Vacuum Hot Pressing Research Articles

Characterization of Microstructure and High Temperature Compressive Strength of Austenitic Stainless Steel (21-4N) through Powder Metallurgy Route

Exposure of the engine valve to high temperatures led to the degradation of the valve material due to microstructural instability and deteriorating mechanical properties. Performance enhancement and alteration in microstructures can be attained through the powder metallurgy route which is a viable method to produce near net shape components. In this current study, the development of austenitic stainless steel (21-4N) through the powder metallurgy route as an alternate material for engine valves was investigated. Mechanical alloying was carried out for the pre-alloyed mixtures and consolidated using vacuum hot pressing. Sintering parameters were fixed at 1200 °C, 50 MPa and at a vacuum level of 10-3 Torr. A scanning electron microscope was used to analyze the morphology of the milled powders. Densities for the hot pressed powders were compared with theoretical densities and found to be around 98–99%. Observations regarding grain size, the presence of austenitic grain, heterogeneous distribution of metal carbides and analysis of chemical composition along the metal matrix were determined using both optical and electron microscopes. X-ray diffraction was carried out for both the consolidated and powder samples. The hot pressed samples exhibited a hardness value of 410 ± 10 Hv. An isothermal compression test for the sintered samples was carried out at a temperature of 650 °C and strain rate of 0.001 s−1. It is showed that the compressive strength of 1380 MPa. An analysis between the room temperature yield strength obtained from hardness measurement and the strengthening mechanism based on the microstructure was conducted. Grain size, dislocation and solid solution are the major strengthening mechanisms which strengthen the material. Overall, the development of valve steel material through the powder metallurgy route exhibited improved metallurgical and mechanical properties in comparison to the corresponding cast product.

Research on Preparation and Mechanical Properties of Multilayer Graphene Reinforced Aluminum Matrix Composite

In order to solve the problem of the dispersion of graphene in aluminum-based composite materials, a high-energy ball milling method was used to prepare graphene/aluminum-based composite powder, and then the graphene/aluminum composite material was obtained by vacuum hot pressing. XRD, SEM, TEM were used to analyze and study the morphology and microstructure of graphene/aluminum-based composite powders, and the tensile mechanical properties, hardness and fracture morphology characteristics of graphene/aluminum-based composites. The research results show that the addition of graphene can reduce the cold welding of aluminum powder during the high-energy ball milling process, and obtain better dispersed graphene/aluminum composite powder; when the content of graphene is 1 wt.%, the composite material The tensile strength can reach 295.62 MPa, the hardness is 113.6HV, which is 24% higher than the pure aluminum tensile strength, and the hardness is increased by 20%; graphene exists in the dimples of the tensile fracture, under the action of tensile force, Graphene can carry load transfer and improve the tensile strength of composite materials, which is the enhancement mechanism of graphene in aluminum-based composite materials.

High transparency Pr:Y2O3 ceramics: A promising gain medium for red emission solid-state lasers

Highly transparent 0.5 and 1.0 at% Pr-doped Y2O3 ceramics were fabricated by vacuum sintering plus hot isostatic pressing (HIP) treatment. The selection of suitable pre-sintering temperatures and right microstructures before HIP was critical to obtain high density of the final sintered bodies. The well-densified ceramics had pore-free microstructures with an average grain size of about 1 µm. It was also found that the charge states of the Pr ions could be changed through regulating the annealing atmospheres, resulting in different absorption and emission characteristics in the visible wavelength region. Annealing in reducing atmosphere (5% H2/95% Ar) favored the formation of Pr3+, resulting in stronger red emissions, while annealing in oxygen atmosphere led to the rise of lattice constant due to the concentration increase of oxygen interstitials. The H2/Ar-annealed 0.5 at% Pr:Y2O3 ceramics exhibited strong red emission at 600–675 nm, which may be a promising gain material for red solid-state lasers.

Improved optical properties of BN powder shielded Ce:YAG ceramics prepared by hot pressing

Yttrium aluminum garnet (YAG) phosphor transparent ceramics prepared via vacuum hot pressing (HP) suffer from carbon contamination owing to the graphite mould used in the HP system. The conventional method was to dope LiF as a decarburizing agent, but carbon contamination cannot be effectively prevented when preparing Ce:YAG ceramics via a solid-phase reaction combined with HP sintering. In this study, we report the fabrication of highly transparent Ce:YAG ceramics via HP sintering with a flexible hexagonal boron nitride (BN-h) wrapping, which prevents carbon contamination of the samples caused by the graphite mould used in the process. High-transmittance Ce:YAG ceramics were prepared at a medium temperature of 1600 °C and a low vacuum pressure of 10−1 Pa.

MOF-derived CoNi@C-silver nanowires/cellulose nanofiber composite papers with excellent thermal management capability for outstanding electromagnetic interference shielding

High-performance electromagnetic interference (EMI) shielding materials with excellent flexibility and superior thermal management properties are ideal candidates for aerospace, communication industry, artificial intelligence, and wearable electronics. Herein, MOF-derived CoNi@C-silver nanowires/cellulose nanofiber (MAg/CNF) composite papers with Janus structure were prepared using a combination of two-step vacuum filtration and hot pressing. The resulting composite papers exhibit distinct electrical differences on both sides. The electrical conductivity of the CoNi@C/CNF side is 8.35 × 10−12 S/cm, while that of the AgNWs/CNF side is 990 S/cm. EMI shielding effectiveness (SE) of the MAg/CNF composite papers reaches 82 dB in the X-band, benefiting from effective double-layer structure, the AgNWs conductive network and the CoNi@C permeability network for adequate dissipation of electromagnetic waves. Excellent EMI shielding performance is maintained even after being subjected to low temperature of −196 °C and bending cycles of 1000 times. In addition, the MAg/CNF composite paper-based electric heaters exhibit satisfactory heating temperatures at absolutely safe operating voltages for the human as well as sufficient stability and repeatability during repeated heating and cooling cycles. Therefore, our fabricated MAg/CNF composite papers with Janus structure present broad application prospects for high-performance EMI shielding and thermal management in low-temperature environment.

Effect of Electroless Cu Plating Ti3AlC2 Particles on Microstructure and Properties of Gd2O3/Cu Composites.

Ti3AlC2 presents a hexagonal layered crystal structure and bridges the gap between metallic and ceramic properties, and Gadolinia (Gd2O3) has excellent thermodynamic stability, which make them potentially attractive as dispersive phases for Cu matrix composites. In this paper, Cu@Ti3AlC2-Gd2O3/Cu composites, Ti3AlC2-Gd2O3/Cu composites, and Gd2O3/Cu composites were prepared by electroless Cu plating, internal oxidation, and vacuum hot press sintering. The microstructure and the effect of the Cu plating on the properties of the Cu@Ti3AlC2-Gd2O3/Cu composites were discussed. The results showed that a Cu plating with a thickness of about 0.67 μm was successfully plated onto the surface of Ti3AlC2 particles. The ex situ Ti3AlC2 particles were distributed at the Cu grain boundary, while the in situ Gd2O3 particles with a grain size of 20 nm were dispersed in the Cu grains. The electroless Cu plating onto the surface of the Ti3AlC2 particles effectively reduces their surfactivity and improves the surface contacting state between the Cu@Ti3AlC2 particles and the Cu matrix, and reduces electron scattering, so that the tensile strength reached 378.9 MPa, meanwhile, the electrical conductivity and elongation of the Cu matrix composites was maintained at 93.6 IACS% and 17.6%.

The Study of Enhanced Mechanical Properties for Cu/Gr/2024Al Composite

2024Al alloy powder as matrix material with nanocopper-modified graphene as reinforcement was studied to explore the effects of graphene on the tissue, hardness, friction performance of the composite. The Cu/Gr/2024Al composites were prepared via three-dimensional mixed powder and vacuum hot press sintering. The results found that the nanocopper-modified graphene could be uniformly distributed in the aluminum alloy matrix, and formed a good binding interface with the matrix material. When the graphene content was 0.75 wt.% and 1.0wt%, the impact yield strength and the hardness reached the maximum of 434.8 MPa and 118.4 HV5, which were 27.24% and 43.11% higher than that of 2024Al respectively. Furthermore, with the increase of nanocopper-modified graphene content, the corrosion resistance of composite materials in 3.5%Cl-concentration solution was improved.

Study on Effect of Milling Time on the Mechanical and Wear Properties of the Za40-B4c-Graphene Hybrid Nanocomposites Fabricated by Vacuum Hot-Pressing

Study on Effect of Milling Time on the Mechanical and Wear Properties of the Za40-B4c-Graphene Hybrid Nanocomposites Fabricated by Vacuum Hot-Pressing

Analysis of The Removal Mechanism of Ferroalloy by Acidithiobacillus Ferrooxidans

Bio-machining technology is widely recognized as an environmentally friendly processing method for metals removal. In order to reveal the removal mechanism of ferroalloy corroded by Acidithiobacillus Ferrooxidans, ferroalloy workpieces were prepared by mixing 50% the powder of Fe with Co, Ni and Cu, and they were sintered with vacuum hot-pressing, A set of material removal experiments were carried out by shaking the ferroalloy with the bacterial strain culture solution of Fe3+ level at 5.5 g/L, and the material removal amount, surface morphology and roughness, and electrochemical characteristics of ferroalloy workpieces were determinated. The results showed that the Fe-based alloys containing Co, Ni, Cu, and Sn can be bio-machined, and the surface roughness of the ferroalloys significantly increased after machining. The material removal amount (MRA) of 8 hours of bio-machining ranked in the order of Fe50Co (42.3) >Fe50Cu (36.4) > Fe50Ni (21.6)>Fe50Sn (14.3). The MRA of each ferroalloy increased with the processing time, and the results could be fitted in a power function with machining time. In conclusion, the activity of the metal, the passivation on the alloy surface and the accelerated corrosion of the inert metal on the active metal were the three most important factors affecting MRA of ferroalloys. The additional reaction between Fe and newly generated metal ions by Fe3+, as well as the uneven reaction layer of workpieces was involved in the process of bio-machining.

Microstructure, tensile properties of SLMed TNT5Zr-0.2O alloys without/with keyholes produced by different Post-processing treatments

Ti-34Nb-13Ta-5Zr-0.2O alloy (hereafter termed TNT5Zr-0.2O) with keyhole defects was fabricated by SLM. The microstructure and tensile properties have been investigated after two post-processing treatments, namely vacuum heat treatment (HT) and hot isostatic pressing (HIP). Microstructure shows both alloys retain beta grain matrix along with discrete nano-sized grain boundary alpha precipitates. The stress concentrations at the edge of keyholes make the voids wall collapse under tension and cracks propagate more easily in that region. Notably, the pore enclosure by HIP is not contribute to a better ductility of this alloy. This explains why the TNT5Zr-0.2O-HT alloy possesses lower UTS (978 ± 19 MPa) and elastic modulus (49 ± 3 GPa), but higher elongation (4.5% ± 0.6%) in comparison with TNT5Zr-0.2O-HIP sample.

A study of preparation and properties of epoxy resin/carbon fiber/phenolic residual carbon composites with adjustable negative permittivity behavior

In recent years, there have been more and more researches on the dielectric properties of carbon-based composites, and its dielectric tunability is a hot research topic. In this paper, epoxy resin/carbon fiber/phenolic residual carbon (EP/CF/C) was prepared by high-temperature sintering, vacuum liquid phase impregnation and hot-press curing methods. The AC conduction behavior, dielectric properties and impedance properties of composites with different CF/C compression ratios (1, 1.5, 2) are studied. With the increase of CF/C compression ratio, the negative dielectric properties of composites are getting stronger and stronger. The results show that when the CF/C compression ratio is 1, the composites occur percolation phenomenon and dielectric constant is negative. When the CF/C compression ratio is 2, the absolute value of the negative dielectric coefficient of the EP/CF/C composite reaches 9.8 × 104, and the conductivity reaches 0.54 S/cm. In addition, impedance analysis has further confirmed that the inductance properties lead to the negative dielectric properties of the material. The adjustable negative dielectric properties of EP/CF/C composites can provide some suggestions for the field of electromagnetic shielding and capacitors.

Fabrication, microstructure and properties of transparent Yb:Y2O3 ceramics from co-precipitated nanopowders

Ytterbium-doped yttrium oxide (5 at.%Yb:Y2O3) nanopowders were synthesized by the co-precipitation method in this work. The influence of the molar ratio of ammonium hydrogen carbonate to metal ions (NH4HCO3/M3+, defined as R) on the properties of the precursors and powders was studied. The powders calcined at 1100 °C from the precursors with different R showed the same phase as the pure cubic Y2O3. However, the agglomerations of the calcined powders were influenced by the R evidently and the 5 at.%Yb:Y2O3 powders with R = 4.0 showed better dispersivity. Using the obtained powders as starting materials, 5 at.%Yb:Y2O3 transparent ceramics were fabricated by vacuum sintering and hot isostatic pressing (HIP) post-treating. The 5 at.%Yb:Y2O3 ceramics with R = 4.0 possessed the best optical quality with the in-line transmittance of 81.2% at 1100 nm with fine grains. In addition, the microstructures of the 5 at.%Yb:Y2O3 transparent ceramics with different R values were also studied and the HIP post-treated ceramics exhibited homogeneous microstructures. Meanwhile, the absorption and emission cross-sections of the 5 at.%Yb:Y2O3 ceramics were calculated to be 0.99 × 10−20 cm2 at 976 nm and 0.97 × 10−20 cm2 at 1031 nm, respectively. The gain spectra also were obtained from the absorption and emission spectra.

High Temperature Oxidation Behaviors of BaO/TiO2 Binary Oxide-Enhanced NiAl-Based Composites.

High temperature lubricating composites have been widely used in aerospace and other high-tech industries. In the actual application process, high temperature oxidation resistance is a very importance parameter. In this paper, BaO/TiO2-enhanced NiAl-based composites were prepared by vacuum hot-press sintering. The oxidation resistance performance of the composites at 800 °C was investigated. The composites exhibited very good sintered compactness and only a few pores were present. Meanwhile, the composite had excellent oxidation resistance properties due to the formation of a dense Al2O3 layer which could prevent further oxidation of the internal substrate; its oxidation mechanism was mainly decided by the outward diffusion of Al and the inward diffusion of O. The addition of BaO/TiO2 introduced more boundaries and made the Kp value increase from 1.2 × 10−14 g2/cm4 s to 3.3 × 10−14 g2/cm4 s, leading to a slight reduction in the oxidation resistance performance of the composites—although it was still excellent.

Failure analysis of metal-polymer-metal sandwich panels with wire mesh interlayers: Finite element modeling and experimental validation

Vacuum hot pressing of a thermoplastic core and metal skin sheets resistance welded to wire meshes is used to fabricate high interface strength metal-polymer-metal sandwich panels. The wire mesh strongly bonds with polymer core from material interlocking and it bonds with metal skin from welding. A finite element (FE) model is developed for characterizing the deformation behavior and failure of fabricated sandwich panels. While, a continuum ductile damage model is used to characterize failure of layers in stretching, a cohesive zone model is used to characterize interface failure. The FE model is validated using tensile tests, peel tests, double lap shear tests, and V-bending experiments. Good match is seen between simulations and experiments. In V-bending, spring back as well as debonding is correctly predicted. Sandwich panels fabricated in this work form an attractive alternative to adhesively bonded panels. The developed finite element model can be used for forming analysis of developed sandwich panels and for optimizing the panel architecture.

In-situ graphene enhanced copper wire: A novel electrical material with simultaneously high electrical conductivity and high strength

Copper-based materials with high electrical conductivity and high strength are strongly desired in energy and electric power fields. Unfortunately, it's almost impossible to obtain scalable and facile copper-based materials with the combined improvement of electrical conductivity and strength through traditional means. Here, by in-situ grown high quality graphene in copper through vacuum hot-press sintering the Cu powder and non-toxic, cheap liquid paraffin, high electrical conductivity and high strength are easily realized. Through further elaborate structure design and morphology control of graphene by altering the copper powder size, the liquid paraffin content and the sintering process parameters, the performance is improved. The uniformly-distributed 3D graphene plays a vital role, which acts as an electron transport channel and strengthens the matrix by grain refinement, dislocation strengthening and load transfer mechanisms. As a result, the cold drawn Cu/Graphene composite wire exhibits high electrical conductivity of 94.85% IACS and high tensile strength of 516 MPa, together with good softening resistance. This work provides a low cost and high-efficiency way to prepare high-performance graphene reinforced Cu matrix composite, and opens a new window for large-scale production of high-performance Cu/Graphene composites with different shapes.

Microstructure Evolution of Ruthenium During Vacuum Hot Pressing

Ruthenium tablets with mean grain size of ~4–5 μm were prepared by vacuum hot pressing (VHP), and tablets with maximum density of 12.2 g cm–3 were obtained with sintering time of 2 h. X-ray diffraction (XRD) revealed that there was a texture change with sintering time. The microstructure of the ruthenium tablets was observed by electron backscatter diffraction (EBSD) and field emission scanning electron microscopy (FSEM). The microstructure evolution of ruthenium with sintering time is discussed.

Effects of Fe content on microstructure and properties of Cu–Fe alloy

Cu–Fe alloys with different Fe contents were prepared by vacuum hot pressing. After hot rolling and aging treatment, the effects of Fe content on microstructure, mechanical properties and electrical conductivity of Cu–Fe alloys were studied. The results show that, when w(Fe)<60%, the dynamic recrystallization extent of both Cu phase and Fe phase increases. When w(Fe)≥60%, Cu phase is uniformly distributed into the Fe phase and the deformation of alloy is more uniform. With the increase of the Fe content, the tensile strength of Cu–5wt.%Fe alloy increases from 305 MPa to 736 MPa of Cu–70wt.%Fe alloy, the elongation decreases from 23% to 17% and the electrical conductivity decreases from 31%IACS to 19%IACS. These results provide a guidance for the composition and processing design of Cu–Fe alloys.

Thermal properties of graphite flakes/aluminum composites: Anisotropy and isotropy

Highly aligned graphite flakes were coated on the surface of aluminum foil by the coating process. Graphite flakes/aluminum composites with laminate structure (GF/Al(L)) and rolling structure (GF/Al(R)) were prepared via vacuum hot pressing. The results showed that the in-plane and out-plane thermal conductivity of the GF/Al(L) composite with 60 vol% GF reached 629 W/(m·K) and 46 W/(m·K), while that of the GF/Al(R) composite were 335 W/(m·K) and 341 W/(m·K), respectively. The in-plane and out-plane coefficient of thermal expansion values were 15.1, 8.1 ppm/K of the GF/Al(L) composite and 3.8, 3.2 ppm/K of the GF/Al(R) composite. The thermal properties of the GF/Al(L) composite exhibited high anisotropy, while that of the GF/Al(R) composite was close to isotropy.

Structure and Properties of WC–Co Composites with Different CrB2 Concentrations, Sintered by Vacuum Hot Pressing, for Drill Bits

Structure and Properties of WC–Co Composites with Different CrB2 Concentrations, Sintered by Vacuum Hot Pressing, for Drill Bits

High strength and conductivity copper/graphene composites prepared by severe plastic deformation of graphene coated copper powder

In order to uniformly disperse graphene in the copper matrix, the complete coating of few-layers high-quality graphene on the surface of the copper powder was realized by chemical vapor deposition, and the graphene coated copper powder is sequentially formed through vacuum hot pressing, hot extrusion, and cold drawing processes. The results show that during the vacuum hot pressing, the metallurgical bonding between original copper powders is hindered by the graphene with a relatively complete structure. And the copper grains are recrystallized and refined in the hot extrusion process, and then stretched into a long and thin fibrous structure during the cold drawing process. Besides, the graphene is partially agglomerated during the hot extrusion process, and is dispersed again during the cold drawing process and gradually oriented along the drawing direction. Finally, the graphene and the copper matrix form a network interpenetrating structure, which improves the mechanical and electrical properties of the composite synergistically. The tensile strength and conductivity of the composite wire drawn to 1 mm reached 573.3 MPa and 5.79 × 107 S/m, respectively, achieving a breakthrough in the performance of copper-based materials.