Dual Mode High Brightness Fiber Laser For Ablation And Drilling Of Aerospace Superalloys

Posted on May 7, 2010 by Steven Glover

By: Mohammed Naeem

Fiber lasers with its high beam quality (M²~ 1.10 are routinely being used for a welding and cutting for a rage of industrial applications. These fiber lasers are very compact and robust and have an edge over lamp pumped Nd: YAG lasers in terms of beam quality and wall plug efficiency (approx 20%).

To date majority of the laser material processing work with these high beam quality lasers has been carried out either with a continuous wave (CW) or modulated output because currently conventional CW fibre lasers have no peak power over maximum average power capability. This is due to the peak power limitations of the diode pump sources used. Significant lifetime degradation occurs if the junction temperature of a laser diode is increased during operation for any significant length of time which would normally be needed for percussion drilling of aerospace alloys (i.e. milliseconds and above). However it may be possible to use these high beam quality lasers to trepan various sizes holes. Unlike percussion drilling where high pulse energies (up to 20 joules) and high peak powers (up 20kW) are needed to drill holes of 0.3-0.75mm diameter. For trepanning applications the main laser requirements are good beam quality with CW/high frequency modulated output to drill holes at reasonable drilling speeds.

First part of the work describes laser trepanning of aerospace alloys with a 400W single mode (SM) fiber. The second part of the work describes laser ablation of aerospace alloys with the same laser. Normally laser ablation of variety of materials is Q-switched lamp pumped or diode pumped solid state lasers. Here we show that it is possible to the CW fiber for ablation work. Turning the SM fiber laser on and off typically produces a relaxation pulse which is 4-5x the CW power as shown in Figure 1, which is very useful for ablating a range of materials.

Figure 1: Typical output waveform with relaxation pulse, when turning the SM fiber laser on and off.

During drilling primary concern to the component designer is achieving adequate airflow through the holes so that the appropriate cooling is provided. Airflow is governed principally by the size and shape of the hole and hence the need for tight control of size, roundness and taper. There are other factors also to consider; holes are often very closely positioned to one another on a component and any deviation in size may adversely encroach on other holes or even weaken the component locally. Excessive bell- mouthing or barreling is therefore undesirable in addition to recast layer and heat-affected zone.

The geometrical features and the metallurgical characteristics of each laser drilled hole generated during the present study were investigated. The cross sections of some of holes drilled with 400W SM fiber laser are highlighted in Figures 2-3.

Figure 2: 1.25mm thick HastalloyX, 0.5mm dia hole very parallel hole with low recast layer (<25µm thick)

Figure 3: 4mm thick Haynes alloy, 3.6mm dia hole, average recast layer < 40µm

The above holes were drilled with SM fiber laser with CW output, however it possible to use the same laser to remove thermal barrier coatings (TBC) prior to laser drilling. The ablation tests were carried out by optimizing the modulation frequency and the peak power in the initial spike of the relaxation pulse. Some of the ablation results are highlighted in Figures 4-5.

Figure 4: HastalloyX alloy with 0.4mm thick TBC coating; material removal rate 13.8mm³/min; 15 kHz, 40MW/cm²

Figure 5: 2mm thick Haynes alloy with 0.5mm thick TBC coating, 15 kHz, 40MW/cm³ material removal rate 14mm³/min; 1mm dia trepanned hole with the same laser after removing the TBC.

The work reported here show that by optimizing laser and processing parameters it is possible to produce good drilling and ablation results (dual processing) with the same SM fiber laser.

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

Ultra Short Pulses Align Nano-Scale Components within a Complete Device

Posted on December 2, 2009 by Steven Glover

By: Peter Bechtold

The trend in manufacturing today is to make everything smaller. Electronic gadgets, mechanical devices – everything is shrinking. Examples that come to mind are tiny MEMS accelerometers built into automotive seat belts, or miniature medical-diagnostic labs-on-a-chip, or one-inch hard-disk drives.

At the same time there is a demand for an increased functionality of devices, so more and more components have to be precisely positioned relative to each other. It is, however, impractical to achieve the precise alignments needed in a single manufacturing step. Instead, the prevailing technique, especially for nanometer-scale manufacturing, is to assemble parts in approximately the correct position, and then make high-precision adjustments to get the positioning within tolerances. Many adjustment processes, such as the laser-based temperature-gradient mechanism can often meet the demands of nano-manufacturing. Unfortunately, there are also cases where these processes are far from ideal, and sometimes they fail altogether. Thus, it is crucial to develop new micro- and especially nano-scale adjustment processes to meet the demands of current and future products.

Within the paper and presentation we will describe such a new process. We use ultrashort laser pulses to induce a micro shockwave in the workpiece, resulting in a highly controllable, sub-nanometer deformation of the piece. The process is effective in a wide variety of materials, and the interaction mechanism is almost non-thermal, which means that there is a negligible thermal impact on the workpiece.

An analysis based on models of ultrashort laser pulse – material interaction will be introduced and compared to experimental results and the long-term stability will be discussed. As another possible application of this process mechanism besides micro adjustment virtually free-form mirrors fabricated by micro shockwave bending will be presented. The topography of such a mirror is depicted in the attached figure. The original substrate material was a totally flat silicon wafer of 300 µm thickness. It was deformed into a four-facetted mirror geometry using micro shockwave bending solely. Such mirrors could be used in a wide variety of optical systems, for wavefront error correction or beam profile modification for instance.

Laser Sintering of Silver Nanoparticles

Posted on November 30, 2009 by Steven Glover

By: Petri Laakso

INTRODUCTION

Roll-to-roll printing itself is not always sufficient for the production of printed electronics components

and systems. Drying and sintering are the bottleneck processes in metal particle based conductor printing. Lasers show high potential for the curing process, especially in the case of nanoparticulate inks. This is due to fact that the typical sintering temperatures for nanoparticles (100–300°C) are only a fraction of the macroscopic melting point of the corresponding materials. This allows paper or plastic substrates to be used.

MATERIALS AND METHODS

Base material in sintering was polyimide. Laserline fiber coupled diode laser with scanhead was used for sintering.

SINTERING RESULTS

The sintering of printed nanoparticle structures using laser treatment has been investigated at VTT. Laser sintering can be utilised in the manufacturing of printed conductor structures such as antennas, circuits and sensors [1, 2]. A drop-on demand printer was used to print patterns with metal-organic silver nanoparticles on a flexible polyimide substrate. Laser sintering was done with a 940 nm CW fibre-coupled diode laser. The process was optimised using different scanning speeds, laser power levels, line separation and repetition rounds. In sintering tests, three different line thicknesses were printed to gauge the effect of line width. Sintering tests were done with Ink 1 using the hatch technique. The laser speed was 1000 mm/s and the line-to-line distance was 0.2 mm. The beam size was 1 mm in diameter. After a series of pre-tests, the optimal range of average power were estimated to be between 20–50 W. Table 1 shows the effect of average power and conductor width on sheet resistance.

Table 1. Sheet resistance values with different average powers on ink 1 with different conductor widths.

	50 W	40 W	30 W	20 W
100 µm	0.16	0.18	0.29	0.52
200 µm	0.17	0.27	0.31	0.59
350 µm	0.16	0.19	0.28	0.38

SUMMARY

Laser sintering of nanoparticle inks seems to be promising curing technique for R2R sintering. Especially in cases

where only part of the substrate need to be cured, laser has the potential for reaching a high processing speed.

Additionally it can allow low-cost low melting point substrates to be used since heating is well-targeted to inks.

Choosing the right ink for the process and keeping the substrate clean are key factors for successful operation.

Sheet resistance values only slightly exceed the values obtained by heat sintering. This result was obtained in preliminary test and can be further optimised.

BUSINESS POTENTIAL

Laser sintering has a high potential for curing metal particle inks on flexible substrates. It offers a fast processing speed and low temperature processing, and therefore, it often represents an improvement over oven sintering.

REFERENCES

[1] Khan, A.; Rasmussen, N.; Marinov.; Svenson, O.: Laser

Sintering of Nanomaterial on Polymer Substrates.

In: journal of Microelectronics and electronic Packaging

(2008)5, 77-86

[2] Ko S.; Pan H.; Grigoropoulos C.; Luscombe C.; Frechet

J.; Poulikakos D.: All-inkjet-printed fl exible

electronics fabrication on a polymer substrate by

low-temperature high-resolution selective laser sintering

of metal nanoparticles, Nanotechnology 18

(2007) 345202.

ACKNOWLEDGEMENTS

Laser sintering work was done in a TEKES funded PESEP -project. The ink jet printing for laser sintering trials was done in this project by Eerik Halonen from the Tampere University of Technology.

Laser Surface Treatment of Engineering Ceramics and the Effects Thereof on Fracture Toughness

Posted on November 10, 2009 by Steven Glover

By: P.P. Shukla and J. Lawrence

Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, LE11 3TU, United Kingdom.

Engineering ceramics offer a wide range of mechanical and thermal properties such as high melting point, corrosion and wear, bending strength, hardness, heat resistance under extreme conditions, physical stability, chemical inertness, very good electrical conductivity combined with their suitability for mass production. These properties are typically superior to conventionally used metals, plastics and glass. Fracture Toughness (K1c) is a measure of the materials resistance to fracture or crack propagation.

Materials with high K1c are much softer and ductile. Those types of material can resist cracks at higher stress levels and loading. Materials with low K1c are much harder, brittle and allow crack propagation at lower stresses and loading. Ceramics also do not mechanically yield as well as metals in comparison which leads to much lower resistance to fracture. Ceramics in comparison with metal and metal alloys have a low K1c, hence, it would be an advantage if the K1c of ceramics could be improved. This could open new avenues for ceramics to be applicable to high demanding applications where metals and metal alloys fail due to their low thermal resistance, co- efficient of friction, wear rate and hardness in comparison with ceramics. Surface treatment of silicon nitride and zirconia engineering ceramics with a CO2 laser and a fibre laser was conducted to identify changes in the fracture toughness. Vickers hardness indentation tests were employed prior to and after the laser treatment to investigate the near surface changes in the hardness of the engineering ceramics. Optical microscopy was then used to observe the near surface integrity, crack lengths and crack geometry within the engineering ceramics. A co-ordinate measuring machine was used to observe the diamond indentations and to measure the lengths of the cracks in the ceramics. Thereafter, computational and analytical methods were employed to determine the K1c.

Comparison of the laser treated surfaces with the as received surfaces showed that the K1c of silicon nitride was raised to 3.07 MPa m1/2 and 1.68 MPa m1/2 for zirconia ceramics when employing the CO2 laser. This was indicating that the CO2 laser reduced the hardness of the near surface layer which softened the ceramic and exhibited more resistance to fracture.

Laser treatment using the fibre laser increased the K1c by 1.8 MPa m1/2 with silicon nitride and 3.14 MPa m1/2 with zirconia ceramics when compared to the as received surfaces. In this case, the hardness of the fibre laser treated samples measured 4 % decrease with silicon nitride and 4% for zirconia. This meant that there was not a significant change in the hardness in comparison to the as received surface. However, the K1c was yet increased due to the reduction in the resulting crack lengths produced from the Vickers indentation test. The reduction in the crack lengths indicated that both ceramics treated by the fibre laser had become more resistive to crack propagation by a possible inducement of residual compressive stress. It can be concluded that the end effect of applying both laser types to the ceramics differ due to the differing wavelength which changed the thermal energy input into the ceramics, the brightness of the two lasers which could have possibly affected the absorption ratio and finally, the way in which the thermal heat is dissipated through the ceramics.

(Paper 202)

Pulse Control improves the micromachining of hard materials

Posted on October 30, 2009 by Steven Glover

By: Sami T. Hendow

Multiwave Photonics

Pulsed fiber lasers are being used extensively in microstructuring of materials. Various reports indicate that the temporal shape and duration of the pulse are important for increasing the material removal rates and cutting speed. In particular, the leading edge, which is often observed in pulsed fiber lasers, is suggested to enhance the process of cutting and material ablation.

In this report, a pulsed MOPA fiber laser is used to demonstrate that significant improvement in ablation quality and cutting speeds of hard materials, such as silicon, can be obtained if the pulse shape, pulse energy, and peak power are controlled and optimized.

An important capability of the MOPA laser used is the freedom to change one of the pulse parameters without affecting the others. This enables us to optimize the process for the application. For example, we can adjust the pulse peak power, while maintaining pulse energy to suite the application, or we can adjust repetition frequency without affecting the shape of the pulse, up to the maximum average power for the laser.

Optimization of the ablation process

The leading edge is observed to enhance material removal and to speed up wafer cutting speeds. However, this is seen to be important only when the pulse has sufficient tail energy. This later segment of the pulse contributes to heating of the surface of the material, which in turn is coupled with the leading edge to enhance material ablation. Alternately, a short pulse with high peak power and no tail, leads only to ablation of a thin layer. This is often attributed to the formation of highly ionized plasma which prevents the remainder of the pulse from reaching the material.

To facilitate the study, the MOPA laser is operated in two separate modes of operation; (1) constant pulse energy as the pulse width is adjusted; and (2) constant output peak power as pulse width and repetition frequency are changed. In all cases, the system is operated with an average power of 10W at the work piece.

The MOPA system is configured to operate with pulses ranging from 10ns to 250ns, and with pulse repetition frequencies from single shot to 750kHz. Pulse energy can be up to 0.5mJ, and peak power of up to 10kW. On target, these translate to the high fluence levels of over 150J/cm², or over 3GW/cm² peak power levels.

Data is presented that demonstrates a factor of two improvements in cutting time and material removal if pulse peak power, energy and shape are controlled and optimized. Other data is presented for scribing and percussion drilling of mono and multi-crystalline silicon, aluminium-coated silicon and copper.

Fig.1. To optimize the process of machining, the pulsed MOPA laser is operated in two configurations: (A) pulses of equal energy but different widths and peak power, and (B) pulses of different widths but with the same peak power.

Paper M209

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