Rapid surface resolidification with a high powered CO2-laser was performed in preparing directionally solidified Al2O3/YAG/ZrO2 ternary eutectic ceramic in situ composite.The effects of laser processing parameters on ...Rapid surface resolidification with a high powered CO2-laser was performed in preparing directionally solidified Al2O3/YAG/ZrO2 ternary eutectic ceramic in situ composite.The effects of laser processing parameters on the solidification microstructure characteristics and thermal properties were studied by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffractometry(XRD)and synthetically thermal analysis(STA).Detailed investigations of the influence of laser power and scanning rate on the preparation and microstructural parameters of the ternary eutectic were presented.Moreover, the eutectic phase separation rule at high temperature was discussed.The results indicate that solidification microstructure of the ternary eutectic composite is greatly influenced by the laser processing parameters.The synthetically thermal analysis shows that the eutectic temperature of ternary Al2O3/YAG/ZrO2 composite is 1 738℃,well matching the phase diagram of Al2O3-Y2O3-ZrO2.展开更多
The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement.A crossover was observed on the 1/M-T curve during the overheating process,indicating that a liqui...The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement.A crossover was observed on the 1/M-T curve during the overheating process,indicating that a liquid-liquid structure transition(LLST)took place in the melt.Based on this information,the effects of LLST on the solidification behavior,microstructure and tribology property were investigated experimentally.The sample solidified with the LLST exhibits significantly different solidification behaviors,i.e.,the nucleation undercooling and the recalescence extent are conspicuously enlarged,and the solidification time is shortened.As a result,the microstructure is effectively refined and homogenized,and the hardness and wear resistance are significantly enhanced.The present work might be helpful for not only theoretically understanding the influence of LLST on the solidification behavior but also providing an alternative approach to tailor the microstructure and properties.展开更多
The dissimilar 2B06 and 7B04 Al alloy joints were prepared by refill friction stir spot welding(RFSSW),and the microstructural evolution and corrosion behavior of the joints were investigated.Based on microstructural ...The dissimilar 2B06 and 7B04 Al alloy joints were prepared by refill friction stir spot welding(RFSSW),and the microstructural evolution and corrosion behavior of the joints were investigated.Based on microstructural analysis,the welded joints exhibit distinct microstructural zones,including the stir zone(SZ),thermomechanically affected zone(TMAZ),and heat-affected zone(HAZ).The grain size of each zone is in the order of HAZ>TMAZ>SZ.Notably,the TMAZ and HAZ contain significantly larger secondary-phase particles compared to the SZ,with particle size in the HAZ increasing at higher rotational speeds.Electrochemical tests indicate that corrosion susceptibility follows the sequence of HAZ>TMAZ>SZ>BM,with greater sensitivity observed at increased rotational speeds.Post-corrosion mechanical performance degradation primarily arises from crevice corrosion at joint overlaps,but not from the changes in the microstructure.展开更多
In order to overcome the embrittlement of metastable titanium alloys caused by the precipitation ofωiso phase during aging,regulation of isothermalωprecipitation was investigated in Ti−15Mo alloy.The results show th...In order to overcome the embrittlement of metastable titanium alloys caused by the precipitation ofωiso phase during aging,regulation of isothermalωprecipitation was investigated in Ti−15Mo alloy.The results show that the sample is brittle when direct aging(A)is applied at 350℃for 1 h after solution treatment(ST).If pre-deformation(D)is performed on the ST sample to induce{332}twins and secondaryα″phase,subsequent aging at 350℃(STDA350)improves the strength to 931 MPa with a good ductility of about 20%maintained.However,when aging is performed at 400℃or 450℃(STDA400/450),the strength can be further improved,but the ductility is dramatically reduced.Atomic-scale characterizations show that the partial collapse ofωphase in the STDA350 sample effectively eliminates aging-induced embrittlement,but complete collapse leads to poor ductility in the STDA400/450 sample.展开更多
In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and...In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode,and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.展开更多
Thin-walled aluminum alloy tube numerical control(NC)bending with small bending radius is a complex process with multi-factor coupling effects and multi-die constraints.A significance-based optimization method of the ...Thin-walled aluminum alloy tube numerical control(NC)bending with small bending radius is a complex process with multi-factor coupling effects and multi-die constraints.A significance-based optimization method of the parameters was proposed based on the finite element(FE)simulation,and the significance analysis of the processing parameters on the forming quality in terms of the maximum wall thinning ratio and the maximum cross section distortion degree was implemented using the fractional factorial design.The optimum value of the significant parameter,the clearance between the tube and the wiper die,was obtained,and the values of the other parameters,including the friction coefficients and the clearances between the tube and the dies,the mandrel extension length and the boost velocity were estimated.The results are applied to aluminum alloy tube NC bending d50 mm×1 mm×75 mm and d70 mm×1.5 mm×105 mm(initial tube outside diameter D0×initial tube wall thickness t0×bending radius R),and qualified tubes are produced.展开更多
The true stress-strain curves of as-cast 7075 aluminum alloy have been obtained by isothermal compression tests at temperatures of 300 500 ~C and strain rates of 0.01 10 s i. The plastic flow instability map is establ...The true stress-strain curves of as-cast 7075 aluminum alloy have been obtained by isothermal compression tests at temperatures of 300 500 ~C and strain rates of 0.01 10 s i. The plastic flow instability map is established based on Gegel B and Murthy instability criteria because the deformed compression samples suggest that the combination of the above two instability criteria has more comprehensive crack prediction ability. And the processing map based on Dynamic Mate- rial Model （DMM） of as-cast 7075 aluminum alloy has been developed through a superposition of the established instability map and power dissipation map. In terms of microstructure of the deformed samples and whether plastic flow is stable or not, the processing map can be divided into five areas： stable area with as-cast grain, stable area with homogeneous grain resulting from dynamic recovery, instability area with as-cast grain, instability area with the second phase and instability area with mixed grains. In consideration of microstructure characteristics in the above five areas of the processing map, the stable area with homogeneous grain resulting from dynamic recovery, namely the temperatures at 425465 ℃ and the strain rates at 0.01^-1 s^-1, is suggested to be suitable processing window for the as-cast 7075 aluminum alloy.展开更多
In order to get a better understanding of the vacuum consumable arc remelting(VAR) processes and thus to optimize them,a 3D finite element model was developed for the temperature fields and heat transfer of titanium a...In order to get a better understanding of the vacuum consumable arc remelting(VAR) processes and thus to optimize them,a 3D finite element model was developed for the temperature fields and heat transfer of titanium alloy ingots during VAR process.The results show that the temperature fields obtained by the simulation are well validated through the experiment results.The temperature distribution is different during the whole VAR process and the steady-state molten pool forms at 329 s for d100 mm × 180 mm ingots.At the initial stage of remelting,the heat dissipation of crucible bottom plays an important role in the whole heat dissipation system.At the middle of remelting,the crucible wall becomes a major heat dissipation way.The effect of cooling velocity on the solidification structure of ingots was investigated based on the temperature fields and the results can well explain the macrostructure of titanium alloy ingots.展开更多
To obtain high-quality aviation forgings of titanium alloys, b forging is an essential processing step which must be considered throughout a production process. In this work, the effect of b forging on the crystal ori...To obtain high-quality aviation forgings of titanium alloys, b forging is an essential processing step which must be considered throughout a production process. In this work, the effect of b forging on the crystal orientation and morphology of lamellar a was experimentally investigated in a two-phase titanium alloy. Strong dynamic recovery during b working resulted in the formation of low-angle grain boundary(LAGBb) inside b grains. The lamellar a can penetrate through the LAGBb, leading to similar intra a LAGBs on subgrain boundaries. Deformation banding occurs at high strain rates, and both diffusive and sharp boundaries of deformation bands can be observed.A continuous change of the b orientation in diffusive boundaries results in the formation of fine and disordered a lamellae without intra-lamellar boundary to hold the Burgers orientation relationship(OR). On sharp boundaries, it is prone to producing continuous grain boundary a(aGB) with a highly similar orientation along the boundaries. Meanwhile, there may exist several lower-angle boundaries within the grain boundary a for a smoother orientation change on the b grain boundary.展开更多
The influence of melt superheating treatment on the solid/liquid （S/L） interface morphology of directionally solidified Ni-based superalloy DZ125 is investigated to elucidate the relationship between melt characteri...The influence of melt superheating treatment on the solid/liquid （S/L） interface morphology of directionally solidified Ni-based superalloy DZ125 is investigated to elucidate the relationship between melt characteristic and S/L interface stability. The results indicate that the interface morphology is not only related to the withdrawal velocity （R） but also to the melt superheating temperature （Ts） when the thermal gradient of solidification interface remains constant for different Ts with appropriate superheating treatment regulation. The interface morphology changes from cell to plane at R of 1.1 μm/s when Ts increases from 1500°C to 1650°C, and maintains plane with further elevated Ts of 1750°C. However, the interface morphology changes from coarse dendrite to cell and then to cellular dendrite at R of 2.25 μm/s when Ts increases from 1500°C to 1650°C and then to 1750°C. It is proved that the solidification onset temperature and the solidification interval undergo the nonlinear variation when Ts increases from 1500°C to 1680°C, and the turning point is 1650°C at which the solidification onset temperature and the solidification interval are all minimum. This indicates that the melt superheating treatment enhances the solidification interface stability and has important effect on the solidification characteristics.展开更多
The effects of solute elements during solidification on the grain size are very important and can be quantified by the growth-restriction parameter Q,and Q possesses the better correlation with the grain size. Based o...The effects of solute elements during solidification on the grain size are very important and can be quantified by the growth-restriction parameter Q,and Q possesses the better correlation with the grain size. Based on the constitutional undercooling generated by the growth of an adjacent grain during the initial solidification,the growth-restriction parameter Q is deduced and a comprehensive physical basis of Q is obtained by using an initial solute distributing equation. For the alloys with more potent nucleants,Q is a suitable predictor of the grain size. For less potent nucleants,the relative grain size(RGS) is a more accurate prediction of the grain size. This prediction coincides with the experimental behaviors for Al-Ti and Al-Cu alloys with lower solute content.展开更多
The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron ...The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy dispersive spectroscope （EDS）, and transmission electron microscopy （TEM）. The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47A17Nb2.5V1.0Cr （at%） alloy solidifying through the peritectic reaction. The β-solidifying Ti46A17Nb2.5V1.0Cr （at%） alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48A17Nb2.5V1.0Cr （at%） alloy is equivalent to that of Ti46A17Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.展开更多
Isothermal compression tests of as-forged 30Si2MnCrMoVE low-alloying ultra-high-strength steel were carried out on a Gleeble 3500 thermal simulator at the deformation temperatures of 950-1150℃and strain rates of 0.01...Isothermal compression tests of as-forged 30Si2MnCrMoVE low-alloying ultra-high-strength steel were carried out on a Gleeble 3500 thermal simulator at the deformation temperatures of 950-1150℃and strain rates of 0.01-10 s^−1.Based on the classical stress-dislocation density relationship and the kinematics of the dynamic recrystallization,the constitutive equations of the work hardening dynamical recovery period and dynamical recrystallization period were developed by using the work hardening curve and Avrami equation,which shows good agreement with the experimental value.Processing maps at the strain of 0.90 were constructed based on dynamic material model and were analyzed combined with microstructure observation under different conditions.The optimum parameter based on the processing maps was obtained and verified by a supplementary experiment.The power dissipation maps and instability maps at strains of 0.05-0.90 were also constructed,and the evolution law was analyzed in detail.The established constitutive equation and hot processing maps can provide some guidance for hot working process.展开更多
The homogeneous liquid was separated into two phases, （Fe, Co）-rich LI and Cu-rich L2, once the melt was undercooled below a liquid-phase separation temperature Tsep. If the duration from Tsep to Tsl （solidificatio...The homogeneous liquid was separated into two phases, （Fe, Co）-rich LI and Cu-rich L2, once the melt was undercooled below a liquid-phase separation temperature Tsep. If the duration from Tsep to Tsl （solidification temperature of LI phase）, termed the liquid-phase separation interval Δt, exceeded a critical value, an eggtype structure was observed. By utilizing differential thermal analyses （DTA）, the solidification process of the undercooled Fe-Co-Cu alloys was studied. Additionally, an immiscible boundary was obtained, which was a convex parabola with a symmetrical axis of XCu=0.52. Depending on the relative amounts of LI and L2, the minor phase was nucleated firstly to form liquid droplets and separated from the original liquids at the beginning of liquid-phase separation.展开更多
Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will tr...Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.展开更多
Solidification behaviour of AA8006 aluminium alloy in suction casting has been investigated by field emission scanning electron microscopy with energy dispersive spectroscopy （EDS）. It is found that there is a secon...Solidification behaviour of AA8006 aluminium alloy in suction casting has been investigated by field emission scanning electron microscopy with energy dispersive spectroscopy （EDS）. It is found that there is a secondary solidification process of the remaining liquid in located region of α-Al dendrites, and the cooling rate influences not only the solidification of the primary α-Al dendrite, but also the secondary solidification process of the remaining liquid. With the primary solidification being responsible for the formation of the relatively large α- Al dendrite, a fine and homogeneous microstructure is observed in the secondary solidification. Furthermore, because of the presence of the fine microstructure, the eutectic reaction is confined into small intergranular areas, inducing the formation of the laminar eutectic phase in the primary solidification region and fibrous eutectic phase in the secondary solidification region. EDS analysis shows that the content of Fe is higher in the secondary solidification region, and the enrichment of the solute element further confirms the existence of the secondary solidification.展开更多
Phase selection and growth characteristics of directionally solidified Al_(2)O_(3)/GdAlO_3(GAP)faceted eutectic ce ramics are investigated over wide ranges of compositions and solidification rates to explore the eutec...Phase selection and growth characteristics of directionally solidified Al_(2)O_(3)/GdAlO_3(GAP)faceted eutectic ce ramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone.Through the obse rvation of the quenched solid-liquid interface,the competitive growth of primary faceted Al_(2)O_(3)phase,prima ry non-faceted GAP phase and Al_(2)O_(3)/GAP eutectic with diffe rent morphologies is detected.Microstructure transitions from wholly eutectic to primary Al_(2)O_(3)(GAP)dendrite plus eutectic and then to wholly eutectic are found in Al_(2)O_(3)-2 O mol%Gd_(2)O_(3)hypoeutectic(Al_(2)O_(3)-26 mol%Gd_(2)O_(3)hypereutectic)ceramics with the increase of solidification rate.The dendrite growth of faceted Al_(2)O_(3)and non-faceted GAP phases are well predicted by KGT model,which have introduced appro p riate dimensionless supersaturationΩto characterize the anisotropic growth of dendrites.Based on the maximum interface temperature criterion,the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al_(2)O_(3)/GAP eutectic ceramics is in good agreement with the experimental results.Besides,the influence of microstructure with these different morphologies on the flexural strength of Al_(2)O_(3)/GAP eutectic ceramics is studied.展开更多
To control the tri-modal microstructure and performance,a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed.The staged isothermal local loading experi...To control the tri-modal microstructure and performance,a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed.The staged isothermal local loading experiment on TA15alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure,i.e.,the generation of lamellarα,content variation of equiaxedα,spatial orientation change of lamellarαand globularization of lamellarα.Considering the laws of these microstructure phenomena,the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions.Then,the developed microstructure model was integrated with finite element(FE)model to predict the tri-modal microstructure in the isothermal local loading forming.Its reliability and accuracy were verified by the microstructure observation at different locations of sample.Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15alloy.展开更多
The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus. The experiments were performed under...The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus. The experiments were performed under a thermal gradient of approximately 45 K.cm-1 and at withdrawal rates ranging from 30 to 200 m.s-1. The experimental results show that the primary and secondary dendritic arm spacings （PDAS and SDAS） decrease when the withdrawal rate is increased. Compared with the theoretical models of PDAS, the results are in good agreement with Trivedi＇s model. The relationships of PDAS and SDAS with withdrawal rates can be described as ;λ1 = 649.7V-0.24±0.02 and λ2 = 281 V-0.32±0.03, respectively. In addition, the size of the λ2 phase significantly decreases with increasing withdrawal rate.展开更多
基金Project(50772090)supported by the National Natural Science Foundation of ChinaProject(04G53048)supported by the Aeronautical Science Foundation of China+4 种基金Project(20040699035)supported by the Specialized Research Fund for the Doctoral Program of Higher Education of ChinaProject(W018101)supported by the Foundation Research Fund of Northwestern Polytechnical University,ChinaProject(2007AMM004)supported by the Opening Project of State Key Laboratory for Advanced Metals and Materials,ChinaProject supported by the Fund of the State Key Laboratory of Solidification Processing in Northwestern Polytechnical University,ChinaProject supported by the Scientific Research Start-up Foundation for Outstanding Persons in Northwestern Polytechnical University,China
文摘Rapid surface resolidification with a high powered CO2-laser was performed in preparing directionally solidified Al2O3/YAG/ZrO2 ternary eutectic ceramic in situ composite.The effects of laser processing parameters on the solidification microstructure characteristics and thermal properties were studied by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffractometry(XRD)and synthetically thermal analysis(STA).Detailed investigations of the influence of laser power and scanning rate on the preparation and microstructural parameters of the ternary eutectic were presented.Moreover, the eutectic phase separation rule at high temperature was discussed.The results indicate that solidification microstructure of the ternary eutectic composite is greatly influenced by the laser processing parameters.The synthetically thermal analysis shows that the eutectic temperature of ternary Al2O3/YAG/ZrO2 composite is 1 738℃,well matching the phase diagram of Al2O3-Y2O3-ZrO2.
基金financially supported by the fund of National Key Laboratory for Precision Hot Processing of Metals(No.6142909200104)Shanghai Sailing Program+2 种基金National Training Program of Innovation and Entrepreneurship for Undergraduates(No.S202010699137)Natural Science Foundation of China(Nos.51690164 and 51801161)the Fundamental Research Funds for the Central Universities。
文摘The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement.A crossover was observed on the 1/M-T curve during the overheating process,indicating that a liquid-liquid structure transition(LLST)took place in the melt.Based on this information,the effects of LLST on the solidification behavior,microstructure and tribology property were investigated experimentally.The sample solidified with the LLST exhibits significantly different solidification behaviors,i.e.,the nucleation undercooling and the recalescence extent are conspicuously enlarged,and the solidification time is shortened.As a result,the microstructure is effectively refined and homogenized,and the hardness and wear resistance are significantly enhanced.The present work might be helpful for not only theoretically understanding the influence of LLST on the solidification behavior but also providing an alternative approach to tailor the microstructure and properties.
基金supported by the National Natural Science Foundation of China (Nos. 52075449, 51975480)。
文摘The dissimilar 2B06 and 7B04 Al alloy joints were prepared by refill friction stir spot welding(RFSSW),and the microstructural evolution and corrosion behavior of the joints were investigated.Based on microstructural analysis,the welded joints exhibit distinct microstructural zones,including the stir zone(SZ),thermomechanically affected zone(TMAZ),and heat-affected zone(HAZ).The grain size of each zone is in the order of HAZ>TMAZ>SZ.Notably,the TMAZ and HAZ contain significantly larger secondary-phase particles compared to the SZ,with particle size in the HAZ increasing at higher rotational speeds.Electrochemical tests indicate that corrosion susceptibility follows the sequence of HAZ>TMAZ>SZ>BM,with greater sensitivity observed at increased rotational speeds.Post-corrosion mechanical performance degradation primarily arises from crevice corrosion at joint overlaps,but not from the changes in the microstructure.
基金the financial support from the National Natural Science Foundation of China (No. 52374380)the China Postdoctoral Science Foundation (Nos. 2023M730234, 2024T171126)。
文摘In order to overcome the embrittlement of metastable titanium alloys caused by the precipitation ofωiso phase during aging,regulation of isothermalωprecipitation was investigated in Ti−15Mo alloy.The results show that the sample is brittle when direct aging(A)is applied at 350℃for 1 h after solution treatment(ST).If pre-deformation(D)is performed on the ST sample to induce{332}twins and secondaryα″phase,subsequent aging at 350℃(STDA350)improves the strength to 931 MPa with a good ductility of about 20%maintained.However,when aging is performed at 400℃or 450℃(STDA400/450),the strength can be further improved,but the ductility is dramatically reduced.Atomic-scale characterizations show that the partial collapse ofωphase in the STDA350 sample effectively eliminates aging-induced embrittlement,but complete collapse leads to poor ductility in the STDA400/450 sample.
基金financially supported by the National Natural Science Foundation of China(NSFC)under grant Nos.51774254,51774253,U1610123,51574207,51574206the Science and Technology Major Project of Shanxi Province under grant No.MC2016-06
文摘In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode,and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.
基金Projects(50905144,50875216)supported by the National Natural Science Foundation of ChinaProject(09-10)supported by the State Key Laboratory of Materials Processing and Die&Mould Technology,ChinaProject(JC201028)supported by the Northwestern Polytechnical University Foundation for Fundamental Research,China
文摘Thin-walled aluminum alloy tube numerical control(NC)bending with small bending radius is a complex process with multi-factor coupling effects and multi-die constraints.A significance-based optimization method of the parameters was proposed based on the finite element(FE)simulation,and the significance analysis of the processing parameters on the forming quality in terms of the maximum wall thinning ratio and the maximum cross section distortion degree was implemented using the fractional factorial design.The optimum value of the significant parameter,the clearance between the tube and the wiper die,was obtained,and the values of the other parameters,including the friction coefficients and the clearances between the tube and the dies,the mandrel extension length and the boost velocity were estimated.The results are applied to aluminum alloy tube NC bending d50 mm×1 mm×75 mm and d70 mm×1.5 mm×105 mm(initial tube outside diameter D0×initial tube wall thickness t0×bending radius R),and qualified tubes are produced.
基金financially supported by the National Science and Technology Major Project of China(No.2009ZX04005-031-11)the EU Marie Curie Actions–Mat Pro Future Project(No.FP7-PEOPLE-2012-IRSES-318968)the‘‘111"Project of China(No.B08040)
文摘The true stress-strain curves of as-cast 7075 aluminum alloy have been obtained by isothermal compression tests at temperatures of 300 500 ~C and strain rates of 0.01 10 s i. The plastic flow instability map is established based on Gegel B and Murthy instability criteria because the deformed compression samples suggest that the combination of the above two instability criteria has more comprehensive crack prediction ability. And the processing map based on Dynamic Mate- rial Model （DMM） of as-cast 7075 aluminum alloy has been developed through a superposition of the established instability map and power dissipation map. In terms of microstructure of the deformed samples and whether plastic flow is stable or not, the processing map can be divided into five areas： stable area with as-cast grain, stable area with homogeneous grain resulting from dynamic recovery, instability area with as-cast grain, instability area with the second phase and instability area with mixed grains. In consideration of microstructure characteristics in the above five areas of the processing map, the stable area with homogeneous grain resulting from dynamic recovery, namely the temperatures at 425465 ℃ and the strain rates at 0.01^-1 s^-1, is suggested to be suitable processing window for the as-cast 7075 aluminum alloy.
基金Project(2007CB613802) supported by the National Basic Research Program of China
文摘In order to get a better understanding of the vacuum consumable arc remelting(VAR) processes and thus to optimize them,a 3D finite element model was developed for the temperature fields and heat transfer of titanium alloy ingots during VAR process.The results show that the temperature fields obtained by the simulation are well validated through the experiment results.The temperature distribution is different during the whole VAR process and the steady-state molten pool forms at 329 s for d100 mm × 180 mm ingots.At the initial stage of remelting,the heat dissipation of crucible bottom plays an important role in the whole heat dissipation system.At the middle of remelting,the crucible wall becomes a major heat dissipation way.The effect of cooling velocity on the solidification structure of ingots was investigated based on the temperature fields and the results can well explain the macrostructure of titanium alloy ingots.
基金the support of the National Natural Science Foundation of China(No.51575449)
文摘To obtain high-quality aviation forgings of titanium alloys, b forging is an essential processing step which must be considered throughout a production process. In this work, the effect of b forging on the crystal orientation and morphology of lamellar a was experimentally investigated in a two-phase titanium alloy. Strong dynamic recovery during b working resulted in the formation of low-angle grain boundary(LAGBb) inside b grains. The lamellar a can penetrate through the LAGBb, leading to similar intra a LAGBs on subgrain boundaries. Deformation banding occurs at high strain rates, and both diffusive and sharp boundaries of deformation bands can be observed.A continuous change of the b orientation in diffusive boundaries results in the formation of fine and disordered a lamellae without intra-lamellar boundary to hold the Burgers orientation relationship(OR). On sharp boundaries, it is prone to producing continuous grain boundary a(aGB) with a highly similar orientation along the boundaries. Meanwhile, there may exist several lower-angle boundaries within the grain boundary a for a smoother orientation change on the b grain boundary.
基金supports from the National Natural Science Foundation of China (Grant No. 50931004)the National Basic Research Program of China (Grant Nos. 2011CB610406 and 2010CB631202)the National High Technology Research and Development Program (Grant No. 2007AA03Z552)
文摘The influence of melt superheating treatment on the solid/liquid （S/L） interface morphology of directionally solidified Ni-based superalloy DZ125 is investigated to elucidate the relationship between melt characteristic and S/L interface stability. The results indicate that the interface morphology is not only related to the withdrawal velocity （R） but also to the melt superheating temperature （Ts） when the thermal gradient of solidification interface remains constant for different Ts with appropriate superheating treatment regulation. The interface morphology changes from cell to plane at R of 1.1 μm/s when Ts increases from 1500°C to 1650°C, and maintains plane with further elevated Ts of 1750°C. However, the interface morphology changes from coarse dendrite to cell and then to cellular dendrite at R of 2.25 μm/s when Ts increases from 1500°C to 1650°C and then to 1750°C. It is proved that the solidification onset temperature and the solidification interval undergo the nonlinear variation when Ts increases from 1500°C to 1680°C, and the turning point is 1650°C at which the solidification onset temperature and the solidification interval are all minimum. This indicates that the melt superheating treatment enhances the solidification interface stability and has important effect on the solidification characteristics.
基金Project(G2000067202-1) supported by the National Basic Research Program of China
文摘The effects of solute elements during solidification on the grain size are very important and can be quantified by the growth-restriction parameter Q,and Q possesses the better correlation with the grain size. Based on the constitutional undercooling generated by the growth of an adjacent grain during the initial solidification,the growth-restriction parameter Q is deduced and a comprehensive physical basis of Q is obtained by using an initial solute distributing equation. For the alloys with more potent nucleants,Q is a suitable predictor of the grain size. For less potent nucleants,the relative grain size(RGS) is a more accurate prediction of the grain size. This prediction coincides with the experimental behaviors for Al-Ti and Al-Cu alloys with lower solute content.
基金financially supported by the National Basic Research Program of China(No.2011CB605503)the Program of Introducing Talents of Discipline to Universities(No.B08040)
文摘The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy dispersive spectroscope （EDS）, and transmission electron microscopy （TEM）. The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47A17Nb2.5V1.0Cr （at%） alloy solidifying through the peritectic reaction. The β-solidifying Ti46A17Nb2.5V1.0Cr （at%） alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48A17Nb2.5V1.0Cr （at%） alloy is equivalent to that of Ti46A17Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.
基金This work was supported by the Shaanxi Key Research and Development Program(No.S2017-ZDYF-ZDXM-GY-0115)Natural Science Basic Research Plan in Shaanxi Province of China(No.2017JM5010)Fundamental Research Funds for the Central Universities of China(No.3102019ZX004).
文摘Isothermal compression tests of as-forged 30Si2MnCrMoVE low-alloying ultra-high-strength steel were carried out on a Gleeble 3500 thermal simulator at the deformation temperatures of 950-1150℃and strain rates of 0.01-10 s^−1.Based on the classical stress-dislocation density relationship and the kinematics of the dynamic recrystallization,the constitutive equations of the work hardening dynamical recovery period and dynamical recrystallization period were developed by using the work hardening curve and Avrami equation,which shows good agreement with the experimental value.Processing maps at the strain of 0.90 were constructed based on dynamic material model and were analyzed combined with microstructure observation under different conditions.The optimum parameter based on the processing maps was obtained and verified by a supplementary experiment.The power dissipation maps and instability maps at strains of 0.05-0.90 were also constructed,and the evolution law was analyzed in detail.The established constitutive equation and hot processing maps can provide some guidance for hot working process.
基金supported by the Natural Science Foundation of China (Grant No.50771084)the Natural Science Foundation of the Education Department of Jiangsu province,China (Grant No.09KJB430004)
文摘The homogeneous liquid was separated into two phases, （Fe, Co）-rich LI and Cu-rich L2, once the melt was undercooled below a liquid-phase separation temperature Tsep. If the duration from Tsep to Tsl （solidification temperature of LI phase）, termed the liquid-phase separation interval Δt, exceeded a critical value, an eggtype structure was observed. By utilizing differential thermal analyses （DTA）, the solidification process of the undercooled Fe-Co-Cu alloys was studied. Additionally, an immiscible boundary was obtained, which was a convex parabola with a symmetrical axis of XCu=0.52. Depending on the relative amounts of LI and L2, the minor phase was nucleated firstly to form liquid droplets and separated from the original liquids at the beginning of liquid-phase separation.
基金supported by the National Natural Science Foundation of China (Grant No. 51771152)the National Key Research and Development Program of China (Grant No. 2018YFB1106800)。
文摘Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.
基金support from Research Fund of the State Key Laboratory of Solidification Processing (No. 42-QP-009)Fundamental Research Fund of Northwestern Polytechnical University (No. JC200929) the 111 Project (No. B08040)
文摘Solidification behaviour of AA8006 aluminium alloy in suction casting has been investigated by field emission scanning electron microscopy with energy dispersive spectroscopy （EDS）. It is found that there is a secondary solidification process of the remaining liquid in located region of α-Al dendrites, and the cooling rate influences not only the solidification of the primary α-Al dendrite, but also the secondary solidification process of the remaining liquid. With the primary solidification being responsible for the formation of the relatively large α- Al dendrite, a fine and homogeneous microstructure is observed in the secondary solidification. Furthermore, because of the presence of the fine microstructure, the eutectic reaction is confined into small intergranular areas, inducing the formation of the laminar eutectic phase in the primary solidification region and fibrous eutectic phase in the secondary solidification region. EDS analysis shows that the content of Fe is higher in the secondary solidification region, and the enrichment of the solute element further confirms the existence of the secondary solidification.
基金supported financially by the National Natural Science Foundation of China(Nos.51822405 and 51472200)the National Key R&D Program of China(Nos.2017YFB1103500 and2018YFB1106600)+5 种基金the Science,Technology and Innovation Commission of Shenzhen Municipality(No.JCYJ20180306171121424)the Research Fund of Equipment Development Department(No.61409230402)the Aeronautics Power Foundation(No.6141B09050337)the Innovation Fund of the Zhejiang Kechuang New Materials Research Institute(No.ZKN-18-P04)the Key R&D Program of Shaan Xi Province(No.2018ZDCXL-GY-09-04)the Research Fund of the State Key Laboratory of Solidification Processing(NPU)(No.2019-QZ-02)。
文摘Phase selection and growth characteristics of directionally solidified Al_(2)O_(3)/GdAlO_3(GAP)faceted eutectic ce ramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone.Through the obse rvation of the quenched solid-liquid interface,the competitive growth of primary faceted Al_(2)O_(3)phase,prima ry non-faceted GAP phase and Al_(2)O_(3)/GAP eutectic with diffe rent morphologies is detected.Microstructure transitions from wholly eutectic to primary Al_(2)O_(3)(GAP)dendrite plus eutectic and then to wholly eutectic are found in Al_(2)O_(3)-2 O mol%Gd_(2)O_(3)hypoeutectic(Al_(2)O_(3)-26 mol%Gd_(2)O_(3)hypereutectic)ceramics with the increase of solidification rate.The dendrite growth of faceted Al_(2)O_(3)and non-faceted GAP phases are well predicted by KGT model,which have introduced appro p riate dimensionless supersaturationΩto characterize the anisotropic growth of dendrites.Based on the maximum interface temperature criterion,the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al_(2)O_(3)/GAP eutectic ceramics is in good agreement with the experimental results.Besides,the influence of microstructure with these different morphologies on the flexural strength of Al_(2)O_(3)/GAP eutectic ceramics is studied.
基金Projects(51605388,51575449)supported by the National Natural Science Foundation of ChinaProject(B08040)supported by the "111" Project,China+1 种基金Project(131-QP-2015)supported by the Research Fund of the State Key Laboratory of Solidification Processing(NWPU),ChinaProject supported by the Open Research Fund of State Key Laboratory of Materials Processing and Die&Mould Technology,Huazhong University of Science and Technology,China
文摘To control the tri-modal microstructure and performance,a prediction model of tri-modal microstructure in the isothermal local loading forming of titanium alloy was developed.The staged isothermal local loading experiment on TA15alloy indicates that there exist four important microstructure evolution phenomena in the development of tri-modal microstructure,i.e.,the generation of lamellarα,content variation of equiaxedα,spatial orientation change of lamellarαand globularization of lamellarα.Considering the laws of these microstructure phenomena,the microstructure model was established to correlate the parameters of tri-modal microstructure and processing conditions.Then,the developed microstructure model was integrated with finite element(FE)model to predict the tri-modal microstructure in the isothermal local loading forming.Its reliability and accuracy were verified by the microstructure observation at different locations of sample.Good agreements between the predicted and experimental results suggest that the developed microstructure model and its combination with FE model are effective in the prediction of tri-modal microstructure in the isothermal local loading forming of TA15alloy.
基金supported by the National Basic Research Program of China(Nos.2010CB631202 and 2011CB610406)the National Natural Science Foundation of China(Nos.50827102,51171151 and 50931004)+1 种基金the Aeronautical Science Foundation of China(No.2010ZE53048)the China Post-doctoral Science Foundation Funded Project(No.20100481361)
文摘The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus. The experiments were performed under a thermal gradient of approximately 45 K.cm-1 and at withdrawal rates ranging from 30 to 200 m.s-1. The experimental results show that the primary and secondary dendritic arm spacings （PDAS and SDAS） decrease when the withdrawal rate is increased. Compared with the theoretical models of PDAS, the results are in good agreement with Trivedi＇s model. The relationships of PDAS and SDAS with withdrawal rates can be described as ;λ1 = 649.7V-0.24±0.02 and λ2 = 281 V-0.32±0.03, respectively. In addition, the size of the λ2 phase significantly decreases with increasing withdrawal rate.
