Pulsed Laser Spot Research Articles

A theoretical study on vertical finite cracks detection using pulsed laser spot thermography (PLST)

A theoretical study on vertical finite cracks detection in low carbon steel (A36) blocks using pulsed laser spot thermography (PLST) was carried out in this paper. The basic principle of PLST was described. The 3D heat conduction model of flux transfer under pulsed laser spot excitation was established and calculated using finite element method (FEM). The effect of laser-crack distance to temperature abrupt jump across the vertical finite cracks was analyzed, and results show that the optimum laser-crack distance is the radius of laser spot. The relationship between geometric parameters of cracks and temperature abrupt jump was studied, and results show larger geometric size of cracks leads to greater peak values of temperature abrupt jump, and tends to stable values at certain size.

Weld bead formation and corrosion behavior of pulsed laser welded zirconium alloy

Pulsed laser spot welding was developed for the fabrication of Zircaloy spacer grid assembly in pressurized light water reactors. The effects of laser welding parameters on the size of the weld beads were discussed. The influence of oxygen and hydrogen uptakes during the welding process on the corrosion resistance was investigated after the corrosion tests. The results indicate that the optimum weld bead size could be obtained by adjusting the pulsed laser parameters, including laser peak power, number of shots, and pulse width. Pulse width should be taken into consideration to control the weld width. When the moisture and oxygen contents were maintained at a low level, i.e., 200 parts per million (ppm) and 30 ppm, respectively, the oxide thickness of specimens corroded for 120 h was about 0.7 μm compared to about 1.5 μm at high moisture and oxygen contents (1,000 and 280 ppm). The combination of the oxidation and hydriding induced cracking was responsible for the crack of the specimens welded under high moisture and oxygen contents, thereby decreasing the corrosion resistance of the welding specimens.

Pulsed laser spot welding of intersection points for Zircaloy-4 spacer grid assembly

A pulsed laser spot welding technology has been developed for joining intersection points of 0.457mm-thick Zircaloy-4 straps. Weld beads size, mechanical properties and microstructure characterization of the weld beads were investigated. The results indicate that peak power of the pulsed laser has significant influence on the penetration depth of weld beads. Pulse width and number of shots should be taken into consideration mainly to control the weld width. The average value of ultimate tensile loads of the intersection points are continuously increasing as the penetration depth and weld width of weld beads increase. Fracture was located at the fusion line and the fracture surface could be characterized as a mixture of ductile and cleavage feature. Column grains and equiaxed grains were observed in fusion zone and heat affected zone, respectively. The fusion zone consists of a mixture of α-Zr and β-Zr phases among which lamellar Widmanstatten structure α-Zr+Zr3Fe was distributed. The Zr(Fe, Cr)2 second phase particles were precipitated inter and intragranular of α-Zr grains.

A retrospective analysis of the management of freckles and lentigines using four different pigment lasers on Asian skin

Background: The risk of post-inflammatory hyperpigmentation (PIH) is increased during freckles and lentigines treatment in Asians. Objective: To determine the effectiveness and safety of using 595-nm long pulsed dye laser (LPDL), 755-nm LP Alexandrite laser, 532-nm QS Nd:YAG laser and 532-nm LP potassium-titanyl-phosphate (KTP) laser for the treatment of freckles or lentigines in Asian patients. Methods: This is a retrospective study of 40 Chinese patients, who were divided into four groups based on treatment modality using four different pigment lasers. Each patient attended between 1 and 4 treatments (mean of 1.8), at 4–6 weeks intervals, depending on clinical response. Lesional clearance and PIH were assessed by two independent clinicians. Results: Statistically significant improvement of global and focal facial pigmentation was found after treatment with LPDL, QS Nd:YAG and LP KTP lasers. No significant improvement was found after LP Alexandrite laser. PIH risk was 20% after LP Alexandrite treatment, 10% with QS Nd:YAG, and absent after LPDL and LP KTP treatment. Conclusion: A long pulse laser and small spot size appear to reduce the risks of lentigines treatment in darker skin types.

Temperature Distribution Simulation for Pulsed Laser Spot Welding of Dissimilar Stainless Steel AISI 302 to Low Carbon Steel AISI 1008

This paper describes the development of a computer model used to analyze the heat flow during pulsed Nd:YAG laser spot welding of dissimilar metals; stainless steel AISI 302 to low carbon steel AISI 1008. The model is built using ANSYS FLUENT 6.3 software where almost all the environments simulated to be similar to the experimental environments. A simulation analysis was implemented, based on conduction heat transfer, out of the key hole where no melting occurs. The effect of laser power and pulse duration was studied. Four peak powers 5, 5.5, 6.5 and 7 kW were varied during pulsed laser spot welding (keeping the energy constant), also the effect of four pulse durations 5, 6, 6.5 and 7 ms (with constant peak power), on the transient temperature distribution and weld pool dimensions was predicted ,using the present simulation. It was found that the present simulation model can give an indication for choosing the suitable laser parameters (i.e. pulse duration, peak power) during pulsed laser spot welding of dissimilar metals.

Temperature Distribution Simulation for Pulsed Laser Spot Welding of Dissimilar Stainless Steel AISI 302 to Low Carbon Steel AISI 1008

This paper describes the development of a computer model used to analyze the heat flow during pulsed Nd:YAG laser spot welding of dissimilar metals; stainless steel AISI 302 to low carbon steel AISI 1008. The model is built using ANSYS FLUENT 6.3 software where almost all the environments simulated to be similar to the experimental environments. A simulation analysis was implemented, based on conduction heat transfer, out of the key hole where no melting occurs. The effect of laser power and pulse duration was studied. Four peak powers 5, 5.5, 6.5 and 7 kW were varied during pulsed laser spot welding (keeping the energy constant), also the effect of four pulse durations 5, 6, 6.5 and 7 ms (with constant peak power), on the transient temperature distribution and weld pool dimensions was predicted ,using the present simulation. It was found that the present simulation model can give an indication for choosing the suitable laser parameters (i.e. pulse duration, peak power) during pulsed laser spot welding of dissimilar metals.

Large spot transpupillary thermotherapy: A quicker laser for treatment of high risk prethreshold retinopathy of prematurity - A randomized study

To compare structural and functional outcome and time efficiency between standard spot sized conventional pulsed mode diode laser and continuous mode large spot transpupillary thermotherapy (LS TTT) for treatment of high risk prethreshold retinopathy of prematurity (ROP). Ten eyes of five preterm babies having bilateral symmetrical high risk prethreshold ROP were included in this study. One eye of each baby was randomized to get either standard spot sized conventional pulsed mode diode laser or continuous mode LS TTT. There was no significant difference between structural or functional outcome in either group. The mean time taken for conventional diode laser was 20.07 minutes, while that for LS TTT was 12.3 minutes. LS TTT was 40% more time efficient than the conventional laser. It may be better suited for the very small fragile premature infants as it is quicker than the conventional laser.

Crack imaging by scanning pulsed laser spot thermography

A new crack imaging technique is presented that is based on second derivative image processing of thermal images of laser heated spots. Experimental results are shown that compare well with those obtained by the dye penetrant inspection method. A 3D simulation has been developed to simulate heat flow from a laser heated spot in the proximity of a crack. A ‘ghost point’ method has been used to deal efficiently with cracks having openings in the micometre range. Results are presented showing the effects of crack geometry and system parameters on thermal images of laser heated spots.

Crack imaging by pulsed laser spot thermography

A surface crack close to a spot heated by a laser beam impedes lateral heat flow and produces alterations to the shape of the thermal image of the spot that can be monitored by thermography. A full 3D simulation has been developed to simulate heat flow from a laser heated spot in the proximity of a crack. The modelling provided an understanding of the ways that different parameters affect the thermal images of laser heated spots. It also assisted in the development of an efficient image processing strategy for extracting the scanned cracks. Experimental results show that scanning pulsed laser spot thermography has considerable potential as a remote, non-contact crack imaging technique.

Scanning laser microscopy of an MgB2 superconducting sensor

The MgB2 detector consists of a 200-nm-thick MgB2 thin-film meander line, a 300-nm-thick SiO protective layer, and 150-nm-thick Nb electrodes. We prepared a straight MgB2 stripline of width 3μm and length 100μm. To understand the position-dependent sensitivity of the MgB2 detector, a scanning pulsed laser spot can be achieved by the combination of the XYZ piezo-drive stages and an optical fiber with an aspheric focusing lens. The observed scanning XY image indicates that the improvement in sensor homogeneity is very important to improve the sensitivity of the MgB2 sensor as a neutron detector.

Using the ultra-long pulse width pulsed dye laser and elliptical spot to treat resistant nasal telangiectasia

Thick linear telangiectasia on the ala nasi and nasolabial crease can be resistant to treatment with the potassium-titanyl-phosphate (KTP) laser and the traditional round spot on a pulsed dye laser (PDL). We evaluated the efficacy of a 3 mm x 10 mm elliptical spot using the ultra-long pulse width on a Candela Vbeam(R) PDL for treatment of PDL- and KTP laser-resistant nasal telangiectasia. Nasal telangiectasia resistant to PDL (12 patients) and KTP laser (12 patients) in 18 patients were treated with a 3 mm x 10 mm elliptical spot on the ultra-long pulse pulsed dye laser (ULPDL) utilising long pulse width [595 nm, 40 ms, double pulse, 30:20 dynamic cooling device (DCD)]. Six patients had previously received treatment with both PDL and KTP laser prior to ULPDL (40 treatments, range1-4, mean 2.2). Complete clearance was seen in ten patients, and eight patients displayed more than 80% improvement after ULPDL treatment. Self-limiting purpura occurred with round spot PDL and erythema with KTP laser and ULPDL. Subtle linear furrows along the treatment sites were seen in three patients treated with the KTP laser. ULPDL treatment delivered using a 3 mm x 10 mm elliptical spot was non-purpuric and highly effective in the treatment of nasal telangiectasia resistant to KTP laser and PDL.

フェムト秒レーザー照射による衝撃波を<br>利用した微細かしめ接合法の提案

Femtosecond laser is a type of ultrashort-pulse laser. Ultrashort-pulse laser irradiation generates high-pressure plasma and shock waves at the surface of the target. If the target is irradiated in water, the shock waves are enough to deform the target plastically. The authors proposed a new microjoining method using the shock waves accompanied by femtosecond laser irradiation. A thin wire was mounted in a fit bore drilled in a plate, and the laser was focused around the edge of the wire end face. The wire was deformed by shock waves generated by many laser pulses. After the laser irradiation, the deformed wire prevented the release of the plate, as well as the rivet connection. Some model tests were performed to examine forming properties and joining strength. Laser pulse energy, spot diameter and laser spot position influenced the deformed shape and joining strength. In the case of irradiation close to the wire edge, the deformed shape and joining strength varied. They were made stable by transferring the laser spot intermittently.

In-Process Monitoring and Adaptive Control during Pulsed YAG Laser Spot Welding of Aluminum Alloy Thin Sheets

In-Process Monitoring and Adaptive Control during Pulsed YAG Laser Spot Welding of Aluminum Alloy Thin Sheets

Time resolved nanosecond vortex dynamics in high Tc superconducting films

Flux dynamics in a high T c superconducting thin film is investigated in the ns time scale. The initial metastable state is determined by applying a pulsed magnetic field to a sample previous cooled below the irreversibility line. Selective local heating of the film with a pulsed laser spot lowers the vortex pinning force suddenly. The magnetic flux rearrangements that follow generate a voltage pulse between two contacts on the zero current biased sample. Detail simulations were performed in the framework of a phenomenological model based on Maxwell Equations with constitutive relations for creep and flux flow that fully describe the observed voltage signals.

Refractive index patterning of tellurite glass surfaces by ultra short pulse laser spot heating

Dot patterns of refractive indices were formed by the laser pulse irradiation on the tellurite glasses. The ternary tellurite glasses of TeO2-Na2O-Al2O3, TeO2-Na2O-GeO2 and TeO2-Na2O-TiO2 doped with 2 mol% of CoO were irradiated by a femtosecond pulse laser beam (800 nm) or by a green light beam (532 nm) from a second harmonic generator of a Q switch pulse YAG laser. The refractive index map of the glass was composed with an He-Ne laser beam by an scanning ellipsometric technique at a resolution of 100 μm × 50 μm, indicating that the spots possessing refractive index lower by about 0.05–0.38 than the surroundings were formed at the region irradiated by the laser beam. The irradiation of the femtosecond laser beam generated the dot patterns roughly equivalent to the beam size. The change of refractive index could be tunable by adjusting laser power, suggesting that the process could be applied to optical recording.

A new criterion for quality monitoring of pulsed laser spot welding using an infrared sensor Part 1: The radiation feature as a criterion for quality monitoring

Many manufactured goods use pulsed laser spot welding in joining and weld quality has to be controlled in particular. This paper suggests a criterion for quality monitoring of pulsed laser spot welding of thin metal sheets using a point infrared (IR) sensor. A new radiation feature is derived from a mathematical model and its properties are examined. Weld quality is closely related to temperature profile and there are many cases that can generate the expected weld quality. Change in weld quality should be detected from the variation in temperature profile. Simulation results show that the radiation feature can be used to detect the change in the initial temperature profile. In the experiments, radiation features and weld cross-sections were examined for various welding conditions. Results show that the radiation feature could detect the dimensional variation of the heat-affected zone (HAZ).

A new criterion for quality monitoring of pulsed laser spot welding using an infrared sensor Part 2: Quality estimation using an artificial neural network

This paper suggests a method for estimating weld quality using a radiation feature. In one experiment the dimensions of the weld joint were examined using the radiation feature. Results show that the feature can be used to estimate weld dimensions. In another experiment, weld strength was estimated using the feature. Since it would be laborious to examine a large number of radiation features and find the explicit relationship, an artificial neural network (ANN) was employed. In experiments, the significant welding parameters were varied within a controllable range and 640 laser spot welds were used for ANN learning. The correlation coefficient between the estimated and the measured strength was as high as 0.98 for learned parts. The other 180 welds were used to appraise the learned ANN. The correlation coefficient between the estimated and the measured strength was as high as 0.95 for the unstudied parts and the mean square error of estimation was as low as 0.78 kgf.

In-Process joint strength estimation in pulsed laser spot welding using artificial neural networks

Pulsed laser spot welding is used in the manufacture of many goods. Because weak joints can lead to product defects, it is important to monitor and control the joint strength precisely. This paper introduces a method to estimate the joint strength of spot welds during the welding process. A point infrared sensor is used to measure temporal radiation on the top face of the spot weld. Because variable measuring conditions affect the radiation power, a scale-free radiation feature is extracted from the measured radiation and used as a monitoring criterion. An artificial neural network (ANN) uses this feature to estimate joint strength. In experiments, significant welding parameters are varied within a controllable range, and 640 weld parts are used for ANN learning. The correlation coefficient between the estimated and measured strength is more than 0.98 for learned parts. Another 180 weld parts are used to appraise the efficiency of the learned ANN, and the mean square error of estimation is 0.78 kgf.

In-Process Joint Strength Estimation in Pulsed Laser Spot Welding Using Artificial Neural Networks

Pulsed laser spot welding is used in the manufacture of many goods. Because weak joints can lead to product defects, it is important to monitor and control the joint strength precisely. This paper introduces a method to estimate the joint strength of spot welds during the welding process. A point infrared sensor is used to measure temporal radiation on the top face of the spot weld. Because variable measuring conditions affect the radiation power, a scale-free radiation feature is extracted from the measured radiation and used as a monitoring criterion. An artificial neural network (ANN) uses this feature to estimate joint strength. In experiments, significant welding parameters are varied within a controllable range, and 640 weld parts are used for ANN learning. The correlation coefficient between the estimated and measured strength is more than 0.98 for learned parts. Another 180 weld parts are used to appraise the efficiency of the learned ANN, and the mean square error of estimation is 0.78 kgf.

A robust in-process monitoring of pulsed laser spot welding using a point infrared sensor

Many manufactured goods employ pulsed laser spot welding in joining and it is important to monitor the weld quality. The weld quality is highly related to the thermal history of the weld and there have been many trials to monitor the quality using an infrared (IR) sensor. Though the emitted thermal radiation is a function of the emissivity and the surface temperature, the measuring condition has an important influence on the detectable radiation. To increase the reliability of monitoring, it is useful to find the robust feature that is immune to the change of measuring condition. This paper suggests an in-process monitoring method using a point IR sensor for the pulsed laser spot welding of thin metal sheet. The suggested monitoring method is robust to the change of the measuring condition and it can detect the variation of the absorbed energy as well as any serious defect in the weldment.