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Kobe Steel, LTD. Proprietary Information1

Technologies for reliable titanium alloy forgings focusing on ultrasonic inspection

in aerospace industry

Yoshinori Ito, Hiroyuki Takamatsu and Keiji KinoshitaKobe Steel, Ltd.

Kobe Steel, LTD. Proprietary Information

Outline of aerospace sector in Kobe Steel

2

VAR MeltingFurnace

Open Die Forging Press

Closed Die Forging Press

Ring RollingMill

Heat treatment Inspection Shipment

• Kobe Steel has integrated production line for titanium alloy products which allows us consistent quality design and quality control.


Long Supply History for Aerospace – 55 years

3

• No. 1 titanium supplier on the aerospace market in Japan.• First delivery for aerospace application was in 1959.

Supply Record for V2500 Jet Engine

(Courtesy IHI)

Fan Cases :More than 10,000 pcs Deliverywith Internal Ingots since 1985

Compressor Disks and Shafts :More than 6,000 pcs Deliverywith Internal Ingots since 1986


New production facilities

4

Japan Aeroforge: 50,000 metric ton forging press Started-up in 2013

• Recently, we have installed new production facilities which allowed us provide large titanium forgings meeting specifications in aircraft industry.

• 50,000 metric ton forging press, Heat Treatment Plant, Inspection Plant, et al.


Technology platform for process design

• Mechanical tests• Inspections

• Forging simulation using FEM• Initial condition

Cycles to Failure

Stre

ss

• Process design is key technology to provide high quality titanium forgings. • Establishment of forging simulation technique and processing window is essential.

• Tensile properties• Fatigue properties• Toughness• Ultrasonic inspection• Metallograph

et al.

• Forging method• Forging condition• Dies shape• Preform shape

et al.

• Temperature, StrainStrain rate

• Shape, Load et al.

Process design

• Processing window

• Based on microstructure, properties et al.


Forging simulation using FEM analysis

Equivalent strain distribution Temperature distribution

Isothermal flow stress curves (Input data in FEM)

Stress-strain curves (Compression test)

Removed the adiabatic heating effect

Optimized boundary condition

0

50

100

150

200

250

300

350

400

450

500

0.0 0.5 1.0 1.5 2.0 2.5

Stre

ss /M

Pa

Strain

850℃

900℃

950℃

1000℃

1050℃

1100℃

0

50

100

150

200

250

300

350

0 0.2 0.4 0.6 0.8

Stre

ss /M

Pa

Strain

850℃

900℃

950℃

1000℃

1050℃

1100℃


Experimental facilities for process design

Thermo-mechanical simulator (Small test piece)

Lab-scale forging press

Microstructural analysis,characterization

Process Microstructure Properties


Technologies of improving ultrasonic detectability

Our goal• Design our forging process (Shapes of dies and preforms, and process

conditions et al.) to optimize mechanical properties and ultrasonic detectability.

• Ultrasonic simulation • UT operation

-1.E-01

-8.E-02

-6.E-02

-4.E-02

-2.E-02

0.E+00

2.E-02

4.E-02

6.E-02

8.E-02

1.E-01

0.E+00 2.E-06 4.E-06 6.E-06 8.E-06 1.E-05

時間（s)

z変

位(a

.u.)

Time /s

Ampl

itude

(a.u

.)


• Processing window


Necessity of ultrasonic inspection

• Each defects in titanium alloys have the potential to cause catastrophic aircraft engine failures.

Ultrasonic inspection

Defects in forgings • Inclusions (Hard alpha et al.)• Cracks• Voids(Rarely occurred, but it is crucial.)

Titanium alloy forgings

Sign

al

Defects?Microstructure

(Noise)?

Time

Surface echo Bottom echo


Influence on ultrasonic detectability

• Clarification of the effect of microstructure on ultrasonic noise.• Establishment of a prediction technique of ultrasonic noise.

Material characteristicsDensity, Wave speed,Attenuation,Grain size, Grain shape,Texture, et al.

Inspection characteristicsFrequency (Wavelength), Diameter of transducer,Focal length, Focussed or flat, et al.

200μm

Microstructure of beta forged Ti-6246 alloy

Beta forged Ti-6246 alloy• It have good strength and toughness balance so it is used for critical parts

(disks et al.) in aircraft engine. But it is well known that beta forged titanium alloys generate high backscattering noise due to microstructure.


F..D

.

(Non-deformation)0% reduction 67% reduction

0

0.005

0.01

0.015

0.02

0.025

230 235 240 245 250R

MS

sig

nal l

evel

/ V

Time/μs

67%

Surface echo Bottom echo

0%

Lab-scale experimental results

β grain boundary

Elongated β grain

200μm200μm

Ultrasonic signals (Measured)Microstructures

F..D. : Forging direction

• Frequency: 5MHz


Lab-scale experimental results

0

0.005

0.01

0.015

0.02

0.025

230 235 240 245 250R

MS

sig

nal l

evel

/ V

Time/μs

: Parallel to the F.D. : Perpendicular to the F.D.

Incident ultrasonic wave

Elongated β grain

Parallel

Perpendicular

F.D

.

F..D. : Forging direction


Computer simulation regarding ultrasonic scattering

Computer simulation based on FEM

Multilayered model(Titanium alloy)

Transducer

Homogeneous body

Air

d m

Planar grain boundaries

Grain (Velocity 1)

-0.10-0.08-0.06-0.04-0.020.000.020.040.060.080.10

0.0 2.0 4.0 6.0 8.0 10.0In

tens

ity o

f sig

nal (

a.u.

) Time/μs

Transmitter pulse Bottom echo

Backscattering

Ultrasonic signals (Simulated)


Computer simulation regarding ultrasonic scattering

Transducer

Pro

paga

tion

dire

ctio

n

Transducer

10mm

10m

m

Case 1 Case 2 Ultrasonic signals (Simulated)

• These simulation models provide similar profiles to experimental results.

Time/s

Am

plitu

de (a

.u.)


Concept of process design for ultrasonic detectability

• Ultrasonic simulationusing FEM

-1.E-01

-8.E-02

-6.E-02

-4.E-02

-2.E-02

0.E+00

2.E-02

4.E-02

6.E-02

8.E-02

1.E-01

0.E+00 2.E-06 4.E-06 6.E-06 8.E-06 1.E-05

時間（s)

z変

位(a

.u.)

Time /s

Ampl

itude

(a.u

.)



Practical application to production-scale parts

Heating

Machining

Hot die forgingFan AC followed by STA

Disc 1: Not applied Disc 2: Applied

• Material : Ti-6246 alloy

General Appearance of disks • Diameter: approx. 700mm• Thickness: approx. 50mm


Practical application to production-scale parts

: Not applied: Applied

C-Scan images

A-Scan images

Disc 1: Not applied Disc 2: Applied

• It allows us to provide forging parts with lower backscattering noise (Higher reliability).

Lower noise (Good)Higher noise (Bad)


Conclusions

• Kobe steel is an integrated manufacturer of titanium forgings, from melting process to final products, and we have essential technologies to provide high quality titanium forgings, such as forging simulation, dies design, process design, et al.

• Besides mechanical properties, ultrasonic detectability is required for critical parts (like disks) in aircraft engine. It is well known that ultrasonic testing for titanium alloy forgings is difficult because ultrasonic backscattering noise due to microstructural features can mask reflection from defects.

• This talk introduced the outline of our technologies to improve the ultrasonic detectability of titanium forgings.


Thank you for your attention.

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