Category Archives: Laser Machining

Effect of grain size on laser machining of alumina

Posted on by
Reply

By: Michael Furlan

Laser machining is filling the niche market created by the increased demand for advanced ceramics and high manufacturing tolerances. The effect of grain size on the laser machinability of alumina (Al2O3) was examined in order to meet the demand of higher tolerances when laser machining advanced ceramics.  Laser lines created in alumina samples fabricated by using progressively increasing sintering times, which results in increased grain size, showed a difference in the amount of damage produced along the edges of the channels that were created, with less damage in smaller-grained samples. The average line width of the samples decreased with increasing grain size. These phenomena can be seen in figure 1. It was hypothesized that these effects are a result of grain boundaries absorbing a higher amount of energy than the bulk. The removal of entire grains due to the cracking mechanism of ablation is not uncommon. If the grain boundaries are absorbing a larger amount of energy relative to the bulk, it is possible that rather than bonds breaking and ablating individual atoms, that the energy absorbed by the boundaries leads to entire grains being ejected from the material.  This improvement with regards to cut quality that smaller grain structured materials have, offers a benefit in many fields ranging from micro-fluidic channels to fine scale micro and nano-machining.

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

Optimiztion of Laser Drilling in Ceramics

Posted on by
Reply

By: Yinzhou Yan

Laser drilling is commonly acknowledged as a hole fabricating technique especially for hard and brittle materials like advanced ceramics. Unfortunately, laser drilled hole are inherently associated with spatter deposition due to the incomplete expulsion of molten ejection from the hole, which enlarges the hole diameter and resolidifies on the workpiece surface around the hole periphery. This defect is more evident in laser percussion drilling thick ceramics that has a high melting boiling point. It causes a low quality of drilled hole and needs a long pre-cutting path in laser cutting to avoid spatter depositing on the cutting path. In our current study, the behaviors of debris and hole diameter in CO2 laser percussion drilling of 95 % alumina ceramic sheets (4.4 mm thick) under different processing parameters were investigated to figure out the effect of main energetic processing parameters on these behaviors. The corresponding processing parameters include laser peck power, pulse duty cycle, pulse repetition frequency and piercing time. The change trend curves of debris and hole diameter with the related parameters were plotted respectively. The combined effects of these parameters were also studied in our work, such as the co-action of peak power and pulse duty cycle, the co-action of peak power and piercing time, the co-action of pulse repetition frequency and piercing time, the co-action of pulse duty cycle and piercing time, and the co-action of pulse duty cycle, pulse repetition frequency and piercing time. The potential mechanism of individual parameter affected on material removal during laser percussion drilling was also discussed based on the experimental result.

The obtained result shows that (1) Laser peak power affected vaporization rate. (2) Pulse duty cycle influenced melt rate. (3) Pulse frequency affected valid heating efficiency in workpiece. Higher pulse frequency caused the debris and hole diameter decreased, and resulted in more symmetrical spatter deposition which contributed to the perfect circularity of the drilled holes. (4) Piercing time influenced drilled depth before entire beam break-through. With increasing peak power, pulse duty cycle or pulse frequency, the piercing time for a complete through-hole could be shorten. (5) On the process qualities (debris and hole diameter), peak power and pulse duty cycle had significant effects, pulse frequency had a lower effect, while piercing time had the lowest effect. Comparatively, peak power had a more significant effect than pulse duty cycle on spatter formation.

From our research, the effects of different processing parameters on quality of laser drilled hole in alumina ceramics were observed. The processing parameters could be further optimized to achieve less debris and finer hole drilling by some basic conclusions from our work. The possibility of controlling debris and hole diameter also leads to numerous benefits especially during closely spaced array laser drilling or short per-cutting path laser cutting for efficiency improvements. Moreover, the method referred in the work is also suitable for studying other materials drilled by laser, which could help technicians optimize processing parameters more effectively.

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

Advanced Beam Steering In Helical Drilling

Posted on by
Reply

By: Henrikki Pantsar1, Petri Laakso2, Mika Aikio2, Jouni Huopana2, Hans Herfurth1, Stefan Heinemann1

1 Fraunhofer USA, Inc. Center for Laser Technology, 46025 Port St, Plymouth, MI 48170

2VTT Technical Research Centre of Finland, Tuotantokatu 2, 53850 Lappeenranta, Finland

Helical laser drilling is a method for producing high quality holes with defined geometries in different materials among industries such as aerospace, medical device manufacturing and electronics. If the aspect ratio of the hole is small, drilling can be done using a fast scanner. However, a special drill head is needed for higher aspect ratio holes and improved precision. The drill head typically comprises wedges or a Dove prism to rotate the laser beam at high velocities. Using a pulsed laser, each pulse removes a portion of the material. Thermal effects and the thickness of the recast layer are significantly smaller than associated with single pulse or percussion drilling.

Combining a galvanometric mirrors together with rotating optics opens up possibilities for drilling and processing which cannot be accomplished with either of those devices separately. In addition to using the helical drill head for precision drilling and adjusting the hole diameter using the scanner mirror angles, it is possible to create non-circular geometric features by combining the movement of scanner mirrors and the rotational movement. Movement away from the origin along the x or y axis on the scanner’s Cartesian coordinate system is translated into the radial coordinate on the polar coordinate system and the angular coordinate is defined by the angle of the prism. In principle, the rotating prism creates a circular beam path which can be scanner at a rate up to 10,000 rpm. Using a sufficiently large prism, the helical drilling device can be stopped to engrave or mark the samples using the same optical setup. There is no need for removing the drill head.

VTT Technical Research Center of Finland has developed an add-on helical drill head which can be attached to typical galvanometric scanners. The head is based on a Dove prism which rotates at 5,000 rpm, creating 10,000 optical rotations per minute. Fraunhofer USA, Center for Laser Technology is currently using such a head to develop laser processes utilizing crystal and fiber based pulsed lasers for expanding the possibilities of helical drilling for industrial applications.

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

C204

High Power Rate Femtosecond Lasers and Novel Dynamics during High Repetition Machining

Posted on by
Reply

By: Andreas Tünnermann

Today, there is a strong need for advanced micro machining tools due to the increasing miniaturization of components and systems. Application examples include the drilling of fuel injection nozzles or the structuring of thin film solar cells. However, the fabrication of structures with micrometer or even nanometer precision is a difficult task. Since a few years, pulsed laser systems are replacing conventional tools for micro machining like edm-machines. However, especially the precise laser micro structuring of metals is typically limited by thermal and or mechanical damage in the surrounding. Here, the ability of ultrashort-pulse lasers to fabricate precise micro structures on solid targets is opening new perspectives. In the past years, the superior quality of ablated holes and patterns produced by femtosecond or picosecond laser pulses compared to nanosecond pulses has been demonstrated. Although the production of high quality and high aspect ratio holes in metals with ultrashort laser pulses is still an open field of research, it already has significant technological impact on industrial applications.

Despite of these benefits, the industrial use of ultrashort pulse lasers has been hindered by their complexity and the limited processing speed which does not allow for cost-effective manufacturing. These disadvantages can be overcome by the novel regeneratively amplified solid state or fiber laser sources, providing high average powers and repetition rates.

Recently, we demonstrated in our laboratories ultrafast fiber amplifier systems with 800-W-average-power and mJ-level-pulse energies. These sources are very promising for industrial micro machining applications because of their compactness, high average power and high repetition rates that enable a significant increase of the processing speed.

Systematic studies of the effect of high repetition rates and high average powers on the processing speed and on the morphology of the structures have been performed. At high repetition rates heat accumulation effects leading to melting and increased heat-affected zones have been observed. In addition, plasma shielding effects have been measured. However, by using the high-power ultrafast fiber laser systems with optimized process control we have been able to machine high quality melt-free, and high aspect ratio micro structures within a few tens of ms. These results clearly demonstrate, that high average power ultrafast fiber amplifiers will open new avenues for the micro manufacturing of solid materials – most recent results will be reported.

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

Paper # M201

High Rate Laser Drilling And Texturing of Silicon

Posted on by
Reply

By: Henrikki Pantsar1, Tim Lauterborn1, Annerose Knorz2, Hans Herfurth1, Stefan Heinemann1

1 Fraunhofer USA, Center for Laser Technlogy,

46025 Port Street, Plymouth, MI 48170

2Fraunhofer Institute for Solar Energy Systems

Heidenhofstrasse 2, 79110 Freiburg, Germany

The greatest challenge for photovoltaic solar cells is to reduce the price per watt for terrestrial applications. In silicon panel production this can be accomplished by economies of scale, developing automation, improving cell efficiency and reducing material costs either by using thinner wafers and/or lower-quality materials. Larger, thinner substrates enable processing of more active area per step and reduce the consumption of material per cell. Back contact solar cell concepts such as the Emitter Wrap Through (EWT) adapt well to these requirements. The respective challenge in manufacturing these cells is the large number of through holes that are needed per cell. One cell can comprise up to 25,000 holes.

Due to the required cycle times drilling techniques such as percussion drilling are not fast enough for production. In order to reach highest possible drilling rates high rate drilling techniques, such as using a scanner with pulse synchronization are needed. This processing strategy allows high beam duty cycles and thus faster processing times. The drilling process has to be optimized to reach best material removal efficiency. In this aspect MOPA fiber lasers have shown to be efficient tools due to their property of allowing independent adjustment of pulse parameter such as pulse width and frequency.

The Center for Laser Technology of Fraunhofer USA, Inc. in Plymouth, MI has developed laser-based techniques for texturing and drilling Si-wafers at extremely high rates using MOPA fiber lasers. Contrary to q-switched lasers, the pulse parameters; pulse energy, pulse width and pulse frequency can be adjusted independently opening a parameter space in which the process can be optimized for specific process or material. Drilling efficiencies of 150 holes per Joule have been demonstrated in 200 μm silicon wafers using process-optimized pulse parameters. Using a 5 W average power a number of approximately 800 holes per second were drilled. The results have been published in the Proceedings of the ICALEO 2009 conference held in Orlando, Fl. Since then, the process has been demonstrated using 20 W laser power reaching more than 3,100 holes per second. The high drilling rate is based on pulse parameter optimization and a unique FPGA based pulse synchronization controller. Further improvement in drilling rates is yet expected with manipulating the pulse shape.

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

Paper 1104