Use of high-energy laser radiation for surface preparation of magnesium for adhesive applications
© Schneider and Wrobel. 2015
Received: 15 August 2015
Accepted: 26 November 2015
Published: 8 December 2015
This paper is intended to demonstrate how the parameters for the surface preparation of magnesium alloys for adhesive bonding can be optimized. The effects of different laser parameters are analyzed by using a combination of advanced sample preparation and ultra-high resolution scanning electron microscopy on a nanoscale level and a specific combination of mechanical tests on the macroscopic level. These data allow a discussion of the physical principles and the key parameters influencing the interaction of laser radiation with the magnesium surface.
KeywordsMicro processing Surface functionalization
Magnesium alloys are very versatile materials. Their high specific strength, combined with easy machining, suggest their application in any kind of lightweight structure . For the use of magnesium in multi-material constructions, i.e. in combination with other materials, efficient joining technologies need to be developed.
Adhesive bonding is a process that is perfectly suited to connect different materials. Due to its position in the electrochemical series (standard potential E0: −2.362 V), magnesium is a base metal . This means that magnesium corrodes very easily, especially in contact with other metals. In this respect the surface pretreatment and corrosion protection are major challenges in order to ensure the durability of the adhesive bond. The functionalization of magnesium surfaces by high-energy laser radiation is a promising and cost saving alternative to mechanical (e.g. grinding) or chemical pretreatment methods like pickling or anodizing .
For this work the magnesium alloy MgAl3Zn (AZ31) was used. This alloy is composed of at least 3.0 % aluminum (Al), 1.0 % zinc (Zn), 0.35 % manganese (Mn) and the rest magnesium (Mg) . The thickness of the metal sheets was 0.5 mm.
For these experiments a Nd:YAG-Laser was used. It has a wavelength of 1064 nm, a maximum mean laser power of 100 W and the laser pulse frequency can be varied between 1 Hz and 50 kHz. Because of the use of pulsed laser radiation with pulse duration of 70 ns, the thermal input is localized directly at the surface. The focusing of the short laser pulses to a footprint of approximately 200 µm in diameter results in very high energy densities, so that the magnesium surface is melted or sublimated within a few microseconds. At the same time, an explosively expanding plasma is generated, which needs the major part of the thermal energy provided by the individual laser pulses. Therefore, only very little heat is transferred into the bulk material . Thus changes in microstructure can be avoided and makes it also possible to process very thin magnesium-substrates and pretreat them for adhesive bonding.
First of all, the surfaces treated by laser were investigated by a scanning electron microscope including energy-dispersive X-ray microanalysis for chemical analysis of the surface (SEM/EDX). The acceleration voltage was 5 kV, the magnification was 100 times, the working distance 25 mm and the scanning time 100 s. This acceleration voltage was selected due to the resulting low penetration depth in the material and to obtain sufficient information from the surface composition. With the above mentioned parameters the maximum penetration depth in accord with the formula of Castaing is 1,2 µm .
Apply pressure at 40 ± 5 psi (0.28 ± 0.03 MPa)
Heat up to 120 °C in 30 min
Hold at (120 ± 3) °C for 60 min
During the process, a vacuum of 500 mbar must be applied.
For the optical inspection of the surfaces a field emission scanning electron microscope (FE-SEM) was used. The cross section samples were prepared with a cross section polisher (CSP) by means of a defocussed argon ion beam.
Results and discussion
The measured values for the weight- % of oxygen behaved quite opposite (see Fig. 4b). That means, that at lower laser pulse frequencies the values were higher than the initial value, then they decreased to a minimum at 11 kHz before they increased to the initial value of the untreated AZ31.
Melting, sublimation and oxidation
Low laser pulse frequency is equivalent to high laser fluence and therefor a high ablation rate, which means significant changes in the surface topography. The natural magnesium oxide is removed and magnesium is sublimated and recondenses on the rough surface. Here it reacts with the ambient oxygen building different types of magnesium oxides (see Fig. 3b–d). The best results to get a nearly homogenous oxide layer on the surface were achieved with a laser pulse frequency of 7 kHz.
Removal of the oxide layer
With increasing laser pulse frequency (decreasing laser fluence) only the natural magnesium oxide layer is removed but no more magnesium is sublimated. That is the reason for the maximum value of magnesium by coinstantaneous measuring the minimum value of oxygen.
Removal of contaminates
By furthermore increasing laser pulse frequencies the available energy is not sufficient anymore for any changes at the metallic surface except the ablation of contaminates. Consequently the initial value of the weight- % of magnesium and of oxygen is measured again.
So it is clearly evident, that the laser treatment with a Nd:YAG-Laser generates an oxide layer on the surface which is essential for the strength of the bonding. This oxide layer has a maximum thickness of approx. 350 nm. It is not possible to create thicker oxide layers with this method, because with every laser treatment the oxide layer is removed and then build again.
The excellent suitability of the laser pretreatment was already visible in these simple tests. However, it is noticeable that there were some significant variations in the average peel strength at one and the same laser parameters. This was a first indication that the curing process by Hot Bonder must be done very carefully in order to prevent a falsification of the measurement results. The results with samples cured in an autoclave were quite similar (not shown here), so that the advantages of the laser treatment become clearly evident.
It was shown that pulsed high energy laser radiation in combination with a film adhesive is a very promising alternative for the pretreatment of thin magnesium sheets compared to conventional methods like SACO-blasting or pickling. The laser pretreatment generates a thin oxide layer on the surface of the magnesium, which results in a very good improved molecular interaction of the adhesive with the surface. All parameter have to be optimized regarding the building of a homogenous oxide layer. The most important parameter is the forward speed of the guided laser beam. The halving of the forward speed increases the peel strength significantly.
By combining destructive tests, innovative preparation methods and FE scanning microscopy (especially fracture analysis of cross sections), the surface pretreatment could be optimized. As a result, high strength of adhesive bonding of magnesium-alloys could be achieved. This fast physical process fulfils all legal requirements concerning environmental protection and occupational safety and also has low operating costs.
NS designed the study, supervised the experimental work and evaluation and wrote the manuscript. CW performed the experimental work and evaluation. All authors read and approved the final manuscript.
We thank Dr. Jens Holtmannspoetter and his team of the WIWeB-Department 310 “Surface Analysis” for performing the SEM analysis and supervising the adhesion experiments. Furthermore we thank Florian Doellel for his technical support for the curing process of the specimens. In addition, we like to thank Prof. Dr.-Ing. Guenther Loewisch for fruitful discussions during the study.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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