In the directed energy deposition(DED)process with high heat input,repeated heating and cooling cycles in the deposited layers have a signifcant efect on the microstructure.Because of the diferences in the cyclic numb...In the directed energy deposition(DED)process with high heat input,repeated heating and cooling cycles in the deposited layers have a signifcant efect on the microstructure.Because of the diferences in the cyclic numbers and peak temperatures from the lower layer to the upper layer,inhomogeneous microstructures are formed in the as-built components.In this work,a cyclic heat treatment(CHT)with gradual cooling was used to simulate the thermal process during the DED process of Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe(Ti5321)near-βTi alloy.The efect of CHT on the microstructural evolution,especially the spheroidization ofαphase,was investigated.As the CHT cycle increased,the volume fraction ofαphase gradually increased from 35.9%after 1 cycle to 60.9%after 100 cycles,and the length ofαphase frst increased and then gradually decreased,while the width ofαphase increased slowly.The aspect ratio ofαphase decreased from 9.90±3.39 after 1 cycle to 2.37±0.87 after 100 cycles,implying that CHT inducedαphase spheroidization.This phenomenon resulted from both the boundary splitting mechanism and the termination migration mechanism during CHT.The evolution of microstructure afects its mechanical properties.As the CHT cycles increased,the hardness increased overall,from 342.8±5.3 HV after 1 cycle to 400.3±3.4 HV after 100 cycles.This work provides a potential method to tailor the microstructure of near-βTi alloys by heat treatment alone,especially for non-deformable additively manufactured metal components.展开更多
Traditional heat treatment methods require a significant amount of time and energy to affect atomic diffusion and enhance the spheroidization process of carbides in bearing steel,while pulsed current can accelerate at...Traditional heat treatment methods require a significant amount of time and energy to affect atomic diffusion and enhance the spheroidization process of carbides in bearing steel,while pulsed current can accelerate atomic diffusion to achieve ultra-fast spheroidization of carbides.However,the understanding of the mechanism by which different pulse current parameters regulate the dissolution behavior of carbides requires a large amount of experimental data to support,which limits the application of pulse current technology in the field of heat treatment.Based on this,quantify the obtained pulse current processing data to create an important dataset that could be applied to machine learning.Through machine learning,the mechanism of mutual influence between carbide regulation and various factors was elucidated,and the optimal spheroidization process parameters were determined.Compared to the 20 h required for traditional heat treatment,the application of pulsed electric current technology achieved ultra-fast spheroidization of GCr15 bearing steel within 90 min.展开更多
Eutectic high-entropy alloys(EHEAs),as a classification of high-entropy alloys(HEAs),have received worldwide interest due to superior fluidity and attractive properties.However,other than the FCC+B2 EHEA system,most o...Eutectic high-entropy alloys(EHEAs),as a classification of high-entropy alloys(HEAs),have received worldwide interest due to superior fluidity and attractive properties.However,other than the FCC+B2 EHEA system,most other reported EHEA systems show inherent brittleness during tensile loading at room temperature,which limits their advanced engineering application.In this work,a novel spheroidization+recrystallization(SR)strategy for synergistic strengthening and plasticizing of the brit-tle CoCrFeNi_(2)(V_(6)B_(3)Si)_(0.149)was proposed.The superior combination of strength and ductility was achieved by tailoring spherical M3 B2+recrystallized FCC duplex phases.Based on this strategy,the yield strength and elongation were improved from 565±15 MPa and 2.3%±0.3%to 841±24-1278±20 MPa and 14.7%±0.5%-22.5%±1.2%,with an increase of 48%-126%and 539%-878%,respectively.The synergistic increment in the strength and ductility of SR-FCC+M_(3)B_(2)EHEAs exceeds all reported fur-ther strengthened FCC+B2 EHEAs.Meanwhile,such simple thermo-mechanical processing is suitable for large-scale industrial production.The high strength results from the back stress provided by the dual heterogeneity of FCC grain sizes and soft FCC/hard M_(3)B_(2).The good ductility is attributed to the dislo-cation movement path released by spheroidized M3 B2 and a more uniform stress distribution caused by the recrystallized FCC.This work provides a new strategy for synergistic strengthening and plasticizing of the brittle EHEAs to meet industrial reliability requirements.展开更多
Laser three-dimensional(3D)printing offers significant advantages in integrating the shape and function of regen-erative tissues through biomimetic manufacturing.However,its effectiveness is limited by the lack of spe...Laser three-dimensional(3D)printing offers significant advantages in integrating the shape and function of regen-erative tissues through biomimetic manufacturing.However,its effectiveness is limited by the lack of specialized biopolymer powders-while solvent methods that use residual solvents produce powders with poor biocompati-bility,mechanical methods result in irregularly shaped crystals.In this study,a biopolymer powder spheroidiza-tion and shaping technology,which utilizes the evolution of irregular powders into spheres with minimal surface free energy in the molten state,is proposed based on the thermodynamic principle of minimum energy.Initially,the motion trajectory and temperature field of the poly(L-lactic acid)(PLLA)powder during spheroidization were quantitatively assessed and optimized using Stokes’law and Fourier’s principle.Subsequently,the cohesive forces and aggregation kinetics of the polymer chains were calculated using molecular dynamics.Finally,based on these calculations,a phase-field model was constructed to simulate the evolution of the spheroidization rate and deduce the optimal parameters for the process.This precise approach enhances PLLA spheroidization control for laser 3D printing,improves part densification and surface quality,and offers a clean and efficient path for preparing high-quality PLLA spheroidized powder for laser 3D printing.展开更多
Abstract： The dynamic spheroidization kinetics behavior of Ti-6.5Al-2Zr-1Mo-1V alloy with a lamellar initial microstructure was studied by isothermal hot compression tests in the temperature range of 750-950℃ and st...Abstract： The dynamic spheroidization kinetics behavior of Ti-6.5Al-2Zr-1Mo-1V alloy with a lamellar initial microstructure was studied by isothermal hot compression tests in the temperature range of 750-950℃ and strain rates of 0.001-10 s^-1. The results show that the spheroidized fraction of alpha lamellae increases with the increase of temperature and the decrease of strain rate. The spheroidization kinetics curves predicted by JMAK equation agree well with experimental ones. The corresponding SEM and TEM observations indicate that the dynamic spheroidization process can be divided into two stages. The primary stage is boundary splitting formed by two competing mechanisms which are dynamic recrystallization and mechanical twin. In the second stage, the penetration of beta phase into the alpha/alpha grain boundaries is dominant, which is controlled in nature by diffusion of the chemical elements such as Al, Mo and V.展开更多
Spheroidizing annealing and torsion testing of 0.027 wt% carbon steel rod were conducted to evaluate spheroidization kinetic behavior at 943 K （670 ℃） under deformed and non-deformed states. Kinetic curves were als...Spheroidizing annealing and torsion testing of 0.027 wt% carbon steel rod were conducted to evaluate spheroidization kinetic behavior at 943 K （670 ℃） under deformed and non-deformed states. Kinetic curves were also predicted using the Johnson-Mehl-Avrami-Kolmogorov equation, and the results agree well with the experimental ones. After spheroidization was performed twice, the spherical cementite and precipitated carbides became smaller and the distribution was more uniform. Comparison of materials subjected to single and double spheroidizing annealing indicated a difference in grain size. Torsion performance was considerably improved under double spheroidization, especially the maximum torque with slight variations.展开更多
During the plasma spheroidization process powders undergo different changes in their microstructures and crystal phases. In this paper, simple calculation of heat transfer between the plasma and a suspended particle w...During the plasma spheroidization process powders undergo different changes in their microstructures and crystal phases. In this paper, simple calculation of heat transfer between the plasma and a suspended particle was performed based on three hypotheses for the purpose of guiding experiments. Experimental investigation of the crystal phases and microstructural changes during the plasma processing was made using silica, alumina and nickel powders as starting materials. It has been revealed from the experimental results that these materials undergo different changes in crystal phases and microstructures, and these changes are essentially determined by the structures, properties and aggregate states of the starting materials.展开更多
Amorphous spherical silica powders were prepared by inductively coupled thermal plasma treatment at a radio frequency of 36.2 MHz. The effects of the added content of hydrogen and nitrogen into argon(serving as the sh...Amorphous spherical silica powders were prepared by inductively coupled thermal plasma treatment at a radio frequency of 36.2 MHz. The effects of the added content of hydrogen and nitrogen into argon(serving as the sheath gas), as well as the carrier gas flow rate, on the spheroidization rate of silica powders, were investigated. The prepared silica powders before and after plasma treatment were examined by scanning electron microscopy, X-ray diffraction, and laser granulometric analysis. Results indicated that the average size of the silica particles increased, and the transformation of crystals into the amorphous state occurred after plasma treatment. Discharge image processing was employed to analyze the effect of the plasma temperature field on the spheroidization rate. The spheroidization rate of the silica powder increased with the increase of the hydrogen content in the sheath gas. On the other hand, the spheroidization rate of the silica power first increased and then decreased with the increase of the nitrogen content in the sheath gas. Moreover, the amorphous content increased with the increase of the spheroidization rate of the silica powder.展开更多
The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to ana...The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.展开更多
The fractal dimension（FD） of surfaces has been widely used to characterize the properties of materials.However,most of the previous researches were concentrated on the correlation between the FD of surfaces and mech...The fractal dimension（FD） of surfaces has been widely used to characterize the properties of materials.However,most of the previous researches were concentrated on the correlation between the FD of surfaces and mechanical properties of materials,such as impact energy and fracture toughness,etc.The aim of this paper is to characterize the spheroidization grade and strength of 15CrMo steel through determination of FD of cementite phase on the basis of two-dimension microstructural image.Two methods,namely slit-island method（SIM） and box-counting method（BCM）,are used to determine the value of FD.It is found that the FD value evaluated by using BCM is generally higher than that evaluated by SIM.This phenomenon may be due to the difference in the principles used in different methods.Whether SIM or BCM is used,the spheroidization grade in 15CrMo steel linearly increases with decreasing the value of FD.The relationship between the FD value,D,and spheroidization grade,Sg,can be approximately expressed as D≌-0.11Sg＋A,where A is a constant value which is depended on the evaluation method.Both the ultimate strength and the yielding strength of 15CrMo steel increase with increasing FD of cementite phase.There may be a common relationship between the FD of cementite phase and strength of 15CrMo steel.When the FD of cementite phase in 15CrMo steel is determined,the strength of this steel can be evaluated.The present paper can provide a novel method to evaluate the strength and spheroidization grade of carbon steel through determination of fractal dimension（FD） of cementite phase.展开更多
Spherical powders with good flowability and high stacking density are mandatory for powder bed additive manufacturing. Nevertheless, the preparation of spherical refractory tungsten and tungsten alloy powders is a for...Spherical powders with good flowability and high stacking density are mandatory for powder bed additive manufacturing. Nevertheless, the preparation of spherical refractory tungsten and tungsten alloy powders is a formidable task. In this paper, spherical refractory metal powders processed by high-energy stir ball milling and RF inductively coupled plasma were investigated. By utilizing the technical route, pure spherical tungsten powders were prepared successfully, the flowability increased from 10.7 s/50 g to 5.5 s/50 g and apparent density increased from 6.916 g cm-3 to 11.041 g cm-3. Alloying element tantalum can reduce the tendency to micro- crack during tungsten laser melting and rapid solidification process. Spherical W-6Ta （%wt） powders were prepared in this way, homogeneous dispersion of tantalum in a tungsten matrix occurred but a small amount of flake-like shape particles appeared after high-energy stir ball milling. The flake-like shape particles can hardly be spheroidized in subsequent RF inductively coupled plasma process, might result from the unique suspended state of flaky particles under complex electric and magnetic fields as well as plasma-particle heat exchange was different under various turbulence models. As a result, the flake-like shape particles cannot pass through the high-temperature area of thermal plasma torch and cannot be spheroidized properly.展开更多
The spheroidization of the lamellar structure can greatly contribute to the superplasticity of the nugget zone(NZ)of Ti alloy welds,which is the key to achieve the integral superplastic forming of welds for the fabric...The spheroidization of the lamellar structure can greatly contribute to the superplasticity of the nugget zone(NZ)of Ti alloy welds,which is the key to achieve the integral superplastic forming of welds for the fabrication of large-scale complex components.However,the spheroidization process is complex and costly since it cannot be obtained generally,unless the lamellae suffers from a large deformation.In this study,the static spheroidization was achieved for the fine lamellae structure in the nugget of a friction stir welded(FSW)Ti-6Al-4V joint,particularly by the annealing without any deformation.The specialα/βinterface obeying a Burgers orientation relationship(BOR)after FSW was first time directly observed,whose effect on the spheroidization was discussed.A new static spheroidization mechanism with the gradual coalescence of the adjacent lamellae was discovered,which we named as“termination coalescence”.There was a slower coarsening rate in the lamellar structure than in the classical equiaxed one,due to the BOR in the lamellae,although both of them exhibited a volume diffusion character during annealing.Consequently,the similar superplasticity can be achieved for the base material and NZ after annealing.This study can provide a new way to the spheroidization and a theoretical basis for the integral superplastic forming of welds during production.展开更多
A cost-effective Fe-enriched eutectic high-entropy alloy(EHEA),Fe35Ni25Cr25Mo15,was designed and prepared to avoid the use of expensive Co that is commonly used in HEAs.However,the as-cast Feenriched EHEA was associat...A cost-effective Fe-enriched eutectic high-entropy alloy(EHEA),Fe35Ni25Cr25Mo15,was designed and prepared to avoid the use of expensive Co that is commonly used in HEAs.However,the as-cast Feenriched EHEA was associated with brittleness.The present work aims to evaluate the possibility and feasibility of spheroidization of the lamellar structure of the EH EA in order to improve the ductility.Due to the high cooling rate of arc-melting,the as-melted Fe35Ni25Cr25Mo15 EHEA was found to be a pseudo eutectic alloy comprised of alternantσphase(Cr(0.22)Mo0.18Fe0.6-type intermetallic)and face centred cubic(FCC)phase.The lamellar structure in the Fe-enriched EHEA remained stable up to 800Κ.The instability of the lamellar structure occurred at temperatures over 800℃,which was resulted from migration of high-density faults(i.e.lamellar termination and ledges in the lamellae).However,the Fe35Ni25Cr25Mo15EHEA still exhibited brittleness even after spheroidization at 1100℃for 168 h due to the formation of the hard and brittleσmatrix in the pseudo Fe35Ni25Cr25Mo15 EHEA as a result of decomposition of the lamellar structure.Therefore,in contrast to the softening of traditional eutectic alloys,spheroidization treatment was considered as invalid to improve the ductility of pseudo-eutectic HEA with high fraction of intermetallic phase.The present work provides a valuable re ference for those who aim to improve the ductility of brittle EHEA through spheroidization.展开更多
We investigate characteristics of multi-arc torches with three pairs of electrodes(three cathodes and three anodes)and their performance on the spheroidization of SiO2 powder.The effect of electrode arrangement,includ...We investigate characteristics of multi-arc torches with three pairs of electrodes(three cathodes and three anodes)and their performance on the spheroidization of SiO2 powder.The effect of electrode arrangement,including adjacent pattern(AD pattern,adjacent electrodes powered by one power supply)and opposite pattern(OP pattern,opposite electrodes powered by one power supply),on the dynamics of arc plasma is investigated based on synchronous acquisition of electrical and optical signals.The results show that both the voltage and spatial distribution of each arc of multiple arcs are more stable compared with those of a single arc.The fluctuation of an arc in multiple arcs mainly comes from the small-scale arc-to-arc restrikes among multiple arcs.Moreover,these arc-to-arc restrikes occur more frequently among multiple arc columns in OP pattern than in AD pattern.Moreover,the high-temperature area of the central region of arc chamber in OP pattern is larger than that in AP pattern.For the spheroidization of SiO2 in this multi-arc generator,the spheronization degrees of plasma treated silica in OP pattern are at least 20%higher than those in AD pattern.展开更多
Arc plasma torch is an effective tool for spheroidization of metallic powders.However,as most conventional plasma torches were not specifically designed for plasma spheroidization,they may exhibit the disadvantages of...Arc plasma torch is an effective tool for spheroidization of metallic powders.However,as most conventional plasma torches were not specifically designed for plasma spheroidization,they may exhibit the disadvantages of the radial injection of powders,large fluctuations in the arc voltage,large gas flow rate,and disequilibrium between multiple plasma jets during the spheroidization process.Therefore,this paper presents a triple-cathode cascade plasma torch(TCCPT)for plasma spheroidization.Its structural design,including three cathodes,a common anode,and three sets of inter-electrodes,are detailed to ensure that powders can be inserted into the plasma jet by axial injection,the arc voltage fluctuations are easily maintained at a low level,and the plasma torches can work at a relatively small gas flow rate.Experimental results showed that the proposed TCCPT exhibits the following characteristics:(1)a relatively small arc voltage fluctuation within 5.3%;(2)a relatively high arc voltage of 75 V and low gas flow rate range of10-30 SLM;(3)easy to be maintained at the equilibrium state with the equilibrium index of the three plasma jets within 3.5 V.Furthermore,plasma spheroidization experiments of SUS304 stainless steel powers were carried out using the proposed TCCPT.Results verified that the proposed TCCPT is applicable and effective for the spheroidization of metallic powders with wide size distribution.展开更多
To control the morphology and particle size of dense spherical molybdenum powder prepared by radio frequency（RF） plasma from irregular molybdenum powder as a precursor, plasma process parameters were optimized in th...To control the morphology and particle size of dense spherical molybdenum powder prepared by radio frequency（RF） plasma from irregular molybdenum powder as a precursor, plasma process parameters were optimized in this paper. The effects of the carrier gas flow rate and molybdenum powder feeding rate on the shape and size of the final products were studied. The molybdenum powder morphology was examined using high-resolution scanning electron microscopy. The powder phases were analyzed by X-ray diffraction. The tap density and apparent density of the molybdenum powder were investigated using a Hall flow meter and a Scott volumeter. The optimal process parameters for the spherical molybdenum powder preparation are 50 g/min powder feeding rate and 0.6 m^3/h carrier gas rate. In addition, pure spherical molybdenum powder can be obtained from irregular powder, and the tap density is enhanced after plasma processing. The average size is reduced from 72 to 62 μm, and the tap density is increased from 2.7 to 6.2 g/cm^3. Therefore, RF plasma is a promising method for the preparation of high-density and high-purity spherical powders.展开更多
In this study,the spheroidization process of large-deformed steel under various conditions was researched.Steel with a high carbon content （1.0％ C） was first treated thermomechanically using multipass rolling.Then ...In this study,the spheroidization process of large-deformed steel under various conditions was researched.Steel with a high carbon content （1.0％ C） was first treated thermomechanically using multipass rolling.Then it underwent spheroidization treatments at different heating temperatures,using various heating times and cooling rates.Spheroidization processes with a lower heating temperature,shorter heating time,or faster cooling rate than those of the traditional process all showed good results,indicating that the spheroidization process was promoted significantly by the large deformation process.Grain refinement and fragmentation of cementite,along with the large deformation process,promoted this spheroidization process.展开更多
Conventionally manufactured 35CrMo cold heading steel must undergo spheroidization annealing before the cold heading process In this paper, different types of deformation processes with various controlled cooling peri...Conventionally manufactured 35CrMo cold heading steel must undergo spheroidization annealing before the cold heading process In this paper, different types of deformation processes with various controlled cooling periods were operated to achieve on-line spheroidal cementite using the Gleeble-3500 simulation technique. According to the measured dynamic ferrite transformation temperature （Ad3）, the deformation could be divided into two types： low temperature deformation at 810 and 780℃; ＂deformation-induced ferrite transformation＂ （DIFT） deformation at 750 and 720℃. Compared with the low temperature deformation, the DIFT deformation followed by accelerated cooling to 680℃ is beneficial for the formation of spheroidal cementite. Samples subjected to both the low-temperature deformation and DIFT deformation can obtain granular bainite by accelerated cooling to 640℃; the latter may contribute to the formation of a fine dispersion of secondary constituents. Granular bainite can transform into globular pearlite rapidly during subcritical annealing, and the more the disperse phase, the more homogeneously distributed globular cementite can be obtained.展开更多
In this work,the corrosion behavior of medium-carbon steels(45,45 Cu and 45 Cuq steels)in acidic chloride environment was investigated.The results indicated that the micro-galvanic effect between the anodic ferrite ma...In this work,the corrosion behavior of medium-carbon steels(45,45 Cu and 45 Cuq steels)in acidic chloride environment was investigated.The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels.For 45 steel,serious pitting corrosion happened in and around the pearlite regions,and a large number of lamellar cementite was fixed in the corrosion pits.Meanwhile,the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion,resulting in a rapidly increase in corrosion rate with time.While for 45 Cu and 45 Cuq steels,macroscopic uniform corrosion occurred,and the cementite accumulation was markedly reduced as compared with 45 steel,thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly.Among these,45 Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion,indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying,thereby significantly improving the corrosion resistance of 45 steel.展开更多
The research aims to provide an alternative to austempering treatment of ductile cast iron with a simple and cost-effective heat-treatment process.This goal was accomplished by applying a simple one-step spheroidizati...The research aims to provide an alternative to austempering treatment of ductile cast iron with a simple and cost-effective heat-treatment process.This goal was accomplished by applying a simple one-step spheroidization heat treatment to the as-cast ductile iron,which would normally possess a coarse pearlitic microstructure to a significant extent.Spheroidization experiments involving isothermal holding below the lower critical temperature(A1)were conducted followed by standard mechanical testing and microstructural characterization for an experimental ductile iron.After improving the spheroidization holding time at a given temperature,the work shows that the ductility and toughness of an as-cast ductile iron can be improved by 90%and 40%,respectively,at the cost of reducing the tensile strength by 8%.Controlled discretization of the continuous cementite network in pearlitic matrix of the ductile iron is deemed responsible for the improved properties.The work also shows that prolonged holding time during spheroidization heat treatment leads to degradation of mechanical properties due to the inhomogenous microstructure formation caused by heterogeneous decomposition and cementite clustering in the material.The main outcome of this work is the demonstration of ductile cast iron’s necking behavior due to spheroidization heat treatment.展开更多
基金sponsored by the National Natural Science Foundation of China(No.52271108)the Foundation of Xi’an Key Laboratory of High-Performance Titanium Alloy(No.NIN-HTL-2022-02)+2 种基金the Natural Science Foundation of Shanghai(No.21ZR1445100)the Shanghai Science and Technology Development Funds(No.22QB1406500)the ECU DVC Strategic Research Support Fund(No.23965).
文摘In the directed energy deposition(DED)process with high heat input,repeated heating and cooling cycles in the deposited layers have a signifcant efect on the microstructure.Because of the diferences in the cyclic numbers and peak temperatures from the lower layer to the upper layer,inhomogeneous microstructures are formed in the as-built components.In this work,a cyclic heat treatment(CHT)with gradual cooling was used to simulate the thermal process during the DED process of Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe(Ti5321)near-βTi alloy.The efect of CHT on the microstructural evolution,especially the spheroidization ofαphase,was investigated.As the CHT cycle increased,the volume fraction ofαphase gradually increased from 35.9%after 1 cycle to 60.9%after 100 cycles,and the length ofαphase frst increased and then gradually decreased,while the width ofαphase increased slowly.The aspect ratio ofαphase decreased from 9.90±3.39 after 1 cycle to 2.37±0.87 after 100 cycles,implying that CHT inducedαphase spheroidization.This phenomenon resulted from both the boundary splitting mechanism and the termination migration mechanism during CHT.The evolution of microstructure afects its mechanical properties.As the CHT cycles increased,the hardness increased overall,from 342.8±5.3 HV after 1 cycle to 400.3±3.4 HV after 100 cycles.This work provides a potential method to tailor the microstructure of near-βTi alloys by heat treatment alone,especially for non-deformable additively manufactured metal components.
基金supported by the National Key R&D Program of China(2020YFA0714900,2023YFB3709903)the National Natural Science Foundation of China(U21B2082,52474410)+6 种基金the Key R&D Program of Shandong Province,China(2023CXGC010406)the Scientific Research Special Project for First-Class Disciplines in Inner Mongolia Autonomous Region(YLXKZX-NKD-001)the International Science and Technology Cooperation Project of Higher Education Institutions in Inner Mongolia Autonomous Region(GHXM-002)the Natural Science Foundation of Inner Mongolia Autonomous Region of China(2024ZD06)the Technology Support Project for the Construction of Major Innovation Platforms in Inner Mongolia Autonomous Region(XM2024XTGXQ16)the Beijing Municipal Natural Science Foundation(2222065)the Fundamental Research Funds for the Central Universities(FRF-TP-22-02C2).
文摘Traditional heat treatment methods require a significant amount of time and energy to affect atomic diffusion and enhance the spheroidization process of carbides in bearing steel,while pulsed current can accelerate atomic diffusion to achieve ultra-fast spheroidization of carbides.However,the understanding of the mechanism by which different pulse current parameters regulate the dissolution behavior of carbides requires a large amount of experimental data to support,which limits the application of pulse current technology in the field of heat treatment.Based on this,quantify the obtained pulse current processing data to create an important dataset that could be applied to machine learning.Through machine learning,the mechanism of mutual influence between carbide regulation and various factors was elucidated,and the optimal spheroidization process parameters were determined.Compared to the 20 h required for traditional heat treatment,the application of pulsed electric current technology achieved ultra-fast spheroidization of GCr15 bearing steel within 90 min.
基金This work was supported by the National Key Research and De-velopment Program of China(No.2018YFA0702901)National Nat-ural Science Foundation of China(No.U20A20278)Liao Ning Revitalization Talents Program(No.XLYC1807047).
文摘Eutectic high-entropy alloys(EHEAs),as a classification of high-entropy alloys(HEAs),have received worldwide interest due to superior fluidity and attractive properties.However,other than the FCC+B2 EHEA system,most other reported EHEA systems show inherent brittleness during tensile loading at room temperature,which limits their advanced engineering application.In this work,a novel spheroidization+recrystallization(SR)strategy for synergistic strengthening and plasticizing of the brit-tle CoCrFeNi_(2)(V_(6)B_(3)Si)_(0.149)was proposed.The superior combination of strength and ductility was achieved by tailoring spherical M3 B2+recrystallized FCC duplex phases.Based on this strategy,the yield strength and elongation were improved from 565±15 MPa and 2.3%±0.3%to 841±24-1278±20 MPa and 14.7%±0.5%-22.5%±1.2%,with an increase of 48%-126%and 539%-878%,respectively.The synergistic increment in the strength and ductility of SR-FCC+M_(3)B_(2)EHEAs exceeds all reported fur-ther strengthened FCC+B2 EHEAs.Meanwhile,such simple thermo-mechanical processing is suitable for large-scale industrial production.The high strength results from the back stress provided by the dual heterogeneity of FCC grain sizes and soft FCC/hard M_(3)B_(2).The good ductility is attributed to the dislo-cation movement path released by spheroidized M3 B2 and a more uniform stress distribution caused by the recrystallized FCC.This work provides a new strategy for synergistic strengthening and plasticizing of the brittle EHEAs to meet industrial reliability requirements.
基金supported by National Key Research and Development Program of China(Grant No.2023YFB4605800)Natural Science Foundation of China(Grant Nos.51935014,52365046,52105352,82072084)+1 种基金JiangXi Provincial Natural Science Foundation of China(Grant No.20224ACB204013)Schig-Qinling Program(Grant No.2022360702014891).
文摘Laser three-dimensional(3D)printing offers significant advantages in integrating the shape and function of regen-erative tissues through biomimetic manufacturing.However,its effectiveness is limited by the lack of specialized biopolymer powders-while solvent methods that use residual solvents produce powders with poor biocompati-bility,mechanical methods result in irregularly shaped crystals.In this study,a biopolymer powder spheroidiza-tion and shaping technology,which utilizes the evolution of irregular powders into spheres with minimal surface free energy in the molten state,is proposed based on the thermodynamic principle of minimum energy.Initially,the motion trajectory and temperature field of the poly(L-lactic acid)(PLLA)powder during spheroidization were quantitatively assessed and optimized using Stokes’law and Fourier’s principle.Subsequently,the cohesive forces and aggregation kinetics of the polymer chains were calculated using molecular dynamics.Finally,based on these calculations,a phase-field model was constructed to simulate the evolution of the spheroidization rate and deduce the optimal parameters for the process.This precise approach enhances PLLA spheroidization control for laser 3D printing,improves part densification and surface quality,and offers a clean and efficient path for preparing high-quality PLLA spheroidized powder for laser 3D printing.
基金Project(2014ZE56015)supported by Aeronautical Science Foundation of ChinaProject(51261020)supported by the National Natural Science Foundation of ChinaProject(Zk201001004)supported by the Open Fund of the Aeronautical Science and Technology Key Laboratory of Aeronautical Material Hot Processing Technology,China
文摘Abstract： The dynamic spheroidization kinetics behavior of Ti-6.5Al-2Zr-1Mo-1V alloy with a lamellar initial microstructure was studied by isothermal hot compression tests in the temperature range of 750-950℃ and strain rates of 0.001-10 s^-1. The results show that the spheroidized fraction of alpha lamellae increases with the increase of temperature and the decrease of strain rate. The spheroidization kinetics curves predicted by JMAK equation agree well with experimental ones. The corresponding SEM and TEM observations indicate that the dynamic spheroidization process can be divided into two stages. The primary stage is boundary splitting formed by two competing mechanisms which are dynamic recrystallization and mechanical twin. In the second stage, the penetration of beta phase into the alpha/alpha grain boundaries is dominant, which is controlled in nature by diffusion of the chemical elements such as Al, Mo and V.
基金This work was supported by the National Natural Science Foundation of China （Grant No. 51271035）.
文摘Spheroidizing annealing and torsion testing of 0.027 wt% carbon steel rod were conducted to evaluate spheroidization kinetic behavior at 943 K （670 ℃） under deformed and non-deformed states. Kinetic curves were also predicted using the Johnson-Mehl-Avrami-Kolmogorov equation, and the results agree well with the experimental ones. After spheroidization was performed twice, the spherical cementite and precipitated carbides became smaller and the distribution was more uniform. Comparison of materials subjected to single and double spheroidizing annealing indicated a difference in grain size. Torsion performance was considerably improved under double spheroidization, especially the maximum torque with slight variations.
基金supported by National Natural Science Foundation of China (No.50574083)
文摘During the plasma spheroidization process powders undergo different changes in their microstructures and crystal phases. In this paper, simple calculation of heat transfer between the plasma and a suspended particle was performed based on three hypotheses for the purpose of guiding experiments. Experimental investigation of the crystal phases and microstructural changes during the plasma processing was made using silica, alumina and nickel powders as starting materials. It has been revealed from the experimental results that these materials undergo different changes in crystal phases and microstructures, and these changes are essentially determined by the structures, properties and aggregate states of the starting materials.
基金financially supported by the National Natural Science Foundation of China (Nos. 21377133, 11535003, and 11405221)
文摘Amorphous spherical silica powders were prepared by inductively coupled thermal plasma treatment at a radio frequency of 36.2 MHz. The effects of the added content of hydrogen and nitrogen into argon(serving as the sheath gas), as well as the carrier gas flow rate, on the spheroidization rate of silica powders, were investigated. The prepared silica powders before and after plasma treatment were examined by scanning electron microscopy, X-ray diffraction, and laser granulometric analysis. Results indicated that the average size of the silica particles increased, and the transformation of crystals into the amorphous state occurred after plasma treatment. Discharge image processing was employed to analyze the effect of the plasma temperature field on the spheroidization rate. The spheroidization rate of the silica powder increased with the increase of the hydrogen content in the sheath gas. On the other hand, the spheroidization rate of the silica power first increased and then decreased with the increase of the nitrogen content in the sheath gas. Moreover, the amorphous content increased with the increase of the spheroidization rate of the silica powder.
基金Item Sponsored by National Natural Science Foundation of China(50271060)Natural Science Foundation of Hebei Province of China(503291)
文摘The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 ℃ and in the strain rate of 0.01 s-1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.
基金supported by National Natural Science Foundations ofChina (Grant Nos. 50835003,50805047,10972078)National Hi-tech Research and Development Program of China (863 Program,Grant No.2009AA04Z421)
文摘The fractal dimension（FD） of surfaces has been widely used to characterize the properties of materials.However,most of the previous researches were concentrated on the correlation between the FD of surfaces and mechanical properties of materials,such as impact energy and fracture toughness,etc.The aim of this paper is to characterize the spheroidization grade and strength of 15CrMo steel through determination of FD of cementite phase on the basis of two-dimension microstructural image.Two methods,namely slit-island method（SIM） and box-counting method（BCM）,are used to determine the value of FD.It is found that the FD value evaluated by using BCM is generally higher than that evaluated by SIM.This phenomenon may be due to the difference in the principles used in different methods.Whether SIM or BCM is used,the spheroidization grade in 15CrMo steel linearly increases with decreasing the value of FD.The relationship between the FD value,D,and spheroidization grade,Sg,can be approximately expressed as D≌-0.11Sg＋A,where A is a constant value which is depended on the evaluation method.Both the ultimate strength and the yielding strength of 15CrMo steel increase with increasing FD of cementite phase.There may be a common relationship between the FD of cementite phase and strength of 15CrMo steel.When the FD of cementite phase in 15CrMo steel is determined,the strength of this steel can be evaluated.The present paper can provide a novel method to evaluate the strength and spheroidization grade of carbon steel through determination of fractal dimension（FD） of cementite phase.
文摘Spherical powders with good flowability and high stacking density are mandatory for powder bed additive manufacturing. Nevertheless, the preparation of spherical refractory tungsten and tungsten alloy powders is a formidable task. In this paper, spherical refractory metal powders processed by high-energy stir ball milling and RF inductively coupled plasma were investigated. By utilizing the technical route, pure spherical tungsten powders were prepared successfully, the flowability increased from 10.7 s/50 g to 5.5 s/50 g and apparent density increased from 6.916 g cm-3 to 11.041 g cm-3. Alloying element tantalum can reduce the tendency to micro- crack during tungsten laser melting and rapid solidification process. Spherical W-6Ta （%wt） powders were prepared in this way, homogeneous dispersion of tantalum in a tungsten matrix occurred but a small amount of flake-like shape particles appeared after high-energy stir ball milling. The flake-like shape particles can hardly be spheroidized in subsequent RF inductively coupled plasma process, might result from the unique suspended state of flaky particles under complex electric and magnetic fields as well as plasma-particle heat exchange was different under various turbulence models. As a result, the flake-like shape particles cannot pass through the high-temperature area of thermal plasma torch and cannot be spheroidized properly.
基金supported by the National Natural Science Foundation of China(Nos.51601194,51471171,and U1760201)the Chinese Academy of Sciences Youth Innovation Promotion Association(No.2021193)+1 种基金the Liaoning Provincial Natural Science Foundation(No.2021-YQ-01)the Liaoning Revitalization Talents Program(No.XLYC2002099)。
文摘The spheroidization of the lamellar structure can greatly contribute to the superplasticity of the nugget zone(NZ)of Ti alloy welds,which is the key to achieve the integral superplastic forming of welds for the fabrication of large-scale complex components.However,the spheroidization process is complex and costly since it cannot be obtained generally,unless the lamellae suffers from a large deformation.In this study,the static spheroidization was achieved for the fine lamellae structure in the nugget of a friction stir welded(FSW)Ti-6Al-4V joint,particularly by the annealing without any deformation.The specialα/βinterface obeying a Burgers orientation relationship(BOR)after FSW was first time directly observed,whose effect on the spheroidization was discussed.A new static spheroidization mechanism with the gradual coalescence of the adjacent lamellae was discovered,which we named as“termination coalescence”.There was a slower coarsening rate in the lamellar structure than in the classical equiaxed one,due to the BOR in the lamellae,although both of them exhibited a volume diffusion character during annealing.Consequently,the similar superplasticity can be achieved for the base material and NZ after annealing.This study can provide a new way to the spheroidization and a theoretical basis for the integral superplastic forming of welds during production.
基金finically supported by the ARC Discovery Project(No.DP180102454)。
文摘A cost-effective Fe-enriched eutectic high-entropy alloy(EHEA),Fe35Ni25Cr25Mo15,was designed and prepared to avoid the use of expensive Co that is commonly used in HEAs.However,the as-cast Feenriched EHEA was associated with brittleness.The present work aims to evaluate the possibility and feasibility of spheroidization of the lamellar structure of the EH EA in order to improve the ductility.Due to the high cooling rate of arc-melting,the as-melted Fe35Ni25Cr25Mo15 EHEA was found to be a pseudo eutectic alloy comprised of alternantσphase(Cr(0.22)Mo0.18Fe0.6-type intermetallic)and face centred cubic(FCC)phase.The lamellar structure in the Fe-enriched EHEA remained stable up to 800Κ.The instability of the lamellar structure occurred at temperatures over 800℃,which was resulted from migration of high-density faults(i.e.lamellar termination and ledges in the lamellae).However,the Fe35Ni25Cr25Mo15EHEA still exhibited brittleness even after spheroidization at 1100℃for 168 h due to the formation of the hard and brittleσmatrix in the pseudo Fe35Ni25Cr25Mo15 EHEA as a result of decomposition of the lamellar structure.Therefore,in contrast to the softening of traditional eutectic alloys,spheroidization treatment was considered as invalid to improve the ductility of pseudo-eutectic HEA with high fraction of intermetallic phase.The present work provides a valuable re ference for those who aim to improve the ductility of brittle EHEA through spheroidization.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11875295 and 11535003)the National Key R&D Program of China(Grant No.2019YFC0119000)+1 种基金Provincial Science and Technology Major Project of Anhui Province,China(Grant No.17030801035)Key Program of 13th Five-Year Plan,CASHIPS,China(Grant No.KP-2017-25).
文摘We investigate characteristics of multi-arc torches with three pairs of electrodes(three cathodes and three anodes)and their performance on the spheroidization of SiO2 powder.The effect of electrode arrangement,including adjacent pattern(AD pattern,adjacent electrodes powered by one power supply)and opposite pattern(OP pattern,opposite electrodes powered by one power supply),on the dynamics of arc plasma is investigated based on synchronous acquisition of electrical and optical signals.The results show that both the voltage and spatial distribution of each arc of multiple arcs are more stable compared with those of a single arc.The fluctuation of an arc in multiple arcs mainly comes from the small-scale arc-to-arc restrikes among multiple arcs.Moreover,these arc-to-arc restrikes occur more frequently among multiple arc columns in OP pattern than in AD pattern.Moreover,the high-temperature area of the central region of arc chamber in OP pattern is larger than that in AP pattern.For the spheroidization of SiO2 in this multi-arc generator,the spheronization degrees of plasma treated silica in OP pattern are at least 20%higher than those in AD pattern.
基金the supports of the Key R&D Program of Advanced Technology of Sichuan Science and Technology Department(No.2020YFG0111)。
文摘Arc plasma torch is an effective tool for spheroidization of metallic powders.However,as most conventional plasma torches were not specifically designed for plasma spheroidization,they may exhibit the disadvantages of the radial injection of powders,large fluctuations in the arc voltage,large gas flow rate,and disequilibrium between multiple plasma jets during the spheroidization process.Therefore,this paper presents a triple-cathode cascade plasma torch(TCCPT)for plasma spheroidization.Its structural design,including three cathodes,a common anode,and three sets of inter-electrodes,are detailed to ensure that powders can be inserted into the plasma jet by axial injection,the arc voltage fluctuations are easily maintained at a low level,and the plasma torches can work at a relatively small gas flow rate.Experimental results showed that the proposed TCCPT exhibits the following characteristics:(1)a relatively small arc voltage fluctuation within 5.3%;(2)a relatively high arc voltage of 75 V and low gas flow rate range of10-30 SLM;(3)easy to be maintained at the equilibrium state with the equilibrium index of the three plasma jets within 3.5 V.Furthermore,plasma spheroidization experiments of SUS304 stainless steel powers were carried out using the proposed TCCPT.Results verified that the proposed TCCPT is applicable and effective for the spheroidization of metallic powders with wide size distribution.
基金financially supported by the 2012 Western Materials Innovation Foundation of China (No. XBCL-1-06)the Science and Technology Coordinating Innovative Engineering Project of Shaanxi Province of China (No. 2014KTCQ01-35)+1 种基金the Natural Science Foundation of Shaanxi Province of China (No. 2014JM6226)the Specialized Research Fund of Education Commission of Shaanxi Province of China (No. 2013JK0905)
文摘To control the morphology and particle size of dense spherical molybdenum powder prepared by radio frequency（RF） plasma from irregular molybdenum powder as a precursor, plasma process parameters were optimized in this paper. The effects of the carrier gas flow rate and molybdenum powder feeding rate on the shape and size of the final products were studied. The molybdenum powder morphology was examined using high-resolution scanning electron microscopy. The powder phases were analyzed by X-ray diffraction. The tap density and apparent density of the molybdenum powder were investigated using a Hall flow meter and a Scott volumeter. The optimal process parameters for the spherical molybdenum powder preparation are 50 g/min powder feeding rate and 0.6 m^3/h carrier gas rate. In addition, pure spherical molybdenum powder can be obtained from irregular powder, and the tap density is enhanced after plasma processing. The average size is reduced from 72 to 62 μm, and the tap density is increased from 2.7 to 6.2 g/cm^3. Therefore, RF plasma is a promising method for the preparation of high-density and high-purity spherical powders.
文摘In this study,the spheroidization process of large-deformed steel under various conditions was researched.Steel with a high carbon content （1.0％ C） was first treated thermomechanically using multipass rolling.Then it underwent spheroidization treatments at different heating temperatures,using various heating times and cooling rates.Spheroidization processes with a lower heating temperature,shorter heating time,or faster cooling rate than those of the traditional process all showed good results,indicating that the spheroidization process was promoted significantly by the large deformation process.Grain refinement and fragmentation of cementite,along with the large deformation process,promoted this spheroidization process.
文摘Conventionally manufactured 35CrMo cold heading steel must undergo spheroidization annealing before the cold heading process In this paper, different types of deformation processes with various controlled cooling periods were operated to achieve on-line spheroidal cementite using the Gleeble-3500 simulation technique. According to the measured dynamic ferrite transformation temperature （Ad3）, the deformation could be divided into two types： low temperature deformation at 810 and 780℃; ＂deformation-induced ferrite transformation＂ （DIFT） deformation at 750 and 720℃. Compared with the low temperature deformation, the DIFT deformation followed by accelerated cooling to 680℃ is beneficial for the formation of spheroidal cementite. Samples subjected to both the low-temperature deformation and DIFT deformation can obtain granular bainite by accelerated cooling to 640℃; the latter may contribute to the formation of a fine dispersion of secondary constituents. Granular bainite can transform into globular pearlite rapidly during subcritical annealing, and the more the disperse phase, the more homogeneously distributed globular cementite can be obtained.
基金financially supported by the Natural Science Foundation of Liaoning Province,China(No.2019JH3/30100037)National Natural Science Foundation of China(Nos.U1867216,51701222)State Key Laboratory of Metal Material for Marine Equipment and Application(No.SKLMEA-K201907)。
文摘In this work,the corrosion behavior of medium-carbon steels(45,45 Cu and 45 Cuq steels)in acidic chloride environment was investigated.The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels.For 45 steel,serious pitting corrosion happened in and around the pearlite regions,and a large number of lamellar cementite was fixed in the corrosion pits.Meanwhile,the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion,resulting in a rapidly increase in corrosion rate with time.While for 45 Cu and 45 Cuq steels,macroscopic uniform corrosion occurred,and the cementite accumulation was markedly reduced as compared with 45 steel,thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly.Among these,45 Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion,indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying,thereby significantly improving the corrosion resistance of 45 steel.
文摘The research aims to provide an alternative to austempering treatment of ductile cast iron with a simple and cost-effective heat-treatment process.This goal was accomplished by applying a simple one-step spheroidization heat treatment to the as-cast ductile iron,which would normally possess a coarse pearlitic microstructure to a significant extent.Spheroidization experiments involving isothermal holding below the lower critical temperature(A1)were conducted followed by standard mechanical testing and microstructural characterization for an experimental ductile iron.After improving the spheroidization holding time at a given temperature,the work shows that the ductility and toughness of an as-cast ductile iron can be improved by 90%and 40%,respectively,at the cost of reducing the tensile strength by 8%.Controlled discretization of the continuous cementite network in pearlitic matrix of the ductile iron is deemed responsible for the improved properties.The work also shows that prolonged holding time during spheroidization heat treatment leads to degradation of mechanical properties due to the inhomogenous microstructure formation caused by heterogeneous decomposition and cementite clustering in the material.The main outcome of this work is the demonstration of ductile cast iron’s necking behavior due to spheroidization heat treatment.
