Ying Li, Xinyue Li, Jiaqi Wang, Jinkai Xu, and Huadong Yu

ObjectiveWaterjet-assisted laser processing technology provides a new direction in the development of laser processing that can effectively mitigate thermal damage to materials during laser processing; thus, it has good prospect for microfabrication applications. During processing, the quality of the material surface can be inspected in real time, and the introduction of microscopic systems can provide a more convenient observation of material processing details as well as improve the corresponding efficiency. However, because of the interference of the waterjet, bubbles are present in the source image captured by the CCD camera, obscuring detailed information and blurring the surface. In addition, the source image sequence is only partially focused because of the limited depth-of-field of the microscope system. To enhance the detail of surface features of materials for the purpose of dehazing, an image processing technique is employed to improve the contrast of an image by emphasizing the respective brightness, saturation, and textural features of local areas. However, conventional dehazing algorithms begin with the image itself. Without considering that the local area processing method has problems including loss of detailed information and excessive contrast enhancement, development of an image dehazing method for the local areas of different images is necessary. In the traditional microscope mode, the effect of extending the depth-of-field can be achieved via component deformation; however, this has reduced efficiency, large equipment size, and low lateral resolution. Currently, image fusion based on the transform domain is a popular research topic, and is one of the most widely used and mature methods in practical applications.MethodsThis study solves the problems of waterjet interference and image fusion. Bubbles were maximally reduced via foreground segmentation and morphological theory, and the detailed information of the source images was enhanced according to the adaptive multiscale Retinex dehazing algorithm. After dividing the image into blocks, the detail index is defined by the standard deviation value of each block. The most suitable Gaussian filter function scale value was determined, and the corresponding weights were calculated to linearly superimpose the single-scale Retinex algorithm of different scales. The source image was decomposed into detail and approximate components using discrete wavelet transform, the detail component was stretched according to the human vision principle, and each discrete wavelet inverse transform was performed according to the fusion rules. As a result, a full-focus image with an extended depth-of-field can be realized.Results and DiscussionsThe experimental system adopted in this work was set up as depicted in Fig. 1, mainly constituting a laser processing and image capture system, and a high-pressure waterjet assist system. The processing and image capture system comprises a laser, waterjet nozzle, and CCD camera. The processed material surface images of the waterjet interference problem are presented in Fig. 5: (a) displays one of the source images captured in the waterjet environment and (b) shows the final processed image. The interference of bubbles on the image information in the processed image appears to fade, and more detailed features are restored. Compared with the source image, the average gradient (AG), standard deviation (SD), and spatial frequency (SF) of the processed image improved by 27.6%, 20.1%, and 4.74%, respectively. Table 3 presents the image quality comparison results of the three source images in the air and waterjet environments. In the waterjet environment, the image quality was significantly lower than that in the air environment, where the SF and AG were reduced by 33.88% and 31.11%, respectively. Figure 6 shows the source images of the material surface collected for waterjet nozzle diameters of 0.4, 0.8, and 1.2 mm as well as the processed images after algorithmic processing. As the diameter of the waterjet nozzle increases, the thicker the flowing water layer on the surface of the sample, and the more limited the image information that can be obtained from the source image. According to Fig. 7(b), the three indicators of the processed image obtained with the diameter nozzle of 0.4 mm reached 95.41%, 71.88%, and 67.29%, respectively. Fig. 8 displays the source images of the material surface collected for waterjet inclination angles of 30°, 45°, and 60°, besides the processed images after algorithmic processing. As the waterjet inclination angle decreases, the thicker the flowing water layer on the surface of the sample, the more limited the image information that can be obtained from the source image. As shown in Fig. 9(b), the three indicators of the processed image considering the 45° angle reached 90.59%,72.69%, and 94.50%, respectively; thus, maximizing the exclusion of the interference of the waterjet, which could restore part of the detailed features.ConclusionsIn waterjet-assisted laser processing, microscopic images of material surfaces are subject to waterjet interference and depth-of-field limitations. Therefore, we propose a waterjet laser processing image-fusion algorithm based on the Retinex dehazing algorithm. The method determines different Gaussian filter function scales for various images, improves the dehazing effect of traditional algorithms, and stretches the detailed components according to human vision. As a result, detail of source image information was enhanced and the image quality was improved. Experiments demonstrate that the algorithm reduces the interference of the waterjet in the source image, enhances the detailed information of the image, and achieves full focus. As the diameter of the waterjet nozzle increases and the inclination angle decreases, the water film on the surface of the material becomes thicker, and the quality of the source image is subsequently reduced, which is difficult for the algorithm to process. However, the experimental results show that with the nozzle diameter of 1.2 mm or an inclination angle of 30°, the processed image still presents most of the detailed features and improved image quality. Thus, the developed algorithm can effectively improve the efficiency of waterjet-assisted laser processing and is expected to be widely used in liquid-assisted laser processing.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402201 (2023)
DOI:10.3788/CJL230985
Chen Xie, Shixian Sun, Erse Jia, and Minglie Hu

ObjectiveMicro-helices are applied in microrobots and chiral metamaterials and require various features for structure fabrication in diverse applications. The femtosecond laser direct writing (FsLDW) technology can fabricate three dimensional (3D) microstructures based on two-photon polymerization (2PP) with sub-diffraction-limited resolution. This technology is used to fabricate micro-helix structures using the widely used point-by-point writing scheme with a single focus. However, this is relatively inefficient because of the repetitive scans along many helical trajectories in the fabrication process. Recently, one-step exposure with structured light has allowed the rapid fabrication of micro-helices, wherein helical beams are specially designed with vortex phases. However, state-of-the-art schemes can only produce microstructures with limited patterns owing to the complex and professional light manipulation techniques. To efficiently fabricate micro-helix structures with flexible features such as diameter, thread number, pitch, and chirality, we propose a scheme based on dynamic multi-focus patterns to fabricate multiple helical microstructures.MethodsMicro-helix structures were fabricated by piling up the multi-focal voxels along helical trajectories. The helical motion of the voxels was divided into two components: a circular motion manipulated by dynamic holograms loaded on the spatial light modulator (SLM), and a linear motion controlled by a z-axis translation stage (Fig. 3). Based on our in-house fabrication system, we adapted the Gerchberg-Saxton (G-S) algorithm to compute dynamic multi-focus holograms on the SLM iteratively. Subsequently, the tightly focused femtosecond multi-focal beam patterns induced polymerization of the photoresist (SU-8). In this method, the diameter and thread number of the micro-helices were determined by the parameters of the hologram alone under fixed exposure conditions. The dynamic holograms with the z-axis translation stage allowed flexible control of the pitch and chirality.Results and DiscussionsThe improved G-S algorithm adapted in our setup generates a well-defined multi-focus in the experiments, showing good consistency with the simulation (Fig. 2). These multi-focused beam patterns allow the flexible fabrication of various micro-helices by piling up the multi-focus voxels in a single helical motion (Fig. 3). Using a four-focus beam as an example, the geometric features of the corresponding voxels in 2PP are experimentally characterized in terms of their diameters and axial sizes (Fig. 4). In this experiment, the threshold power is determined as 0.01 mW for every single focus under 5 ms exposure time. Micro-helices are fabricated under the aforementioned exposure conditions. However, insufficient adhesion leads to the detachment of these microstructures with high aspect ratios from the silicon substrates (Fig. 5). To address this issue, another polymerized thin film is deposited on the substrate, which is cured using a UV lamp before proceeding with 2PP. This process significantly enhances the adhesion between the microstructures and substrate. Additionally, the capillary forces occurring during the developing step of post-processing distort the microstructures by pulling down the helical threads (Fig. 5). This issue is resolved by drying the micro-helices in supercritical carbon dioxide. Finally, five sets of dynamic multi-focus holograms are used to fabricate the micro-helices with different features (Fig. 6). The spiral diameter of these microhelices ranges from 2.4 to 19.2 μm as the number of threads increases from one to eight, with adjustable chirality and pitch. Introducing supporting micro-ribs also significantly enhances the stiffness of the micro-helices, which enables the height of the micro-helices to be up to 30 μm (Fig. 7). A possible reason for the different thread widths under different pitches in the voxel-stacking model is analyzed (Fig. 8).ConclusionsIn this paper, we propose a dynamic hologram scheme to fabricate micro-helices using 2PP. In our scheme, the helical motion of the focus in conventional direct laser writing can be divided into linear and circular motions. Linear axial motion is controlled by a mechanical translation stage, and circular motion is manipulated by a set of dynamically programmable holograms. We also prove that the improved G-S algorithm is effective in generating multi-focus femtosecond laser patterns with flexibly controlled parameters to achieve circular motion. Therefore, well-defined dynamic multi-focus patterns in combination with axial mechanical motion can allow the fabrication of micro-helices with flexible control over diameters, thread numbers, pitches, and chiralities. Compared with the FsLDW method based on the single focus, the multi-focus parallel writing scheme can increase the fabrication efficiency by N times for micro-helices with N threads. Additionally, this hologram-based scheme offers advantages in terms of system cost, as it eliminates the need for an expensive motion controller for sophisticated spiral mechanical movements. The proposed flexible and economical method of micro-helix fabrication holds great potential for various applications, such as microrobots, chiral metamaterials, and bioengineering.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402402 (2023)
DOI:10.3788/CJL230772
[in Chinese], [in Chinese], [in Chinese], [in Chinese], [in Chinese], [in Chinese], [in Chinese], and [in Chinese]

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2416008 (2023)
DOI:10.3788/CJL231377
Qianhao Wang, Hualong Zhao, Xiaojun Yang, Wenlong Wen, and Yi Li

ObjectiveThis study investigates the prevalent process problems, such as “microcracking” and “induced streaking,” in the femtosecond laser processing of hard and brittle transparent materials. The study employs the femtosecond time-resolved pump-probe shadow imaging technique to visualize the electron dynamics during the femtosecond laser multi-pulse ablation of quartz glass. Particularly, the plasma filament evolution at the early stage of laser pulse ionization (before 700 fs) is analyzed. The multi-pulse-induced microstructures distribute the filament formation regions on both sides of the microstructure with respect to the axial direction of the light pulse propagation. The distribution on both sides is primarily due to the refraction of the light pulse by the sidewalls of the microstructure, while that on the axis is caused by the difference in the shape of the bottom and sidewalls of the microstructure, creating the light range difference. The empirical results show that the pulse train induces a remodeling effect of the microstructure on the subsequent light field during multi-pulse processing, affecting the distribution of the plasma filament formation region and energy deposition—the core mechanism responsible for common process problems.MethodsA femtosecond time-resolved pump-probe shadow imaging setup was built to capture the propagation and ionization process of a single subsequent pulse beneath the microstructure induced by irradiating the material with 219 fs pulses. First, the actual spatial location of the focus was determined by imaging the shadow of the air-ionized plasma at the focus. After that, the power density at the material surface was obtained for different focus positions. The distinctive “V” and “inverted trapezoid” shapes were obtained after controlling the relative positions of the laser focus and the material. Second, the ionization process of femtosecond time-resolved propagation of the 220th pulse under different microstructures was obtained by modulating the time delay between pump and probe beams. Finally, the ionized filament-forming regions in the transient ionization images were compared with the process defects to reveal the formation mechanism of the process defects.Results and DiscussionsThe propagation and ionization process of the 220th pulse is observed using femtosecond time-resolved pump-probe shadow imaging (Figs.5.6.9.10). The physical mechanisms governing process problems such as “microcracking” in micromachining of hard and brittle materials are revealed. The light-field remodeling effect, guided by various morphology microstructures, leads to energy deposition and the mechanism of generating common process problems. In the context of multi-pulse processing, the influence of energy deposition (propagation and ionization) is determined by the linear refractive index of the material, a nonlinear refractive index that varies with the light intensity, plasma defocusing effect, microstructure morphology, and the focusing conditions in conjunction with the laser fluence on the material surface. Moreover, the relaxation time of ionized free electron number density during light field propagation is determined to be less than 300 fs across diverse microstructures.ConclusionsUnder multi-pulse irradiation, the remodeling effect of different microstructural morphologies on the subsequent light field orchestrates the nonlinear ionization process. In the case of the V-shaped structure, the formation process is accompanied by decreasing tilt angle of the sidewalls, guiding the ionization filamentation direction of the subsequent light field and sweeping across the sidewall region. It corresponds to the areas of “microcracks” and “induced stripes” on both sides of the microstructure. Conversely, the “inverted trapezoidal” structure yields strong ionization filament formation at its relatively flat bottom center region owing to the ionization effect at the bottom of the structure. For the “inverted trapezoidal” structure, a strong ionization effect occurs at the center of the relatively flat bottom, which is the root cause of the fragmentation at the bottom. Overall, the light-field remodeling effect, facilitated by different morphology microstructures, plays a crucial role in energy deposition and governing the common process problems. This result offers directions for optimizing machining processes, such as selecting the number of pulses and controlling the focal feed. Additionally, the high-temporal-resolution pump-probe shadow imaging technique holds promise for predicting fragmentation regions in different morphologies of hard and brittle transparent materials, serving as a powerful tool for online monitoring of high-end processing equipment.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402101 (2023)
DOI:10.3788/CJL230834
[in Chinese], [in Chinese], [in Chinese], [in Chinese], and [in Chinese]

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2416003 (2023)
DOI:10.3788/CJL231247
Chunyan Dong, Xiaoyu Zhang, Dehua Gu, Siwu Shao, Jinlong Liu, Liangxian Chen, Chengming Li, and Junjun Wei

ObjectiveDiamond has high chemical stability, mechanical properties, high carrier mobility, and thermal conductivity, which has broad application prospects in many frontier fields. Diamond with micropores has good application prospects in high-precision lead forming and high-power microwave device heat dissipation. This study established the variation law of diamond hole patterns and defect characteristics under different laser powers. The diamond hole patterns and defect characteristics are vital in implementing the heat dissipation currently used in substrates for high-frequency electronics. The basic conditions can only be provided for the subsequent microporous metallization when the pore type meets the requirements, and the surface graphite residue and the pore wall are smooth.MethodsIn this study, laser technology was used to process micropores on self-supporting polycrystalline diamond films. By adjusting the laser power, the influence of laser power on diamond microporous molding was studied, the reaction mechanism of laser-diamond interaction was discussed, and the removal mechanism of diamond was analyzed. Field emission environment scanning electron microscopy was used for morphology analysis. Laser confocal scanning microscopy was used to measure microwell contours. Laser Raman spectroscopy and X-ray photoelectron spectroscopy were used for surface composition characterization to analyze the influence of laser power on the outer and inner surface of micropores and the causes of defects. The material removal mechanism and microhole forming process during laser microhole processing were revealed by introducing diamond ablation threshold analysis.Results and DiscussionsMorphology analysis was conducted by field emission environment scanning electron microscopy, and the results showed that the microporous surface had significant sedimentary layers and spherical deposits. When the power reaches 17.6 W, the surface of the micropores is damaged and fractured, and there is an obvious stripe structure at the fracture location, which may be formed by the interconnection of crack propagation caused by thermal stress. The diamond-layered deposits begin to fall off when the microporous surface sedimentary layer is partially detached during the fracture of the diamond surface layer [Fig. 1(d)], and the thermal stress on the microporous surface is greater than the van der Waals force between the sedimentary layer and the diamond. In contrast, as the laser power increases, the thickness of the sediment layer on the diamond surface also increases, which may also be another cause of layered sediment shedding due to the difference in thermal expansion between the layered deposit and the diamond substrate. The topography of the inner surface of the micropores (Fig. 3) shows that a fine graphite layer covers the top of the micropores, and the graphite layer reduced from top to bottom. Simultaneously, significant cracks and flaky shedding of the inner surface can be observed. The microwell profile was measured by laser confocal scanning microscopy, and further analysis of the change of micropore taper showed that the inner surface of the upper end of the micropores was rough. The microporous taper decreased with the increase of laser power. In the downward energy transfer process, it is absorbed by the diamond and generates graphite. The energy received at the lower end of the micropores is reduced, and the diamond micropores finally take on a cone shape. Moreover, because the degree of graphitization increases with the increase of laser power, the thickness of the graphite layer on the surface and inner surface of the diamond increases, which improves the absorption of laser energy; hence, at high power, the energy difference between the upper and lower ends of the micropores increases, which in turn leads to an increase in taper.Laser Raman spectroscopy and X-ray photoelectron spectroscopy were used to characterize the surface composition, and it was proved that the main components of the surface and inner surface sedimentary layers of diamond after laser processing were graphite and then proved that the diamond underwent phase transition under the action of the laser. The effect of power on stress was analyzed by Raman spectral peak shift calculation. With the increase of laser power, the compressive stress on the diamond's outer surface and inner surface increases, and the amplitude of the increase is the same. Finally, introducing diamond ablation threshold analysis reveals the material removal mechanism and micropore forming process in laser microporous processing. The ablation threshold of polycrystalline diamond is 3.16 J/cm2, and the corresponding average power is 2.23 W. A phase change reaction begins on the diamond surface when the laser energy is above the ablation threshold. The increase in laser power provides more energy for the phase change reaction, and the amount of diamond removal increases, which produces more defects. The reaction is terminated when the energy is absorbed below the ablation threshold.ConclusionsThe results show that the outer surface of the micropore is damaged when the laser power is high, and the micropore's inner surface also has a striped structure. The degree of graphitization on the outer surface and inner micropore surface increased with the laser power. The taper of the microporous type decreases with the increase of laser power, and the verticality of the micropores was improved. The stress on the inner surface of the micropore during laser processing is greater than that on the edge position of the micropore.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402404 (2023)
DOI:10.3788/CJL230739
Haiyang Long, Zhen Dong, Bingwen Lu, Xingchen Yan, Rucheng Ma, Qing Ma, Guorui Zhao, and Changming Qiu

ObjectiveHigh-entropy alloys (HEAs) exhibit excellent properties owing to their four unique effects and have great application potential in marine, nuclear power, new energy, and other fields. However, multielement HEAs fabricated using laser cladding (LC) tend to form a single solid-solution phase. The matching of strength and ductility is a key problem that must be urgently solved to promote the application of LC HEA coatings. In this study, FeCoNiCr HEA coating was reinforced by adding different contents of hard WC particles, which contributed significantly to the matching of the high ductility of the FCC-structured HEAs and the high hardness of the WC particles. The addition of hard particles to FCC-structured HEAs provides a new method to eliminate the mismatch between strength and ductility.MethodsTwo types of powders were mixed at a certain mass percentage, and the mixing process was conducted using a planetary ball mill. The LC process parameters were optimized by varying the laser power and scanning speed. The LC coatings were cut into cubic samples for microstructure characterization. The forming quality was observed using optical microscopy (OM). An X-ray diffractometer was used for the phase composition analysis. The scanning electron microscopy (SEM) was used to characterize the sample microstructures, and the wear tests were performed using a wear-testing machine. The corrosion resistance of the coatings was measured using an electrochemical workstation. Based on the experimental results, the effect of WC content on changes in the microstructure, microhardness, wear resistance, and corrosion resistance of the FeCoNiCr high-entropy alloy was systematically investigated.Results and DiscussionsIn order to fully combine the high ductility of FCC-structured HEAs with the high hardness of WC particles, 10%?60% (mass fraction) WC/FeCoNiCr HEA composite coatings were fabricated. As shown in (Fig.3), the crack-free 60% WC-reinforced FeCoNiCr HEA composite coating was successfully manufactured. Furthermore, it was demonstrated that with an increase in the WC content, the content of the FCC phase in the FeCoNiCr HEA composite coatings decreased, whereas the carbide precipitation content increased, as shown in Fig.4. Figure 8 confirms that the wear resistance of the composite coatings with 60% WC was approximately 233% higher than that of the coatings without WC addition.ConclusionsIn this study, the influence of different WC contents on the microstructures and properties of LC FeCoNiCr HEA coatings was investigated. According to the results of the flaw detection experiments, the composite coatings were well formed with no macrocracks. With an increase in the WC content, the phase composition of the coating gradually changed from a single FCC phase to the multi-phase of FCC phase, WC, W2C, and Co4W2C phases. Moreover, the grains of the coatings were refined with the addition of WC, and block- and fishbone-like structures appeared in the 60% WC composite coatings. With 60% WC addition, the average microhardness of the coating cross-section reached 501 HV0.2, which was about 186% higher than that of pure FeCoNiCr HEAs coating. Although the addition of WC continuously improved the wear resistance of the composite coatings, the corrosion resistance of the composite coatings gradually decreased, mainly due to the decrease in FCC phase with good corrosion resistance in the FeCoNiCr HEAs.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402206 (2023)
DOI:10.3788/CJL230637
Gaoyang Mi, Yiming Jiang, Chunming Wang, Mingyang Zhang, and Qiubao Ouyang

ObjectiveThe 7-series aluminum alloy is a heat-treatable aluminum alloy widely used in aerospace, rail transit, and other fields because of its excellent specific strength. Laser beam swing welding is a new method developed from conventional laser-welding technology. It can reduce the temperature gradient, stabilize the welding process, and inhibit the formation of pores and other defects in the weld. The addition of alloying elements is currently the focus of research for improving the weld performance. Many researchers worldwide have shown that adding rare-earth elements or Zr, Ti, and other elements to the weld can improve the mechanical properties. However, research on the effect of Ti addition on the microstructure and properties of 7075 aluminum alloy weld is not yet comprehensive. In this study, a systematic investigation of the effect of the thickness of a Ti metal interlayer on the microstructure and mechanical properties of 7075 aluminum alloy weld joints is reported.MethodsIn this study, 2 mm thick 7075 aluminum alloy and Ti foils of different thicknesses were used. First, welds with different thicknesses of Ti metal intermediate layers were prepared using laser beam swing welding. Then, methods, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD), were used to analyze the effects of different Ti interlayer thicknesses on the microstructures and phases of the weld metal. Finally, by analyzing the fracture location and considering the changes in the tensile strength of the joint, the influence of adding different thicknesses of Ti metal intermediate layers on the mechanical properties of the joint was determined.Results and DiscussionsWhen the thickness of Ti intermediate layer is 0.02 mm, the Ti content in the liquid metal is relatively low. During the cooling solidification process, Al grains precipitate simultaneously with TiAl3, and the TiAl3 phase forms a short rod-shaped distribution near the interface of the aluminum grains during the solidification process. When the thickness of Ti intermediate layer is 0.03 mm, the liquid metal first precipitates a high-melting-point TiAl3 phase during the solidification process. The precipitated phase is small and cross shaped. As the temperature continues to decrease, the TiAl3 phase dispersed in the molten pool becomes the substrate for heterogeneous nucleation. When the thickness of Ti intermediate layer is 0.04 mm, excess Ti cannot be dissolved in liquid Al, and a large area of unmelted Ti is retained. At the interface between this phase and the Al grains, a short rod-shaped TiAl3 phase forms to envelop the unmelted Ti. When the thickness of the Ti intermediate layer is 0.02 mm, the influence of Ti on the fusion line is relatively small. The fusion line on both sides of the weld becomes the weak position of the welded joint owing to the reduction in alloy element segregation and the strengthening phase. When the thickness of the Ti interlayer increases to 0.03 mm, the interface between a small amount of Ti gathering area and the Al grains in the weld becomes the crack source. When the thickness of the Ti interlayer continues to increase to 0.04 mm, a large number of cracks are generated in a large area of the Ti gathering area, and these cracks continue to extend and connect with each other, resulting in joint failure.ConclusionsBy studying the effects of Ti intermediate layer thickness on the weld formation, microstructure, and mechanical properties of the weld after adding different thicknesses of Ti metal intermediate layers in 7075 aluminum alloy during laser beam swing welding, the following conclusions can be drawn.(1) With the increase in Ti intermediate layer layer thickness from 0.02 to 0.04 mm, a short rod-like TiAl3 phase, cross-shaped TiAl3 phase, and large area of Ti gathering area appear in the weld. The EBSD results indicate that the microstructure of the weld area with the addition of the Ti intermediate layer consists of fine equiaxed grains. When a 0.03 mm thick Ti intermediate layer is added, the average equivalent circular diameter of the equiaxed grain area in the weld is 3.34 μm. The weighted average value of the grain area is 1.7% of the grain size of the base material.(2) The fine TiAl3 phase is mainly distributed within the Al grains owing to the formation of the high-melting-point TiAl3 phase as the heterogeneous core of the Al grains during the solidification process and the segregation of Zn, Mg, and Cu at the grain boundaries, resulting in the formation of hard and brittle phases, such as Al2CuMg, which weakens the grain boundaries.(3) As the thickness of the intermediate Ti metal layer increases from 0.02 to 0.04 mm, the average tensile strength of the joint shows a pattern of first increasing and then decreasing. When the thickness of the Ti intermediate layer is 0.03 mm, the average tensile strength is the highest, reaching about 377.8 MPa, which is 69% of the base material strength.(4) When the thickness of the Ti intermediate layer is 0.02 mm, the weak position of the joint are located at the grain boundaries of the fusion zone and the heat-affected zone. When the thickness of the Ti intermediate layer is 0.03 mm, the weak position of the joint is in the fine equiaxed crystal zone on both sides of the weld. When the thickness of Ti interlayer is 0.04 mm, the weak position of the joint is in the Ti gathering area in the middle of the weld.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402102 (2023)
DOI:10.3788/CJL230887
Jiaming Yu, Yongqiang Yang, Trofimov Vyacheslav, Di Wang, Jinhui Huang, Yan Wang, and Hanxiang Zhou

ObjectiveLaser powder bed fusion (LPBF) technology, a laser additive manufacturing (AM) technology based on powder bed melting, slices a three-dimensional model layer-by-layer through a computer. The laser scans the powder surface according to the two-dimensional slice profile and then the material stacks layer by layer to fabricate three-dimensional metal parts. Parts fabricated by LPBF technology have high dimensional accuracy and surface quality and can obtain nearly 100% relative density. In this study, we fabricate Sn-3.0Ag-0.5Cu (SAC305) material using LPBF technology without a protective atmosphere, which reduces the volume of equipment, improves the portability of equipment, and verifies the improvement of the mechanical properties of the LPBF fabricated alloy, providing the possibility for parts processing and printing under special conditions.MethodsDifferent process parameters for LPBF printing of SAC305 cube samples and tensile specimens are designed. Following this design phase, an analytical balance is employed to measure the density of the sample. Subsequently, a conversion to calculate the relative density of the sample is performed. To support this calculation, the conductivity of the samples is measured using an eddy current conductivity tester. A three-dimensional microscopic system is used to observe the surface topography and to measure the roughness. After polishing the sample, a metallographic microscope is used to observe pores, cracks, and other defects. X-ray diffraction (XRD) is used to analyze the phase compositions of SAC305 powder and fabricated samples. An electronic testing machine is used to test the tensile properties of tensile specimens, and the fracture morphology is observed.Results and DiscussionsSAC305 samples are successfully printed by LPBF in an air environment without a protective atmosphere. The top surface of the fabricated sample is yellow-brown, and the surface is free of cracks, obvious holes, warpage, collapse, and other defects; furthermore, the formability is good. When the laser energy density is high, owing to the excessive local instantaneous energy input, the resulting spatter particles fall onto the surface, causing internal defects or spheroidized particles. When the laser energy density decreases, the spheroidization phenomenon decreases, and the amount of unmelted powder increases, as shown in Fig.3. When the laser energy density reaches approximately 20 J/mm3, the density of the sample is high, and only a few tiny holes are present inside, as shown in Figs.4 and 5. The tensile strength reaches 85.29 MPa when the scanning speed is 700 mm/s, and the laser power is 30 W, as shown in Fig.6. The powder, along with the formed structures, primarily comprises of the β-Sn phase and Ag3Sn phase. The oxygen inside the fabricated sample is randomly distributed in the form of tin oxide, and no obvious aggregation occurs, as illustrated in Fig.9. According to the scanning electron microscope (SEM) diagram of the longitudinal profile of the fabricated sample, there is no obvious oxide film layer inside the sample, and oxygen is evenly distributed. The energy dispersive spectrometer (EDS) line scanning is performed over a range of multiple layers along the built direction, as shown in Fig.10. The oxides, rather than being enriched at the molten pool boundary, are distributed in the molten pool stage owing to the influence of heat input from subsequent layers. During the fabrication of the SAC305 alloy using LPBF without a protective atmosphere, less spatter is generated because of the low laser power, and most of the spatter is powder spatter, as shown in Fig.11.ConclusionsUnder an unprotected atmosphere, a violent oxidation reaction gradually occurs when the laser energy density exceeds 20 J/mm3, resulting in an increase in the surface roughness of the sample and even fabrication failure. When the laser energy input is lower than 15 J/mm3, the internal manifestation of the fabricated sample is loose and the powder in the scanning area does not fully melt, resulting in low relative density and poor mechanical properties of the sample. The best process parameters are a laser power of 30 W and a laser scanning speed of 700 mm/s.The relative density of the LPBF fabricated SAC305 sample reaches 98%, while its tensile strength and elongation are 85.29 MPa and 15.37%, respectively. The mechanical properties are better than those of the cast-fabricated samples, in which the tensile strength increases by 108%. The conductivity of the fabricated sample reaches 14.99% of international annealing copper standard (IACS), which meets the general conductivity requirements for this material.In the process of LPBF fabricating SAC305 alloy without a protective atmosphere, less spatter is generated owing to the low laser power, and most of the spatter is powder. The interior of the sample is composed of β-Sn, oxide particles, and a small amount of Ag3Sn. Owing to the high cooling rate of the LPBF printing process, the microstructural grains of the fabricated SAC305 alloy are smaller than those by the casting process, which improves the tensile strength of the material.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402303 (2023)
DOI:10.3788/CJL230675
Xueping Ding, Qi Zhang, and Honglin Ma

ObjectiveLaser wire vacuum additive manufacturing (LWVAM) is a highly promising metallic additive manufacturing technology. However, Ti6Al4V alloys fabricated via LWVAM have inferior corrosion resistance due to the higher fraction of acicular α′ phase and larger residual stress from rapid melting and solidification in the laser additive manufacturing process. Previous studies have shown that annealing treatment can improve corrosion resistance in Ti6Al4V parts fabricated by laser additive manufacturing, but their strength and hardness are reduced. As is known, solution-aging treatment (SAT) can not only improve the strength and hardness of titanium alloy, but also its ductility and toughness. Therefore, this study investigates the effect of solution-aging treatment on the corrosion resistance of Ti6Al4V via laser wire vacuum additive manufacturing to explain the underlying reasons for the changes in corrosion resistance based on microstructure analysis.MethodsTi6Al4V samples are fabricated by laser wire vacuum additive manufacturing. The solution-aging treatment parameters are shown in Table 1. To simplify the description, the as-fabricated sample and solution-aging treatment samples are labeled AM, SAT500 and SAT600, respectively. The microstructure of samples is observed by scanning electron microscope (SEM) with electron back-scatter diffraction (EBSD). The electrochemical tests, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization, are conducted in NaCl water solution with mass fraction of 3.5%. An X-Ray photoelectron spectroscopy (XPS) surface analysis system is used to analyze the elemental composition and valence state of the passive film.Results and DiscussionsThe potentiodynamic polarization results show that the corrosion current density value of the AM sample is about 74.83 times and 7.39 times higher than that of the SAT500 and SAT samples, respectively, indicating that SAT is conducive to improving the corrosion resistance of the sample fabricated by LWVAM. Compared to the SAT600 sample, the SAT500 sample has better corrosion resistance. These conclusions are also borne out by EIS test results. XPS results show that the intensity of TiO2 is much stronger than that of Ti2O3 and TiO, suggesting that TiO2 is the primary component in the passive film. In addition, it is found that in solution-aging treatment samples, there is more TiO2 than in the AM sample (Fig.5). Previous studies showed that high-valence oxides had stabler and better protection. Therefore, solution-aging treatment is helpful to obtain better corrosion resistance. The SAT500 sample has the highest TiO2 content, indicating that the SAT500 sample has the best corrosion resistance. EBSD results show that after solution-aging treatment, the acicular α′ phase in the AM sample changes to the lamellar α phase. Compared to the SAT500 sample, the α lath is coarser for the SAT600 sample (Fig.6). It is also suggested that after solution-aging treatment, local misorientation is significantly reduced and the dislocation density and residual stress decrease significantly (Fig.9). In addition, it is found that the number fraction of low angle grain boundary (corresponding to the misorientation angle of 2°?15°) decreases from 0.343 (AM sample) to 0.203 (SAT500 sample) and 0.105 (SAT600 sample) (Fig.10). As is known, acicular α′ phase is a high energy phase with inferior corrosion resistance. After solution-aging treatment, the decrease of α′ phase is thus beneficial to improve the corrosion resistance. In addition, the dislocation density is prone to corrosion due to the high activation energy. The decrease of dislocation density is thus useful to enhance the corrosion resistance due to the solution-aging treatment. Moreover, it has been suggested that grain boundaries are prone to corrosion due to higher lattice distortion than in the grain interior. Furthermore, the high angle grain boundary energy is higher than that of the low angle grain boundary. The high angle grain boundaries are more vulnerable to corrosive solution attack. In fact, owing to the higher energy, grain boundaries are preferential sites for the nucleation of protective layers. Therefore, a higher number fraction of high grain boundary is found in the solution-aging treatment sample with better corrosion resistance. In addition, it is found that in the SAT600 sample, the αlathis coarser than in the SAT500 sample, leading to the inferior corrosion resistance of the SAT600 sample.ConclusionsIn the present study, the effect of solution-aging treatment on the corrosion resistance of Ti6Al4V via laser wire vacuum additive manufacturing is investigated. Results show that after solution-aging treatment, the corrosion current density of the samples decreases and the passive film formed on the solution-aging treatment sample consists of more high-valence oxide TiO2,which has stabler and better protection. The reason is due to less acicular α′ phase and more high angle grain boundary in solution-aging treatment samples, showing solution-aging treatment to be conducive to improving corrosion resistance. Moreover, compared with the SAT500 sample, the αlathis coarser in the SAT600 sample, leading to inferior corrosion resistance.

Dec. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 24 2402305 (2023)
DOI:10.3788/CJL230634
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