Qin Y. Micromanufacturing Engineering and Technology
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CHAPTER 9 Micro-Hydroforming 161
10
Laser-Assisted Micro-Forming
Jens Holtkamp
INTRODUCTION
Forming is one of the basic production technologies,
and due to the potential and low costs of the process
is widely spread in industrial applications. Never-
theless, there are specific limitations regarding the
workpiece materials and the maximum strain
within the forming operation. Brittle and high
strength materials cannot be accommodated with-
out high process complexity or low product quality.
By heating the used materials prior to the form-
ing process, with the corresponding change of
their material properties, these drawbacks can
be eliminated.
The use of laser radiation as the heat source
enables short heating times, due to the associated
high energy density. In addition, there are other
advantages such as contactless heating, good focu-
sability and good controllability lasers are therefore
becoming a promising alternative to existing tech-
nologies such as induction or conductive heating.
In principle, every forming technique can be
enhanced by laser technology. The basics for its
integration and some exemplified processes are
described in the following sections.
SYSTEM TECHNOLOGY
Heating of the workpiece can be achieved by dif-
ferent methods: conductive through the contact of
a heated tool, convective within a convection
oven, or inductive by swirling currents generated
in the material.
Pre-process heating of the workpiece outside of
the tool leads to an increase of temperature within
the whole part. Temperature gradients are not
adjustable; therefore, local heating of selected areas
of the part is not possible. Heating of micro-
components is difficult due to their low mass. This
lowheatcapacityleadstoacoolingdownafterthe
transport into and the contact with the tool.
The disadvantage of methods based on heat
convection is the bad controllability of the form-
ing temperature and the resulting long cycle times.
Similarly to the pre-heating method, the heating
of local areas of the workpiece is not possible.
By using a laser as the heat source, the induced
energy and the resul ting temperatures can be eas-
ily controlled by laser power. Heating of selected
areas is possible by forming the laser beam. The
high energy density of the radiation and the direct
absorption enable short cycle times. Therefore, a
laser is a promising tool to achieve the potential of
micro-forming at elevated temperatures.
The laser system causes the main additional
cost in comparison to the cost of conventional
machinery. Due to its low pric e in comparison
with that of other laser systems, a diode laser is
often favored, but there are also other effective
systems such as fiber lasers. If absorpt ion within
transparent workpieces is intended, the wave-
length of the laser has to be adapted.
The machinery used for laser-assisted pro-
cesses is based on conventional forming machines.
The system is then, beside the laser itself,
enhanced with components for the integration
and control of the laser. One basic additional ele-
ment is an optical system which is placed inside
the tool. Adapted to a fiber which conn ects the
CHAPTER
162
laser with this system, it guides the radiation
through the forming tool onto the surface of the
workpiece to be heated. If necessary, the shape of
the radiation can be adapted to the geometry of
the workpiece.
Figure 10-1 presents a schematic drawing of an
exemplified configuration.
The laser radia tion is first collimated and then
reflected on a dichroic mirror through a focal lens
towards the sheet metal surface.
By means of an additional camera, which is
arranged coaxially with the optical path, the posi-
tion of the laser radiation can be displayed on a
monitor. This enables an easy adjustment of the
tool to the optical system and the possibility of
process monitoring.
Knowledge of, and the possibility to control,
the process temperatures arising are important for
reproducible and accurate process results. Addi-
tional sensors can be used such as thermocouples
or pyrometers to detect these temperatures and, if
necessary, to submit them to a controller. The
value of a pyrometer depends on the emittance
of the material and its surface propert ies. There-
fore, it is necessary to ‘teach’ the system before
using new materials.
Filtering is necessary to divide the radiation
into the wavelengths that are important for the
corresponding detectors: while the camera
requires wavelengths in the visible range, the
pyrometer detects a range just above 1 mm.
If met al is used as the workpiece material, a
circular shape of the laser focus leads to a non-
uniform temperature distribution. Hot spots are
generated in the middle of the area to be heated,
whereas the border area remains cold due to the
high thermal conductivity of metals. By using an
Axicon within the beam bath, this drawback can
be substantially eliminated.
An Axicon is a rotationally symmetric optical
element which consists of a cylindrical part and a
cone, as shown in Fig. 10-2. The Axicon creates an
annulus as the focus geometry. As a result, a more
homogeneous temperature distribution can be
achieved. Depending on the angle of the Axicon
and the focal lengths of the lenses used, the ring
diameter can vary.
In addition to the optical system, the forming
tool also has to be adapted, in such a way that the
radiation can be guided onto the workpiece or the
punch, depending on the workpiece material. For
stamping operations the radiation can be easily
FIGURE 10-2 An Axicon.
FIGURE 10-1 Example of an optical path.
CHAPTER 10 Laser-Assisted Micro-Forming 163
guided through the tool matrix. No additional
elements are necessary, but it is important to
remove the punched parts from the optical path
at the end of the operation. Other forming pro-
cesses require ‘tool windows’. These can be made
of sapphire or fused silica which are both, on the
one hand, hard enough for the occurr ing pro cess
forces and, on the other hand, transparent for the
laser radiation (as long as it is not in the infrared
range). Through a breakout in the tool frame, the
laser radiation is transmitted through the sap-
phire/glass onto the workpiece or the punch.
All devices – the laser and the press – are con-
trolled by software which controls the laser,
depending on the position of the punch relative
to the tool and the required temperature, and
switches it off after a defined distance or time
has been reached.
In order to obtain short heating times and to
maintain a constant temperature, a temperature
controller can be used. The controller compares
the signal of a thermal sensor with the demanded
temperature and determines an according value
for the laser system.
PROCESSES
Stamping
Stamping is a well-established process which
enables the production of sheet metal or plastic
parts in very short cycle times (up to 40 parts per
second). In addition, the production of precise
and complicated contours is possible. The process
is characterized by a high material utilization [3].
The main components of a stamping tool usu-
ally consist of the punch and the female die (the
matrix), sometimes with an additional pressure
pad to build up compressive strength in the work-
piece. The cutting forces are transferred from the
surface of the punch and the female die to the
sheet metal, which leads to an elastic deforma-
tion. With increasing force the deformation
resistance is overcome and the elastic limit of the
material is reached, whereupon the punch pene-
trates into the sheet metal [7]. This plastic defor-
mation without separation leads to the rounded
edges at the material surface [3]. Due to the plastic
deformation in the cutting direction, the material
yields in lesser-loaded areas and leads to roll-over.
With increasing punch displacement, the roll-over
passes into a smooth shearing zone. Within zones
of high material stressing, cracks appear. This
leads to cleavage fracture and the creation of the
fracture zone [7] .
Figure 10-3 shows the three phases of the
stamping process [5].
The process, the workpiece material and the
production accuracy of the tool are crucial for
the quality of the stamped part. While a high
shearing zone is intended, different defects occur
on nearly every part. Figure 10-4 shows the dif-
ferent defects of the cut edge.
The height of the roll-over depends on the cut-
ting clearance and the yield/stress ratio . This ratio
is defined as the quotient of the yield stress and
the tensile strength. The roll-over increases
with increasing cutting clearance and yield/stress
ratio. Moreover, increase in cutting clearance also
affects the fracture zone.
FIGURE 10-3 Process sequence.
164 CHAPTER 10 Laser-Assisted Micro-Forming
The height of the burr, an indication of the
manufacturing quality, depends on the properties
of the workpiece material. Whereas ductile mate-
rials, due to their form ability, create a high burr;
the burr is smaller on brittle materials [5].
Figure 10-5 shows the active tool parts for the
stamping operation of gearwheels.
Depending on the temperature of the metal
workpiece, the processes are divided into cold,
warm and hot forming. Cold forming takes place
at ambient temperature. During the process the
mechanical properties of the workpiece materials
change, with the strength being most affected. The
formability decreases, while the forming resistance
increases, with a reduced cross-section.
Within the hot-forming processes the temper-
ature of the workpiece is above the recrystalliza-
tion temperature, which is estimated to be around
40 to 50% of the melting temperature [8] accord-
ing to the Tamman rule. Hereby, the formab ility
can be increased, whereas the required forming
forces are lower.
Warm forming takes place close to, but below,
this recrystallization temperature. Even a small
temperature increase leads to a reduction of the
yield strength within the material [9]. Higher tem-
peratures lead to higher flexibility of dislocation
and crystal regenerations, which causes a reduc-
tion of the dislocation density and the strain hard-
ening of the crystal lattice. In comparison to
recrystallization, the size and the position of the
crystallites remain unchanged.
The influence of different forming tempera-
tures on the material properties are shown in
Fig. 10-6. The yield stress and maximum strain
are displayed for room and for elevated tempera-
tures. This points out that greater working tem-
peratures ease plastic forming by reducing the
yield stress. The reduction of the yield stress can
be explained with the oscillation of atom crystals.
The possibility to move is constricted at room
temperature but increases at elevated tempera-
tures until the melting point is reached [6].
Hot forming enables processes to be carried out
that cannot be implemented at room temperature.
FIGURE 10-5 Active tool elements (manufactured by Fraunhofer IPK).
FIGURE 10-4 Defects of the cut edge.
CHAPTER 10 Laser-Assisted Micro-Forming 165
Nevertheless, it is not an entirely satisfactory
alternative to cold forming due to its disadvan-
tages: e.g. bad surface quality, inaccuracy in size
and high tool loading because of the high tem-
peratures. Since the whole tool is heated and not
just the forming zone, the whole component is
thermally loaded [10].
Warm forming uses the potentials of both the
cold- and hot-forming processes. These poten-
tials are the constant strain hardening and good
surface quality of cold forming and the low pro-
cess forces and the high formability of hot form-
ing [2]. H igh-alloyed steels are then plastically
formable with constant quality and high accu-
racy in size.
As an example, the magnesium alloy AZ31
(MgAl
3
Zn; 3.5312) is chosen as the test material.
The main alloying additions are aluminum and
zinc. Due to their addition, the tensile strength
and hardness are increased. However, the increased
micro-porosity is disadvantageous [4] for forming
operations.
Due to the hexagonal arrangement of the crys-
tals of magnesium, the formability at room tem-
perature is very low. Above 225
C the formability
increases through the activation of additional
gliding planes [1].
The measured force/displacement curves con-
firm the influence of the elevated workpiece tem-
peratures. The maximum punching force can be
reduced by 30% (Fig. 10-7).
Figure 10-8 shows two pictures made by a
scanning electron microscope. The quality of both
parts can be assessed concerning defects of form.
The cold-formed part has significant defects.
Many teeth are roughly removed, rather than
smoothly sheared. The burr and the large fracture
zones are indicators of bad quality. The warm-
FIGURE 10-6 Forming temperature versus yield stress
and maximum strain.
FIGURE 10-7 Force path.
FIGURE 10-8 Gearwheels stamped without (left) and with (right) laser assistance.
166 CHAPTER 10 Laser-Assisted Micro-Forming
formed gearwheel has nearly no burr and fracture
zones, but has a high shearing ratio.
Embossing
Conventional technologies for the manufacturing
of micro-structured components with geometries
below 100 mm are injection molding and hot
embossing. Both technologies are mainly limited
to polymer materials and require long cycle times
due to different temperatures during forming and
mold release. The se drawbacks can be eliminated
by laser-assisted hot embossing, where high
energy density laser radiation is used for a selec-
tive and fast heating of the tool and the work-
piece. Using controlled irradiation of the tool sur-
face, tool temperatures of more than 500
C can
be achieved within seconds. At these temperatures
even glass and metals can be structured with this
technology. In comparison to other heating tech-
nologies such as induction heating, laser-assisted
embossing enables the use of tool materials with
low thermal and electrical conductivity such as
ceramics.
In the use of transparent workpiec e mat erials
such as glass or polymers, the laser radiation
heats the structured die which, after achieving
the required temperature, is t hen pressed onto
the workpiece and heats it via heat conduction.
In the case of non-transparent workpiece mate-
rials such as m etals, the radiation is directly
absorbed within the workpiece. The cold die
then penetrates into the heated m etal workpiece.
With this technology micro-structures from
200 nm to several tens of micrometers can be
created at cycle times of below one minute.
The process sequence f or transparent materials
is shown in Fig. 10-9.
The laser radiation is guided onto the struc-
tured surface of the punch. Transparent tool
inserts, integrated into the low er tool part, enable
the radiation to directly access through the tool
onto the die.
The workpiece is heated to the forming tem-
perature, which is between the glass-transition
temperature and the melting point. The tool then
applies pressure on, and penetrates into, the
workpiece. During the holding time, the material
yields to the punch structure. The de-embossing
phase is the most critical dur ing the process: the
structures are often destroyed in this phase due to
tearing or overstretching. After the laser is
switched off, the punch cools down again to
below the heat-transition temperature. Since only
the structured face side is heated, the cooling
phase is shorter in comparison to that for conven-
tional technologies.
The advantages of laser-assisted hot embossing
can be summarized as follows:
1. Tooling materials are independent of thermal
or electrical conductivity;
2. Short heating times due to high energy density
of laser radiation;
3. Short cooling times by heating the near-surface
area only;
4. Accurate measurement of the embossing tem-
perature;
5. Selective heating of single areas of the compo-
nent.
Metals. As an example, the magnesium alloy
AZ31 is used. A step-like structure with an edge
length of 100 mm is pressed into the material both
FIGURE 10-9 Process sequence in hot embossing.
CHAPTER 10 Laser-Assisted Micro-Forming 167
at room and elevated temperature. Figure 10-10
shows the top of the punch and the resulting
metallographic cross-sections of the embossed
geometry.
Starting with a force of 5.9 kN, only the top of
the geometry can be formed into the sheet metal.
With increasing punch force, the penetration
depth also increase s. Even at 15.1 kN it is not
possible to reproduce the hole structure in the
sheet metal, as shown in the middle picture. When
the pressing force is increased further, the tool
breaks. By heating the sheet metal with laser radi-
ation, it is possible to reproduce the structure with
just 6.3 kN of applied force. In addition, the roll-
over that occurred in the cold-formed part can be
avoided. Thus, highly precise parts used for
micro-fluidic applications, for example, can be
produced.
Plastics. Amorphous thermoplastics are hard
and brittle at room temperature. Above the soft-
ening or the glass-transition temperature they
transform into a plastic condition in whi ch they
can be formed.
The tooling requirements are low due to the
low embossing temperatures of 150–250
C. Dif-
ferent technologies can be used for tool
manufacturing, such as lithography, whereby
structure sizes in the nanometer range can be
obtained with high flexibility regarding the geom-
etry. Figure 10-11 shows two imprints into acrylic
glass, both embossed with cycle times of below
one minute. The left-hand side picture has an
overall size of 6 6 mm. It consists of pins with
a structure size of around 15 microns. The picture
on the right-hand side has a diameter of 7 mm.
The structure size of 200 nm leads to the colored
effects visible on the picture.
Glass. In comparison to plastics, the material
glass has a higher optical quality, which is impor-
tant for different applications such as camera
lenses, sensors or innovative lighting systems
where aberration has to be avoided. The charac-
teristic properties of glass are, among others, high
mechanical strength, and environmental and tem-
perature resistance, as well as joining ability. Due
to its hardness and brittleness, this material is
difficult to process. The mass manufacture of pre-
cise glass components is done by blank molding,
but it is not possible to manufacture micro-optical
functions such as diffractive optical elements on
3D components with this technology.
Laser-assisted hot embossing is a suitable pro-
cess for the cost-efficient processing of glass
within short cycle times. Tool materials with high
FIGURE 10-10 Hot embossing of magnesium (AZ31).
168 CHAPTER 10 Laser-Assisted Micro-Forming
temperature stability are necessary, which can be
carbides, ceramics or high alloy HSS.
If high temperature gradients appear on the
glass surface, the risk of glass breakage increases.
A plane heat input is therefore necessary. Glass
material SK57 is a low melting point glass with a
transition temperature of T
g
= 493
C. Starting at
this temperature, imprints are possible with heat-
ing times of below one minute.
Figure 10-12 shows embossed imprints made
with this technology with structure sizes of
between 30 and 80 microns.
Bonding of Plastic with Metal
and Ceramic – LIFTEC
The ongoing trend regarding an increased level of
integration in many technical products and the
increased use of plastics as construction material
is a challenge for many manufacturer s regarding
the connection of dissimilar materials, such as
plastics with metals. The requirements for con-
sumer products as well as for technical compo-
nents are a flexible joining technique with short
cycle times and a broad field of application.
Until now, the connection of these materials
has been performed by gluing, screwed fastening
or the so-called mold-in technique. Hereby the
properties of the particular process define the area
of application. Clamped or screwed joints enab le
a detachable connection while form-closed con-
nections are able to transmit high power free of
clearance. Gluing is applied for large areas. A
complex preparation of the components is neces-
sary for all mechanical connections. The most
commonly used connection technique for plastics
with metal is the mold-in technique during injec-
tion molding. The mechanical component is
40µm
50µm
FIGURE 10-12 Hot embossing of glass (SK57).
FIGURE 10-11 Hot embossing of acrylic glass.
CHAPTER 10 Laser-Assisted Micro-Forming 169
placed in an adapted tool prior to the injection-
molding process. An optimal process result
requires tight tolerances of the tool and high pre-
cision components. The part handling is difficult.
A subsequent join t of plastic with metal is not
possible.
A process for a subsequent joint is the so-called
post-molding technology, which is mainly used
for thread inserts which are heated by induction
and then pressed into the plastic component.
However, the whole component may be heated,
and ceramics cannot be processed. The position-
ing of the inductor is often difficult and the heat
input not sufficient for small structure sizes.
Figure 10-13 displays a comparison of LIFTEC
with other joining techniques.
LIFTEC
– an acronym for ‘laser-ind uced
fusion technology’ – is a newly developed process.
It is based on every thermoplastic being transpar-
ent or at least translucent in the unpigmented
state. Based on this fact, a metal or ceramic com-
ponent or a part of it is heated with laser radiation
through the plastic part. The component is
pressed onto the plastic part and heats this by heat
conduction. After reaching sufficient plasticity
the component penetrates into the plastic part.
By choosing an appropriate geometry, a form-
closed connection can be obtained. This can be a
bump, a drilling or a groove, for example. The
material displaced upwards leads to unwanted
bulging on the surface of the part. This can be
eliminated or at least reduced by an additional
cavity or by drilling within the part where the
plastic can yield.
An inevitable element of this technology is a
component with a higher melting point in com-
parison to the plastic join partne r. Possi ble mate-
rials are mainly metals and ceramics, but can also
be a temperature-resistant plastic such as Teflon
or even wood.
Advantages of the technique are:
1. Short cycle times;
2. High mechanical strength;
3. Non-loosening connection, free from backlash;
4. Low requirements regarding tolerances and
positioning accuracy;
5. No pre- or post-processing necessary;
6. Post-process assembly;
7. Synergy effects by material combination.
Figure 10-14 shows the sequence of this pro-
cess:
1. Positioning and applying pressure;
2. Heating the metal part throu gh the plastic com-
ponent with laser radiation;
3. Penetration into the plastic component after
exceeding the glass transition temperature;
4. Cooling down and the creation of positive
locking.
When plastics are used that are not transparent
for the laser radiation, the process can be adapted
in the way that the radiation does not transmit
through the plastic part, but is guided laterally to
the component, as shown in Fig. 10-15.
Heating with laser radiation is, in comparison
to heating concepts already in use, quasi-indepen-
dent of the heat conductivity and the electrical
conductivity of the material used. This means
that, in addition to metal, ceramic materials can
FIGURE 10-13 Comparison of different joining techniques.
170 CHAPTER 10 Laser-Assisted Micro-Forming