Flat-top Laser Research Articles

High power laser powder bed fusion of Ti6Al4V alloy: The control of defects, microstructure, and mechanical properties

Laser powder bed fusion (LPBF) is the most widely used metal additive manufacturing technology, but it still faces challenges in manufacturing efficiency. To address the issue, high-power LPBF (HP-LPBF) which employs kilowatt-level lasers emerges in recent years. In this study, a 4 kW Flat-top laser was used for the HP-LPBF of Ti6Al4V alloy. The samples in as-built state and annealed states (750 °C/2h, 850 °C/2h, 950 °C/2h, 1050 °C/2h) were investigated in terms of defects, microstructure, and mechanical properties. Results show that keyhole mode melting is avoided by adopting the Flat-top laser, so the relative density of as-built Ti6Al4V is generally positively correlated with the employed laser energy density. The minimum laser energy density to obtain the high-density (≥99.9%) sample is 50.0 J/mm3, and the highest build rate is 288 cm3/h. The microstructure in as-built state exhibits a unique alternating pattern of "bright bands/dark bands". The dark bands consist of needle-shaped α′ martensite, while the bright bands consist of needle-shaped α+β, which results from the decomposition of α′ under the in-situ annealing effect of the 4 kW Flat-top laser. After annealing at 750 °C, most of the residual α′ decomposes into needle-shaped α+β. As the annealing temperature increases from 750 °C to 1050 °C, the microstructure undergoes an evolution from needle-shaped α+β to lamellar α+β then to lamellar α+β with some globular α and finally to duplex α+β. An optimum strength-ductility balance is achieved after annealing at 850 °C, both the tensile strength and the elongation exceed the standard values of Ti6Al4V forgings.

Hot Crack Formation Mechanism and Inhibition of a Novel Cobalt-Based Alloy Coating during Laser Cladding.

Laser cladding provides advanced surface treatment capabilities for enhancing the properties of components. However, its effectiveness is often challenged by the formation of hot cracks during the cladding process. This study focuses on the formation mechanism and inhibition of hot cracks in a novel cobalt-based alloy (K688) coating applied to 304LN stainless steel via laser cladding. The results indicate that hot crack formation is influenced by liquid film stability, the stress concentration, and precipitation phases. Most hot cracks were found at 25°-45° high-angle grain boundaries (HAGBs) due to the high energy of these grain boundaries, which stabilize the liquid film. A flat-top beam, compared to a Gaussian beam, creates a melt pool with a lower temperature gradient and more mitigatory fluid flow, reducing thermal stresses within the coating and the fraction of crack-sensitive, high-angle grain boundaries (S-HAGBs). Finally, crack formation was significantly inhibited by utilizing a flat-top laser beam to optimize the process parameters. These findings provide a technical foundation for achieving high-quality laser cladding of dissimilar materials, offering insights into optimizing process parameters to prevent hot crack formation.

Precision isolation and cultivation of single cells by vortex and flat-top laser ejection.

Single-cell isolation stands as a critical step in single-cell studies, and single-cell ejection technology based on laser induced forward transfer technology (LIFT) is considered one of the most promising methods in this regard for its ability of visible isolating single cell from complex samples. In this study, we improve the LIFT technology and introduce optical vortex laser-induced forward transfer (OV-LIFT) and flat-top laser-induced forward transfer (FT-LIFT) by utilizing spatial light modulator (SLM), aiming to enhance the precision of single-cell sorting and the cell's viability after ejection. Experimental results demonstrate that applying vortex and flat-top beams during the sorting and collection process enables precise retrieval of single cells within diameter ranges of 50 μm and 100 μm, respectively. The recovery rates of Saccharomyces cerevisiae and Escherichia coli DH5α single cell ejected by vortex beam are 89 and 78%, by flat-top beam are 85 and 57%. When employing Gaussian beam sorting, the receiving range extends to 400 μm, with cultivation success rates of S. cerevisiae and E. coli DH5α single cell are 48 and 19%, respectively. This marks the first application of different mode beams in the ejection and cultivation of single cells, providing a novel and effective approach for the precise isolation and improving the viability of single cells.

Novel flat-top laser-aided cold metal transfer additive manufacturing for titanium alloy: Arc characteristics, microstructure, and tensile properties

In this paper, a novel flat-top laser-aided cold metal transfer additive manufacturing (F-LCAM) method is proposed for titanium alloys to restrict internal defects and anisotropy of mechanical properties in large-scale additive-manufactured titanium structures. The proposed method uses a flat-top laser to stabilise the drifting cathode spot and suppress the irregular shape of the weld deposit by promoting thermionic emission on the molten pool surface. Compared to the cold metal transfer additive manufacturing (CMT-AM) process, the auxiliary flat-top laser promotes the wettability of the molten pool and increases the contact angle of deposition from 95° to 142° in an F-LCAM sample. The non-destructive X-ray testing (NDT) results of multi-bead multilayer deposition showed that the defects caused by the pool weld bead morphology were effectively suppressed. Attributed to fluid flow in the melt pool enhanced by the flat-top laser, the dendritic arms were broken, columnar β grains in the F-LCAM sample were finer than in CMT-AM sample, and the aspect ratios of prior-β grains decreased from approximately 6 to 1. The F-LCAM sample exhibited a higher elongation and lower anisotropy than the CMT-AM sample. Defects were observed on the fracture surfaces of the CMT-AM vertical samples, which reduced the elongation of the CMT-AM samples to lower than that of the F-LCAM sample. The ultimate tensile strength, yield strength, and elongation of the multi-bead multilayer deposition met the standards of as-forged Ti6321 titanium alloy, indicating that the Ti6321 titanium alloy structure fabricated by the F-LCAM process has acceptable tensile properties for engineering applications.

Improving grating duty cycle uniformity: amplitude-splitting flat-top beam laser interference lithography.

Laser interference lithography is an effective approach for grating fabrication. As a key parameter of the grating profile, the duty cycle determines the diffraction characteristics and is associated with the irradiance of the exposure beam. In this study, we developed a fabrication technique amplitude-splitting flat-top beam interference lithography to improve duty cycle uniformity. The relationship between the duty cycle uniformity and irradiance of the exposure beam is analyzed, and the results indicate that when the beam irradiance nonuniformity is less than 20%, the grating duty cycle nonuniformity is maintained below ±2%. Moreover, an experimental amplitude-splitting flat-top beam interference lithography system is developed to realize an incident beam irradiance nonuniformity of 21%. The full-aperture duty cycle nonuniformity of the fabricated grating is less than ±3%. Amplitude-splitting flat-top beam interference lithography improves duty cycle uniformity, greatly reduces energy loss compared to conventional apodization, and is more suitable for manufacturing highly uniform gratings over large areas.

Wavefront-enhanced laser-induced breakdown spectroscopy (WELIBS) utilizing a crystalline silicon wafer for a flat-top IR laser beam

On passing through a thin crystalline silicon wafer, the profile of an infrared laser beam is changed from a quasi-Gaussian to a flat-top one, improving the LIBS technique's analytical performance. That is the proposed (WELIBS) Wavefront-enhanced laser-induced breakdown spectroscopy approach.

Material flow and heat transfer characteristics in flat-top laser composite arc-based directed energy deposition

In this study, a novel flat-top laser composite arc-based directed energy deposition process is proposed, and the material flow and heat transfer behaviors are investigated for both arc-only and hybrid flat-top laser modes. Transient CFD models are developed to simulate the energy of the laser and arc using the surface heat source and double-ellipsoid body heat source models, respectively, and validated through experimentation. The results show that the high-temperature metal fluid flows outward from the center of the molten pool and then backward in the front part. The flat-top laser does not alter the flow field pattern but allows for the spread of more hot liquid metal towards the lateral edges. Furthermore, it is discovered that the flat-top laser reduces the contact angle and promotes a stable molten pool temperature.

Minimizing Defect on Ti Thin Film Laser Ablation using Square Flat-top NIR Femtosecond Laser for Thin Film Transistor of µ-LED Repair

Minimizing Defect on Ti Thin Film Laser Ablation using Square Flat-top NIR Femtosecond Laser for Thin Film Transistor of µ-LED Repair

Experimental cooling rates during high-power laser powder bed fusion at varying processing conditions

High-power laser power bed fusion (HP-LPBF) with a large flat-top laser beam allows additive manufacturing of components at much higher build-up rates than conventional LPBF, since thicker powder layers can be processed. This makes this technology attractive for industry due to the augmented productivity. Here, we have utilized HP-LPBF to fabricate Al-33Cu (wt%) specimens at differing layer thickness and laser power. Based on the average spacing of the lamellae of the eutectic microstructure, the cooling rate inherent to HP-LPBF was experimentally determined as a function of the processing conditions. At the lowest layer thickness (50 µm) and laser power (500 W), the cooling rate amounts to about 90000 K/s, whereas it drops to about 15000 K/s for HP-LPBF with the largest layer thickness (200 µm) and highest laser power (1000 W). When the base plate is additionally pre-heated, the cooling rate prevailing during solidification decreased to about 5000 K/s. Our findings demonstrate that relatively low cooling rates are effective during HP-LPBF at high build-up rates being tantamount to high productivity. It should be considered that such drastic changes in cooling rate may strongly affect the microstructure formation, the properties and hence performance of the corresponding additively manufactured component.

Effect of interfacial thermal history on bonding mechanism of laser assisted joining of QP980-CFRTP with adjustable flat-top rectangular laser beam

Carbon fiber reinforced thermoplastic composite (CFRTP) - metal hybrid structures, which have been increasingly used in the industry, can significantly reduce structure weight and energy consumption. In this paper, an adjustable flat-top rectangular diode laser beam was used as a heat source to join the quenched and portioned QP980 steel and CFRTP in a lap joint configuration. Compared to conventional laser beam scanning process in the literatures, the rectangular laser beam provides a relatively uniform temperature distribution at interface. The relationship between interfacial thermal history and the joint performance was explored. Experimental results showed that thermal history had a significant impact on interfacial chemical compositions and the formation of joint defects via affecting the resin state, resulting in change of the joint fracture behaviors. Furthermore, higher interfacial temperature and sufficient heating/holding time contributed to the complete melting and diffusing of resin matrix on QP980 surface, filling the micro pores of the interface, and promoting the chemical bonding between QP980 and CFRTP. On this basis, a novel strategy to achieve excellent interfacial bonding via adjusting the interfacial thermal history was proposed to produce a high-quality joint with the peak load higher than 10 kN and the shear strength of 22 MPa.

Drilling of cylindrical holes in Crown glass by a short-pulse flat-top CO2 laser beam

We investigated cylindrical hole drilling in a crown glass with a high thermal expansion coefficient of 94 × 10–7 K−1 and a low melting point of 724 °C using a short-pulse CO2 laser with a flat-top beam, and also examined the drilling characteristics. The short laser pulse consisted of a pulse spike with a pulse width of 276 ns and a pulse tail with a length of 56.9 µs at a repetition rate of 200 Hz. The laser beam had a flat-top profile with an estimated M2 parameter of 13.5 and a diameter of 12.5 mm before a focusing lens. The flat-top beam was focused by the focusing lens, which had a focal length of 12.7 mm, on the glass surface at a focus offset of −0.20 mm to +0.40 mm. The incident flat-top beam produced conical holes at focus offsets of −0.20 mm to 0.00 mm and produced cylindrical holes at focus offsets of +0.20 mm to +0.40 mm. The hole depth of the cylindrical holes was 109 μm to 434 μm, the surface hole diameter was 150 μm to 366 μm, and the aspect ratio, defined as the ratio of the hole depth to the surface hole diameter, was 0.30 to 2.89. The hole depth was influenced by the focus offset and the total irradiation fluence, whereas the surface hole diameter, the taper angle and the ratio of the surface hole diameter to the irradiation diameter were influenced by the focus offset only. The ratio of the surface HAZ (Heat affected zone) diameter to the irradiation diameter was influenced by both the focus offset and the total irradiation fluence.

Experimental study on the pressure characteristics of laser-induced shock waves under different energy distribution

Laser shock peening has been widely used in anti-fatigue strengthening of metals. The different types of lasers (Flattop laser and Gaussian laser) used have influence on the pressure characteristics of shock wave and the strengthening effect. In this work, using PDV (photonic doppler velocimetry) system, the difference of peak pressure of Flattop laser-induced shock wave and Gaussian laser-induced shock wave is studied experimentally and theoretically. Research shows that the peak pressure of shock wave induced by Gaussian laser is higher than that of Flattop laser, because of the infinitesimal power density in the center of Gaussian laser spot is higher than that of Flattop laser. Furthermore, at the same power density, Gaussian laser can implant a larger and deeper residual stress field in the material.

Noise analysis of the atomic superheterodyne receiver based on flat-top laser beams.

Since its theoretical sensitivity is limited by quantum noise, radio wave sensing based on Rydberg atoms has the potential to replace its traditional counterparts with higher sensitivity and has developed rapidly in recent years. However, as the most sensitive atomic radio wave sensor, the atomic superheterodyne receiver lacks a detailed noise analysis to pave its way to achieve theoretical sensitivity. In this work, we quantitatively study the noise power spectrum of the atomic receiver versus the number of atoms, where the number of atoms is precisely controlled by changing the diameters of flat-top excitation laser beams. The results show that under the experimental conditions that the diameters of excitation beams are less than or equal to 2 mm and the read-out frequency is larger than 70 kHz, the sensitivity of the atomic receiver is limited only by the quantum noise and, in the other conditions, limited by classical noise. However, the experimental quantum-projection-noise-limited sensitivity this atomic receiver reaches is far from the theoretical sensitivity. This is because all atoms involved in light-atom interaction will contribute to noise, but only a fraction of them participating in the radio wave transition can provide valuable signals. At the same time, the calculation of the theoretical sensitivity considers both the noise and signal are contributed by the same amount of atoms. This work is essential in making the sensitivity of the atomic receiver reach its ultimate limit and is significant in quantum precision measurement.

Fabrication of Single Crystals by Laser Powder Bed Fusion with a Flat-Top Laser Beam

Fabrication of Single Crystals by Laser Powder Bed Fusion with a Flat-Top Laser Beam

Effect of scan strategy on the formation of a pure nickel single-crystal structure using a flat-top laser beam via laser powder bed fusion

ABSTRACT In this study, the role of a scan strategy in the fabrication of a single crystal (SX) structure of face-centered cubic (fcc) pure Ni using a flat-top laser profile by a laser powder bed fusion (LPBF) process was investigated. A flat-top laser with a uniform heat intensity across the beam profile was employed to fabricate the SX structure with a <001> texture parallel to the build direction (║BD). A meander scan strategy with an XY–90° scan rotation was required to achieve the flat-top-derived SX structure. The meander scanning strategy promoted the epitaxial <001> fcc growth along the direction ║BD, leading to the <001>║BD texture formation. Meanwhile, the XY–90° scan rotation induced the epitaxial growth of <001> fcc phase on the hatch direction (HD)–scan direction (SD) plane in the direction deviated by approximately 45° relative to the SD. This 45° deviation relative to the SD occurred to accommodate the beam movement and the circular beam geometry on the HD – SD plane, resulting in the preferred configurations of <011>║SD and HD. Thus, this paper reports the importance of meander scanning with a 90° scan-rotation strategy to yield a flat-top-LPBF-derived SX structure with <001>║BD and <011>║SD and HD textures.

Modeling of epitaxial growth of single crystal superalloys fabricated by directed energy deposition

Single crystal (SX) nickel-based superalloys have been used for turbine blades fabrication in recent years because of their superior mechanical performance at high temperature. The restoration of SX blades is significant considering the high cost to re-manufacture the blade. Directed energy deposition (DED), as a laser additive manufacturing technique, is a promising method for repairing the SX blades. Continuous epitaxial growth from the substrate to deposited layers is vital for successful restoration of SX blades. In this work, a novel single crystal texture cellular automata (SITCA) method is developed to simultaneously predict the columnar to equiaxed transition (CET) and crystal growth direction at the molten pool scale. Based on the grain envelope simplification, the SITCA method introduces a random variable to trace the growth trajectory, and therefore indicates the partition of the growth regions in the molten pool. A continuous nucleation model is introduced for nucleation simulation, and the crystal growth is simulated using the Kurz–Giovanola–Trivedi (KGT) model. The validation of SITCA model is performed by casting solidification cases under uniform temperature and uniform temperature gradient. The SITCA method is then integrated with a thermo-fluid molten pool simulation using finite element method to comprehensively compute the epitaxial growth. SX remelting experiments are carried out with different laser scanning speeds using a flat top laser source, and compared with the numerical results. The experimental and numerical results both indicate that increasing laser scanning speed will increase the ratio of crystals growing in the [001] direction while facilitate the CET on the other hand. This work shows the capacity of the SITCA method to predict epitaxial growth in DED process, and we anticipate to deploy the SITCA method to complex SX additive manufacturing.

Thermal-induced effects on ultrafast laser filamentation in ethanol

Fs laser filamentation in ethanol is studied for bottom-up nanomaterials synthesis at different laser repetition rate. Measurements of energy loss, visible conical images, transmitted beam profile and the optical emission spectra show that laser repetition rate affects ultrafast laser filamentation in ethanol significantly because of the induced thermal processes. Firstly, near the threshold laser energy for self-focusing, high laser repetition rate could interfere with the process, via thermal defocusing that inhibits the onset of filamentation. An enlarged, near flat-top laser beam is produced for the conditions that favor thermal lensing. Secondly, at high laser energy, thermal defocusing effect is saturated as convective heat dissipation occurs. Thus, self-focusing prevailed and led to filamentation at all repetition rates. The strong convection at high laser repetition rate increases the rate of reactions for nanodiamonds synthesis, as the flow of fresh ethanol precursor to the localized filamentation zone is enhanced.

Blue laser welding of laminated electrical steels: Dynamic process, weld bead characteristics, mechanical and magnetic properties

Laminated electrical steels are the key material structure of the stator and rotator of a motor, where high-quality welding is crucial for improving the performance of the electric machine. However, it is challenging to achieve a desired balance among the magnetic properties, mechanical properties, and welding efficiency for the laminated electrical steels at present. This work first adopted a 450 nm flat-top blue laser with a maximum power of 2000 W, which has a higher primary absorption rate to Fe compared to that of the conventionally used 980-1080 nm infrared laser, to join the laminated electrical steels. The dynamic process, weld bead characteristics, mechanical properties, and magnetic properties of the blue laser welding of laminated electric steels were investigated. The results show that a flat-top and large-spot blue laser can make the molten pool have a self-stabilizing ability to keep the continuity when encountering the gap and unsmooth surface. Moreover, a further improved self-stabilizing ability is observed for a small welding speed. The largest depth and width of the blue laser weld bead in this work are 2.30 and 0.57 mm, respectively, with a shear strength of up to 1700 N, which are achieved at the laser power of 1500 W and welding speed of 10 mm/s. In addition, the work also validated that the blue laser could significantly reduce the splashing in welding of electrical steel laminations and improve the welding efficiency, which provides a promising solution for high-performance electrical machine manufacturing.

Study on Melting Kinetics of CF/PEEK Composites Irradiated by Flat Top Laser

Study on Melting Kinetics of CF/PEEK Composites Irradiated by Flat Top Laser

Focus on lasers, imaging, microscopy, and nanoscience

Focus on lasers, imaging, microscopy, and nanoscience