Continuous fiber laser was used to carry out laser welding process experiments of dissimilar materials butt joints between 5083 aluminum alloy and TA15 titanium alloy with the same thickness of 4 mm. The morphology and composition of the microstructure were analyzed by scanning electron microscope (SEM), backscattered electron detector and energy spectrometer (EDS). It was observed that there were a large quantity of intermetallic compound phases with different microscopic morphologies and uneven distribution within the interface reaction zone when laser focusing on titanium side. Moreover, the reasons for the formation of different morphological characteristics were explain from thermal and mechanical aspects. Furthermore, tensile performance test was carried out under the normal temperature, and the fracture morphology was analyzed to determine the characteristics of brittle fracture. The fracture occurred in the TiAl phase at the interface, and the highest tensile strength was 150.73 MPa.

To decrease the laser welding defects of 6061 aluminum alloy lap joints, laser wobble welding was employed, and the effect of the laser beam wobble on the weld formation, microstructure and microhardness was studied. Results show that the weld formation is instability, and a large number of porosities exist in the center of the weld for the lap joints without laser wobble. By using the laser wobble welding, the weld formation is improved, and the porosities are decreased. When the wobble amplitude and frequency are set as 1.0-1.5 mm and 100-300 Hz, respectively, the weld formation is good, and the porosity is less than 0.6%. Compared to the lap joints without wobble welding, the weld shape of wobble weld joints is changed from Y-shaped to U-shaped, and the grain refinement occurs. The size of columnar crystal is reduced from 380 μm to 130 μm, and the size of equiaxed crystal is decreased from 120 μm to 75 μm. Micro hardness is increased from 60 HV to 65 HV by using laser wobble welding.

Melting tracks and fusion surfaces formed by the “line by line-layer by layer” scanning in selective laser melting can greatly affect the quality of the product. Based on the process principle of selective laser melting, the optimized range of line energy density was obtained by single line scanning. The mesoscopic surface morphology of overlapped melting tracks formed by multi-line scanning was further analyzed by combining the average re-melting rate. Results show that the melting tracks are relatively continuous and the line width is relatively uniform when the linear energy density is between 0.070 and 0.217 J/mm. Small average re-melting rate of multi-line scanning will result in large surface height difference, and if the average re-melting rate is too large, the surface cracks will occur. When the average re-melting rate is between 18% and 50%, a relatively smooth surface can be obtained.

In this paper, a three-dimensional mesoscopic model of the laser energy absorption behavior in the powder layer is established based on the ray tracing using COMSOL software. The effects of Ti6Al4V powder particle size, powder layer thickness, powder layer spatial distribution and particle size distribution on energy absorptivity in selective laser melting (SLM) are investigated. Simulation conclusions indicate that the absorptivity of closed-packing model is between 63.06% and 63.24% with particle size increasing from 5 μm to 60 μm; while the absorptivity of loose-random-stacked is between 69.37% and 70.46%, and the fluctuation of absorptivity along the scanning path sharply increases from 0.68% to 1.99% when the particle size of the powder increases from 20 μm to 25 μm. The absorptivity is insensitive to the changes in powder particle size; however, considering the stability of absorptivity during the SLM, the particle size of the powder should be less than 25 μm. In the close-packed model, the absorption of laser energy in the powder layer mainly occurs on the surface of the powder layer, and the energy penetration depth is about 30 μm. The energy accumulation in the pores appears at substrate and the depth of one powder particle size, and the energy penetration depth in the loose-random-stacking model is twice the powder particle size, which is the optimal spreading thickness. The absorptivity of the powder layer with a Gaussian particle size distribution and the powder layer with a uniform particle size distribution are 69.2% and 70.3%, respectively, and there is a possibility of mutual substitution between two particle size distributions. The results of this study can provide theoretical basis for the preparation of powder materials in SLM. At the same time, the developed calculation model can be extended to calculate the absorptivity of laser energy in the powder layer composed by other materials.

The specimens of 316L austenitic stainless steel prepared by selective laser melting (SLM) were heat-treated, and the wear resistance, hardness and abrasion width of 316L stainless steel were observed by using the MFT-R4000 abrasion and wear tester, microhardness measurement and analysis system and optical microscope (OM). The results show that: with the annealing temperature increases, the hardness of the specimen are decreasing in the Z direction; the hardness difference of SLM specimens and specimens after 880 ℃ heat treatment is small; Y direction hardness difference is small. As the load increases, the width of the abrasion marks increases, and oxidation wear, adhesive wear and abrasive wear exist in the friction process, while cracks exist on the surface of the abrasion marks. In a certain range of load, load has a certain influence on the friction coefficient. With the load increases, the wear heat rises, the oxide film is generated, the friction coefficient decreases, and the grinding chip is plastic plowing. When the load is greater than 20 N, the friction coefficient rises, and the grinding chip is brittle flaking.

Carbon fiber reinforced plastics is composed of carbon fiber and resin matrix. Owing to the big difference in thermal properties between the carbon fiber and matrix, the laser processing of carbon fiber reinforced plastics in the air will cause damage problems such as large heat affected zone, fiber expansion, fiber pull-out and section material delamination, etc. Therefore, the control of its thermal damage has become a key issue in the laser processing of carbon fiber reinforced plastics. This paper reviews the current domestic and overseas research status of carbon fiber reinforced plastics composite processed with laser, which focuses on the advantages and disadvantages of the two processing methods, i.e., under gas-assisted and water-assisted lasers. Finally, this paper analyzes and summarizes the quality of laser processing in the two cases. In the aid of gas, different types of gas, pressures, phosgene coaxial and paraxial assist will bring different processing effects. There are three forms of water assisted, i.e., water jet, water layer and water conduction processing. This paper summarizes the mechanism of gas/liquid assisted laser processing of carbon fiber reinforced plastics, which aims to balance the difference in the absorption of laser energy between carbon fiber and matrix. This paper can provide a reference for laser low-damage processing of carbon fiber reinforced plastics.

Hydraulic turbine runners and blades are the core components of the Hydraulic turbine, but they are prone to cavitation in varying degrees during long-term operation. Laser additive remanufacturing can be used as an effective repair method for such defects due to its high efficiency, low heat input and good bonding performance. In this paper, a three-dimensional finite element model for laser additive remanufacturing of hydraulic turbine blades is developed, and the transient distribution laws of temperature field and stress field in the laser additive remanufacturing area are calculated. The results show that the influence of laser additive remanufacturing process on the instantaneous temperature and stress of hydraulic turbine blades is mainly concentrated near the additive remanufacturing area. The heat affected range of single laser additive remanufacturing is about 10 mm, and the heat affected range of four laser additive remanufacturing increases to 25 mm. The deformation generated in this process is very slight. The maximum deformation generated by the central cross section is 0.15 mm when the four-channel laser additive remanufacturing is completed, which has little effect on the overall turbine blade.

In order to optimize the process parameters of laser preparation of surface micro-pit texture and improve the tribological performance of cylinder liner-piston ring friction pair, nanosecond pulse laser was used to prepare elliptical micro-pit texture on the surface of chrome-plated piston ring, which explored the influence of laser power, processing times, scanning speed and filling line spacing on the morphology and size of the micro-pits. The results show that increasing the laser power and repetitive processing times, reducing the scanning speed and filling line spacing can effectively increase the depth of the micro-pits. The length of the micro-pit length decreases with the increase of laser power, and it first increases and then decreases with the increase of processing times. The scanning speed and the spacing of the filling lines have a low influence on the length of the micro-pit length. When the processing parameters are laser power 9 W, repeated scanning three times, scanning speed 200 mm/s, filling line spacing 1 μm, and laser repetition frequency 30 kHz, a target with a long radius of 200 μm, a short radius of 150 μm and a depth of 38 μm can be machined on the surface of the chrome-plated piston ring micro-pits. The research results provide guidance for the practical application of laser preparation of micro-pit texture on the surface of piston ring and improvement of tribological properties of friction pairs.

Non-line-of-sight imaging technology is a new type of optical imaging technology, which is mainly used to detect hidden objects in corners. Aiming at the problem of slow speed data acquisition caused by complex imaging equipment and the need for scene scanning, a transmission imaging experimental device using nanosecond laser and APD array detector is proposed, which can achieve fast data acquisition without scene scanning. Compared with the previous system, the data acquisition time is greatly shortened. Aiming at the problem that the image reconstruction results are poor due to the data collected only at sparse angles or limited angles, maximize likelihood estimate maximization (MLEM) iterative algorithm is proposed for 3D image reconstruction of hidden objects. Experimental results show that: The MLEM iterative algorithm can well reconstruct the shape of the hidden object and effectively suppress the artifacts brought by the traditional back projection algorithm. Compared with the back projection algorithm, the structure similarity (SSIM) value of the hidden object at the depth of 150 cm was improved by 0.232 1, 0.387 8 and 0.438 9 in the third, sixth and tenth iterations, respectively. At the depth of 190 cm, the SSIM value of the results of three iterations (the 3rd, 6th and 10th iterations) increased by 0.314 6, 0.401 3 and 0.431 3, respectively. The results show that MLEM algorithm can greatly improve the reconstructed image quality.

In order to explore the ripple formation mechanism in laser polishing, 718 Alloy and 304 stainless steel were polished by nanosecond fiber laser, and the ripple effect caused by laser polishing was investigated. Results show that there is a great difference between the ripple produced by laser polishing (the scale is tens of microns) and the laser pulse action distance (10 μm). Moreover, the ripple amplitude decreases with the decrease of laser energy, while the wavelength of the ripple changes little with the laser energy. According to the experimental results, the generation of ripple is a complex process involving the absorption of laser energy, melt flow, material evaporation and even pulse sequence. The source of ripple is the variation of melt flow caused by the laser ablation alternately to the melt and high temperature solid. The influence mechanism of laser energy on ripple is as follows: when a very high energy laser pulse is used, the input energy would be consumed by melt gasification, thus the increase of melt thickness as well as the ripple changes very little with energy. When the laser energy is in the range with less gasification, the decrease of laser energy would lead to the decrease of melt thickness as well as ripple amplitude.

Using lasers to achieve high beam quality and high peak power output are research hotspots in recent years. This paper adopts the method of matching the refractive index of the core layer and the cladding layer and uses the mode selection of mode competition. By simulating the relative gain of each mode of the crystal waveguide core layer, the cut-off size of the core layer under different saturated light intensities in the cavity is calculated. A large-core size crystal waveguide was fabricated, and the output characteristics of the large-core size crystal waveguide active Q-switched pulse laser was experimentally studied. By using a 1.0% (atomic percent) Yb:YAG with core size of 320 μm×400 μm, 0.5% (atomic percent) Er:YAG with cladding size of 7 mm×30 mm and length of 77 mm single cladding rectangular crystal waveguide, and a BBO Electro optic Q-switch, the pulse energy of 1.29 mJ@10 kHz, the pulse width of 10 ns were obtained, and the M2 value of beam quality was 1.15×1.10. Experimental results show that the high peak power pulse output with near diffraction limit can be obtained by the actively Q-switched pulse laser with large core size diameter crystal waveguide.

In order to identify the observation efficiency and evolution process of the wake vortex produced by ARJ21 in the near-ground stage, appropriate experiments were performed at shuangliu airport, and a rapid identification method for wake vortex was presented based on radial velocity observed by CDL. As a result, wake vortex evolution processes of ARJ21 in the near-ground stage were analyzed accordingly. Results show that the wake vortex of the ARJ21 is generated near the ground and then diffuses downwards and outwards, and the circulation of wake vortex gradually decreases with time. However, under the influence of a specific near-ground effect, the secondary vortex will induces the main vortex to rebound, with the height of the vortex core rising from 13 m to 18 m. Meanwhile, crosswind conditions will accelerate the dissipation of ARJ21 wake vortex and push it out the runway. The duration of wake vortex is reduced from 70 s in calm wind to 42 s in crosswind.

In order to solve the problem of vehicle information acquiring, system and method for vehicle feature recognition are designed based on laser scanning 3D imaging. 3D point clouds are obtained via a 3D scanning system based on a 2D laser scanner carried on an electric turntable. Method for system self-demarcation is developed based on point clouds, which can automatically calculate the placement parameters of the system. Moreover, vehicle feature recognition method is presented, in which the point clouds are segmented into different classification, and characteristic parameters are calculated. System control and data process software is designed and realized based on QT, in which system control, data collection and process, data exchange and control communication are realized. Experiments are presented in scene of car loader, experimental results show that the system can reach the measure accuracy of 5 cm and has a short time cast less than 40 s, which can be applied in vehicle feature recognition for car loader.

Decontamination is one of the important links of nuclear and biochemical protection. Using laser cleaning technology as decontamination means has the unique advantages of high decontamination efficiency, low support requirements, no decontamination agent pollution and so on. It can avoid the disadvantages of water + decontamination agent and other traditional decontamination methods, such as large logistics burden, corrosion equipment, restricted service environment and so on. Laser cleaning technology is a new research field of nuclear and biochemical decontamination. In this paper, the development history and research progress of laser cleaning technology are summarized. The research and application of laser cleaning technology in the decontamination of toxic and harmful substances, such as radioactive dust, chemical agents and biological agents, are summarized. Laser decontamination is effective for pollutants on a single substrate, but the laser decontamination parameters of different decontamination objects and pollutants need to be further studied. Laser cleaning, as a new method, has a good prospect in the field of nuclear and biochemical cleaning. Finally, the research and development of laser cleaning in the field of nuclear and biochemical decontamination are prospected.

Slab CO2 laser with diffusion-cooling and RF pumping has the advantages of high efficiency, small size and good beam quality, and this kind of laser has been applied in laser precision annealing, industrial processing and some other fields. High-power RF pumped, diffusion-cooled slab CO2 laser with more than kilowatts power output plays an important role in the precision processing and micro-electronic equipment because of its high beam quality and high stability. In this paper, the progress of CO2 laser technology, focusing on research trends of the RF excited slab structure with high power output, was analyzed in detail, and the development stages and applications of this technology was summarized. Finally, the research tendency of slab CO2 laser was prospected.