We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the...We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the reaction. Thereby, we established mathematical models on two scales, respectively, preform and reactor. These models were used for the numerical simulation of the process of ceramic matrix composites densified by isothermal chemical vapor infiltration(ICVI). The models were used to carry out a systematic study on the influence of process conditions and the preform structure on the densification behaviors. The most important findings of our study are that the processing time could be reduced by about 50% without compromising the quality of the material, if the processing temperature is 950-1 000 ℃ for the first 70 hours and then raised to 1 100 ℃.展开更多
The three-dimensional physical model of the randomly packed powder material irradiated by the laser beam was established,taking into account the transformation of the material phase,the melt spreading and the interact...The three-dimensional physical model of the randomly packed powder material irradiated by the laser beam was established,taking into account the transformation of the material phase,the melt spreading and the interaction of the free surface of the molten pool and the recoiling pressure caused by the material evaporation during the selective laser melting.Influence of the processing parameters on the thermal behavior,the material evaporation,the surface morphology and the densification behavior in the connection region of the molten pool and the substrate was studied.It was shown that the powder material underwent the transformation from the partial melting state to the complete melting state and finally to the overheating state with the applied laser energy density increasing from 167 J/mm^(3) to 417 J/mm^(3).Therefore,the solidified track ranged from the discontinuous tracks with the rough surface to the continuous tracks with residual porosities,then to the continuous and dense tracks and terminally to the fluctuated tracks with the increase in the laser energy density.Meanwhile,the laser energy effect depth was maintained the positive relationship with the laser energy density.The vortex velocity obtained in the free surface of the molten pool towards to the rear region in the opposite laser scan direction promoted the melt convection to the edge region of the molten pool as the laser energy density was higher than 277 J/mm^(3),demonstrating the efficient energy dissipation from the center of the irradiation region to the whole part of the molten pool and the attendant production of the sufficient melt volume.Therefore,the efficient spreading of the molten pool and the metallurgical bonding ability of the melt with the substrate was obtained at the optimized laser energy density of 277 J/mm^(3).However,the severe material evaporation would take place as the melt was overheated,resulting in the formation of the residual pores and poor surface quality.展开更多
Densification behavior of nanocrystalline Mg2Si (n-Mg2Si) with grain size about 30-50 nm was investigated by hot-pressing at 400℃. The results indicated that the densification process of n-Mg2Si exhibited three lin...Densification behavior of nanocrystalline Mg2Si (n-Mg2Si) with grain size about 30-50 nm was investigated by hot-pressing at 400℃. The results indicated that the densification process of n-Mg2Si exhibited three linear segments: p〈0.3 GPa, 0.3 GPa〈p〈1.2 GPa, and p〉1.2 GPa determined by Heckel formula, among which the third fast increasing segment in high pressure range p〉1.2 GPa has seldom been reported in conventional coarse-grained polycrystalline materials. Nevertheless, in the whole pressure range (0.125-1.500 GPa) investigated the densification behavior of n-Mg2Si can be well described by a Kawakita formula p/C=(1/a)p+ 1/(ab) with constant α=0.452 being in good agreement with the initial porosity of the compact.展开更多
B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass with different Al_2O_3 contents(1mol%, 3mol%, 5mol%, and 7mol%) was prepared, and it was intended to be used as lead-free and low-melting glass sealants for solid oxide fuel cells....B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass with different Al_2O_3 contents(1mol%, 3mol%, 5mol%, and 7mol%) was prepared, and it was intended to be used as lead-free and low-melting glass sealants for solid oxide fuel cells. The effects of Al_2O_3 content on the structures, thermal properties, and sintering behaviors of the B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass were investigated in detail. The Al_2O_3 content largely influenced the structures and thermal properties of the glass. When the Al_2O_3 content 5mol%, the transition temperature of the glass decreased with the Al_2O_3 content, while the crystallization temperature increased with the Al_2O_3 content. However, higher Al_2O_3 content degraded the stability of the glass. The B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass with 5 mol% Al_2O_3 content exhibits the optimal sintering densification characteristics and can be used as glass sealants for solid oxide fuel cells.展开更多
The healing behavior of micropores in powder metallurgy (P/M) 316L stainless steel during hot forging and subsequent heat treatment was studied. The results showed that hot forging can improve the homogeneity of the...The healing behavior of micropores in powder metallurgy (P/M) 316L stainless steel during hot forging and subsequent heat treatment was studied. The results showed that hot forging can improve the homogeneity of the pore size and enhance the relative density of material in varying degree due to different forging temperatures. As a re- sult of deformation and diffusion bonding at high temperature, the irregular pores were spheroidized and finally turned into stable inner grain pores. The comparison of compression behavior between P/M and wrought dense mate rials has shown that the pores can either be the obstacles of dislocation movement or be the nucleation sites accelera- ting the reerystallization according to the difference of deformation temperatures.展开更多
Alumina whisker reinforced zirconia ceramic composite was prepared by both hot oscillatory pressing(HOP)and conventional hot pressing(HP).The results show that compared with HP,HOP can significantly increase the final...Alumina whisker reinforced zirconia ceramic composite was prepared by both hot oscillatory pressing(HOP)and conventional hot pressing(HP).The results show that compared with HP,HOP can significantly increase the final density and densification rate of the material.Analysis of densification kinetics reveals that the predominant densification mechanism transits from grain boundary sliding in the beginning to the diffusion in the later stage.The main effect of the oscillating pressure is to increase the densification rate in the process of grain boundary sliding.The current study suggests that HOP is a promising technique for densifying whisker reinforced ceramics.展开更多
In the present study, a series of in situ TiB/Ti6Al4V composites were fabricated using selective laser melting. The formability, microstructure evolution and mechanical properties of the as-built samples added with di...In the present study, a series of in situ TiB/Ti6Al4V composites were fabricated using selective laser melting. The formability, microstructure evolution and mechanical properties of the as-built samples added with different contents of TiB2 were studied. It is found that the densification level is related to both the content of TiB2 and laser energy density. The added TiB2 reinforcement particle can spontaneously react with titanium and then form the TiB phase. The needle-like TiB phase tends to transform into dot-like particles with the decrease in energy density. Additionally, with the increase in TiB2 content, the TiB phase is coarsened due to the increased nucleation rate and more reactions. The grain morphology is found to largely depend on the translational speed of solid–fluid interface determined by the temperature gradient and cooling rate. Also, the microhardness of the as-built TiB/Ti6Al4V composites is obviously improved. More interestingly, as the energy density increases, the microhardness of the as-built TiB/Ti6Al4V composites firstly increases and then decreases due to the synergy of grain size and different morphologies and distribution of TiB phases. The wear resistance of TiB/Ti6Al4V composites is far superior to that of Ti6Al4V alloy owing to the increased microhardness resulted from the uniform distribution of the hard TiB phase in the matrix.展开更多
B_(x)C-TiB_(2)ceramic composites were fabricated via reactive spark plasma sintering using TiC and B as the raw materials.The impact of B/TiC mole ratios on the phase compositions,densification behaviors,microstructu...B_(x)C-TiB_(2)ceramic composites were fabricated via reactive spark plasma sintering using TiC and B as the raw materials.The impact of B/TiC mole ratios on the phase compositions,densification behaviors,microstructure,and mechanical properties of the ceramic composites were investigated.The results showed that the stoichiometry of‘B_(x)C’could be tailored by changing initial boron content and the obtained B_(4.5)C,B_(6.5)C and B_(8.5)C phases have the same crystal structure(R-3 m).The excess of B enhanced the reaction between TiC and B,which released a large amount of hot energy and promoted the densification of the composites.The TB8.5 composite sintered at 1900°C had the best comprehensive mechanical properties,with hardness and flexural strength of 40.36 GPa and 551 MPa,respectively.The formation of nano-sized TiB_(2)grains induced by reaction were beneficial for improving the mechanical properties of these composites.展开更多
基金Funded by the National Natural Science Foundation of China(No.51472092)
文摘We studied the characteristics of two-scale pore structure of preform in the deposition process and the mass transfer of reactant gas in dual-scale pores, and observed the physiochemical phenomenon associated with the reaction. Thereby, we established mathematical models on two scales, respectively, preform and reactor. These models were used for the numerical simulation of the process of ceramic matrix composites densified by isothermal chemical vapor infiltration(ICVI). The models were used to carry out a systematic study on the influence of process conditions and the preform structure on the densification behaviors. The most important findings of our study are that the processing time could be reduced by about 50% without compromising the quality of the material, if the processing temperature is 950-1 000 ℃ for the first 70 hours and then raised to 1 100 ℃.
基金We are grateful for the financial support from the National Key Research and Development Program“Additive Manufacturing and Laser Manufacturing”(Nos.2016YFB1100101,2018YFB1106302)the National Natural Science Foundation of China(Nos.51790175,51735005)+3 种基金the Fundamental Research Funds for the Central Universities(No.NC2020004),the financial support from the Innovation Fund of National Engineering and Research Center for Commercial Aircraft Manufacturing(No.COMAC-SFGS-2016-33238)National Natural Science Foundation of China for Creative Research Groups(Grant No.51921003)The 15th Batch of“Six Talents Peaks”Innovative Talents Team Program“Laser Precise Additive Manufacturing of Structure-Performance Integrated Lightweight Alloy Components”(No.TD-GDZB-001)(Jiangsu Provincial Department of Human Resources and Social Security of China)2017 Excellent Scientific and Technological Innovation Teams of Universities in Jiangsu“Laser Additive Manufacturing Technologies for Metallic Components”(Jiangsu Provincial Department of Education of China).
文摘The three-dimensional physical model of the randomly packed powder material irradiated by the laser beam was established,taking into account the transformation of the material phase,the melt spreading and the interaction of the free surface of the molten pool and the recoiling pressure caused by the material evaporation during the selective laser melting.Influence of the processing parameters on the thermal behavior,the material evaporation,the surface morphology and the densification behavior in the connection region of the molten pool and the substrate was studied.It was shown that the powder material underwent the transformation from the partial melting state to the complete melting state and finally to the overheating state with the applied laser energy density increasing from 167 J/mm^(3) to 417 J/mm^(3).Therefore,the solidified track ranged from the discontinuous tracks with the rough surface to the continuous tracks with residual porosities,then to the continuous and dense tracks and terminally to the fluctuated tracks with the increase in the laser energy density.Meanwhile,the laser energy effect depth was maintained the positive relationship with the laser energy density.The vortex velocity obtained in the free surface of the molten pool towards to the rear region in the opposite laser scan direction promoted the melt convection to the edge region of the molten pool as the laser energy density was higher than 277 J/mm^(3),demonstrating the efficient energy dissipation from the center of the irradiation region to the whole part of the molten pool and the attendant production of the sufficient melt volume.Therefore,the efficient spreading of the molten pool and the metallurgical bonding ability of the melt with the substrate was obtained at the optimized laser energy density of 277 J/mm^(3).However,the severe material evaporation would take place as the melt was overheated,resulting in the formation of the residual pores and poor surface quality.
基金the National Natural Science Foundation of China under grant No. 50371081.
文摘Densification behavior of nanocrystalline Mg2Si (n-Mg2Si) with grain size about 30-50 nm was investigated by hot-pressing at 400℃. The results indicated that the densification process of n-Mg2Si exhibited three linear segments: p〈0.3 GPa, 0.3 GPa〈p〈1.2 GPa, and p〉1.2 GPa determined by Heckel formula, among which the third fast increasing segment in high pressure range p〉1.2 GPa has seldom been reported in conventional coarse-grained polycrystalline materials. Nevertheless, in the whole pressure range (0.125-1.500 GPa) investigated the densification behavior of n-Mg2Si can be well described by a Kawakita formula p/C=(1/a)p+ 1/(ab) with constant α=0.452 being in good agreement with the initial porosity of the compact.
基金Funded by the Jiangxi Provincial Department of Education(KJLD13008)the National Natural Science Foundation of China(number 51362020)the Research Fund for the Doctoral Program of Higher Education(20123601110006)
文摘B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass with different Al_2O_3 contents(1mol%, 3mol%, 5mol%, and 7mol%) was prepared, and it was intended to be used as lead-free and low-melting glass sealants for solid oxide fuel cells. The effects of Al_2O_3 content on the structures, thermal properties, and sintering behaviors of the B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass were investigated in detail. The Al_2O_3 content largely influenced the structures and thermal properties of the glass. When the Al_2O_3 content 5mol%, the transition temperature of the glass decreased with the Al_2O_3 content, while the crystallization temperature increased with the Al_2O_3 content. However, higher Al_2O_3 content degraded the stability of the glass. The B_2O_3-SiO_2-ZnO-BaO-Al_2O_3 glass with 5 mol% Al_2O_3 content exhibits the optimal sintering densification characteristics and can be used as glass sealants for solid oxide fuel cells.
基金Item Sponsored by National Basic Research Program of China(2012CB619600)
文摘The healing behavior of micropores in powder metallurgy (P/M) 316L stainless steel during hot forging and subsequent heat treatment was studied. The results showed that hot forging can improve the homogeneity of the pore size and enhance the relative density of material in varying degree due to different forging temperatures. As a re- sult of deformation and diffusion bonding at high temperature, the irregular pores were spheroidized and finally turned into stable inner grain pores. The comparison of compression behavior between P/M and wrought dense mate rials has shown that the pores can either be the obstacles of dislocation movement or be the nucleation sites accelera- ting the reerystallization according to the difference of deformation temperatures.
基金We thank the financial support from the National Natural Science Foundation of China(Grant Nos.52072344 and U1904180)Excellent Young Scientists Fund of Henan Province(Grant No.202300410369)Henan Province University Innovation Talents Support Program(Grant No.21HASTIT001).
文摘Alumina whisker reinforced zirconia ceramic composite was prepared by both hot oscillatory pressing(HOP)and conventional hot pressing(HP).The results show that compared with HP,HOP can significantly increase the final density and densification rate of the material.Analysis of densification kinetics reveals that the predominant densification mechanism transits from grain boundary sliding in the beginning to the diffusion in the later stage.The main effect of the oscillating pressure is to increase the densification rate in the process of grain boundary sliding.The current study suggests that HOP is a promising technique for densifying whisker reinforced ceramics.
基金the Civil Aerospace Pre-research Project:research on additive manufacturing of core components in the liquid rocket enginethe Fundamental Research Funds for the Central Universities(Nos.2019kfyXMPY005 and 2019kfyXKJC042)。
文摘In the present study, a series of in situ TiB/Ti6Al4V composites were fabricated using selective laser melting. The formability, microstructure evolution and mechanical properties of the as-built samples added with different contents of TiB2 were studied. It is found that the densification level is related to both the content of TiB2 and laser energy density. The added TiB2 reinforcement particle can spontaneously react with titanium and then form the TiB phase. The needle-like TiB phase tends to transform into dot-like particles with the decrease in energy density. Additionally, with the increase in TiB2 content, the TiB phase is coarsened due to the increased nucleation rate and more reactions. The grain morphology is found to largely depend on the translational speed of solid–fluid interface determined by the temperature gradient and cooling rate. Also, the microhardness of the as-built TiB/Ti6Al4V composites is obviously improved. More interestingly, as the energy density increases, the microhardness of the as-built TiB/Ti6Al4V composites firstly increases and then decreases due to the synergy of grain size and different morphologies and distribution of TiB phases. The wear resistance of TiB/Ti6Al4V composites is far superior to that of Ti6Al4V alloy owing to the increased microhardness resulted from the uniform distribution of the hard TiB phase in the matrix.
基金support from the National Natural Science Foundation of China(51901063),the Fundamental Research Funds for the Central Universities of China(JZ2021HGTB0094 and PA2021GDGP0059)is greatly acknowledged。
文摘B_(x)C-TiB_(2)ceramic composites were fabricated via reactive spark plasma sintering using TiC and B as the raw materials.The impact of B/TiC mole ratios on the phase compositions,densification behaviors,microstructure,and mechanical properties of the ceramic composites were investigated.The results showed that the stoichiometry of‘B_(x)C’could be tailored by changing initial boron content and the obtained B_(4.5)C,B_(6.5)C and B_(8.5)C phases have the same crystal structure(R-3 m).The excess of B enhanced the reaction between TiC and B,which released a large amount of hot energy and promoted the densification of the composites.The TB8.5 composite sintered at 1900°C had the best comprehensive mechanical properties,with hardness and flexural strength of 40.36 GPa and 551 MPa,respectively.The formation of nano-sized TiB_(2)grains induced by reaction were beneficial for improving the mechanical properties of these composites.