Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect...Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel.This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF.The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction,but it reduces the average size of the cellular microstructure from 0.60 to 0.35μm.The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample,resulting in inadequate fusion and keyhole defects respectively.The optimal scanning speed for fabricating samples was determined to be 800 mm/s,which obtained the highest room temperature tensile strength and elongation,with the ultimate tensile strength measured at(1088.3±2.0)MPa and the elongation of(16.76±0.10)%.Furthermore,the mechanism of the evolution of surface morphology,defects,and energy input were clarified,and the relationship between cellular microstructure size and mechanical properties was also established.展开更多
An analytical method to investigate the morphological evolution of the cellular mi-crostructure is explored and proposed. The method is essentially based on the Es-helby 's micromechanics theory, and it is extende...An analytical method to investigate the morphological evolution of the cellular mi-crostructure is explored and proposed. The method is essentially based on the Es-helby 's micromechanics theory, and it is extended so as to be applied for a material system containing inclusions with high volume fraction, by employing the average stress field approximation by Mori and Tanaka. The proposed method enables us to discuss a stable shape of precipitate in the material system, which must be influenced by many factors: e.g., volume fraction of precipitate; Young's modulus ratio and lattice misfit between matrix and precipitate; external stress field in multiaxial state; and heterogeneity of plastic strain between matrix and precipitate. A series of numerical calculations were summarized on stable shape maps. The application of the method to predict the γ' rafting in superalloys during creep showed that the heterogeneity of plastic strain between matrix and precipitates may play a significant role in the shape stability of the precipitate. Furthermore, it was shown that the method was successfully applied to estimate the morphology of the cellular microstructure formed in CMSX-4 single crystal Ni-based superalloy.展开更多
Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mech...Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances,while its potential role in microstructure architecting during post-heat treatment is rarely explored.In this work,we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300)steel and revealed how this microstructure influences austenite reversion upon aging.Segregation of Ti and Mo is experimentally detected at cell boundaries.It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging,whereas much less austenite is found in conventionally treated 18Ni(300)steels.The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible.Phase-field simulations suggest austenite grows in a fast,partitionless manner along cell boundaries where the chemical driving force for austen-ite reversion is substantially enhanced by Ti and Mo segregations,but in a sluggish,partitioning manner towards cell interiors.Contrary to conventional views that austenite fraction should be confined to avoid strength reduction,current SLM-fabricated 18Ni(300)steel containing∼13%cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite.The design of austen-ite also shows its potential to enhance fracture toughness.The current study demonstrates that cellular structures could substantially alter austenite reversion behavior,providing a new route for microstructure architecting in additively manufactured steels.展开更多
A dynamic observation on the Sm (Co, Cu, Fe, Zr)_(7.4) permanent magnetic alloy with a 1000 kV HVEM and a study on the effect of Zr by Mossbauer effect are carried out. The magnetic property of the above magnets by po...A dynamic observation on the Sm (Co, Cu, Fe, Zr)_(7.4) permanent magnetic alloy with a 1000 kV HVEM and a study on the effect of Zr by Mossbauer effect are carried out. The magnetic property of the above magnets by powder metallurgy is Br=1.12 T, _iH_c=1078 kA/m, (BH)_(max)=243.6 kJ/m^3. It is found that the centers of the cellular structure. which plays an important role for _iH_c, form at 460℃ and grow up during 5000℃ to 700℃. The intrinsic coercivity of the alloy rises up with the gradual perfection, the size and amount increment of cellular structure. The Mossbauer experiment showed the addition of Zr induced atoms to enter into 2:17 phase from 1:5 phase, which raised the content and magnetic difference of the two phases. Adding Zr speeded up Fe atom to move to Co_3 crystal position from Co_1 position, hence raised the single-axis anisotropy of the alloy. The two effects are both beneficial for the rise of _iH_c.展开更多
Despite the success of cementless hip stem,stress shielding still presents a serious problem leading to bone resorption.Stems incorporating porous cellular structures represent a promising solution.Therefore,this stud...Despite the success of cementless hip stem,stress shielding still presents a serious problem leading to bone resorption.Stems incorporating porous cellular structures represent a promising solution.Therefore,this study validates the finite element models of titanium(Ti)alloy(Ti-6Al-4V)porous stem and effective porous stems.Several effective porous stems with strut thicknesses 0.33 mm-1.25 mm(18%-90%porosity)under different loading conditions were analyzed.The results of finite element models revealed that changing the load type and porosity affect stress shielding.Climbing loads yield the maximum stress levels while walking loads result in the lowest stresses in the stems.Furthermore,the point load results in the maximum stress shielding and micromotions(-19% to 18%,40μm to 703μm),as compared to walking(-17.5% to 3%,35μm to 242μm)and climbing loads(-7% to 1.6%,30μm to 221 μm).Finally,effective porous stems of strut thickness 0.87 mm exhibit the lowest stress shielding signals(<5%)under all loading conditions.展开更多
基金financially supported by the National Natural Science Foundation of China(Nos.U2141205,52371002,and 52374366)the Fundamental Research Funds for the Central Universities(Nos.06109125 and 06930007)Fundamental Research Funds for the Central Universities(No.FRF-BD-23-02).
文摘Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel.This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF.The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction,but it reduces the average size of the cellular microstructure from 0.60 to 0.35μm.The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample,resulting in inadequate fusion and keyhole defects respectively.The optimal scanning speed for fabricating samples was determined to be 800 mm/s,which obtained the highest room temperature tensile strength and elongation,with the ultimate tensile strength measured at(1088.3±2.0)MPa and the elongation of(16.76±0.10)%.Furthermore,the mechanism of the evolution of surface morphology,defects,and energy input were clarified,and the relationship between cellular microstructure size and mechanical properties was also established.
基金supported by the Ministry of Education,Japan,as Grant-in-Aid for Scientific Research(No.12650072 and 15360046)are greatly acknowledged
文摘An analytical method to investigate the morphological evolution of the cellular mi-crostructure is explored and proposed. The method is essentially based on the Es-helby 's micromechanics theory, and it is extended so as to be applied for a material system containing inclusions with high volume fraction, by employing the average stress field approximation by Mori and Tanaka. The proposed method enables us to discuss a stable shape of precipitate in the material system, which must be influenced by many factors: e.g., volume fraction of precipitate; Young's modulus ratio and lattice misfit between matrix and precipitate; external stress field in multiaxial state; and heterogeneity of plastic strain between matrix and precipitate. A series of numerical calculations were summarized on stable shape maps. The application of the method to predict the γ' rafting in superalloys during creep showed that the heterogeneity of plastic strain between matrix and precipitates may play a significant role in the shape stability of the precipitate. Furthermore, it was shown that the method was successfully applied to estimate the morphology of the cellular microstructure formed in CMSX-4 single crystal Ni-based superalloy.
基金the National Key R&D program of China(Grant Nos.2022YFB3705200 and 2021YFB3702301)the National Natural Science Foundation of China(Grant No.52171008)+2 种基金the National Key R&D program of China(Grant No.2022YFE0110800)the National Natural Science Foundation of China(Grants Nos.51922054 and U1808208)the Mobility Programme from the Sino-German Center(Grant No.M-0319).
文摘Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances,while its potential role in microstructure architecting during post-heat treatment is rarely explored.In this work,we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300)steel and revealed how this microstructure influences austenite reversion upon aging.Segregation of Ti and Mo is experimentally detected at cell boundaries.It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging,whereas much less austenite is found in conventionally treated 18Ni(300)steels.The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible.Phase-field simulations suggest austenite grows in a fast,partitionless manner along cell boundaries where the chemical driving force for austen-ite reversion is substantially enhanced by Ti and Mo segregations,but in a sluggish,partitioning manner towards cell interiors.Contrary to conventional views that austenite fraction should be confined to avoid strength reduction,current SLM-fabricated 18Ni(300)steel containing∼13%cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite.The design of austen-ite also shows its potential to enhance fracture toughness.The current study demonstrates that cellular structures could substantially alter austenite reversion behavior,providing a new route for microstructure architecting in additively manufactured steels.
基金Project supported by the State Key Laboratory of Magnetism Institute of Physics, Academia Sinica.
文摘A dynamic observation on the Sm (Co, Cu, Fe, Zr)_(7.4) permanent magnetic alloy with a 1000 kV HVEM and a study on the effect of Zr by Mossbauer effect are carried out. The magnetic property of the above magnets by powder metallurgy is Br=1.12 T, _iH_c=1078 kA/m, (BH)_(max)=243.6 kJ/m^3. It is found that the centers of the cellular structure. which plays an important role for _iH_c, form at 460℃ and grow up during 5000℃ to 700℃. The intrinsic coercivity of the alloy rises up with the gradual perfection, the size and amount increment of cellular structure. The Mossbauer experiment showed the addition of Zr induced atoms to enter into 2:17 phase from 1:5 phase, which raised the content and magnetic difference of the two phases. Adding Zr speeded up Fe atom to move to Co_3 crystal position from Co_1 position, hence raised the single-axis anisotropy of the alloy. The two effects are both beneficial for the rise of _iH_c.
文摘Despite the success of cementless hip stem,stress shielding still presents a serious problem leading to bone resorption.Stems incorporating porous cellular structures represent a promising solution.Therefore,this study validates the finite element models of titanium(Ti)alloy(Ti-6Al-4V)porous stem and effective porous stems.Several effective porous stems with strut thicknesses 0.33 mm-1.25 mm(18%-90%porosity)under different loading conditions were analyzed.The results of finite element models revealed that changing the load type and porosity affect stress shielding.Climbing loads yield the maximum stress levels while walking loads result in the lowest stresses in the stems.Furthermore,the point load results in the maximum stress shielding and micromotions(-19% to 18%,40μm to 703μm),as compared to walking(-17.5% to 3%,35μm to 242μm)and climbing loads(-7% to 1.6%,30μm to 221 μm).Finally,effective porous stems of strut thickness 0.87 mm exhibit the lowest stress shielding signals(<5%)under all loading conditions.