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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
3Kobe Steel, LTD Proprietary Information
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.
4Kobe Steel, LTD Proprietary Information
Ti-6Al-4V bolts
Thread rollingor
machiningHot forging
(1)
(2) Machining only
Manufacturing costs are higher than cold forging.
5Kobe Steel, LTD Proprietary Information
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.
6Kobe Steel, LTD Proprietary Information
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.
7Kobe Steel, LTD Proprietary Information
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.
8Kobe Steel, LTD Proprietary Information
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.
9Kobe Steel, LTD Proprietary Information
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
10Kobe Steel, LTD Proprietary Information
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)
11Kobe Steel, LTD Proprietary Information
A bolt made from Ti-15-3-3-3- Trial production was successful.10,000 bolts were made by this cold forging process.
10mm
12Kobe Steel, LTD Proprietary Information
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
13Kobe Steel, LTD Proprietary Information
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)
14Kobe Steel, LTD Proprietary Information
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
15Kobe Steel, LTD Proprietary Information
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)
16Kobe Steel, LTD Proprietary Information
- 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