Qin Y. Micromanufacturing Engineering and Technology

Подождите немного. Документ загружается.

The machine enables the micro-stamping/

forming of sheet-metal parts (ideally for sheet

metals of a thickness of less than 100 mm). The

machine has a maximum working space of

400 mm  400 mm with a flexible set-u p, due

its modular design (the ram-driven form/power

is changeable, without the need to change other

machine set-ups; four machine-frame columns

and supports to the ram guiding bridge can be

repositioned according to the requirements, as

well as the sheet metal feeder, and the part car-

rier). The bridge for guiding the ram is separated

from the main machine frame, and hence it is not

affected significantly by the deflection of the main

frame and by vibration. Other innovations

include monitoring the displacement directly on

the tooling (therefore the punch stroke can be

controlled more accurately), transporting the

miniature/micro-parts directly out of the dies by

a part carrier, a new vacuum/compression air

chamber design, a new sheet metal holding

design, etc. The machine design was supported

by finite element dynamics analysis, which led to

the development of a bench-top machine that has

very good dynamic performance and machine sta-

bility (no connection to the bench is needed, and

no significant vibration is felt on the shop floor).

Micro-forming may also be effected with

micro-machines or similar set-ups, especially for

research purposes. This category of machines is of

much smaller size, compared to that of conven-

tional, large-scale presses. The development of

this type of machine has attracted much interest

from researchers during the last ten years [18–19].

Various new concepts are being experimented

with to design and fabricate prototypes of new

micro-machines. Force may be effected with

linear-motor actuation, piezoelectric actuation,

piezoelectric/hydraulic actuation, electromag-

netic launch and impact, etc. A micro-machine

prototype using a working principle of incremen-

tal micro-forming ha s also been developed in

Japan [14]. Small dents in sheet metal can be gene-

rated by hitting the metal with a small punch

installed at the end of a swinging arm. By repeated

hammering, incremental deformation can be

achieved across small areas of thin sheets. Other

developments include the use of combinations of

piezo-actuating with hydraulic devices to amplify

the punch stroke [20–21], e.g. the piezoelectric-

driven press developed by Zentrum Fertigung-

stechnik Stuttgart (ZFS), Germany. A mechanical

micro-press has been devel oped by the Mechani-

cal Engineering Laboratory [22], Ministry of

Trade and Industry, Japan. The machine has a size

of 111 mm  66 mm  170 mm, and is powered

by an AC servo motor of 100 W rated power

which can generate a force of up to 3 kN.

The transmission is effected by a ball screw/nut

structure, plus timing pulleys and belt s. A micro-

progressive die enables four blanking and two

bending strokes. The stroke and speed of the

machine can be controlled and 60 strokes per

minute achieved.

DESIGN CONSIDERATIONS

FOR MICRO-SHEET METAL PARTS

FOR FORMING

Various metals are possible for making micro-

sheet parts, including copper, brass, stainless steel,

low carbon, mild and high strength steel, alumi-

num, nickel, etc. Fine grain sheet metals are pref-

erable for micro-sheet forming, due to the reasons

explained in Manufacturing Processes and Funda-

mentals, above. The description/characterization

of material properties has to refer to the scaling

factor for a particular part, material, process and

tooling, the values of which are normally expected

to be obtained through a series of tests/measure-

ments. The strength, relationship of stress and

strain, anisotropy factor, rate-dependent proper-

ties, springback behavior, thermal properties, etc.

of the metal to be used are required to be corrected

from the descriptions/definitions, which are based

normally on a macro-scale.

General product design considerations include:

1. Thin sheet metal parts/structures should have

sufficient strengths and stiffness, in relation to

the functional and performance require-

ments;

2. Thin sheet metal parts should have good

dimensional accuracy, with reference to the

applications;

CHAPTER 8 Forming of Micro-Sheet-Metal Components 141

3. Thin sheet metal parts should have good sur-

face finish, and precoating may be needed,

depending on the application;

4. Relatively low cost manufacturing may be a

key factor to be considered;

5. The design should consider the manufactur-

ing constraints to the smallest features and

tolerances, taking tool-making and tooling

construction capabi lities into account;

6. The design should consider the manufactur-

ing constraints to the closeness of neighboring

smaller features (minimum web sizes) and the

number of features;

7. The design should consider the specific aspect

ratios achievable in stamping, considering

size effects in micro-stamping;

8. The design should consider specific bending

radius, considering size effects in micro-bend-

ing;

9. The design should consider the limiting spe-

cific drawing ratios for deep drawing, and the

feasibilities of redrawing, considering size

effects in micro-deep drawing;

10. The design should consider the specific

embossing/coining/d enting ratios achievable

in embossing/coining/incremental forming,

considering size effects in these processes; and

11. The design should consider proper material

selection an d the availability of suitable mate-

rials, together with the ‘size-effects’ associ-

ated with their mechanical properties, sur-

face/interface properties, and their influence

on process desi gn and tool design.

MICRO-SHEET FORMING – A CASE

STUDY

In the following case study, the forming of a micro-

sheetpart–asheetmetalspring(Fig. 8-11), is

described. The part has overall dimensions of

2.4 mm  3.0 mm, a sheet thickness of 50 mm,

and it is used in a micro-device. First, the part design

was assessed and individual features identified for

punching, blanking and bending/forming. Sub-

sequently, the stamping techniques were examined

in detail before developing a suitable strip layout

for progressive die stamping of the part. Five-stage

stamping was employed, which included:

1. Punching to create carrier tabs and holes for

pilot pins;

2. Blanking to create two side slots;

3. Forming of a center curve (bending);

4. Forming of edge features (bending/bottoming);

5. Blanking of the final pa rt.

FIGURE 8-11 The sample part (left) and an FEA study on one of the micro-dies (right).

142 CHAPTER 8 Forming of Micro-Sheet-Metal Components

The punch and die designs were supported

with FE analysis (Fig. 8-11). A high speed tool

steel (hardened to a value of 62-64 HRC) was

selected to fabricate the individual punches and

dies, which were created using mechanical mill-

ing, followed by m-EDM, and then precision

grinding to allow the required tolerances to be

achieved. A dedicated micro-forming tool system

with a footprint of 250 mm  160 mm was

designed and fabricated (Fig. 8-12), with the

individual punches and dies designed as inserts.

The tooling incorporates a specially designed

blank-holder to hold the strip during the stamp-

ing and four high precision ball race guide units

as the primary means of aligning the punches

and dies. The formed features were generated

using polyurethane pads and profiled punches.

The pads under pressure adopt the negative

feature of the solid punch and form the material

of the strip.

The micro-forming tool was designed for use in

a dedicated micro-forming machine (Fig. 8-12),

capable of providing high stamping rates (up to

1000 strokes per minute) while providing accu-

rate control over the ram position. A load cell and

positional encoder are integrated into the machine

to allow for data collection from the stamping

process and, additionally, for monitoring of the

punch force to indicate breakages or problems in

the system.

Brass and stainless steel were both used to pro-

duce the micro-spring (Fig. 8-13). The scrap was

collected through a vacuum system which is con-

nected to the die blocks directly, while the parts

were transported out of the final-stage die via a

tape/reel transport arrangement [8].

FIGURE 8-12 The micro-forming tool (left) and the micro-forming machine (right), developed at the University of

Strathclyde.

FIGURE 8-13 Samples of the formed micro-components

in brass and stainless steel

CHAPTER 8 Forming of Micro-Sheet-Metal Components 143

TECHNOLOGICAL

COMPETITIVENESS

AND OPERATIONAL ECONOMICS

Micro-fabrication in the past dealt with largely

silicon-based materials due to economic consi-

derations and the unique properties of these mate-

rials for micro-devices and systems. There are now

increased demands on non-silicon materials, e.g.

polymers and metals, due to the need of multi-

functional materials for the evolvement of multi-

functional micro-systems and devices. There

are various technologies available for the micro-

fabrication of metals, among these micro-sheet

forming renders unique advantages over other

technologies. At the same time, micro-sheet form-

ing also competes with other technologies in the

manufacture of micro-sheet metal parts, including

photonic etching and electro-forming technology,

electron-beam lithography etching, laser micro-

machining. It will be difficult for micro-sheet form-

ing to compete with photo-chemical etching/

electro-forming, in the areas of fabricating 2D

and 2.5D parts with smaller dimensions and finer

features such as several to tens of microns in

size, and thinner metal sheets, e.g. below 10 to 20

microns. Micro-sheet forming of these may be ex-

tremely expensive, or almost impossible to achieve.

It is also difficult to compete with laser machining,

in terms of the flexibility of the process set-up, the

type of materials that can be processed, and when

finer geometries are involved. Micro-sheet form-

ing, however, is able to produce metal parts with

better integrity, and less deleterious effects on the

properties of the material. It is also ideal for pro-

ducing various 3D structured parts and connec-

tions. Most importantly, it can be implemented at

a mass production scale (for suitable component

forms) for which other processes/technologies

cannot compete, if corresponding tooling techno-

logies and machine capabilities are able to meet

the manufacturing requirements. Good examples

of applications include the high speed stamping of

lead frames for electronics products and the stamp-

ing of micro-laminates for micro-motors, etc.

Table 8-1 presents a comparison of the costs

quoted for using photo-chemical etching and

micro-stamping, to produce a 2D flat sheet metal

part.

Without consid ering other merits of micro-

sheet forming, just referring to the manufacture

of this particular part, the data presented in this

table suggests that the investment on tooling for

the first batch production is extremely high for

micro-sheet forming. However, for producing

over one million pieces of this part, micro-sheet

forming does have an advantage. Considering the

feasibility of producing 1000 pieces of such a part

(simple micro-blanking) per minute with micro-

stamping, the advantage of micro-sheet forming

in mass production is evident. Such an example

does, however, not suggest a general calculating

principle, in terms of comparison of costing. It

should depend on the actual component form to

be considered. Currently, there is a trend in com-

bining photo-chemical etching, laser machining

and micro-sheet forming, in a process chain, to

achieve the greatest efficiency of manufacturing.

The engineering applications of micro-sheet

forming are not isolated issues. To enable realiza-

tion of the potential of micro-sheet forming, in

terms of its technological competitiveness and

economic advantages, one should be able to man-

age the whole manufacturing and operation chain

properly, including, in the chain, material supply

and characterization of the material properties,

TABLE 8-1

Company Technology Quality Quotation Other Costs

A Photo-chemical etching 10,000 £600 Tooling £125

B Photo-chemical etching 10,000 £488 Tooling £395

C Micro-stamping 10,000 £349 Tooling £20,000 (first batch)

144

CHAPTER 8 Forming of Micro-Sheet-Metal Components

stock preparation, forming process select ion/

design, forming tool selection/design and fabrica-

tion, machine selection/set-up, process/machine/

tool control, material/parts/tool handling, post-

processing, linking to other processes/equipment,

etc. The quality of the sheet metal and the quality

of the tooling are extremely important, especially

for the forming of thin sheet metals (thickness

below 100 microns), smaller part sizes (several

millimeters) and finer features (sub-millimeters).

Fine grain sheet metals could be expensive and the

tooling cost could be extremely high, while the

tool life could be very short, due to the fragility

of the small tools employed, such as slender

punches, etc. Therefore, the development of over-

all consideration for the manufacture of a partic-

ular part is needed, which should take a balanced

view of the manufacturing economics.

REFERENCES

[1] M. Geiger, M. Kleiner, R. Eckstein, N. Tiesler, U.

Engel, Microforming, Annals of CIRP 50(2) (2001)

445–462.

[2] F. Vollerston, Z. Hu, H. Schulze Niehoff, C. Theiler,

State of the art in microforming and investigations

into micro-deep drawing, J. Mats. Proc. Tech 151

(2004) 70–79.

[3] U. Engel, S. Gei

rfer, Microforming technology –

on the way to industrial application, Keynote paper,

Proc. 1st Int. Conf. on Micro-Manufacturing,

Urbana-Champaign, USA (Sept. 2006) 21–30.

[4] EU Masmicro Consortium, Project Technical

Reports (2004-2008), NMP2-CT-2004-500095.

[5] Y. Qin, Micro-forming and miniature manufacturing

systems – development needs and perspectives, Key-

note paper, 11th Int. Conf. of Metal Forming, Sept.

2006, J. Mats. Proc. Tech. 177 (2006) 8–18.

[6] Y. Qin, Development of an integrated manufacturing

facility for mass-manufacture of miniature/micro-

products, Keynote paper, Proc. 1st Int. Conf. on

Micro-Manufacturing, Urbana-Champaign, USA

(Sept. 2006) 35–40.

[7] Y. Qin, Advance in micro-manufacturing research

and technological development, and challenges/

opportunities for micro-mechanical machining, Key-

note paper, The Cutting Tool Congress 2007, Milan

(Nov. 2007) 1–8.

[8] Y. Qin, Y. Ma, C. Harrison, A. Brockett, M. Zhou,

J. Zhao, F. Law, A. Razali, R. Smith and J. Eguia,

Development of a new machine system for the

forming of micro-sheet-products, Proc. of the ESA-

FORM Conf.

[9] M. Geiger, F. Vollertsen, R. Kals, Fundamentals on

the manufacturing of sheet metal microparts, Annals

of the CIRP 45(1) (1996) 277–282.

[10] M. Gr

€

unbaum, J. Breitling and T. Altan, Influence of

high cutting speeds on the quality of blanked parts,

Report No. ERC/NSM-S-96-19 (1996) The Ohio

State University.

[11] S.S. Kima, C.S. Hana, Y.-S. Lee, Development of a

new burr-free hydro-mechanical punching, J. of

Mater. Process. Technol 162–163 (2005) 524–529.

[12] N. Matsushita, Laser micro-bending for precise

micro-fabrication of magnetic disk-drive compo-

nents, Int. Sympo. on Laser Precision Microfabrica-

tion, No. 4, Munich 5063 (2003) 24–29.

[13] H.-W. Jeong, S. Hata, A. Shimokohbe, Microform-

ing of three-dimensional microstructures from thin-

film metallic glass, J. of Microelectromechanical

Systems 12(1) (2003) 42–52.

[14] Y. Saotome, T. Okamoto, An in-situ incremental

microforming system for three-dimensional shell

structures of foil materials, J. Mats. Proc. Tech 113

(2001) 636–640.

[15] S.I. Oh, S.H. Rhim, B.Y. Joo, S.M. Yoon, H.J. Park, T.

H. Choi, Forming of micro channels with ultra thin

metal foil by cold isostatic pressing, 5th Japan-Korea

Joint Symposium on Micro-Fabrication (2005) 50–54.

[16] http://www.bruderer.com/bruderer-high-perfor-

mance-presses/bsta-300, visited in May 2009.

[17] http://www.schulergroup.com/de/05_Extras/

02_Publikationen/Broschueren_Anlagen_Herstel-

lung_Elektrobleche/Download_Anlagen_Herstel-

lung_Elektrobleche/extras_download_flyer_schnell-

laeuferpressen_stanzrapid_linearantrieb_d.pdf.

[18]N.Mishima,K.Ashida,T.Tanikawa,H.Mae-

kawa, Design of a microfactory, Proceedings of

the ASME Design Engineering Technical Confer-

ence – 7th Design for Manufacturing Conference,

Sept. 29–Oct. 2, Montreal, Qu e., Can ada 3 (2002)

103–110.

[19] Y. Okazaki, N. Mishima and K. Ashida, Microfac-

tory and micro machine tools, presented in the 1st

Korea-Japan Conference o n Positioning Technol-

ogy, Oct. 15-17, KIMM, Daejeon, Korea (20 02)

1/6-6/6.

[20] J.H. Park, K. Yoshida, Y. Nakasu, S. Yokota, A

resonantly-driven piezoelectric micropump for

microfactory, Proc. ICMT20002, Kitakyushu

(2002) 417–422.

[21] A. Hess, Piezoelectric driven press for production of

metallic microparts by forming, ACTUATOR 2000,

7th International Conference on New Actuators,

June 19-21, Bremen (2000) 427–430.

[22] http://www.mel.go.jp/.

CHAPTER 8 Forming of Micro-Sheet-Metal Components 145

Micro-Hydroforming

Christoph Hartl

INTRODUCTION

Hydroforming is a metal forming technology

based on the application of pressurized liquid

media t o generate defined workpiece shapes

from tubular materials or sheet metals. Numer-

ous industries, for example the manufacturers

of automotive components and the piping

industries, which are providing mass products,

apply this t echnology productively today [1].

This successful adoption of hydroforming tech-

nology results from the advantages that hydro-

forming offers in compa rison to c onventional

techniques such as the assembly of stampings

by welding. Hydroforming provides the possi-

bility to form hollow complex-shaped compo-

nents with integrated structures from single ini-

tial workpieces, combined with improvements

in stiffness and strength behavior due to the

reduction of welding seams, and with reduced

assembly costs [1].

Due to an increasing integration of micro-

system technology into products of electronics,

telecom and medical devices for an ever-grow-

ing market, efficient and timesaving production

technologies for micro-components become

increasingly more important. Against this back-

ground, metal forming plays a decisive role for

the mass production of respective components

because it c an offer the required productivity

and accuracy. Concerning the mass produc-

tion of hollow-shaped micro-components, this

applies to hydroforming technology in cases

where the manufacture of such parts currently

still largely relies on time-consuming techniques

based on the removal of material, either by

chemical or mechanical means. Important

existing technologies for the manufacture of

complex-shaped hollow miniature and micro-

components made f rom metals are, for exam-

ple, techniques using electrodeposition of m etal

materials on subsequently removed mandrels

(electroforming) [2] or rapid manufacturing

techniques such as selective laser me lting of

metal powder ( SLM) [3]. Although these pro-

cesses can offer the fabrication of high preci-

sion components, their application is limited to

small and low volume production due to a

comparatively high cycle time. Techniques

working with the assembly of micro-surface

structured component halves (for example,

structured by laser or etching processes) which

are used for the production of hollow-shaped

metal parts are predominantly restricted to pla-

nar workpieces. Also micro-injection molding

techniques (MIM) [4] enable the manufacture

of complex-shaped components; however, these

are limited to those with comparatively simple

hollow inner shapes due to restricted pos sibi li-

ties in core design.

The design and optimization of micro-hydro-

forming processes require knowledge of the fun-

damentals to determine the necessary process

loads, to estimate feasibility, and to obtai n an

improved comprehension of influences on the reli-

ability and quality of component manufacturing.

Additionally, size effects have to be taken into

account when scal ing down conventional hydro-

forming processes to micro-size. The objective of

CHAPTER

146

this chapter is to provide an overview of the

respective hydroforming fundamentals and

detailed information relevant for the practical

application of micro-hydroforming.

PRINCIPLE OF HYDROFORMING

AND PROCESS VARIANTS

Concerning existing hydroforming processes, a

general distinction is to be drawn between the

forming of tubular material such as straight as

well as bent tubes or profiles, and the forming

of sheet material, for example single or multiple

sheets. Currently, tubular material is predomi-

nantly applied for the manufacture of hydro-

formed components. The principle of these hydro-

forming processes is represent ed in Fig. 9-1. At the

beginning of the proces s the initial part is placed

into a die cavity which corresponds to the final

shape of the component. The dies are closed with

the closing force F

while the tube is internally

pressurized by a liquid medium with internal pres-

sure p

to effect the expansion of the component.

Additionally the tube ends are axially compressed

by sealing punches with axial force F

to force

material into the die cavity. The component

is formed under the simultaneously controlled

action of internal pressure and axial force.

Water/oil emulsions are typically used media to

apply the internal pressure, which is usually

increased to 1200 up to 4000 bar. The necessary

amount of internal pressure is influenced signifi-

cantly by the wall thickness of the component and

the material strength and hardening, as well as by

the component shape [1].

Further mechanical loads can be applied to the

workpiece, depending on the part and process

type. An example is the hydroforming of

T-shaped components, as shown in Fig. 9-2. This

process requires an additional counter-punch

with a suitable control of the counter force F

during the forming process. The counter-punch

acts on the end of the expanded protrusion and

is displaced by the workpiece when the hereby

exerted force achieves the current level of the

counter-force.

FIGURE 9-2 Principle of T-piece hydroforming.

FIGURE 9-1 Hydroforming principle.

CHAPTER 9 Micro-Hydroforming 147

Regarding the currently applied hydroform-

ing process types, in [5] a first classification has

been developed considering the acting stress

state within the formed workpiece region and

the specific characteristics of the expanded

geometry. Based on this work, classifying engi-

neering standards have been enhanced and

updated r egarding the description of hydroform-

ing processes, for example the engineering stan-

dard of manufacturing technologies DIN 8580,

published by the German Institute for Standard-

ization (DIN).

For conventional hydroforming processes the

integration of additional manufacturing opera-

tions in the hydrofor ming process itself is used

to improve productivity. Industrial hydroformi ng

tools are often equipped with numerous piercing

units to create holes for bolts, drain holes, refer-

ence points, collar formed holes, etc. [6]. Addi-

tionally, assembly operations within the hydro-

forming process have been shown to be feasible,

for example, cam shafts [7,8].

SEMI-FINISHED PRODUCTS

AND MATERIALS

Currently, predominantly steel alloys and alumi-

num alloys are used as materials for the required

semi-finished products in hydroforming produc-

tion. Copper and brass alloys are used for hydro-

formed products in the piping and sanitary indus-

try. The applied alloys correspond in the majority

of cases to materials which are used for common

cold forming processes such as deep drawing or

mass forming. In principle, all metal materials

with sufficient formability are suitable for semi-

finished products in hydroforming processes. A

fine-grained struc ture combined with large

amounts of uniform elongation and elongation

at fracture and a large strain-hardening coefficient

are advantageous in the feasible expansion of the

initial workpiece, achievable without the occur-

rence of material instabilities. The strength of the

final component is improved by a distinctive

work-hardening of the formed material; however,

work-hardening also causes an increase in the

required forming loads.

Steel alloys used or tested for conventional

hydroforming components are ductile low carbon

steels, case-hardened steels, heat trea table steels,

ferritic and austenitic stainless steels as well as

high strength and ultra-high strength steels, for

example [9]. In general, tubular steel materials

which are used for hydroforming applications

are produced from flat sheet material by continu-

ous roll forming and longitudinal high frequency

welding to close the roll formed tubular cross-

section. Tubes with circular cross-section as well

as profiles which differ from a circular shape are

able to be generated by the roll forming process

using appropriate roll forming tools. However,

predominantly semi-finished products with circu-

lar cross-sections are currently in use for the

hydroforming production of steel components.

Typical dimensions of conventionally hydro-

formed steel tubes are outer diameters d

between

about 20 mm and 140 mm with ratios of wall

thickness to outer diameter t

between about

0.012 and 0.16. Regarding micro-hydroforming,

the market currently provides roll formed and

welded metal micro- tubes with minimal outer

diameters of about 0.2 mm and minimal wall

thickness of abo ut 0.03 mm.

When selecting appropriate tubes for hydro-

forming processes, a distinction is to be drawn

between tubes without an annealing process

after cold forming by roll forming or drawing,

tubes drawn with a small resulting strai n after a

preceding annealing process and tubes annealed

after the final cold forming operation. Drawing

processes, following the roll forming operation,

serve for the adjustment of the final tube diam-

eter and/or wall thickness as well as providing an

increase in strength due to work-hardening

effects.

Drawn and non-annealed tubes comm only

provide reduced formability in hydroforming pro-

cesses, depending on the characteristics of the

steel alloy used and the amount of strain induced

by the drawing operation. Tubes which have been

drawn with a small resulting strain after annealing

show a cold form ability within certain limits. The

most extensive cold formability is obtained by

the use of tubes which have been annealed after

148 CHAPTER 9 Micro-Hydroforming

the final cold forming operation such as roll form-

ing or drawing.

To avoid premature bursting of the workpiece

within the hydroforming process, a highly satis-

factory weld seam quality is required for roll

formed and welded tubes. It is recommended

to avoid locating the we ld seam in the final

hydroformed component within areas where

excessive tensile stresses due to the expansion

are acting on the component during the hydro-

forming process.

Figure 9-3 shows an example of a hydroformed

micro-prototype part made from a solution

annealed stainless steel tube. The initial tube with

an outer diameter of 0.8 mm and a wall thickness

of 0.04 mm had been manufactured by con tinu-

ous roll forming and subsequent drawing and

annealing processes.

Concerning the use of aluminum alloys for

conventional hydroforming applications, work-

hardening aluminum 5000 alloys are currently

used when priority is given to a high amount of

formability and corrosion resist ance, whereas

precipitation-hardening aluminum 6000 alloys

are applied for components requiring high

strength, e.g. [10]. In general, tubes made from

aluminum 5000 alloys are manufa ctured from flat

sheet material by continuous roll forming with

longitudinal welding, whereas aluminum 6000

alloys are produced as extruded profiles.

Extruded profiles offer advantages in design flex-

ibility for complex cross-sections with sharp cor-

ners, multiple hollows and flanges. However, the

reduced formability of these semi-finished pro-

ducts has to be considered when designing a

respective hydroformi ng component. Addition-

ally, the selection of extruded material for hydro-

formed micro-components is currently restricted

by the minimal cross-sectional dimensions that

can be produced by the relevant industries. Cur-

rently, the manufacturing of micro-extrude d pro-

files as semi-finished products is the subje ct of

several investigations, for example [11].

Due to their high strength-to-weight ratio,

magnesium alloys offer a great potential for

weight-reduced components. However, the use

of these alloys in forming processes working at

room temperature is limited due to their hexago-

nal atomic structure. An improvement in form-

ability is achieved by the use of increased tempera-

tures, above about 200



C, when additional

gliding planes become activated. Against this

background, various investigations into the con-

ventional hydroforming of semi-finished products

made from magnesium alloys by the use of an

elevated temperature have been carried out during

the last few years, e.g. [12].

The design of hydroforming proces ses as well

as the monitoring of semi-finished product quality

in hydroforming production require suitable and

reliable methods to obtain material parameters

characterizing the forming behavior. Concerning

conventional tube hydroforming, predominantly

traditional material-testing methods are currently

in use, such as tensile tests, mechanical expansi on

methods, and grid analysis. However, the suitabil-

ity of these methods is often limi ted, as the typical

biaxial stress state in hydroforming processes is

not, or is only approximately, reproduced.

Most common in use to characterize the form-

ing behavior of the applied tubular material is the

tensile test which is a standardized uniaxial mate-

rial test method. A distinction is to be drawn

between the application of this test to the initial

sheet material before roll forming and the appli-

cation to the roll formed and welded workpieces.

Testing the initial sheet material means that

changes in material properties due to the

manufacturing process of the tube remain uncon-

sidered.

A method for strain analyses in hydroformed

components consists in the application of circular

or quadratic grids on the surface of the initial

semi-finished product. The measured distortion

of the individual grid elements at the hydroformed

workpiece enables the determination of local

FIGURE 9-3 Micro-hydroformed component.

CHAPTER 9 Micro-Hydroforming 149

strains, which provides an assessment of the

hydroforming process when comparing the ana-

lyzed strains with the forming limit curve of the

respective tube material, e.g. [13]. There are

restrictions in the use of this method in micro-

hydroforming processes due to the minimal appli-

cable grid size on micro-tubes.

An example of a standardized mechanical

expansion testing method is the cone test, where

the end of the investigated tube is expanded by a

conical punch until fracture occurs. This test

enables the principal determination of formabil-

ity, for example to compare different batches of

tubular material. Also, failures at the tube surface

or within the weld seam are able to be detected.

When applying this test method it has to be taken

into consideration that variations in friction con-

ditions or unequal prepared surface roughness

at the tube end face influence the initiation of

fracture of the expanded tube section. Figure 9-4

shows results of mechanically expanded micro-

tubes made from stainless steel AISI 304. These

solution-annealed tubes with an outer diameter of

0.8 mm and a wall thickness of 0.04 mm showed

a possible expansion of the initial tube diameter

by this test method of about 33% [14].

To improve methods for the characterization

of tubes for hydroforming applications, several

investigations have been carried out into tube

expansion tests working with an inner pressuriza-

tion of the tested tube, which is clamped at its ends

according to Fig. 9-5. This bulge test enables the

determination of the bursting pressure p

, the

pressure-dependent expansion diameter d(p

)

and the achievable expansion diameter d

under

the biaxial tensile stress state. Strategies to deter-

mine the material properties of tubes as well as

their yield curves based on the bulge test have

been developed, for example in [15] and [16].

When applying the bulge test, it has to be taken

into consideration that the ratio of the expanded

tube length l

to the tube diameter d

influences

the required pressure to expand a tubular speci-

men, if the ratio l

is below a certain limit

[17,18]. The bulge test device shown in Fig. 9-5

has been developed for the testing of micro-tubes

with outer diameters below 1 mm and is suitable

to apply up to 4000 bar of internal pressure [19].

PROCESS CHAIN FOR PART

PRODUCTION

Depending on the component design, the in-

dustrial production of hydroforming parts re-

quires several additional manufacturing steps in

addition to the hydroforming process itself.

Figure 9-6 shows schematically a typical process

chain for the manufacture of hydroformed com-

ponents.

FIGURE 9-4 Expansion cone test and experimental results.

FIGURE 9-5 Bulge test device for micro-tubes.

150 CHAPTER 9 Micro-Hydroforming