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Heat treatment using laser radiation
F R A U N H O F E R I N S T I T U T E F O R L A S E R T E C H N O L O G Y I LT
DQS certified by
DIN EN ISO 9001
Reg.-No. DE-69572-01
Fraunhofer-Institut
für Lasertechnik ILT
Director
Prof. Dr. Reinhart Poprawe M.A.
Steinbachstraße 15
52074 Aachen, Germany
Phone +49 241 8906-0
Fax +49 241 8906-121
www.ilt.fraunhofer.de
Fraunhofer Institute for Laser Technology ILT
With about 300 employees and more than 11,000 m² of usable
floorspace the Fraunhofer Institute for Laser Technology ILT is
worldwide one of the most important development and con-
tract research institutes of its specific field. The activities cover
a wide range of areas such as the development of new laser
beam sources and components, precise laser based metrology,
testing technology and industrial laser processes. This includes
laser cutting, caving, drilling, welding and soldering as well as
surface treatment, micro processing and rapid prototyping.
Furthermore, the Fraunhofer ILT is engaged in laser plant
technology, process control, modeling as well as in the entire
system technology. We offer feasibility studies, process
qualification and laser integration in customer specific manu-
facturing lines. The Fraunhofer ILT is part of the Fraunhofer-
Gesellschaft with more than 80 research units, 17,000 em-
ployees and an annual research budget of 1.5 billion euros.
Subject to alterations in specifications and other technical information. 11/2009.
hardening edges, ribs or grooves precisely. Large areas can be
hardened using rectangular dimensions up to a width of 100 mm.
High-power lasers in the wavelength range of around 1 µm
(Nd:YAG, diode lasers, fiber lasers) normally do not require an
absorber layer on the workpiece to increase absorption. Take for
example the surface layer hardening of torsion springs used for
door hinges. Wear occurs at the contact area between the tor-
sion springs and the guide rollers. Using dual-beam technology,
the contact area is hardened over an area of 170° and a length
of 10 - 12 mm with diode-laser radiation. The bulk properties
of the torsion springs are retained. The process is used in 3-shift
operation to produce around eight million springs a year.
Softening
Heat treatment using laser radiation can also be used for spe-
cific softening, e.g. of high-strength steels. These steels exhibit
a complex microstructure made up of martensite, austenite,
perlite, ferrite and carbides. The proportion of martensite
largely determines the strength. The softening mechanism is
based on the tempering or partial austenitization with subse-
quent ferrite-perlite transformation. The softening can be used
to improve the forming properties of steels. High-strength
steels are increasingly being used in the automotive industry
for body or chassis parts on the basis of their outstanding
mechanical properties. These steels are generally cold-formed
in the high-strength state in which they were delivered.
However, the higher strength limits the degree to which they
can be formed so that cracks may appear in areas of high
deformation degree. As a result, certain components
cannot be made out of high-strength steels. Local softening
using laser radiation increases the formability in areas of high
deformation degrees. In this state, the B-pillar of a car body
can be manufactured using a cold-forming process without
any cracks developing.
Another application of softening involves the recrystallization
of thin metal sheets. The cold-formed material is temporarily
heated using the laser beam until the grain structure has been
fully renewed, enabling the material to be cold-formed again.
Homogenous recrystallization across the strip thickness of
0.3 mm with a strip throughput-speed of up to 65 m/min has
been achieved for cold-rolled strips of a copper/iron alloy.
Annealing
Laser radiation can also be used for annealing processes where
only local treatment is required, or where furnace treatment
is not an option due to the resulting distortion. One such
application is stress relief annealing of components and tools
that are repaired by means of laser cladding. High residual
tensile stresses normally develop in the laser-clad areas, with
the potential to cause premature fatigue if oscillating loads are
involved. This risk can be countered with local heat treatment,
with creep and diffusion processes reducing the internal stress.
Annealing can also be used to specifically alter electromagne-
tic properties. The domain structures can be refined by means
of laser heat treatment of electric steel strip, which is used in
applications such as transformers, enabling hysteresis losses to
be reduced significantly.
Forming
Laser radiation allows metal sheets to be formed in a flexible
process without any physical contact. Forming can be pro-
duced either thermally by inducing a temperature field and,
hence, a mechanical stress field, or non-thermally by means
of laser beam-induced shockwaves, which are generated by
the explosive evaporation of an absorption layer. This process
requires beam sources with pulse lengths of 3 - 30 ns and
pulse intensities between 1012 - 1014 W/m2.
Contacts
Dr. Andreas Weisheit
Phone +49 241 8906-403
Dr. Konrad Wissenbach
Phone +49 241 8906-147
1 Hardening of torsion springs
2 Annealing of a stator sheet
3 Local hardening of a cold
formed part
4 Hardening of linear guide rails
using four-beam system
5 Local softening of a sheet
made of high strength steel
Heat treatment using laser radiationLaser radiat ion is ideal ly suited to the precise, local heat treatment of metal l ic mater ia ls , enabl ing the
specif ic modif icat ion of propert ies. The Fraunhofer Inst i tute for Laser Technology ILT develops customized
solut ions for var ious appl icat ions.
The Process
During heat treatment with laser radiation, the material is
heated locally to a temperature below the melt temperature.
The wall thickness determines whether just the surface layer
or, in the case of sheet metal, the entire cross-section is
heated. Unlike furnace treatment, this technique invariably
involves a short-time heat treatment with cycle times in the
region of a few seconds. The heating rate, the maximum
temperature and the cooling rate can be set specifically via
temperature control.
Surface Layer Hardening
When hardening a component made out of hardenable steel
or cast iron, a surface layer is austenitized for a short period.
The induced heat quickly flows into the cold bulk volume during
cooling. As a result of this quenching effect, the austenite
is transformed into martensite. This transformation can be
adjusted up to a depth of approximately 1 mm. The formation
of martensite is associated with an increase in hardness, which,
in turn, improves the component‘s wear-resistance properties.
The microstructure of the bulk volume remains unaffected
so that, for instance, toughness and wear resistance can be
combined to the best possible effect. The compressive residual
stresses induced during martensite formation can also be uti-
lized to improve the fatigue properties of components subjected
to oscillating loads. Using beam-forming optics, the laser beam
can be adjusted specifically to the task in hand. A circular laser
beam with a surface of a few square millimeters is ideal for
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