Fabrication of Ag Micro‑dots with Laser Interference Induced Forward Transfer Technology and Their SERS Property
Huijuan Shen, Zhankun Weng, Changli Li, Liqiang Deng, and Taikun Han
ObjectiveSurface-enhanced Raman scattering (SERS) plays an important role in trace detection and other fields of research. The use of periodic micro-nano structures has been a popular method for realizing high-performance SERS substrates. Traditional preparation technologies can prepare micro-nano structures with high precision and excellent quality. However, most of them have the limitations of strict environmental requirements, low efficiency, and high material dependence; therefore, it is necessary to develop alternative micro-nano periodic structure preparation techniques. Laser-induced forward transfer (LIFT) has been used to transfer almost all types of materials; however, it is limited in preparing periodic micro-nano structures. Laser interference lithography (LIL) has the advantage of high efficiency in preparing periodic micro–nano structures with large areas. Therefore, combining LIFT with LIL technology, a novel technique of laser interference-induced forward transfer (LIIFT) is proposed to transfer the periodic Ag micro-dots under three-beam laser interference in this study, which can overcome the shortcomings of traditional technologies and realize large-area metal micro-nano array structure manufacturing in a rapid, low-cost manner. Finally, the SERS properties of the Ag micro-dots transferred via LIIFT are tested and analyzed.MethodsThe donor film to be transferred consists of a quartz substrate, a polyimide(PI)sacrificial layer film mixed with carbon nanoparticles (CNPs@PI), and an Ag film. The preparation process of the CNPs@PI thin film is described in Ref.[16], and the Ag film is evaporated onto the CNPs@PI film (Fig.1). The principle of transferring a micro- dot via LIIFT is shown in Fig.2. The azimuth angles (0°, 120°, and 240°) and transverse-magnetic (TM) mode of the polarization direction are chosen. Moreover, the effect of the interference period on the transferred microstructure is discussed. Finally, the Raman spectra of RhB on the transferred Ag micro-dots with different periods (9, 11, and 15 μm) are tested to analyze the SERS property of the transferred Ag micro-dots.Results and DiscussionsThe transferred Ag micro-dot structure is observed on the receiving substrate, as shown in Fig. 3(a1)?(a3). While the interference periods of the three beams are 9 μm, the outline of the micro-dot structure can be observed, but there are many overlapping areas between adjacent micro-dots, as shown in Fig.3(a1). While the interference period increases to 11 μm, a micro-dot with clear edge can be observed, as shown in Fig.3(a2). Upon increasing the period to 15 μm, the clarity of the micro-dot is further improved [Fig.3(a3)]. These results, combined with Fig. 3(b1)?(b3), indicate that an increasing number of nanoparticles are distributed in the middle regions of a single micro-dot, leading to an improved resolution of each micro-dot with an increase in the period. The average diameter of the nanoparticles in the consistent area in the middle regions of the micro-dots with different periods is 129?141 nm, as shown in Fig.3(c1)?(c3). These results, combined with the statistical results of the numbers of Ag nanoparticles in the consistent area (Fig.4), indicate that, with an increase in the interference period, the average size of the Ag nanoparticles changes slightly, whereas the number of nanoparticles increases significantly. This is because the maximum intensity and contrast of the three-beam interference light field increase with the laser interference period (Fig.5), which affects the transferable mass of the Ag film and the distribution of Ag nanoparticles on the receiving substrate.The SERS characteristics of Ag micro-dot substrates with periods of 9, 11, and 15 μm are tested by selecting Rhodamine B(RhB) as the analyte with a concentration of 10-3 mol?L-1 and compared with the RhB Raman spectrum on the bare Si substrate. The results show that, while using an Si substrate covered with the transferred Ag nanoparticle micro-dots, the Raman intensity of RhB is significantly enhanced compared with that of the bare Si substrate. Simultaneously, the Raman intensity of RhB on the Ag nanoparticle micro-dot substrate increases with the micro-dot period (Fig.7). This can be attributed to the local surface plasmon resonance effect. With the increase in interference period, the density of the transferred Ag nanoparticles increases evidently, which can create more and stronger “hot spots,” making it easier to excite strong Raman signals.ConclusionsPeriodic Ag nanoparticle micro-dots are realized efficiently with three-beam laser interference LIIFT. The distribution of Ag nanoparticles in each micro-dot is controlled by adjusting the laser interference period. With the increase in the period of the laser interference from 9 μm to 15 μm, the density of Ag nanoparticles in the micro-dot increases. Finally, the Ag nanoparticle micro-dot substrates prepared by the three-beam LIIFT technology show significant SERS characteristics. Moreover, the Raman intensity of RhB on the transferred Ag micro-dots increases with the period of the micro-dots. This study demonstrates the potential application of LIIFT technology in the field of SERS chips.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602406 (2024)
  • DOI:10.3788/CJL231081
Experimental Research on Waterjet‑Guided Laser Processing of Microholes in Single Crystal Ni‑Based Superalloys
Shunshan Wang, Hongchao Qiao, Zhihe Cao, Jinsheng Liang, Dongyu Han, and Jibin Zhao
ObjectiveWith the rapid development of the aerospace, ship, power, and energy fields, single-crystal Ni-based superalloys have been widely used in aeroengine and gas turbine components because of their excellent comprehensive performance. This has resulted in an increase in the quality requirements for related microhole structures, which has translated to higher processing technology requirements. Waterjet-guided laser drilling technology, when compared with other traditional microhole processing techniques, such as electrochemical machining, electrical discharge machining, and “dry laser” processing, has the advantages of a large working distance, no thermal damage, neat cutting, and no obvious taper. However, the high specific strength and low thermal conductivity of single-crystal Ni-based superalloys make them prone to defects such as poor microhole surface morphologies and large tapers during processing. Hence, it is crucial to investigate the effects of the processing parameters on the microhole surface morphologies and taper for high-quality machining of superalloy microholes.MethodsThis study investigates the mechanism and experimental research of waterjet-guided laser drilling of the single-crystal Ni-based superalloy, DD91. First, the effects of the laser single-pulse energy, scanning speed, feed time, and scanning time on the surface morphologies and tapers of microholes are studied by setting up single-factor experiments. Then, based on the single-step spiral scanning mode [Fig.2(a)], a multistep spiral scanning mode drilling method [Fig.2(b)] is proposed to improve the defects of poor microhole surface morphologies and large tapers. In the multistep spiral scanning mode, the coupled energy beam repeatedly scans the innermost circle (circle 1) N times, cut across the material to form a prefabricated hole at the center of the microhole, and then scans the second circle (circle 2) to the outermost circle (circle N) N times with a single-step spiral scanning mode to complete the processing of the filling circle and hence widen the aperture and improve the microhole geometry. Finally, the quality of microhole machining via the single/multistep spiral drilling methods is compared under the appropriate processing parameters. The microhole surface morphologies are observed using optical microscope, the entrance and exit apertures are measured via ultra-depth-of-field microscope, and the corresponding taper is calculated.Results and DiscussionsDuring waterjet-guided laser trepanning on metals, material removal is dominated by laser ablation through mechanisms such as photothermal mechanisms, including material melting, evaporation, and sublimation. The water jet, with its high heat capacity, can provide good heat management as well as clean molten material and debris from the ablation zone (Fig.3). As the laser single-pulse energy increases, the material removal rate also increases, which enlarges the exit diameters and causes the taper to increase (Fig.5). A pulse energy that is too low will lead to serious microhole surface morphology damage (Fig.4). With an increase in scanning speed, the ablation time per unit area decreases, which leads to a worsening of the circularity of the hole (Fig.7), a decrease in the exit diameter, and an increase in the taper (Fig.6). As shown in Fig. 9, the entrance diameters of the microholes are all steady at approximately 1025 μm, regardless of how many feeds are applied. The exit diameters increase with an increasing number of feeds and reach a saturation value (approximately 1000 μm) after the feed time is over 6 (Fig.9). Multiple feeds can improve the circularity of the microhole (Fig.8). When the scanning time is 1, the microhole taper is smallest, but the dimensional accuracy is low. With an increase in the scanning times, the quality of the microhole deteriorates, the entrance aperture decreases linearly, the exit aperture first decreases and then becomes saturated, and the taper of the microhole first increases and then decreases (Figs.10 and 11). Based on the above results, the appropriate processing parameters are selected to compare the quality of microhole machining via the single/multistep spiral drilling methods. The surface morphologies and taper of the microhole processed using the multistep spiral drilling method are obviously improved (Fig.12 and Table 2). This is because a prefabricated hole at the center of the microhole can discharge debris and water from the bottom of the hole, reduce the interference with laser transmission, and improve the surface morphologies and taper of the microhole.ConclusionsThe variations in the laser single-pulse energy, scanning speed, feed time, and scanning time on the surface morphologies and taper of microholes using the single spiral drilling method are investigated. A multistep spiral scanning mode drilling method is proposed to improve the defects of poor microhole surface morphologies and large tapers caused by the single-step spiral scanning mode. The quality of microhole machining using the single/multistep spiral drilling methods is compared under appropriate processing parameters. The experiments indicate that increasing the single-pulse energy and reducing the scanning speed can improve the surface morphology of microholes and reduce the microhole taper. With an increase in the feed times, the surface morphology of the microhole gradually improves, and the microhole taper initially decreases and then saturates. As the number of scanning rounds increases, the surface morphology of microhole gradually deteriorates, and the microhole taper first increases and then decreases. The taper of microholes processed using the multistep spiral method is only 0.29°, which is a 70% reduction compared to that using the single-step spiral method, and the dimensional deviation and roundness are controlled within 20 μm.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602404 (2024)
  • DOI:10.3788/CJL231117
Microstructure and Strength-Toughness of FSP-Assisted Laser Deposited AlSi10Mg Alloy
Haisheng Zhao, Feng Zhang, Chengchao Du, Xudong Ren, Xiangyu Wei, and Junjie Gao
ObjectiveIn recent years, laser additive manufacturing based on direct laser deposition has attracted widespread attention because of its flexibility and efficiency. This technology has a wide range of applications and high additive manufacturing efficiency. It is widely used in the aerospace, rail transit, and ship component maintenance equipment fields. However, high-strength aluminum alloys such as those in the Al-Zn-Mg-Cu series have a high content of alloying elements. During the solidification process, the semi-solid range of the alloy may exceed 100 ℃, which can easily leave gaps between aluminum grains. The α-Al layer of "liquid film" generates cracks under the action of thermal stress, making laser deposition repair difficult. AlSi10Mg alloy, as a cast aluminum alloy, has the characteristics of a short semi-solid range and high strength, and is suitable for additive manufacturing and the laser repair of high-strength aluminum alloy components. However, during the laser deposition process, process fluctuations often cause defects such as pores to appear in the components, leading to cracks and ultimately component failure during use. Therefore, exploring a method to eliminate pores in components produced using AlSi10Mg laser deposition is of great significance for improving the mechanical properties and service life of AlSi10Mg components.MethodsAtomized AlSi10Mg alloy powder with a particle size ranging from 53 μm to 150 μm is adopted. During the laser deposition process, the laser power is 2700 W, deposition speed is 600 mm/min, powder feed rate is 5.8 g/min, overlap amount is 2.5 mm, argon flow rate is 5 L/min, and protective argon amount is 20 L/min, resulting in a single-layer thickness of 0.5 mm. After depositing eight layers to achieve a thickness of 4 mm, stir friction treatment is performed on the deposited AlSi10Mg alloy. The height of the mixing needle of the mixing head is 4 mm, with a four-prism shape and diameter of 6 mm at the end of the prism. During the stirring friction treatment process, the rotational speed is 800 r/min, stirring speed is 100 mm/min, and variation in the stirring friction treatment passes is 5 mm. Subsequently, the laser deposition of eight-layer AlSi10Mmg alloy is continued on the surface of the AlSi10Mg alloy after the stir friction machining, and then stir friction machining is used.Wire cutting is used to cut the AlSi10Mg alloy into five samples, and stir friction-assisted laser deposition is conducted, followed by room-temperature rolling treatment. Rolling deformation values of 20%, 46%, and 68% are achieved on three of the samples. The five tensile specimens of the AlSi10Mg alloy are treated as mentioned above, and their strength and elongation values are measured using a tensile testing machine. After vibration polishing, the five metallographic samples are observed using a scanning electron microscope and backscattered electron diffractometer, and their microhardness values are measured. A thin film sample of the AlSi10Mg alloy is prepared and its microstructure is observed using a transmission electron microscope after electrolytic double spraying. The fracture of the tensile specimen is observed using the scanning electron microscope.Results and DiscussionsThe hardness values of the AlSi10Mg alloy in the five different states are listed in Table 1. It can be observed that the hardness of the deposited AlSi10Mg alloy is approximately 109 HV. Because at high temperatures, the solid solubility of the Si element in the α-Al matrix is relatively high, and when the temperature rapidly drops, it is difficult for the Si element to recover from α-Al matrix, and a large number of Si atoms on α-Al matrix play a role in solid solution strengthening. The Al matrix plays a role in solid solution strengthening. After stir friction processing, the solid solution strengthening effect is significantly weakened, and the hardness of the AlSi10Mg alloy decreases to 75 HV. Based on the hardness values of the rolled AlSi10Mg specimens listed in Table 2, it can be observed that the rolling process improves the effects of dislocation strengthening and fine grain strengthening in the AlSi10Mg alloy, ultimately increasing the hardness of the laser-deposited AlSi10Mg alloy after stir friction processing to 116 HV. As shown in Fig. 9, after stir friction processing, the strength of the AlSi10Mg alloy is close to 200 MPa, and the elongation distribution is 33%?40%. It can be seen that stir friction processing can simultaneously improve the strength and plasticity of the laser-deposited AlSi10Mg alloy. Figure 10 shows that there are a large number of dimples in the tensile fracture surface of the AlSi10Mg alloy in the laser deposition state and stir friction processing state, indicating that the fracture mode of both AlSi10Mg alloy specimens is the plastic fracture mode. The research on hardness shows that the strength and elongation of the laser deposited AlSi10Mg alloy cannot reach high levels. However, after stir friction processing, the larger shoulder pressure and stirring effect eliminate the porosity defects in the alloy, reduce the stress concentration, and thus significantly increase its elongation.ConclusionsAfter friction stir processing, the columnar α-Al and eutectic phases in the laser deposited AlSi10Mg become equiaxed α-Al grains and Si particles, while the Mg2Si precipitate phase is significantly refined. After rolling, when the deformation of the AlSi10Mg alloy increases to 68%, dislocation strengthening further refines the grains.After rolling, the dislocation strengthening effect in the alloy is significantly enhanced. In addition, increasing the rolling amount also brings about a fine grain strengthening effect. Therefore, the hardness of the AlSi10Mg alloy can be increased to 116 HV at most, exceeding the microhardness of the laser deposited AlSi10Mg alloy.Although the solid solution strengthening effect in the laser deposited AlSi10Mg alloy is remarkable, the solidification defects in the alloy lead to the formation of early cracks during the tensile process, which results in an alloy strength of less than 200 MPa and an elongation of less than 20%. After friction stir processing, the strength and toughness of the AlSi10Mg alloy are simultaneously improved, with a strength close to 200 MPa and an elongation of 33%?40%. After rolling, the dislocation strengthening effect of the AlSi10Mg alloy gradually increases, and its strength continues to rise, reaching a maximum of approximately 400 MPa. The localized hardening area in the alloy leads to a decrease in its plastic deformation ability, and the elongation gradually decreases to 25%.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602308 (2024)
  • DOI:10.3788/CJL230976
Monitoring of Warping Deformation of Laser Powder Bed Fusion Formed parts
Jintang Chen, Kai Zhang, Tingting Liu, Zhiyong Zou, Jiansen Li, Huiliang Wei, and Wenhe Liao
ObjectiveLaser powder bed fusion (LPBF) is an additive manufacturing (AM) process that has the advantages of forming complex-shaped parts and cutting costs. It is widely used in the aerospace, medical equipment, weapons manufacturing, and other industries. However, in the LPBF process, the material powder is repeatedly heated and melted under the effect of laser energy and then cooled and solidified, which facilitates the formation of a large thermal gradient and thermal stress in the parts, leading to warping deformation. This type of deformation significantly affects the dimensional accuracy and mechanical properties of parts. By combining sensor signal acquisition with data analysis, deformation defects can be detected during AM to reduce production costs and improve the quality of formed parts. The radiant light signal of the molten pool is sensitive to the thickness of the powder layer during the LPBF process, which may reflect the warping deformation that has already occurred. It is also correlated with the temperature of the molten pool, reflecting the peak temperature at that location, and is related to the temperature field of the sample. Therefore, it has the potential to monitor the thermal stress during warping deformation. To study the relationship between thermal stress-induced warping deformation and the radiant light signal of the molten pool, a method for monitoring warping deformation in the LPBF process by acquiring the radiant light signal of the molten pool is explored in this study. In this study, an overhanging sample is formed during the experiment, and the radiation signal of the molten pool is collected and analyzed. The results show that the radiant light signal can not only monitor warping deformation but also reflect formation process of warping deformation to a certain extent.MethodsTo collect and compare the radiation light signal of the molten pool during the forming process of the warped and normal samples, T-shaped overhanging structure samples are formed (Fig.2), and five samples with three different support structures and sizes are designed for the experiment (Table 1). In this process, three sensors collect the radiation intensity signals from the molten pool, and an upper computer records the coordinate data of the laser spots (Fig.1). After data alignment, each light intensity value corresponds to the coordinates of the laser spot during scanning. To further explain the variation trend of the light intensity signal along the long side (Y-direction) of the sample, the scanning section of the sample is divided into regions, and the average light intensity of each region is calculated. Three measurement points are selected on the sample, and the heights of the measurement points relative to the substrate plane are measured using a coordinate apparatus.Results and DiscussionsNo evident warping deformation is observed in the forming process of samples S80-1 and S80-2, whereas the warping deformations of samples S25-1, S25-2, and S20 are larger (Fig.6). This result indicates that samples with smaller support areas are prone to warping deformation; however, no noticeable linear correlation is observed. The normal samples S80-1 and S80-2 produce a larger average light intensity at both ends, with a minimum value of 0.93 V, while warped samples S25-1, S25-2 and S20-1 produce lower light intensity at the same area (Fig.7). This phenomenon indicates that sample warping can be distinguished from the light signal of the molten pool. The light intensity distribution of the first overhanging layer is different between the warped and normal samples. The light intensity of the warped sample in the region where the corresponding lower layer is solid is significantly higher than that in other regions, forming a “wave peak” in the curve (Fig.8). The above phenomena indicate a correlation between the radiant intensity distribution and peak temperature at the corresponding position and reveal that the evolution trend of the light intensity between the layers of the samples with the same geometric structure. The light intensity of the normal sample fluctuates more between layers, whereas that of the warped sample fluctuates less (Fig.9).ConclusionsIn this study, three types of overhanging samples with different structures are formed, and the radiation light signal of the molten pool is collected. Combined with sample deformation measurements and statistical methods, the data are analyzed, and the following conclusions are obtained:1) In the layer after warping deformation, the light intensity of the warped specimen decreases significantly in the warped region, while the distribution of the light intensity of the normal specimen is uniform without a notable gradient.2) For the warped specimen, when the overhanging layer has just been formed, and the deformation has not yet occurred, the light intensity "crest" corresponding to the central solid region of the specimen is quite different from the light intensity in other regions of the layer.3) The interlayer evolution trends of the light-intensity values of the warped and normal samples are different. With an increase in the number of formed layers, the influence of the overhanging structure on the light intensity signal gradually decreases, and the light intensity tends to stabilize after the fifth layer.4) A sample with a smaller support area is more likely to produce warping deformation, but no notable linear correlation exists between these two factors.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602306 (2024)
  • DOI:10.3788/CJL231062
Effects of Heat Treatment on Microstructure and Mechanical Property of ATI 718Plus by Laser Additive Manufacturing
Yiqun Ren, Yuechen Wu, Shuai Chang, Liqun Li, and Minqing Wang
ObjectiveLaser melting deposition is used to prepare ATI 718Plus samples to study the effects of three heat treatment regimes on their microstructure evolution, hardness, and room temperature tensile properties. These regimes include direct aging heat treatment, solutionizing and aging heat treatment at 982 ℃, and high-temperature solutionizing and aging heat treatment at 1020 ℃. The aim is to elucidate the phase transformation behavior and mechanical property changes of laser additive ATI 718Plus under different heat treatment regimes and provide guidance for the selection of heat treatment processes used in the laser additive manufacturing of ATI 718Plus.MethodsThe plasma rotating electrode process is used with ATI 718Plus powder with particle diameter of 45?105 μm to prepare wrought ATI 718Plus superalloy substrates. The experiments are performed on the laser additive manufacturing system shown in Fig. 2, which consists of a 4000 W continuous wave fiber laser, an inert atmosphere processing chamber, a coaxial nozzle, and a powder feeding device. A well-formed ATI 718Plus sample is prepared using a unidirectional reciprocating scanning method with the following parameters: a laser power of 1200 W, scanning speed of 0.8 m/min, protective gas flow rate of 10 L/min, carrier gas flow rate of 15 L/min, and powder feed rate of 13 g/min. The geometric dimensions of each sample are 50.0 mm×58.0 mm×2.5 mm. Three heat treatment regimes are employed, as shown in Fig. 2(b). The analyzed samples are mechanically ground with SiC paper and polished using diamond suspensions and a colloidal silica suspension to prepare metallographic samples. Then, the polished samples are etched with No.2 waterless Kailing's reagent for optical microscope and scanning electron microscope (SEM) investigations. Uniaxial tensile tests are carried out at room temperature using a universal testing machine with a constant displacement rate of 1 mm/min.Results and DiscussionsAfter laser deposition, a large number of Laves phase areas form in the interdendritic region (Fig. 3). This hard and brittle phase deteriorates the mechanical properties of the additive-manufactured ATI 718Plus samples. The as-deposited sample mainly exhibits an epitaxial growth columnar dendritic morphology, with a large number of brittle long-chain Laves phases precipitated between dendrites, which consumes a significant amount of Nb, Mo, and other strengthening elements, severely reducing the mechanical properties of the as-deposited sample. After the direct aging heat treatment, the long-chain Laves phase morphology remains unchanged, and the η and γ′ phases precipitate heavily between dendrites. The solution and aging heat treatment system can effectively reduce the size and content of the Laves phase. With an increase in the solution temperature, the size and content of the Laves and η phases gradually decrease, and the γ′ phase uniformly precipitates. The hardness significantly increases after heat treatment (Table 2), but the hardness differences between the three heat treatments are relatively small. The room temperature tensile properties are shown in Fig. 8. Compared to the as-deposited sample, after heat treatment the samples exhibit significant increases in both the yield strength and tensile strength, while the elongation at fracture decreases and then increases. The yield and tensile strengths increase by 67.7% and 51.9% after the direct aging heat treatment, respectively, while the elongation at fracture decreases by 13%. After the solution aging (SA) heat treatment at 982 ℃, although the strength improvement is not as significant as that after the direct aging treatment, the yield and tensile strengths still increase by 63.6% and 45.6%, respectively. At the same time, the elongation at fracture increases by 3% compared to that of the as-deposited state. The strength improvement is the smallest after the 1020 ℃ SA, with a yield strength increase of only 62.0% and tensile strength increase of 34.2%, but the plasticity is significantly improved, with an elongation at fracture increase of 25.8% compared to that of the as-deposited state.ConclusionsThe strength and hardness values of the ATI 718Plus additive samples significantly increase after heat treatment. The best match between strength and plasticity is obtained after high-temperature solution and aging heat treatment at 1020 ℃. Compared with those of the as-deposited state, the tensile strength and elongation at the fracture of the sample increase by 34.2% and 25.8%, respectively, after the 1020 ℃ solution and aging heat treatment.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602305 (2024)
  • DOI:10.3788/CJL231244
Influence of Heat Treatment on Microstructure and Mechanical Properties of Laser Selective Melting Bimetal Structure Connection Interface
Siyuan Zhang, Youzhao Zhang, Xiangwei Li, Tao Zhang, Chao Yuan, and Shuyan Zhang
ObjectiveSelective laser melting (SLM) technology offers a wide range of design freedom, high density, and strong metallurgical bonding; thus, it is highly suitable for processing workpieces with complex shapes. A conformal cooling mold formed via SLM can improve the cooling efficiency and decrease the injection cycle time. However, only a few types of mold steel materials are suitable for 3D printing because of long processing time and high costs. First, conventional processes can be employed to manufacture conventional parts; subsequently, complex parts can be built using SLM. With this approach, the manufacturing efficiency can be improved and costs can be reduced. In this study, a new type of 3D printing die steel material (AM40) is deposited on a commercial H13 substrate using SLM. The effects of heat treatment (HT) on the microstructure and mechanical properties of AM40/H13 bimetallic structural materials are studied, and the deformation and cracking behaviors of the bimetallic molds are revealed.MethodsIn this study, AM40 steel powder and annealed H13 steel sheets are used. SLM is used to deposit AM40 onto the H13 substrate. Subsequently, quenching and tempering are performed to study the effects of the heat treatment. The particle size distribution is characterized using a laser particle size analyzer, whereas the microstructure and fracture morphology are characterized using optical microscope (OM) and scanning electron microscope (SEM). The grain morphology, orientation, and local misorientation of the bonding interface are characterized using electron backscatter diffraction (EBSD). Additionally, a Vickers microhardness tester is employed to measure the microhardnesses of the as-built and heat-treated samples. Tensile tests are performed using a fatigue testing machine.Results and DiscussionsNo crack defect is observed at the interface of the as-built AM40/H13 bimetallic structure and the unique Marangoni molten pool at the interface (Fig. 7). Moreover, fine cellular and columnar martensite structures are observed in the AM40 region (Fig. 8). The microstructure of H13 is coarsened austenite (Fig. 5), and the distinct microstructural inhomogeneity is observed at the bonding interface. After quenching and tempering, the characteristics of the molten pool disappear, and uniform lath martensite microstructures form in the H13 region (Fig. 8). The inhomogeneity of the grain size and misorientation at the interface are eliminated (Fig. 10). Moreover, the diffusion width of element at the interface increases from 440 μm to 500 μm (Fig. 9). Additionally, the hardness of the as-built AM40/H13 at the bonding interface is 642 HV, which is higher than those of AM40 (529 HV) and H13 (202 HV). The average hardness of HT-AM40/H13 at the bonding interface decreases to 480 HV (Fig. 11), thus indicating that the hardness difference between AM40 and H13 is eliminated by the heat treatment. The tensile strength of HT-AM40/H13 increases significantly from 644 MPa to 1436 MPa (Fig. 12). Furthermore, some dimples, along with a cleavage pattern, are observed in the fracture (Fig. 14), thus indicating that the fracture mode is a combination of ductile and brittle. The increase in the tensile strength and ductility of the heat-treated AM40/H13 bimetallic alloy is analyzed based on the microstructure and fracture morphology of the bonding interface.ConclusionsIn this study, the as-built AM40/H13 bimetallic structure does not exhibit crack defects at the interface, and the microstructure is heterogeneous. Marangoni convection and cellular and columnar structures are observed in the weld pool at the interface. The alloying elements are evenly distributed at the interface, thus indicating good metallurgical bonding. After heat treatment, the grain size and dislocation density near the interface are similar, thus eliminating the inhomogeneity of the interface structure. The elements at the interface diffuse, and the diffusion width increases by 60 μm. The hardness at the as-built AM40/H13 bimetallic H13 side is the lowest (202 HV), followed by that at the AM40 side (529 HV); by contrast, the interface hardness is the highest (642 HV). Tensile deformation and cracking of the bimetal preferentially occur at the H13 side, with a strength of 644 MPa and fracture elongation of 29%, thus indicating ductile fracture. After heat treatment, the hardness of H13 increases to 483 HV, which is equivalent to that of AM40 (479 HV) after heat treatment, and the inhomogeneity of the hardness is eliminated. In addition, the tensile strength of HT-AM40/H13 increases significantly from 644 MPa to 1436 MPa, which is between those of AM40 and H13. The fracture is preferentially located at the AM40 side, far from the interface. Further, some dimples and cleavage patterns are observed, thus indicating that the fracture mode is a combination of ductile and brittle.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602304 (2024)
  • DOI:10.3788/CJL231025
Parameters and Microstructure Evolution of TiC/TC4 Composites Formed by Selective Laser Melting
Hongkang Huang, Xia Luo, Yuhong Dai, Xin He, Yunzhong Liu, Bensheng Huang, and Zhou Fan
ObjectiveTitanium matrix composites have attracted considerable attention because of their high modulus of elasticity, high specific strength, high wear resistance, and excellent high-temperature durability. Most studies on titanium matrix composites (TMCs) focus primarily on the in-situ formed TiC reinforced composites. However, few studies have focused on the direct addition of TiC-reinforced titanium matrices. The manners in which the size, morphology, and distribution of TiC evolve during the SLM process and how they affect the microstructure and mechanical properties remain unclear. In this study, TiC/TC4 composites with directly added nanoscale TiC particles are successfully prepared by selective laser melting (SLM), and the microstructure evolution under different volume energy densities is investigated. Further, the TiC evolution during SLM and its influence on the microstructure and microhardness are analyzed. Thus, the findings of this study can provide the support for SLM preparation of titanium composites.MethodsHerein, nanoscale TiC (diameter of 50?150 nm) and TC4 are selected as the reinforced phase and matrix, respectively. The composite powder with TiC uniformly embedded on the surface of the TC4 powder is obtained by low-energy ball milling. Subsequently, the TiC/TC4 composites are prepared via SLM with different volume energy densities (29?97 J/mm3). The forming quality and microstructures at different volume energy densities are observed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped energy disperse spectroscope (EDS). The grain size and crystal orientation are investigated using electron backscattering diffractometer (EBSD), and the phase compositions are measured using X-ray diffraction (XRD). Finally, the microhardness is measured using a digital microhardness tester.Results and DiscussionsThe optimized volume energy densities for the SLM formed TiC/TC4 composites are in the range of 50?70 J/mm3, with a relative density of 99.7% (Fig.3). Owing to the enrichment of TiC in the melt pool boundary zone, the microstructure of the composites exhibits a special double-sized grain distribution in the cross section (Fig.6). Owing to the rapid cooling characteristics of the SLM process, TiC cannot be sufficiently dissolved. Therefore, the SEM and EBSD results reveal three types of reinforcement: undissolved TiC, eutectic TiC, and precipitated TiC. Undissolved TiC is distributed primarily at the boundaries of coarse β equiaxed grains, eutectic TiC is distributed primarily in the boundaries of irregular eutectic β grains, and precipitated TiC is distributed primarily in the grains. With an increase in volume energy density, the chain-like eutectic TiC gradually transforms to rod-like eutectic TiC (Figs.7 and 8), the size of precipitated TiC inside the grain gradually increases, and the sizes of longitudinal and transverse α'-Ti gradually increase.ConclusionsThe optimal volume energy density for the formation of TiC/TC4 composites by SLM is 50?70 /mm3, and the relative density is 99.7% within this parameter range. TiC is enriched in the melt-pool boundary region under a strong temperature gradient and Marangoni convection. The microstructure of the composite has a special double-size grain distribution in the cross section, consisting of primary β equiaxed grains and irregular eutectic regions growing on the periphery. In the longitudinal section, the molten pool is a fish scale, and some chain structures exist in the molten pool that grow from the direction of heat flow to the horizontal direction. With an increase in volume energy density, the size of primary β equiaxed grains decreases, outer-ring irregular eutectic region expands, and morphology of fish scales becomes sharp. The microhardness initially decreases and then increases, essentially reaching 385?392 HV in the optimal molding process window. TiC in the composites is composed primarily of undissolved TiC (distributed near the primary β grain boundaries), eutectic TiC (distributed in the eutectic β grain boundaries in a chain or rod-like network), and precipitated TiC (distributed in the grain in a granular manner). With an increase in volume energy density, the difference in TiC size and quantity inside and outside the molten pool increases, chain distribution of eutectic TiC changes to rod, and the size of TiC in the grains increases. Further, no obvious orientation relationship between eutectic TiC and β-Ti is observed; however, a distinct orientation relationship between eutectic and in-grain TiC and α'-Ti exists: {11?20} α'-Ti∥{110}TiC.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602301 (2024)
  • DOI:10.3788/CJL231156
Simulation and Experimental Study on Laser Cleaning of Surface Paint Layer of Aluminum Alloy Skin for Civil Aircraft
Tiangang Zhang, Yu Li, Junhao Zou, Zhiqiang Zhang, and Yanan Liu
ObjectiveCivil aircrafts operate under severe conditions, leading to issues such as peeling and cracking in the aircraft paint layer. This makes localized or comprehensive paint removal and surface maintenance pivotal during C-check or D-check procedures. Current paint removal methods, such as manual grinding and chemical stripping, are widely used. However, they have significant shortcomings. Manual grinding often lacks precision and can damage the aircraft surface, while chemical stripping is complicated and environmentally detrimental. These challenges hinder sustainable and efficient advancements in the civil aviation industry. Laser paint removal has emerged as a promising solution, offering high precision, reduced pollution, and automation possibilities. It is rapidly gaining global attention. However, a knowledge gap exists in understanding the exact mechanism of paint removal during laser ablation, particularly regarding the impacts of single-pulse thermal stress and plasma-induced removal. This study bridges this gap by first determining the vaporization point and strength limit of the paint layer. Then, single-pulse laser ablation simulations are compared with experimental results to better understand thermal stress paint removal during laser cleaning. This research sheds light on paint removal mechanisms and evaluates the impacts of varying scanning speeds on laser paint removal efficiency. Hence, the aim is to offer valuable insights and references for enhancing the use and development of laser paint removal techniques on civil aircraft skin.MethodsIn this study, the vaporization point and strength limit of the paint layer are first determined via thermogravimetry and stress-strain experiments. Subsequently, a finite element analysis of the single-pulse laser ablation-thermal stress paint removal process is conducted using the COMSOL software. Experiments on laser paint removal, both single-pulse and multi-pulse, at varying scanning speeds (ν=1000, 900, 800, and 700 mm/s), are performed on the composite paint system of the LY12 aluminum alloy substrate using nanosecond pulsed fiber lasers. Then, a white light interferometer (WLI) is employed to inspect and analyze the crater profile created by the single-pulse laser. An optical microscope (OM) is used to characterize the resulting surface and cross-sectional morphology from the multi-pulse line scanning laser paint removal. Additionally, a scanning electron microscope (SEM) and an energy-dispersive X-ray spectroscope (EDS) are utilized to analyze the microstructure and compositional changes on the cleaned surface after the paint is removed.Results and DiscussionsAfter the completion of single-pulse laser irradiation (t=200 ns), the paint ablation process does not cease. The accumulated heat causes the surface temperature of the residual paint layer to further increase until t=750 ns. When the temperature falls below its vaporization point, the ablation paint removal process concludes (Figs. 6 and 7). At this moment, the action depth of thermal stress generated by the coupling of the temperature field on the surface of the residual paint layer gradually increases from the bottom to the edge of the crater, while the corresponding values progressively decrease (Fig. 9). The maximum value (Fig. 9, σmax=2.7×107 Pa) approaches the strength limit of the paint layer [Fig. 11(a), σ=2.68×107 Pa], leading to a U-shaped stress-damage zone on the surface of the residual paint layer (Fig. 12). This results in physical damage, such as delamination and fragmentation of the residual paint layer (Fig. 13). During the laser ablation process, both plasma impact and shielding effects coexist. The plasma impact causes the actual width of the ablated crater to be larger than the simulated results, while the shielding effect results in the experimental ablation depth of the crater being smaller than the simulated results (Fig. 14). In the process of single-pulse laser paint removal, the ablation and plasma paint removal effects gradually intensify before the laser irradiation ends and then gradually weaken after the irradiation completes. However, the thermal stress paint removal effect remains unchanged (Fig. 15). In the multi-pulse surface scanning laser paint removal experiments, the actual spot overlap rate is higher than the theoretical value (Fig. 17), resulting in the removal depth of the paint layer being greater than that of the single-pulse results. As the scanning speed gradually reduces, the removal effect of the paint layer gradually enhances due to ablation, the plasma effect gradually weakens, and the depth of paint removal and the deposition amount of β-type copper phthalocyanine along with the functional oxidized particles gradually increase [Figs. 16(a), (c), (e), and (g), and Table 10].ConclusionsIn this study, the single-pulse nanosecond laser thermo-mechanical coupling paint removal transient process is simulated using the COMSOL software. The results show that the behavior of ablative removal of paint layers does not finish at the end of one pulse cycle, and the heat accumulated on the cleaning surface extends the ablation process by 550 ns. The simulated thermal stress value is slightly larger than the experimentally determined tensile limit of the paint layer. This causes the residual paint layer on the cleaning surface to produce a delamination and cracking zone ranging from 0.6 μm to 2.8 μm. The laser ablation paint removal process triggers plasma impact and shielding. This makes the experimental crater paint removal width larger and the depth smaller than the simulation results. In the single pulse laser paint removal process, the ablation and plasma paint removal effects gradually intensify before the laser irradiation ends and then gradually weaken after the irradiation ends, but the thermal stress paint removal effect remains unchanged. As the scanning speed gradually decreases, the ablative paint removal effect strengthens, the plasma effect weakens, the thickness of the paint layer removal increases, and the amount of functionally oxidized particles deposited on the paint layer also increases. When ν=1000 mm/s, the topcoat is partially removed and the primer is slightly damaged. The topcoat is removed cleanly, and the primer is partially removed when ν=900 mm/s. However, for ν=800 mm/s and 700 mm/s, the area and depth of the residual primer continue to decrease with the reduction in the scanning speed, and the oxidized film is exposed.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602209 (2024)
  • DOI:10.3788/CJL231079
Effect of Laser Cleaning Process on Microstructure and Fatigue Properties of Aviation Aluminum Alloy
Cheng Cheng, Weichen Yu, Yao Ding, Taiwen Qiu, Shuangchao Fu, and Yunyang Tang
ObjectiveAfter an aircraft is in service for a certain period, various factors, such as external forces, light, and humidity, affect the paint layer on its skin surface. This leads to aging, cracking, peeling, and other phenomena. Hence, removing the original coating on the metal material surface becomes a necessary step before repainting it. Laser cleaning offers advantages, including non-contact, environmental friendliness, precision, and no secondary pollution. It can replace traditional mechanical polishing and chemical paint removal methods, enhancing the cleanliness of aircraft surface paint layers. Due to the strict safety requirements of aircraft, it is crucial to understand whether the use of lasers for removing surface paint layers impacts the fatigue properties of the aircraft. In this study, a nanosecond pulse laser is used to clean aviation aluminum alloys coated with a fluid-resistant epoxy primer coating. Subsequently, the effects of the laser cleaning process on the microstructure and fatigue properties of the aluminum alloys are examined.MethodsA pulsed laser was used to remove the surface coating of the aviation aluminum alloys. The effects of the laser cleaning process on the microstructure, mechanical properties, and fatigue properties of the aviation aluminum alloys were examined via appearance inspection, optical microscope (OM), scanning electron microscope (SEM), in situ temperature detection, mechanical property detection, and high cycle fatigue test.Results and DiscussionsThe results show that when the laser power and pulse frequency are 80 W and 100 kHz, the epoxy primer coating on the surface of the aluminum alloy can be removed. However, some residual paint layer remains on the surface of the sample. When the laser power and pulse frequency are 500 W and 500 kHz, the maximum surface temperature does not exceed 115 ℃, and no obvious heat affected zone is observed. Laser cleaning increases the surface roughness, with partial ablation and melting occurring within the depth range of 10 μm. After laser cleaning, the hardness of the material increases. As the laser power, frequency, and energy density increase, the rate of hardness increase decelerates. The tensile property results indicate that the tensile strength, yield strength, and elongation of the sample after laser cleaning are slightly lower than those of the blank sample. Through high-cycle fatigue testing, when compared to those of the blank sample, the fatigue properties of the painted sample after laser cleaning decrease by 11.76%. This mainly stems from the increased surface roughness caused by laser cleaning. However, after anodizing and painting treatments, laser cleaning does not further exacerbate the fatigue damage caused by anodizing.ConclusionsAnalyses are conducted on the appearance, microstructure, roughness, in-situ temperature, mechanical properties, and fatigue performance of the samples after laser cleaning. With a laser power and pulse frequency of 80 W and 100 kHz, a residual paint layer remains on the sample surface. However, at elevated levels of 500 W and 500 kHz, oxidation might appear on the substrate surface. The process of laser cleaning tends to increase surface roughness, causing partial ablation and melting within the depth range of 10 μm. The surface temperature during this procedure increases in tandem with the increase in laser power and pulse frequency. But even at peaks of 500 W and 500 kHz, the maximum surface temperature stays below 115 ℃. After cleaning, the material hardness increases. However, as the laser power, frequency, and energy density increase, the increase in hardness decelerates. There is a minor reduction in the sample tensile strength, yield strength, and elongation. When compared to the untreated samples, those cleaned by laser but not anodized or painted show a reduction in fatigue properties by 9.34%. In comparison, samples that undergo anodizing and painting processes after cleaning experience a reduction in fatigue properties by 13.84%. Specifically, after painting, laser cleaning results in a decrease in fatigue properties by 11.76%. Notably, laser cleaning does not further increase the fatigue damage due to anodizing.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602205 (2024)
  • DOI:10.3788/CJL231088
Ultrasonic Rolling Strengthening and Wear Properties of Laser Remelted High Manganese Steel
Enlan Zhao, Yuxing Peng, Jiaxiang Man, Xiang Liu, and Haifeng Yang
ObjectiveHigh manganese steel (HMnS) has good deformation hardening properties. Under impact load, its surface rapidly hardens, thereby improving surface hardness but maintaining good toughness inside. However, under low stress wear conditions, it often exhibits a low hardening behavior accompanied by early surface wear. The pre-hardening treatment of the HMnS surface can improve its mechanical properties under low impact and low stress conditions. Therefore, scholars have proposed various surface pre-hardening treatment methods for HMnS, such as mechanical shot peening, explosive hardening, laser shock, and ultrasonic shock. Laser remelting is the process of using a laser beam to melt the surface of a material and improve its microstructure and mechanical properties through the rapid solidification of the molten pool. Unlike the equiaxed grains of cast HMnS, laser remelted HMnS often forms typical columnar and equiaxed dendritic structures due to the high temperature gradient and high cooling rate during solidification. Therefore, exploring the hardening behavior and wear resistance of laser remelted HMnS under ultrasonic rolling is of great significance.MethodsThis study used continuous cast Mn13 steel plate for laser remelting, and its cross-sectional microstructure was equiaxed grains. The laser power used was 700 W, the laser wavelength was about 960 nm, the scanning speed was 5 mm/s, the spot diameter was 1.2 mm, and the overlap rate was 50%. To prevent oxidation during laser remelting, high-purity argon with volume fraction of 99.99% gas was selected as the protective gas. An ultrasonic rolling strengthening device was used to treat the surface of HMnS after laser remelting with an amplitude of 4 μm. The vibration frequency was 40 kHz, and the static pressures were 100 N and 200 N, respectively. The samples were sequentially ground, polished, and corroded using silicon carbide sandpaper, metallographic grinder, and aqua regia solution. Measurement and analysis of laser remelted HMnS before and after ultrasonic rolling were carried out using field emission electron probe microanalyser, electron backscatter diffractometer, field emission scanning electron microscope, roughness profilometer, Vickers hardness tester, pin disc rotary friction and wear tester, and three-dimensional profilometer.Results and DiscussionsDuring the laser remelting, due to the high cooling rate and temperature gradient, the solidification structure consists of columnar and equiaxed dendrites, without obvious defects such as cracks and pores and without precipitation of cementite. After ultrasonic rolling with static pressures of 100 N and 200 N, the surface hardness increases by 120.48% and 173.82%, respectively. It can be seen that the microstructure of laser remelted HMnS also has deformation hardening, especially with outstanding surface hardness properties. The wear test shows that without ultrasonic rolling, the depth and width of the wear marks are the highest. In contrast, the depth and width of the wear marks are the lowest when the static pressure of ultrasonic rolling is 100 N. The volume wear rate without ultrasonic rolling is 6.945×10-5 mm3/(N·m), and those under ultrasonic rolling with static pressure of 100 N and 200 N are 4.93×10-5 mm3/(N·m) and 5.95×10-5 mm3/(N·m), respectively. The ultrasonic rolling hardening mechanism of laser remelted HMnS is as follows. During the ultrasonic rolling, the surface of laser remelted HMnS undergoes severe plastic deformation, which is essentially dislocation slip and deformation twinning. Normally, high-frequency ultrasonic rolling can obtain nanograins on the surface of the material. Unlike the equiaxed grain structure of cast HMnS, laser remelted HMnS has a high interdendritic Mn content, while the intra-dendrite Mn content is lower. So the stacking fault energy within the dendrites is lower than that between the dendrites, making it easier to form twins within the dendrites. Twins can still expand between adjacent dendrites, forming twins that can penetrate multiple dendrites. The results indicate that the small angle grain boundaries and Mn segregation do not inhibit the formation and expansion of twinning. Due to the small angle grain boundaries and Mn segregation that can hinder the movement of dislocations, laser remelted HMnS exhibits good deformation hardening behavior.ConclusionsThis study uses laser remelting technology to obtain non-uniform solidification structure on the surface of HMnS, and investigates the hardening behavior and wear resistance of non-uniform solidification structure of HMnS under ultrasonic rolling. The solidification structure of laser remelted HMnS is composed of thinner equiaxed dendrites and columnar dendrites growing perpendicular to the bonding surface. There are many small angle grain boundaries formed in the solidification structure, and there is Mn segregation at the small angle grain boundaries. The non-uniform structure of laser remelted HMnS forms a dense twinning and thin severe plastic deformation layer under ultrasonic rolling, indicating its twinning hardening behavior. The thickness of the severe plastic deformation layer is 3?4 μm. The wear test shows that when the static pressure of ultrasonic rolling is 100 N, the twinning hardening and severe plastic deformation of the surface significantly increase the surface hardness of HMnS, and the volume wear rate is reduced by 29.01% compared to that of the surface without ultrasonic rolling. The wear mechanism is light adhesive wear and abrasive wear.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602201 (2024)
  • DOI:10.3788/CJL231092