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

info@ilt.fraunhofer.de

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

andreas.weisheit@ilt.fraunhofer.de

Dr. Konrad Wissenbach

Phone +49 241 8906-147

konrad.wissenbach@ilt.fraunhofer.de

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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