Effects of Heat Source Angle on Weld Formation and Porosity Defects of Laser-MIG Hybrid Welding of 6A01 Aluminum Alloy
Han Xiaohui, Zhang Zhiyi, Ma Guolong, Wu Laijun, Song Xiaoguo, Wang Houqin, and Tan Caiwang
ConclusionsAluminum alloy profiles based on laser-MIG hybrid welding were studied for high-speed trains, and the influence of different heat source angles on the welding characteristics, including appearance, pores and melt flowing, was investigated. Results show that the weld depth decreases with an increase in the angle between the laser and the welding direction. When the laser source angle is increased from 82.5° to 110°, the penetration depth and weld width decrease by 50% and 25%, respectively. Moreover, increasing the heat source angle is conducive for reducing the weld pores. When the laser source angle is increased from 82.5° to 97.5°, the weld porosity decreases from 3% to 0%. Moreover, the droplet transfer mode was observed under different heat source angles and the length of the molten pool was promoted by increasing the angle between the laser and the welding direction. Fluent simulation results suggested that the heat source angle has significant effects on energy propagation. With an increase in the angle between the laser and the welding direction, the propagation of energy in the depth direction will decrease. The heating range increases with increasing arc angle, which is beneficial for prolonging the solidification time of molten pools and the eliminating weld pores.
  • Jan. 11, 2022
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202020 (2022)
  • DOI:10.3788/CJL202249.0202020
Algorithm and Experiment of Silicon Wafer Multifocus Laser Stealth Dicing
Zhang Huaizhi, Xu Jiaming, Zhang Lantian, and Qin Yingxiong
ConclusionsTo realize the multifocus stealth dicing of silicon wafers, an axial multifocus algorithm with a large numerical aperture is proposed. The number of on-axis foci, energy, and the interval between the foci can all be adjusted by changing the Fourier series coefficients. For the multifocus light field with equal energy, the simulation results show that the energy utilization rate is over 90% and the light intensity uniformity is over 90%. The phase diagram is loaded onto the spatial light modulator to simultaneously generate three focuses inside the silicon wafer. The 250 μm thick silicon wafer is successfully diced at a time by selecting the corresponding laser parameters and the movement speed of the displacement table. Because spherical aberration increases with machining depth, developing a multifocus laser dicing method for simultaneous aberration correction is essential in future work.
  • Jan. 11, 2022
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202018 (2022)
  • DOI:10.3788/CJL202249.0202018
Microstructure and Mechanical Properties of Laser Cladded Ti-Al-(C, N) Composite Coating on TC4 Surface
Feng Jinyu, Xiao Huaqiang, Xiao Yi, You Chuanchuan, Zhao Xinxin, Tian Yuxin, and Zhou Xuan
ConclusionsThe composite coating had a good metallurgical combination with the substrate. The matrix of the coating showed a compact TixAly phase with short rod structure, and a large number of TiC1-xNx phase dendrites mainly distributed on the matrix. During the melting process, Al atoms diffused into the TiC1-xNx phase dendrites, resulting in the formation of a layer of MAX phase at the dendrites' edges. TiC and Ti2AlC in the composite coating were gradually replaced by TiN and Ti2AlN as the AlN/TiC content ratio in the mixed powder increased. The solid solution TiC1-xNx changed from solid solution N in TiC to solid solution C in TiN, that is, from TiC0.7N0.3 to TiC0.3N0.7,and finally to TiC0.2N0.8. The coating's hardness was increased to 2.04 times of the TC4 matrix by forming a fine and dense structure and a dispersed reinforcing phase with the proper powder ratio. The Ti-Al-(C, N) composite coating is expected to improve the surface properties of titanium alloy, allowing it to adapt to more difficult working conditions.
  • Jan. 11, 2022
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202015 (2022)
  • DOI:10.3788/CJL202249.0202015
Effect of Solute Redistribution Coefficient on Solute Distribution in Laser Cladding
Zhang Caowei, Ge Honghao, Fang Hao, Zhang Qunli, and Yao Jianhua
ConclusionsA three-phase solidification model based on the volume-averaged method with considerations of non-equilibrium dynamic solute redistribution coefficient and constant solute redistribution coefficient is established. The morphologies of the temperature field and flow field are the same when compared to the simulation results of the two solidification models, as is the variation trend of solute field concentration. However, the dynamic solute redistribution coefficient solidification model predicts the solute distribution in the deposition layer with less error and is closer to the experimental data. In the laser cladding process, the solidification rate of the molten pool varies with the change in the solute redistribution coefficient, which leads to the instability of the solute distribution process and obvious element segregation. Our findings indicate that the dynamic solute redistribution coefficient has a significant impact on solute distribution in the deposition layer. To more accurately predict the solute distribution in the deposition layer, the change in the solute redistribution coefficient should be considered in the laser cladding process simulation.
  • Jan. 11, 2022
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202012 (2022)
  • DOI:10.3788/CJL202249.0202012
Nanosecond Pulsed Laser-Induced Controllable Oxidation of TiAl Intermetallic Alloys
Zhao Guolong, Xia Hongjun, Li Liang, Wang Min, and He Ning
Objective Poor rigidity of micro milling tools and a high milling force are the main causes of low machining efficiency, poor surface integrity, and severe tool wear in micro milling TiAl intermetallic alloys. In this study, an innovative hybrid machining process comprising laser-induced controllable oxidation assisted micro milling was proposed to address these problems. In the proposed process, a controllable oxidation reaction occurs in the cutting zone, and loose oxides, which are easy to cut, could be synthesized during the hybrid machining, thereby decreasing the milling force and achieving a mass removal rate. Subsequently, micro milling would be applied to the subsurface materials and high quality microstructures would be manufactured. Most importantly, in this study, nanosecond pulse laser-induced oxidation of TiAl intermetallic alloys was studied, and the influence of laser machining parameters together with an assisted gas atmosphere on the oxidation behavior was investigated. The micro-zone oxidation mechanisms of workpiece materials under both laser irradiation and oxidizer were investigated in detail, and the forming mechanisms of loose oxidation were studied. A control strategy of loose oxidation was proposed; then, the oxidation behavior was adjusted subjectively. The results of this study will provide both theoretical and technical supports in micro milling of TiAl intermetallic alloys.Methods TiAl intermetallic alloys were used in this work (Fig. 1). Laser-induced oxidation experiments were performed with high precision nanosecond (ns) pulsed laser equipment composed of a pulsed ytterbium fiber laser (YLP-F20, IPG Photonics Corporation) and CNC air floating platform. The laser spot diameter and pulse repetition frequency were fixed at 57 μm and 20 kHz, respectively. Laser-induced oxidation experiments were performed in a 99.5% pure oxygen-rich atmosphere and an injection velocity of 5 L/min. The laser energy density was varied from 6.86 J/cm 2 to 11.76 J/cm 2, and the laser scanning speed was 1 mm/s, 3 mm/s, 6 mm/s, and 12 mm/s (Table 3). The oxidation behavior in the atmosphere of air, argon (Ar), and nitrogen (N2) under the same laser parameters was studied. A scanning electron microscope (SEM, Hitachi S-4800) was used to observe the morphologies and cross-sections of both the oxide layer and sub-layer. The hardness of TiAl alloys before and after laser-induced oxidation was measured with a Vickers diamond pyramid indenter (HVS-50) with a static load of 196 N and a loading time of 15 s. The phase compositions with the laser energy density after laser irradiation were detected by X-ray diffraction (XRD, Bruker D8). Cu-K(α) radiation with a scanning step of 0.02° and a sweep speed of 6 (°)/min were used. Results and Discussions At the fixed laser pulse repetition frequency and laser spot diameter, the absorbed energy of the irradiated surface increased as the laser energy density increased. When the laser energy density was greater than the ablation threshold of the irradiated material, the oxidation reaction between the irradiated material and oxygen-rich atmosphere occurred, producing the titanium oxides. However, when the laser energy density was too high, the thermal effect accumulated on the surface of the irradiated material ablated the generated oxide (as shown in Fig. 5). The varied laser energy density significantly influenced the topographies of the sub-layer. At low laser energy density, the subsurface was flat, and residual oxides as well as micro-cracks existed. At lower laser energy density, the oxide layer primarily included low valent titanium oxides, such as TiO2 and Ti2O3, as well as Ti3O5 and Al2O3. As the laser energy density increased, stable and high valent titanium oxides were produced, and the phase compositions primarily consisted of anatase TiO2, rutile TiO2, and Al2O3 (Fig. 6). At high laser energy density, the subsurface had a recasting-layer and many tiny micro craters together with large cracks (Fig. 7). In addition, the thickness of the oxide layer and sub-layer increased as the laser energy density increased (Fig. 8). Moreover, the low laser scanning speed produced better oxidation results compared with the results produced under high scanning velocity at the fixed laser energy density and repetition frequency (Fig. 9). It was noted that at low scanning speed, the thickness of the oxide layer was better than that at high scanning speed (Fig. 10). Furthermore, the irradiated material had better oxidation results under the oxygen-rich atmosphere, compared with other assisted gas atmospheres (Fig. 11).Conclusions In this paper, the oxidation behavior of the irradiated material was studied under changing laser energy densities. All other laser parameters remained unchanged. In the oxygen-rich environment, the accumulated energy absorbed by TiAl material increased gradually as the laser energy density increased, which further promoted the oxidation reaction. In addition, the thickness of the generated oxide layer gradually increased. However, when the laser energy density was more than 9.80 J/cm 2, the produced oxides started to melt and a dense recast layer was formed. The heat-affected zone generated by thermal diffusion expanded rapidly and the thickness of sub-layer increased dramatically. At high laser energy density, the oxide layer was primarily composed of anatase TiO2, rutile TiO2, and Al2O3. For the varied range of laser parameters, the oxidation result was better at a lower laser scanning speed. However, the laser scanning speed and assisted gas atmospheres other than the oxygen-rich environment had no effect on the thickness of the sub-layer. Overall, at laser energy density of 8.82 J/cm 2 and laser scanning speed of 1 mm/s, as well as in an oxygen-rich environment, TiAl intermetallic alloys had better oxidation results, where the thickness of the oxide layer and sub-layer was 66 μm and 22 μm, respectively. After laser irradiation, the hardness of the sub-layer (200 HV) was lower than that of the substrate (365 HV, Table 1), which indicated that the laser-induced oxidation can improve the micro machinability of TiAl intermetallic alloys and promote the service life of micro end mills.
  • Jan. 04, 2022
  • Chinese Journal of Lasers
  • Vol.48 Issue, 22 2202021 (2021)
  • DOI:10.3788/CJL202148.2202021
Effect of Defocus Distance on Formability of CX Maraging Stainless Steel by Selective Laser Melting
Zhang Liangliang, Wang Minjie, Zhang Jiaqi, Liu Jianye, Niu Liuhui, and Wang Jinhai
Objective As one of the most promising additive manufacturing technologies, selective laser melting (SLM) is commonly used in metal mold forming. However, there are few types of materials used for SLM forming of the metal mold. Most die steels are prone to crack and porosity because of the effect of carbon content, limiting the application of SLM in metal mold manufacturing. A new type of maraging stainless steel, SS-CX (corrax stainless steel, referred to as CX stainless steel), can exhibit excellent mechanical strength and good corrosion resistance through the intermetallic compound precipitation and has a lower carbon content, which is considered to be an ideal candidate material for manufacturing metal mold. Because of the novelty of CX stainless steel, its SLM forming has not been systematically studied. The process parameters of SLM forming have been widely studied. Among them, defocus distance as one of the important parameters is rarely reported. The spot size and energy density can be adjusted, and the molten pool shape can be effectively controlled by changing the defocus distance, which is helpful to improve the production efficiency and obtains high-density parts. This study reports the CX stainless steel samples formed through SLM based on defocus parameters, combined with microstructure observation, phase analysis and experimental research, and the sample’s printing quality and forming performance. We believe that the research results obtained will provide a valuable reference for the SLM forming of CX stainless steel and help expand SLM’s range of materials used for metal mold manufacturing.Methods First, the SLM forming process of CX stainless steel is optimized and a reasonable process window is established by conducting the single weld channel test combined with the cross-section observation. Then, the square and tensile specimens are formed through SLM based on different defocus distances. The effects of defocus distance on the sample’s density, hardness, and surface roughness are analyzed through optical microscopy and scanning electron microscopy. Then, the microstructure and phase composition of the sample are studied using metallurgical microscope and X-ray diffraction. The effect of the mechanical properties of the sample is studied before and after heat treatment. The samples’ microstructure evolution and strengthening mechanism after solution, aging, and solution aging heat treatment are then investigated using metallographic observation, scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and hardness testing. Furthermore, the variation of mechanical properties of the sample before and after heat treatment is investigated in combination with the tensile test.Results and Discussions In the SLM forming process window, the welding channel in the stable melting region is continuous and straight and the cross section shows a fine wetting effect (Fig.6). The density and hardness of the sample are first increased and then decreased with the change of defocus distance, whereas the variation of surface roughness is opposite (Fig.12). The main composition of the sample is martensite and austenite. The grain refinement is visible as the defocus distance increases, which is beneficial in promoting martensitic transformation. Simultaneously, the tensile fracture transitions from quasi-cleavage to ductile fracture (Fig.18), the number of dimples increases, and the mechanical properties considerably improve. However, excessive defocus distance leads to incomplete powder melting and reduces the sample’s mechanical properties (Table 4). In addition, some differences are present in the microstructure and tensile fracture morphology of different heat-treated samples. After solution aging heat treatment, the boundary of the welding channel disappears; a large number of lath martensite exist in the structure. Meanwhile, the hard second phase particles of NiAl are precipitated to produce a precipitation strengthening effect. Consequently, the hardness and tensile properties of the sample are considerably improved, the tensile fracture appears as river-like morphology with a few shallow deformation dimples, exhibiting quasi-cleavage fracture characteristics (Fig.27).Conclusions The single weld channel test is used in this study to determine the SLM process window of CX stainless steel, which includes severe melting, stable melting, and incomplete melting regions. The molten liquid phase, for example, exhibits a good melt-wetting effect in the stable melting region. The shorter defocus distance causes an excessively high laser energy density, molten pool instability, and increased spheroidization. The results show that the density and hardness of the sample are reduced and the surface roughness is increased. The tensile characteristic shows quasi-cleavage fracture. With the increase in the defocus distance, the suitable energy density and spot size are conducive to forming a good metallurgical bond between the adjacent weld channels and layers and the sample’s mechanical properties are improved. Under the condition of 3.5 mm defocus distance, the sample’s maximum cross-section and longitudinal-section hardness are 35.94 HRC and 36.82 HRC, respectively, and the surface roughness is 7.315 μm. The tensile fracture mechanism is transformed into ductile fracture characteristics, and the maximum tensile strength is 1218 MPa. Simultaneously, the sample’s mechanical properties are considerably improved after the solution aging heat treatment due to the precipitation and precipitation strengthening effect of the hard second phase NiAl. The maximum hardness of the cross section and longitudinal section is 43.17 HRC and 44.52 HRC, respectively, and the tensile strength is 1746 MPa, which is 43.35% higher than that of the printed sample. When the defocus distance increases excessively, the laser energy density and penetration depth decrease and the liquid melt’s diffusion and infiltration effects become poor. Unmelted metal powder is present between the layers, resulting in the decrease of the density and mechanical properties of the sample.
  • Jan. 04, 2022
  • Chinese Journal of Lasers
  • Vol.48 Issue, 22 2202020 (2021)
  • DOI:10.3788/CJL202148.2202020
Influences of Laser Etching on Optical and Electrical Properties of Ag/FTO/AZO Thin Films
Zhou Yunlong, Lei Min, Wang Chenlin, Xu Qian, Li Baojia, and Huang Lijing
ConclusionsIn this study, laser etching using a laser energy density of 0.7 J/cm2 resulted in a grating structure with an appropriate height and total side area on the Ag/FTO/AZO film surface, which could effectively improve the anti-reflection ability of the film. Furthermore, the additional laser annealing effect produced during laser etching promoted grain growth in the film, reduced crystal defects, and resulted in less light and carrier scattering loss and higher carrier mobility. Therefore, the film's optical transmittance and electrical conductivity were enhanced. The resulting film received the highest figure-of-merit (2.80×10-2 Ω-1), indicating that its overall quality was superior to that of the original AZO film (2.58×10-2 Ω-1).
  • Dec. 30, 2021
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202007 (2022)
  • DOI:10.3788/CJL202249.0202007
Prediction and Control on the Surface Roughness of Metal Powder using Selective Laser Melting
Song Jianfeng, Song Younian, Wang Wenwu, Lang Yuedong, Dong Yonggang, and Yang Ruixing
ConclusionsThe morphology of the sample’s upper surface is primarily related to the melting width, and there is slight powder sticking, spheroidization, and splashing in some areas; the morphology of the up-skin and vertical surfaces is greatly affected by the powder sticking and spheroidization, which is related to the width of the melt channel, Furthermore, the hatch distance, powder diameter, and forming angle are directly related. The powder adheres more on the up-skin surface than the vertical surface regardless of the process parameters, and most of the powder does not melt, which decreases the surface quality of the oblique side. The liquid phase area of the molten pool increases with the increase in the line energy density. Hence, the melt width is directly proportional to the laser power and line energy density and inversely proportional to the scanning speed. The larger the melt width, the smaller the upper surface roughness; as the line energy density increases, the energy accumulation effect increases the surface temperature of the molded part. Moreover, the temperature rise on the up-skin and vertical surfaces cause the powder spheroidization and sticking phenomenon in the contact area and the powder pile substantially grows in size, thereby increasing the surface roughness value of the SLM molded part. The predicted value of theoretical roughness and the actual measured value are basically the same under different process parameters. Additionally, the prediction error of the upper, vertical, and up-skin surfaces are less than 20%, 30%, and 15%, respectively, which can be used to predict the surface roughness of 316L stainless steel SLM molded metal powder parts under different process parameters.
  • Dec. 27, 2021
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202008 (2022)
  • DOI:10.3788/CJL202249.0202008
Laser Powder Feeding Additive Manufacturing of Ternary Blade with Abnormity Base Plane
Li Kuan, Shi Tuo, Shi Shihong, Fu Geyan, Wang Mingyu, Zhang Rongwei, and Liu Guang
ConclusionsThe streamline layering technique is proposed in this study to tackle the ternary blade laser cladding formation trajectory problem. The main base surface is evenly spaced along the radial direction to obtain arc slices perpendicular to the radial direction, which are then evenly spaced along the scanning direction to obtain the segmentation unit. The streamlined layering technique is proposed in this study to solve the problem of ternary blade structural parts layering, bending, and inclination, and to accomplish the accumulation of ternary blade forming parts. The inspection results of the formed parts are as follows: the surface of the ternary blade-formed parts is smooth, and the average surface roughness value is less than 4.065 μm, which effectively reduces the step effect; it achieves a good metallurgical combination with the irregular base surface, and the average thickness of the formed parts is 5.97 mm. The relative errors of thickness and torsion angle are from -1.4% to 1.03% and -4.67%, respectively. The forming accuracy is high. The heat accumulation at the laser molten pool is evident as the height of the formed part increases. Further, the microhardness reduces as the microstructure increases; the microhardness of the formed part ranges from 348.3 to 360.4 HV, and the metallographic structure is uniformly dense with no evident holes or cracks.
  • Dec. 13, 2021
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202019 (2022)
  • DOI:10.3788/CJL202249.0202019
Microstructures and High-Temperature Friction and Wear Properties of Laser Cladded Fe-Ni-Cr Gradient Composite Coating for Brake Disc
Shi Xiaoyu, Wen Daosheng, Wang Shouren, Wang Gaoqi, and Zhang Mingyuan
ConclusionsIn this study, a Fe-Ni-Cr composite coating was applied to the surface of gray cast iron using the laser cladding method. Its phase and microstructure were investigated, as well as sliding friction and wear tests were performed at room-temperature and high-temperature. The specific conclusions were as follows: the main phases of the coating are γ-(Fe, Ni), Fe5C2,F3C, and Cr7C3. F3C had an excellent bonding property and was the main component of the transition zone between the coating and the substrate, resulting in a good metallurgical bonding between the coating and the substrate. Due to the undercooling degree, the microstructure of the coating from the heat-affected zone to the top of the coating gradually changed from columnar crystals to cellular crystals, dendrites, and equiaxed crystals, which had the effect of fine-grain strengthening and improving the coating’s hardness and wear resistance. At room temperature, the substrate’s wear mechanism was primarily abrasive wear, while at high temperatures, it was oxidation and adhesion wear. The wear mechanisms of the coating and substrate materials at room temperature were mainly abrasive wear. The thermal oxidation reaction became more severe as the test temperature rose, and the composite oxide film formed on the coating’s surface and alleviated adhesive wear and abrasive wear, which was one of the reasons for the coating’s ability to reduce friction coefficient and wear loss.
  • Dec. 13, 2021
  • Chinese Journal of Lasers
  • Vol.49 Issue, 2 0202017 (2022)
  • DOI:10.3788/CJL202149.0202017