Friday, July 29, 2016
VOLUME -27 NUMBER 4
Publication Date: 04/1/2012
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ARCHIVE >  April 2012 Issue >  Special Features: Assembly and Packaging > 

Laser Marking: Scratching the Surface
U20 laser system set up for test.

In the past, CO2 and Nd: YAG lasers dominated the marking, cutting and welding of industrial tooling and manufactured goods. These laser types were the first to be brought to the marketplace and they were quite useful for their time. Focusing solely on the laser marking side of these technologies, each unit was causing Heat Affected Zones (HAZ), Recasting/Remelt Layers, Stress Risers, and/or, Micro-Cracking.

HAZ is the area of base material where the microstructure and properties were altered by the heat intensive marking operation. This heating, then subsequent cooling causes this change in the surrounding area of the marking. Recasting/re-melt layer is the surrounding area of the marking affected by the heat.
Tubing as marked in test.


Stress risers and micro-cracking are the location of a part where stress can be concentrated. Parts, or anything for that matter, are strongest when forces are distributed evenly. If marking damages the surface or causes drastic surface irregularities, the stress is no longer distributed evenly, but rather focused on the specific region and this reduction in surface area can propagate cracking and ultimately will lead to part failure.

Outdated Specs
Many specs of the past which are still in use today, (e.g. SAE AS478) have been written to almost exclude laser marking systems in spite of the many changes in both the industry and the technology being offered. Today, direct part marking is being done more by that of Nd: YVO4 and Fiber Laser Technologies as they offer more in terms of life cycles and marking controls. CO2 lasers are still very prominent for cutting and still very useful for marking woods, papers, acrylics, leathers, fabrics and other materials typically used in industries like Gifts and Awards. Nd: YAG has mostly been phased out as the Nd: YVO4 laser marking systems offer far better life cycles for both the crystal and the diode, while also offering the same benefits for direct part marking.

Fiber Lasers
Fiber Lasers have really taken off in the last few years with the claim of 50,000 to 100,000 hours Mean Time Between Failure (MTBF) but there are limitations and issues that this technology causes when it comes to marking sensitive components or when there is a requirement for surface marking, without any surface degradation. These issues and specs have inhibited companies from utilizing laser marking and they are instead using Dot Peen, Chemical/Acid Etching, or Ink-Jet printing. Each of these methods requires constant maintenance, while using up consumables, and do not offer the same benefits that laser marking does. Dot Peen is fast but it is impact marking and will cause micro-cracking and stress risers, and only provides the ability to mark basic alpha-numerics and low grade 2D Data Matrices. Chemical/Acid etching requires the disposal of the chemicals after use and also creates the need for stencils to be made after 20-30 markings. If serialization is needed, a new stencil will need to be made each time and the marking is not consistent which increases the reject rate on finished goods. Ink-Jets are fast and offer the full RGB or CMYK color swatches, but they require consumable inks, daily maintenance, and the ink must be baked on afterwards to ensure permanence. However, in time, these markings will fade out or in some cases, will completely disappear.
Photomicrograph showing laser-drilled holes that form letters.


To settle these disparate theories, RMI Laser had a third party perform metallurgical surface analysis on three different laser marks. These markings were performed on a 1-in. Titanium tube that is used for fuel lines on airplanes, chosen because they have very strict standards for surface irregularities and are currently not utilizing lasers for marking in the existing assembly or automated processes. The markings were performed by RMI Laser's U-20, 20 Watt Nd: YVO4 laser marker, the UF-30, 30 Watt Fiber laser marker, and the UM-1, 1 Watt Nd: YAG laser marker. There were two markings each, on two different tubes, one which would be a faster cycle time and lesser contrast, and the other with extremely high contrast and slower cycle times.

This test shows that the UF-30 Fiber Laser is causing the most surface irregularities and the largest amount of re-melt or recast layer. The numbers are nearly twice that of the UM-1 Nd: YAG and the U-20 Nd: YVO4 laser marking systems. The conclusion here is that the fiber laser, whether marking lightly and fast or harder and slow, still causes too much surface degradation in the form of HAZ and recasting/re-melt layers.

The UM-1 and U-20 are very similar in spite of the difference of 19 watts of output power. It can now be determined that wattage is not the key factor for causing this surface damage. The major factor here is the pulse duration of these units — the amount of time it takes from the beginning of the pulse to the very end of the laser pulse. The UM-1 and U-20 have pulse durations of 8 nanoseconds (ns). The UF-30 or fiber lasers in general, have a pulse duration of 100ns, more than 10 times that of the other two units being tested. This means that each and every pulse sits on the part 10 times longer and that excessive heat is what is causing the surface irregularities and degradation to be more severe.

The next step during this process is to actually put the marked sample tubes through Impulse Cycling Testing or Impulse Fatigue Testing as this is the primary test to ensure that the markings will not cause any of the defect issues mentioned earlier. The testing complies with SAE Aerospace Standard AS603B, Revised 2009-02, ANSI NCSL Z540-1, and ISO 17025:2005 specifications. Since the UF-30 did not pass the original tests, and the UM-1 is not fast enough to meet most of the cycle times needed for automation, the U-20 was the only system tested for this process. There was an unmarked control tube and two tubes laser marked by RMI Laser's U-20. The tubes were marked with 12 digit part number, date code, 10 digit serial number and the pressure rating of the tube. The test itself was for 200,000 cycles which replicates the life of a typical fuel line and was completed without any failures. Thus, the U-20 laser marking system is capable of performing high contrast and high speed markings without concern for HAZ, recasting/re-melt layers, stress risers, or micro-cracking.

We have thus learned that laser marking systems are capable of performing markings on sensitive components or parts that require markings without surface degradation. However, not all laser marking technologies are ideal for such applications and caution must be used when deciding what type of marking system to use.

When comparing laser marking to other marking methods, lasers really do stand out. There are no consumables or disposal involved, there is lower cost of ownership, they require little maintenance, are digitally driven, easy to automate and integrate, provide precision marking control, marking versatility, and reduced cycle times.

Contact: RMI Laser, LLC, 106 Laser Drive, Building #2, Lafayette, CO 80026 303-664-9000 fax: 303-664-9090 E-mail: info@rmilaser.net Web:
http://www.rmilaser.net

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