Dynamic spheroidization kinetics behavior of Ti-6.5Al-2Zr-1Mo-1V alloy with lamellar microstructure

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.

Effect of Plasma Spheroidization Process on the Microstructure and Crystallographic Phases of Silica, Alumina and Nickel Particles

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.

Characterizing the Spheroidization Grade and Strength of 15CrMo Steel through Determining Fractal Dimension

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.

Spheroidization of silica powders by radio frequency inductively coupled plasma with Ar–H2 and Ar–N2 as the sheath gases at atmospheric pressure

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.

Study on the RF inductively coupled plasma spheroidization of refractory W and W-Ta alloy powders

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.

Characteristics of DC arcs in a multi-arc generator and their application in the spheroidization of SiO2

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.

Spheroidization of molybdenum powder by radio frequency thermal plasma

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.

Spheroidization process in large-deformed high-carbon steel

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.

Design and characteristics of a triple-cathode cascade plasma torch for spheroidization of metallic powders

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.

Effect of spheroidization of cementite in ductile cast iron

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.

On-line spheroidization process of medium-carbon low-alloyed cold heading steel

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.

Spheroidization and Ostwald Growth of Carbide in Isothermal Process of Hot-Deformed High Carbon Chromium Cast Steel

In isothermal spheroidizing process,the spheroidization and growth of the carbide formed in hot-deformed high-carbon chromium cast steel at high temperature were investigated.The results showed that the spheroidizing growth of carbide proceeds in such a way that the bigger carbide particles swallow the smaller ones,and the short rhabdoid carbides dissolve and are spheroidized by itself.When the samples were held at 720℃ for more than 3 h,the spheroidization is not obvious.The feature of the process is the size increment and the amount decrement of carbide particles.The empirical equation for growth rate of carbides was obtained.The volume fraction of carbides keeps constant.The growth process agrees well with Ostwald Ripening Law.

Spheroidization behavior of dendritic b.c.c. phase in Zr-based β-phase composite

The spheroidization behavior of the dendritic b.c.c, phase dispersed in a bulk metallic glass （BMG） matrix was investigated through applying semi-solid isothermal processing and a subsequent rapid quenching procedure to a Zr-basedβ-phase composite. The Zr-based composite with the composition of Z%62Ti138NbsoCuegNi5.6Be125 was prefabricated by a water-cooled copper mold-casting method and characterized by X-ray diffraction （XRD） and scanning electron microscope （SEM）. The results show that the composite consists of a glassy matrix and uniformly distributed fine dendrites of theβ-Zr solid solution with the body-centered-cubic （b.c.c.） structure. Based on the differential scanning calorimeter （DSC） examination results, and in view of the b.c.c.β-Zr to h.c.p, α-Zr phase transition temperature, a semi-solid holding temperature of 900 ℃ was determined. After reheating the prefabricated composite to the semi-solid temperature, followed by an isothermal holding process at this temperature for 5 min, and then quenching the semi-solid mixture into iced-water; the two-phase microstructure composed of a BMG matrix and uniformly dispersed spherical b.c.c.β-Zr particles with a high degree of sphericity was achieved. The present spheroidization transition is a thermodynamically autonomic behavior, and essentially a diffusion process controlled by kinetic factors; and the formation of the BMG matrix should be attributed to the rapid quenching of the semi-solid mixture as well as the large glass-forming ability of the remaining melt in the semi-solid mixture.

Ultra-low-carbon steel spheroidization and torsion

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.

Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

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.

Static spheroidization and its effect on superplasticity of fine lamellae in nugget of a friction stir welded Ti-6Al-4V joint

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.

Spheroidization behaviour of a Fe-enriched eutectic high-entropy alloy

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.

The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution

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.

Porous nanofibrous dressing enables mesenchymal stem cell spheroid formation and delivery to promote diabetic wound healing

Delayed and nonhealing of diabetic wounds imposes substantial economic burdens and physical pain on patients.Mesenchymal stem cells(MSCs)promote diabetic wound healing.Particularly when MSCs aggregate into multicellular spheroids,their therapeutic effect is enhanced.However,traditional culture platforms are inadequate for the efficient preparation and delivery of MSC spheroids,resulting in inefficiencies and inconveniences in MSC spheroid therapy.In this study,a three-dimensional porous nanofibrous dressing(NFD)is prepared using a combination of electrospinning and homogeneous freeze-drying.Using thermal crosslinking,the NFD not only achieves satisfactory elasticity but also maintains notable cytocompatibility.Through the design of its structure and chemical composition,the NFD allows MSCs to spontaneously form MSC spheroids with controllable sizes,serving as MSC spheroid delivery systems for diabetic wound sites.Most importantly,MSC spheroids cultured on the NFD exhibit improved secretion of vascular endothelial growth factor,basic fibroblast growth factor,and hepatocyte growth factor,thereby accelerating diabetic wound healing.The NFD provides a competitive strategy for MSC spheroid formation and delivery to promote diabetic wound healing.

Spheroid construction strategies and application in 3D bioprinting

Tissue engineering has been striving toward designing and producing natural and functional human tissues.Cells are the fundamental building blocks of tissues.Compared with traditional two-dimensional cultured cells,cell spheres are threedimensional(3D)structures that can naturally form complex cell–cell and cell–matrix interactions.This structure is close to the natural environment of cells in living organisms.In addition to being used in disease modeling and drug screening,spheroids have significant potential in tissue regeneration.The 3D bioprinting is an advanced biofabrication technique.It accurately deposits bioinks into predesigned 3D shapes to create complex tissue structures.Although 3D bioprinting is efficient,the time required for cells to develop into complex tissue structures can be lengthy.The 3D bioprinting of spheroids significantly reduces the time required for their development into large tissues/organs during later cultivation stages by printing them with high cell density.Combining spheroid fabrication and bioprinting technology should provide a new solution to many problems in regenerative medicine.This paper systematically elaborates and analyzes the spheroid fabrication methods and 3D bioprinting strategies by introducing spheroids as building blocks.Finally,we present the primary challenges faced by spheroid fabrication and 3D bioprinting with future requirements and some recommendations.