To research the solidification behavior and microstructures of a laser remelting/solidification layer on anisotropic Nd_(15)Fe_(77)B_(8 )sintered magnets with their magnetization direction parallel to X, Y, Z-axis res...To research the solidification behavior and microstructures of a laser remelting/solidification layer on anisotropic Nd_(15)Fe_(77)B_(8 )sintered magnets with their magnetization direction parallel to X, Y, Z-axis respectively, their surfaces (parallel to XOY plane) were scanned by 5 kW Roffin-Sinar 850 type of CO_(2) laser along Y axis. The rapid solidification of the molten alloy in the layer results in three distinct zones. The transition zone close to the unmolten portion of a magnet (substrate), consists of the columnar Nd_(2)Fe_(14)B phase (matrix), the 10.0%~15.1% dendrite primary iron phase dispersing in the matrix, and the Nd-rich phase along Nd_(2)Fe_(14)B grain boundaries. The columnar crystal zone in the middle of the layer consists of the long columnar Nd_(2)Fe_(14)B grains and their grain boundary Nd-rich phase. And the dendrite crystal zone near the free surface of the layer consists of dendrite Nd_(2)Fe_(14)B grains and their grain boundary Nd-rich phase. When the laser scanning velocity is lower, the growing direction of the microstructures in the layer tends to the laser scanning direction step by step. When the velocity is not lower than 25 mm·s^(-1), the laser remelting/solidification layer thins and the columnar crystal zone comprises almost the whole layer. Under this condition, on the substrate with its magnetization direction along X or Y-axis respectively, the columnar Nd_(2)Fe_(14)B grains in the layer grow in the direction of Z-axis (that is their long-axis along Z-axis), their alignment of the easy magnetization axis [001] is parallel to the magnetization direction of the substrate correspondingly; but on the substrate with its magnetization direction along Z-axis, the columnar Nd_(2)Fe_(14)B grains in the transition zone grow at an angle of 30°~50° between Z-axis and their long-axis. And the columnar Nd_(2)Fe_(14)B grains in the columnar crystal zone gradually tend to the Z-axis,and their easy magnetization axis [001] arrange in the range of 0°~360° of the plane perpendicular to their long-axis.展开更多
A heat and mass transfer modelling containing phase transformation dynamics is made for pure metals and binary alloys under pulsed laser processing. The nonequilibrium effects of processing parameters and physical pro...A heat and mass transfer modelling containing phase transformation dynamics is made for pure metals and binary alloys under pulsed laser processing. The nonequilibrium effects of processing parameters and physical properties are evaluated on the melting and solidification of pure metals (Al, Cu, Fe and Ni) and Al Cu alloys. It is shown that the energy intensity of laser beam and physical properties of metals and the solute concentration of alloys have important effect on the interface temperature, melting and solidification velocity, melting depth and non equilibrium partition coefficient. This situation is resulted from the interaction of heat transfer, redistribution of solute, solute trapping and growth kinetics.展开更多
Materials Lao.8Sro.2Gao.83Mgo.17_xCox03_6 with x = 0, 0.05, 0.085, 0.10, and 0.15 are synthesized by laser rapid solidification. It is shown that the samples prepared by laser rapid solidification give rise to unique ...Materials Lao.8Sro.2Gao.83Mgo.17_xCox03_6 with x = 0, 0.05, 0.085, 0.10, and 0.15 are synthesized by laser rapid solidification. It is shown that the samples prepared by laser rapid solidification give rise to unique spear-like or leaf-like microstructures which are orderly arranged and densely packed. Their electrical properties each show a general depen dence of the Co content and the total conductivities of Lao.8Sro.2Gao.83Mgo.085Coo.08503_6 prepared by laser rapid solidification are measured to be 0.067, 0.124, and 0.202 S.cm-1 at 600, 700, and 800 ℃, respectively, which are much higher than by conventional solid state reactions. Moreover, the electrical conductivities each as a function of the oxy gen partial pressure are also measured. It is shown that the samples with the Co content values 〈 8.5 mol% each exhibit basically ionic conduction while those for Co content values 〉 10 mol % each show ionic mixed electronic conduction under oxygen partial pressures from 10-16 atm (1 atm = 1.01325 x 105 Pa) to 0.98 atm. The improved ionic conductivity of Lao.sSro.2Gao.83Mgo.085Coo.08503 prepared by laser rapid solidification compared with by solid state reactions is attributed to the unique microstructure of the sample generated during laser rapid solidification.展开更多
This paper reports that the rapid solidification of mixed Li2B4O7 and KNbO3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO3 crystals have been...This paper reports that the rapid solidification of mixed Li2B4O7 and KNbO3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.展开更多
This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- ...This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- crostructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210 mm/s. An implicit expression of the critical solidification velocity for the cellular-planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M-S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652 mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the η phase. This indicates that HGAS was successfully achieved in the present experiments.展开更多
The ultra-fine microstructrue of cast iron has been studied under rapid solidification by laser rapid meltingand solidifying method and SEM and TEM technology and results show cementite, martensite, austenite and ultr...The ultra-fine microstructrue of cast iron has been studied under rapid solidification by laser rapid meltingand solidifying method and SEM and TEM technology and results show cementite, martensite, austenite and ultra-fineperlite phases can be obtained under rapid solidification and many dislocations have been the found in cementite.展开更多
Rapid solidification of Ti-50 at.%Al peritectic alloy is realized by laser melting technique at diferent conditions of laser power and scanning speed. The temperature field and the cooling rate under the corresponding...Rapid solidification of Ti-50 at.%Al peritectic alloy is realized by laser melting technique at diferent conditions of laser power and scanning speed. The temperature field and the cooling rate under the corresponding conditions are derived from the finite element simulation.℃omparing the measured pool size with the simulated result, the laser absorptivity of Ti-50 at.%Al peritectic alloy at diferent conditions can be deduced to establish the relationships between the laser absorptivity, the laser power and the scanning speed. The morphology evolution and the phase selection of Ti-50 at.%Al peritectic alloy are described by the temperature gradient and the cooling rate. With the increase of temperature gradient and cooling rate, β phase replaces α phase to become the leading growth phase. And the growth of α phase experiences the transition from facet to non-facet manner, while β phase is refined. To understand the underlying mechanism of the competition growth can bring benefit to the industrial application of Ti-Al alloy.展开更多
The microstructure of a laser-melted Fe-4% C-10% Sn alloy has been studied.A non-crystalline phase was found in the upper part of the laser-melted zone:At the bottom of the melted zone,however,the microcrystalline zon...The microstructure of a laser-melted Fe-4% C-10% Sn alloy has been studied.A non-crystalline phase was found in the upper part of the laser-melted zone:At the bottom of the melted zone,however,the microcrystalline zone which consists of α-Fe and a bet phase was observed.Fine twinning martensite exists in the other area of the melted zone.展开更多
The Fe-Ni-Cr coating laver has been alloyed with 45 steel base metal by transverse.flow type CO_2 gas laser of maximum output 5kW.The characteristics of the melted zone and the es- sence of the“bright band”has been ...The Fe-Ni-Cr coating laver has been alloyed with 45 steel base metal by transverse.flow type CO_2 gas laser of maximum output 5kW.The characteristics of the melted zone and the es- sence of the“bright band”has been investigated using the electron microscopy.The results show that the“bright band”belongs to the melted zone and is a vertical section of the plane crystal,The width of the“bright band”equals the height of the plane crystal,which decreases with the increase of laser beam scanning rate and the decrease of laser power.This has been explained in terms of constitutional supercooling G/R.展开更多
The construction and application of traditional high-strength 7075 aluminum alloy(Al7075) through selective laser melting(SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical ...The construction and application of traditional high-strength 7075 aluminum alloy(Al7075) through selective laser melting(SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical issue, in this study, Si is employed to assist the SLM printing of high-strength Al7075. The laser energy density during SLM is optimized, and the eff ects of Si element on solidification path, relative density, microstructure and mechanical properties of Al7075 alloy are studied systematically. With the modified solidification path, laser energy density, and the dense microstructure with refined grain size and semi-continuous precipitates network at grain boundaries, which consists of fine Si, β-MgSi, Q-phase and θ-AlCu, the hot cracking phenomenon and mechanical properties are eff ectively improved. As a result, the tensile strength of the SLM-processed Si-modified Al7075 can reach 486 ± 3 MPa, with a high relative density of ~ 99.4%, a yield strength of 291 ± 8 MPa, fracture elongation of(6.4 ± 0.4)% and hardness of 162 ± 2(HV) at the laser energy density of 112.5 J/mm~3. The main strengthening mechanism with Si modification is demonstrated to be the synergetic enhancement of grain refinement, solution strengthening, load transfer, and dislocation strengthening. This work will inspire more new design of high-strength alloys through SLM.展开更多
Distribution control and formation mechanism of gas inclusions formed in directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods are explored during laser floating zone melting. In atmospheric environment,...Distribution control and formation mechanism of gas inclusions formed in directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods are explored during laser floating zone melting. In atmospheric environment, highly-dense bubble-free eutectic rods are well fabricated at low solidification rate(<25μm/s). Gas inclusions form intermittently when the solidification rate is in the range of 25-50 μm/s,but produce continuously at higher solidification rates(100-200 μm/s). The gas inclusions exhibit an elongated finger-like pattern along the growth direction, which of the maximum value of diameter first increases and then decreases with increasing the solidification rate. Meanwhile, the volume fraction of gas inclusions increased gradually with the solidification rate. Based on the effect of surface tension gradient, heterogeneous nucleation of gas bubbles is evaluated to be the primary formation mechanism of gas inclusions.展开更多
The inherent drawbacks of selective laser melting technique including serious micro-pore and element microsegregation problems destroy the mechanical property of the component.To overcome this problem,a new approach,c...The inherent drawbacks of selective laser melting technique including serious micro-pore and element microsegregation problems destroy the mechanical property of the component.To overcome this problem,a new approach,circular beam oscillation,was successfully applied in the SLMed Inconel 718 samples including single tracks,thin walls and cuboid samples.On one hand,circular beam oscillation reduces the micro-pores in molten pools and cuboid samples,increasing the relative density of the cuboid sample to 99.95%.On the other hand,circular beam oscillation suppresses the element microsegregation,reducing the formation of Laves phases in SLMed Inconel 718 samples.Moreover,circular beam oscillation enhances the<001>texture of thin walls and the<101>texture of cuboid samples.The improvement of formability and microstructure of the SLMed samples with oscillation is closely related to cooling rate,thermal gradient and stirring effect during the solidification process.Therefore,circular beam oscillation shows the possibility to overcome the key bottlenecks of the traditional SLM technology and to realize a further industrial application of SLM technology.展开更多
The arrangements of the easy magnetization axis [001] of columnar Nd2Fe14B crystals in the laser scanned layer on anisotropic sintered Nd15Fe77B8 magnets were investigated by XRD and the Bitter method. The results sho...The arrangements of the easy magnetization axis [001] of columnar Nd2Fe14B crystals in the laser scanned layer on anisotropic sintered Nd15Fe77B8 magnets were investigated by XRD and the Bitter method. The results show that the common effects of both the heat flux and the substrate magnetization orientation constrain the columnar Nd2Fe14B solidified from the laser melting pool to form the c-axis texture orientated with the same direction as that of the substrate, when the geometric relationship between the heat flux in the laser scanning layer and c-axis texture orientation of the substrate is perpendicular to each other, and if the laser scanning velocity is no less than 25 mm·min-1. The c-axes of columnar Nd2Fe14B crystals are no longer randomly distributed in the plane normal to their preferential growing direction as they are randomly done in both ingots cooled by water-cooling copper mould and directionally solidified Nd-Fe-B rods.展开更多
Al-27%Cu-5.3%Si ternary eutectic alloy was melted using a YAG laser and then solidified while being acoustically levitated. A maximum undercooling to 195 K (0.24 TL) was achieved with a cooling rate of 76 K/s. The sol...Al-27%Cu-5.3%Si ternary eutectic alloy was melted using a YAG laser and then solidified while being acoustically levitated. A maximum undercooling to 195 K (0.24 TL) was achieved with a cooling rate of 76 K/s. The solidification microstructure was composed of (Al+θ+Si) ternary eutectics and (Al+θ) pseudobinary eutectics. During acoustic levitation, the (Al+θ+Si) ternary eutectics are refined and the (Al+θ) pseudobinary eutectics have morphological diversity. On the surface of the alloys, surface oscillations and acoustic streaming promote the nucleation of the three eutectic phases and expedite the cooling process. This results in the refinement of the ternary eutectic microstructure. During experiments, the reflector decreases with increasing alloy temperature, and the levitation distance always exceeds the resonant distance. Because of the acoustic radiation pressure, the melted alloy was flattened, and deformation increases with increasing sound pressure. The maximum aspect ratio achieved was 6.64, corresponding to a sound pressure of 1.8×104 Pa.展开更多
A type of polymer-coated molybdenum powder used in selective laser sintering technology was prepared by coating polymer on molybdenum particles and frozen grinding techniques, with the maximum particle diameter of 71 ...A type of polymer-coated molybdenum powder used in selective laser sintering technology was prepared by coating polymer on molybdenum particles and frozen grinding techniques, with the maximum particle diameter of 71 μm. The laser sintering experiments of polymer-coated molybdenum powder were conducted by using the self-developed selective laser sintering machine (HLRP-350I). The method of microscopic analysis was used to investigate the dynamic laser sintering process of polymer-coated molybdenum powder. Based on the study, the laser sintering mechanisms of polymer-coated molybdenum powder were presented. It is found that the mechanism is viscous flow when the laser sintering temperature is between 100 ℃ and 160 ℃, which can be described by a two-sphere model; and the mechanism is melting /solidification when the temperature is above 160 ℃.展开更多
As a typical dual-phase eutectic high entropy alloy(EHEA),AlCoCrFeNi_(2.1)can achieve the fair matching of strength and ductility,which has attracted wide attention.However,the engineering applications of as-cast AlCo...As a typical dual-phase eutectic high entropy alloy(EHEA),AlCoCrFeNi_(2.1)can achieve the fair matching of strength and ductility,which has attracted wide attention.However,the engineering applications of as-cast AlCoCrFeNi_(2.1)EHEAs still face challenges,such as coarse grain and low yield strength resulting from low solidification rate and temperature gradient.In this study,selective laser melting(SLM)was introduced into the preparation of AlCoCrFeNi_(2.1)EHEA to realize unique strength-ductility balance,with emphasis on investigating the effects of processing parameters on its eutectic microstructure and properties.The results show that the SLM-ed samples exhibit a completely eutectic structure consisting of ultra-fine face-centered cubic(FCC)and ordered body-centered cubic(B2)phases,and the duplex microstructure undergoes a morphological evolution from lamellar structure to cellular structure as laser energy input reducing.The SLM-ed AlCoCrFeNi_(2.1)EHEA presents an excellent match of high tensile strength(1271 MPa),yield strength(966 MPa),and good ductility(22.5%)at room temperature,which are significantly enhanced by the ultra-fine grains and heterogeneous structure due to rapid solidification rate and high temperature gradient during SLM.Especially,the yield strength increment of~50%is realized with no loss in ductility as compared with the as-cast samples with the same composition.On this basis,the precise complex component with excellent mechanical properties is well achieved.This work paves the way for the performance improvement and complex parts preparation of EHEA by microstructural design using laser additive manufacturing.展开更多
基金ProjectsupportedbytheNaturalScienceFundationofJiangxi(CA99110901)andZhejiang(M503096andM502166) Province
文摘To research the solidification behavior and microstructures of a laser remelting/solidification layer on anisotropic Nd_(15)Fe_(77)B_(8 )sintered magnets with their magnetization direction parallel to X, Y, Z-axis respectively, their surfaces (parallel to XOY plane) were scanned by 5 kW Roffin-Sinar 850 type of CO_(2) laser along Y axis. The rapid solidification of the molten alloy in the layer results in three distinct zones. The transition zone close to the unmolten portion of a magnet (substrate), consists of the columnar Nd_(2)Fe_(14)B phase (matrix), the 10.0%~15.1% dendrite primary iron phase dispersing in the matrix, and the Nd-rich phase along Nd_(2)Fe_(14)B grain boundaries. The columnar crystal zone in the middle of the layer consists of the long columnar Nd_(2)Fe_(14)B grains and their grain boundary Nd-rich phase. And the dendrite crystal zone near the free surface of the layer consists of dendrite Nd_(2)Fe_(14)B grains and their grain boundary Nd-rich phase. When the laser scanning velocity is lower, the growing direction of the microstructures in the layer tends to the laser scanning direction step by step. When the velocity is not lower than 25 mm·s^(-1), the laser remelting/solidification layer thins and the columnar crystal zone comprises almost the whole layer. Under this condition, on the substrate with its magnetization direction along X or Y-axis respectively, the columnar Nd_(2)Fe_(14)B grains in the layer grow in the direction of Z-axis (that is their long-axis along Z-axis), their alignment of the easy magnetization axis [001] is parallel to the magnetization direction of the substrate correspondingly; but on the substrate with its magnetization direction along Z-axis, the columnar Nd_(2)Fe_(14)B grains in the transition zone grow at an angle of 30°~50° between Z-axis and their long-axis. And the columnar Nd_(2)Fe_(14)B grains in the columnar crystal zone gradually tend to the Z-axis,and their easy magnetization axis [001] arrange in the range of 0°~360° of the plane perpendicular to their long-axis.
文摘A heat and mass transfer modelling containing phase transformation dynamics is made for pure metals and binary alloys under pulsed laser processing. The nonequilibrium effects of processing parameters and physical properties are evaluated on the melting and solidification of pure metals (Al, Cu, Fe and Ni) and Al Cu alloys. It is shown that the energy intensity of laser beam and physical properties of metals and the solute concentration of alloys have important effect on the interface temperature, melting and solidification velocity, melting depth and non equilibrium partition coefficient. This situation is resulted from the interaction of heat transfer, redistribution of solute, solute trapping and growth kinetics.
基金supported by the National Natural Science Foundation of China(Grant No.10974183)the Fund for Science and Technology Innovation Team of Zhengzhou City,China(Grant No.2011-3)the Postdoctoral Research Sponsorship in Henan Province,China(Grant No.2011002)
文摘Materials Lao.8Sro.2Gao.83Mgo.17_xCox03_6 with x = 0, 0.05, 0.085, 0.10, and 0.15 are synthesized by laser rapid solidification. It is shown that the samples prepared by laser rapid solidification give rise to unique spear-like or leaf-like microstructures which are orderly arranged and densely packed. Their electrical properties each show a general depen dence of the Co content and the total conductivities of Lao.8Sro.2Gao.83Mgo.085Coo.08503_6 prepared by laser rapid solidification are measured to be 0.067, 0.124, and 0.202 S.cm-1 at 600, 700, and 800 ℃, respectively, which are much higher than by conventional solid state reactions. Moreover, the electrical conductivities each as a function of the oxy gen partial pressure are also measured. It is shown that the samples with the Co content values 〈 8.5 mol% each exhibit basically ionic conduction while those for Co content values 〉 10 mol % each show ionic mixed electronic conduction under oxygen partial pressures from 10-16 atm (1 atm = 1.01325 x 105 Pa) to 0.98 atm. The improved ionic conductivity of Lao.sSro.2Gao.83Mgo.085Coo.08503 prepared by laser rapid solidification compared with by solid state reactions is attributed to the unique microstructure of the sample generated during laser rapid solidification.
基金supported by the National Natural Science Foundation of China (Grant Nos 50331040 and 50802105)the Innovation Funds from Shanghai Institute of Ceramics, Chinese Academy of Sciences (Grant No SCX0623)
文摘This paper reports that the rapid solidification of mixed Li2B4O7 and KNbO3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.
基金Project supported by the National Natural Science Foundation of China (Grant Nos 50201012 and 50471065).
文摘This paper reports on laser surface remelting experiments performed on a Zn-2wt.%Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the mi- crostructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210 mm/s. An implicit expression of the critical solidification velocity for the cellular-planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M-S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652 mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the η phase. This indicates that HGAS was successfully achieved in the present experiments.
文摘The ultra-fine microstructrue of cast iron has been studied under rapid solidification by laser rapid meltingand solidifying method and SEM and TEM technology and results show cementite, martensite, austenite and ultra-fineperlite phases can be obtained under rapid solidification and many dislocations have been the found in cementite.
基金supported by the National Natural Science Foundations of China Nos.51271149 and 50901060)the National Basic Research Program of China(No.2011CB610404)+3 种基金the National Aerospace Science Foundation of China(No.2010ZF53059)the NPU Foundations for Fundamental Research(No.NPUJC20110279)the Fund of the Innovation Base of Graduate Students of NPUsupported partly by the National Research Foundation of Korea(No.2012-0009451)
文摘Rapid solidification of Ti-50 at.%Al peritectic alloy is realized by laser melting technique at diferent conditions of laser power and scanning speed. The temperature field and the cooling rate under the corresponding conditions are derived from the finite element simulation.℃omparing the measured pool size with the simulated result, the laser absorptivity of Ti-50 at.%Al peritectic alloy at diferent conditions can be deduced to establish the relationships between the laser absorptivity, the laser power and the scanning speed. The morphology evolution and the phase selection of Ti-50 at.%Al peritectic alloy are described by the temperature gradient and the cooling rate. With the increase of temperature gradient and cooling rate, β phase replaces α phase to become the leading growth phase. And the growth of α phase experiences the transition from facet to non-facet manner, while β phase is refined. To understand the underlying mechanism of the competition growth can bring benefit to the industrial application of Ti-Al alloy.
文摘The microstructure of a laser-melted Fe-4% C-10% Sn alloy has been studied.A non-crystalline phase was found in the upper part of the laser-melted zone:At the bottom of the melted zone,however,the microcrystalline zone which consists of α-Fe and a bet phase was observed.Fine twinning martensite exists in the other area of the melted zone.
文摘The Fe-Ni-Cr coating laver has been alloyed with 45 steel base metal by transverse.flow type CO_2 gas laser of maximum output 5kW.The characteristics of the melted zone and the es- sence of the“bright band”has been investigated using the electron microscopy.The results show that the“bright band”belongs to the melted zone and is a vertical section of the plane crystal,The width of the“bright band”equals the height of the plane crystal,which decreases with the increase of laser beam scanning rate and the decrease of laser power.This has been explained in terms of constitutional supercooling G/R.
基金financially supported by the Joint Fund Project of Equipment Pre-research of Education Ministry(Grant No.6141A02033230)。
文摘The construction and application of traditional high-strength 7075 aluminum alloy(Al7075) through selective laser melting(SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical issue, in this study, Si is employed to assist the SLM printing of high-strength Al7075. The laser energy density during SLM is optimized, and the eff ects of Si element on solidification path, relative density, microstructure and mechanical properties of Al7075 alloy are studied systematically. With the modified solidification path, laser energy density, and the dense microstructure with refined grain size and semi-continuous precipitates network at grain boundaries, which consists of fine Si, β-MgSi, Q-phase and θ-AlCu, the hot cracking phenomenon and mechanical properties are eff ectively improved. As a result, the tensile strength of the SLM-processed Si-modified Al7075 can reach 486 ± 3 MPa, with a high relative density of ~ 99.4%, a yield strength of 291 ± 8 MPa, fracture elongation of(6.4 ± 0.4)% and hardness of 162 ± 2(HV) at the laser energy density of 112.5 J/mm~3. The main strengthening mechanism with Si modification is demonstrated to be the synergetic enhancement of grain refinement, solution strengthening, load transfer, and dislocation strengthening. This work will inspire more new design of high-strength alloys through SLM.
基金supported financially by the Science, Technology and Innovation Commission of Shenzhen Municipality (No. JCYJ20180306171121424)the National Key R&D Program of China (Nos. 2018YFB1106600 and 2017YFB1103500)+5 种基金the National Natural Science Foundation of China (Nos. 51822405 and 51472200)the Aeronautics Power Foundation (No. 6141B09050337)the Research Fund of Equipment Development Department (No. 61409230402)the Key R&D Program of ShaanXi Province (No. 2018ZDCXL-GY-0904)the Innovation Fund of the Zhejiang Kechuang New Materials Research Institute (No. ZKN-18-P04)the Research Fund of the State Key Laboratory of Solidification Processing (NPU) (No. 2019QZ-02)。
文摘Distribution control and formation mechanism of gas inclusions formed in directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods are explored during laser floating zone melting. In atmospheric environment, highly-dense bubble-free eutectic rods are well fabricated at low solidification rate(<25μm/s). Gas inclusions form intermittently when the solidification rate is in the range of 25-50 μm/s,but produce continuously at higher solidification rates(100-200 μm/s). The gas inclusions exhibit an elongated finger-like pattern along the growth direction, which of the maximum value of diameter first increases and then decreases with increasing the solidification rate. Meanwhile, the volume fraction of gas inclusions increased gradually with the solidification rate. Based on the effect of surface tension gradient, heterogeneous nucleation of gas bubbles is evaluated to be the primary formation mechanism of gas inclusions.
基金financially supported by the Pre-research Fund Project of Ministry of Equipment and Development of China though Program(No.61409230301)the Fundamental Research Funds for the Central Universities through Program(Nos.2019kfy XMPY005 and 2019kfy XKJC042)。
文摘The inherent drawbacks of selective laser melting technique including serious micro-pore and element microsegregation problems destroy the mechanical property of the component.To overcome this problem,a new approach,circular beam oscillation,was successfully applied in the SLMed Inconel 718 samples including single tracks,thin walls and cuboid samples.On one hand,circular beam oscillation reduces the micro-pores in molten pools and cuboid samples,increasing the relative density of the cuboid sample to 99.95%.On the other hand,circular beam oscillation suppresses the element microsegregation,reducing the formation of Laves phases in SLMed Inconel 718 samples.Moreover,circular beam oscillation enhances the<001>texture of thin walls and the<101>texture of cuboid samples.The improvement of formability and microstructure of the SLMed samples with oscillation is closely related to cooling rate,thermal gradient and stirring effect during the solidification process.Therefore,circular beam oscillation shows the possibility to overcome the key bottlenecks of the traditional SLM technology and to realize a further industrial application of SLM technology.
基金This work was financially supported by the Zhejiang Provincial Natural Science Fund of China (No.M503096) and by Ministry of Science and Technology (No.2004CCA04000).
文摘The arrangements of the easy magnetization axis [001] of columnar Nd2Fe14B crystals in the laser scanned layer on anisotropic sintered Nd15Fe77B8 magnets were investigated by XRD and the Bitter method. The results show that the common effects of both the heat flux and the substrate magnetization orientation constrain the columnar Nd2Fe14B solidified from the laser melting pool to form the c-axis texture orientated with the same direction as that of the substrate, when the geometric relationship between the heat flux in the laser scanning layer and c-axis texture orientation of the substrate is perpendicular to each other, and if the laser scanning velocity is no less than 25 mm·min-1. The c-axes of columnar Nd2Fe14B crystals are no longer randomly distributed in the plane normal to their preferential growing direction as they are randomly done in both ingots cooled by water-cooling copper mould and directionally solidified Nd-Fe-B rods.
基金supported by the National Natural Science Foundation of China (50971105)
文摘Al-27%Cu-5.3%Si ternary eutectic alloy was melted using a YAG laser and then solidified while being acoustically levitated. A maximum undercooling to 195 K (0.24 TL) was achieved with a cooling rate of 76 K/s. The solidification microstructure was composed of (Al+θ+Si) ternary eutectics and (Al+θ) pseudobinary eutectics. During acoustic levitation, the (Al+θ+Si) ternary eutectics are refined and the (Al+θ) pseudobinary eutectics have morphological diversity. On the surface of the alloys, surface oscillations and acoustic streaming promote the nucleation of the three eutectic phases and expedite the cooling process. This results in the refinement of the ternary eutectic microstructure. During experiments, the reflector decreases with increasing alloy temperature, and the levitation distance always exceeds the resonant distance. Because of the acoustic radiation pressure, the melted alloy was flattened, and deformation increases with increasing sound pressure. The maximum aspect ratio achieved was 6.64, corresponding to a sound pressure of 1.8×104 Pa.
基金Project(50675210) supported by the National Natural Science Foundation of ChinaProject(03022) supported by the Key Science Research Program of Education Ministry of ChinaProject(200410250) supported by the Youth Science Foundation of Shanxi Province, China
文摘A type of polymer-coated molybdenum powder used in selective laser sintering technology was prepared by coating polymer on molybdenum particles and frozen grinding techniques, with the maximum particle diameter of 71 μm. The laser sintering experiments of polymer-coated molybdenum powder were conducted by using the self-developed selective laser sintering machine (HLRP-350I). The method of microscopic analysis was used to investigate the dynamic laser sintering process of polymer-coated molybdenum powder. Based on the study, the laser sintering mechanisms of polymer-coated molybdenum powder were presented. It is found that the mechanism is viscous flow when the laser sintering temperature is between 100 ℃ and 160 ℃, which can be described by a two-sphere model; and the mechanism is melting /solidification when the temperature is above 160 ℃.
基金financially supported by the National Natural Science Foundation of China(Nos.51690163,51822405,52130204,and 52174376)Science,Technology and Innovation Commission of Shenzhen Municipality(No.JCYJ20180306171121424)+3 种基金Joint Research Funds of the Department of Science&Technology of Shaanxi Province and NPU(No.2020GXLH-Z-024)Fundamental Research Funds for the Central Universities(No.D5000210902)Research Fund of the State Key Laboratory of Solidification Processing(NPU)(No.2019-QZ-02)Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University(No.CX2021056 and CX2021066)。
文摘As a typical dual-phase eutectic high entropy alloy(EHEA),AlCoCrFeNi_(2.1)can achieve the fair matching of strength and ductility,which has attracted wide attention.However,the engineering applications of as-cast AlCoCrFeNi_(2.1)EHEAs still face challenges,such as coarse grain and low yield strength resulting from low solidification rate and temperature gradient.In this study,selective laser melting(SLM)was introduced into the preparation of AlCoCrFeNi_(2.1)EHEA to realize unique strength-ductility balance,with emphasis on investigating the effects of processing parameters on its eutectic microstructure and properties.The results show that the SLM-ed samples exhibit a completely eutectic structure consisting of ultra-fine face-centered cubic(FCC)and ordered body-centered cubic(B2)phases,and the duplex microstructure undergoes a morphological evolution from lamellar structure to cellular structure as laser energy input reducing.The SLM-ed AlCoCrFeNi_(2.1)EHEA presents an excellent match of high tensile strength(1271 MPa),yield strength(966 MPa),and good ductility(22.5%)at room temperature,which are significantly enhanced by the ultra-fine grains and heterogeneous structure due to rapid solidification rate and high temperature gradient during SLM.Especially,the yield strength increment of~50%is realized with no loss in ductility as compared with the as-cast samples with the same composition.On this basis,the precise complex component with excellent mechanical properties is well achieved.This work paves the way for the performance improvement and complex parts preparation of EHEA by microstructural design using laser additive manufacturing.