Low-power Pulsed Laser Research Articles

Pulsed-Laser-Triggered Piezoelectric Photocatalytic CO2 Reduction over Tetragonal BaTiO3 Nanocubes.

The recombination of photoinduced carriers in photocatalysts is considered one of the biggest barriers to the increase of photocatalytic efficiency. Piezoelectric photocatalysts open a new route to realize rapid carrier separation by mechanically distorting the lattice of piezoelectric nanocrystals to form a piezoelectric potential within the nanocrystals, generally requiring external force (e.g., ultrasonic radiation, mechanical stirring, and ball milling). In this study, a low-power UV pulsed laser (PL) (3W, 355nm) as a UV light source can trigger piezoelectric photocatalytic CO2 reduction of tetragonal BaTiO3 (BTO-T) in the absence of an applied force. The tremendous transient light pressure (5.7 × 107 Pa, 2.7W) of 355nm PL not only bends the energy band of BTO-T, thus allowing reactions that cannot theoretically occur to take place, but also induces a pulsed built-in electric field to determine an efficient photoinduced carrier separation. On that basis, the PL-triggered piezoelectric photocatalytic CO2 reduction realizes the highest reported performance, reaching a millimole level CO yield of 52.9mmol g-1 h-1 and achieving efficient photocatalytic CO2 reduction in the continuous catalytic system. The method in this study is promising to contribute to the design of efficient piezoelectric photocatalytic reactions.

Research on 316 stainless steel low-power pulsed laser-induced arc additive manufacturing by different deposition routes

In this paper, a low-power pulsed laser-induced arc heat source is used to complete the wire-filled additive manufacturing of multi-layer multi-pass stainless steel walls. The low-power pulsed laser can induce and compress the arc to achieve the concentration of arc energy. This paper systematically studied the effects of different deposition paths on the microstructure and mechanical properties. The results show that the parallel deposition mainly consists of columnar grains, and the grain size of the fine grain area is about 10 μm. The longitudinal tensile strength of the parallel part can reach 620 Mpa, and that elongation is 46%. The horizontal tensile strength is 590 MPa and the elongation is only 40%. The alternating deposition part is mainly composed of oriented dendrites, and the grain size of the fine crystal area is about 12 μm. While the average tensile strength of the alternate part is 585 MPa, the average elongation is 38%. The parallel deposition part has better anisotropy, and the isotropy of the alternate deposited part is better. A parallel deposition path is recommended to choose if the component is required to serve under the unidirectional stress state. An alternate deposition path should be adopted if the component is required to serve under the multidirectional stress state.

Mechanism Analysis of Nanosecond Pulse Laser Etching of SiCp/Mg Composites.

Due to the introduction of silicon carbide reinforcement, the physical and cutting properties of SiCp/Mg composites are very different from those of metal composites. Nanosecond pulse laser processing is more efficient than traditional processing for SiCp/Mg composites. A low-power pulsed fiber laser was used to etch 3.0 mm thick SiCp/Mg composites. The effect of low laser power (0~50 W) on the morphology and heat-affected zone of the SiCp/Mg composite after etching was studied. The results show that when the laser power increases, the material accumulation at the ablation end of the machining surface becomes more and more serious. With the increase in power, the differences in ablation width and ablation depth on the surface of composite materials do not increase proportionally. When the laser power increases gradually, the width of the heat-affected zone increases in the direction of the perpendicular laser beam and reaches the maximum value at the etched end.

An Efficiency Improvement Driver for Master Oscillator Power Amplifier Pulsed Laser Systems

The master oscillator power amplifier (MOPA) pulsed laser, one of the popular topologies for high-power fiber laser systems, is widely applied in industrial machining laser systems. In MOPA, the low-power pulsed laser, stimulated from a seed laser diode, is amplified by the high- power optical energy from pump laser diodes via the gain fiber. Generally, the high-power pump laser diodes are driven by lossy linear current drivers. The switched mode current drivers boost the driver efficiency but suffer from pulse energy consistency due to the current switching ripple. In this paper, a laser driver system that varies the switching frequency of current source to synchronize with pulsed laser repetition rate is analyzed and implemented. Experimental results are demonstrated using a 20 W pulsed fiber laser system.

Effect of auxiliary heating process on low power pulsed laser wire feeding deposition

Low-power pulsed laser wire-feeding deposition has the advantages of low cost and less accumulation of heat. At the same time, the low laser power leads to the prominent problem of unfused defects in the deposited samples and the appearance of cold crack defects. In this study, a combined heating additive manufacturing process method combining the substrate induction heating and hot wire processes is proposed for this problem, and the corresponding experimental device is developed. Through an experiment on the influence of the auxiliary heating process on the deposition of pulsed laser wire feeding, the focus is on the improvement effect of the combined heating process on the deposition of low-power pulsed laser wire feeding. By comparing the processing experiments of thin-walled wall samples under different auxiliary heating process conditions, the influence of auxiliary heating process parameters on the geometry, microstructure, microhardness, and defects of the sample is revealed. The experimental process data and metallographic analysis results of the sample show that the combined heating process not only improves the energy input in the deposition process, but also reduces the temperature gradient between the deposited sample and the substrate, improves the laser absorption rate of the substrate, increases the size of the deposition channel and molten pool, reduces the surface roughness of the sample, and enhances the interlayer remelting, A more uniform macro shape contour is obtained. The open partial melting defect is effectively restrained, the porosity of the deposited sample is reduced, and crack defects are eliminated.

Research on aluminum alloy welding process based on high frequency and low power pulsed Laser-MIG hybrid welding

Taking 2219 aluminum alloy as the welding research base metal, a high frequency and low power pulsed laser-MIG hybrid welding system was established. When the laser pulse frequency ranges from 500 Hz to 1500 Hz and the laser-wire distance ranges from 1 to 2 mm, the effects of high-frequency and low-power pulsed laser on the arc shape, metal transfer behavior, and weld formation in the hybrid welding process were compared and analyzed by using high-speed camera system and electrical signal acquisition system. The results show that as the pulse frequency increases, the arc is gradually compressed, and the compression reaches the maximum when the frequency is 1500 Hz. In the test, the limit distance of laser attracted arc is 2 mm and the limit distance of laser stabilized arc is 1.5 mm. From the collected electrical signal, it can be seen that the welding current is reduced by 5.8A after adding laser irradiation. Since the welding power source is a constant voltage source, the welding voltage is almost constant, so the heat input of the welding arc to the base metal and droplets is reduced to a certain extent. When the laser is biased laterally along the welding wire, the weld morphology also shifts from the center to the laser irradiation point, which further verifies the guiding effect of the laser on the arc. In addition, the addition of the laser effectively improves the stability of the welding process, the weld seam is formed uniformly, and the oscillation of the molten pool caused by the laser accelerates the discharge of gas in the molten metal, which effectively reduces the pore defects.

Inducing thermionic emission from lanthanum hexaboride probes in Magnum-PSI

For thermionic emission rates exceeding the incident plasma electron flux, recent theory proposes an inverse sheath regime, with promising properties for future application in fusion edge plasmas. With the aim of inducing thermionic emission in fusion-relevant plasma conditions, several lanthanum hexaboride probes were heated in the linear plasma generator Magnum-PSI. During exposures at low plasma power and additional pulsed laser heating, the probe’s floating potential was reduced by up to 12%, providing a possible indication of thermionic emission. However, these observations coincided with rapid erosion of probe material, attributed to enhanced lanthanum self-sputtering. During follow-up experiments with helium plasmas at electron temperatures around 1 eV, the lanthanum ion impact energy and sputtering yield were reduced, and rapid erosion was avoided, thus confirming the thesis of self-sputtering. A parameter scan of plasma power resulted in LaB 6 surface temperatures up to 2450 °C, exceeding the theoretical inverse sheath threshold temperature by over 300 °C. However, the probe’s floating potential did not deviate from reference measurements using a probe with high electronic work function, indicating absence of strong thermionic emission. This apparent discrepancy is attributed to the effects of probe surface modifications as observed during these experiments: impurity deposition, erosion and cavity formation. These modifications possibly affected the LaB 6 electronic work function, thereby keeping the inverse sheath threshold out of reach. In conclusion, although LaB 6 has one of the lowest work functions available, the inverse sheath threshold conditions could not be reached with the present setup in Magnum-PSI. Surface modifications thus do form a limiting factor for the application of LaB 6 in fusion-relevant plasma conditions. Moreover, the window of stable operation for LaB 6 in dense hydrogen plasmas is limited below ∼ 1.5 eV, and does not overlap with the conditions expected in the edge region of future fusion devices like ITER. • LaB 6 probes were heated up to 2450 °C under fusion-relevant plasma conditions. • Surface modifications limit the effective operation of powder-pressed LaB 6 . • Lanthanum self-sputtering in dense plasma conditions leads to rapid erosion. • The LaB 6 operation window does not overlap with ITER divertor conditions.

Influence of laser power and welding speed on dendrite structure growth of low power pulsed laser welded super alloy

The influence of different welding parameters like welding speed and laser power on microstructure and the mechanical properties of low power pulsed Nd: Yag laser welded Nickel based super alloy inconel 617 sheets of thickness 1.5 mm has been investigated. In the present study, the welding of the specimens was done in a single pass aimed to full depth penetration. The microstructure of the various samples at fusion zone and at heat affected zone was characterized by field effect scanning electron microscope (FESEM) while the weld size of the welded metal at different parameters were measured using optical microscope (OM). The microhardness in the various zones for the welded sample has also been investigated. The microhardness of the welded samples was compared with base metal after experimental observations. The XRD technique was used to analyze the crystallography of the material at (2θ degree). The material has been used for the present study because of its exceptional properties of heat resistant and corrosion resistant. The Nd: YAG pulsed laser has been used because of its overlapping factor during welding that reduces porosity in the weld zone.

Pulsed laser irradiation of a nanoparticles sensitised RDX crystal

The laser initiation of secondary explosives presents tactical, safety and environmental advantages over traditional ignition systems utilising primary explosives. In this paper, direct ignition or initiation by pulsed laser was investigated of cyclotrimethylenetrinitramine (RDX) crystals doped with gold nanoparticles (GNPs) for the first time. A nano-seconds pulsed laser was used for direct irradiation of the crystals at the GNP's surface-plasmon-resonance wavelength of 532 nm. RDX crystals were recrystallised from a solution containing GNPs and characterised by visual microscopy for surface coating and subsurface doping. A high-speed camera was used to observe and capture the modes of interaction between the GNPs-doped RDX crystals and pulsed laser irradiation with a new classification method. Laser fluence thresholds for ignition and initiation were established. The GNPs sensitized RDX to pulsed laser irradiation at this selected wavelength is 3 orders of magnitude more sensitive than pure RDX. Direct initiation of the GNPs doped RDX crystal at laser fluence of 0.04 J/cm2 led to the total consumption of a crystal sample, which was unreproducible with pure RDX. The results show that GNPs are effective optical sensitizers for direct initiation of RDX crystal by low-power pulsed laser.

Al-WC composite coating on AZ91D Mg alloy by low power pulsed laser cladding

Al-WC composite coating on AZ91D Mg alloy by low power pulsed laser cladding

Microstructure and properties of Ti-6Al-4V fabricated by low-power pulsed laser directed energy deposition

Thin-wall structures of Ti-6Al-4V were fabricated by low-power pulsed laser directed energy deposition. During deposition, consistent with prior reports, columnar grains were observed which grew from the bottom toward the top of melt pool tail. This resulted in a microstructure mainly composed of long and thin prior epitaxial β columnar grains (average width ≈200 μm). A periodic pattern in epitaxial growth of grains was observed, which was shown to depend upon laser traverse direction. Utilizing this, a novel means was proposed to determine accurately the fusion boundary of each deposited layer by inspection of the periodic wave patterns. As a result it was applied to investigate the influence of thermal cycling on microstructure evolution. Results showed that acicular martensite, α' phase, and a small amount of Widmanstätten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanstätten α laths under layer-wise thermal cycling. Tensile tests showed that the yield strength, ultimate tensile strength and elongation of Ti-6Al-4V thin wall in the build direction were 9.1%, 17.3% and 42% higher respectively than those typically observed in forged solids of the same alloy. It also showed the yield strength and ultimate tensile strength of the transverse tensile samples both were ˜13.3% higher than those from the build direction due to the strengthening effect of a large number of vertical β grain boundaries, but the elongation was 69.7% lower than that of the build direction due to the uneven grain deformation of β grains.

Pulsed laser enhanced metal transfer of aluminum alloy in GMAW

Pulsed laser enhanced metal transfer of gas metal arc welding (GMAW) is a recent modification of conventional GMAW that applies a relatively low-power pulsed laser to the droplet. Previously current-independent metal transfer of carbon steel, in free-flight or short-circuiting mode, was achieved by utilizing single-side pulsed laser irradiation on the droplet. However, there are few studies on laser enhanced aluminum alloy metal transfer of GMAW due to the high reflectivity of aluminum alloy. In addition, the low melting point of aluminum alloy makes it possible to cut off the welding wire (irradiate on the wire) directly by pulsed laser with high energy density, instead of irradiating on the solid-liquid interface of the droplet. Hence, the laser-enhanced aluminum alloy metal transfer process is studied in this paper. The results demonstrate that when the pulsed laser irradiated the solid-liquid interface of the droplet, the laser was found to lead to a significant increase in the stability of welding process as an additional detaching force and change the transfer mode and flight path of the droplet. Both the short-circuiting (SC) transfer and the spray transfer processes were promoted by changing the stress state of the droplet, and the transfer frequency was effectively increased. Moreover, when the pulsed laser irradiated the welding wire, the welding wire was cut off and the arc climbed to the laser incident point. The cut wire-droplet (W-D) changed into a molten droplet by the heating of the arc and entered into the molten pool. A new spray transfer balance formed at the incident point and the restriction that the laser must impinge on the solid-liquid (S-L) interface of the droplet was released.

The Analysis on the Formation of Porosity During Pulsed Laser–Induced TIG Hybrid Welding of 6061 Aluminium Alloy at High Welding Speed

Reducing welding defects and improving welding manufacturing speed are important for the development of automated welding technology. Low-power pulsed laser–induced TIG hybrid welding method was used to join 6061-T6 aluminium alloy. The formation mechanism of porosity during the high speed welding process was investigated in different parameters, such as pulse frequency, pulse duration, pulse energy and arc current. The results showed that the pulse duration made influence on the energy fluctuations in the welding process, and there was an interaction between laser frequency and welding speed. The energy state of the pulsed laser became a key factor affecting the welding defect and appropriate arc current can effectively decrease the porosity generation.

Effect of CO2 Laser and 1.23% Acidulated Phosphate Fluoride on Acid Resistance and Fluoride Uptake of Human Tooth Enamel: An In Vitro Assessment

To evaluate the effectiveness of CO2 laser treatment before applying 1.23% acidulated phosphate fluoride (APF), through topically applied 1.23% APF solution, and after applying 1.23% APF on acid resistance and fluoride uptake of the enamel. Sixty non-carious human premolars were extracted due to the orthodontic reason and stored in distilled water solution under refrigeration. Using a water-cooled diamond disc, enamel slabs of 4 mm × 4 mm × 1.5 mm were cut from the buccal surface of each tooth. Sixty samples were randomly divided into one control group and five test groups of 10 premolars each. Solution was prepared for wet chemical analysis followed by fluoride analysis that was carried out using a fluoride ion selective electrode (Thermo Scientific Orion 4-Star Plus ISE Meter). The weight of enamel (WE) was determined from the amount of calcium (Ca) etched away considering the fact that the Ca content of the human enamel is 37.4 wt%. The subgroups were statistically analyzed using ANOVA for fluoride determination and evaluation of acid resistance. There was a significant increase in acid resistance of enamel slabs when treated individually or in combination of a low-power CO2 laser and 1.23% APF solution. Application of 1.23% APF solution after low-power CO2 laser treatment showed maximum increase in acid resistance. Application of a low-power pulsed CO2 laser through topically applied 1.23% APF solution resulted in a detrimental effect of the human tooth enamel with resultant decrease in acid resistance. High fluoride uptake does not necessarily indicate increased acid resistance. The present study provides evidence that a low-power CO2 laser can be used effectively in combination with topically applied 1.23% APF solution in order to make the enamel more resistant to acid attack, thereby helping in controlling dental caries.

Effect of relative position in low-power pulsed-laser–tungsten-inert-gas hybrid welding on laser-arc interaction

In laser-arc hybrid welding, the appropriate position between the laser beam and arc will be helpful in forming an intensive interaction and significantly increasing penetration depth. In this paper, first, an orthogonal experiment with three factors and four levels were carried out on the inclination angle of the welding torch, the arc length, and the distance between laser beam and TIG electrode, Dla, to find the optimal positional parameters with the strongest laser-arc interaction. The composite plasma behaviors and interaction mechanism between laser and arc were then analyzed under different position parameters; on this basis, the influence of laser power on the laser-arc interaction was further analyzed. Results reveal that the interaction is a decided difference under different positions. When the distance Dla is small, the arc will be significantly compressed; when the laser acts on the arc center, the pre-heating of the arc will greatly improve the laser absorption rate, a shorter arc length will help to further increase the penetration. When the distance Dla, the arc length, and the welding torch angle are 1.5, 1 mm, and 45°, respectively, the laser-arc interaction reaches its strongest level and the weld penetration is the deepest. In this case, the arc tail is obviously shortened, and the laser-arc composite plasma volume in the central region is greatly increased. Further, when the laser-power percentage is 10%–50%, the weld profile shows the characteristics of laser heat-conduction welding; when the laser-power percentage is 50%–80%, the weld profile shows the characteristics of laser deep penetration welding; and when the laser-power percentage is 80%–100%, the valid penetration is shallow and accompanied by a large splash due to the very short laser duration. But the weld profile should also be characterized by typical laser deep-penetration welding when the duration of laser action is enough long.

Surface quality and forming characteristics of thin-wall aluminium alloy parts manufactured by laser assisted MIG arc additive manufacturing

An innovative additive manufacturing (AM) system using low power pulsed laser assisted MIG arc welding (L-M) was proposed to manufacture metal products. With the purpose of revealing how width and height dimension of the manufactured thin-wall component are affected by the laser power, the present study has been carried out. The width decreased with the increasing of the laser power within a certain range of laser power, and the height increased proportionally under the equal deposition rate. The width and height fluctuation reduced while adding the low power laser, both the standard deviation decreased by more than 50％ when the laser power was 400 W. The coefficient of materials utilization was up to 91.12%, and increased by more than 15% while using L-M based AM to fabricate thin-wall parts with a proper laser power. In comparison with the common GMAW-based AM method, the L-M based AM method shows feasibility to manufacture a narrower thin-wall component with better surface quality and higher stability, and also with higher deposition efficiency.

Al-TiC in situ composite coating fabricated by low power pulsed laser cladding on AZ91D magnesium alloy

Al + (Ti + B4C) composite coating was cladded on AZ91D magnesium alloy by a low power pulsed Nd-YAG laser. The Ti+B4C mixed powder is with the ratio of Ti: B4C = 5:1, which was then mixed with Al powder by weight fraction of 10%, 15% and 20%, respectively. Scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction were used to study the microstructure, chemical composition and phase composition of the coating. Results showed that the coating had satisfied metallurgical bonding with the magnesium substrate. Al3Mg2, Al12Mg17, Al3Ti and TiC were formed by in-situ reaction. The coatings have micro-hardness of 348HV, which is about 5–6 times higher than that of AZ91D. The wear resistance and corrosion resistance of the coatings are enhanced with the addition of the mixed powder.

Laser transmission welding of polylactide to aluminium thin films for applications in the food-packaging industry

Laser transmission welding is a suitable technology to join thin films of similar or dissimilar materials without any addition of chemical solvents or adhesives. This process represents a very important opportunity in the case of packaging applications (for example in food and pharmaceutical sectors) where the realisation of strong welds by avoiding the contact between the thermal source and the processed materials and, furthermore, without using any third material that could contaminate the contents, is reliable and relevant.The aim of this paper is to prove the feasibility of the laser transmission welding of polylactide to aluminium thin films by means of laser transmission welding through the use of a low power pulsed wave fibre laser. Laser joint samples were realised, analysed by optical microscopy to reveal possible defects and to evaluate the weld width and tested to measure the mechanical tensile strength. An accurate relationship between the joint quality and both the welding speed and the k-factor, which represents the delivered energy per unit length and affects the bonding mechanism at the interface, was determined. The achieved feasibility area is extremely narrow and possible only for the higher value of the average power. The joint tensile strength was proven to be in a proportional relationship with the effective bonded area and reached satisfactory values.

Effect of activating flux on laser penetration welding performance of galvanized steel

To study the feasvhibility of activating welding technique in increasing the width of low-power pulsed laser penetration weld joint and to improve the efficiency of low-power laser welding while reducing cost, theoretical analysis and experimental verification are conducted in this study to determine the effects of activating flux Cr2O3 on the welding performance of DC56D galvanized steel plate for a vehicle body. Research data on the variation in weld width, joint microstructure, mechanical properties, distribution of elements, and corrosion resistance are acquired. Results indicate that activating flux can effectively increase the YAG laser energy absorption of the steel plate and melting of parent metals, thus increasing the weld width. Although the joint microstructure is dominated by refined isometric crystals and columnar crystals, the heat-affected zone expands slightly because of the activating flux. No evident diffusion and segregation of active elements are observed in the weld joint. The involvement of activating flux does not change the relative contents and distribution of chemical components of the weld. The welded test piece with activating flux is superior to that without activating flux in terms of tensile strength, microhardness, and corrosion resistance, indicating that the addition of activating flux effectively improves the performance of low-power laser penetration welding test pieces.

Selective Targeting and Restrictive Damage for Nonspecific Cells by Pulsed Laser-Activated Hyaluronan-Gold Nanoparticles.

Herein, we describe an approach that immobilizes low-molecular-weight hyaluronic acid (low-MW HA) on the surface of gold nanoparticles (GNPs), which can serve as a cellular probe and photodamage media, to evaluate the selectivity and efficiency of HA-based GNPs (HGNPs) as a mediator of laser-induced photothermal cell damage. In addition, it is known that solid tumors contain a higher content of low-MW HA than normal tissues. Thus, we used low-MW HA rather than high-MW HA used in other studies. In the present study, we conjugated low-MW HA, which is a linear polysaccharide with a disaccharide repeat unit, to prevent a reduction of the ligand-receptor binding efficiency in contrast to the conjugation of protein or peptides, which have unique three-dimensional structures. Three cell lines-MDA-MB-435 S (with CD44), MDA-MB-453 and NIH/3T3 (both are without CD44)-were investigated in the study, and qualitative observations were conducted by dark-field microscopy and laser scanning confocal microscopy (LSCM). In addition, quantitative measurements calculated using inductively coupled plasma emissions were taken for comparison. Our results showed that within the same treatment time, the uptake dosage of HGNPs by the MDA-MB-435 S cells was higher than that by the MDA-MB-453 and NIH 3T3 cells. Meanwhile, HGNPs uptake by the untreated MDA-MB-435 S cells was higher than that of MDA-MB-435 S cells with CD44 blocked by antibodies or silencing CD44 expression. This result implies that receptor-mediated endocytosis can enhance the cellular uptake of HGNPs. In addition, when exposed to a low-power pulsed laser, the former cell morphologies showed a more laser-induced giant plasma membrane vesicles (GPMV) than the latter morphologies. Therefore, this study utilized the specific photothermal property of HA-modified GNPs with laser-induced blebs to create a possible new method for medical applications.