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1Kobe Steel, LTD Proprietary Information

Development of Cold Forging Technology for Beta-Titanium

October 2006

NORIKAZU MATSUKURATITANIUM TECHNOLOGY DEPARTMENT

KOBE STEEL, LTD.

2Kobe Steel, LTD Proprietary Information(Reference: Japan Titanium Association)

Various bolts and parts made with titanium


Titanium materials for bolts and parts

Kind of material Tensile strength Young’s Modulus

Unalloyed titanium　　 300 ~ 500 MPa 105 GPa

Ti-6Al-4V (annealed) 900 MPa　　 115 GPa(ST-A) 1200 MPa

Ti-6-4 is generally used for high strength bolts.


Ti-6Al-4V bolts

Thread rollingor

machiningHot forging

(1)

(2) Machining only

Manufacturing costs are higher than cold forging.


Feature of Ti-15V-3Cr-3Sn-3Al (1)

Upset : 20%Upset : 60% Upset : 80%

Good cold workability

High tensile strength due to agingTensile strength 750~800MPa (annealed)

Aging1000~1200MPa

Ti-15-3-3-3 is a good material for high strength bolts.

- No cracks are generated even if Ti-15-3-3-3 is deformed to 80%.- Ti-15-3-3-3 can be cold forged.


Feature of Ti-15V-3Cr-3Sn-3Al (2)0.02

0.01

00 20000 40000 60000 80000 100000

Time [s]

Tota

l str

ain

at room temperatureCP-Ti (0.8σ0.2)

Ti-6Al-4V (0.8σ0.2)

Ti-15V-3Cr-3Sn-3Al (0.8σ0.2)

Creep curve of titanium materials (annealed)(Ref : E. Sato : J. JILM., 55 (2005), 604-609)

- Since Ti-15-3-3-3 does not start creep at room temperature, it is suitable for high strength bolts.


Why is cold forging difficult for Ti-15-3-3-3 ?The Young’s modulus is low. (~80GPa / less than half that of steel)

- It is easy to gall with the diewhen the reduction and hollowing process are performed.

die

material

reduction

die material

hollowing

Thermal conductivity is low. (8.1W/m K / one tenth that of steel)- The temperature of the contact surface between material

and die is increased locally by friction heat, so it is easy to gall.


Ti-15V-3Cr-3Sn-3Al boltsPOINTS- galling prevention coating for the material

- cooling the material and the die effectively

Trial production

By coating materials with the appropriate oxide layer and lubricant, galling can be prevented.


The cold forging process of the bolts

Stage 1 chamfer the edge of the shaftStage 2 reduce the shaft (forward extrusion)Stage 3 upset the head partStage 4 reduce the head part and form the head shapeStage 5 hollow the head part (backward extrusion) and

expand the flange

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5


Parts former

Production equipment

Material(Ti-15-3-3-3)

diepunch

#1#2#3#4#5

#1#2#3#4#5

back and forth

(Reference: SAKAMURA MACHINE CO., LTD)


A bolt made from Ti-15-3-3-3- Trial production was successful.10,000 bolts were made by this cold forging process.

10mm


Relationship between the tensile properties of Ti-15-3-3-3 bolts and aging condition.

- Tensile strength without aging is about 900 MPa.- Though maximum tensile strength exceeds 1200 MPa, the elongation declines about 1%.

- Tensile strength of 1000~1100MPa is appropriate for Ti-15-3-3-3 bolts

Relationship between tensile properties and aging condition in a Ti-15-3-3-3 material

600

800

1000

1200

1400

0 4 8 12 16 20 24 28aging time [h]

450°C480°C

570°C

510°C540°C

Tens

ile s

treng

th [M

Pa]

600

800

1000

1200

1400

0 4 8 12 16 20 24 28aging time [h]

450°C480°C

570°C

510°C540°C

0.2%

pro

of s

treng

th [M

Pa]

05

101520

0 4 8 12 16 20 24 28Aging time [h]

Elo

ngat

ion

[%]

450°C480°C

570°C

510°C540°C

0

800

900

1000

1100

1200

1300

1400

1 2 3 4Aging temperature [°C]

Tens

ile s

treng

th [M

Pa] as cold forging

800°C ST and aging770°C ST and agingaging only

without aging 570540510

breaking belowthe normal

0

5

10

15

1 2 3 4Aging temperature [°C]

Elon

gatio

n [%

]

without aging 570540510


Microstructures of the cross section of screw part(a) 800 degree C solution treatment and 570 degree C aging(b) 570 degree C aging

- The microstructure of (a) is coarse, so the elongation is low.- The microstructure of (a) is fine, so the elongation is not declined.

0.2mm

1mm

(a)

1mm

0.2mm(b)


Relationship between aging temperature and tensile properties ofTi-15-3-3-3 smooth and notch specimens - When the tensile strength of smooth specimen exceeds 1200MPa,

the tensile strength of notch specimen declines from the peak. Then the bolts cannot be used by the tensile strength over 1200MPa.

600

800

1000

1200

1400

1600

1 2 3 4 5 6Aging temperature [°C]

Stre

ngth

[MPa

]

tensile strength (smooth)tensile strength (notch)

1000MPaclass bolt

1100MPaclass bolt

05

10152025

1 2 3 4 5 6Aging temperature [°C]

Elon

gatio

n [%

]

elongation(smooth)

540without aging 570 480500510


The relation between temperature and tensile properties of Ti-15-3-3-3 bolts

- A tensile strength of 1000~1100MPa is appropriate for Ti-15-3-3-3 bolts considering the balance between tensile strength and elongation.

0

2

4

6

8

10

12

14

800 900 1000 1100 1200 1300Tensile strength [MPa]

Elon

gatio

n [%

]

as cold forged(without heat treatment)


- Cold forging of Ti-15-3-3-3 bolts is enabled by improvements of galling prevention coating, that is the appropriate oxide layer and lubricant.

- The trial production of 10,000 bolts by cold forging was successful.So, it is expected that the cost of high strength titanium bolts become lower than that of Ti-6-4.

- Ti-15-3-3-3 bolt has a strength of 1000~1100MPa and it is expected to contributes to weight reduction of the parts.

Summary