This paper discusses an experimental investigation into the fluidity of AZ91D-1 wt.%Ca O magnesium melt via induction for thin-section investment casting.Plaster molds with thin spiral cavities(0.5 to 1.5 mm square se...This paper discusses an experimental investigation into the fluidity of AZ91D-1 wt.%Ca O magnesium melt via induction for thin-section investment casting.Plaster molds with thin spiral cavities(0.5 to 1.5 mm square sections)were designed and manufactured to assess the impact of casting conditions on filling length,as magnesium alloys cause severe melting and melt-mold exothermic reactions,making investment casting challenging.Combinations of traditional Mg-mold reaction mitigation techniques,such as applying a protective mold coating(Yttria)and vacuum,were examined to determine their role in the filling process.The results suggest that when induction is employed to melt reactive alloys,these methods are not always beneficial,as initially thought.Particularly at higher melt temperatures,the combination of Yttria-coated molds with low-pressure vacuum induction significantly reduce fluidity:vacuum induced melt levitation which promotes oxidation with the residual atmosphere;and Yttria-coating cracking due to thermal stress during the mold fabrication slows filling and promotes significant melt-mold reaction.This study shows that best results to investment cast thin-sections are obtained by avoiding both vacuum and protective coatings,providing a viable route for the precision manufacturing of stent biomedical devices.展开更多
Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present resear...Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented ＂CRC＂ equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE （WE54） alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction （fs） in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.展开更多
To investigate the influence of high magnetic field (HMF) on the solidification microstructure of Cu-25wt.%Ag alloy, the Cu-25wt.%Ag alloy was prepared under HMF of 12 T, and for comparison, the alloy solidified witho...To investigate the influence of high magnetic field (HMF) on the solidification microstructure of Cu-25wt.%Ag alloy, the Cu-25wt.%Ag alloy was prepared under HMF of 12 T, and for comparison, the alloy solidified without HMF was also fabricated. Macro and microstructures of the alloys were observed using the stereomicroscope, and scanning electron microscope, field emission scanning electron microscopy. The weight percentages of the pro-eutectic and eutectic, Cu phase and Ag phase in eutectic, and precipitates of Ag phase in pro-eutectic were analyzed by using of IPP software. Results show that the morphology of the column dendrites changes into cellular dendrites and the grains are refined under HMF of 12 T. Meanwhile, the thickness of the eutectic wall increases, but the sizes of Cu phase and Ag phase and the eutectic lamellar spacings are decreased. The Ag precipitates in the Cu matrix become coarser and sparser. The weight percentage variation of the phases in the microstructure and the Cu-Ag binary phase diagram reveals that the eutectic point moves to the left of the eutectic point in the equilibrium condition and the supersaturated solid solubility of Ag decreases under HMF.展开更多
The microstructure in welding heat-affected zones of 5 wt.% manganese steels was studied, and its effect on impact toughness was analyzed. The simulated coarse-grained heat-affected zone (CGHAZ) had the lowest impact ...The microstructure in welding heat-affected zones of 5 wt.% manganese steels was studied, and its effect on impact toughness was analyzed. The simulated coarse-grained heat-affected zone (CGHAZ) had the lowest impact toughness of ~39 J at — 40℃ because of coarse-grained structure and least volume fraction of retained austenite (RA) of 1.2 vol.%. The impact toughness of simulated intercritical heat-affected zone (ICHAZ) and fine-grained heat-affected zone (FGHAZ) were ~165 and ~45 J, respectively, at — 40℃. The effective grain size of simulated FGHAZ was smaller than that of the simulated ICHAZ. Furthermore, microstructural investigation revealed that the simulated FGHAZ and ICHAZ had similarity in volume fraction and stability of RA. However, tempered martensite was present in ICHAZ and absent in FGHAZ. It is proposed that the presence of tempered martensite contributed to good impact toughness in simulated ICHAZ.展开更多
Thermal sprayed Ni-5wt.% Al coating was fabricated on the substrate of 6061-T6 aluminum alloy by twin-wire arc spraying. Experimental results indicated that the average value of bond strength was around 46. 90 MPa, th...Thermal sprayed Ni-5wt.% Al coating was fabricated on the substrate of 6061-T6 aluminum alloy by twin-wire arc spraying. Experimental results indicated that the average value of bond strength was around 46. 90 MPa, the average hardness was 240 HV and the average value of surface roughness was about O. 14 mm. Friction and wear test results showed that the dry friction coefficient of Ni-5wt.% Al coating firstly decreased, and then tended to a slight increase after 200 cycles. In the early abrasion stage, adhesion wear played the key role for wear mechanics of Ni-5wt.% Al coating, but gradually abrasive wear became to replace adhesion wear.展开更多
The high efficiency of Ce addition in grain refinement ofδ-ferrite in a cast Fe–4 wt.%Si alloy was verified.In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite,the convention...The high efficiency of Ce addition in grain refinement ofδ-ferrite in a cast Fe–4 wt.%Si alloy was verified.In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite,the conventional directional solidification technique,which enabled to freeze the solid–liquid interface to room temperature,was used to investigate the interfacial morphology and solute redistribution in the liquid at the front of the interface,together with thermodynamic calculation of the equilibrium partition coefficients of Ce and Si in Fe–4 wt.%Si–Ce system using the Equilib module and the FsStel database in FactSage software system.Metallographic examination using a laser scanning confocal microscope showed a transition of the solid–liquid interface from planar to cellular in the Fe–4 wt.%Si alloy after adding 0.0260 wt.%Ce during the directional solidification experiment.Further,electron probe microanalysis revealed an enhanced segregation of Si solute in the liquid at the front of the solid–liquid interface due to the Ce addition.This solute segregation is considered as the cause of planar to cellular interface transition,which resulted from the creation of constitutional supercooling zone.Thermodynamic calculation indicated that Ce also segregated at the solid–liquid interface and the Ce addition had negligible effect on the equilibrium partition coefficient of Si.It is reasonable to consider that the contribution of Ce to the grain refinement ofδ-ferrite in the cast Fe–4 wt.%Si alloy as a solute was marginal.展开更多
The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys a...The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.展开更多
In the present study, the tested hypereutectic Al-21wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modification effects and mechanis...In the present study, the tested hypereutectic Al-21wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modification effects and mechanism of P＋Ce complex modifier on the Si phase of hypereutectic Al-21wt.%Si alloy were studied, and the aging precipitation behavior after modification was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles are significantly refined after modification, while the needle-like eutectic silicon crystals become fibrous and short. It was found that the mechanism of phosphorus modification on the primary silicon can be attributed to heterogeneous nucleation of AlP, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modified hypereutectic Al-21wt.%Si alloy, there existed some strengthening phases such as AI4Cu9, Al2Cu, AlCu3, and Al57Mn12. The P＋Ce complex modifier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21wt.%Si alloy was modified using 0.08%wt.P ＋ 0.6wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and elevated temperatures are optimized （Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ℃）.展开更多
To explore the corrosion properties of magnesium alloys, the chemical behavior of a high strength Mg_(97)Zn_(1)Y_(2)-1 wt.%Si C alloy in different corrosion environments was studied. Three solutions of 0.2 mol·L^...To explore the corrosion properties of magnesium alloys, the chemical behavior of a high strength Mg_(97)Zn_(1)Y_(2)-1 wt.%Si C alloy in different corrosion environments was studied. Three solutions of 0.2 mol·L^(-1) NaCl, Na_(2)SO_(4) and NaNO_(3) were selected as corrosion solutions. The microstructures, corrosion rate, corrosion potential, and mechanism were investigated qualitatively and quantitatively by optical microscopy(OM), scanning electron microscopy(SEM), immersion testing experiment, and electrochemical test. Microstructure observation shows that the Mg_(97) Zn_(1)Y_(2)-1 wt.%Si C alloy is composed of α-Mg matrix, LPSO(Mg_(12) ZnY) phase and Si C phase. The hydrogen evolution and electrochemical test results reflect that the Mg_(97)Zn_(1)Y_(2)-1 wt.%SiC in 0.2 mol·L^(-1) Na Cl solution has the fastest corrosion rate, followed by Na_(2)SO_(4) and NaNO_(3) solutions, and that the charge-transfer resistance presents the contrary trend and decreases in turn.展开更多
W-2 wt.%Y2O3 composite material with uniform distribution of yttrium element was fabricated through processes of mechanical alloying （MA） and spark plasma sintering （SPS）. The relevant productions were characteriz...W-2 wt.%Y2O3 composite material with uniform distribution of yttrium element was fabricated through processes of mechanical alloying （MA） and spark plasma sintering （SPS）. The relevant productions were characterized by scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. The XRD showed that the W-2 wt.%Y2O3 composite powder, including tungsten matrix and Y2O3 particles, was refined to nanometer sizes during the MA process. The SEM and TEM micrographs showed that the MA produced composite powder presented a lamellar morphology and contained many dislocations and microcracks. The EDS showed that the Y and O elements were uniformly distributed in the W matrix after mechanically alloying for 15 h. The W-2 wt.%Y2O3 composite material with uniform distribution of yttrium was obtained by sintering of the MA produced composite powder.展开更多
Hypereutectic Al-40 wt.%Si alloys were fabricated by the combination of gas atomization and spark plasma sintering(SPS) technology. The effects of holding time(15-60 min) on phase composition, microstructure, density,...Hypereutectic Al-40 wt.%Si alloys were fabricated by the combination of gas atomization and spark plasma sintering(SPS) technology. The effects of holding time(15-60 min) on phase composition, microstructure, density,mechanical properties of Al-Si alloys were investigated by XRD, SEM, a hydrostatic balance, an automatic micro hardness tester and a universal tensile testing machine. The results showed that homogenous distribution of ultrafine primary Si and high density of alloys can be obtained at holding time of 30 min. Compared with primary Si(3.7 μm)fabricated by gas atomization, the average size increased from 5.17 to 7.72 μm with the increase of holding time during SPS process. Overall, the relative density, maximum tensile strength and Vickers hardness of 94.9%, 205 MPa and HV;196.86 were achieved at holding time of 30 min, respectively. In addition, all the diffraction peaks were corresponded to α-Al or β-Si and no other phase can be detected. Finally, the densification process of SPS was also discussed.展开更多
基金financed by National Funds through the Portuguese funding agency,FCT–Funda??o para a Ciência e a Tecnologia,within the strategic projects UIDB/04436/2020,UIDB/00481/2020 and LA/P/0063/2020(DOI 10.54499/LA/P/0063/2020)。
文摘This paper discusses an experimental investigation into the fluidity of AZ91D-1 wt.%Ca O magnesium melt via induction for thin-section investment casting.Plaster molds with thin spiral cavities(0.5 to 1.5 mm square sections)were designed and manufactured to assess the impact of casting conditions on filling length,as magnesium alloys cause severe melting and melt-mold exothermic reactions,making investment casting challenging.Combinations of traditional Mg-mold reaction mitigation techniques,such as applying a protective mold coating(Yttria)and vacuum,were examined to determine their role in the filling process.The results suggest that when induction is employed to melt reactive alloys,these methods are not always beneficial,as initially thought.Particularly at higher melt temperatures,the combination of Yttria-coated molds with low-pressure vacuum induction significantly reduce fluidity:vacuum induced melt levitation which promotes oxidation with the residual atmosphere;and Yttria-coating cracking due to thermal stress during the mold fabrication slows filling and promotes significant melt-mold reaction.This study shows that best results to investment cast thin-sections are obtained by avoiding both vacuum and protective coatings,providing a viable route for the precision manufacturing of stent biomedical devices.
基金financially supported by the National Basic Research Program of China(No.2013CB632202)
文摘Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented ＂CRC＂ equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE （WE54） alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction （fs） in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.
基金supported by the National High-Tech Research and Development Program of China(No.2007AA03Z519)the Talents Introduction and Discipline Innovation Program for Higher Education(No.B07015)the Program of "High Grade Numerical Control Machine Tool and Basic Equipment" Major Science and Technology Project(No.2012ZX04010031)
文摘To investigate the influence of high magnetic field (HMF) on the solidification microstructure of Cu-25wt.%Ag alloy, the Cu-25wt.%Ag alloy was prepared under HMF of 12 T, and for comparison, the alloy solidified without HMF was also fabricated. Macro and microstructures of the alloys were observed using the stereomicroscope, and scanning electron microscope, field emission scanning electron microscopy. The weight percentages of the pro-eutectic and eutectic, Cu phase and Ag phase in eutectic, and precipitates of Ag phase in pro-eutectic were analyzed by using of IPP software. Results show that the morphology of the column dendrites changes into cellular dendrites and the grains are refined under HMF of 12 T. Meanwhile, the thickness of the eutectic wall increases, but the sizes of Cu phase and Ag phase and the eutectic lamellar spacings are decreased. The Ag precipitates in the Cu matrix become coarser and sparser. The weight percentage variation of the phases in the microstructure and the Cu-Ag binary phase diagram reveals that the eutectic point moves to the left of the eutectic point in the equilibrium condition and the supersaturated solid solubility of Ag decreases under HMF.
基金Authors gratefully acknowledge the support from National Natural Science Foundation of China (No. 2015AA03A501).
文摘The microstructure in welding heat-affected zones of 5 wt.% manganese steels was studied, and its effect on impact toughness was analyzed. The simulated coarse-grained heat-affected zone (CGHAZ) had the lowest impact toughness of ~39 J at — 40℃ because of coarse-grained structure and least volume fraction of retained austenite (RA) of 1.2 vol.%. The impact toughness of simulated intercritical heat-affected zone (ICHAZ) and fine-grained heat-affected zone (FGHAZ) were ~165 and ~45 J, respectively, at — 40℃. The effective grain size of simulated FGHAZ was smaller than that of the simulated ICHAZ. Furthermore, microstructural investigation revealed that the simulated FGHAZ and ICHAZ had similarity in volume fraction and stability of RA. However, tempered martensite was present in ICHAZ and absent in FGHAZ. It is proposed that the presence of tempered martensite contributed to good impact toughness in simulated ICHAZ.
文摘Thermal sprayed Ni-5wt.% Al coating was fabricated on the substrate of 6061-T6 aluminum alloy by twin-wire arc spraying. Experimental results indicated that the average value of bond strength was around 46. 90 MPa, the average hardness was 240 HV and the average value of surface roughness was about O. 14 mm. Friction and wear test results showed that the dry friction coefficient of Ni-5wt.% Al coating firstly decreased, and then tended to a slight increase after 200 cycles. In the early abrasion stage, adhesion wear played the key role for wear mechanics of Ni-5wt.% Al coating, but gradually abrasive wear became to replace adhesion wear.
基金The authors are very grateful to the funding support from the National Natural Science Foundation of China(Grant Nos.51761034 and 51261018)the Natural Science Foundation of Inner Mongolia in China(Grant Nos.2017MS0512 and 2020BS05018).
文摘The high efficiency of Ce addition in grain refinement ofδ-ferrite in a cast Fe–4 wt.%Si alloy was verified.In order to further understand the solute effect of Ce on the grain refinement of δ-ferrite,the conventional directional solidification technique,which enabled to freeze the solid–liquid interface to room temperature,was used to investigate the interfacial morphology and solute redistribution in the liquid at the front of the interface,together with thermodynamic calculation of the equilibrium partition coefficients of Ce and Si in Fe–4 wt.%Si–Ce system using the Equilib module and the FsStel database in FactSage software system.Metallographic examination using a laser scanning confocal microscope showed a transition of the solid–liquid interface from planar to cellular in the Fe–4 wt.%Si alloy after adding 0.0260 wt.%Ce during the directional solidification experiment.Further,electron probe microanalysis revealed an enhanced segregation of Si solute in the liquid at the front of the solid–liquid interface due to the Ce addition.This solute segregation is considered as the cause of planar to cellular interface transition,which resulted from the creation of constitutional supercooling zone.Thermodynamic calculation indicated that Ce also segregated at the solid–liquid interface and the Ce addition had negligible effect on the equilibrium partition coefficient of Si.It is reasonable to consider that the contribution of Ce to the grain refinement ofδ-ferrite in the cast Fe–4 wt.%Si alloy as a solute was marginal.
文摘The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.
基金funded by the National Natural Science Foundation of China(51371077)
文摘In the present study, the tested hypereutectic Al-21wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modification effects and mechanism of P＋Ce complex modifier on the Si phase of hypereutectic Al-21wt.%Si alloy were studied, and the aging precipitation behavior after modification was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles are significantly refined after modification, while the needle-like eutectic silicon crystals become fibrous and short. It was found that the mechanism of phosphorus modification on the primary silicon can be attributed to heterogeneous nucleation of AlP, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modified hypereutectic Al-21wt.%Si alloy, there existed some strengthening phases such as AI4Cu9, Al2Cu, AlCu3, and Al57Mn12. The P＋Ce complex modifier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21wt.%Si alloy was modified using 0.08%wt.P ＋ 0.6wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and elevated temperatures are optimized （Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ℃）.
基金Financially supported by the National Natural Science Foundation of China (51665012)the Jiangxi Province Science Foundation for Outstanding Scholarship (20171BCB23061,2018ACB21020)the Primary Research & Development Plan of Jiangxi Province (20192BBEL50019)。
文摘To explore the corrosion properties of magnesium alloys, the chemical behavior of a high strength Mg_(97)Zn_(1)Y_(2)-1 wt.%Si C alloy in different corrosion environments was studied. Three solutions of 0.2 mol·L^(-1) NaCl, Na_(2)SO_(4) and NaNO_(3) were selected as corrosion solutions. The microstructures, corrosion rate, corrosion potential, and mechanism were investigated qualitatively and quantitatively by optical microscopy(OM), scanning electron microscopy(SEM), immersion testing experiment, and electrochemical test. Microstructure observation shows that the Mg_(97) Zn_(1)Y_(2)-1 wt.%Si C alloy is composed of α-Mg matrix, LPSO(Mg_(12) ZnY) phase and Si C phase. The hydrogen evolution and electrochemical test results reflect that the Mg_(97)Zn_(1)Y_(2)-1 wt.%SiC in 0.2 mol·L^(-1) Na Cl solution has the fastest corrosion rate, followed by Na_(2)SO_(4) and NaNO_(3) solutions, and that the charge-transfer resistance presents the contrary trend and decreases in turn.
文摘W-2 wt.%Y2O3 composite material with uniform distribution of yttrium element was fabricated through processes of mechanical alloying （MA） and spark plasma sintering （SPS）. The relevant productions were characterized by scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, X-ray diffraction （XRD）, and transmission electron microscopy （TEM）. The XRD showed that the W-2 wt.%Y2O3 composite powder, including tungsten matrix and Y2O3 particles, was refined to nanometer sizes during the MA process. The SEM and TEM micrographs showed that the MA produced composite powder presented a lamellar morphology and contained many dislocations and microcracks. The EDS showed that the Y and O elements were uniformly distributed in the W matrix after mechanically alloying for 15 h. The W-2 wt.%Y2O3 composite material with uniform distribution of yttrium was obtained by sintering of the MA produced composite powder.
基金Project(18JS060) supported by the Shaanxi Key Laboratory of Nano-materials and Technology,ChinaProject(2018JQ5087) supported by Natural Science Basic Research Plan of Shaanxi Province,China。
文摘Hypereutectic Al-40 wt.%Si alloys were fabricated by the combination of gas atomization and spark plasma sintering(SPS) technology. The effects of holding time(15-60 min) on phase composition, microstructure, density,mechanical properties of Al-Si alloys were investigated by XRD, SEM, a hydrostatic balance, an automatic micro hardness tester and a universal tensile testing machine. The results showed that homogenous distribution of ultrafine primary Si and high density of alloys can be obtained at holding time of 30 min. Compared with primary Si(3.7 μm)fabricated by gas atomization, the average size increased from 5.17 to 7.72 μm with the increase of holding time during SPS process. Overall, the relative density, maximum tensile strength and Vickers hardness of 94.9%, 205 MPa and HV;196.86 were achieved at holding time of 30 min, respectively. In addition, all the diffraction peaks were corresponded to α-Al or β-Si and no other phase can be detected. Finally, the densification process of SPS was also discussed.
