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Procedia Engineering 207 (2017) 2334–2339

International
International Conference
Conference on
on the
the Technology
Technology of
of Plasticity,
Plasticity, ICTP
ICTP 2017,
2017, 17-22
17-22 September
September 2017,
2017,
Cambridge, United Kingdom
Cambridge, United Kingdom

Factors influencing the forming characteristics in micro tube

hydroforming by ultra high-forming pressure
Ken-ichi Manabeaa*,
Ken-ichi Manabe *, Hideki
Hideki Sato
b
Satob,, Kenta
Kenta Itai
a
Itaia,Marko
,Marko Vilotic
c
Viloticc,Kazuo
,Kazuo Tada
Tadaa
a

a Graduate
Graduate School
School of
of Science
Science and
and Engineering,
Engineering, Tokyo
Tokyo Metropolitan
Metropolitan University,
University, 1-1 Minamioosawa, Hachioji,
Hachioji, Tokyo,192-0397,Japan
Tokyo,192-0397,Japan
a
1-1 Minamioosawa,
b Ibid.., Presently, Hitachi Ltd.
b
Ibid.., Presently, Hitachi Ltd.
c Faculty of Technical Sciences, University of Novi Sad, 21000 Novi Sad, Serbia
c
Faculty of Technical Sciences, University of Novi Sad, 21000 Novi Sad, Serbia

Abstract
Abstract

The
The focus
focus of
of this
this study
study is
is micro
micro tube
tube hydroforming
hydroforming (MTHF)
(MTHF) forfor fabricating
fabricating complex
complex shaped
shaped tubular
tubular or
or three-dimensional hollow
three-dimensional hollow
micro
micro components, which is a scaled down technology of tube hydroforming. In our previous study, a new MTHF system with
components, which is a scaled down technology of tube hydroforming. In our previous study, a new MTHF system with
ultra-high pressure
ultra-high pressure generator
generator was
was developed
developed andand aa T-
T- and
and cross-shaped
cross-shaped micro
micro tube
tube hydroforming
hydroforming of of phosphorous-deoxidized
phosphorous-deoxidized copper
copper
tube
tube with
with diameter
diameter ofof 500
500 µm
µm and
and thickness
thickness of of 100
100 µm
µm was
was successful.
successful. InIn this
this study,
study, aa micro
micro T-shaped
T-shaped hydroforming
hydroforming as as well
well as
as aa
micro cross-shaped hydroforming are carried out to confirm influential factors on micro forming characteristics
micro cross-shaped hydroforming are carried out to confirm influential factors on micro forming characteristics of MTHF. of MTHF.
Lubrication
Lubrication condition,
condition, material
material property,
property, formed
formed shape
shape and
and grain
grain size
size are
are mainly
mainly considered
considered as
as influential
influential factors,
factors, and
and discussed.
discussed.
© 2017
© 2017 The
The Authors.
Authors. Published
Published by
by Elsevier
Elsevier Ltd.
Ltd.
© 2017 The Authors.
Peer-review Published byofElsevier
under responsibility
responsibility Ltd.
the scientific
scientific committee of of the
the International
International Conference
Conference on on the
the Technology
Technology
Peer-review
Peer-review under
under responsibility of the
of the committee
scientific committee of the International Conference on the Technology of Plasticity.
of Plasticity
of Plasticity..
Keywords:
Keywords: Micro
Micro tube
tube hydroforming;
hydroforming; Forming
Forming characteristics;
characteristics; High
High internal
internal pressure;
pressure; Influential
Influential factors;
factors;

1.
1. Introduction
Introduction

According
According to
to the
the forecast,
forecast, market
market of
of micro-electro
micro-electro mechanical
mechanical systems
systems (MEMS)
(MEMS) industry
industry has
has been
been growing
growing in
in
medical,
medical, electronic,
electronic, measuring
measuring instrument
instrument and
and communication
communication fields
fields [1].
[1]. Since
Since the
the demand
demand around
around the
the world
world is
is
increasing,
increasing, miniaturization
miniaturization of
of metal
metal working
working processes
processes is
is increasing
increasing too.
too. Micro
Micro metal
metal forming
forming technologies,
technologies, such
such as
as

*
* Corresponding
Corresponding author.
author. Tel.:
Tel.: +82-42-677-2712;
+82-42-677-2712; fax:
fax: +82-42-677-2701.
+82-42-677-2701.
E-mail address: manabe@tmu.ac.jp
E-mail address: manabe@tmu.ac.jp

1877-7058
1877-7058 ©© 2017
2017 The
The Authors.
Authors. Published
Published by
by Elsevier
Elsevier Ltd.
Ltd.
Peer-review under responsibility of the scientific committee
Peer-review under responsibility of the scientific committee of
of the
the International
International Conference
Conference on
on the
the Technology
Technology of
of
Plasticity
Plasticity..

1877-7058 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity.
10.1016/j.proeng.2017.10.1004
 Ken-ichi Manabe et al. / Procedia Engineering 207 (2017) 2334–2339 2335
2 Ken-ichi Manabe/ Procedia Engineering 00 (2017) 000–000

micro blanking, micro extrusion, micro bending, micro forging and micro deep drawing, were developed starting from
2000 [2]. In the micro metal forming processes, micro tube hydroforming (MTHF) is expected to become important
and essential microforming processes. However, it is not easy for the process to be simply scaled down. Difficulties
took place during manufacturing of miniaturized high pressure supply system without fluid leakage, high precision
fine die for MTHF, reducing tool stiffness and because of so-called scale effects [3, 4]. Therefore, it was necessary to
clarify above-mentioned problems in order to successfully perform MTHF. Following studies have been carried out
until now.
Zhung et al. conducted the different bulge shapes MTHF experiments on 0.8mm diameter stainless steel tube with
0.04mm wall thickness. Influence of grain size effect on the thinning and necking behavior was demonstrated by a
plain strain crystal plasticity finite element (FE) simulation [4].
In regard to MTHF system, Hartl et.al [5] developed a MTHF system with a spindle driven pressure intensifier
which enables to apply up to 400MPa for volume-production on demand. They produced different shapes of micro
hydroformed samples with small expansion from stainless steel of 0.8mm and 0.04mm wall thickness. Ngaile et al.
[6] proposed and developed a new floating die assembly concept for MTHF process. In their experiments, bulged
forming, including typical protrusion-type hydroforming for Y- and T- shapes, was conducted for stainless steel tubes
(1.0 and 2.0mm outer diameter with 0.1 and 0.2mm wall thickness, respectively). Shairayori [7, 8] conducted cross
shape tube hydroforming for small size aluminum alloy and copper tubes of 8mm diameter with 0.5mm and 0.8mm
wall thicknesses. The hydroforming experiment with larger protrusion height was successful.
For small size and micro tubes, microtubes with outer diameter less than 0.8mm were not used in previous tube
hydroforming studies. Therefore, in our recent study, we used 0.5mm outer diameter micro tube of with 0.1 mm wall
thickness for a cross shape MTHF process [9, 10]. The experiments were successful and only process window and
failure modes [11] for the cross shape MTHF process were obtained and clarified using a new designed MTHF system.
After that, the T-shape micro tube hydroforming (MTHF) process for 500m outer diameter and 100m wall thick
copper microtube is investigated experimentally and numerically [12]. In particular, the fundamental microforming
characteristics and failure modes, as well as forming limits, were considered in order to create the process window
diagram. As mentioned above, the fundamental microforming characteristics and failure modes of T- and cross-shaped
MTHF was clarified; however, the influential factors in MTHF have not been confirmed yet.
In this study, some factors influencing microforming characteristics are focused on in T- and cross-shapes MTHF
processes. Materials, hydroformed shape, lubrication condition and grain size are considered as influential factors in
the experiment.

2. Experimental

2.1. Materials used

Material used are phosphorous-deoxidized copper tube (C1220-H) and stainless steel tube (SUS304) with 500µm
outer diameter and wall thickness of 100 µm. Fig. 1 shows stress-strain curves of microtubes obtained from the tensile
tests. It is seen that the tensile strength of stainless steel tube is around two times higher than that of copper tube and
the elongation shows opposite characteristic. The length of test piece for MTHF experiment is 3.2mm.

2.2. Micro Tube Hydroforming (MTHF) setup and experimental procedure

The structure and assembly of experimental setup for T- and cross-shapes MTHF are the same as the cross-shape
MTHF system [9-12]. Fig. 2 shows schematic of MTHF system for T- and cross-shaped microcomponents. Fluid
media for internal pressure is provided by ultrahigh pressure generator, and the fluid medium insert into the inside of
tube through the micro notches on the surface of axial punch. [9, 10]. Axial feeding for microtube can be executed by
manually rotating screw feeder. The die for the MTHF is split in two parts symmetrically on left-right along axial
cross section of T- and cross-shaped geometry. The three trials are carried out for each experiment.
 Ken-ichi Manabe/ Procedia Engineering 00 (2017) 000–000 3
2336 Ken-ichi Manabe et al. / Procedia Engineering 207 (2017) 2334–2339

1400
Stainless steel
1200
(SUS304)

Nominal stress /MPa

1000
800 Phosphorous-
600 deoxidized copper
400
(C1220)
200
0
0 0.02 0.04 0.06 0.08
Nominal strain

Fig. 1. Stress-strain curves of microtubes obtained by tensile test.

a b Die split line

φ=0.51

r=0.3
t=0.1

D=φ0.5

L=0.51

Die split line

φ=0.51

r=0.3
t=0.1

D=φ0.5

L=0.51

Fig. 2. Schematic of MTHF system for T- and cross-shaped microcomponents. (a) MTHF system; (b) T- and cross shaped die.

Fig. 3 shows the loading path adopted in this study. This indicates a basic loading path pattern that includes sealing
stage (1) in which axial feeding continues up to L1 without fluid leakage, pressurization stage (2) where internal
pressure is increased until a certain level ph under the constant axial penetrationL1, and feeding stage (3) where axial
feeding is increased up to L2 under the constant ph. In this experiment, required axial feeding that prevents the leakage
of pressurized fluid is 𝛥𝛥L1=150µm for copper tube and 𝛥𝛥L1=450µm for stainless steel tube. As lubrication conditions,
no lubricant and fluorocarbon sprayed conditions are chosen in the experiments.

(3)Feeding stage
Internal pressure p

(2)Pressurization
stage
(1)Sealing stage

𝛥𝛥 𝛥𝛥
Axial feeding of each punch ΔL
Fig. 3. Loading path used in MTHF process.
 Ken-ichi Manabe et al. / Procedia Engineering 207 (2017) 2334–2339 2337
4 Ken-ichi Manabe/ Procedia Engineering 00 (2017) 000–000

3. Results and discussion

3.1. Influence of materials

Fig. 4 shows the appearance and the axial cross section of T-shaped hydroformed part at different deformation
stages for copper tube. It is observed that buckling and folding deformation occur at the bottom side at the different
MTHF stages. On the other hand, it can be observed for stainless steel microtube that buckling deformation does not
occur at the bottom side despite of similar hydroforming stage as shown in Fig. 5. Copper microtube has lower yield
stress and tensile strength than stainless steel microtube. This means that copper microtube easily buckles under axial
compression compared with stainless steel microtube. It is seen that high tensile strength and strain hardening material
has good formability for asymmetric T shape MTHF deformation.

a b c
500μm
500μm

Fig. 4. Appearance of T-shaped parts and axial cross section for copper microtube. (a) ph=10MPa, L=900m; (b) ph=50MPa, L=450m;
ph=120MPa, L=700m.

Fig. 5. Appearance of T-shaped hydroformed part and axial cross section for stainless steel tube (ph=70MPa, L1=450μm  2=750μm).

3.2. Influence of lubrication condition

Fig. 6 shows the influence of lubrication condition in T-shaped MTHF process on microformability. The bulging
height in fluorocarbon sprayed condition is two time higher than that in no lubrication condition. Friction, which
normally become a problem in micro scale due to the friction size effect, is also the important factor in MTHF to
improve the bulging characteristics.

a b

Fig. 6. Influence of lubrication condition in T-shaped MTHF process. (a) fluorocarbon sprayed using; (b) no lubrication.
2338 Ken-ichi Manabe et al. / Procedia Engineering 207 (2017) 2334–2339
Ken-ichi Manabe/ Procedia Engineering 00 (2017) 000–000 5

3.3. Influence of formed shape

Fig. 7 shows the appearance and axial cross section for T- and cross shaped samples. In case of cross-shape forming,
deformation flow takes place in both opposite two directions easily as shown in Fig. 7(b). However, in case of T-
shape forming, the deformation is one way and much thickening occurs because the bottom side is no flow and only
in compressive as shown in Fig. 7(c).
From the above difference of deformation flow in both formed shapes, we can suggest that total bulging height is
different between both formed shapes. Fig. 8 shows the relation between bulging height and axial feeding in both
shapes MTHF processes. The bulging height of T-shape forming is higher than that of cross-shape forming. It can be
easily understood that the material flow at bottom side is shifted to the bulging direction in T-shape forming. It means
the bulging height in T-shape forming is higher than that in cross-shape forming.

a b c
100μm
500μm

500μm 500μm

Fig. 7. (a) appearance of T- and cross-shaped micro samples; (b) one side of axial cross section for cross-shaped sample;
(c) axial cross section for T-shaped sample.

One side height for cross-shape

Hf
h=(Hf －tube dia.)/ 2

1400
T- Cross-
1200
shape shape
1000 T-shape
height h/ μm

h=1.3437L-186.61 Success
800 Wrinkling & buckling
600 Fracture
400 h Cross-shape
200 h=0.852L-120.58
0
0 250 500 750 1000 1250
Axial feeding ΔL / μm
Fig. 8. Relation between bulging height and axial feeding in cross- and T-shape MTHFs.

3.4. Grain size of micro tubes

Fig. 9 shows the microstructures of initial copper microtube and at the bulged crown part of micro T-shaped sample
fabricated by MTHF process (ph=50MPa, ΔL1=150m, ΔL=580m). Despite the fact that original microtube has
approximately 20 grains along to thickness direction and its microstructure is lamellar texture, the microstructure after
processing also has 20 grains along to thickness and the microstructure has not changed much. It is almost similar to
the initial state. Therefore, it is suggested that the deformation behavior could be the same as macro-scale deformation
because the size effects on material and friction do not occur for the materials.
a b

Fig. 9. Microstructures of initial microtube and micro T-shaped sample fabricated by MTHF process (ph=50MPa, ΔL1=150m, ΔL=580m).
(a) initial; (b) after MTHF process.
 Ken-ichi Manabe et al. / Procedia Engineering 207 (2017) 2334–2339 2339
6 Ken-ichi Manabe/ Procedia Engineering 00 (2017) 000–000

4. Conclusions

T- and cross-shapes MTHF experiments were carried out to confirm the dominant influencing factors in T- and
cross- shapes MTHF processes in this study. Materials, lubrication condition, hydroformed shape and grain size are
focused on as influential factors for microforming characteristics in the experiment. For the purpose, phosphorous-
deoxidized copper tube (C1220-H) and stainless steel tube (SUS304) with 500 µm outer diameter and wall thickness
of 100 µm are employed in this study. It was found that:
• Stainless steel with high tensile strength has high microformability compared with copper microtube, especially
for T-shape MTHF process.
• Lubrication condition is an important factor for high micro formability even though MTHF process is in fluid
pressure environment.
• It is found that the formed shape manages the material flow so that T-shape MTHF gains larger bulging height
compared with cross-shape MTHF.
• When a microtube that a number of grains at wall thickness is over 20, it is confirmed size effect does not occur in
MTHF.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number JP16K14440. The authors express their thanks to
Naoya Kikuchi and Kanta Sasaki of Tokyo Metropolitan University for conducting additional experimental work and
microstructural observation of the samples, respectively.

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