Laser-directed energy deposition(L-DED)is an advanced additive manufacturing technology primarily adopted in metal three-dimensional printing systems.The L-DED process is characterized by various defects,thus necessit...Laser-directed energy deposition(L-DED)is an advanced additive manufacturing technology primarily adopted in metal three-dimensional printing systems.The L-DED process is characterized by various defects,thus necessitating the extensive use of in-situ monitoring to enable real-time adjustments of process parameters by detecting molten-pool features.To address the challenge of accurately extracting the molten-pool morphology from an undetached spatter,an innovative monitoring method based on the U-Net(U-shaped network)is proposed herein.A lightweight architecture accelerates the processing speed,whereas an enhanced loss function incorporating weight maps augments the segmentation precision.The model performance is evaluated by comparing its segmentation accuracy and processing speed with those of the conventional U-Net,using the mean intersection over union(MIoU)as the segmentation metric.The improved model demonstrates superior segmentation accuracy at the interface between the molten pool and spatter,with a peak MIoU of 0.9798 achieved on the test set.Furthermore,this model processes each image in an extremely short time of 17.9 ms.Using this segmentation algorithm,the error in extracting the molten-pool width from single-track experiments is within 0.1 mm.The proposed method for monitoring the molten-pool morphology is suitable for deployment in online monitoring systems,thus providing a foundation for subsequent process-parameter regulation.展开更多
The wide application of additive-manufactured Ti alloys is impeded by coarse columnar grains along the building direction and thus the severe anisotropy of mechanical properties.To address this issue,a novel multiallo...The wide application of additive-manufactured Ti alloys is impeded by coarse columnar grains along the building direction and thus the severe anisotropy of mechanical properties.To address this issue,a novel multialloying CoCrMoSi strategy has been developed to produce near-equiaxed grains of a modified Ti6Al4V(TC4)alloy for laser-directed energy deposition(LDED)based on computational thermodynamic and experimental approaches.The results show that the microstructure of the TC4alloy consists of large columnar β grains and α/α'laths with a high aspect ratio of 5.73,exhibiting a strong anisotropy of tensile properties.In contrast,the TC4-1.5%CoCrMoSi alloy is characterized by mixed columnarequiaxed β grains and near-equiaxed β grains with increased CoCrMoSi additions to 4.5%.Additionally,the α/α'laths are successively refined with the increase of CoCrMoSi content,showing an aspect ratio of smaller than4.31.However,an excess addition of CoCrMoSi leads to the formation of micro voids.After multi-alloyingCoCrMoSi,the number density of twins increases remarkably with a substantially reduced width,because of the increased lattice distortion and dislocation density together with the reducedβ→αphase transformation temperature.The anisotropy of the tensile properties can be effectively eliminated by adding 3 wt%CoCrMoSi with an exemplary strength-ductility combination,superior to the LDEDed-modified TC4 alloy in the literature reporting the tensile properties along both horizontal(X)and vertical(Z)directions.The underlaying mechanisms for the evolution of the microstructure and the tensile properties induced by multi-alloying CoCrMoSi were discussed in detail.展开更多
The unique structure and formation mechanism of medium-entropy alloys(MEAs)generally result in bet-ter comprehensive properties than traditional alloys.However,the strength-ductility trade-offremains a bottleneck,whic...The unique structure and formation mechanism of medium-entropy alloys(MEAs)generally result in bet-ter comprehensive properties than traditional alloys.However,the strength-ductility trade-offremains a bottleneck,which limits their applications.In this study,we designed novel high-performance CrNiCu x MEAs with a heterophase composition by incorporating a Cu-rich phase,and they were fabricated using laser-directed energy deposition(LDED).The results show that synergistic strengthening from multiple phases significantly improved the mechanical properties of the alloys,resulting in a tensile strength of 675 MPa and a ductility of 34.4%,demonstrating an excellent combination of high tensile strength and ductility.The improved mechanical properties of the CrNiCu x medium-entropy alloys are primarily due to the heterophase interfacial strengthening mechanism.In the alloy,numerous semi-coherent and coher-ent interfaces formed between the Cr-rich phase,Cu-rich phase,and the matrix,creating extensive lattice distortions at the interfaces.An increase in the Cu-rich phase content promoted the interaction between phases,enhancing the strain energy of the alloy and the barrier strength of the interfaces.The calcu-latedτint values,ranging from approximately 5.92-6.69 GPa,are significantly higher than those found in traditional alloys,providing a benchmark for designing new high-performance medium-entropy alloys.展开更多
Laser additive-manufactured (AM) metallic components typically have superior uniaxial tensile strength to their conventional processing counterparts. However, the strength and toughness trade-off for most AM-fabricate...Laser additive-manufactured (AM) metallic components typically have superior uniaxial tensile strength to their conventional processing counterparts. However, the strength and toughness trade-off for most AM-fabricated metallic parts remains unsolved. Generally, the heat treatment processes can enhance the elongation and toughness of as-deposited AM samples. In this work, the fracture toughness of high-power (7600 W) laser directed energy deposition Ti-6Al-4V (Ti64) + heat treatment (short as Ti64 DED-HT) samples, were studied using fracture property tests. Combining electron backscatter diffraction (EBSD), confocal laser scanning microscope, and fractal geometry theory, we investigated their fracture mechanism and proposed a new prediction model between plane-strain fracture toughness (K_(Ic)) and conventional tensile properties. The results show that the plane-strain fracture toughness value in four states (two scanning speeds and two directions) is 81.3 ± 0.7 MPa m^(1/2), higher than that of the wrought counterparts (∼65 MPa m^(1/2)). This high plane-strain fracture toughness results from the combination of relatively fine columnar β grains and coarse α laths of the deposited parts after a specific heat-treated process. Combined with a confocal laser scanning microscope and fractal geometry analysis theory, we found that the rough surface profile leads to high fractal dimension values. In addition, we proposed a modified analytical prediction model, which can effectively predict the plane-strain fracture toughness value of AM Ti64 titanium alloys. These findings provide a guideline for obtaining a high strength-toughness and reliably predicting its KIc value in AM titanium alloys.展开更多
A thermofluidic-metallurgical integrated model is proposed to investigate the thermofluidic transport and solidification characteristics of 316 L stainless steel under conventional laser-directed energy deposition(CL-...A thermofluidic-metallurgical integrated model is proposed to investigate the thermofluidic transport and solidification characteristics of 316 L stainless steel under conventional laser-directed energy deposition(CL-DED)and high-speed laser-directed energy deposition(HL-DED).Compared to the CL-DED strategy,the weaker Marangoni convection effect produced by the HL-DED strategy results in a lower flow velocity within the pool.A higher scanning speed can shorten the laser-scanning distance when the melt pool reaches a steady-state temperature and reduce the temperature gradients in the solid-phase zone at the rear of the melt pool.Because of the higher scanning speed and lower heat input per unit length,the HL-DED strategy can significantly decrease the flatness ratio of the melt pool,average the growth direction,and average the morphology indicator of the solidification interface,but can remarkably increase the average solidification velocity and cooling rate.Under the HL-DED strategy,the grain morphologies from the top to the bottom of the pool change from fully equiaxed grains to columnar/equiaxed mixed grains and then to fully columnar grains.However,fully equiaxed grains are not observed using the CL-DED strategy because of the larger morphological indicator.Finer grains are induced by the higher cooling rate in the HL-DED strategy.展开更多
The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experi...The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experimental characterizations were performed to examine the impact of IHT on the micros true ture and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy(MEA).The results show that the intensity of the IHT is proportional to the utilized laser energy density(LED).As the LED increased,significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT.However,the high LED leads to severe hot cracking within the as-built MEA,resulting in inferior ductility.By decreasing LED,the hot cracking was effectively eliminated.Meanwhile,low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization,resulting in a heterogeneous grain structure characterized by multi-scale-sized grains.This structure provides significant hetero-deformation-induced hardening during plastic deformation,enabling the alloy to have a sustainable work-hardening capacity.We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate ME As with excellent metallurgical quality and mechanical performance.展开更多
Tungsten(W)particle-reinforced nickel(Ni)-based composites were fabricated via laser-direct energy de-position(L-DED).The influence of the W particle size on the microstructure and mechanical properties of the deposit...Tungsten(W)particle-reinforced nickel(Ni)-based composites were fabricated via laser-direct energy de-position(L-DED).The influence of the W particle size on the microstructure and mechanical properties of the deposited samples was systematically studied.The results indicate that refining the W particle size could refine theγ-Ni grains and subgrains,thin the(Ni,Cr)_(4)W interface layer,and increase the disloca-tion density of the intergranular matrix,thus improving the tensile strength and elongation of the L-DED samples.As W particle size decreased from 75 to 150μm to 6.5-12μm,the tensile strength and elonga-tion of the deposited samples increased by 150 MPa and 2.9 times to 1347.6±15.7 MPa and 17.5±0.4%,respectively.Based on the properties of the interface(Ni,Cr)_(4)W,a load-transfer efficiency factor suitable for this composite was proposed and the load-transfer strengthening formula was optimized.A quanti-tative analysis of the strengthening mechanisms was established considering load-transfer strengthening,Hall-Petch strengthening,thermal-mismatch strengthening,and solid-solution strengthening.The calcu-lated contribution of each strengthening mechanism to the yield strength and theoretical calculations were in good agreement with the experimental data.The article breaks the bottleneck of poor plasticity of W particle-reinforced Ni-based composites prepared by L-DED and provides a theoretical basis for the construction design of W particle-reinforced Ni-based composites with excellent mechanical properties.展开更多
激光定向能量沉积(laser directed energy deposition,LDED)凭借高效率与工艺柔性,正成为解决室温高脆性和高活性的TiAl4822(Ti-48Al-2Cr-2Nb)合金传统工艺难加工、难制备大型复杂构件问题的关键途径,以充分发挥其航空发动机等高温轻质...激光定向能量沉积(laser directed energy deposition,LDED)凭借高效率与工艺柔性,正成为解决室温高脆性和高活性的TiAl4822(Ti-48Al-2Cr-2Nb)合金传统工艺难加工、难制备大型复杂构件问题的关键途径,以充分发挥其航空发动机等高温轻质部件的理想材料潜力。然而,LDED过程中快速熔融-凝固循环会产生极大的温度梯度和残余应力,从而导致构件开裂,但目前尚无成熟手段能够完全抑制裂纹产生。本工作利用整体高温辅助LDED制备出30 mm×25 mm×6 mm致密无裂纹的TiAl4822合金薄壁构件,并对其宏观形貌、微观组织、孔隙率及显微硬度进行研究。研究结果表明:在常温条件下,LDED制备的TiAl4822合金薄壁样件易发生以解理为主的脆性断裂,显微组织以细小等轴晶为主;引入800℃整体高温辅助后,沉积层晶粒定向生长为自下而上倾斜的柱状晶,孔隙率从0.05%降至0.008%,孔径分布更均匀,表面未见宏观裂纹;与此同时,显微硬度由常温样件的390.46HV_(0.2)降至354.94HV_(0.2),这主要归因于在高温辅助条件下晶粒长大、晶界减少及析出相中γ相的含量相对增加。因此,整体高温辅助不仅有效抑制裂纹与大尺寸孔隙的产生,还优化微观组织均匀性,为TiAl4822合金的高致密、高性能制备提供新途径。展开更多
在金属材料激光增材制造过程中,预测出热历史的变化,可有效地提高成形质量,确保成形零件的尺寸高精度。采用网格激活和边界调整的有限差分方法,建立了能在线预测激光定向能量沉积(laser directed energy deposition,LDED)温度场随时间...在金属材料激光增材制造过程中,预测出热历史的变化,可有效地提高成形质量,确保成形零件的尺寸高精度。采用网格激活和边界调整的有限差分方法,建立了能在线预测激光定向能量沉积(laser directed energy deposition,LDED)温度场随时间和空间变化的三维瞬态数学模型。预测出熔池中心温度和熔池中心后方不同处的温度。最后通过沉积实验对模型计算出的温度进行了验证,模拟耗时仅为实际加工时间的83.5%。展开更多
基金supported by National Natural Science Foundation of China(Grant Nos.52305440,52204263)Natural Science Foundation of Changsha City(Grant Nos.kq2208272,kq2208274)+1 种基金Tribology Science Fund of the State Key Laboratory of Tribology in Advanced Equipment(Grant SKLTKF22B09)National Key Research and Development Program of China(2022YFB3706902).
文摘Laser-directed energy deposition(L-DED)is an advanced additive manufacturing technology primarily adopted in metal three-dimensional printing systems.The L-DED process is characterized by various defects,thus necessitating the extensive use of in-situ monitoring to enable real-time adjustments of process parameters by detecting molten-pool features.To address the challenge of accurately extracting the molten-pool morphology from an undetached spatter,an innovative monitoring method based on the U-Net(U-shaped network)is proposed herein.A lightweight architecture accelerates the processing speed,whereas an enhanced loss function incorporating weight maps augments the segmentation precision.The model performance is evaluated by comparing its segmentation accuracy and processing speed with those of the conventional U-Net,using the mean intersection over union(MIoU)as the segmentation metric.The improved model demonstrates superior segmentation accuracy at the interface between the molten pool and spatter,with a peak MIoU of 0.9798 achieved on the test set.Furthermore,this model processes each image in an extremely short time of 17.9 ms.Using this segmentation algorithm,the error in extracting the molten-pool width from single-track experiments is within 0.1 mm.The proposed method for monitoring the molten-pool morphology is suitable for deployment in online monitoring systems,thus providing a foundation for subsequent process-parameter regulation.
基金financially supported by the National Natural Science Foundation of China(No.52375341)Hunan Provincial Natural Science Foundation(No.2022JJ30494)
文摘The wide application of additive-manufactured Ti alloys is impeded by coarse columnar grains along the building direction and thus the severe anisotropy of mechanical properties.To address this issue,a novel multialloying CoCrMoSi strategy has been developed to produce near-equiaxed grains of a modified Ti6Al4V(TC4)alloy for laser-directed energy deposition(LDED)based on computational thermodynamic and experimental approaches.The results show that the microstructure of the TC4alloy consists of large columnar β grains and α/α'laths with a high aspect ratio of 5.73,exhibiting a strong anisotropy of tensile properties.In contrast,the TC4-1.5%CoCrMoSi alloy is characterized by mixed columnarequiaxed β grains and near-equiaxed β grains with increased CoCrMoSi additions to 4.5%.Additionally,the α/α'laths are successively refined with the increase of CoCrMoSi content,showing an aspect ratio of smaller than4.31.However,an excess addition of CoCrMoSi leads to the formation of micro voids.After multi-alloyingCoCrMoSi,the number density of twins increases remarkably with a substantially reduced width,because of the increased lattice distortion and dislocation density together with the reducedβ→αphase transformation temperature.The anisotropy of the tensile properties can be effectively eliminated by adding 3 wt%CoCrMoSi with an exemplary strength-ductility combination,superior to the LDEDed-modified TC4 alloy in the literature reporting the tensile properties along both horizontal(X)and vertical(Z)directions.The underlaying mechanisms for the evolution of the microstructure and the tensile properties induced by multi-alloying CoCrMoSi were discussed in detail.
基金supported by the National Natural Science Foundation of China(Grant No.U2341254)the National Natural Science Foundation of China(Grant No.52071124),the Natural Science Foundation of Jiangsu Province(No.BK20230502)the Jiangsu Funding Program for Excellent Postdoctoral Talent(No.2022ZB547).
文摘The unique structure and formation mechanism of medium-entropy alloys(MEAs)generally result in bet-ter comprehensive properties than traditional alloys.However,the strength-ductility trade-offremains a bottleneck,which limits their applications.In this study,we designed novel high-performance CrNiCu x MEAs with a heterophase composition by incorporating a Cu-rich phase,and they were fabricated using laser-directed energy deposition(LDED).The results show that synergistic strengthening from multiple phases significantly improved the mechanical properties of the alloys,resulting in a tensile strength of 675 MPa and a ductility of 34.4%,demonstrating an excellent combination of high tensile strength and ductility.The improved mechanical properties of the CrNiCu x medium-entropy alloys are primarily due to the heterophase interfacial strengthening mechanism.In the alloy,numerous semi-coherent and coher-ent interfaces formed between the Cr-rich phase,Cu-rich phase,and the matrix,creating extensive lattice distortions at the interfaces.An increase in the Cu-rich phase content promoted the interaction between phases,enhancing the strain energy of the alloy and the barrier strength of the interfaces.The calcu-latedτint values,ranging from approximately 5.92-6.69 GPa,are significantly higher than those found in traditional alloys,providing a benchmark for designing new high-performance medium-entropy alloys.
基金supported by the National Key Re-search and Development Program of China(No.2022YFB4602301)the National Natural Science Foundation of China(No.52275381)+2 种基金the Key Research and Development Program of Shaanxi Province(No.2021LLRH-08)the 73rd batch of China Postdoctoral Science Foundation General Financial Support(No.2023MD734199)the Shaanxi Provincial Natural Science Basic Research Program(No.2023-JC-QN-0551).
文摘Laser additive-manufactured (AM) metallic components typically have superior uniaxial tensile strength to their conventional processing counterparts. However, the strength and toughness trade-off for most AM-fabricated metallic parts remains unsolved. Generally, the heat treatment processes can enhance the elongation and toughness of as-deposited AM samples. In this work, the fracture toughness of high-power (7600 W) laser directed energy deposition Ti-6Al-4V (Ti64) + heat treatment (short as Ti64 DED-HT) samples, were studied using fracture property tests. Combining electron backscatter diffraction (EBSD), confocal laser scanning microscope, and fractal geometry theory, we investigated their fracture mechanism and proposed a new prediction model between plane-strain fracture toughness (K_(Ic)) and conventional tensile properties. The results show that the plane-strain fracture toughness value in four states (two scanning speeds and two directions) is 81.3 ± 0.7 MPa m^(1/2), higher than that of the wrought counterparts (∼65 MPa m^(1/2)). This high plane-strain fracture toughness results from the combination of relatively fine columnar β grains and coarse α laths of the deposited parts after a specific heat-treated process. Combined with a confocal laser scanning microscope and fractal geometry analysis theory, we found that the rough surface profile leads to high fractal dimension values. In addition, we proposed a modified analytical prediction model, which can effectively predict the plane-strain fracture toughness value of AM Ti64 titanium alloys. These findings provide a guideline for obtaining a high strength-toughness and reliably predicting its KIc value in AM titanium alloys.
基金supported by National Natural Science Foundation of China(Grant No.52365041)Guizhou Provincial Basic Research Program(Natural Science)(Grant No.QKHJC-ZK[2023]017)+2 种基金Guizhou Provincial Talent Project(Grant No.QKH-GCC[2022]007–1)Guizhou Province Science and Technology Foundation(Grant No.BQW[2024]011)Guizhou University Fund Project(Grant No.[2024]03).
文摘A thermofluidic-metallurgical integrated model is proposed to investigate the thermofluidic transport and solidification characteristics of 316 L stainless steel under conventional laser-directed energy deposition(CL-DED)and high-speed laser-directed energy deposition(HL-DED).Compared to the CL-DED strategy,the weaker Marangoni convection effect produced by the HL-DED strategy results in a lower flow velocity within the pool.A higher scanning speed can shorten the laser-scanning distance when the melt pool reaches a steady-state temperature and reduce the temperature gradients in the solid-phase zone at the rear of the melt pool.Because of the higher scanning speed and lower heat input per unit length,the HL-DED strategy can significantly decrease the flatness ratio of the melt pool,average the growth direction,and average the morphology indicator of the solidification interface,but can remarkably increase the average solidification velocity and cooling rate.Under the HL-DED strategy,the grain morphologies from the top to the bottom of the pool change from fully equiaxed grains to columnar/equiaxed mixed grains and then to fully columnar grains.However,fully equiaxed grains are not observed using the CL-DED strategy because of the larger morphological indicator.Finer grains are induced by the higher cooling rate in the HL-DED strategy.
基金financially supported by the funding from Guangdong Province Basic and Applied Research Key Projects (No.2020B0301030001)National Key R&D Programme,Ministry of Science and Technology of China (Nos. 2018YFB1105200 and 2019YFA0209904)the National Natural Science Foundation of China (Nos.52371106 and 52371025)。
文摘The distinctive intrinsic heat treatment(IHT)originating from cyclic reheating in the laser-directed energy deposition(LDED) has attracted growing attention in recent years.In this investigation,simulations and experimental characterizations were performed to examine the impact of IHT on the micros true ture and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy(MEA).The results show that the intensity of the IHT is proportional to the utilized laser energy density(LED).As the LED increased,significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT.However,the high LED leads to severe hot cracking within the as-built MEA,resulting in inferior ductility.By decreasing LED,the hot cracking was effectively eliminated.Meanwhile,low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization,resulting in a heterogeneous grain structure characterized by multi-scale-sized grains.This structure provides significant hetero-deformation-induced hardening during plastic deformation,enabling the alloy to have a sustainable work-hardening capacity.We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate ME As with excellent metallurgical quality and mechanical performance.
基金supported by the Key Projects of the National Natural Science Foundation of China(Nos.92066201 and 92266101)the Jiangxi Provincial Key R&D Programme Projects(No.20212BBE51011).
文摘Tungsten(W)particle-reinforced nickel(Ni)-based composites were fabricated via laser-direct energy de-position(L-DED).The influence of the W particle size on the microstructure and mechanical properties of the deposited samples was systematically studied.The results indicate that refining the W particle size could refine theγ-Ni grains and subgrains,thin the(Ni,Cr)_(4)W interface layer,and increase the disloca-tion density of the intergranular matrix,thus improving the tensile strength and elongation of the L-DED samples.As W particle size decreased from 75 to 150μm to 6.5-12μm,the tensile strength and elonga-tion of the deposited samples increased by 150 MPa and 2.9 times to 1347.6±15.7 MPa and 17.5±0.4%,respectively.Based on the properties of the interface(Ni,Cr)_(4)W,a load-transfer efficiency factor suitable for this composite was proposed and the load-transfer strengthening formula was optimized.A quanti-tative analysis of the strengthening mechanisms was established considering load-transfer strengthening,Hall-Petch strengthening,thermal-mismatch strengthening,and solid-solution strengthening.The calcu-lated contribution of each strengthening mechanism to the yield strength and theoretical calculations were in good agreement with the experimental data.The article breaks the bottleneck of poor plasticity of W particle-reinforced Ni-based composites prepared by L-DED and provides a theoretical basis for the construction design of W particle-reinforced Ni-based composites with excellent mechanical properties.
文摘在金属材料激光增材制造过程中,预测出热历史的变化,可有效地提高成形质量,确保成形零件的尺寸高精度。采用网格激活和边界调整的有限差分方法,建立了能在线预测激光定向能量沉积(laser directed energy deposition,LDED)温度场随时间和空间变化的三维瞬态数学模型。预测出熔池中心温度和熔池中心后方不同处的温度。最后通过沉积实验对模型计算出的温度进行了验证,模拟耗时仅为实际加工时间的83.5%。
