Monthly Archives: October 2009

Marking and micro-machining with nanosecond pulsed fiber lasers

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By: Dr Jack Gabzdyl

The new generation of ns fiber lasers offer a range of highly flexible compact beam sources with tailored beam quality options to give a further dimension for process enhancement. Combined with increases in peak powers and pulse energies new applications beyond the standard marking and micro-machining have opened. MOPA designs with directly modulated seeds also allow control of the pulse shape and duration using a range of preset pulse waveforms adding further flexibility.

The fundamental beam quality has a significant effect on many applications and beam quality should be taken in the context of fitness for purpose.

A comparative study clearly shows the impact of the spot size on marking and drilling applications. Marks made by single pulses on anodised aluminium and drilled holes in ceramic show strong correlation between spot size and focused spot diameters. Changing the M2 from 1.2 to 3.2 more than doubles the effective spot size and hence mark feature.

Additionally the beam quality can impact the depth and hole profile that is achieved. The single moded pulsed laser generates a narrow and deep high aspect ratio hole while the higher moded lasers generate progressively wider and shallower holes given the same number of laser pulses.

Other characteristics such as pulse energy and peak power are also prime requirements for applications such as deep engraving and processing reflective materials.

Deep metallic engraving has conventionally been an application dominated by the high pulse energy, low rep rate lamp pumped YAG laser. Fiber lasers lacked both the peak power and pulse energy required by this application, but with >20kW peak power and pulse energy >1.25mJ SPI’s 40W HM laser is well suited to this application. The higher mode does result in a larger spot size but can be mitigated by using higher beam expansion and this is more than compensated for by the additional pulse energy and peak power available. The requirements for deep engraved marks in the engineering industry are growing and typical examples are alphanumeric codes and 2D data matrices. Modest material removal rates of circa 3mm3/min are achievable.

The higher peak powers can also enable the processing of reflective metal such as copper and brass.

Nanosecond fiber lasers are maturing and elevating themselves from just simple marking lasers to far more capable micromachining tools.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Cover glass influence on high power fiber laser induced focus shift

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By: D. Reitemeyer1, T. Seefeld1, F. Vollertsen1, J. P. Bergmann2
1 BIAS – Bremer Institut für angewandte Strahltechnik GmbH, Bremen, 28359, Germany
2 Jenoptik Automatisierungstechnik GmbH, Jena, 07745, Germany

In materials processing with solid state laser sources a fiber optical waveguide is used to transport the
beam to the work station where the beam is focused on the work piece by a processing head. The high
beam quality of state of the art diode pumped solid state lasers allows concentrating the high output
power in a small focus diameter. This forwards the advantage of laser beam welding, the locally
concentrated energy input which enables precise processing.
Along with today’s high beam quality new challenges arise in the systems technology with increasing
requirements for the processing head. The high energy density in the laser beam path heats up the beam
guiding and forming optics, despite the fact that they have an absorption level below 1%. The
temperature increases when fume and spatter from the welding process are locally enhancing the
absorption on the cover glass which protects the beam outlet of the welding head. Figure 1a shows the
measured temperature distribution on the surface, at 5 kW a maximum rise of 170 K was detected.
This temperature rise influences the refractive index and the surface shape. Figure 1b shows the surface
deformation of a contaminated cover glass at 5 kW laser power measured by the fringe reflexion
technique. This influences the optical properties of the optics and thus the beam geometry at the work
piece. Figure 2 shows the influence on the focus’ position and diameter in dependence of the laser
power. The so called focus shift effect may influence the precision of the process.
At ICALEO 2009 a new way to analyze the influences on the focus shift will be presented. This work is
part of a project where BIAS and Jenoptik Automatisierungstechnik GmbH are working on new
approaches for systems technology for material processing with high brightness laser sources.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Welcome to Laser Insights

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Laser Insights is a feature that gives insight into the very latest developments of Laser Materials Processing and the possible applications. These overviews are designed to give you insight into the content and applications of the papers presented at our conferences and workshops.

LIA conferences and workshops are considered worldwide to be the best forum for exchanging up-to-date, technical information on laser technology, applications, and safety. No matter what the laser-related question, the answer can be found at an LIA conference.

To view a complete list of LIA conferences and workshops click here.

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Surface Plasmon Polaritons for Micro and Nano-Texturing of Metal Surfaces

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BY: L. Mellor, S.P. Edwardson, W. Perrie, G. Dearden, K.G. Watkins

Laser Group, Department of Engineering, The University of Liverpool, Liverpool L69 3GQ, UK

A study has been carried out on the production of micron and sub-micron features on the surface of a range of metallic materials through laser illumination. Possible application areas for such features include the biotechnology, communications, and aerospace industries for applications such as control of surface wetting angle. Modification of the surfaces was attributed to the production of Surface Plasmon Polaritons (SPPs) along the sample surface, resulting in a ripple effect with features smaller than the wavelength of the incoming laser radiation. Periodic ripple structures were produced on the surfaces of two industrially relevant materials; M42 High Speed Steel, used in the manufacture of various cutting tools, and Ti-6Al-4V, used in aerospace engine and frame components. For this work a high power ultra fast laser system by Fianium was used. This system provided picosecond pulses at 1064nm with pulse energies up to 10mJ with selectable repetition rates from 200 kHz down to the single pulse level. The pitch of the periodic features produced with this system was typically 1mm with depths ranging from 100-500nm into the sample. The high repetition rate of the Fianium system allowed for the production of these surface features at traverse speeds up to 500mms-1 allowing for rapid sample coverage. Alteration of the beam polarisation via a half wave plate allowed the orientation of the ripple structures to be controlled. Samples were textured with ripples in one orientation, then after altering the beam polarisation sections of the pre machined area were re-exposed to varying numbers of laser pulses. Re-exposed areas showed ripple growth corresponding to the new polarisation orientation between 5-10 pulses, below this it was found that under certain processing parameters it was possible to completely erase the ripple features. This gave us a high degree of control over the production of these ripple features allowing writing, over writing and erasing.

Textured surfaces were then analysed for changes to their water contact angle. Areas were textured with an increasing level of ripple coverage with initial results showing a steady reduction of the contact angle as the coverage increased. Maximum contact angle changes of up to 600 in Ti-6Al-4V and 250 in HSS occurred when complete ripple coverage was achieved. In the case of Ti-6Al-4V the resulting surface was highly hydrophilic giving rapid spreading of water with a contact angle of approximately 150. Results such as these are a positive indication that gaining control over the production of these ripple structures at high enough speeds to make them industrially attractive could lead to some interesting applications.

a)
b)
c)
10mm
10mm

Overlap patterns produced on a) Ti-6Al-4V; background pattern produced at 2.5mJ/pulse, 200kHz, 500mms-1. Spot produced with polarisation rotated approximately 450 at 2.5mJ with a single pulse b) HSS; background pattern produced at 2.5mJ/pulse, 200kHz, 500mms-1. Spot produced with ten 1.2mJ pulses rotating beam polarisation by 600 and c) effect of ripple patterns on the water contact angle of Ti-6Al-4V. Left side of the sample has been modified with ripple patterns and displays a high level of hydrophilicity.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Prevention of porosity by oxygen in partial penetration laser and laser-gma hybrid welding

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By: Susumu Tsukamoto1, Lin Zhao1, Goro Arakane1, Tomohiro Sugino2

1 National Institute for Materials Science, 2IHI Corp.

High power and high brightness solid state lasers such as fibre and disc lasers have a great potential to weld heavy section plate members.  In this case, one of the major problems is formation of some weld defects such as porosity and hot cracking.  Especially, the porosity is easily formed in deep partial penetration laser welds.  From this point of view, prevention of the porosity in partial penetration fibre laser and fibre laser-GMA hybrid welding has been attempted in the present study.

Most attractive characteristic of the laser welding is narrow and deep weld penetration. It is attributed to formation of narrow and long keyhole by evaporation of the materials as shown in Fig.1.  However, the keyhole is basically unstable, if the length is larger than the circumference.  Then, the keyhole tends to be closed during deep penetration laser welding.  If the keyhole is closed near the root, the separated keyhole tip forms the bubble and remains as a porosity.  Thus, the porosity is formed by keyhole instability.  Stabilisation of the keyhole should be effective to prevent the porosity.

In the previous paper, we revealed that the porosity was successfully prevented by periodical change in the laser power (laser power modulation) in deep penetration CO2 laser welding.  It is because the laser power modulation can stabilise the keyhole at optimum frequency and waveform.  Then, we applied this technique to fibre laser and hybrid welding at first.  The results indicate that the power modulation can effectively prevent the porosity in fibre laser welding, but a lot of porosities are still remained in the hybrid welds.  On the other hand, the porosity is effectively suppressed with a small amount of oxygen addition in the shielding gas both in fibre laser and hybrid welding as shown in the x-ray radiographs of Fig.2. Observation of dynamic keyhole behaviour using an in-situ x-ray transmission imaging system confirmed that the keyhole was stabilised by a small amount of oxygen addition.  Spatter generation can also be reduced by oxygen in fibre laser welding due to stabilisation of the keyhole.  Thus, oxygen can stabilise the laser welding process and then prevent the weld defects.

Fig.1 Formation of keyhole in laser welding.

Fig.2  X-ray radiographs of hybrid welds with or without oxygen in the shielding gas.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

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