Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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APPLICATION-ORIENTED TECHNOLOGIES Laser cleaning and surface modification <strong>of</strong> <strong>aluminium</strong> – first step to a green plant Edwin Büchter, Clean-Lasersysteme GmbH Advanced industrial lasers have evolved well beyond simple cutting and welding applications. Laser technology now <strong>of</strong>fers an industrial de-coating and surface cleaning solution that is cost-effective as well as responsive to environmental concerns. From <strong>the</strong> automated cleaning <strong>of</strong> moulds to precise de-coating to <strong>oxide</strong> removal, laser surface treatments are proving to be an attractive option to traditional labour intensive methods. Besides simple cleaning application <strong>the</strong> laser technology is also capable to apply modifications to <strong>the</strong> substrate surface such as roughening or changing <strong>the</strong> metallurgical structure. Using a laser for surface modification means substituting critical conventional chemical media blasting process by cleaning with laser light. In <strong>the</strong> past decade laser cleaning and surface treatment systems have generated significant interest as a viable alternative to conventional cleaning and paint removal technologies. Research <strong>of</strong> mobile, reliable, and powerful laser systems for cleaning and paint removal operations began in <strong>the</strong> late eighties with <strong>the</strong> modification <strong>of</strong> weld or cutting lasers into laser systems for surface preparation. This approach did not meet <strong>the</strong> requirements for a surface preparation laser, which are significantly different than for cutting and welding. In <strong>the</strong> early nineties, research took place around <strong>the</strong> world for more efficient, reliable laser systems for surface preparation work. It took ano<strong>the</strong>r few years to develop <strong>the</strong> technology from a laboratory system to a system capable for use in day-to-day industrial operations. Today, laser systems are widely used for various surface preparation tasks in many industries. Those tasks include: mould cleaning, paint removal, joining pretreatment, oil and grease removal, and many more. Operating principle The laser generates a directed and monochromatic beam <strong>of</strong> light. In order to create a high power density, <strong>the</strong> laser beam is also typically tightly focused. In <strong>the</strong> focal point, <strong>the</strong> intense laser beam is absorbed by <strong>the</strong> contamination or paint and <strong>the</strong>rmally incinerates or sublimates <strong>the</strong> target material, i. e. paint or con- Operating principle (Nd:YAG): pulsed laser vaporizes <strong>the</strong> absorbing ‘dirt’ <strong>layer</strong>, scanned beam covers and cleans <strong>the</strong> whole surface until <strong>the</strong> blank substrate reflects <strong>the</strong> laser radiation taminations. This incineration or vaporization will, in combination with <strong>the</strong> resulting micro <strong>the</strong>rmal shockwave, remove <strong>the</strong> target material as long as <strong>the</strong> target material is able to absorb <strong>the</strong> laser energy. The better <strong>the</strong> target material absorbs <strong>the</strong> energy, <strong>the</strong> faster it can be removed. Colour, chemical composition and thickness <strong>of</strong> <strong>the</strong> target <strong>layer</strong> all have a direct impact on <strong>the</strong> effectiveness <strong>of</strong> <strong>the</strong> process. The removal process automatically stops once a metal substrate is reached since metal surfaces reflect <strong>the</strong> laser beam and do not generally absorb <strong>the</strong> laser energy. The heat transfer into <strong>the</strong> substrate material can be a critical factor. To minimize this effect, many laser equipment manufacturers use pulsed laser sources. The laser intensity, known as <strong>the</strong> laser power per beam spot, is a critical parameter for <strong>the</strong> heat transfer into <strong>the</strong> substrate material. Very short laser pulses with a pulse duration <strong>of</strong> only a few nanoseconds (ns) in combination with a very small focus diameter (less <strong>the</strong>n 500 μm) result in a minimal heat transfer into <strong>the</strong> substrate material. Under normal operating conditions and with <strong>the</strong> right process parameters, damage to <strong>the</strong> substrate material can be eliminated. The heat transfer factor <strong>of</strong> continuous wave laser systems is much higher and might result in substrate temperatures that will damage <strong>the</strong> substrate. Test results with a Precise automated paint stripping / manual paint stripping for repair applications 48 ALUMINIUM · EAC CONGRESS 2011
Surface modification and de-coating <strong>of</strong> anodized <strong>layer</strong>s for electrical bonding Laser modification on <strong>aluminium</strong> surfaces for adhesive bonding and improved wettability prior to painting handheld pulsed Nd:YAG laser with an average laser power <strong>of</strong> 500 W (peak power <strong>of</strong> over 400 kW) on an <strong>aluminium</strong> sheet resulted in maximum substrate temperatures <strong>of</strong> 80°C. Pulsed laser systems generate laser power levels well beyond <strong>the</strong> average power <strong>of</strong> <strong>the</strong> laser source. A pulsed 150 W solid-state laser will generate a peak pulse power <strong>of</strong> over 160 kW. This high peak power and <strong>the</strong> above mentioned beam parameter results in a power intensity removing many target materials with acceptable production rates. Currently, <strong>the</strong>re are three different kinds <strong>of</strong> laser sources available for surface preparation works. The main difference is <strong>the</strong> laser generation and <strong>the</strong> resulting beam delivery configuration. Respecting <strong>the</strong> automation and integration possibilities, solid-state Nd:YAG laser <strong>of</strong>fer special advantages in industrial, automated applications such adhesive bonding preparation, pint stripping, mould cleaning and pre- / post treatment for welding and brazing. The operative wavelength <strong>of</strong> 1,064 nanometres (nm) lies within <strong>the</strong> transmission bandwidth <strong>of</strong> common optical glass and enables <strong>the</strong> use <strong>of</strong> fibre optic cables for beam delivery. Fibre optics dramatically increases <strong>the</strong> flexibility <strong>of</strong> <strong>the</strong> laser system. Nd:YAG lasers can be used for work on hard to reach areas. Currently, <strong>the</strong> maximum fibre optic length is 150 ft. Nd:YAG lasers are nearly maintenance free and very simple to operate. Pulsed systems for surface preparation use reach average laser power levels <strong>of</strong> up to 1,000 W. Sustainable technology Due to <strong>the</strong> media free cleaning with light, cleanLaser <strong>of</strong>fer several advantages. These advantages, which are leading to sustainable savings, are: Laser cleaning <strong>of</strong> <strong>aluminium</strong> car body parts for welding preparation APPLICATION-ORIENTED TECHNOLOGIES • No media and chemical materials required • Energy consumption far low (typically 5 kW or less) • Up to 80% lower running costs and waste <strong>of</strong> resources significant reduced • No noise and dirt allocation leading to better and safer workplaces. This combination <strong>of</strong> saving resources an reduction <strong>of</strong> costs will have a sustainable long term effect in industrial cleaning and surface applications. In October 2010, <strong>the</strong> founders <strong>of</strong> cleanLA- SER have been awarded with Europe’s highest doped environmental award, <strong>the</strong> German environmental award (Deutscher Umweltpreis). Clean Laser has also been nominated for <strong>the</strong> Clean Tech Media Award 2011. Current application fields for cleanLaser technology Currently, a wide range <strong>of</strong> industrial application fields are applicable for cleanLaser surface treatment and cleaning technology: • Cleaning <strong>of</strong> metal parts, especially aluminium for following production steps • Pre-treatment prior to painting • Adhesive bonding and pre-treatment <strong>of</strong> <strong>aluminium</strong> for long term stability • Welding and brazing preparations • (Precise) de-coating and ablation <strong>of</strong> <strong>layer</strong>s for joining or electrical bonding <strong>of</strong> plated <strong>aluminium</strong> • Modification and structuring <strong>of</strong> (metal) surfaces for enhanced performance. cleanLaser Systems Clean-Lasersysteme GmbH, an ISO 9001: 2008 certified laser system manufacturer <strong>of</strong>fers a wide range <strong>of</strong> powerful laser systems for surface applications. These systems are available with different end effectors. Laser optics are available for manual use and as well as for automated applications. Due to cleanLaser’s fibre coupling technology, <strong>the</strong> compact basic laser unit can be connected to a laser optics by using an up to 50 m long flexible fibre cable. The flexibility <strong>of</strong>fers easy integration as well as suitable power ranges for applications in an industrial environment. Advantages such as in-line cleaning are evident. High efficient laser systems mainly based on Q-switched solid state laser sources guaranty short laser pulse impact times for sensitive cleaning by nothing else <strong>the</strong>n laser light. Current laser systems are available in a power range between 20 to 1,000 Watts, ready for serving nearly every speed and cycle time demand. � ALUMINIUM · EAC CONGRESS 2011 49 Audi AG
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Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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