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
Heat treatment using laser radiation
Heat treatment using laser radiation
L a s e r r a d i a t i o n i s i d e a l l y s u i t e d t o t h e precise, local heat treatment of metallic materials, enabling the
spe c if ic m odifi c a ti o n o f p ro p e rti e s . T h e F raunhofer I ns ti tute for L as er Tec hnol ogy I LT dev el ops cust om ized
solut ions f or v a ri o u s a p p l i ca ti o n s .
The Process
hardening edges, ribs or grooves precisely. Large areas can be
hardened using rectangular dimensions up to a width of 100 mm.
During heat treatment with laser radiation, the material is
High-power lasers in the wavelength range of around 1 m
heated locally to a temperature below the melt temperature.
(Nd:YAG, diode lasers, fiber lasers) normally do not require an
The wall thickness determines whether just the surface layer
absorber layer on the workpiece to increase absorption. Take for
or, in the case of sheet metal, the entire cross-section is
example the surface layer hardening of torsion springs used for
heated. Unlike furnace treatment, this technique invariably
door hinges. Wear occurs at the contact area between the tor-
involves a short-time heat treatment with cycle times in the
sion springs and the guide rollers. Using dual-beam technology,
region of a few seconds. The heating rate, the maximum
the contact area is hardened over an area of 170 and a length
temperature and the cooling rate can be set specifically via
of 10 - 12 mm with diode-laser radiation. The bulk properties
temperature control.
of the torsion springs are retained. The process is used in 3-shift
operation to produce around eight million springs a year.
Surface Layer Hardening
Softening
When hardening a component made out of hardenable steel
or cast iron, a surface layer is austenitized for a short period.
Heat treatment using laser radiation can also be used for spe-
The induced heat quickly flows into the cold bulk volume during
cific softening, e.g. of high-strength steels. These steels exhibit
cooling. As a result of this quenching effect, the austenite
a complex microstructure made up of martensite, austenite,
is transformed into martensite. This transformation can be
perlite, ferrite and carbides. The proportion of martensite
adjusted up to a depth of approximately 1 mm. The formation
largely determines the strength. The softening mechanism is
of martensite is associated with an increase in hardness, which,
based on the tempering or partial austenitization with subse-
in turn, improves the components wear-resistance properties.
quent ferrite-perlite transformation. The softening can be used
The microstructure of the bulk volume remains unaffected
to improve the forming properties of steels. High-strength
so that, for instance, toughness and wear resistance can be
steels are increasingly being used in the automotive industry
combined to the best possible effect. The compressive residual
for body or chassis parts on the basis of their outstanding
stresses induced during martensite formation can also be uti-
mechanical properties. These steels are generally cold-formed
lized to improve the fatigue properties of components subjected
in the high-strength state in which they were delivered.
to oscillating loads. Using beam-forming optics, the laser beam
However, the higher strength limits the degree to which they
can be adjusted specifically to the task in hand. A circular laser
can be formed so that cracks may appear in areas of high
beam with a surface of a few square millimeters is ideal for
deformation degree. As a result, certain components
cannot be made out of high-strength steels. Local softening
Forming
using laser radiation increases the formability in areas of high
deformation degrees. In this state, the B-pillar of a car body
Laser radiation allows metal sheets to be formed in a flexible
can be manufactured using a cold-forming process without
process without any physical contact. Forming can be pro-
any cracks developing.
duced either thermally by inducing a temperature field and,
hence, a mechanical stress field, or non-thermally by means
Another application of softening involves the recrystallization
of laser beam-induced shockwaves, which are generated by
of thin metal sheets. The cold-formed material is temporarily
the explosive evaporation of an absorption layer. This process
heated using the laser beam until the grain structure has been
requires beam sources with pulse lengths of 3 - 30 ns and
fully renewed, enabling the material to be cold-formed again.
pulse intensities between 1012 - 1014 W/m2.
Homogenous recrystallization across the strip thickness of
0.3 mm with a strip throughput-speed of up to 65 m/min has
Contacts
been achieved for cold-rolled strips of a copper/iron alloy.
Dr. Andreas Weisheit
Annealing
Phone +49 241 8906-403
andreas.weisheit@ilt.fraunhofer.de
Laser radiation can also be used for annealing processes where
only local treatment is required, or where furnace treatment
Dr. Konrad Wissenbach
is not an option due to the resulting distortion. One such
Phone +49 241 8906-147
application is stress relief annealing of components and tools
konrad.wissenbach@ilt.fraunhofer.de
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 electromagnetic 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.
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
Fraunhofer Institute for Laser Technology ILT
With about 300 employees and more than 11,000 m of usable
floorspace the Fraunhofer Institute for Laser Technology ILT is
worldwide one of the most important development and contract research institutes of its specific field. The activities cover
DQS certified by
a wide range of areas such as the development of new laser
DIN EN ISO 9001
beam sources and components, precise laser based metrology,
Reg.-No. DE-69572-01
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
Fraunhofer-Institut
qualification and laser integration in customer specific manu-
fr Lasertechnik ILT
facturing lines. The Fraunhofer ILT is part of the Fraunhofer-
Director
Gesellschaft with more than 80 research units, 17,000 em-
Prof. Dr. Reinhart Poprawe M.A.
ployees and an annual research budget of 1.5 billion euros.
Steinbachstrae 15
52074 Aachen, Germany
Phone +49 241 8906-0
Fax +49 241 8906-121
info@ilt.fraunhofer.de
www.ilt.fraunhofer.de
Subject to alterations in specifications and other technical information. 11/2009.
