metals

Editorial
Metal Micro-Forming
Ken-ichi Manabe
Department of Mechanical Systems Engineering, Tokyo Metropolitan University, 1-1 Minamiosawa,
Hachioji-shi 192-0397, Japan; manabe@tmu.ac.jp




Received: 8 June 2020; Accepted: 16 June 2020; Published: 18 June 2020


1. Introduction
Metal micro-forming is the technological field of micro-manufacturing. It is a microfabrication
technology for micro parts that takes advantage of the excellent mechanical and functional characteristics
peculiar to metal, and is a vital forming technology that has been scaled down to the microscale range
suitable for mass production. Yole Développement [1] predicts that the compound annual growth
rate (CAGR) of the micro electromechanical systems (MEMS) market in micro-manufacturing from
2018 to 2024 is 43.6% for telecom applications, 7.5% for medical applications and 11.0% for industry
applications. These are mainly related to the semiconductor market. Similarly, the market prediction
for metal micro-components with the excellent characteristics of metals is also expected to grow.
Investigations into metal micro-forming technology have been developing over the last 20 years.
These studies include research and development activity for micro-bulk forming, micro-sheet metal
forming, micro-hydroforming, laser-assisted micro-forming, surface engineering for die and tooling,
embossing and micro-sintering. Among them, the elucidation of the size effect associated with
scale-down in each processing method is one of the central issues, and its fundamental research and
new special processing technology has been developed.
The purpose of this Special Issue is to collect expert views and contributions on the present
achievements on metal micro-forming, and through this issue, to find present challenges and to offer
ideas for possible solutions.

2. Contributions
In this Special Issue, there are nine papers related to conventional metal forming processes,
and two basic testing and evaluation methods (surface roughening and evaluation of micro-tribological
characteristics with lubricants); a total of 11 papers. Their processing is categorized into nine methods:
tube forming, wire drawing, wire rolling, orbital forging, ring-shape forging, channel forming and
embossing. As a basic common characterization, it contains two themes: the micro-tribological
characteristics of surface texture on a micro die and the surface roughening evaluation test of raw
material. Some techniques are assisted by temperature (high-frequency induction heating and
resistance heating) and others by ultrasonic vibration, hydroforming and shock wave liquid medium
high-energy-rate processing. Examples of copper, titanium, aluminum, stainless steel and the metallic
powders of stainless steel and aluminum are used for medical and communication fields.
So far, relatively few review articles on the forming technology of micro-tubes have been
published. It is welcome, therefore, that Hartl [2] reviews and classifies the recent progress in
research and development on the fabrication and secondary metalworking methods of micro-tubes.
The various micro tube manufacturing methods include various heat assisted dieless drawing methods,
micro tube manufacturing by conventional technology, micro tube fabrication for special tubes and the
latest manufacturing technology for micro hydroforming technology for T-shapes and cross shapes,
laser heating assisted bending processing, micro tube processing technology using severe plastic
deformation and micro tube characteristic evaluation methods. A total of 117 papers are surveyed and

Metals 2020, 10, 813; doi:10.3390/met10060813 www.mdpi.com/journal/metals

summarized. Yasui et al. [3] focus on micro T-shape hydroforming of 0.5 mm diameter micro-tubes and
examine the effects of the friction and the length of micro-tube on their hydroformability and material
flow, using both experiments and finite element analysis. They confirm that, for a short micro-tube,
the friction effect is suppressed and high hydroformability is obtained, but by increasing the length of
the tube and friction, the hydroformability decreases.
For microwires, Hwang and Liu [4] examine the effects of processing temperature and the oxide
layer on the mechanical properties of the drawn wire in microwire dieless drawing by high-frequency
induction heating using fine, 1 mm diameter stainless steel wire. Additionally, the influence of the
drawing speed and the processing temperature on the drawing limit is examined by the finite element
dieless drawing analysis. Xie et al. [5] examine the size effect of microstructure on the deformation
characteristics of microwires with a flat rolling outer diameter of 1.5 to 0.8 mm in flat rolling. The effect
of the surface layer effect on rolling resistance and the effect of wire diameter on the development of
surface roughness.
For micro forging, Presz [6] investigates the material flow related to ultrasonic orbital micro
forging, the material flow in different die shapes was clarified in detail by the application of 20 kHz
vibration with a 16 µm longitudinal amplitude at surface of punch nose to an aluminum cylinder,
with a diameter of 1 mm. Yang and Shimizu [7] develop an innovative resistance heating method
using a surface-modified punch and evaluate the heating characteristics. The coating effect of the die
in the ring-shape forging is tested by applying it to Ti, Cu, SUS304 materials using a die coated with
0.5 and 1 µm of AlCrSiN film, to improve heating characteristics.
As an application of high-energy rate micro-forming technology, Liu et al. [8] develop a laser
impact liquid flexible micro-forming process, which was taken up and applied to the forming of
a microchannel of copper foil material. The fabricated shape was compared with finite element
analysis results.
In terms of the micro metal compaction for micro sintering, Emadinia et al. [9] perform micro
hot embossing using aluminum and stainless steel metal powders with carbon nanotube (CNTs),
and examine the optimum conditions of the powder component blending ratio to make the green
compact uniform, and to improve the surface properties of the compact.
On the other hand, Shimuzu et al. [10] carry out research on basic micro-tribological characteristics
related to metal micro-forming technology, investigating the lubrication effect of dimple texture with
a diameter of 10, 50, 100 µm provided on the die surface, which is important for the tribological
characteristics in sheet forming, the bending and ironing experiment of 0.1 mm thick SUS304 foil
material. In addition to the finite element forming analysis and computational fluid dynamics analysis
(CFD) on die contact lubrication condition, an in-situ observation of lubricant flow is performed to
confirm the effect of a dimple array on the die. Furushima et al. [11] propose a new compression test
method for thin sheet metals, to characterize the surface roughening evolution under compressive
strain, and confirm the surface roughening behavior during the straining of thin sheet.
For micro-die manufacturing, Shiratori et al. [12] develop a manufacturing method of
micro-meshing punch array for micro-embossing on copper thin sheet by a low-temperature
plasma printing process. A 180 µm × 180 µm square prism array of a thin copper plate is formed
by micro-embossing.

3. Conclusions
This Special Issue provides a total of 11 articles from seven countries, including one review
paper. From these collected papers, it may be seen that micro-forming research in the world is steadily
progressing and developing, and it is possible to get a glimpse of the present situation and the academic
and technological issues. I hope that advanced metal micro-forming technology will further progress
through the elucidation of material deformation behavior in microscale, and that this Special Issue will
contribute to prosperous future research.
Metals 2020, 10, 813 3 of 3

I would like to express sincere thanks to all the contributors to this Special Issue. Additionally, to
the reviewers for making useful comments, and the Metal Editorial staff for prompt publishing.

Conflicts of Interest: The author declares no conflict of interest.

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