Huang and China Steel and Aluminum Corp. in 1991 has resulted in the capability in producing superior superplastic 7075 Al sheets (1500% without back pressure at 5008C and 2 10ÿ4sÿ1, as shown in Fig. 1)
starting from the DC ingot product of China Aluminum Company in Kaoksiung [9]. The materials have an average grain size of 5 mm and were equivalent in comparison with the commercial products fabricated by Kobe Steel or Super-form company. For the 8090 Al-Li alloys, high-temperature superplasticity (HTSP, 600% without back pressure at 500± 5408C and 2 10ÿ4sÿ1) or low-temperature
superplasti-city (LTSP, 710% without back pressure at 3508C and 8 10ÿ4sÿ1, as shown in Fig. 2) has been developed in
1993 [3,7], starting from the Alcan 8090 non-superplastic thick plates to 50 mm. The (sub)grain dimension before SP loading was around 0.7 0.5 0.2 mm, as shown in Fig. 3. Transformation from LTSP to HTSP can be easily done [8]. Meanwhile, the LTSP materials have superior properties over the HTSP counterparts in four aspects [7], (1) a higher room temperature strength (510 MPa vs. 450 MPa) before SP loading, (2) a much higher post-SP strength (500 MPa vs. 380 MPa) meaning little degradation after loading, (3) a considerably narrower surface solute depletion layer (7 mm vs. 100 mm), and (4) a much ®ner grain size after 500% SP straining (3.7 mm vs. 22 mm). The improved post-SP properties would be of great values in commercial applications.
Recent involvements in Huang's laboratory were the exploration in high strain rate superplasticity (HRSP) in aluminum matrix composites, dispersion strengthened
alu-Fig. 1. Examples of the Al-Zn-Mg 7075 Al tensile specimens before and after superplastic loading at 450±5008C and 2 10ÿ4sÿ1, with a
maximum elongation of 1500%.
Fig. 2. Examples of the Al-Li-Cu-Mg 8090 Al tensile specimens before and after superplastic loading at 3508C and 8 10ÿ4sÿ1, with a
minum alloys, or pure Al systems. Examples include the 6061 alloys reinforced by different sizes of SiC particles from 3 mm down to 30 nm (HRSP of 300% at 5808C and 5 10ÿ1sÿ1), pure Al (HRSP of 250% at 600±6408C and
1 100sÿ1), and 5083 alloy (LTSP of 400% at 2508C and
2 10ÿ3sÿ1). The ®ne-grained superplastic materials also
tend to possess good combinations of high strength and ductility at room temperature. For example, 510 MPa and 9% for the LTSP 8090 alloy, 570 MPa and 3% for the HRSP 2024/15%SiC(p)composite, 460 MPa and 4% for the HRSP
6061/15%SiC(p) composite, and 520 MPa and 5% for the
LTSP 5083 alloy. Finally, a collaboration research with Bampton originally at Rockwell was initiated in 1994, studying the superplasticity of super 2 Ti3Al base alloy
(Ti-25atAl-10Nb-3V-1Mo) [12,23]. A high elongation of 1500% was achieved from the 2 mm sheets when loaded at 9608C and 2 10ÿ4sÿ1, as presented in Fig. 4. Recent
transmission electron microscopy (TEM) results suggested that the phases present during superplastic loading the ordered 2, B2 and O phases, instead of the originally expected ordered 2 and disordered .
Professor Chang's group in National Tsing Hua Univer-sity also studied the superplasticity of self cast binary or quaternary Al-Li-Cu-Mg alloys [55]; an elongation of 629% was reported at 5008C and 1.5 10ÿ3sÿ1 without adding
back pressure. Yeh and his student in National Tsing Hua University processed the Zn-22Al-0.5Cu alloys [57] using
his patented reciprocal extrusion method [58], producing ®ne-grain structures with an average grain size of 0.4 mm after 10-times extrusion passes. The resulting Zn-Al alloys exhibited 2000% superplastic elongation at 2658C and 7 10ÿ2sÿ1, as presented in Fig. 5, a typical high-strain
rate superplastic behavior in extra ®ne-grained materials. In Tatung Institute of Technology, Yang and his students studied the role of each step during thermomechanical treatments for the Zn-22Al and Al-40 Zn alloys [45]. A
high-superplastic elongation around 1000% was obtained for these alloys at 250±2708C and 10ÿ3sÿ1.
Kao in National Sun Yat-Sen University examined the mechanically alloyed (MA) Al/Al3Ti system [18,22], made
from elemental pure Al and Ti powders with compositions of Al-4Ti, Al-8Ti, together with extra®ne oxides/carbides
Fig. 3. TEM micrograph showing the (sub)grain structure of the LTSP 8090 Al sheet before SP loading, seen from the rolling plane. The (sub)grain shape was not completely spherical with a dimension of 0.7 0.5 0.2 mm. After SP straining to 50%, the grains gradually become equiaxial.
Fig. 4. Examples of the super 2 Ti3Al-Nb-Mo-V tensile specimens
before and after superplastic loading at 900±10008C and 2 10ÿ4sÿ1,
with a maximum elongation of 1500%.
Fig. 5. Examples of the Zn-22Al-0.5Cu tensile specimens before and after superplastic loading at 2568C and 7 10ÿ2sÿ1, with a maximum
originated from the process control agent of CH3(CH2)16COOH. The resulting oxide/carbide
disper-soids, Al3Ti particle size and matrix grain size were
30 nm, 1 mm, respectively (Fig. 6). The MA alloys were further hot extruded and then superplastically loaded. The optimum superplastic elongation of 200% was obtained from the Al-Ti at 6208C and 50 sÿ1, as presented in
Fig. 7. The group of Chen and Lui in National Cheng Kung University has started to explore the superplastic behavior of the Al/Al3Ni system [59,60]. The Al/Al3Ni eutectic alloys
were prepared by directional solidi®cation, followed by hot extrusion to result in ®ne-grained substructures with strong [1 1 1] ®ber texture. The highest superplastic elongation obtained so far at 5008C and 1 10ÿ2sÿ1 was 300%, as
shown in Fig. 8. Such an extended deformation at relatively lower temperature (5008C compared with the much higher temperature of 600±6408C for similar pure Al systems reinforced by Al3Ti [22] or AlN [62]) was believed to
originated from the `strain softening resistance' instead
of strain rate sensitivity (0.25±0.3) [60]. Continuous inves-tigations are now undergoing, and higher superplastic elon-gations and higher strain rate sensitivity values could be expected when loaded at higher temperatures and strain rates.
In development of superplastic intermetallic compounds, the achievement made by the Koo's group in National Taiwan University [2,51] in producing superplastic Ti3
Al-Nb and TiAl-Al-Nb alloys should be addressed. The materials were prepared by noncomsumable electrode vacuum arc remelting furnace, followed by two-stage hot rolling. A maximum superplastic elongation of 1100% for the Ti3
Al-10Nb regular 2 alloy loaded at 9758C and 2 10ÿ4sÿ1
was reported (Fig. 9). Texture in¯uence on the superplastic behavior was observed in their study and the underlying reasons were discussed [42]. Another trial was the team led by Chu in National Ocean University [61]. The coarse-grained (700±800 mm) Fe-27atAl was found to exhibit superplasticity at temperature above 7008C, with optimum elongation up to 300% and strain rate sensitivity of 0.25± 0.3. After superplastic straining, the grain structure became ®ner to 100±200 mm through continuous grains boundary migration [61].
Fig. 6. TEM micrograph showing the grain structure of the MA Al-Al3Ti
alloy before SP loading. The equiaxial grain size was around 0.5 mm. (courtesy of Prof. Kao in Sun Yat-Sen University)
Fig. 7. Examples of the MA Al-Al3Ti tensile specimens before and after superplastic loading at 6208C and 5 10ÿ1to 5 101sÿ1, with a maximum
elongation 200. (courtesy of Prof. Kao of Sun Yat-Sen University).
Fig. 8. Examples of the Al-Al3Ni tensile specimens before and after
superplastic loading at 5008C and 1 10ÿ2sÿ1, with a maximum
Currently, a research group of seven professors is formed, lasting for three years from 1997 to 2000, headed by Huang and sponsored by National Science Council of Taiwan. Major effects are aimed on the development and character-ization of ®ne-grained materials, such as aluminum com-posites exhibiting HTSP and HRSP and aluminum alloys possessing LTSP and HRSP. The thermomechanical treat-ments include (1) conventional rolling and reheat method, (2) equal channel angular extrusion, (3) reciprocal extru-sion, (4) asymmetrical rolling, (5) wedge-shape forging, and (6) directional solidi®cation plus extrusion. The materials examined so far comprise 6061/SiC(p), Al/Ai3Ni, pure Al,
Al-Si, 5083 and 6061 alloys. 2.2. SPF/DB practices
The research group centered in National Taiwan Uni-versity (Chuang, Cheng, Koo, Wang, and Yang) places more weight on SPF/DB and applications. Titanium base alloy (Ti-6Al-4V, Ti3Al-Nb) and stainless steels are of more
interest, mostly in collaboration with Materials Develop-ment Center of Chung-Shan Institute of Military Science and Technology and National Science Council. Major break-through includes the successful fabrication of high pressure vessel, spherical coolant containers for infrared detectors, ¯oating balls for ¯uid level control in chemical industry, internal-gas blow forming using inorganic pow-ders, small-scaled SPF/DB straight- or arc-rib reinforced hollow structures, SPF/DB golf heads, and a special design for electronic products [46].
A 5-year research project on SPF/DB was conducted from 1991 to 1996, led by Professor Chuang and sponsored by National Science Council of ROC. Subprogram I con-ducted by Chuang [26±29,32±37,40,41,43,46,48] investi-gated the general aspects of SPF/DB. Numerous
demo-workpieces were made. For example, the Ti-6Al-4V high pressure vessel (1 mm thick) shown in Fig. 10 was made by SPF, compared with the conventional 6061 Al counterpart (5 mm thick) fabricated by extensive machining plus braz-ing. This vessel was designed to be ®lled with coolant for soldier-used infrared detectors. The vessels were ®rst super-plastically blow-formed followed by laser welding. With the compatible strength, the new products were much lighter than the previous 6061 ones, which was considered to be important in terms of solider carriage. Another example was
Fig. 9. Examples of the regular 2 Ti3Al-Nb tensile specimens before and after superplastic loading at 10008C and 2 10ÿ4sÿ1with a maximum elongation
of 1081% (courtesy of Prof. Koo of Taiwan University).
Fig. 10. Ti-6Al-4V high pressure vessel made by SPF/laser-welding, used to fill coolant for soldier-carrying infrared detectors. The laser weld line is indicated by arrows.
the Ti-6Al-4V spherical coolant containers for infrared detectors equipped in middle-range missiles (Fig. 11), ori-ginally made by SUS 410 stainless steel via machining and fusion welding. The SPF/DB coolant containers can sustain a high pressure of 41 MPa (6000 psi). Furthermore, SPF/DB Ti-6Al-4V ¯oating balls for ¯uid level control in chemical industry were produced [27], as demonstrated in Fig. 12. The ¯oating balls have the same function as the plastic balls used in ¯ush toilet, but can suffer higher corrosive ¯uid.
A new SPF forming mean via the decomposition reaction of CaCO3C powders was proposed [32,37,48] by Chuang's
laboratory. The forming gas was not provided through inputting outside Ar gas, instead was given by the internal CO2gas inside the formed piece resulted from the
decom-position of CaCO3powders into CaO and CO2at 9278C for
Ti-6Al-4V and 9858C for Superdux 6 stainless steel sheets. Meanwhile, it was found that the (MgCO3)4Mg(OH)2xH2O
powders can be decomposed into CO2and H2O, and could
be used as gas source for the 8090 and 7475 Al alloys at
5008C. A forming gas pressure to 1.4 MPa (200 psi) or above can be achieved at the corresponding temperatures, as depicted in Fig. 13. Another effort was the low-pressure diffusion with the insertion of a superplastic interlayer between two non-superplastic Ti or stainless sheets [33,34]. For example, the general DB pressure of 7 MPa needed for bonding two non-superplastic Ti-6Al-4V sheets can be reduced down to 2 MPa with a superplastic Ti-6Al-4V thin interlayer in-between.
Subprogram II directed by Cheng [30,31,38,50] fabri-cated the 3-layer perpendicular-rid (Fig. 14) or slanting-rid (Fig. 15) strengthened titanium structures, using the self-made forming setup and a commercial program ABAQUS/ STANDARD. In order to ensure the ef®ciency of the SPF/ DB workpiece ®nite element analyses were performed in terms of product design, SPF/DB process design, and SPF pressure and thickness simulation. The program dealt with the determination of the eigenfrequency and eigenmodes, local bucking of the blade face, local stress concentration, and impact response. Joint efforts with a local golf club factory in fabrication and computer simulation of Ti golf heads have been and are still being made using the patented know-how [44].
Subprogram III headed by Wang [47] examined the effects of temporary hydrogen charging on the superplastic behavior of Ti base alloys and shape memory Cu-Zn-Al-Zr alloys. It was found that under an optimum combination of hydrogenation temperature and hydrogen concentration SPF of Ti-6Al-4V alloys proceeded at much higher rates. Subprogram IV directed by Koo [42,51] developed the Ti3Al-Nb superplastic sheets with 8±12atNb, as introduced
Fig. 11. Ti-6Al-4V spherical coolant containers for infrared detectors made by SPF/DB, used in middle-range missiles.
Fig. 12. Ti-6Al-4V floating balls made by SPF/DB used for fluid level control in chemical industry.
Fig. 13. The internal gas pressure (a) from the decomposition reaction of CaCO3C powders, and (b) resulted from the decomposition reaction of
(MgCO3)4Mg(OH)2H2O powders as a function of temperature: (± ± ±)
above. Subprogram V directed by Yang [49] emphasized the solid state and transient liquid phase diffusion bonding of various aluminum alloys. Interlayers of Zn were prepared through mill rolling (to 20 mm in thickness) or electroplating (3±10 mm) methods. Parameters under examination included the surface roughness, interlayer thickness, base alloy composition, bonding temperature, time, pressure, and vacuum condition. Optimum shear lap bond strength, using solid DB under a vacuum of 10ÿ4Torr, to 90% of that of the
base alloy has been achieved for the 7475 Al system. In addition to solid state and transient liquid phase diffusion for superplastic materials, brazing [26,40,41], electron beam welding [6,17,20], and laser beam welding [16,17,20] have been utilized in joining superplastic thin sheets in National Taiwan University (Chuang) and Sun Yat-Sen University (Huang). Solidi®cation cracking and poros-ity were identi®ed to be the major detrimental factors for aluminum superplastic sheets. Typically, a joining ef®-ciency of the weld strength above 85% of the parent materials can be obtained. However, recent observations suggested that even though the weld strength can be sus-tained, the toughness of a notched electron-beam-welded bar specimen tended to be only 30% that of the parent alloy. Precaution should be taken care of.
The group under Professor Lee in National Central Uni-versity took advantage of the Murduck SPF hot press and performed systematic blow forming for aluminum and titanium alloys. Due to the size of their hot press, much larger workpiece can be made. Sandwich panels of three- or four-piece Ti-6Al-4V SPF/DB structures have been tried [52±54] with special efforts on die and gas tube design. Extensive computer simulation work on the dimple or grooving effect in SPF/DB pieces was conducted [31].
Hwang's and Huang's groups in National Sun Yat-Sen University also made great efforts on SPF/DB of aluminum alloys. A series of self-written computer programs for pressure pro®les and thickness distributions, based on ®nite difference methods, have been completed in simula-tion and comparison with experimental observasimula-tions [2,5,11,14,19,24]. The program incorporated the strain hardening exponent (n-value) additionally into the consti-tutive equation for the simulation, i.e., the equation K0"n_"m was used. The model also incorporates the
non-uniform thinning effect during free bulging [19]. In addition, the effects of different friction coef®cients at different locations of the formed part can be considered, using different friction coef®cients, such as 0 (complete sliding), 0.01±0.1 (different degrees of partial sliding),
Fig. 14. Examples of the three-layer, perpendicular-rid strengthened Ti-6Al-4V structure using SPF/DB technique. (courtesy of Prof. Cheng of Taiwan University)
Fig. 15. Examples of the three-layer slanting-rid strengthened Ti-6Al-4V structure using SPF/DB technique (courtesy of Prof. Cheng of Taiwan University).
or 1 (complete sticking) for each selected position [5,11,19,24]. It was found that the lubrication effect after the onset of SPF becomes different at different positions of the formed part. Due to the lubricant sliding downwards, there is almost no lubrication at the die entry (hence 1) and near complete lubrication and free specimen sliding at the central bottom region (hence 0). The comparison between theoretically predicted and experimentally mea-sured thickness distribution for SPF aluminum sheets is shown in Fig. 16. The best agreement was found in those cases where 1 is used for the die entry and side wall regions and 0.01 for the central bottom regime. Finally, the variable strain rate path for blow forming has been examined [14]; a similar work has also been published by the Yang's group [39].
2.3. Superplasticity mechanisms
Chang and Yeh in National Tsing Hua University pro-posed to use the curvature of tensile-loaded specimens to de®ne the superplastic behavior [55], instead of the common superplastic elongation due to the lack of universally spe-ci®ed gauge length. The curvature can describe the degree of necking within the gauge section. Furthermore, through the derived equations for curvature, strain rate sensitivity and tensile elongation, systematic relationship can be revealed [56] as depicted in Fig. 17. The dotted line on the top refers to the maximum tensile elongation with a given strain rate sensitivity m-value; e.g., a maximum elongation of 600 is theoretically predicted for m 0.5 under the given geometry conditions.
The rate controlling constitutional equations have been examined in several superplastic materials in Huang's and Kao's group in National Sun Yat Sen University. The combined roles of dislocation slip creep and grain boundary sliding (GBS) were considered for the LTSP 8090 and super
2 alloys [7,23]. The activation of cooperative and indivi-dual GBS during each stage of superplastic tensile straining or superplastic forming, as well as the topography of grain boundary sliding offsets, grain rotation angles, formation of striated bands or ®bers, cavity distribution, cavity formation mechanisms, and contribution of grain boundary sliding or grain separation to the overall strain, were traced using scanning or transmission electron microscopy [7,25] shown, e.g., in Fig. 18. Special emphasis was laid on the deforma-tion mechanisms during the initial stage of superplastic straining, and the role of liquid phase observed in HRSP materials. Chen and Lui's group in National Cheng Kung University found high elongation with ideal plastic behavior following the initial work hardening stage in their strongly [1 1 1] textured Al-Al3Ni alloys [60]. Interestingly, the
strong ®ber texture did not get altered signi®cantly after 300% superplastic elongation [59,60] contrary to most superplastic alloys. The extended elongation was believed
Fig. 16. The thickness distribution of the 8090 Al-Li-Cu-Mg superplastic thin sheet close-die SPFed at 5258C to a depth-to-diameter ratio of 1/3. The experimental data fit best with the computer simulation using non-uniform thinning during free bulging and friction coefficients of 1 for the die entry and die wall region and 0.01 for the die bottom regime.
Fig. 17. The relationship of tensile superplastic elongation, specimen gauge curvature radius, and strain rate sensitivity m-values. The dashed line represents the theoretically predicted superplastic elongations based on the characteristic m-values (courtesy of Prof. Chang of Tsing Hua University).
Fig. 18. Typical SEM micrograph of the surface topography observed from a SPFed 8090 Al-Li-Cu-Mg superplastic thin sheet after a thickness strain of 1.0. The grain boundary sliding, grain rotation, prescribed maker line offset, formation of striated bands and fibers, and cavitation distribution were under examination.
anisotropic superplasticity, i.e., a higher superplastic elon-gation when loading the specimen along its rolling direc-tion.
3. Industry applications
The R and D progress has been announced to local industrial circles through a few open seminars. The one in 1994 held in National Taiwan University attracted over 150 attendants from 80 local manufacture companies, including ®elds in aerospace and aircraft manufacture, military, steel making, forging, electric power, toy, micro-electronics, golf club maker, heat treatment, and ship build-ing, etc.
Nevertheless, limited applications are underway as yet. In Aero Industry Development Center, major efforts were made on the SPF process design, post-SPF material property evaluation and quality control for a number of aluminum aircraft parts. Examples include the cover box measuring 390 304 92 mm and the ¯ap substructure measuring 660 450 20 mm [63]. A 200-pages industry working guideline for SPF was recently completed for reference, comprising seven chapters, i.e., SPF development and applications, aluminum superplasticity, aircraft SPF-part design, processing and simulation analysis, die and tool design, SPF processing technology, and sample illustration. In Chung Shan Institute of Military Science and Tech-nology, the major SPF product under fabrication is the front-head covers for missiles, as shown in Fig. 19. The assembly was ®rst made by hot pressing under optimum superplastic conditions using superplastic Ti-6Al-4V, followed by elec-tron beam welding in vacuum. The SPF/DB Ti-6Al-4V spherical coolant containers for infrared detectors (Fig. 11) have been routinely made in production lines for middle-range missiles by Chung Shan Institute of Mili-tary Science and Technology. Another example is the hollow fan Hub (Fig. 20) made by Shan-Tung aerospace company, using DB technique from Ti-6Al-4V forged rings. The hollow fan hub is a key piece in the Pratt and Whitney JT9D gas turbine engine, and Boeing B747 and B767 as well as DC-10. Diffusion bonding was applied for bonding a spacer (for dovetail and turbine fans) onto this hollow hub
measuring 915 mm in outer diameter, 203 mm in inner diameter and 254 mm in thickness.
Metal Industry Development Center has set SPF/DB techniques as one of the ®ve center development goals.
Fig. 19. The (a) front view, and (b) back view of the front-head covers for missiles made of Ti-6Al-4V superplastic sheets (courtesy of Chung Shan Institute of Science and Technology).
Fig. 20. The hollow fan hub made from Ti-6Al-4V forged rings, applying the DB technique for bonding a spacer (for dove tail and turbine fans) onto this hollow hub measuring 915 mm in outer diameter, 203 mm in inner diameter and 254 mm in thickness (courtesy of Shan-Tung aerospace company).
companies which continue the effort, but the motivation seems to decrease due to the recent drop in sale prices. Some special applications using SPF coupled with different means of bonding techniques in electronic industry seem to have promising potential. Due to business competition and patent restriction, it is not suitable to disclose related information yet. Finally, the production of extra ®ne-grained aluminum alloys has attracted the attention of local aluminum com-panies lately.
4. Closing remarks
The R and D progress made during the past 10 years has advanced the understanding for superplasticity from near zero to a status critical for future directions. Compared with the fast growing global research achievements in super-plasticity, the advance during the past decade in Taiwan has not been impressive. The internal gas pressure forming was an interesting invention and unique process alternative, which should earn its credit. The continuous efforts on developing superplastic materials, particularly aluminum alloys, might be the main achievement on the worldwide standard. It is expected that further breakthrough along this direction should be accomplished within the next few years, using extensive thermomechanical treatments, equal channel angular extrusion, or reciprocal extrusion. In addi-tion to producing superplastic aluminum alloys, it is sug-gested that parallel explorations on several new related ®elds should be initiated, these includes, for example, the superplastic-like behavior of supercooled liquid observed in bulk amorphous Zr-Al-Ti-Cu-Ni (or similar) alloys [64,65] and the `near-net-shape' injection molding technique for Mg base alloys.
However, if still no mass production of SPF/DB parts could be achieved, research along this line would soon shrink appreciably. Potential utilization in Taiwan might be in either the electronic or the motorcycle factories. It is hoped that the application of superplastic sheets to shielding covers for electronic instruments can be spread in Taiwan soon. As the HRSP and LTSP are sup-posed to be simultaneously present in Al-Li or Al-Ci base alloys processed via equal channel angular extrusion at 200±4008C [66], applications in motorcycle or auto-mobile industry could have chance for a breakthrough. A few Taiwanese companies seem to have willingness in following the future movement of Japanese major companies.
related information from Professors Cheng, Koo, Lee, Yang, Kao, Hwang, Chen, Chang, Yeh, Mr. Tien of Aero Industry Development Center, and Mr. Shyr of Shan-Tung Aerospace Company are gratefully acknowl-edged. It is also our pleasure to acknowledge the diligent commitments of numerous graduate students in this interesting ®eld. This paper is sponsored under the project No. NSC 87±2216-E-110±017.
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