Search

Wire arc additive manufacturing (WAAM) is an emerging technology in the manufacturing industry, which uses a welding arc as an energy source to fuse metal wire and deposit layer by layer. In order to promote its manufacture precision, stability, and repeatability, it’s crucial to develop a feedback control strategy for WAAM. This research implements vision-based feedback control for the layer width during the WAAM process. A WAAM system is developed using a robot and CMT welder with a visual sensing system. The dynamics of the layer width in WAAM process is modeled experimentally. An ARX dynamic model is built. Based on this model, a model predictive control (MPC) strategy is derived to regulate the WAAM process. Feedback control experiments were conducted to verify the tracking and robustness performance of the proposed MPC algorithm.

As an advanced and highly efficient welding method, Keyhole Tungsten Inert Gas (keyhole TIG) welding has drawn wide interests from the manufacturing industry. In order to improve its manufacturing quality and automation level, it’s necessary to develop an online monitoring system for the keyhole TIG welding process. This study developed a visual monitoring system, which utilized an HDR welding camera to monitor the welding pool and keyhole during keyhole TIG welding process. A state of the art Convolutional neural network (Resnet) was developed to recognize different welding states, including good weld, incomplete penetration, burn through, misalignment and undercut. In order to improve the diversity of training dataset, image augmentation was performed. To optimize the training process, a metric learning strategy of center loss was introduced. Furthermore, visualization methods, including guided Grad-CAM, feature map and t-SNE were applied to understand and explain the effectiveness of deep learning process. This study will lay a solid foundation for the development of on-line monitoring system of keyhole TIG.

Cu2ZnSnS4 is a promising and noticeable material for the absorber layer of thin film solar cells. Suitable off-stoichiometic composition of pure kesterite phase is profitable for the photoelectron properties, but such compositions tend to cause the appearance of secondary phases. The stable composition range of pure kesterite phase is difficult to confirm by experiment. In this paper, calculation models of single kesterite phase and other possible phases in Cu-Zn-Sn-S alloy system were constructed and the formation energies E were calculated based on first-principle method. The stability of single kesterite phase with off-stoichiometric compositions was deduced by comparing its E with that of different phase combinations in Cu-Zn-Sn-S alloy system. Following conclusions were drawn. When Cu/(Zn+Sn) ≤ 0.778, single kesterite phase is not as stable as any of phase combinations because of its smallest E, so secondary phases definitely appear. When Cu/(Zn+Sn) = 0.778–0.882, the secondary phases should also appear on equilibrium condition, while they may not appear on non-equilibrium condition because the E difference is relative small. When Cu/(Zn+Sn) = 1.000–0.882, single kesterite phase should be the stablest phase and the secondary phases should not appear on equilibrium condition. All experimental results we could find are in accordance with the calculated kesterite phase stable range. Such a range can provide a profitable guidance for the composition design of Cu-Zn-Sn-S alloys for thin film solar cells. The calculation model and method can be further extended for the prediction of the phase stability in other alloy systems.

An approach based on millisecond pulsed laser is proposed for thermal fatigue property evaluation in this paper. Cyclic thermal stresses and strains within millisecond interval are induced by complex and transient temperature gradients with pulsed laser heating. The influence of laser parameters on surface temperature is studied. The combination of low pulse repetition rate and high pulse energy produces small temperature oscillation, while high pulse repetition rate and low pulse energy introduces large temperature shock. The possibility of application is confirmed by two thermal fatigue tests of compacted graphite iron with different laser controlled modes. The developed approach is able to fulfill the preset temperature cycles and simulate thermal fatigue failure of engine components.

For hot metal forming at high temperatures, lubricant is required to reduce friction and wear, as well as improve the product surface quality. Inorganic compound has been regarded as a candidate to replace the traditional oil/water lubricant for the forming process. Friction and wear behaviour of three inorganic compounds, including sodium polyphosphate, sodium carbonate, sodium borate were evaluated under ball-on-disc tests at high temperature for high speed steel/silicon steel counterparts to simulate the hot rolling process. The results show that all the three compounds can reduce the friction and wear. Sodium polyphosphate is more corrosive with the oxide scale than others, which is believed to contribute to the descaling of red scale of silicon (Si) steel. Mechanisms by which the inorganic compounds improve the tribological behaviours were discussed here. Simulation results indicate that the molecules of sodium polyphosphate and sodium borate show a much higher binding energy on the iron oxide surface than that of sodium carbonate. The binding energy is mainly contributed from the ionic and the covalent interactions. The ionic part comes from the interaction between the Na+ cations and the oxide surface, while the covalent part comes from the chemical bonds between Fe and oxygen of the lubricants. Both of them contribute to the adhesion of the lubricants on the steel surface and are vital to the friction and wear performance of the lubricants.

Adequate lubrication for the sliding contacts is responsible for the energy saving and long service life. In this work, load-dependence of tribological behaviours of sodium carbonate coating on the stainless steel was evaluated by ball-on-disc friction test at high temperatures. The results suggested that this sodium carbonate coating showed promising lubrication even under high applied load of 1800 N (Hertz pressure is 6.14 GPa). Apart from the improvement of lubricity, it can also inhibit adhesive wear that transfer oxide scale from the disc to the ball under dry sliding condition. The desirable lubrication and load-carrying capability were attributed to the frictional heat induced in-situ formed sodium carbonate melt that produced an easily sheared tribo-interface, as well as the reconstruction of the oxide scale. The sodium carbonate coating with a superior load capability at high temperature is initially reported here. This finding is believed to bring new perspective of high-temperature, extreme-pressure lubrication design with the easily accessible carbonate salt.

As one of obvious electrolyte design technologies of Li ion batteries (LIBs) to meet durable high-rate capability at elevated temperatures for battery electric vehicles, this study assesses the superiority of gel polymer electrolyte (GPE) based on experimental results supporting its working mechanism. Our previous study indicated that degradation of cathode electrode under high-rate cycling at elevated temperature was a major cause of the decrease in performance of LIBs, and the single full cells (SFCs) with a GPE designed from dipentaerythritol hexaacrylate and methyl ether methacrylate was re-verified to have superior 3.0C cycling performance at 80 °C. The superiority of the GPE is studied from comparing mid-voltages of discharge profiles of the SFCs, observing the cross-sectional morphology of the electrodes by field emission scanning electron microscopy, assessing the interacting force among the electrolyte components by thermogravimetric analysis, and examining each resistance component of the SFCs by electrochemical impedance spectroscopy. Gel polymerization of liquid electrolyte results in a significant increase of durable high-rate capability of LIBs due to the mechanisms of not only its buffering effect on solvating process of Li+ ions being extracted from the active materials during high rate operation, but also higher thermal stability of electrolyte components, lower susceptibility of the ionic conductivity of the electrolyte to a temperature change, and lower energy barrier to breakup of solvated structures during conducting of Li+ ions in gel polymer matrix.

Grain refinement and homogenization of Cr phase were achieved by laser surface melting (LSM) method, and the properties of Cu-Cr alloy were significantly improved. In this study, LSM of Cu-50Cr alloy (wt.%) was conducted with a high power density (106–107 W/cm2) laser beam, the microstructure and the properties of melt layer were investigated. The size of Cr phase was effectively refined from hundreds of micron scale to several micron scale, and the average size of Cr particles decreased to a few hundred nanometers. High cooling rate effectively inhibited coarsening effect on the Cr particles during liquid phase separation. Spherical Cr particles were dispersed in the melt layer with a thickness of 165 ± 20 μm. Microhardness was obviously enhanced and the maximum hardness was 232HV, which was 2.8 times that of the substrate. Arc duration of the LSM treated contacts increased up to 18%. The withstanding voltage of the fixed and the moving contact increased to 28.7% and 35.4%, respectively. The results show that LSM is an effective method to refine the microstructure of Cu-Cr alloy, and it is a promising modification method for electrical Cu-Cr vacuum contacts.