Monthly Archives: October 2009

Comparative analysis of GMA, laser, and laser-GMA hybrid welding

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By: Siltanen Jukka

Laser and laser hybrid welding processes have attracted interest during the last few years in engineering industry. The basic reason is the need to increase the productivity in order to be more competitive in global markets. In comparison to traditional arc welding processes, laser welding has several benefits such as low heat input resulting in low amount of distortion, deep weld penetration and high welding speed, high quality of welds and possibility to weld a wide spectrum of materials. However, laser welding means that the groove and processing tolerances become tighter as the focal point diameter of the laser beam used for deep penetration welding is very small. In one view this can be seen as a benefit, because it improves the accuracy of a product or component.

Laser hybrid welding (combination of laser and arc welding) permits looser groove tolerances compared to laser welding. The use of filler metal enables the filling of gaps formed during welding and due to inaccuracies in edge preparation or other earlier manufacturing phases as well as gives the possibility to mix the weld metal such to achieve the desired mechanical properties. Another restriction of laser welding is the high investment cost of the laser source and the welding system with a high accuracy. On the other hand, during the recent years the prices of the laser sources have lowered, but still they are a lot more expensive than the arc welding sources.

In 2006 Rautaruukki Oyj (www.ruukki.com) started a project called Rlaser, the aim of which was to find new ways to use laser technologies and discover new business areas and markets. All three business divisions (Construction, Engineering, and Metals) of Rautaruukki Oyj participate in the project. The project has several independent sub-projects focused on the research areas like mechanical properties of the welds and products, optimizing of the manufacturing chain, economical and technical comparison of the production methods and measuring techniques. The conference paper “Comparative analysis of GMA, laser, and laser-GMA hybrid welding” in ICALEO 2009 is especially focused on the mechanical properties of the joint of mild steel S355. Figure 1 shows the testing arrangement of laser-GMA hybrid welding.

Currently, Rautaruukki Oyj concentrates increasingly on the manufacturing of high and ultra high strength steels (up to 1100 MPa yield strength) and therefore in further studies these steels have an essential role. The use of laser and laser hybrid welding as a joining method is very attractive for such special steels. This is due to the established limits for welding energy and cooling time which are accomplished easily with the laser and laser hybrid welding.

Figure 1. Laser-GMA hybrid welding of mild steel S355

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

Electrical-Mechanical-Systems

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By: Norbert Lorenz

Over the past few years the interest in Micro-Electro-Mechanical-Systems (MEMS) and Micro-Optical-Electrical-Mechanical-Systems (MOEMS) from research institutions, industrial firms and the press has risen considerably. Of particular interest are lab-on-a chip devices where many subsystems (ICs and MEMS) are assembled to a functional system for medical and bio-photonic applications. To date the main focus has been on the devices themselves; the research and development of low cost, high reliability assembly and packaging has been rather limited, and as a result current MEMS packaging costs can reach up to 90% of the overall device costs.

MEMS are sensors and actuators on a micro- and nanometre scale incorporating mechanical elements and electronics. They are normally built on a common silicon substrate through microfabrication technology, although alternative technologies using different materials are being increasingly developed. MEMS are commonly known for the realisation of “lab-on-a-chip” devices where many subsystems (ICs and MEMS) are assembled together to make a functional system.

The Heriot-Watt group has demonstrated the feasibility of laser-based glass frit bonding of a range of miniature packages, important for MEMS and related applications. The laser provides localised heat energy, preventing damage to temperature-sensitive materials or dis-assembly of other components of the overall system. Glass frit is an ideal material to use, as it conforms well to the surfaces being joined, provides good hermeticity, and the temperatures required are relatively modest.

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

Carbon Nanotubes (CNTs)

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By: Masoud Mahjouri-Samani

Limitations of current technology and demands for smaller, faster, more efficient and cost effective electronic and optical devices have provoked scientists and researchers around the world to find new materials which satisfy their needs. The extraordinary characteristics and properties of carbon nanotubes (CNTs) have made them promising candidates for fabricating nano-scale electronics and optical devices such as transistors and sensors.

Although tremendous progress has been made in the growth and fabrication of CNTs and CNT-based devices, CNT-based devices have not yet been mass-produced due to lack of sufficient methods for controlling the location, orientation and the type of the CNTs grown on the samples. Current synthesis techniques provide CNTs of different electronic types, including semiconductor, semimetal and metal, which play different roles in applied fields, such as molecular conductors for metallic tubes and transistors for semiconducting tubes. Therefore, controlled production of mono-dispersed CNTs with the same electronic properties is highly desired in developing CNT-based electronics and photonics.

Works have been reported attempting to achieve precise discerning of CNTs of different electronic properties, including preferential growth of s-CNTs, post-growth separation, and selective removal of metallic tubes. However, several drawbacks, such as liquid processing and polymer wrapping, degrade the quality of the CNTs and prevent their practical applications. Also, there are very limited investigations on the in-situ selective separation of CNTs deposited on solid substrates, which are frequently used in device fabrication. Therefore, a new approach is desired to achieve in-situ efficient separation of surface deposited CNTs according to their electronic properties and still preserve their pristine properties.

In this study, an optically controlled approach is developed to remove m-CNTs selectively from CNT mixtures deposited on SiO2 surfaces through well tuned laser irradiation process. The mechanism behind the process is attributed to the free electron movement and optical near-field effects under the electromagnetic field generated by the laser irradiation. Due to the distinct electronic properties, m-CNTs and s-CNTs react to the laser irradiation in different ways. For m-CNTs, existing electrons in conduction bands respond readily to the electromagnetic field from the laser irradiation. However, electrons in s-CNTs have to overcome the band-gaps by absorbing photons of matching energy and transit from valance bands to conduction bands. Therefore, m-CNTs are more responsive to the incident lights than the s-CNTs, and yield thermal effects. Through this process, m-CNTs were selectively heated and preferentially oxidized when exposed to air. This method demonstrates a simple and efficient approach for processing s-CNT based devices through optical selection.

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

Effect of grain size on laser machining of alumina

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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

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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