Pure magnesium is a very promising material in the fields of biomedical and engineering.Obtaining pure magnesium with superior mechanical properties has consistently been a significant challenge in the area of materia...Pure magnesium is a very promising material in the fields of biomedical and engineering.Obtaining pure magnesium with superior mechanical properties has consistently been a significant challenge in the area of materials science.This study focuses on investigating the processing method and strengthening mechanism of pure magnesium by ultra-precision cutting.The research results show that the pure magnesium grains were significantly refined after ultra-precision cutting.The average grain size reduced from∼24μm to nanometers,and the average nano-hardness increased from 1.02 GPa to 2.82 GPa.Amorphous pure magnesium structure and body-centered cubic(BCC)lattice pure magnesium were reported.Molecular dynamics(MD)simulation confirmed that the high shear strain and hydrostatic pressure during ultra-precision cutting was the origin of amorphization and lattice transformation.The amorphous phase and a significant number of long-period stacking-ordered(LPSO)phases inside the pure magnesium were responsible for the high hardness after ultra-precision cutting.展开更多
Hole-making for Carbon Fiber Reinforced Plastics(CFRP)/Ti-6Al-4V stacks is crucial for the assembling strength of aircraft structure parts.This work carried out experimental work for helical milling(HM)of the stacks w...Hole-making for Carbon Fiber Reinforced Plastics(CFRP)/Ti-6Al-4V stacks is crucial for the assembling strength of aircraft structure parts.This work carried out experimental work for helical milling(HM)of the stacks with sustainable cooling/lubrication(dry,MQL and cryogenic)conditions.Cutting forces and temperatures at the CFRP layer,Ti-6Al-4V layer and the interface of stacks were obtained by a developed measuring system.The temperatures in CFRP machining at cryogenic condition varied from-167℃to-94℃,which were much lower than those at dry and MQL conditions.The maximum temperature near the interface of stacks for the ninth hole was higher than 240℃due to heat conduction from Ti-6Al-4V layer.The hole quality,hole diameter and tool wear mechanism at different cooling/lubrication conditions were presented and discussed.MQL condition generated mainly extrusion fracture for the fibers,due to the reduced friction effect compared with dry condition.MQL was helpful to reduce the feed mark at the hole surface of Ti-6Al-4V alloy.The flank wear of cutting edge at MQL condition was better than those at dry and cryogenic conditions.Cryogenic cooling contributed to better CFRP surface with smaller delamination and hole entrance damage due to the increased resin strength and fiber brittleness.The damage near the entrance of CFRP were analyzed by the contact state of cutting edges and fibers.Additionally,hole diameters near the exit of CFPR layer were larger than other test positions.This work provided feasible processes for improving hole quality and tool life in hole-making of CFRP/Ti-6Al-4V stacks.展开更多
Polycrystalline materials are extensively employed in industry.Its surface roughness significantly affects the working performance.Material defects,particularly grain boundaries,have a great impact on the achieved sur...Polycrystalline materials are extensively employed in industry.Its surface roughness significantly affects the working performance.Material defects,particularly grain boundaries,have a great impact on the achieved surface roughness of polycrystalline materials.However,it is difficult to establish a purely theoretical model for surface roughness with consideration of the grain boundary effect using conventional analytical methods.In this work,a theoretical and deep learning hybrid model for predicting the surface roughness of diamond-turned polycrystalline materials is proposed.The kinematic–dynamic roughness component in relation to the tool profile duplication effect,work material plastic side flow,relative vibration between the diamond tool and workpiece,etc,is theoretically calculated.The material-defect roughness component is modeled with a cascade forward neural network.In the neural network,the ratio of maximum undeformed chip thickness to cutting edge radius RT S,work material properties(misorientation angle θ_(g) and grain size d_(g)),and spindle rotation speed n s are configured as input variables.The material-defect roughness component is set as the output variable.To validate the developed model,polycrystalline copper with a gradient distribution of grains prepared by friction stir processing is machined with various processing parameters and different diamond tools.Compared with the previously developed model,obvious improvement in the prediction accuracy is observed with this hybrid prediction model.Based on this model,the influences of different factors on the surface roughness of polycrystalline materials are discussed.The influencing mechanism of the misorientation angle and grain size is quantitatively analyzed.Two fracture modes,including transcrystalline and intercrystalline fractures at different RTS values,are observed.Meanwhile,optimal processing parameters are obtained with a simulated annealing algorithm.Cutting experiments are performed with the optimal parameters,and a flat surface finish with Sa 1.314 nm is finally achieved.The developed model and corresponding new findings in this work are beneficial for accurately predicting the surface roughness of polycrystalline materials and understanding the impacting mechanism of material defects in diamond turning.展开更多
Friction energy consumption is the primary cause of energy loss in rolling bearings,and friction characteristics are critical indicators of rolling bearing quality.To comprehensively understand the friction characteri...Friction energy consumption is the primary cause of energy loss in rolling bearings,and friction characteristics are critical indicators of rolling bearing quality.To comprehensively understand the friction characteristics of ball bearings,the equivalent friction coefficient was proposed,and a reliable measurement method was studied.This new solution addressed the difficulty of measuring the friction characteristics of ball bearings highlighted by friction torque.The angular speeds of various components in the rolling bearings were derived using a quasistatic approach.The angular speed relationships among various components of the rolling bearings were subsequently analyzed.A kinetic energy dissipation model for the measuring system was ultimately obtained.A novel apparatus for measuring the rolling bearing equivalent friction coefficient was established.The spindle only underwent angular speed attenuation due to friction of the tested bearing without the use of power,and the variation in kinetic energy was monitored in real time with a highprecision speed sensor.After that,the equivalent friction coefficients of the measured bearings were examined.The effects of speed,load,and lubrication on the equivalent friction coefficient of the tested bearing were studied.The findings demonstrated that,to some extent,the equivalent friction coefficient increased with an increase in spindle speed and decreased with increasing load.The equivalent friction coefficient also increased with increasing kinematic viscosity of the lubrication oil,and the friction coefficient for dry friction was greater than that with 50 oil(with a kinematic viscosity of 50 mm^(2)/s)but slightly lower than that with 70 oil(with a kinematic viscosity of 70 mm^(2)/s).With this method,an accurate and comprehensive understanding of the friction characteristics of ball bearings is achieved.展开更多
Implant-associated infections are generally difficult to cure owing to the bacterial antibiotic resistance which is attributed to the widespread usage of antibiotics.Given the global threat and increasing influence of...Implant-associated infections are generally difficult to cure owing to the bacterial antibiotic resistance which is attributed to the widespread usage of antibiotics.Given the global threat and increasing influence of antibiotic resistance,there is an urgent demand to explore novel antibacterial strategies other than using antibiotics.Recently,using a certain surface topography to provide a more persistent antibacterial solution attracts more and more attention.However,the clinical application of biomimetic nano-pillar array is not satisfactory,mainly because its antibacterial ability against Gram-positive strain is not good enough.Thus,the pillar array should be equipped with other antibacterial agents to fulfill the bacteriostatic and bactericidal requirements of clinical application.Here,we designed a novel model substrate which was a combination of periodic micro/nano-pillar array and TiO2 for basically understanding the topographical bacteriostatic effects of periodic micro/nano-pillar array and the photocatalytic bactericidal activity of TiO2.Such innovation may potentially exert the synergistic effects by integrating the persistent topographical antibacterial activity and the non-invasive X-ray induced photocatalytic antibacterial property of TiO2 to combat against antibiotic-resistant implant-associated infections.First,to separately verify the topographical antibacterial activity of TiO2 periodic micro/nano-pillar array,we systematically investigated its effects on bacterial adhesion,growth,proliferation,and viability in the dark without involving the photocatalysis of TiO2.The pillar array with sub-micron motif size can significantly inhibit the adhesion,growth,and proliferation of Staphylococcus aureus(S.aureus)and Escherichia coli(E.coli).Such antibacterial ability is mainly attributed to a spatial confinement size-effect and limited contact area availability generated by the special topography of pillar array.Moreover,the pillar array is not lethal to S.aureus and E.coli in 24 h.Then,the X-ray induced photocatalytic antibacterial property of TiO2 periodic micro/nano-pillar array in vitro and in vivo will be systematically studied in a future work.This study could shed light on the direction of surface topography design for future medical implants to combat against antibiotic-resistant implant-associated infections without using antibiotics.展开更多
基金the National Natural Science Foundation of China(Nos.52175430 and 52105478)for their support of this work.
文摘Pure magnesium is a very promising material in the fields of biomedical and engineering.Obtaining pure magnesium with superior mechanical properties has consistently been a significant challenge in the area of materials science.This study focuses on investigating the processing method and strengthening mechanism of pure magnesium by ultra-precision cutting.The research results show that the pure magnesium grains were significantly refined after ultra-precision cutting.The average grain size reduced from∼24μm to nanometers,and the average nano-hardness increased from 1.02 GPa to 2.82 GPa.Amorphous pure magnesium structure and body-centered cubic(BCC)lattice pure magnesium were reported.Molecular dynamics(MD)simulation confirmed that the high shear strain and hydrostatic pressure during ultra-precision cutting was the origin of amorphization and lattice transformation.The amorphous phase and a significant number of long-period stacking-ordered(LPSO)phases inside the pure magnesium were responsible for the high hardness after ultra-precision cutting.
基金co-supported by the National Key Research and Development Program(No.2017YFE0111300)Natural Science Foundation of China(No.51575384 and No.51675369)。
文摘Hole-making for Carbon Fiber Reinforced Plastics(CFRP)/Ti-6Al-4V stacks is crucial for the assembling strength of aircraft structure parts.This work carried out experimental work for helical milling(HM)of the stacks with sustainable cooling/lubrication(dry,MQL and cryogenic)conditions.Cutting forces and temperatures at the CFRP layer,Ti-6Al-4V layer and the interface of stacks were obtained by a developed measuring system.The temperatures in CFRP machining at cryogenic condition varied from-167℃to-94℃,which were much lower than those at dry and MQL conditions.The maximum temperature near the interface of stacks for the ninth hole was higher than 240℃due to heat conduction from Ti-6Al-4V layer.The hole quality,hole diameter and tool wear mechanism at different cooling/lubrication conditions were presented and discussed.MQL condition generated mainly extrusion fracture for the fibers,due to the reduced friction effect compared with dry condition.MQL was helpful to reduce the feed mark at the hole surface of Ti-6Al-4V alloy.The flank wear of cutting edge at MQL condition was better than those at dry and cryogenic conditions.Cryogenic cooling contributed to better CFRP surface with smaller delamination and hole entrance damage due to the increased resin strength and fiber brittleness.The damage near the entrance of CFRP were analyzed by the contact state of cutting edges and fibers.Additionally,hole diameters near the exit of CFPR layer were larger than other test positions.This work provided feasible processes for improving hole quality and tool life in hole-making of CFRP/Ti-6Al-4V stacks.
基金National Natural Science Foundation of China(Nos.52175430,51935008 and 52105478)China National Postdoctoral Program for Innovative Talents(BX20200234)Open Fund of Tianjin Key Laboratory of Equipment Design and Manufacturing Technology(EDMT)for the support of this work。
文摘Polycrystalline materials are extensively employed in industry.Its surface roughness significantly affects the working performance.Material defects,particularly grain boundaries,have a great impact on the achieved surface roughness of polycrystalline materials.However,it is difficult to establish a purely theoretical model for surface roughness with consideration of the grain boundary effect using conventional analytical methods.In this work,a theoretical and deep learning hybrid model for predicting the surface roughness of diamond-turned polycrystalline materials is proposed.The kinematic–dynamic roughness component in relation to the tool profile duplication effect,work material plastic side flow,relative vibration between the diamond tool and workpiece,etc,is theoretically calculated.The material-defect roughness component is modeled with a cascade forward neural network.In the neural network,the ratio of maximum undeformed chip thickness to cutting edge radius RT S,work material properties(misorientation angle θ_(g) and grain size d_(g)),and spindle rotation speed n s are configured as input variables.The material-defect roughness component is set as the output variable.To validate the developed model,polycrystalline copper with a gradient distribution of grains prepared by friction stir processing is machined with various processing parameters and different diamond tools.Compared with the previously developed model,obvious improvement in the prediction accuracy is observed with this hybrid prediction model.Based on this model,the influences of different factors on the surface roughness of polycrystalline materials are discussed.The influencing mechanism of the misorientation angle and grain size is quantitatively analyzed.Two fracture modes,including transcrystalline and intercrystalline fractures at different RTS values,are observed.Meanwhile,optimal processing parameters are obtained with a simulated annealing algorithm.Cutting experiments are performed with the optimal parameters,and a flat surface finish with Sa 1.314 nm is finally achieved.The developed model and corresponding new findings in this work are beneficial for accurately predicting the surface roughness of polycrystalline materials and understanding the impacting mechanism of material defects in diamond turning.
基金supported by the National Natural Science Foundation of China(Nos.52175430 and 52105478)the Graduate Innovation Foundation Program of Tianjin(No.2021YJSB178).
文摘Friction energy consumption is the primary cause of energy loss in rolling bearings,and friction characteristics are critical indicators of rolling bearing quality.To comprehensively understand the friction characteristics of ball bearings,the equivalent friction coefficient was proposed,and a reliable measurement method was studied.This new solution addressed the difficulty of measuring the friction characteristics of ball bearings highlighted by friction torque.The angular speeds of various components in the rolling bearings were derived using a quasistatic approach.The angular speed relationships among various components of the rolling bearings were subsequently analyzed.A kinetic energy dissipation model for the measuring system was ultimately obtained.A novel apparatus for measuring the rolling bearing equivalent friction coefficient was established.The spindle only underwent angular speed attenuation due to friction of the tested bearing without the use of power,and the variation in kinetic energy was monitored in real time with a highprecision speed sensor.After that,the equivalent friction coefficients of the measured bearings were examined.The effects of speed,load,and lubrication on the equivalent friction coefficient of the tested bearing were studied.The findings demonstrated that,to some extent,the equivalent friction coefficient increased with an increase in spindle speed and decreased with increasing load.The equivalent friction coefficient also increased with increasing kinematic viscosity of the lubrication oil,and the friction coefficient for dry friction was greater than that with 50 oil(with a kinematic viscosity of 50 mm^(2)/s)but slightly lower than that with 70 oil(with a kinematic viscosity of 70 mm^(2)/s).With this method,an accurate and comprehensive understanding of the friction characteristics of ball bearings is achieved.
基金supported by the Natural Science Foundation of Tianjin(General Program,No.18JCYBJC19500)the Independent Innovation Fund of Tianjin University(No.2019XZS-0014)the Research Grants Council of Hong Kong(No.HKUST615408).
文摘Implant-associated infections are generally difficult to cure owing to the bacterial antibiotic resistance which is attributed to the widespread usage of antibiotics.Given the global threat and increasing influence of antibiotic resistance,there is an urgent demand to explore novel antibacterial strategies other than using antibiotics.Recently,using a certain surface topography to provide a more persistent antibacterial solution attracts more and more attention.However,the clinical application of biomimetic nano-pillar array is not satisfactory,mainly because its antibacterial ability against Gram-positive strain is not good enough.Thus,the pillar array should be equipped with other antibacterial agents to fulfill the bacteriostatic and bactericidal requirements of clinical application.Here,we designed a novel model substrate which was a combination of periodic micro/nano-pillar array and TiO2 for basically understanding the topographical bacteriostatic effects of periodic micro/nano-pillar array and the photocatalytic bactericidal activity of TiO2.Such innovation may potentially exert the synergistic effects by integrating the persistent topographical antibacterial activity and the non-invasive X-ray induced photocatalytic antibacterial property of TiO2 to combat against antibiotic-resistant implant-associated infections.First,to separately verify the topographical antibacterial activity of TiO2 periodic micro/nano-pillar array,we systematically investigated its effects on bacterial adhesion,growth,proliferation,and viability in the dark without involving the photocatalysis of TiO2.The pillar array with sub-micron motif size can significantly inhibit the adhesion,growth,and proliferation of Staphylococcus aureus(S.aureus)and Escherichia coli(E.coli).Such antibacterial ability is mainly attributed to a spatial confinement size-effect and limited contact area availability generated by the special topography of pillar array.Moreover,the pillar array is not lethal to S.aureus and E.coli in 24 h.Then,the X-ray induced photocatalytic antibacterial property of TiO2 periodic micro/nano-pillar array in vitro and in vivo will be systematically studied in a future work.This study could shed light on the direction of surface topography design for future medical implants to combat against antibiotic-resistant implant-associated infections without using antibiotics.
