Structure and Performance of Fe-N_x-C Catalyst for Oxygen Reduction Reaction Prepared by Vacuum Casting Method and the Second Pyrolysis

Affordable non-precious metal（NPM） catalysts played a vital role in the wide application of polymer electrolyte membrane fuel cells（PEMFC）. In current work, a facile vacuum casting reacting method based on vacuum casting was introduced to prepare Fe-N_x-C oxygen reduction reaction（ORR） catalysts with high efficient in acid medium. The catalysts were prepared with ammonium ferrous sulfate hexahydrate（AFS） and 1,10-phenanthroline monohydrate utilizing homemade mesoporous silica template. The heat treatment and its influence on structure and performance were systematically evaluated to achieve superior ORR performance and some clues were found. And 850 ℃ was found to be the best temperature for the first and second pyrolysis. The linear sweep voltammetry（LSV） results showed that there were only 18 mV slightly negative shifts of half-wave potential（E_（1/2）） of the optimal catalyst（749 mV） compared with the commercial Pt/C（20 μg·Pt·cm^-2）. Besides, I850 R also showed better electrochemical stability and methanol-tolerance than that of Pt/C. All evidences proved that our vacuum casting reacting strategy and heat treatment process were prospective for the future R＆D of high performance Fe-N_x-C ORR catalysts.

Vacuum assisted high-pressure die casting of AZ91D magnesium alloy at different slow shot speeds

The effects of vacuum assistance on the microstructure and mechanical properties of high-pressure die cast AZ91 D alloy at different slow shot speeds were evaluated. Plate-shaped castings of AZ91 D alloy were carried out on a TOYO BD-350V5 cold chamber die casting machine incorporated with a self-improved TOYO vacuum system. It was found that the vacuum pressure in the die cavity at the beginning of mold filling increases with the increase of slow shot speed, following a cubic polynomial curve, resulting in a decline in the porosity-reduction ability of vacuum assistance with the increase of slow shot speed. The externally solidified crystal（ESC） contents in conventional and vacuum die castings behave similar against the slow shot speed. The tensile properties of vacuum die castings were strongly influenced by the ESC content at relative low slow shot speeds. With the increase of slow shot speed, the influence of the gas porosity level in vacuum die castings would get prominent.

Effect of casting vacuum on thermodynamic and corrosion properties of Fe-based glassy alloy

The effect of casting vacuum on thermodynamic and corrosion properties of Fe61Co7Zr8Mo5W2B17 in shape of cylinder of 3 mm in diameter and ribbon of 20?40μm in thickness and 2?3 mm in width were investigated with X-ray diffraction (XRD), differential scanning calorimetry (DSC), dilatometer (DIL), scanning electron microscopy (SEM) and electrochemical station. It is found that high casting vacuum can improve the glass forming ability (GFA), the contraction degree during heating, and the pitting resistance of the glassy alloy, which can be ascribed to the fact that the dissolution of tungsten in the melt is improved under the high casting vacuum.

Rotary bending fatigue behavior of A356 –T6 aluminum alloys by vacuum pressurizing casting

Vacuum pressurizing casting technique, providing better mould filling and inter-dendritic feeding, can reduce the porosity greatly in cast aluminum alloys, and improve the fatigue properties. The rotary bending fatigue properties of A356-T6 alloys prepared by vacuum pressurizing casting were investigated. The S-N curve and limit strength 90 MPa under fatigue life of 107 cycles were obtained. The analyses on the fatigue fractography and microstructure of specimens showed that the fatigue fracture mainly occurs at the positions with casting defects in the subsurface, especially at porosities regions, which attributed to the crack propagation during the fatigue fracture process. Using the empirical crack propagation law of Pairs-Erdogon, the quantitative relationship among the initial crack size, fatigue life and applied stress was established. The fatigue life decreases with an increase in initial crack size. Two constants in the Pairs-Erdogon equation of aluminum alloy A356-T6 were calculated using the experimental data.

Design and Verification of an Auxiliary System for High Vacuum Die Casting

Vacuum die casting is the optimal method to produce high quality aluminum alloy components.At present,there are still very few systematic studies on vacuum die casting theory and equipment design.On the basis of the existing theories of the vacuum die casting pumping and venting systems,a simplified model is established in this research.The model has an aggregate unit consisted of ＂vacuum pump ＋ buffer tank＂ and a cylindrical container（including the shot sleeve,cavity and exhaust channel）.The theoretical analysis is carried out between the cavity pressure and the pumping time under different volume models.An auxiliary system for high vacuum die casting is designed based on the above analysis.This system is composed of a vacuum control machine and a new vacuum stop valve.The machine has a human-computer control mode with ＂programmable logic controller（PLC） ＋ touch screen＂ and a real-time monitoring function of vacuum degree for buffer tank and die cavity.The vacuum stop valve with the ＂compressed gas ＋ piston rod ＋ labyrinth groove＂ structure can realize the function of whole-process vacuum venting.The new system shows great advantages on vacuuming the cavity with a much faster speed by making tests on an existing die casting mold and a 250 t die casting machine.A die cavity pressure less than 10 kPa can be reached within 0.8 s in the experiment and the porosity of castings can be greatly decreased.The systematic studies on vacuum die casting theory and equipment have a great guiding significance for high vacuum die casting,and can also be applied to other high vacuum forming in related theoretical and practical research.

Characterization of A390 aluminum alloy produced at different slow shot speeds using vacuum assisted high pressure die casting

The effects of vacuum assistance on the microstructure and mechanical properties of high pressure die cast A390alloy at different slow shot speeds were evaluated.Plate-shaped specimens of hypereutectic A390aluminum alloy were produced on a TOYO BD?350V5cold chamber die casting machine incorporated with a self-improved TOYO vacuum system.According to the results,the vacuum pressure inside the die cavity increased linearly with the increasing slow shot speed at the beginning of mold filling.Meanwhile,tensile properties of vacuum die castings were deteriorated by the porosity content.In addition,the average primary silicon size decreased from23to14μm when the slow shot speed increased from0.05to0.2m/s,which has a binary functional relationship with the slow shot speed.After heat treatment,microstructural morphologies revealed that needle-shaped and thin-flaked eutectic silicon particles became rounded while Al2Cu dissolved intoα(Al)matrix.Furthermore,the fractography revealed that the fracture mechanism has evolved from brittle transgranular fracture to a fracture mode with many dimples after heat treatment.

Effect of depressurizing speed on mold filling behavior and entrainment of oxide film in vacuum suction casting of A356 alloy

The effect of depressurizing speed on mold filling behavior and entrainment of oxide film of A356 alloy was studied. Themold filling behavior and velocity fields were recorded by water simulation with particle image velocimetry. The results show thatthe gate velocity first increased dramatically, then changed with the depressurizing speed: the gate velocity increased slowly atrelatively high depressurizing speed; at reasonable depressurizing speed, the gate velocity kept unchanged; while at lowerdepressurizing speed, the gate velocity decreased firstly and then kept unchanged. High gate velocity results in melt falling backunder gravity at higher speed. The falling velocity is the main factor of oxide film entrainment in vacuum suction casting. The designcriterion of depressurizing rate was deduced, and the A356 alloy castings were poured to test the formula. The four-point bend testand Weibull probability plots were applied to assessing the fracture mechanisms of the as-cast A356 alloy. The results illuminate amethod on designing suitable depressurizing speed for mold filling in vacuum suction casting.

Surface composites fabricated by vacuum infiltration casting technique

Alumina （Al2O3） particles reinforced copper matrix surface composites were fabricated on the bronze substrate using the vacuum infiltration casting technique. Three cases were obtained in the vacuum infiltration casting technique： no infiltration, partial infiltration and full infiltration （the thickness of preforms do not exceed 3.5mm）. The reason of no infiltration is that the vacuum degree is not enough so that the force acting on the liquid metal is lower than the resistance due to the surface tension. Partial infiltration is because of somewhat lower vacuum degree and pouring temperature. Full desired infiltration is on account of suitable infiltration casting conditions, such as vacuum degree, pouring temperature, grain size and preheating temperature of the preform. The most important factor of affecting formation of surface composites is the vacuum degree, then pouring temperature and particle size. The infiltration mechanism was discussed on the bases of different processing conditions. The surface composite up to 3.5 mm in thickness with uniformly distributed Al2O3 particles could be fabricated via the vacuum infiltration casting technique.

Fractal model for evaluating heat transfer of high temperature porous corundum shell in vacuum investment casting

Under vacuum, heat transfer in porous corundum shell of investment casting depends on the characteristics of the solid materials and the spatial arrangement of solids and pores. In this study, we present a modified fractal approach to model the pore structure of corundum shell and to describe its influence on the thermal conductivity. We assumed that there is no heat convection in the shell. A sectioned view of porous corundum shell was studied and used to describe the geometric structure and to calculate the fractal dimension d. Based on the fractal dimension d, we obtained the relationship between volumetric solid content and pore arrangement in different measure scales. A heat transfer model was thus established using a network of resistors in which we applied an equivalent approach to calculate the effective thermal conductivity of real porous corundum shell that include the effects of heat conduction and heat radiation of solid. From the obtained results we discuss these effects on the effective thermal conductivity including the scale of measurement, the structure of pore and the temperature. At last these results were compared with other empirical model, which computed by assuming even porosity in which effect of pore structure was not being considered. Though the thermal conductivity calculated essentially in agreement with that obtained from empirical model, model used in this study is more close to the real heat transfer process.

Effect of multi-step slow shot speed on microstructure of vacuum die cast AZ91D magnesium alloy

Two multi-step （two-step and three-step） slow shot speeds were used in the vacuum die casting process of AZ91D magnesium alloy. The vacuum pressure variation in the die cavity before mold filling was monitored by using a pressure sensor. The microstructures of the produced castings were analyzed with optical microscope and image analysis software. The experimental results demonstrate that, the vacuum pressure in the die cavity at the beginning of mold filling is significantly reduced by using three-step slow shot speed, resulting in a low gas porosity level in the produced castings. At an appropriate multi-step slow shot speed, the dwell time of the liquid metal in the shot sleeve before mold filling can be reduced and the flow of the liquid metal in the shot sleeve at the later stage of the slow shot process can be restrained, which cause a low externally solidified crystal content in the produced castings.

Double oxide film defects in Ni-based super-alloy castings

A new class of defects has been recently discovered, which are regarded as double oxide film defects. In this study, a number of Ni-based vacuum-cast test bars were investigated. The fractures in the broken test bars were observed by Scanning Electron Microscopy （SEM）. Energy Dispersive Spectroscopy （EDS）, as well as Wavelength Dispersive Spectroscopy （WDS） were used to characterize the chemical elements of the fracture＇s surface. Observation by SEM revealed the presence of inclusions identified as films that appeared to have initiated the growth of carbides. This study has added to the evidence that oxide film defects do exist in Ni-based super-alloy in certain vacuum casting conditions. It provides a reference for researchers＇ further study on the defects, and provides a possible direction for researchers to improve casting technology so as to remove these defects.

Numerical simulation of solidification morphologies of Cu-0.6Cr casting alloy using modified cellular automaton model

The purpose of this study is to predict the morphologies in the solidification process for Cu-0.6Cr(mass fraction,%)alloy by vacuum continuous casting(VCC)and verify its accuracy by the observed experimental results.In numerical simulation aspect, finite difference(FD)method and modified cellular automaton(MCA)model were used to simulate the macro-temperature field, micro-concentration field,nucleation and grain growth of Cu-0.6Cr alloy using real data from actual casting operations.From the observed casting experiment,the preliminary grain morphologies are the directional columnar grains by the VCC process.The solidification morphologies by MCAFD model are in agreement with the result of actual casting experiment well.

Effect of different processing parameters on interfacial heat-transfer behavior in high-pressure die-casting process

Vacuum die casting can reduce the'air entrapment'phenomenon during casting process.Based on the temperature measurements at metal-die interface with different processing parameters,such as slow shot speed(VL),high shot speed(VH),pouring temperature(Tp)and initial die temperature(Tm),inverse method was developed to determine the interfacial heat transfer coefficient(IHTC).The results indicate that a closer contact between the casting and die could be achieved when the vacuum system is used.It is found that the vacuum could strongly increase the values of IHTC and decrease the grain size in castings.The IHTC could have a higher peak value with increasing the Tp from680to720℃or the VL from0.1to0.4m/s.In addition,the influence of the VH and Tm on IHTC could be negligible.

Effect of centrifugal counter-gravity casting on solidification microstructure and mechanical properties of A357 aluminum alloy

To investigate the influence of Centrifugal Counter-gravity Casting(C3) process on the solidification microstructure and mechanical properties of the casting, A357 aluminum alloy samples were produced by different process conditions under C3. The results show that C3 has better feeding capacity compared with the vacuum suction casting; and that the mechanical vibration and the convection of melts formed at the centrifugal rotation stage suppress the growth of dendrites, subsequently resulting in the refinement of grains and the improvement of mechanical properties, density and hardness. A finer grain and higher strength can be obtained in the A357 alloy by increasing centrifugal radius and rotational speed. However, casting defects will appear near the rotational axis and the mechanical properties will decrease once the rotational speed exceeds 150 r·min-1.

Properties of vacuum cast CuCr25 and CuCr25Te contact material

The CuCr25 and CuCr25Te contact materials were manufactured by vacuum casting process.The microstructures of two alloys were observed by metallographic microscope,the electrical conductivity,density and hardness were measured.The tensile test was done by universal testing machine while the fractography was observed by SEM,the breaking current tests were carried out in Weil Synthetic Circuit with arc-igniting branch in Xi’an Jiaotong University.The results show that the microstructures and physical parameters change after adding Te element.The tensile strength decreases and the toughness turns bad,which is propitious to improve the anti-welding property.But the breaking current capacity of CuCr25Te alloy is inferior to CuCr25 alloy.Thus,adding Te element has both advantages and disadvantages on main properties of CuCr contact material.

A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper

High pressure die casting copper is used to produce rotors for induction motors to improve efficiency.Experiments were carried out for a special"step-shape"casting with different step thicknesses.Based on the measured temperature inside the die,the interfacial heat transfer coefficient(IHTC)at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem.The IHTC peak value was 4.5×10^3-11×10^3 W·m^-2·K^-1 under the basic operation condition.The influences of casting pressure,fast shot speed,pouring temperature and initial die surface temperature on the IHTC peak values were investigated.Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate.An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.

Density and solidification feeding model of vacuum counter-pressure cast aluminum alloy under grade-pressuring conditions

The density of vacuum counter-pressure cast aluminum alloy samples under grade-pressuring condition was studied. The effect of grade pressure difference and time on the density of aluminum alloys was discussed, and the solidification feeding model under grade-pressuring condition was established. The results indicate the grade-pressured solidification feeding ability of vacuum counter-pressure casting mainly depends on grade pressure difference and time. With the increase of grade pressure difference, the density of all the aluminum alloy samples increases, and the trend of change in density from the pouring gate to the top location is first decreasing gradually and then increasing. In addition, in obtaining the maximum density, the optimal grade-pressuring time is different for samples with different wall thicknesses, and the solidification time when the solid volume fraction of aluminum alloy reaches about 0.65 appears to be the optimal beginning time for gradepressuring.

Synergistic effects of composition and heat treatment on microstructure and properties of vacuum die cast Al-Si-Mg-Mn alloys

The purpose of this study was to prepare high-quality Al-Si-Mg-Mn alloy with a good combination of strength and ductility employing the vacuum-assisted high-pressure die cast process. An orthogonal study of heat treatments was conducted to design an optimized T6 heat treatment process for both Al-10%Si-0.3%Mg-Mn and Al-11%Si-0.6%Mg-Mn alloys. The results demonstrate that no obvious blisters and warpage were observed in these two alloys with solid solution treatment. After the optimal T6 heat treatment of 530°C×3 h + 165°C×6 h, Al-11%Si-0.6%Mg-Mn alloy has better mechanical properties, of which tensile strength, yield strength and elongation reached 377.3 MPa, 307.8 MPa and 9%, respectively. The improvement of mechanical properties can be attributed to the high density of needle-like β″(Mg_5Si_6) precipitation after aging treatment and the fine and spherical eutectic Si particles uniformly distributed in the α-Al matrix.

High-tenacity in-situ Ti/Zr-based bulk metallic glasses composites fabricated by industrial high-pressure die casting

The glass-forming ability and mechanical properties of metallic glasses and their composites are well known to be sensitive to the preparation conditions and are highly deteriorated by industrial preparing conditions such as low-purity raw materials and low vacuum.Here,we showed that a series of in-situ bulk metallic glass composites(BMGCs)which exhibit excellent ductility and segmental work hardening were successfully developed utilizing a high vacuum high-pressure die casting(HV-HPDC)technology along with industrial-grade raw materials.The tensile properties of these BMGCs are systematically investigated and correlated with the alloy microstructure.As compared with the copper mold suction casting method,the volume fraction difference of the dendrite phase for the BMGCs with the same composition is not significant when fabricated by the HV-HPDC,whereas the size of theβ-phase is generally larger.Insitu BMGCs with the composition of Ti_(48)Zr_(20)(V_(12/17)Cu_(5/17))19 Be 13 obtained by the HV-HPDC process show ductility up to 11.3%under tension at room temperature and exhibit a certain amount of work hardening.Two conditions need to be met to enable the BMGCs,which are prepared by vacuum die-casting to retain favorable ductility:(1)The volume fraction ofβphase stays below 62%±2%;(2)The equiaxed crystals with a more uniform size in the range of 5-10μm.Meanwhile,the results of the present study provided guidance for developing BMGCs with good ductile properties under industrial conditions.

Manufacturing and High Heat Flux Testing of Brazed Flat-Type W/CuCrZr Plasma Facing Components

Water-cooled flat-type W/Cu Cr Zr plasma facing components with an interlayer of oxygen-free copper（OFC） have been developed by using vacuum brazing route.The OFC layer for the accommodation of thermal stresses was cast onto the surface of W at a temperature range of 1150oC-1200 oC in a vacuum furnace.The W/OFC cast tiles were vacuum brazed to a Cu Cr Zr heat sink at 940 oC using the silver-free filler material Cu Mn Si Cr.The microstructure,bonding strength,and high heat flux properties of the brazed W/Cu Cr Zr joint samples were investigated.The W/Cu joint exhibits an average tensile strength of 134 MPa,which is about the same strength as pure annealed copper.High heat flux tests were performed in the electron beam facility EMS-60.Experimental results indicated that the brazed W/Cu Cr Zr mock-up experienced screening tests of up to 15 MW/m^2 and cyclic tests of 9 MW/m^2 for 1000 cycles without visible damage.