Journal of Central South University

Sun, Wei-gao; Yan, Tian-yang; Wang, Yu-heng; Ji, Ling-fei

doi: 10.1007/s11771-022-5141-3pmid: N/A

The influence of the picosecond (ps) pulsed burst with a nanosecond scale of temporal separation (50 ns) on filamentary traces in sapphire substrate is investigated. The spatiotemporal evolution of the filamentary plasma string induced by sub-pulses of the burst-mode is revealed according to the analysis of the instantaneous photoluminescence images. Due to the presence of residual plasma, the energy loss of sub-pulse during the balancing of self-focusing effect is reduced, and thus refreshes the plasma via refocusing. The refreshed plasma peak generated by the subsequent subpulse appears at relatively low density positions in the formed filamentary plasma string, which results in more uniform densities and less spatial overlap among the plasma peaks. The continuity and uniformity of the filamentary trace in sapphire are enhanced by the burst-mode. Besides, the burst filamentary propagation can also remain effective when the sub-pulse energy is below the self-focusing threshold. Based on this uniform and precise energy propagation mode, the feasibility of its use for the laser lift-off (LLO) process is verified.

Zhao, Yi-zhe; Hong, Ming-hui

doi: 10.1007/s11771-022-5145-zpmid: N/A

Metal superhydrophobic surfaces with anisotropic wettability and adhesion have become more and more important due to their promising applications. Herein, we report a new fabrication strategy through a combination of pulsed laser ablation and low-temperature annealing post-processing. An inclined cone structure array is made on stainless steel surfaces, and then 120 °C low-temperature annealing is applied. Such surface displays excellent mechanical durability and anisotropic superhydrophobicity. It is demonstrated experimentally that the contact angle of water droplets on the surface is different along the parallel (167° ±2°) and perpendicular directions (157° ±2°) of the inclined cone structure. The sliding behaviors of water droplets and mechanical durability of the inclined cone structures are studied. These surfaces obtained in a short time with environmentally friendly fabrication can be applied in industries for water harvesting, droplet manipulation, and pipeline transportation.

Luo, Fang-fang; Liu, Peng; Qiu, Tie-cheng; Zhai, Yong-ping; Wang, Xian-wei; Guo, Ting; Liu, Jia-bin; Wang, Zu-yong

doi: 10.1007/s11771-022-5155-xpmid: N/A

Poly(ε-caprolactone) (PCL) holds unique bioresorbability and competent biomechanical properties for tissue-engineering application. However, PCL is hydrophobic intrinsically and poor in cell-biomaterial interaction. In this study, we prepared a composite based on PCL and bioactive tantalum (Ta) to understand the effects of direct laser micropatterning on composite surface properties. The PCL/Ta composite after preparation was surface-patterned by femtosecond laser and characterized with surface morphology, crystal structure, chemical composition, wettability and cellular response of fibroblast. It was found that laser micropatterning enlarged the difference of wetting properties (∼15°) on PCL and PCL/Ta surfaces. The wetting changes was dependent on both material composition and laser-machined geometry. The blending of Ta enhanced surface wettability with prolonged contact time on the laser-machined line and rectangle microarrays. In vitro culture results showed beneficial effects of laser micropatterning on cell morphology of the fibroblasts. On the PCL/Ta surfaces with line and rectangle microarrays, the cells were more likely to bridge the sidewalls of the microgrooves, showing adaptive 3D morphologies to the micro/nano topographies on the sidewalls. These findings are envisaged to facilitate surface design and micropattern optimization for favorable tuning the cell response to biomedical PCL/Ta composites.

Butkus, Antanas; Skliutas, Edvinas; Gailevičius, Darius; Malinauskas, Mangirdas

doi: 10.1007/s11771-022-5153-zpmid: N/A

Here we report a femtosecond laser direct writing (a precise 3D printing also known as two-photon polymerization lithography) of hybrid organic-inorganic SZ2080™ pre-polymer without using any photo-initiator and applying ∼100 fs oscillator operating at 517 nm wavelength and 76 MHz repetition rate. The proof of concept was experimentally demonstrated and benchmarking 3D woodpile nanostructures, micro-scaffolds, free-form micro-object “Benchy” and bulk micro-cubes are successfully produced. The essential novelty underlies the fact that non-amplified laser systems delivering just 40–500 pJ individual pulses are sufficient for inducing localized cross-linking reactions within hundreds of nanometers in cross sections. And it is opposed to the prejudice that higher pulse energies and lower repetition rates of amplified lasers are necessary for structuring non-photosensitized polymers. The experimental work is of high importance for fundamental understanding of laser enabled nanoscale 3D additive manufacturing and widens technology’s field of applications where the avoidance of photo-initiator is preferable or is even a necessity, such as micro-optics, nano-photonics, and biomedicine.

Ma, Xu-feng; Sun, Yao-ning; Cheng, Wang-jun; Chong, Zhen-zeng; Huang, Liu-fei; Meng, A.-cong; Jiang, Li-heng

doi: 10.1007/s11771-022-5162-ypmid: N/A

In order to study the corrosion resistance of high-speed laser cladding (HLC) coating while improving production efficiency, a CoCrFeNiMo0.2 high-entropy alloy (HEA) coating was prepared by HLC. The optimized parameters of HLC are laser power of 880 W, scanning speed of 18 m/min, overlapping ratio of 60%, and powder feed speed of 3 r/min. Then, the surface roughness, microstructure, phase composition, element distribution, and electrochemical properties in 3.5 wt% NaCl solution of the coatings were analyzed, respectively. The local surface roughness of the CoCrFeNiMo0.2 HEA coating was found to be 15.53 µm. A distinct metallurgical bond could be observed between the coating and the substrate. Compared to the conventional laser cladding (CLC), the results of electrochemical tests showed that CoCrFeNiMo0.2 HEA coating exhibited a significant passivation. The corrosion current density of 5.4411×10−6 A·cm−2 and the corrosion potential of −0.7445 V for the HLC coating were calculated by the Tafel extrapolation method. The CLC coating’s corrosion current density and corrosion potential are 2.7083×10−5 A·cm−2 and −0.9685 V, respectively. The HLC coating shows a superior corrosion resistance, crucially due to the uniform and fine grains. Under various complex and harsh working conditions, this method can be widely used in the field of repairing and remanufacturing of corrosion-proof workpieces.

Li, Ruo-zhou; Guo, Lyu-jiu; Yang, Ming-qing; Qu, Ke; Yan, Jing

doi: 10.1007/s11771-022-5167-6pmid: N/A

Laser processing provides highly-controlled modification and on-demand fabrication of plasmon metal nanostructures for light absorption and photothermal convention. We present the laser-induced forward tansfer (LIFT) fabrication of silver nanomembranes in control of light absorption. By varying the hatch distance, different morphologies of randomly distributed plasmon silver nanostructures were produced, leading to well-controlled light absorption levels from 11% to 81% over broadband. The anti-reflection features were maintained below 17%. Equilibrated and plain absorptions were obtained throughout all absorption levels with a maximum intensity fluctuation of ±8.5% for the 225 µJ cases. The 45 µJ pulse energy can offer a highly equilibrated absorption at a 60% absorption level with an intensity fluctuation of ±1%. Pattern transfer was also achieved on a thin tape surface. The laser-transferred characters and patterns demonstrate a localized temperature rise. A rapid temperature rising of roughly 15 °C can be achieved within 1 s. The LIFT process is highly efficiently fabricated with a typical speed value of 103 to 105 cm2/h. The results indicated that LIFT is a well-controlled and efficient method for the production of optical films with specific absorption levels.

Chen, Jian-qiang; Xie, Xiao-zhu; Peng, Qing-fa; He, Zi-yu; Hu, Wei; Ren, Qing-lei; Long, Jiang-you

doi: 10.1007/s11771-022-5136-0pmid: N/A

Ablation threshold is an important concept in the study of femtosecond laser micro- and nano-machining. In this paper, the ablation experiments of three kinds of surface roughness 4H-SiC substrates irradiated by femtosecond laser were carried out. The feature thresholds were systematically measured for three surface roughness SiC substrates and found in the modification and annealing regions ranging from coincidence (Ra=0.5 nm) to a clear demarcation (Ra=5.5 nm), eventually being difficult to identify the presence of the former (Ra=89 nm). Under multi-pulse laser irradiation, oriented ripple structures were generated in the annealing region, where deep subwavelength ripples (about 110 nm, Λ ≈ 0.2λ) can be generated above substrates with surface roughness higher than 5.5 nm. We investigated the effect of surface roughness on the ablation morphology, ablation threshold, and periodic structures of femtosecond laser ablation of 4H-SiC substrates, while the ablation threshold was tended to decrease and stabilize with the increase of pulse number N⩾500.

Huang, Hai-peng; Hao, Ben-tian; Ye, De-jun; Gao, Hao; Li, Liang

doi: 10.1007/s11771-022-5163-xpmid: N/A

Laser cleaning is a highly nonlinear physical process for solving poor single-modal (e.g., acoustic or vision) detection performance and low inter-information utilization. In this study, a multi-modal feature fusion network model was constructed based on a laser paint removal experiment. The alignment of heterogeneous data under different modals was solved by combining the piecewise aggregate approximation and gramian angular field. Moreover, the attention mechanism was introduced to optimize the dual-path network and dense connection network, enabling the sampling characteristics to be extracted and integrated. Consequently, the multi-modal discriminant detection of laser paint removal was realized. According to the experimental results, the verification accuracy of the constructed model on the experimental dataset was 99.17%, which is 5.77% higher than the optimal single-modal detection results of the laser paint removal. The feature extraction network was optimized by the attention mechanism, and the model accuracy was increased by 3.3%. Results verify the improved classification performance of the constructed multi-modal feature fusion model in detecting laser paint removal, the effective integration of acoustic data and visual image data, and the accurate detection of laser paint removal.

Shi, Wen-tian; Li, Ji-hang; Liu, Yu-de; Liu, Shuai; Lin, Yu-xiang; Han, Yu-fan

doi: 10.1007/s11771-022-5135-1pmid: N/A

This experiment obtained different laser energy density (LED) by changing SLM molding process parameters. The surface morphology, surface quality, and microstructure of as-fabricated samples were studied. The effects of scanning speed, hatching space, and laser power on surface quality were analyzed, and the optimal LED range for surface quality was determined. The results show that pores and spherical particles appear on the sample’s surface when low LED is applied, while there are lamellar structures on the sides of the samples. Cracks appear on the sample’s surface, and the splash phenomenon increases when a high LED is taken. At the same time, a large amount of unmelted powder adhered to the side of the sample. The surface quality is the best when the LED is 150–170 J/mm3. The preferred hatch space is currently 0.05–0.09 mm, the laser power is 200–350 W, and the average surface roughness value is (15.1±3) µm. The average surface hardness reaches HV404±HV3, higher than the forging standard range of HV340–HV395. Increasing the LED within the experiment range can increase the surface hardness, yet an excessively high LED will not further increase the surface hardness. The microstructure is composed of needle-like α′-phases with a length of about 20 µm, in a crisscross ‘N’ shape, when the LED is low. The β-phase grain boundary is not obvious, and the secondary-phase volume fraction is high; when the LED is high, the α′-phase of the microstructure is in the form of coarse slats, and the secondary-phase is composed of a small amount of secondary α′-phase, the tertiary α′-phase and the fourth α′-phase disappear, and the volume fraction of the secondary-phase becomes low.

Wu, Jia-jun; Huang, Zheng; Qiao, Hong-chao; Wei, Bo-xin; Zhao, Yong-jie; Li, Jing-feng; Zhao, Ji-bin

doi: 10.1007/s11771-022-5158-7pmid: N/A

In this work, the nickel-based powder metallurgy superalloy FGH95 was selected as experimental material, and the experimental parameters in multiple overlap laser shock processing (LSP) treatment were selected based on orthogonal experimental design. The experimental data of residual stress and microhardness were measured in the same depth. The residual stress and microhardness laws were investigated and analyzed. Artificial neural network (ANN) with four layers (4-N-(N-1)-2) was applied to predict the residual stress and microhardness of FGH95 subjected to multiple overlap LSP. The experimental data were divided as training-testing sets in pairs. Laser energy, overlap rate, shocked times and depth were set as inputs, while residual stress and microhardness were set as outputs. The prediction performances with different network configuration of developed ANN models were compared and analyzed. The developed ANN model with network configuration of 4-7-6-2 showed the best predict performance. The predicted values showed a good agreement with the experimental values. In addition, the correlation coefficients among all the parameters and the effect of LSP parameters on materials response were studied. It can be concluded that ANN is a useful method to predict residual stress and microhardness of material subjected to LSP when with limited experimental data.