This paper concerns high-power laser welding of thick plates for the ship industry. Thermomechanical behavior during laser welding of 16-mm marine steel EH40 using 25-kW laser power was investigated ...by a 3D finite element model. The objective is to analyze the effects of weld collapse and hump on the residual stress-induced thermal cycle. A double-cylindrical source model was proposed to simulate the transient distribution of temperature field. Heat flow distribution area is a cylinder, radial heat flow presents a Gaussian distribution, while heat flow peak in the direction of thickness is decaying then increasing exponentially. The predicted weld geometry had good agreement with the actual results. When collapse and hump were considered, simulation error of temperature distribution was only 1.54%. In addition, cooling curves obtained from the thermal simulation were incorporated into the continuous cooling transformation diagram of EH40 to explain the evolution mechanism of microstructure. It was shown that collapse and hump affected the values and distribution trend of residual stress in different thickness, especially in the high gradient stress zone near the weld center. The collapse mainly affects the residual stress distribution on the top surface, while the hump affects that on the bottom surface. Both of them have little influence on the residual stress in the middle thickness area. The cold contraction of weld metal and the stress concentration caused by weld shape during the cooling process are the fundamental reasons that collapse and hump affect the distribution of welding residual stress.
The contact-type displacement and angular sensors were improved and utilized in weld seam trajectory detection. A detection–compensation–tracking system was developed. The mechanical part of this ...system was installed and independent of the robot, which can realize the detection of right-left deviation and up–down offset of the weld path. In the experiment, the position coordinates of the detection point in weld groove were calculated and weld seam tracking was carried out simultaneously owing to its single control system. When the absolute interpolation algorithm was adopted, the average error of width deviation and depth deviation were 0.1817 mm and 0.1449 mm, respectively.
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Understanding the stability of monolayer transition metal dichalcogenides in atmospheric conditions has important consequences for their handling, life-span, and utilization in applications. We show ...that cryogenic photoluminescence spectroscopy (PL) is a highly sensitive technique to the detection of oxidation induced degradation of monolayer tungsten disulfide (WS2) caused by exposure to ambient conditions. Although long-term exposure to atmospheric conditions causes massive degradation from oxidation that is optically visible, short-term exposure produces no obvious changes to the PL or Raman spectra measured at either room temperature or even cryogenic environment. Laser processing was employed to remove the surface adsorbents, which enables the defect states to be detected via cryogenic PL spectroscopy. Thermal cycling to room temperature and back down to 77 K shows the process is reversible. We also monitor the degradation process of WS2 using this method, which shows that the defect related peak can be observed after one month aging in ambient conditions.
In this paper, the forming mechanism of weld cross sections (WCSs) was studied via thermal analysis. The melting of a WCS was first dominated by heat convection from the flowing melt until the WCS ...had the max cross section in the transient temperature field. Then, the melting was dominated by heat conduction from the residual heat in the weld pool, giving rise to an increase in middle width but a decrease in upper and bottom width. This indicated that the WCS obtained from the transient temperature field could not represent the section after solidification. Therefore, thermal analysis results should be validated using the WCS obtained from the maximal temperature field. This WCS was dependent upon the max temperature of each node over time. Compared with the former WCS, the latter one showed better adaptability in terms of multi-process parameters when the thermal analysis results were validated.
Considering laser power, defocus length, and welding speed, a Taguchi experiment (25 groups) was designed to investigate the 30-kW-laser-welding processing of 16-mm-thick marine steel EH40. The ...fusion zone (FZ) microstructure was martensite, whereas the microstructure of the base metal (BM) was ferrite and pearlite. The microstructure of the heat-affected zone (HAZ) near the FZ was martensite, and that near the BM was martensite and ferrite. It was found that a large size (width, depth, and depth-width ratio), low hardness, and weak corrosion resistance can be easily obtained under a high laser power and low welding speed. The welding speed was a major factor affecting the weld width, and the effects of laser power and defocus length were small. The influence of laser welding, welding speed, and defocus length on the weld depth increases successively. Owing to the martensite phase in the weld, the hardness of the FZ and most of the HAZ (519-647 HV) was more than twice that of the BM (255 HV). The hardness of most of the HAZ was higher than that of the FZ because of a decrease in grain size. The corrosion resistance was found to become weaker for the same reason.
With the rapid development of robotics, wheeled mobile robots are widely used in smart factories to perform navigation tasks. In this paper, an optimal trajectory planning method based on an improved ...dolphin swarm algorithm is proposed to balance localization uncertainty and energy efficiency, such that a minimum total cost trajectory is obtained for wheeled mobile robots. Since environmental information has different effects on the robot localization process at different positions, a novel localizability measure method based on the likelihood function is presented to explicitly quantify the localization ability of the robot over a prior map. To generate the robot trajectory, we incorporate localizability and energy efficiency criteria into the parameterized trajectory as the cost function. In terms of trajectory optimization issues, an improved dolphin swarm algorithm is then proposed to generate better localization performance and more energy efficiency trajectories. It utilizes the proposed adaptive step strategy and learning strategy to minimize the cost function during the robot motions. Simulations are carried out in various autonomous navigation scenarios to validate the efficiency of the proposed trajectory planning method. Experiments are performed on the prototype "Forbot" four-wheel independently driven-steered mobile robot; the results demonstrate that the proposed method effectively improves energy efficiency while reducing localization errors along the generated trajectory.
This paper investigates robust path tracking issue of the four‐wheel independent driven robot (FWIDR) under time‐varying system uncertainties and unavoidable external disturbances. A robust optimal ...integral sliding mode tracking control (OISMTC) scheme based on double feedback recurrent neural network (DFRNN) is proposed for the FWIDR system. Firstly, the presented OISMTC scheme modifies nominal optimal control part by exploiting an additional integral term to improve the tracking accuracy. Then, the designed DFRNN utilizes a double feedback loops structure to enhance the robustness against large system uncertainties by learning to approximate nonlinear systems. The adaptive law of the DFRNN is presented based on the Lyapunov theory to obtain favourable approximation performance in the presence of the time‐varying operating conditions. Moreover, the asymptotic stability of the resultant FWIDR system is guaranteed by mathematical analysis. Finally, practical experiments are conducted to demonstrate the advantages of the proposed DFRNN‐OISMTC method.
The contact spacer is the core component of flexible tactile sensors, and the performance of this sensor can be adjusted by adjusting contact spacer micro-hole size. At present, the contact spacer ...was mainly prepared by non-quantifiable processing technology (electrospinning, etc.), which directly leads to unstable performance of tactile sensors. In this paper, ultrathin polyimide (PI) contact spacer was fabricated using nanosecond ultraviolet (UV) laser. The quality evaluation system of laser micro-cutting was established based on roundness, diameter and heat affected zone (HAZ) of the micro-hole. Taking a three factors, five levels orthogonal experiment, the optimum laser cutting process was obtained (pulse repetition frequency 190 kHz, cutting speed 40 mm/s, and RNC 3). With the optimal process parameters, the minimum diameter was 24.3 ± 2.3 μm, and the minimum HAZ was 1.8 ± 1.1 μm. By analyzing the interaction process between nanosecond UV laser and PI film, the heating-carbonization mechanism was determined, and the influence of process parameters on the quality of micro-hole was discussed in detail in combination with this mechanism. It provides a new approach for the quantitative industrial fabrication of contact spacers in tactile sensors.
The mechanism of ultraviolet (UV) nanosecond laser cutting of thermoplastic films and the influence of process parameters on process quality are systematically discussed. The photothermal effect ...plays a dominant role in the interaction between the UV-nanosecond laser and thermoplastic materials. In this photothermal reaction, a heat source with the focal point as the core is formed, around which a thermal carbonization layer, a thermal melting layer, and a thermal expansion layer are formed in order from the inside to the outside. Among them, the thermal carbonization layer is not prevalent, and the thermal melting layer and thermal expansion layer are prevalent. The process quality can be adjusted by adjusting the cutting speed, the laser power, and the repetition number of cuts to regulate the process of heat generation and heat dissipation. In the effective range, the faster the cutting speed and the lower the laser power, the smaller the kerf width and heat-affected zone (HAZ) width. Within a certain range, the depth of kerf can be increased by increasing the repetition number of cuts.
Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors, and their 2D structure results in large binding energies for excitons, trions, and biexcitons. The ability to ...explore many-body effects in these monolayered structures has made them appealing for future optoelectronic and photonic applications. The band structure changes for bilayer TMDs with increased contributions from indirect transitions, and this has limited similar in-depth studies of biexcitons. Here, we study biexciton emission in bilayer WS2 grown by chemical vapor deposition as a function of temperature. A biexciton binding energy of 36 ±4 meV is measured in the as-grown bilayer WS2 containing 0.4% biaxial strain as determined by Raman spectroscopy. The biexciton emission was difficult to detect when the WS2 was transferred to another substrate to release the stain. Density functional theory calculations show that 0.4% of tensile strain lowers the direct band gap by about 55 meV without significant change to the indirect band gap, which can cause an increase in the quantum yield of direct exciton transitions and the emission from biexcitons formed by two direct gap excitons. We find that the biexciton emission decreases dramatically with increased temperature due to the thermal dissociation, with an activation energy of 26 ± 5 meV. These results show how strain can be used to tune the many-body effects in bilayered TMD materials and extend the photonic applications beyond pure monolayer systems.
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