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Top 20 Most Read Articles
April 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Weld strength of laser-welded hot-press-forming steel J. Laser Appl. 24, 022004 (2012); http://dx.doi.org/10.2351/1.3699080 (6 pages) Online Publication Date: 02 April 2012
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There is an increasing demand for ultrahigh-strength steel in automotive industries to facilitate the manufacture of lightweight vehicles without compromising their security features. Although transformation-induced plasticity and dual phase steels have strengths of under 1 GPa, boron-alloyed steel obtained after the hot-press process has a strength of over 1500 MPa. Al–Si-coated steel has been developed to prevent excessive oxidation during high-temperature processes, but the Al–Si coating is known to lower weld properties. In this research, the laser weldability of hot-press-forming steels with and without Al–Si coating was examined. The specimens were laser-welded in butt and overlap joints by using disk and fiber lasers. The influence of the process parameters on the weld strength was investigated.
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Laser dressing of metal bonded diamond blades for cutting of hard brittle materials J. Laser Appl. 24, 022002 (2012); http://dx.doi.org/10.2351/1.3685300 (5 pages) Online Publication Date: 28 February 2012
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In this study, the authors investigate the laser dressing of metal bonded diamond blades by means of laser pulses with different pulse durations and wavelengths. Conventional dressing suffers from excessive blade wear, whereas laser dressing enables precise removal of the bonding metal to generate the required chip space of protruding diamond grains. The challenge for processing this composite material is to find appropriate level of ablation for the bonding metal without damaging the diamond grains through cracking or graphitization. For worn out blades, the influence of pulse duration and wavelength on laser dressed surface topography and bonding metal removal is studied. The experiments are performed with 532 and 1064 nm laser radiation with pulse durations from 12 ps to 20 ns. The blade surface topography and bonding metal removal of the dressed blades are measured with a confocal microscope, where the protruding diamond grains are identified by scanning electron microscope examinations. The dressing results show the bonding metal removal for the available laser sources in the studied pulse duration range, with the increasing number of protruding diamond grains for shorter pulse durations. The results indicate a significant increase of blade lifetime for laser dressed blades compared to conventional dressing methods.
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Energetic efficiency of remote cutting in comparison to conventional fusion cutting J. Laser Appl. 24, 022007 (2012); http://dx.doi.org/10.2351/1.3697813 (7 pages) Online Publication Date: 12 April 2012
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The remote cutting technique provides an enormous potential in terms of cutting speeds when working on thin sheets. Even on contour cutting speeds about 100 m/min are realizable. Working without any cutting gas, the material of the cutting kerf must be vaporized partially. It is evident that the energy input must be higher than for pure melting of the cutting kerf’s material. In order to characterize laser cutting processes in terms of energetic efficiency, the severance energy can be used. This parameter depends on the necessary laser power to cut a defined sheet thickness at a certain cutting speed. Hitherto, it was used to compare different laser cutting processes using different laser beam sources when cutting straight lines. In order to characterize different processes for cutting real contours, the geometry of the part to be cut is also important. The complexity of a part can be characterized by the agility. The target of this paper is to combine the severance energy and the agility in order to compare the remote cutting and the conventional fusion cutting energetically. It will be shown where remote cutting can be applied very efficiently to produce parts in a flexible and highly productive way and what the application area of conventional fusion cutting with fast axis is.
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Laser ablation of polyetheretherketone films for reversible wafer bonding J. Laser Appl. 24, 022003 (2012); http://dx.doi.org/10.2351/1.3693530 (5 pages) Online Publication Date: 30 March 2012
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A reversible wafer bonding method has been developed that enables high-temperature processing of thinned silicon wafers. The silicon wafers are bonded to Pyrex carriers using a polyetheretherketone (PEEK) film, which melts at 343 °C, and provides a very strong bond. Debonding is accomplished by UV laser ablation through the Pyrex carrier and can be facilitated by coating the Pyrex wafer with Teflon. Most of the PEEK film remains on the silicon wafer after debonding and is removed with solvents. Precoating the silicon with germanium/tetraethyl orthosilicate (TEOS) might enable PEEK removal without solvents. This germanium/TEOS layer lifts off with the PEEK film during laser ablation.
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J. Laser Appl. 24, 022005 (2012); http://dx.doi.org/10.2351/1.3701047 (7 pages) Online Publication Date: 05 April 2012
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Microstructures fabricated on the surface of solar glass have the potential to improve the performance of solar cells. In this paper, in order to overcome the high transmittance to 1064 nm center wavelength fiber laser irradiation and realize high efficiency process on transparent glass substrates, different absorber materials, including alumina powder, alumina ceramic wafers, and copper sulphate solutions, were applied for dry and wet etching under the irradiation of 1064 nm pulsed fiber laser respectively. The laser fluence was varied from 7 to 10 J/cm2 with a pulse repetition rate of 20 kHz. The morphology of trenches etched by means of laser induced backside dry etching (LIBDE) and by laser induced backside wet etching was measured using a scanning electronic scope, and compared from the aspects of etch depth and width, as well as the roughness. On the basis of this comparison, a higher etch rate can easily be obtained by dry etching, while lower roughness is a feature of wet etching. The mechanism of LIBDE of solar glass was investigated by demonstrating the procedure of dry etching using alumina ceramic wafer. Moreover, the etch threshold fluence was estimated to be 7.47 J/cm2 by extrapolation. Both types of laser induced backside etching techniques, wet and dry, show the evidence of effective microprocessing on solar glass.
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Review of laser hybrid welding J. Laser Appl. 17, 2 (2005); http://dx.doi.org/10.2351/1.1848532 (13 pages) Online Publication Date: 09 February 2005
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In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more efficiently in the future. |
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A review of ultrashort pulsed laser ablation of materials J. Laser Appl. 10, 18 (1998); http://dx.doi.org/10.2351/1.521827 (11 pages)
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The use of ultrashort pulsed lasers in materials processing is an emerging technology. These lasers have the capability to ablate materials precisely with little or no collateral damage, even with materials that are impervious to laser energy from conventional pulsed lasers. The extreme intensities and short timescale at which ultrashort pulsed lasers operate differentiate them from other lasers. The means of ultrashort pulsed laser generation is discussed; included are a survey of pulse compressor techniques with solid state lasers and a brief discussion of excimer-dye lasers. This is followed by a discussion of specific examples of ultrashort pulsed machining of specific materials, along with mechanistic details. Optical breakdown mechanisms, including electron avalanche ionization and multiphoton absorption are discussed. It is shown that as pulse width increases and intensity decreases, laser damage becomes a stochastic process in which the ultrashort pulsed, high intensity light causes optical breakdown over a very narrow range. This, along with the lack of significant thermal conduction, greatly improves the precision of ultrashort pulsed lasers in micromachining applications. © 1998 Laser Institute of America. |
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Laser texturization to improve absorption and weld penetration of aluminum alloys J. Laser Appl. 24, 012002 (2012); http://dx.doi.org/10.2351/1.3659991 (7 pages) Online Publication Date: 02 December 2011
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In the present work, laser texturization treatments have been performed for the first time on aluminum alloys to increase their absorption and weld penetration. Adjusting the experimental conditions, laser texturization increases the roughness and decreases the diffuse and specular reflectance of surfaces. The textured samples were subsequently subjected to bead-on-plate laser weld treatments with a high power diode laser. Taking the weld beads of sandblasted samples as reference, depth improvements percentages around 20% are reached in some textured samples. Laser texturization has demonstrated to increase the weld penetration ability of aluminum alloys, constituting a potential tool to reduce the energetic requirements of the laser welding process.
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Ultra-high speed disk laser cutting of carbon fiber reinforced plastics J. Laser Appl. 24, 012007 (2012); http://dx.doi.org/10.2351/1.3673521 (8 pages) Online Publication Date: 30 December 2011
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Laser cutting of carbon fiber reinforced plastics (CFRPs) has been known to be difficult due to the thermal damage to CFRP material. Therefore, with the objectives of evaluating the cutting possibility for long- or short-fiber pellet CFRP sheets of 3 mm in thickness and obtaining narrower heat affected zone (HAZ) for better cut quality, ultra-high speed cutting was conducted by using a continuous-wave high brightness disk laser. The increase in the laser power rendered the cutting time shorter and the kerf depth deeper for CFRP full cut. It was revealed that CFRP cut with narrow HAZ of less than 50 μm could be produced under the conditions of the laser powers of 2–5 kW, the cutting speed of 5 m/s, the time interval between repeated runs of 1 s, and the defocused distance of 0 mm. It was also confirmed that short fiber pellet CFRP was more easily cut and suffered less thermal damage in comparison with long fiber one. Furthermore, it was demonstrated that the tensile strength of fully laser-cut CFRP was almost equal to that of the base CFRP specimen. This
means the laser-cut surfaces of the CFRP sheet were not degraded in mechanical properties.
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The role of electron–phonon coupling in ultrafast laser heating J. Laser Appl. 17, 63 (2005); http://dx.doi.org/10.2351/1.1848522 (6 pages) Online Publication Date: 09 February 2005
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Most of the ultrafast laser heating analysis to date has been accomplished with a constant electron–phonon coupling factor (G). Due to the significant changes in the electron and lattice temperature caused by high-power laser heating, G could be temperature dependent. In this article a phenomenological temperature-dependent G is introduced to simulate ultrafast laser heating in metals. The electron temperature and the ablation depth computed with the temperature-dependent G compare well with experimental data. |
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Laser lap welding of zinc coated steel sheet with laser-dimple technology J. Laser Appl. 22, 87 (2010); http://dx.doi.org/10.2351/1.3485596 (5 pages) Online Publication Date: 29 September 2010
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Laser beam welding technology has been widely used to weld automobile components, especially for tailored blank welding. In order to provide best corrosion resistance to the welded sheet metal parts, zinc coated steel sheets are normally used. The zinc coating poses no issues to the butt joining of the sheet metals. However, when laser welding technology is applied to lap joint of these sheets, the welding process is not straightforward. Special techniques must be employed to allow the venting of the zinc vapor that is generated at the interface between the paired sheets. Many efforts have been attempted around the world trying to develop a practical technique for laser lap welding of zinc coated steel sheets. Most of the developed technologies has some success but with limitations—extra cost for equipment and process or limited convenience of implementation. In this paper, a new concept of laser lap welding with laser dimpling technology is described. This lap welding technique was implemented successfully in a robotic laser welding system in the laboratory environment and is capable of incorporation into manufacturing processes. |
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Influence of the beam profile formulation when modeling fiber-guided laser welding J. Laser Appl. 23, 042005 (2011); http://dx.doi.org/10.2351/1.3633221 (9 pages) Online Publication Date: 22 September 2011
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During deep penetration laser welding, the focused laser beam determines the vapor capillary, called keyhole, and in turn the whole process physics. Beside spot diameter and Rayleigh length, the beam profile is another important but hardly explored part of the focused laser beam. The focusing of fiber-guided Yb:fiber, Nd:YAG or diode laser beams creates the complex situation that the beam has a top-hat profile in the focal plane but toward the far field transforms to a Gaussian beam. Such power density distribution was measured for a focused high power Yb:fiber laser beam and then approached by three different beam formulations. The beam formulations were then applied to model the keyhole shape during laser welding. Although a second order beam model approached the measured beam significantly more accurately, the first order Gaussian beam was similarly suitable to predict the keyhole shape as long as the central beam domains do not interact with the material, which occurs only for low focal plane positions. A hypothetical top-hat beam would cause a different, steeper keyhole shape. Consequently, a Gaussian beam is still a suitable formulation for a wide range of welding parameters.
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J. Laser Appl. 24, 022001 (2012); http://dx.doi.org/10.2351/1.3685299 (6 pages) Online Publication Date: 28 February 2012
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Nitrogen-doped diamond was synthesized in open air using laser-assisted combustion flame method. A wavelength-tunable CO2 laser was used to resonantly excite the vibration modes of ammonia molecules, which were added into the diamond forming combustion flame. The wavelength of the CO2 laser was tuned to match frequencies of the NH wagging mode of the ammonia molecules. High efficiency energy coupling was achieved at laser wavelengths of 9.219, 10.35, and 10.719 μm, which are related to a rotational–vibrational transition (1084.63 cm−1), and splitting of the NH wagging mode (υ2+, 932.51 cm−1 and υ2−, 968.32 cm−1). Vibrational excitations of the ammonia molecules under these wavelengths actively intervenes the reaction courses, which steers the chemical reaction in the combustion flame and eventually promotes nitrogen concentration in the deposited diamond films. Concentration of the doped nitrogen atoms reaches up to 1.5 × 1020 atoms/cm3 in the diamond films deposited with a laser wavelength of 9.219 μm. Optical emission spectroscopy and mass spectrometry were used to study the evolution of chemical reactions with and without laser excitations.
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Effect of atmosphere above specimen on welding results during remote welding J. Laser Appl. 23, 022007 (2011); http://dx.doi.org/10.2351/1.3567959 (7 pages) Online Publication Date: 19 April 2011
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Remote welding has already been applied in the automobile industry [
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Femtosecond laser machined microfluidic devices for imaging of cells during chemotaxis J. Laser Appl. 23, 042001 (2011); http://dx.doi.org/10.2351/1.3614405 (5 pages) Online Publication Date: 04 August 2011
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Microfluidic devices designed for chemotaxis assays were fabricated on fused silica substrates using femtosecond laser micromachining. These devices have built-in chemical concentration gradient forming structures and are ideally suited for establishing passive diffusion gradients over extended periods of time. Multiple gradient forming structures, with identical or distinct gradient forming characteristics, can be integrated into a single device, and migrating cells can be directly observed using an inverted microscope. In this paper, the design, fabrication, and operation of these devices are discussed. Devices with minimal structure sizes ranging from 3 to 7 μm are presented. The use of these devices to investigate the migration of Dictyostelium discoideum cells toward the chemoattractant folic acid is presented as an example of the devices’ utility.
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Welding of thick stainless steel plates up to 50 mm with high brightness lasers J. Laser Appl. 23, 022002 (2011); http://dx.doi.org/10.2351/1.3567961 (7 pages) Online Publication Date: 28 March 2011
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Thick-section stainless steels are widely used in the components and structures for nuclear power plants. Laser welding is being considered as a high-efficiency method instead of arc welding for some components, so as to improve the production efficiency and reduce the residual stresses of the heat-affected zone. In this paper, multipasses narrow-gap welding of 50 mm thick Type 316L plates with an 8 kW disk laser was first investigated. The effect of welding conditions on the weld bead geometry and welding defects was studied. It shows that lack of fusion could be prevented by optimizing the relationship between laser power intensity and the deposited metal volume. Butt joint of 50 mm thick plates with narrow gap could be performed with eight-layers welding at laser power of 6 kW and welding speed of 0.4 m/min. In order to reduce the weld passes further, gas jet assisted laser welding was tried to weld thick Type 316L plates with a 10 kW fiber laser. The result shows that butt-joint welding of 40 mm plates without filler wire could be carried out at 0.3 m/min welding speed with no porosity or other welding defects. As for 50 mm thick plate, a good weld bead could be obtained with bead-on-plate welding from both sides at 0.2 m/min welding speed.
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J. Laser Appl. 23, 032005 (2011); http://dx.doi.org/10.2351/1.3597830 (7 pages) Online Publication Date: 30 June 2011
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Spatter, the ejection of melt from a weld pool, is a major problem whenever it occurs in a welding process. The ejection of droplets from the weld metal results in a weld with underfill, undercuts, craters, blowholes, or blowouts—all of which can have a detrimental effect on the mechanical properties of the weld. This paper presents a systematic description of the different types of spatter phenomena which occur during laser welding. A categorization system is proposed to facilitate the comparison and combination of research findings on spatter. This should allow researchers in this area to act as a more effective team in future.
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J. Laser Appl. 21, 154 (2009); http://dx.doi.org/10.2351/1.3184429 (8 pages) Online Publication Date: 13 August 2009
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Inert gas assisted laser cutting of 10 mm stainless steel plate and 4 mm aluminum sheet was performed with a 5 kW fiber laser. The effects of laser power, cutting speed, focal point position, and assist gas pressure on the cutting performance and cut quality were investigated. Clean cut surfaces without or with minimal dross were achieved with some combinations of process parameters and attempts were made to define parameter windows in terms of cutting speed and laser power for good quality cutting. The maximum cutting speeds for acceptable cut quality were determined at different power levels. The range at which complete through cutting could be achieved (so-called parameter window) was limited upwards by insufficient power intensity to obtain through cutting at high cutting speeds and downwards by heat conduction at slow cutting speeds. The effects of focal point position and assist gas pressure on the striation pattern (cut surface roughness) were also examined. Low surface roughness was achieved with the focal point position inside the workpiece showing the need for a wider kerf for better melt ejection in thick-section metal cutting. There was also a reduction in surface roughness with increase in assist gas pressure, but there was no significant reduction in surface roughness above the gas pressure of 16 bar, which could be due to the gas flow dynamics inside the narrow cut kerf at high assist-gas pressures. |
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J. Laser Appl. 24, 022006 (2012); http://dx.doi.org/10.2351/1.3701400 (11 pages) Online Publication Date: 11 April 2012
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Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000–4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3% and 6% of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperature, even with unexpected variations of the emissivity during the observed process, and to give a relevant value of this emissivity. A validation of a comsol numerical model of the heating cycle of W, Ta, Nb, V rods has been obtained by comparison with the measurement.
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Weld strength improvement for Al alloy by using laser weaving method J. Laser Appl. 22, 116 (2010); http://dx.doi.org/10.2351/1.3499456 (4 pages) Online Publication Date: 19 October 2010
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Laser welding of aluminum alloys usually produces linear welds that have shear-tensile strength lower than that of the base metal. This is usually due to porosity resulting from an unstable keyhole and cracking which is related to the high crack sensitivity of many of these alloys. In this work laser weaving is used to improve the joint strength of laser welds. In this research, lap welds and bead-on-plate welds were produced on 1 mm thick aluminum alloy sheets using various weaving schemes. When the weaving width was 4 mm, crack propagation was reduced, compared with a weaving width of 2mm. No cracks occurred when the weaving frequency was 5 Hz for weaving widths of 2 and 4 mm. The shear-tensile strengths were measured according to the weaving parameters such as weaving width and frequency. The laser weaving increased the fusion line length, therefore improving the joint strength. |
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