Increasing Laser Power Density Research Articles

Two-color thermal imaging of the melt pool in powder-blown laser-directed energy deposition

This work demonstrates an experimental technique to image melt pool temperature fields with a commercial color camera for laser-directed energy deposition (L-DED) processes. The technique relies on two-color thermography to construct spatially-resolved temperature fields and is demonstrated on 316L stainless steel (SS) melt pools from solidus to near-boiling temperatures. This two-color thermal imaging system negates the need for a priori knowledge of melt pool emissivity or the camera’s view factor and was validated with a calibrated tungsten filament lamp between temperatures of 1220 K and 2850 K. In-situ temperature measurements are combined with ex-situ cross-sectional geometry to determine the material’s effective absorptivity and coefficient of temperature-dependent surface tension, which is responsible for Marangoni convection, in a multi-physics computational fluid dynamics (CFD) model. Measured peak melt pool temperatures are important for understanding the molten convection within the melt pool, and measured cooling rates can be related to the resulting part microstructure. Peak temperatures from 1750 K to 3000 K show that below the boiling point, temperature increases with increasing laser power density and decreases with increasing scanning velocity. Thermal images are used to estimate cooling rates in the melt pool tail, which increase with the ratio of scanning velocity to power. Our thermal imaging strategy will advance measurement science in L-DED processes by validating multi-physics CFD models, quantifying cooling rates, providing real-time feedback control, and allowing process mapping of critical melt pool behaviors.

Studies on the Platinum Thick Film Sensor Conformally Written by Laser Micro-Cladding: Formability, Microstructure, and Performance

In this paper, laser micro-cladding technology (LMC) was conducted to prepare high-temperature Pt thick film sensors in situ. The formability, microstructure, sintering mechanism, and electrical properties of the LMCed Pt thick films were first studied systematically. Results indicated that with the increase of laser power density, the sintering degree of the Pt thick film increased obviously, improving its adhesion strength and reducing its resistivity. However, when the laser power density exceeded the threshold, holes or grooves were formed in the Pt film, leading to the degeneration of its properties. A Pt thick film with good adhesion strength, excellent conductive networks, and the minimum resistivity (46 ± 2 μΩ·cm) was obtained at a laser power density of 1.37 × 106 W·cm-2. Then, Pt thick film temperature sensors (including Pt thermal resistance temperature (RTD) and Pt-Pt10%Rh thermocouple sensors) were conformally prepared by LMC. Their temperature-sensing performance became stable after the initial high-temperature calibration, with a linearity of 0.9985 for the RTD with a TCR of 2.46 × 10-3/°C (at 920 °C) and a linearity of 0.9905 for the thermocouple with a Seebeck coefficient of 9.7 μV/°C, both of which are better than that made by direct DC magnetron sputtering deposition. Therefore, this work provides a novel feasible way to conformally integrate high-performance Pt film sensors in situ.

Improvement of hydrogen embrittlement resistance of 2205 duplex stainless steel by laser peening

The hydrogen embrittlement (HE) resistance of 2205 duplex stainless steel (DSS) treated with laser peening (LP) with different laser power densities was studied. The results show that LP changes the morphologies and distribution of ferrite phase and austenitic phase, thus changes the path of hydrogen transportation and diffusing. LP-induced grain refinement provides more tortuous grain boundaries that increases the difficulty of hydrogen atoms to penetrate them. The beneficial LP-induced microstructures interacts (e.g. dislocation entanglements, dislocation walls, mechanical twins) and helps to trap the hydrogen atoms, reducing their mobility ability. The hydrogen determination test provides direct evidence that LP reduced the amount of hydrogen penetration into the material. In addition, the tensile fracture exhibits that the average depth of the brittle region was inversely proportional to the laser power density, suggesting that an increase in laser power density can reduce the HE sensitivity of 2205 DSS.

Reversible charge-polarity control for a photo-triggered anti-ambipolar In2Se3&WSe2 heterotransistor.

Based on the charge-polarity control, a novel anti-ambipolar heterotransistor is proposed based on a special In2Se3&WSe2 van der Waals heterostructure. Unlike traditional logic transistors, our anti-ambipolar heterotransistor can treat an optical signal as an input to change its operating state, that is, with the switching of the optical signal, it shows a reversible polarity change between anti-ambipolar and P-type. Moreover, with the increase of laser power density from 0 to 4.4 mW cm-2, the current value corresponding to the anti-ambipolar peak of the device (Ipeak) shifts from 0 to 4.3 nA, and the voltage value corresponding to the anti-ambipolar peak of the device (Vpeak) can shift from -7 to -5.67 V. These phenomena demonstrate that the charge neutrality point of the anti-ambipolar heterotransistor can be selectively varied with the change of laser power density. In addition, the aforementioned device possesses a high Ion/Ioff ratio of about 104 (405 nm, 4.4 mW cm-2) at 0 V. These properties indicate that our unique anti-ambipolar In2Se3&WSe2 heterotransistor can be employed in photo-triggered inverters for future optoelectronic circuits, and it has great potential to improve the integration of overall chip circuits by implementing optoelectronic logic functions in a unit.

Efficient preparation of the inclined La1-xCaxMnO3 films with excellent light-induced transverse thermoelectric performance based on MOCVD process

The efficient preparation of inclined La1-xCaxMnO3 (LCMO) films with excellent light-induced transverse thermoelectric (LITT) performance on c-axis miscut 12° SrTiO3 single crystal substrates is achieved based on the home-designed metal-organic chemical vapor deposition (MOCVD) system. The LCMO films with the thickness of 300 nm are prepared in 1 min, and the preparation efficiency reaches 300 nm/min. Under the radiation of 248 nm pulsed laser with the duration time of 28 ns, the voltage signal of LCMO films shows high sensitivity of 3.13 V/mJ and fast response time of 28 ns, superior to the LITT performance of the reported LCMO under the irradiation of pulsed laser with similar wavelength and duration time. The improvement of LITT performance originates from good electrical conductivity and biaxial texture. Additionally, under the radiation of 900 nm continuous laser, the obvious response voltages can be detected in the LCMO films, and the voltage amplitudes increase linearly with increasing laser power densities. These results demonstrate the application potential of the light detectors composed of LCMO films based on the efficient preparation of MOCVD, and may provide a good prospect for developing low-cost and high-performance light detectors based on the LITT effect.

High-temperature fatigue crack growth characteristics of IN718 Ni-based alloy treated by laser peening

The characteristics of fatigue crack growth (FCG) of laser peened IN718 Ni-based alloy at high temperature with different laser power densities were studied. The residual stress relaxation and micro-structure evolution of the samples at high temperature after laser peening (LP) were analyzed by X-ray diffraction technique and scanning electron microscopy (SEM), respectively. Meanwhile, SEM was used to characterize the fracture morphologies of the samples. The strengthening mechanism of LP on the high-temperature FCG resistance of IN718 Ni-based alloy was discussed. The results showed that large compressive residual stress, fine grains and complex grain boundaries induced by LP could significantly enhance the resistance of FCG at 700 °C. As a result, the FCG rate was decreased and fatigue life of the sample was increased. In addition, the laser power density was found to be inversely proportional to the average spacing of fatigue striations and directly proportional to the size and depth of dimples, which indicated that the increase in laser power density can improve the high-temperature FCG resistance of IN718 Ni-based alloy.

Damage characteristics of continuous-wave laser ablation triple-junction solar cells

The effect of laser irradiation on triple-junction GaAs cells studied via cell irradiation experiments was carried out using a continuous-wave laser with a wavelength of 808 nm. In the experiments, a thermal imaging system was used to measure the surface temperature of the cell, and the surface morphology of the irradiated cell was analyzed. The output performance of the irradiated cell was measured by a source meter, and the results show that the change rate of cell surface temperature increased with increasing laser power density. When the cell damage intensifies, the output characteristics of the cell gradually decay, and the cell loses its photoelectric effect.

Microstructure and Mechanical Properties of Sn58Bi Components Prepared by Laser Beam Powder Bed Fusion

As a low melting point alloy, Sn58Bi alloy plays a unique role in many fields. However, the brittleness of Sn58Bi alloy is a limitation that has to be addressed for wider applications. According to previous studies, third element addition is favorable for improved ductility, which is attributable to structure refinement. Therefore, laser beam powder bed fusion technology was adopted to prepare Sn58Bi alloy components. Additionally, the as-printed specimens presented more refined structures compared to the as-cast specimen, so they showed better plasticity. The Sn58Bi alloy showed excellent formability when specimens were prepared by LBPBF between 40–80 W. The density of specimens nearly remained stable above laser power higher than 40 W, and CT scanning could not detect internal defects, so the Sn58Bi alloy specimen was likely to be well fabricated at high laser power. Within the laser power range of 40–80 W, with the variation of laser power or scanning velocity, the laser energy density changed accordingly, and the mechanical strength of specimens was improved with the increase of laser power density. This strength change was probably related to the microstructure evolution and internal residual stress in the printing process.

Effect of laser peen forming process parameters on bending and surface quality of Ti-6Al-4V sheets

Laser peen forming (LPF) is a metal forming process that utilizes laser-induced mechanical shock waves to form desired shapes or modify bent structures. The present work focuses on the applicability of LPF to Ti-6Al-4V sheets, to identify an optimal LPF process parameter window and achieve desired bending without compromising the surface quality within the peened region. The effect of LPF process parameters, i.e. laser power density, overlap, type of sacrificial overlay, and the number of peening sequences was investigated for specimens with different thicknesses. The laser power density and number of peening sequences were the most influential parameters that affect the bending of the specimens. Using sacrificial overlay has a significant effect on the bending and surface quality of the specimens. Surface quality after LPF was assessed by measuring the roughness in the peened region. In experiments without a sacrificial overlay, a black titanium oxide residue on the peened region was observed and additionally, small micro-cracks were found in the near surface region. Further characterization of the peened region revealed that the average crack length increased with increase in laser power density. Two possible LPF process parameter combinations were identified to obtain bending in the peened region, where LPF with sacrificial overlay resulted in no surface damage. Furthermore, residual stresses were determined at various LPF process parameters by incremental hole-drilling method in the peened region.

The effect of laser shock processing on the microstructures and properties of 2060 Al[sbnd]Li alloys

The microstructures, mechanical properties and corrosion resistance of 2060 AL-Li alloy subjected to laser shock peening (LSP) with power densities of 3, 4 and 5 GW·cm−2 were investigated comprehensively. Microhardness, residual stress, wear resistance and fatigue life were tested on LSP alloys to analyze the mechanical properties and the corrosion behavior was investigated by electrochemical test. X-ray diffraction (XRD) and field emission transmission electron microscopy (FE-TEM) were used to investigate the surface microstructure evolution. The results show that LSP leads to a high magnitude of surface compressive residual stress and a significant increase of dislocation density near the surface in the plastic deformation layer. Additionally, with the increase of laser power density, the microhardness, compressive residual stress and surface roughness increase, while the friction coefficient and wear rate decrease compared to pristine alloy. The fatigue life of the sample with a laser power density of 4 GW·cm−2 reaches the highest value of 6.00 × 104 times, approximately twice that of the pristine sample (2.98 × 104 times), which is mainly attributed to the surface residual compressive stress, high density dislocations and surface hardening owing to the LSP. The corrosion resistance of the LSP alloys modestly decreases in a 3.5% NaCl solution compared with pristine alloy, which mainly ascribes to the increase of surface roughness.

Raman study of photoinduced changes in Cd-doped amorphous GeSe2 films

For Cd-doped amorphous GeSe2 films Raman spectra reveal a considerable decrease of the amount of edge-sharing GeSe4 tetrahedra and an increasing number of ethane-like Se3Ge–GeSe3 structural units compared to the undoped films. A Raman peak with a frequency close to that of the CdSe LO phonon observed for the Cd-doped films at increasing laser power density (Pexc ≥ 100 kW/cm2) provides evidence for the photoinduced formation of CdSe nanoparticles in the GeSe2:Cd films due to photoenhanced diffusion.

Study of microstructure and wear properties of laser borided Inconel 718

• Effect of laser power density on microstructure and wear properties of laser borided Inconel 718 is studied. • The borided layer is consisting of borides of Ni, Cr and Fe together with γ (Ni-Cr-Fe) matrix phase. • Formation of boride phases leads to the significant improvement in hardness (∼5 times) and wear resistance (∼8 times). • In the borided layer, microstructural morphology is changed from cellular to equiaxed dendrites. • The phase stability of borided layer is discussed based on phase equilibria information and thermo-kinetic theory. Effect of laser power density (LPD) on microstructure and tribological properties of laser borided Inconel 718 is studied. Boriding is carried out using laser powder bed fusion based additive manufacturing process at three LPDs of 17.0, 22.6 and 28.3 kW cm −2 . Laser power density played significant role on surface roughness, homogeneity and depth of the borided layer. Higher laser power density led to lower surface roughness, improved homogeneity and higher depth of borided layer. Structural, chemical and microstructural characterization of the borided layer confirmed the formation of Nickel boride (Ni 2 B/Ni 3 B), Chromium boride (CrB/Cr 2 B) and Iron boride (FeB/Fe 2 B) along with the matrix γ (Ni-Cr-Fe) phase in borided layer. The content of γ phase in borided layer is found to be increased with increase in LPD due to dilution effect. The presence of different boride phases in the borided layer caused significant improvement in hardness (∼5 times) and wear resistance (∼8 times) than the substrate Inconel 718 for LPD of 17.0 kW cm −2 . At higher LPD, due to increased content of soft γ phase, both hardness and wear resistance are reduced. Across the borided layer, microstructural morphology is changed from cellular to equiaxed dendrite with a clear columnar to equiaxed transition. The formation of different boride phases and microstructural evolution in the borided layer is discussed in view of available phase equilibria information of Ni-Cr-Fe-B system and thermo-kinetic theory. The findings of present study will be helpful in developing the Ni baser super alloys with improved tribological properties for advanced technological applications under extreme service conditions.

Laser heating conditions for copper sphere implantation into borosilicate glass

Implanting metals in glass can serve many different purposes, ranging from enhancing the electro-optical properties to modifying the mechanical properties of the substrate. One approach involves the use of continuous laser illumination to implant metal spheres by transferring particles from a metal foil in direct contact with one side of the glass substrate. The process, which occurs in a very short period time (in tens of milliseconds), is unidentified: due to the bright thermal emissions, it cannot be directly observed. In this study, the continuous laser heating conditions required for implanting copper spheres in borosilicate glass were investigated via spectroscopy and numerical calculations to estimate the temperature of the copper foil. Emission spectroscopy revealed that the temperature during implantation reached 5100 K, which largely exceeded the temperature threshold for laser absorption (∼2000 K). Under such conditions, the plasma forms and enables the migration of copper particles into the glass host. The plasma propagation speed increased with increasing laser power density. The copper sphere was implanted when the plasma propagation speed was 20–50 mm/s. The copper sphere could not be implanted, and fiber fuse occurred when the plasma propagation speed was higher than 50 mm/s.

Correlation between X-Ray diffraction pattern and microstructure of surface of E690 high-strength steel induced by laser-shock processing

To examine the relationship between the X-ray Diffraction (XRD) pattern and microstructure of the surface of E690 high-strength steel induced by laser shocks, X-ray diffractometry and transmission electron microscopy (TEM) were used to detect the laser-impact zone on the steel surface to obtain its diffraction pattern and microstructural morphology, followed by the establishment of a surface microstructure response model of the E690 high-strength steel under laser shock. The grain size of the impacted surface decreased with an increase in laser power density (LPD). When the LPD reached 4.07 GW/cm2, Ostwald ripening was observed in the TEM images and the grain size increased during the later stages of spinodal decomposition. When the LPD was 5.09 GW/cm2, nanograins were detected on the surface of the laser-impacted area. The X-ray diffraction result on the surface of the E690 high-strength steel was consistent with the microstructural analysis using TEM, which proves that the microstructural response of E690 high-strength steel under laser shock can be described by the surface microstructure response model.

Deformation and Fracture Behaviors in Microscale Laser Dynamic Flexible Forming: The Case of Fabricating Micro-Channel on Copper Foil

The case of fabricating micro-channel on copper foil using micro-scale laser dynamic flexible forming (μLDFF) is investigated in this work. To study the deformation and fracture behaviors, the copper foils are subjected to increasing laser power density until the occurrence of fracture. When laser power density ranges from 0.110 GW/cm2 to 0.441 GW/cm2, smooth micro-channel features are formed on copper foils, and the depth of the micro-channel gradually increases. Melting marks are observed at the micro-die entrance of the sample with 0.441 GW/cm2 laser power density. When laser power density is 0.544 GW/cm2, partial fractures are observed. The surface of fracture region is rougher than that of micro-cracks and no micro-cracks regions, which is caused by the higher strain in fracture region. And the fracture edge is irregular, which is caused by the soft effect of material. Complete fracture occurs when laser power density is 0.691 GW/cm2.

Ion-exchanged LTA zeolite derived nepheline phase NaAlSiO4:Eu2+ ceramic phosphor for laser illumination

A reliable yellow phosphor converter that can be efficiently excited by a 405 nm bluish violet laser is in high demand for laser illumination applications. A NaAlSiO4:Eu2+ phosphor with a quantum efficiency reaching 92% was obtained using LTA zeolite as the raw material. NaAlSiO4:Eu2+ ceramics with suitable porosities for laser illumination were prepared from the phosphor powders via spark plasma sintering. The ceramics lost only 2% of the quantum efficiency compared to the powders, maintained good thermal quenching properties (30% drop at 150 °C), and showed good thermal conductivity (2.02 W‧m−1‧K−1). The NaAlSiO4:Eu2+ ceramic with 405 nm bluish violet lasers, with the increase in laser power density to 9.15 W/mm2, exhibited an increasing luminous flux (23.83–70.26 lm) and maintained a stable luminous efficacy (47.7–46.8 lm/W), and the temperature distribution of the ceramic remained uniform and stable under long-time laser irradiation. This indicates that the nepheline-phase NaAlSiO4:Eu2+ ceramic is a promising material for laser illumination.

Investigation on grain size effect and forming mechanism of laser shock hydraulic microforming of copper foil

Laser shock hydraulic microforming (LSHM) is a new microforming that combines the advantages of laser shock microforming and impact hydroforming. LSHM uses monopulse laser to drive the rubber layer for squeezing liquid medium to form metal foils. The size effect and forming mechanism under this process will surely introduce new features, and they were explored in terms of maximum bulge height, surface topography, thickness distribution, microhardness, microstructure, and elastic recovery. When N=t/d (thickness/grain size)>1, the N value decreases, the maximum bulge height and thickness thinning increase, and the roughening of surface topography is not obvious. When N 4.3, this case accords with inverse Hall−Petch tendency because twin nucleation is dominant. When N 1 due to the increase in the scale of grain atoms. The increase in laser power density can appropriately reduce the elastic recovery ratio because of grain refinement.

Removal mechanism of 2.5-dimensional carbon fiber reinforced ceramic matrix composites processed by nanosecond laser

C/SiC composite material is widely used in aerospace fields because of its excellent properties; however, it is difficult to be removed and processed. In this paper, the 2.5-dimensional C/SiC composite material was ablated by nanosecond laser to explore the laser removal mechanism. Firstly, the laser ablation experiment was carried out to investigate the gradual process of ablation morphology with the increasing laser power density. Secondly, the characteristics of ablation holes were analyzed, and the effects of laser processing parameters and fiber orientation on the ablation morphology and size were studied. In addition, the ablation morphology of needle-punched fiber area was analyzed to expound the unique ablation removal characteristics of 2.5-dimensional C/SiC composites. At last, ablation removal mechanism of 2.5-dimensional C/SiC composites was summarized, and the model of ablation removal process was established. The results indicated that ablation morphology was evolved from nothing to ablation mark, to fiber expansion, to fiber shrinkage and tension, to resolidification, to removal holes, and to increasing holes with laser power density. Besides, the ablation morphology in needle-punched fiber area was different from elliptic ablation morphology in the 0° or 90° fiber area, the ablation holes in the needle-punched fiber area were smaller and deeper, and the ablation holes were approximately circular. The results would provide theoretical basis and technical support for processing functionalized structure of 2.5-dimensional C/SiC composites by nanosecond laser, such as blind holes, grooves, and square holes.

Material removal characteristics of TC4 alloy by continuous wave irradiation at low pressure

The material removal characteristics of TC4 alloy by continuous wave laser irradiation at 1064 nm at an air pressure of 101 Pa is investigated. The morphology of the molten region on the surface of the alloy after irradiation with different laser parameters is studied. The diameter of the molten region and the mass loss rate of the Target are measured in dependence on the laser parameters. With increasing laser power density from 10.5 kW/cm2 to 34.5 kW/cm2 and irradiation duration from 50 ms to 800 ms the morphology of the irradiated spot changes from hollow structure, to a crater and then to a platform, the diameter of the molten region increases from 0.65 mm to 4.16 mm, and the mass loss increases from 0.10 mg to 1.43 mg. With increasing laser irradiation duration, the increasing rate of the diameter of the molten region decreases. The mass loss rate increases with higher laser power densities and irradiation durations. Especially, the morphology of the molten region at low pressure is significantly different from that in atmospheric environment.

The Effects of Laser Parameters and the Ablation Mechanism in Laser Ablation of C/SiC Composite

The effects of laser parameters and the ablation mechanism in laser ablation of a carbon fiber reinforced silicon carbide (C/SiC) composite are investigated in the present study. Six different power densities are provided, as well as six levels of pulse numbers, and then ablation experiments are conducted for the C/SiC composite, induced by a pulsed laser. Based on the experimental results, the characteristics of surface morphology and ablation behavior are discussed. It is revealed that the surface morphology of the C/SiC composite under laser irradiation usually includes three regions: the center region, the transition region, and the border region. With the increase of laser power density, the ablation of the center region becomes severe, surface cracks occur, and more spherical SiC particles are found in the transition region. As for scenarios involving multiple pulses, the damage occurs in the center region at low power density limits, within the first two layers below the surface. However, if the power density is relatively high, an ablation pit occurs in the center region when the pulse number is larger than 50. Meanwhile, the transition region and the border region diminish with increase of the pulse number. It is noted that both the power density and pulse number have noticeable effects on surface morphology and ablation behavior during laser ablation, which is helpful for material design and performance evaluation of C/SiC composites.