Laser powder bed fusion(L-PBF) has attracted significant attention in both the industry and academic fields since its inception, providing unprecedented advantages to fabricate complex-shaped metallic components. The ...Laser powder bed fusion(L-PBF) has attracted significant attention in both the industry and academic fields since its inception, providing unprecedented advantages to fabricate complex-shaped metallic components. The printing quality and performance of L-PBF alloys are infuenced by numerous variables consisting of feedstock powders, manufacturing process,and post-treatment. As the starting materials, metallic powders play a critical role in infuencing the fabrication cost, printing consistency, and properties. Given their deterministic roles, the present review aims to retrospect the recent progress on metallic powders for L-PBF including characterization, preparation, and reuse. The powder characterization mainly serves for printing consistency while powder preparation and reuse are introduced to reduce the fabrication costs.Various powder characterization and preparation methods are presented in the beginning by analyzing the measurement principles, advantages, and limitations. Subsequently, the effect of powder reuse on the powder characteristics and mechanical performance of L-PBF parts is analyzed, focusing on steels, nickel-based superalloys, titanium and titanium alloys, and aluminum alloys. The evolution trends of powders and L-PBF parts vary depending on specific alloy systems, which makes the proposal of a unified reuse protocol infeasible. Finally,perspectives are presented to cater to the increased applications of L-PBF technologies for future investigations. The present state-of-the-art work can pave the way for the broad industrial applications of L-PBF by enhancing printing consistency and reducing the total costs from the perspective of powders.展开更多
The arc ignition system based on charring polymers has advantages of simple structure,low ignition power consumption and multiple ignitions,which bringing it broadly application prospect in hybrid propulsion system of...The arc ignition system based on charring polymers has advantages of simple structure,low ignition power consumption and multiple ignitions,which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite.However,charring polymers alone need a relatively high input voltage to achieve pyrolysis and ignition,which increases the burden and cost of the power system of micro/nano satellite in practical application.Adding conductive substance into charring polymers can effectively decrease the conducting voltage which can realize low voltage and low power consumption repeated ignition of arc ignition system.In this paper,a charring conductive polymer ignition grain with a cavity geometry in precombustion chamber,which is composed of PLA and multiwall carbon nanotubes(MWCNT)was proposed.The detailed ignition processes were analyzed and two different ignition mechanisms in the cavity of charring conductive polymers were revealed.The ignition characteristics of charring conductive polymers were also investigated at different input voltages,ignition grain structures,ignition locations and injection schemes in a visual ignition combustor.The results demonstrated that the ignition delay and external energy required for ignition were inversely correlated with the voltages applied to ignition grain.Moreover,the incremental depth of cavity shortened the ignition delay and external energy required for ignition while accelerated the propagation of flame.As the depth of cavity increased from 2 to 6 mm(at 50 V),the time of flame propagating out of ignition grain changed from 235.6 to 108 ms,and values of mean ignition delay time and mean external energy required for ignition decreased from 462.8 to 320 ms and 16.2 to 10.75 J,respectively.The rear side of the cavity was the ideal ignition position which had a shorter ignition delay and a faster flame propagation speed in comparison to other ignition positions.Compared to direct injection scheme,swirling injection provided a more favorable flow field environment in the cavity,which was beneficial to ignition and initial flame propagation,but the ignition position needed to be away from the outlet of swirling injector.At last,the repeated ignition characteristic of charring conductive polymers was also investigated.The ignition delay time and external energy required for ignition decreased with repeated ignition times but the variation was decreasing gradually.展开更多
Laser powder bed fusion(LPBF)is an advanced manufacturing technology;however,inappropriate LPBF process parameters may cause printing defects in materials.In the present work,the LPBF process of Ti-6.5Al-3.5Mo-1.5Zr-0...Laser powder bed fusion(LPBF)is an advanced manufacturing technology;however,inappropriate LPBF process parameters may cause printing defects in materials.In the present work,the LPBF process of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy was investigated by a two-step optimization approach.Subsequently,heat transfer and liquid flow behaviors during LPBF were simulated by a well-tested phenomenological model,and the defect formation mechanisms in the as-fabricated alloy were discussed.The optimized process parameters for LPBF were detected as laser power changed from 195 W to 210 W,with scanning speed of 1250 mm/s.The LPBF process was divided into a laser irradiation stage,a spreading flow stage,and a solidification stage.The morphologies and defects of deposited tracks were affected by liquid flow behavior caused by rapid cooling rates.The findings of this research can provide valuable support for printing defect-free metal components.展开更多
For the on-orbit flight missions,the model of orbit prediction is critical for the tasks with high accuracy requirement and limited computing resources of spacecraft.The precession-nutation model,as the main part of e...For the on-orbit flight missions,the model of orbit prediction is critical for the tasks with high accuracy requirement and limited computing resources of spacecraft.The precession-nutation model,as the main part of extended orbit prediction,affects the efficiency and accuracy of on-board operation.In this paper,the previous research about the conversion between the Geocentric Celestial Reference System and International Terrestrial Reference System is briefly summarized,and a practical concise precession-nutation model is proposed for coordinate transformation computation based on Celestial Intermediate Pole(CIP).The idea that simplifying the CIP-based model with interpolation method is driven by characteristics of precession-nutation parameters changing with time.A cubic spline interpolation algorithm is applied to obtain the required CIP coordinates and Celestial Intermediate Origin locator.The complete precession nutation model containing more than 4000 parameters is simplified to the calculation of a cubic polynomial,which greatly reduces the computational load.In addition,for evaluating the actual performance,an orbit propagator is built with the proposed simplified precession-nutationmodel.Compared with the orbit prediction results obtained by the truncated series of IAU2000/2006 precession-nutation model,the simplified precession-nutation model with cubic spline interpolation can significantly improve the accuracy of orbit prediction,which implicates great practical application value in further on-orbit missions of spacecraft.展开更多
Owing to their remarkable flexibility and favorable cost-effectiveness,industrial robots have found extensive applications to cutting of materials across sophisticated manufacturing fields.However,the structurally low...Owing to their remarkable flexibility and favorable cost-effectiveness,industrial robots have found extensive applications to cutting of materials across sophisticated manufacturing fields.However,the structurally low rigidity of these robots renders the tool tip prone to substantial oscillations during machining processes,significantly impacting product fabrication quality.The objective of this study is to present a novel methodology employing magnetorheological dampers for mitigating vibrations during robotic milling operations.Specifically,a new type of ring nested Magneto-Rheological Foam Damper(MRFD)working in the squeeze mode is developed.Firstly,the MRFD’s structure is designed considering the vibrational characteristics of robotic milling.Subsequently,a damping force model of the MRFD is derived.The feasibility of the MRFD’s structural design is validated by the finite element analyses,which is instrumental in comprehending the influence of structural parameters on the electromagnetic characteristics of the MRFD.Next,a prototype of the MRFD is fabricated selecting appropriate parameters.Finally,a series of excitation and milling experiments are conducted on a KUKA KR500 robot.The outcomes demonstrate a substantial reduction(37%-69%)in radial vibration amplitude at the tool tip during robotic milling,highlighting the effectiveness of the developed MRFD.This research endeavor has introduced a pioneering avenue and framework for vibration control in robotic milling,offering a novel paradigm for enhancing the precision of robotic machining.展开更多
The arc ignition based on charring conductive polymers has advantages of simple structure,low ignition power consumption and restart capacity,which bringing it broadly application prospect in hybrid propulsion system ...The arc ignition based on charring conductive polymers has advantages of simple structure,low ignition power consumption and restart capacity,which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite.In order to optimize the performance of arc ignition system,it is essential to have a deeper understanding of the ignition processes and ignition characteristics of charring conductive polymers.In this paper,the thermal decomposition,electrical conductivity and thermal conductivity characteristics of charring conductive polymers with different conductive additives and matrix materials were comprehensively evaluated.An experimental investigation was conducted to determine the ignition behaviors and characteristics of different charring conductive polymers in a visual ignition combustor.The experiment result showed that the ignition delay and external energy required for ignition are negatively correlated with voltage and initial temperature of the ignition grain,but positively correlated with oxidizer flow velocity.Compared with charring conductive polymers containing multi-walled carbon nanotube,the ignition delay of charring conductive polymers with carbon black is significantly higher and the pyrolysis time is relatively longer.However,the ignition and initial flame propagation of charring conductive polymers with carbon black is more violent and more inclined to carbon particle ignition.Finally,the restart characteristic of different charring conductive polymers was studied.The ignition delay and external energy required for ignition of different charring conductive polymers all reduced with the increasing of the number of ignitions.However,the ignition characteristics would not change a lot after repeated ignition.展开更多
Having accurate values of the dynamic parameters is necessary to characterize the dynamic behaviors of mechanical systems and for the prediction of their responses.To accurately describe the dynamic characteristics of...Having accurate values of the dynamic parameters is necessary to characterize the dynamic behaviors of mechanical systems and for the prediction of their responses.To accurately describe the dynamic characteristics of industrial robots(IRs),a new method for dynamic parameter identification is proposed in this study with the goal of developing a real IR dynamics model that combines the multibody system transfer matrix method(MSTMM)and the nondominated sorting genetic algorithm-II(NSGA-II).First,the multibody dynamics model of an IR is developed using the MSTMM,by which its frequency response function(FRF)is calculated numerically.Then,the experimental modal analysis is conducted to measure the IR's actual FRF.Finally,the objective function of the minimum errors between the calculated and measured eigenfrequencies and FRFs are constructed to identify the dynamic parameters of the IR by the NSGA-II algorithm.The simulated and experimental results illustrate the effectiveness of the methodology presented in this paper,which provides an alternative to the identification of IR dynamic parameters.展开更多
Carbon fiber reinforced plastic(CFRP)and aluminum stacks are widely used in aviation industry due to light weight and high performance.Millions of rivet holes need to be drilled on body materials,and more than 80%of f...Carbon fiber reinforced plastic(CFRP)and aluminum stacks are widely used in aviation industry due to light weight and high performance.Millions of rivet holes need to be drilled on body materials,and more than 80%of fatigue cracks occur at the connection holes,so the damage and residual stress of hole surface have crucial effect on the riveting fatigue life of CFRP/aluminum stacks and the flight performance.Recently,robotic rotary ultrasonic drilling(RRUD)technology is a promising method to machine the stacks.However,the hole surface strengthening mechanism in RRUD and the service performance of the riveting joint are not verified.Thus,in this paper,the hole surface strengthening mechanism of RRUD for CFRP/aluminum stacks is investigated,a theoretical residual stress model is established,and the fatigue life experiment of riveted joints is conducted.Firstly,analysis on residual stress in RRUD is carried out with consideration of strengthening force and cutting temperature.Residual stress model is established based on the calculation of elastic stress,plastic stress and stress release.Validation experiment results show that ultrasonic vibration changes residual stress from tensile stress to compressive stress.At the same time,comparative damage analysis of CFRP hole exit and hole surface in robotic conventional drilling(RCD)and RRUD is presented.Finally,fatigue strength experiments of riveted joints are conducted for performance verification.Experimental results indicate that fatigue life of single-hole riveted joints is increased by 68%with ultrasonic vibration,and four-hole riveted joint arranged according to aerospace design standards is increased by more than 86%.展开更多
Laser powder bed fusion(LPBF)of high-strength Al alloys is challenging due to the formation of both hot and cold cracks.In the present work,highly dense and crack-free AA2024 samples could be additively manufactured v...Laser powder bed fusion(LPBF)of high-strength Al alloys is challenging due to the formation of both hot and cold cracks.In the present work,highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass(MG)powders.The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1μm and relatively coarser grains.The grain refinement can be associated with the formation of Al3Zr particles,serving as the heterogeneous nucleation sites for theα-Al matrix.Complete routes for columnar-equiaxed-transition(CET)have been revealed by tailoring the concentration of nucleation particles and solidification conditions.CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling.Between these two regions,columnar grains epitaxially grow with orientations determined by the thermal gradient.The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains.The as-fabricated AA2024/5%MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement,surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders.The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.展开更多
Currently,industrial robots are considered as an alternative towards traditional machine tools.Especially for the large-scale parts milling,robotic flexibility and low cost make it possess the irreplaceability.However...Currently,industrial robots are considered as an alternative towards traditional machine tools.Especially for the large-scale parts milling,robotic flexibility and low cost make it possess the irreplaceability.However,the milling chatter caused by its weak rigidity hampers robotic application and promotion severely in aviation industry.Rotary ultrasonic milling(RUM)technology with one-dimensional axial vibration has been proven and approved on avoiding robotic chatter.Based on this,the research of project team demonstrates that longitudinal-torsional composite ultrasonic milling(CUM-LT)involving separation characteristic has a greater advantage than RUM in terms of chatter suppression.Thereby,the CUM-LT as a new means is applied to avoid processing vibration of robotic milling system.And its influence mechanism on stability improvement of weak stiffness processing system is clarified.Meanwhile,the approaches to strengthen separation effect are provided innovatively.Moreover,a new analysis method of robotic CUM-LT(RCUM-LT)stability is proposed on the basis of ultrasonic function angles.The simulation and experimental results indicate that compared with robotic RUM(RRUM),stability regions of separated RCUM-LT(SRCUM-LT)and unseparated RCUM-LT(URCUM-LT)are improved by 124.42%and39.20%,respectively.The addition of torsional ultrasonic energy has a wonderful effect on the milling chatter suppression of low stiffness robots.展开更多
Relative navigation is crucial for spacecraft noncooperative rendezvous,and angles-only navigation using visible and infrared cameras provides a feasible solution.Herein,an angles-only navigation algorithm with multis...Relative navigation is crucial for spacecraft noncooperative rendezvous,and angles-only navigation using visible and infrared cameras provides a feasible solution.Herein,an angles-only navigation algorithm with multisensor data fusion is proposed to derive the relative motion states between two noncooperative spacecraft.First,the design model of the proposed algorithm is introduced,including the derivation of the state propagation and measurement equations.Subsequently,models for the sensor and actuator are introduced,and the effects of various factors on the sensors and actuators are considered.The square-root unscented Kalman filter is used to design the angles-only navigation filtering scherne.Additionally,the Clohessy-Wiltshire terminal guidance algorithm is introducedto obtain the theoretical relative motion trajectories during the rendezvous operations of two noncooperative spacecraft.Finally,the effectiveness of the proposed angles-only navigation algorithm is verified using a semi-physical simulation platform.The results prove that an optical navigation camera combined with average accelerometers and occasional orbital maneuvers is feasible for spacecraft noncooperative rendezvous using angles-only navigation.展开更多
In recent years,industrial robots have received extensive attention in manufacturing field due to their high flexibility and great workspace.However,the weak stiffness of industrial robots makes it extremely easy to a...In recent years,industrial robots have received extensive attention in manufacturing field due to their high flexibility and great workspace.However,the weak stiffness of industrial robots makes it extremely easy to arouse chatter,which affects machining quality inevitably and generates noise pollution in severe cases.Compared with drilling,the chatter mechanism of robotic countersinking is more complex.The external excitation changes with cutting width and depth in countersinking.This characteristic results in time-varying and nonlinearity of robotic countersinking dynamics.Thus,it is urgent to propose a new method of chatter suppression and provide an accurate stability analysis model.As a new special machining technology,rotary ultrasonic machining has been proved to improve robotic drilling and milling stability effectively.Based on this,robotic rotary ultrasonic countersinking(RRUC)is proposed to improve the robotic countersinking stability in this paper.A three-dimensional stability domain method of RRUC is established.First,the countersinking process was divided intoρparts.The dynamic model of every unit was constructed based on ultrasonic function angle(γ)and dynamic chip area.Then,the stability region of RRUC is obtained based on the semi-discrete method(SDM).Compared with the robotic conventional countersinking(RCC),RRUC improves the stability by 27%.Finally,the correctness and effectiveness of the stability region model are proved by robotic ultrasonic countersinking experiments.展开更多
Eutectic ceramics with exceptional hardness,elevated Young’s modulus,and impressive fracture toughness have become indispensable materials for various demanding industrial applications in extreme environments.In this...Eutectic ceramics with exceptional hardness,elevated Young’s modulus,and impressive fracture toughness have become indispensable materials for various demanding industrial applications in extreme environments.In this work,in-situ synthesis of high-entropy Al_(2)O_(3)/(Y_(0.2)Eu_(0.2)Er_(0.2)Yb_(0.2)Lu_(0.2))_(3)Al_(5)O_(12)/ZrO_(2)ceramics was successfully achieved with onestep slurry-based laser powder bed fusion(LPBF).The phase formation,microstructure morphologies,phase distribution,crystallographic characteristics,and mechanical properties were systematically investigated.The typical microstructure of a shell–core architecture resembling cell-like features consists of both crystalline and amorphous phases.The selective aggregation of constituents is the combined outcome of solute exclusion and convective transport mechanisms.The crystallographic orientation relationships inside the shell may include■ZrO_(2)//RE_(3)Al_(5)O_(12)and RE_(3)Al_(5)O_(12)//ZrO_(2).During the rapid cooling of LPBF,the amorphous phase is an intermediate metastable product of crystal structure adjustment(REAlO_(3),RE_(4)Al_(2)O_(9)→amorphous phase→RE_(3)Al_(5)O_(12)).Owing to the slower diffusion rate and higher transition energy threshold of Eu elements,more Eu elements are retained in the amorphous phase.Structural analysis indicates that the remarkable mechanical properties can be attributed to several factors,including the significant dissociation energy,strong cationic field,and stress transfer among Al_(2)O_(3),RE_(3)Al_(5)O_(12),ZrO_(2),and the amorphous phase.Additionally,the cocktail effect and plastic deformation mechanism of the high-entropy shell contribute to these properties.展开更多
基金supported by the Fundamental Research Funds for the Central Universities (Grant No. AE89991/403)National Natural Science Foundation of China (Grant No. 52005262)+1 种基金Natural Science Foundation of Jiangsu Province (BK20202007)National Key Research and Development Program of China (2022YFB4600800)。
文摘Laser powder bed fusion(L-PBF) has attracted significant attention in both the industry and academic fields since its inception, providing unprecedented advantages to fabricate complex-shaped metallic components. The printing quality and performance of L-PBF alloys are infuenced by numerous variables consisting of feedstock powders, manufacturing process,and post-treatment. As the starting materials, metallic powders play a critical role in infuencing the fabrication cost, printing consistency, and properties. Given their deterministic roles, the present review aims to retrospect the recent progress on metallic powders for L-PBF including characterization, preparation, and reuse. The powder characterization mainly serves for printing consistency while powder preparation and reuse are introduced to reduce the fabrication costs.Various powder characterization and preparation methods are presented in the beginning by analyzing the measurement principles, advantages, and limitations. Subsequently, the effect of powder reuse on the powder characteristics and mechanical performance of L-PBF parts is analyzed, focusing on steels, nickel-based superalloys, titanium and titanium alloys, and aluminum alloys. The evolution trends of powders and L-PBF parts vary depending on specific alloy systems, which makes the proposal of a unified reuse protocol infeasible. Finally,perspectives are presented to cater to the increased applications of L-PBF technologies for future investigations. The present state-of-the-art work can pave the way for the broad industrial applications of L-PBF by enhancing printing consistency and reducing the total costs from the perspective of powders.
基金the Fundamental Research Funds for the Central Universities(Grant No.30920041102)National Natural Science Foundation of China(Grant No.11802134).
文摘The arc ignition system based on charring polymers has advantages of simple structure,low ignition power consumption and multiple ignitions,which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite.However,charring polymers alone need a relatively high input voltage to achieve pyrolysis and ignition,which increases the burden and cost of the power system of micro/nano satellite in practical application.Adding conductive substance into charring polymers can effectively decrease the conducting voltage which can realize low voltage and low power consumption repeated ignition of arc ignition system.In this paper,a charring conductive polymer ignition grain with a cavity geometry in precombustion chamber,which is composed of PLA and multiwall carbon nanotubes(MWCNT)was proposed.The detailed ignition processes were analyzed and two different ignition mechanisms in the cavity of charring conductive polymers were revealed.The ignition characteristics of charring conductive polymers were also investigated at different input voltages,ignition grain structures,ignition locations and injection schemes in a visual ignition combustor.The results demonstrated that the ignition delay and external energy required for ignition were inversely correlated with the voltages applied to ignition grain.Moreover,the incremental depth of cavity shortened the ignition delay and external energy required for ignition while accelerated the propagation of flame.As the depth of cavity increased from 2 to 6 mm(at 50 V),the time of flame propagating out of ignition grain changed from 235.6 to 108 ms,and values of mean ignition delay time and mean external energy required for ignition decreased from 462.8 to 320 ms and 16.2 to 10.75 J,respectively.The rear side of the cavity was the ideal ignition position which had a shorter ignition delay and a faster flame propagation speed in comparison to other ignition positions.Compared to direct injection scheme,swirling injection provided a more favorable flow field environment in the cavity,which was beneficial to ignition and initial flame propagation,but the ignition position needed to be away from the outlet of swirling injector.At last,the repeated ignition characteristic of charring conductive polymers was also investigated.The ignition delay time and external energy required for ignition decreased with repeated ignition times but the variation was decreasing gradually.
基金Supported by Development of a Verification Platform for Product Design,Process and Information Exchange Standards in Additive Manufacturing (Grant No.2019-00899-1-1)Ministry of Science and Technology of the People’s Republic of China (Grant No.2017YFB1103000)+1 种基金National Natural Science Foundation of China (Grant No.51375242)Natural Science Foundation of Jiangsu Province (Grant No.BK20180483)。
文摘Laser powder bed fusion(LPBF)is an advanced manufacturing technology;however,inappropriate LPBF process parameters may cause printing defects in materials.In the present work,the LPBF process of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy was investigated by a two-step optimization approach.Subsequently,heat transfer and liquid flow behaviors during LPBF were simulated by a well-tested phenomenological model,and the defect formation mechanisms in the as-fabricated alloy were discussed.The optimized process parameters for LPBF were detected as laser power changed from 195 W to 210 W,with scanning speed of 1250 mm/s.The LPBF process was divided into a laser irradiation stage,a spreading flow stage,and a solidification stage.The morphologies and defects of deposited tracks were affected by liquid flow behavior caused by rapid cooling rates.The findings of this research can provide valuable support for printing defect-free metal components.
基金The authors would like to express gratitude for supporting funding from the Natural Science Foundation of China(No.51905272).
文摘For the on-orbit flight missions,the model of orbit prediction is critical for the tasks with high accuracy requirement and limited computing resources of spacecraft.The precession-nutation model,as the main part of extended orbit prediction,affects the efficiency and accuracy of on-board operation.In this paper,the previous research about the conversion between the Geocentric Celestial Reference System and International Terrestrial Reference System is briefly summarized,and a practical concise precession-nutation model is proposed for coordinate transformation computation based on Celestial Intermediate Pole(CIP).The idea that simplifying the CIP-based model with interpolation method is driven by characteristics of precession-nutation parameters changing with time.A cubic spline interpolation algorithm is applied to obtain the required CIP coordinates and Celestial Intermediate Origin locator.The complete precession nutation model containing more than 4000 parameters is simplified to the calculation of a cubic polynomial,which greatly reduces the computational load.In addition,for evaluating the actual performance,an orbit propagator is built with the proposed simplified precession-nutationmodel.Compared with the orbit prediction results obtained by the truncated series of IAU2000/2006 precession-nutation model,the simplified precession-nutation model with cubic spline interpolation can significantly improve the accuracy of orbit prediction,which implicates great practical application value in further on-orbit missions of spacecraft.
基金co-supported by the Natural Science Foundation of Jiangsu Province(No.BK20230092)by the National Natural Science Foundation of China(Nos.52375500,52075256,and U22A20204).
文摘Owing to their remarkable flexibility and favorable cost-effectiveness,industrial robots have found extensive applications to cutting of materials across sophisticated manufacturing fields.However,the structurally low rigidity of these robots renders the tool tip prone to substantial oscillations during machining processes,significantly impacting product fabrication quality.The objective of this study is to present a novel methodology employing magnetorheological dampers for mitigating vibrations during robotic milling operations.Specifically,a new type of ring nested Magneto-Rheological Foam Damper(MRFD)working in the squeeze mode is developed.Firstly,the MRFD’s structure is designed considering the vibrational characteristics of robotic milling.Subsequently,a damping force model of the MRFD is derived.The feasibility of the MRFD’s structural design is validated by the finite element analyses,which is instrumental in comprehending the influence of structural parameters on the electromagnetic characteristics of the MRFD.Next,a prototype of the MRFD is fabricated selecting appropriate parameters.Finally,a series of excitation and milling experiments are conducted on a KUKA KR500 robot.The outcomes demonstrate a substantial reduction(37%-69%)in radial vibration amplitude at the tool tip during robotic milling,highlighting the effectiveness of the developed MRFD.This research endeavor has introduced a pioneering avenue and framework for vibration control in robotic milling,offering a novel paradigm for enhancing the precision of robotic machining.
基金Supported by the Fundamental Research Funds for the Central Universities,China(No.30920041102)the National Natural Science Foundation of China(No.11802134).
文摘The arc ignition based on charring conductive polymers has advantages of simple structure,low ignition power consumption and restart capacity,which bringing it broadly application prospect in hybrid propulsion system of micro/nano satellite.In order to optimize the performance of arc ignition system,it is essential to have a deeper understanding of the ignition processes and ignition characteristics of charring conductive polymers.In this paper,the thermal decomposition,electrical conductivity and thermal conductivity characteristics of charring conductive polymers with different conductive additives and matrix materials were comprehensively evaluated.An experimental investigation was conducted to determine the ignition behaviors and characteristics of different charring conductive polymers in a visual ignition combustor.The experiment result showed that the ignition delay and external energy required for ignition are negatively correlated with voltage and initial temperature of the ignition grain,but positively correlated with oxidizer flow velocity.Compared with charring conductive polymers containing multi-walled carbon nanotube,the ignition delay of charring conductive polymers with carbon black is significantly higher and the pyrolysis time is relatively longer.However,the ignition and initial flame propagation of charring conductive polymers with carbon black is more violent and more inclined to carbon particle ignition.Finally,the restart characteristic of different charring conductive polymers was studied.The ignition delay and external energy required for ignition of different charring conductive polymers all reduced with the increasing of the number of ignitions.However,the ignition characteristics would not change a lot after repeated ignition.
基金Natural Science Foundation of Jiangsu Province,Grant/Award Number:BK20230092State Key Laboratory of Mechanics and Control for Aerospace Structures(Nanjing University of Aeronautics and astronautics),Grant/Award Number:MCAS-S-0423G05+1 种基金National Natural Science Foundation of China,Grant/Award Numbers:52375500,U22A20204,52305540Jiangsu Provincial Key Research and Development Program,Grant/Award Number:BE2022078。
文摘Having accurate values of the dynamic parameters is necessary to characterize the dynamic behaviors of mechanical systems and for the prediction of their responses.To accurately describe the dynamic characteristics of industrial robots(IRs),a new method for dynamic parameter identification is proposed in this study with the goal of developing a real IR dynamics model that combines the multibody system transfer matrix method(MSTMM)and the nondominated sorting genetic algorithm-II(NSGA-II).First,the multibody dynamics model of an IR is developed using the MSTMM,by which its frequency response function(FRF)is calculated numerically.Then,the experimental modal analysis is conducted to measure the IR's actual FRF.Finally,the objective function of the minimum errors between the calculated and measured eigenfrequencies and FRFs are constructed to identify the dynamic parameters of the IR by the NSGA-II algorithm.The simulated and experimental results illustrate the effectiveness of the methodology presented in this paper,which provides an alternative to the identification of IR dynamic parameters.
基金the the Project on the Technological Leading Talent Teams Led by Frontiers Science Center for Complex Equipment System Dynamics(No.FSCCESD220401)the Jiangsu Funding Program for Excellent Postdoctoral Talent(No.2022ZB264).
文摘Carbon fiber reinforced plastic(CFRP)and aluminum stacks are widely used in aviation industry due to light weight and high performance.Millions of rivet holes need to be drilled on body materials,and more than 80%of fatigue cracks occur at the connection holes,so the damage and residual stress of hole surface have crucial effect on the riveting fatigue life of CFRP/aluminum stacks and the flight performance.Recently,robotic rotary ultrasonic drilling(RRUD)technology is a promising method to machine the stacks.However,the hole surface strengthening mechanism in RRUD and the service performance of the riveting joint are not verified.Thus,in this paper,the hole surface strengthening mechanism of RRUD for CFRP/aluminum stacks is investigated,a theoretical residual stress model is established,and the fatigue life experiment of riveted joints is conducted.Firstly,analysis on residual stress in RRUD is carried out with consideration of strengthening force and cutting temperature.Residual stress model is established based on the calculation of elastic stress,plastic stress and stress release.Validation experiment results show that ultrasonic vibration changes residual stress from tensile stress to compressive stress.At the same time,comparative damage analysis of CFRP hole exit and hole surface in robotic conventional drilling(RCD)and RRUD is presented.Finally,fatigue strength experiments of riveted joints are conducted for performance verification.Experimental results indicate that fatigue life of single-hole riveted joints is increased by 68%with ultrasonic vibration,and four-hole riveted joint arranged according to aerospace design standards is increased by more than 86%.
基金supported by the National Natural Science Foundation of China(Nos.52175330 and 51805267)the Frontier Leading Technology Basic Research Project of Jiangsu(No.BK20202007)the Fundamental Research Funds for the Central Universities(No.30921011202).
文摘Laser powder bed fusion(LPBF)of high-strength Al alloys is challenging due to the formation of both hot and cold cracks.In the present work,highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass(MG)powders.The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1μm and relatively coarser grains.The grain refinement can be associated with the formation of Al3Zr particles,serving as the heterogeneous nucleation sites for theα-Al matrix.Complete routes for columnar-equiaxed-transition(CET)have been revealed by tailoring the concentration of nucleation particles and solidification conditions.CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling.Between these two regions,columnar grains epitaxially grow with orientations determined by the thermal gradient.The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains.The as-fabricated AA2024/5%MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement,surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders.The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.
基金co-supported by the National Natural Science Foundation of China(Nos.91860132,51861145405,52075265)。
文摘Currently,industrial robots are considered as an alternative towards traditional machine tools.Especially for the large-scale parts milling,robotic flexibility and low cost make it possess the irreplaceability.However,the milling chatter caused by its weak rigidity hampers robotic application and promotion severely in aviation industry.Rotary ultrasonic milling(RUM)technology with one-dimensional axial vibration has been proven and approved on avoiding robotic chatter.Based on this,the research of project team demonstrates that longitudinal-torsional composite ultrasonic milling(CUM-LT)involving separation characteristic has a greater advantage than RUM in terms of chatter suppression.Thereby,the CUM-LT as a new means is applied to avoid processing vibration of robotic milling system.And its influence mechanism on stability improvement of weak stiffness processing system is clarified.Meanwhile,the approaches to strengthen separation effect are provided innovatively.Moreover,a new analysis method of robotic CUM-LT(RCUM-LT)stability is proposed on the basis of ultrasonic function angles.The simulation and experimental results indicate that compared with robotic RUM(RRUM),stability regions of separated RCUM-LT(SRCUM-LT)and unseparated RCUM-LT(URCUM-LT)are improved by 124.42%and39.20%,respectively.The addition of torsional ultrasonic energy has a wonderful effect on the milling chatter suppression of low stiffness robots.
基金supported by the China Aerospace Science and Technology Corporation Eighth Research Institute Industry-University-Research Cooperation Fund(SAST 2020-019).
文摘Relative navigation is crucial for spacecraft noncooperative rendezvous,and angles-only navigation using visible and infrared cameras provides a feasible solution.Herein,an angles-only navigation algorithm with multisensor data fusion is proposed to derive the relative motion states between two noncooperative spacecraft.First,the design model of the proposed algorithm is introduced,including the derivation of the state propagation and measurement equations.Subsequently,models for the sensor and actuator are introduced,and the effects of various factors on the sensors and actuators are considered.The square-root unscented Kalman filter is used to design the angles-only navigation filtering scherne.Additionally,the Clohessy-Wiltshire terminal guidance algorithm is introducedto obtain the theoretical relative motion trajectories during the rendezvous operations of two noncooperative spacecraft.Finally,the effectiveness of the proposed angles-only navigation algorithm is verified using a semi-physical simulation platform.The results prove that an optical navigation camera combined with average accelerometers and occasional orbital maneuvers is feasible for spacecraft noncooperative rendezvous using angles-only navigation.
基金the Project on the Technological Leading Talent Teams Led by Frontiers Science Center for Complex Equipment System Dynamics(No.FSCCESD220401)the National Natural Science Foundation of China(No.52075265).
文摘In recent years,industrial robots have received extensive attention in manufacturing field due to their high flexibility and great workspace.However,the weak stiffness of industrial robots makes it extremely easy to arouse chatter,which affects machining quality inevitably and generates noise pollution in severe cases.Compared with drilling,the chatter mechanism of robotic countersinking is more complex.The external excitation changes with cutting width and depth in countersinking.This characteristic results in time-varying and nonlinearity of robotic countersinking dynamics.Thus,it is urgent to propose a new method of chatter suppression and provide an accurate stability analysis model.As a new special machining technology,rotary ultrasonic machining has been proved to improve robotic drilling and milling stability effectively.Based on this,robotic rotary ultrasonic countersinking(RRUC)is proposed to improve the robotic countersinking stability in this paper.A three-dimensional stability domain method of RRUC is established.First,the countersinking process was divided intoρparts.The dynamic model of every unit was constructed based on ultrasonic function angle(γ)and dynamic chip area.Then,the stability region of RRUC is obtained based on the semi-discrete method(SDM).Compared with the robotic conventional countersinking(RCC),RRUC improves the stability by 27%.Finally,the correctness and effectiveness of the stability region model are proved by robotic ultrasonic countersinking experiments.
基金supported by the National Natural Science Foundation of China(No.52005262)the National Key R&D Program of China(No.2022YFB4600800).
文摘Eutectic ceramics with exceptional hardness,elevated Young’s modulus,and impressive fracture toughness have become indispensable materials for various demanding industrial applications in extreme environments.In this work,in-situ synthesis of high-entropy Al_(2)O_(3)/(Y_(0.2)Eu_(0.2)Er_(0.2)Yb_(0.2)Lu_(0.2))_(3)Al_(5)O_(12)/ZrO_(2)ceramics was successfully achieved with onestep slurry-based laser powder bed fusion(LPBF).The phase formation,microstructure morphologies,phase distribution,crystallographic characteristics,and mechanical properties were systematically investigated.The typical microstructure of a shell–core architecture resembling cell-like features consists of both crystalline and amorphous phases.The selective aggregation of constituents is the combined outcome of solute exclusion and convective transport mechanisms.The crystallographic orientation relationships inside the shell may include■ZrO_(2)//RE_(3)Al_(5)O_(12)and RE_(3)Al_(5)O_(12)//ZrO_(2).During the rapid cooling of LPBF,the amorphous phase is an intermediate metastable product of crystal structure adjustment(REAlO_(3),RE_(4)Al_(2)O_(9)→amorphous phase→RE_(3)Al_(5)O_(12)).Owing to the slower diffusion rate and higher transition energy threshold of Eu elements,more Eu elements are retained in the amorphous phase.Structural analysis indicates that the remarkable mechanical properties can be attributed to several factors,including the significant dissociation energy,strong cationic field,and stress transfer among Al_(2)O_(3),RE_(3)Al_(5)O_(12),ZrO_(2),and the amorphous phase.Additionally,the cocktail effect and plastic deformation mechanism of the high-entropy shell contribute to these properties.
