Laser Based Surface Treatment of Aluminium Alloys for

LASER BASED SURFACE TREATMENT OF ALUMINIUM ALLOYS FOR
ADHESIVE BONDING
Stefan Böhm, Klaus Dilger, Elisabeth Stammen
IFS – Institute of Joining and Welding, Technical University Braunschweig, Langer Kamp 8,
38106 Braunschweig, Germany, Phone +49 531 391 7820, [email protected]
Abstract
A surface preparation and pre-treatment is usually necessary when aluminium is adhesively
bonded. For this there are several reasons: on the one hand the improvement of the adhesion
and thus the increase of the bond strength and on the other hand the improvement of the
resistance to aging, [Rud03]. E.g. Cr based pre-treatments are very well established and
extremely effective prior to bonding and for corrosion protection. Till 2007, international
legislation requires their replacement by environmental friendly systems.
Therefore, in the context of a public sponsored project, a group of industrial companies and
university institutes work on a pre-treatment especially for aluminium, which is laser based
(solid state laser) and which avoids disadvantages of existing methods.
In this paper the authors will show the principle mode of action of the laser-based pretreatment and exemplary results at the aluminium alloys AlMgSi0.5 6040, AlMgSi0.5 6016 and
AlMgSi0.5 6043 (anodised). Further, first applications of the laser pre-treatments in the
German industry will be shown.
General Surface Preparation and Pre-treatment
Looking on the aluminium surface the need of a surface preparation to remove the dirt layer
and to establish a homogenous surface by using a special pre-treatment is obvious.
Although the surface preparation step is an established process in industrial production it still
causes high labour costs and the integration in a modern serial production is difficult.
Mechanical, chemical and electrochemical pre-treatments and also physical surface pretreatment procedures are very common. Beside techniques like Silicoater [Sch02, Lei98] and
Silane coated corundum (SACO –procedure) [Bud99, Nie97], the laser irradiation has been
carried out to improve adhesion.
Laser Surface Treatment- General
Usually, laser irradiation is used e.g. for cleaning, structuring, chemical and morphological
modification and curing of material surfaces. As a pre-treatment for adhesive bonding laser
radiation generates a oxide and grease free surface with a possible micro-structuring. A wet
chemical cleaning is unnecessary. Laser ablation works noise- and touch-free with small
running costs. A precise controlling makes the integration in serial productions possible.
The functional principle of removing coating layers e.g. dirt by laser radiation is the
following: The coating layer is removed by vaporisation through absorbing the laser spot. The
blank base material reflects the laser radiation, so that the ablation process stops automatically.
Very short laser pulses cause a very little thermal influence on the base material. It is possible
to treat the metal without damaging or melting of the base material.
Laser radiation offers the possibility to treat the material surfaces selectively (in contrast to
blasting processes) with a high flexibility (fast and easy change of the laser parameters) in
order to improve the adhesion characteristics. This has the advantage in comparison to other
procedures that only the required surfaces need to be processed.
To treat a surface using a laser, the pulsed laser is scanned via a GALVO-deflection unit in
line shape onto the surface of the substrate.
Results
In this paper the authors show exemplary results for laser pre-treatment of the aluminium
alloys AlMgSi0.5 6040, AlMgSi0.5 6016 and AlMgSi0.5 6043 (anodised).
The experimental setup was according to DIN EN 1465 (one-side overlapped, adhesive shear
tension samples). The samples where bonded together with two different adhesives: Betamate
1496 (Dow automotive) is a 1 component epoxy resin, Teromix 6700 (Henkel Teroson) a 2
component –PUR. These adhesives a common products in the automobile industry.
The aluminium was treated with different laser-parameters: Parameter 1 melts the surface,
Parameter 2 only cleans the surface without melting and Parameter 3 is a combination of 1
and 2. Additionally, a more homogenous Parameter 6 (generated from Parameter 1 with a
varied fibre-coupling) was analysed. In order to analyse the influence of the surrounding
medium, laser pre-treatment was done in atmosphere, under inert gas, under oxygen and in
combination with the Pyrosil –technique (Innovent Corp.)[Pat]. In some tests the samples
were bonded not only directly after pre-treatment but also after 1 and 24 hours to analyse the
influence of the new established oxide layer.
The adhesive bonds were tested un-aged and aged by a modified VDA 621-415 ( 10 weeks
climate changing test in combination with a salt spray test according to DIN 50021).
Selected results for AlMgSi0.5 6040 with Teromix 6700
Fig.1:
Selected results for AlMgSi0.5 6040 with Teromix 6700, bonded directly after pre-treatment.
To be continued.
The positive influence of a laser pre-treatment is obvious. For un-aged samples, all laserparameters and surrounding mediums achieve better results than bonding without laser pretreatment. After the VDA ageing test, laser parameter 6 gives as good results as the
combination with the Pyrosil-technique for parameter 1 and 3. The laser parameter differ in
laser power, pulse duration, scanning frequency and line per area.
The exclusive use of laser radiation to improve adhesion in this case is satisfactory. This
effect can be traced back to the fact that Parameter 6 establishes a very homogenous surface.
The natural oxid layer is completely removed and so bondline-corrosion can be eliminated.
Selected results for AlMgSi0.5 6016 with Betamate 1496
Fig.2: Selected results for AlMgSi0.5 6016 with Betamate 1496, bonded directly after pre-treatment
To be continued.
For un-aged samples the positive effect of laser pre-treatment is small. After aging the
realised result for samples with a Parameter 1 pre-treatment is convincing. Samples for
Parameter 6 are still in process; for this we expect good results as well.
Selected results for AlMgSi0.5 6043 with Teromix 6700
Fig. 3: Selected results for AlMgSi0.5 6043 with Teromix 6700, bonded directly after pre-treatment
Anodised AlMgSi0.5 6043 usually provides a good aging stability without other pre-treatments.
After cleaning and homogenising the surface with a laser pre-treatment it is possible to
improve these results. We consider a more obvious effect possible for Parameter 6.
Additional surface pre-treatment
Apart from the exclusive use of laser radiation to improve adhesion there is also the
possibility to use laser radiation- induced chemical reactions. A possible process consists of
the application of primers, drying of such primers and afterwards laser treatment. This
procedure is state of the art for aluminium (called “CLP”, Ciba Specialty Chemicals Inc.)
[Bor00, Bro98] but in comparison to a direct handling without adhesion agent, the additional
process step decreases the direct implementation in serial production.
In our test series we checked out the influence of a combination of silane- and laser pretreatment for AlMgSi0.5 6016 with Betamate 1496. We found a stronger decrease of adhesion
after aging for the not silane pre-treated samples than for the combination (silane/laser or
laser/silane/laser). But numerical the combination is not as good as the simple laser pretreatment.
First application of laser pre-treatment in German industry
As a result of this public promoted project two model work stations are established so far. For
this applications, a special robot laser stylus which includes the laser and the adhesive allot
step was developed.
Acknowledgement
This work was supported by a grant from the Bundesministerium für Bildung und Forschung
(BMBF) Germany within the project “Integration eines Laservorbehandlungssystems in eine
klebtechnische Serienfertigung –IntLasKleb“, which is managed by the “Projektträger PFT”,
Karlsruhe, Germany within the “Forschung für die Produktion von Morgen”- program. The
authors would like to express their thanks.
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