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Diffusion Bonding Performance Test Results for New Titanium Alloys (BAOTI

and VSMPO Fine Grain Titanium 6Al-4V, TIMETAL 54M and ATI 425) Using

Different Temperatures

Daniel SANDERS 1, Larry HEFTI

1, Al BRYANT

2, Shanying ZENG

2, Heping GUO

3,

Pengtao GAI 3, Zhiqiang LI

3

1 BOEING Company – P.O. Box 3707 MS5K63 – SEATTLE, WA, USA

[email protected] 2 BOEING Research & Technology China – BEIJING, PR CHINA

3 BAMTRI – Beijing Aeronautical Manufacturing Technology Research Institute – BEIJING, PR CHINA

Abstract

A joint development program was performed by Boeing and Beijing Aeronautical Manufacturing

and Technology Institute (BAMTRI) for testing the diffusion bonding (DB) performance of fine

grain titanium alloy 6Al-4V produced by Verknaya Salda Metallurgical Production Association

(VSMPO) and Baoji Titanium Industry Co., Ltd. (BAOTI), and also Titanium Metals

Corporation (TIMET) alloy TIMETAL 54M and Allegheny Technologies Incorporated (ATI)

alloy ATI 425. These materials have all been identified by their manufacturers as exhibiting

superplastic performance at a lower temperature range, 775 to 815°C, than conventional titanium

6Al-4V, which is typically superplastic formed at 875 to 925°C. The objective of this study is to

determine if the different alloys could be diffusion bonded together with themselves and to each

other at different temperatures. This report covers the first phase of the test program, which is to

test the DB capabilities of the alloys to themselves. The test includes macroscopic and

microscopic evaluations of the resulting bond lines.

Keywords : titanium, superplastic forming, SPF, diffusion bonding, DB, 6Al-4V, fine grain,

TIMETAL 54M, ATI 425.

Introduction

Jet engine designers are working towards the introduction of new materials and operational

improvements that are creating more fuel efficient thrust for aircraft. One of the most obvious

changes is the much larger size of the outer diameter of the high-bypass ratio engines, which is

leading aircraft manufacturers to a need for the manufacture of larger engine related components.

Not visible to the naked eye, but of much more importance for the engine efficiency, is the higher

operating temperatures that the newer engine cores are now operating at. Materials that have

traditionally been used for certain engine components, such as fiber reinforced graphite epoxy

composite, may no longer be acceptable with the higher service temperature requirement.

Superplastic Forming (SPF) and Diffusion Bonding (DB) of titanium may be a practical

alternative to other types of high temperature materials for certain parts, as the material size

limitation of titanium sheet is no longer a significant factor because of the new capabilities of

joining sheets together using friction stir welding (FSW) [1] [2] [3]. It is anticipated that

SPF/DB sandwich structure will be used to a greater extent in future aerospace products.

Under proper pressure and inert atmospheric conditions, DB normally occurs close to the SPF

temperature [4] [5] [6] [7]. Due to their lower SPF temperature requirement, it was theorized

that DB could be accomplished with several of these relatively new titanium alloys at a

temperature lower than conventional 6Al-4V titanium traditionally requires (> 900 deg. C).

Experimental Procedure

The two sheet DB test was accomplished by obtaining the four different titanium sheet materials

with a thickness range between 3 mm to 4.5 mm. The sheets were sheared into 75 mm x 75 mm

squares, deburred and then cleaned using a nitric hydrofluoric “pickle” acid bath in order to

remove contaminants and oxides. Pairs of the squares were mated together and the edges were

TIG welded to make a seal, reference Figure 1. Prior to completely welding the edges, a gas tube

was inserted between the two sheets on one of the edges and welded in place such that a vacuum

(10EE-01Pa) could be drawn between them. With the vacuum established, the tube was sealed

such that the vacuum was left between the two squares. An initial screening test performed

without a vacuum showed that it would be necessary to use the vacuum in order to avoid the

formation of alpha case, which causes dis-bonding. The parts were marked for identification

using a vibrating marker. The outside faces of each part were coated with a high temperature

graphite lubricant and Yttria in order to prevent bonding with the driver sheet that applied the

bonding pressure.

Figure 1. The image on the left shows two of the DB test coupons being TIG welded along an

edge in order to make an air tight seal. The picture on the right shows a completed test coupon

prior to bonding with the welded tube used to pull a vacuum prior to final sealing.

A bonding pressure of 2 MPa was used to force the coupon squares together in order to achieve

bonding. The pressure was applied for a hold period of 3 hours for all of the tests. The method

used to apply the gas pressure was to place four of the coupons onto the bottom surface of an

SPF Tray forming die, reference Figure 2, which was loaded into one of BAMTRI’s SPF presses,

reference Figure 3. A driver sheet of titanium was SPF formed down into the bottom of the Tray

pan, which in turn imparted pressure to the top of the coupons, which were sandwiched between

the bottom of the Tray die and the SPF formed driver sheet part. Additional time was allocated

for the actual forming of the Tray, such that the DB coupons were ensured of having a solid 3

hours of DB pressure time.

Figure 2. BAMTRI’s superplastic forming Figure 3. The SPF Tray die used for bonding.

press that was used to heat the SPF die and

form the SPF Trays for DB pressurization.

The two process parameters that were varied during this press were the material type and the DB

temperature. The temperatures that were tested are shown below:

1. 789 deg. C (1450 deg. F)

2. 815 deg. C (1500 deg. F)

3. 843 deg. C (1550 deg F)

4. 871 deg. C (1600 deg F)

5. 900 deg. C (1650 deg F)

Results

After DB was completed, the test coupons each had a small piece from the center of the bonded

area as shown in Figure 4. The excised material was mounted and polished for macroscopic and

microscopic investigation along the bond line. Photographs of the bond line that show a

representative surface were made for each of the material types and temperatures. A subjective

evaluation of the bond line quality was made based upon the apparent percentage of bonding and

dis-bonding that was observed at each temperature. The DB criteria used for the evaluation was

a numerical score of 1 to 5, with 5 being complete DB (no porosity), 3 being half surface bonding

and 1 being nearly total dis-bonding. The results of the evaluation are given in Table 1. Photos

taken of the bond line of each material taken at various magnifications are shown in Figures 5, 6,

7 and 8.

Looking at the DB quality evaluation in Table 1, it can be seen that the VSMPO fine grain 6Al-

4V exhibited the best bonding at the lowest operating temperature, however there was an

anomaly in the test data for this material at the 815 C bonding temperature (dis-bonds) that could

not be explained. The ATI 425 titanium also bonded well at the lower temperatures, with total

bonding being achieved as low as 815 deg. C. The minimum DB temperature for the TIMETAL

54M was found to be 871 deg. C. and for the BAOTI fine grain material it was 843 deg. C.

The TIMETAL 54M alloy exhibited an unusual elongated grain structure along the bond line

after the 843 deg. C to 871 deg. C bonding was completed, reference coupons number T3 and T4.

The TIMETAL 54M sheet samples had been prepared in the laboratory with an unconventional

surface grinding, which resulted in a shallow surface layer with a critical amount of residual cold

work that caused the microstructure to recrystallize to a large grain size. At the 900 deg. C

temperature any large grains that existed were fully recrystalized into equi-axed structure. The

ATI 425 alloy also exhibited elongated grain structure along the bond line for the 843 deg. C

bonding temperature, but not to extent seen for the TIMETAL 54M. The ATI 425 also

recrystalized into a uniform equi-axed grain structure at the higher temperatures.

Figure 4. After DB, representative samples were cut from the center of each of the test coupons.

Titanium Alloy Name Diffusion Bonding Quality

Bonding Temperature BAOTI 6Al-

4V F.G. TIMETAL 54M ATI 425 VSMPO 6Al-

4V F.G.

789 C (1450 F) 1 1 4 5

815 C (1500 F) 2 2 5 3

843 C (1550 F) 5 4 5 5

871 C (1600 F) 5 5 5 5

900 C (1650 F) 5 5 5 5

Table 1. The subjective DB quality at various temperatures is shown, with a score of 1 being the

poorest bonding (near total dis-bond) and 5 being completely bonded. All coupons were bonded

with a pressure of 2 MPa for 3 hours.

Figure 5. Material: BAOTI FG Batch 1 B1-1 B1-2 B1-3 B1-4 B1-5

788℃ / 2MPa

/3h

815℃ / 2MPa

/3h

843℃ / 2MPa

/3h

871℃ / 2MPa

/3h

900℃ / 2MPa

/3h

50X

100X

200X

300X

400X

Figure 6. Material: TIMETAL 54M T1 T2 T3 T4 T5

788℃ / 2MPa

/3h

815℃ / 2MPa

/3h

843℃ / 2MPa

/3h

871℃ / 2MPa

/3h

900℃ / 2MPa

/3h

50X

100X

200X

300X

400X

Figure 7. Material: ATI 425 A1 A2 A3 A4 A5

788℃ / 2MPa

/3h

815℃ / 2MPa

/3h

843℃ / 2MPa

/3h

871℃ / 2MPa

/3h

900℃ / 2MPa

/3h

50X

100X

200X

300X

400X

Figure 8. Material: VSMPO 6Al-4V FG

V1 V2 V3 V4 V5

788℃ / 2MPa

/3h

815℃ / 2MPa

/3h

843℃ / 2MPa

/3h

871℃ / 2MPa

/3h

900℃ / 2MPa

/3h

50X

100X

200X

300X

400X

Summary and Conclusion

During this study, four titanium alloys were tested for their diffusion bonding performance at

various temperatures using a 2 MPa bonding pressure and a 3 hour hold time. Of these, the

VSMPO fine grain 6Al-4V exhibited the best DB capabilities at the lowest temperature tested,

788 deg. C. The ATI 425 alloy performed nearly as well as the VSMPO fine grain in the same

temperature range. The TIMETAL 54M and the BAOTI fine grain 6Al-4V material required a

higher DB temperature to achieve complete bonding.

From previous reports in the literature (cited in the Introduction) it is known that the VSMPO

fine grain material exhibits excellent superplasticity and DB characteristics at temperatures above

788 deg. C. With these new findings, it appears that the ATI 425 material has DB capabilities

similar to that of the VSMPO fine grain. The TIMETAL 54M and BAOTI fine grain 6Al-4V

both showed excellent DB results, but at a slightly higher temperature range. It is noteworthy

that the BAOTI fine grain 6Al-4V and the TIMETAL 54M material used in this test were each

taken from the first test batch of each organization’s development program, so additional

improvements are to be expected in the future.

All of the materials tested appear to be suitable for superplastic forming combined with diffusion

bonding (SPF/DB). However, it is important to point out that these are the results of a small

number of test coupons. Additional DB testing is needed to validate this data.

The authors wish to acknowledge the BAOTI, VSMPO, Allegheny and TIMET companies for

providing test material to accomplish this project.

References

1. D.G. Sanders, et al, “Characterization of Superplastically Formed Friction Stir Weld in

Titanium 6Al-4V: Preliminary Results”, ASM Journal of Materials Engineering and

Performance, 17 (2) (2007), 187-192.

2. D.G. Sanders, M. Ramulu, P.D. Edwards, “Superplastic Forming of Friction Stir Welds in

Titanium Alloy 6Al-4V: Preliminary Results”, Materialwissenschaft und Werkstofftechnik,

Wiley-VCH, 39, No. 4-5, (2008), 353-357.

3. D.G. Sanders, et al, “Superplastically Formed Friction Stir Welded Tailored Aluminum and

Titanium Blanks for Aerospace Applications”, ASM Journal of Materials Engineering and

Performance, 19 (4) (2010), 515-520.

4. P.N. Comley, “Lowering the Heat – The Development of Reduced SPF Temperature Titanium

Alloys for Aircraft Production,” Superplasticity in Advanced Materials ICSAM 2003, ed. R.I.

Todd (Switzerland: Trans Tech Publications, 2004), 233-238.

5. L.D. Hefti, “Advances in the Superplastically Formed and Diffusion Bonded Process,”

Superplasticity in Advanced Materials ICSAM 2006, ed. K.F. Zhang, Switzerland: Trans Tech

Publications (2007), 87-92.

6. P.N. Comley and L.D. Hefti, U.S. Patent 7,533,794 (2009).

7. T.J. Connelly, K.W. Dunstan, W.T. Williams, P.N. Comley and L.D. Hefti, U.S. Patent

Application 20070102494 (2007).

EOT_RT_Sub_Template.ppt | 1/6/2009 | 1BOEING is a trademark of Boeing Management Company.Copyright © 2009 Boeing. All rights reserved.

Diffusion Bonding Performance Test Results for New Titanium Alloys (BAOTI and VSMPO Fine

Grain Titanium 6Al-4V, TIMET 54M and ATI 425) Using Different Temperatures

Daniel SANDERS 1, Larry HEFTI

1, Shanying ZENG

2, Al BRYANT

2, Heping GUO

3,

Pengtao GAI 3, Zhiqiang LI

3

1BOEING Company – P.O. Box 3707 MS5K63 – SEATTLE, WA – USA

[email protected] 2

BOEING Research & Technology China – BEIJING, PR CHINA3BAMTRI – Beijing Aeronautical Manufacturing Technology Research Institute, BEIJING - PR CHINA

Engineering, Operations & Technology | Boeing Research & Technology Metallic Materials & Processes

EOT_RT_Sub_Template.ppt | 2Copyright © 2009 Boeing. All rights reserved.

• Superplastic Forming Ti for Aerospace

• Diffusion Bonding Ti Applications

• Diffusion Bonding at Lower Temperatures Using New Alloys

Topics for Today



Superplastic Forming (SPF)



Monolithic SPF Stiffened Structures

Built-Up Assy.

SPF Replacement

Monolithic SPF Parts With Integral Stiffening



Unit #1 loaded on the APU test stand.

SPF Ti Frames Titanium Skin

787 Dreamliner first flight.

787 Auxiliary Power Unit Tailcone – SPF Parts



This test part was built from 50 mm thick titanium 6Al-4V plate. Integral stiffeners were machined before forming.

SPF Forming Thick Titanium Plate



Moving Towards Larger Titanium SPF Parts

This part has six friction stir welds that cannot be seen after SPF forming.



Diffusion Bonding (DB) of Titanium



•Testing showed that the material would diffusion bond to itself at 775°C.

•Also diffusion bonds to other alpha-beta alloys at this temperature which is important since standard grain materials typically require around 900°C to fully diffusion bond.

125 µm

Diffusion Bonding of Fine Grain 6Al-4V



•Representative cross section of the parts shows a smooth outer surface and the superplastically formed stiffeners. Area between the stiffeners is diffusion bonded – no fasteners required.

Fabrication of SPF/DB Details



•SPF/DB side panels are produced for the Heat Shields using fine grain for the beaded inner surface and standard grain for the outer surface.

•Outer surface resists mark-off, pulling in of the material during bead forming, at this temperature.

Fabrication of SPF/DB Details



Forward Heat Shield Assembly

•Assembly includes SPF and SPF/DB parts which use the fine grain 6Al-4V material. Also an aft assembly is constructed in the same manner.



Installed Heat Shield Assemblies

•Estimated to save 15% in cost and weight over castings. Actually saved 20% in weight.



Fine Grain 6Al-4V

• Joint research by Boeing and VSMPO yielded a fine grain version of 6Al-4V with a grain size of about 1 µm.

• Advantages: Forms at 775 C instead of 900C, improved die life, less heat radiation on machine operators, less alpha case.

Production parts made with fine grain Ti 6-4.



SPF Tool Life

Die used at 900 deg C for ashort period of time. Note the heavy scale.

Die used at 775 deg C for along period of time. Note the smooth surface.



Diffusion Bonding (DB) of Titanium Alloys At Lower Temperature Test

1. VSMP Fine Grain Ti 6Al-4V (Baseline)2. BAOTI Fine Grain Ti 6Al-4V3. TIMET 54M4. Allegheny ATI 425

DB Test – Titanium Materials



Experimental Procedure

TIG weld two 75mm x75 mm titanium piecestogether around edges.

Gas tube to pull avacuum betweenthe 2 sheets.

75 mm



SPF Form Pan to DB Coupons

SPF Press at BAMTRI

SPF Die used for DB bonding samples

• Coupons were held at 2 MPa gas pressure for 3 hours after the SPF pan was fully formed to squeeze them together for DB.



Temperature Range Tested

1. 789 deg. C (1450 deg. F) 2. 815 deg. C (1500 deg. F)3. 843 deg. C (1550 deg F) 4. 871 deg. C (1600 deg F)5. 900 deg. C (1650 deg F)

All coupons were diffusion bonded at a pressure of 2 MPa for 3 hours. Test coupons excised from center of part.



Bond Line After Bonding Process – ATI 425

Material: ATI 425

A1 A2 A3 A4 A5

788C / 2MPa /3h

815C / 2MPa /3h

843C / 2MPa /3h

871C / 2MPa /3h

900C / 2MPa /3h

100X

200X

300X



Bond Line After Bonding Process - VSMPO

Material: VSMPO Fine Grain

V1 V2 V3 V4 V5

788C / 2MPa /3h

815C / 2MPa /3h

843C / 2MPa /3h

871C / 2MPa /3h

900C / 2MPa /3h

100X

200X

300X



Bond Line After Bonding Process - BaoTi

Material: BaoTi FG Batch 1

B1-1 B1-2 B1-3 B1-4 B1-5

788C / 2MPa /3h

815C / 2MPa /3h

843C / 2MPa /3h

871C / 2MPa /3h

900C / 2MPa /3h

100X

200X

300X



Test Results – TIMET 54M

100X

200X

300X

Material: TIMET 54M

T1 T2 T3 T4 T5

788C / 2MPa /3h

815C / 2MPa /3h

843C / 2MPa /3h

871C / 2MPa /3h

900C / 2MPa /3h



Rating of the DB Zone

Bonding TemperatureBaoTi 6Al-4V

F.G. TIMET 54M ATI 425VSMPO 6Al-4V

F.G.789 C (1450 F) 1 1 4 5

815 C (1500 F) 2 2 5 3

843 C (1550 F) 5 4 5 5

871 C (1600 F) 5 5 5 5

900 C (1650 F) 5 5 5 5

Titanium Alloy Name Diffusion Bonding Quality

A subjective examination was made of the bond line, with a score of 1 being little or no diffusion bonding and a score of 5 being total bonding..



Summary

• Lower temperature superplastic forming and diffusion bonding titanium alloys are needed to produce higher quality parts at less cost.

• New alloys from VSMPO, TIMET, ATI and BaoTi show promise for lower temperature SPF and DB.

• All alloys tested are diffusion bondable to themselves at 843 deg. C (1550 deg F) or above.

• Testing of these alloys for DB between one another is currently in work.

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