Development of novel tooling materials
for next generation air mobility components
with intricate structure

Hiromichi T. Fujii*, Naoki Sakaguchi, Haruyasu Ohno, Kotaro Ona* fujii@shst.co.jp
Principal researcher
SHINHOKOKU STEEL CORP.

September 21-24, 2020 / www.theCAMX.org

Invar alloy used as a tooling material of CFRP moldingInvar alloy✓✓✓Fe-36mass%Ni alloy
Low thermal expansion (R.T. ~ 200ºC)
Tooling material for CFRP components
in aerospace industry

Moldingtemp.

Requirements for tooling material✓✓✓Large differenceClose dimensional change to CFRP
between room and molding temps.
(Match the CTE* values of production
and tooling materials)
High durability
Thermal stability* Coefficient of thermal expansion

Small difference

Comparison of relative thermal expansion curves to a dimension atroom temperature between tooling materials.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 3September 21-24, 2020 / www.theCAMX.org

Background and research motivation
Conventional airplane✓✓Next-gen. air mobility✓✓Large components (Small curvature)
Low-volume production

Small components(Large curvature, intricate structure)
Mass production

Air bike “XTURISMO®”(A.L.I. Technologies webpage: http://ali.jp/)

Expected issues✓✓

Social acceptance
Mass production of small components

Flying car “SkyDrive”(CARTIVATOR webpage: http://cartivator.com/)

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 4September 21-24, 2020 / www.theCAMX.org

Press molding with advanced invar alloy molds
Next-gen. CFRP tooling✓ Tooling formed in near-net shape(As-cast structure)✓ High durability(Decrease in aged deterioration)✓ Thermal stability(Precise molding of intricate structure)✓ Maintenance free
(Automation of factory, mass production)
Press molding with the advanced invar alloy molds that allow mass
production of the small CFRP components with intricate structure.

Cast invar alloys with supreme durability can be a powerful tool for next gen. air mobility components

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 5September 21-24, 2020 / www.theCAMX.org

Control of thermal expansion characteristics
Thermal expansion characteristics
depend on …
✓✓
✓Curie temp.

Controllable thermal expansion✓

Thermal expansion curves of various low thermal expansion alloys. Low
thermal expansion characteristics can be obtained below Curie point.

Chemical composition
Thermal treatment
Curie temperature

Enable to match the CTE between CFRP
and invar type alloys in the desired
temperature range

Current issues✓Low yield strength in cast alloys caused bycoarse columnar grains structure

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 6

September 21-24, 2020 / www.theCAMX.org

Mechanical properties of cast invar type alloys
(a) As-cast

As-cast Fe-based alloys✓✓10mm(b) Forged10mm

Coarse columnar grains structure formed
during solidification process
Deterioration of mechanical properties

Plastic working✓✓✓Typical strengthening techniqueFine and equiaxed grains structure
Difficult to obtain intricate structure

Optical microscope images of grain microstructures in (a) as-cast and (b)forged Fe63Ni32Co5 (super invar alloys).

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 7

September 21-24, 2020 / www.theCAMX.org

Strengthening technique without plastic workingBailey-Hirsch relationship

Reverse martensitic transformation✓✓

Hall-Petch relationship*✓

A metallurgical phenomenon that occurs
in Fe-Ni alloys
High dislocation density region

An inverse relationship between grain
size and yield strength

Bailey-Hirsch relationship**✓An inverse relationship between
dislocation density and yield strength* E.O. Hall: Proc. Phys. Soc. Lond., 64 (1951), 747.
** J.E. Bailey and P.B. Hirsch: Philos. Mag., 5 (1960), 485.

Hall-Petch relationship

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 8September 21-24, 2020 / www.theCAMX.org

Thermal treatment developed for strength improvement
 Developed cast invar alloy
Casting

Near-net shape

Developed
thermaltreatment

Fine grains

Finishprocessing

High rigidity
Low CTE10mm

 Conventional cast invar alloy
Casting

Near-net shape

Thermaltreatment

Coarse grains

Finishprocessing

Low CTE10mm

 Forged invar alloy
Casting

IngotForging

High rigidity

Fine grains
Precisionmachining

Thermaltreatment

Finishprocessing

Low CTE

10mm

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 9September 21-24, 2020 / www.theCAMX.org

Objective

To reveal the effects of reverse martensitic transformation andannealing on the mechanical properties in the low thermal expansion
cast alloys for the development of novel tooling materials.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 10

September 21-24, 2020 / www.theCAMX.org

Sample preparation
Chemical composition
SymbolChemical composition

Type of alloys

Target molding
temperature [ºC]Average CTE in conventionalalloys [10-6 / ºC]

I-200

Fe65.1+xNi34.9-x

Invar

200

2.3 (R.T. ~ 200 ºC)

SI-300Fe64.2+x+yNi31.0-xCo4.8-ySuper invar

300

4.1 (R.T. ~ 300 ºC)

K-400

Fe55.3+x+yNi28.2-xCo16.5-y

Kovar

400

5.0 (R.T. ~ 400 ºC)

Thermal treatment

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 11

September 21-24, 2020 / www.theCAMX.org

Results and discussion

September 21-24, 2020 / www.theCAMX.org
September 21-24, 2020 / www.theCAMX.org

Control of Curie temperature
Curie temperatures in low thermal
expansion alloys✓✓

Linearly decrease with increasing Fe
content both in SI-300 and K-400
Change from 180ºC to 360ºC and from350ºC to 450ºC in SI-300 and K-400,
respectively

Low thermal expansion characteristics✓Enable to be maintained up to 360ºCand 450ºC in SI-300 and K-400,
respectively
Relationship between Curie temperature and Fe contents in SI-300 and K-400.
Curie temperatures were estimated from the inflection points of thermalexpansion curves.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 13

September 21-24, 2020 / www.theCAMX.org

Thermal expansion behavior of developed alloys
Low thermal expansion characteristics✓✓

Maintained below Curie temperature ineach alloy
Applicable in wide molding temperature
range in press molding

Chemical composition✓Adjusted by 0.1% accuracy for causing
martensitic transformation at liquid
nitrogen temperature

Relative thermal expansion curves to a dimension at 20 ºC(room temperature) obtained from I-200, SI-300, K-400 and atypical invar alloy Fe65Ni35.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 14

September 21-24, 2020 / www.theCAMX.org

Microstructural evolution during thermal treatment

Optical micrographs observed in(a) as-cast, (b) as-cryogenic
treated and (c) thermally treatedspecimens obtained from theI-200 (x = 1.7) cast block.
(d) shows a magnified micrographof (b).
as-castas-cryogenic treated

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thermally treated

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 15

September 21-24, 2020 / www.theCAMX.org

Randomization of crystallographic orientations
As-cast invar alloy✓✓

Sharp (100) textureThermally treated cast invar alloy✓✓Random texture

Coarse columnar grains structure
Sharp (100) texture

Fine and equiaxed grains structure
Randomly distributed grains

Controlled grains microstructure✓Low thermal expansion cast alloy withstable and superior mechanical properties

XRD profiles obtained from the as-cast and thermally treated(cryogenic treated and annealed) SI-300 specimens.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 16

September 21-24, 2020 / www.theCAMX.org

Maintenance of low thermal expansion characteristics
Effect of thermal treatment on thermalexpansion characteristics✓

Few effects were observed between
thermal expansion curves of as-cast andthermally treated specimens

Cryogenic treatment and annealing✓Powerful technique to obtain low thermal
expansion cast alloys with high strength

Comparison of relative
thermal expansion
curves to the dimensions
at 20 ºC (roomtemperature) between
the as-cast and thethermally treated I-200,
SI-300 and K-400
specimens.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 17

September 21-24, 2020 / www.theCAMX.org

Effects of thermal treatment on mechanical properties
Increase in tensile strength bythermal treatment✓✓Applied stress in thermallytreated specimens is higher
than that in as-castspecimens
Increase in strength is causedby grain refinement(a) Stress-strain curves obtained from the as-cast and the thermally treated I-200, SI-300 and K400 specimens at 200 ºC, 300 ºC and 400 ºC, respectively. (b) A magnified view of the region in
low strain range indicated by orange square in (a).

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 18

September 21-24, 2020 / www.theCAMX.org

Improvement of mechanical propertiesDrastic improvement of yield strength✓✓Improved more than 2.2 times bythermal treatment
Higher than forged alloys with finegrains structure

Improvement of Young’s modulus✓Improved by thermal treatment in SI-300and K-400

0.2% yield strength and Young’s modulus of the as-cast and the thermallytreated I-200, SI-300 and K-400 specimens estimated from the stress-strain
curves.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 19

September 21-24, 2020 / www.theCAMX.org

Improvement of lifetime
Drastic decrease in aged deterioration
✓✓

✓



Stress-strain curves in low strain range obtained from the as-cast and thethermally treated K-400 specimens at 400 ºC.

40 MPa is assumed to be locally applied bystress concentration during press molding
Cyclic applied stress during mass
production will produce aged deteriorationof tooling materials
No micro-strains are accumulated inthermally treated cast invar alloys

Expected technological benefits✓✓✓

Drastic improvement of tooling lifetime
Stable products quality
Automation of factory

A NEW CAMXFOR A NEW TIME

H.T. FujiiSHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 20

September 21-24, 2020 / www.theCAMX.org

Technological advantages of developed invar alloys
Tooling lifetime cost✓✓✓Long lifetime(Decrease in replacement cost)
Stable components quality(Decrease in maintenance cost)
High recyclability(Decrease in disposal cost)

Summary of lifetime cost in each tooling material for small CFRP
component with an intricate structure.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 21

September 21-24, 2020 / www.theCAMX.org

Summary✓ Yield strength of the low thermal expansion cast alloys at desired CFRP molding
temperature can drastically increase by the application of reverse martensitictransformation.

✓ Low thermal expansion cast alloy with high strength can be a powerful tool inmolding of small CFRP components with intricate structure for next gen. airmobility. In particular, the developed alloys will allow for precise production ofcomponents with high level of safety that enables to social acceptance.

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 22

September 21-24, 2020 / www.theCAMX.org

Hiromichi T. Fujii, Ph.D.
新報国製鉄株式会社fujii@shst.co.jp

SHINHOKOKU STEEL CORPORATION

Principal researcher
SHINHOKOKU STEEL CORP.

CAMX 2020 発表論文
題 目： “Development of Novel Tooling Materials for Next Generation Air Mobility Components with Intricate Structure”
著 者： Hiromichi T. Fujii, Naoki Sakaguchi, Haruyasu Ohno, and Kotaro Ona
年： 2020頁： TP20-0062 (14 pages)
概 要；
Thermal expansion characteristics between room and molding temperatures were analyzed in low thermal expansion alloys developed asnovel tooling materials for small CFRP components with intricate structure. Low thermal expansion characteristics of the developed alloys
could be maintained up to 400 ºC. The developed alloys prepared by casting were processed by cryogenic treatment and annealing toimprove the mechanical properties. The mechanical properties were examined by tensile tests of round-bar specimens. Yield stress of theprocessed alloys showed more than twice of as-cast alloys. The drastic improvement of the mechanical properties was found to be attributed
to the grain refinement and the high density dislocations through microstructural characterization. Cryogenic treatment and annealing
process was also found to possibly reduce variation of mechanical properties. These discoveries suggested that the developed alloys could
improve the tooling lifetime and reduce the manufacturing cost in CFRP molding for next generation air mobility.
URL: https://www.nasampe.org/store/viewproduct.aspx?ID=17609307

A NEW CAMXFOR A NEW TIME

H.T. Fujii

SHINHOKOKU STEEL CORP.

fujii@shst.co.jp

Page 24

September 21-24, 2020 / www.theCAMX.org