Densification Of Preforms Research Articles

Effects of temperature inversion on densification in chemical vapor infiltration

AbstractIn a classical chemical vapor infiltration (CVI) process, the competing effects of chemical kinetics and reagent gas transport lead to non‐uniform depositions such that outer layers of a preform densify faster leaving the core highly porous. Currently, CVI must be performed at a sufficiently low temperature to achieve good densification quality which leads to high processing time and cost. Volumetric heating of the preform, especially through microwaves, can create temperature inversion such that the core is hotter than the outer surface and potentially, overcome the challenges associated with isothermal CVI. Direct numerical simulations (DNS) of densification under various such temperature distributions indicate that microwave heating in CVI processing can lead to better (uniform) densification of porous preforms. The role of key parameters describing the temperature distributions on the densification behavior is investigated. Strategic temporal control of the temperature distribution shows that processing times can be reduced by almost half while maintaining a good densification quality similar to that of low‐temperature isothermal processing. Inside‐out densification due to the inverted temperature profile is a key distinguishing characteristic of microwave assisted CVI.

Experimental Investigations of Al-Cr3C2 Composite Preform Densification and Deformation

This investigation on strain hardening, densification and workability of the Sintered aluminium- Chromium Carbide composition of (Al-Cr3C2 of 2, 4 and 6%) preforms subjected to upsetting were investigated in this research. Industrial practitioners needed the workability data and densification mechanisms to plan and envisage the failure strains. In the current study, under triaxial stress state conditions Al-Cr3C2 preform with primary preform densities and various aspect ratios were compressed. Strain hardening, densification behaviors of aluminium- Chromium Carbide be investigated by gradually increasing the load till the fracture occurs. The outcome of adding Cr3C2 to Al and the impact of aspect ratio on formability was also extensively investigated. We looked at the parameters of stress ratio, instant varying strain rate, work hardening exponent, instantaneous density coefficient and densification attained.

Exploration of the Sparck Plasma Sinter Technique for the Manufacture of Implants for Orthopedic Applications: Development of A High-Performance Nbti-Swcnts Nanocomposite Welded Tv6a Alloy and Densification of Preforms

Nanocomposite Ti 50 wt% Nb with and without reinforcement of carbon nanotubes were successfully fabricated by ball milling of Ti, Nb and SWCNTs (single walled carbon nanotubes) nanopowder mixture folloyed by field actived spark plasma sintering process (FASPS) for biomedical application, superconductivity and magnetism in the ITER Project. The use of brittle Ti powder, instead of ductile elemental powder, led to significant increment in the yield of mechanically alloyed powder. The powder consisted of homogeneously distributed nano-sized Ti/Nb particles together with micronsized pure Nb particles. Sintering of the powders under low temperature and pressure conditions (1273 K, 1473 K and P=50 Mpa) resulted in the fine-grained heterogeneous microstructure consisting of α and β phases. On the other hand, sintering at higher temperatures (1473 K) resulted in a relatively coarse-grained chemically homogeneous microstructure with almost complete phase. Coarse-grained homogeneous β NbTi alloy exhibited higher average hardness as compared to that of heterogeneous fine grained microstructures. An attempt has been made to illustrate the correlation between the microstructural characteristics and mechanical properties of the sintered Ti50Nb compacts. To studies and to develop the influence of SWCNTs on the behavior of the nanocomposite TiNb/SWCNTs and the welding joint interface between TA6V and nanocomposite by sparck plasma sintering. At last, the continuation of the work has been discussed and prepared, planning dynamic flexion tests to measure the NbTi, NbTi/CNTs, NbTi/SWCNTs,/TA6V fatigue limit and developing a computer processing chain in order to customize prostheses respecting patients’ morphology. The fluid ethanchied lubrification of the prosthesis articulation is currently also in study and preparation. Then, a change of scale made possible by the use of an industrial size SPS machine led to the realization of types of blade perform according to patent morphology. These results led to the filing of a patent.

Investigation of Gas-Phase Processes of Obtaining Fiber-Reinforced Organomorphic Ceramic Composites with SiC-Matrix

The isothermal process of chemical vapor deposition for the densification with a SiC-matrix of organomorphic carbon-fiber preforms obtained by carbonization of compressed fibers of oxidized polyacrylonitrile is investigated. Such preforms are characterized by a high (up to 70%) porosity and uniform pore size (the reduced pore diameter ranges from several micrometers to tens of micrometers). Technological parameters of the process of obtaining ceramic-matrix composites (CMC) were optimized by a combination of experimental studies and numerical simulations. Experimental samples of CMC were obtained using a non-halogen precursor, methylsilane CH3SiH3, and their residual porosity was determined. For the numerical study of the gas-phase process of densification of preforms, a 1D model was used. The simulation results were compared with experimental observations.

Influences of impact energy on the densification and mechanical properties of powder metallurgical Fe–C–Cu preforms during a powder-forged process

Accurate impact energy control is important for the success of the powder-forged (P/F) technique. This paper address how impact energy influences the densification and mechanical properties of powder metallurgical (P/M) preforms during a P/F process by combining finite element simulations and experiments. The simulated results coincide well with the experimental results. Results reveal that impact energy strongly influences the P/M preform densification by controlling their stress states and strain rate. The densification continuously proceeds throughout the P/F processing by the three steps of micro-pore compression, segmentation, and closure, while the densification uniformity gradually deteriorates in the earlier free upsetting step and then significantly improves in the later closed compressing step. The increased impact energy within the range below 56.3J/cm3 induces a higher strain rate, and thus facilitates the P/M preform densification notably. Inversely, the excessive impact energy not only has less influence on the densification but also creates exceptionally high forging pressure. The notably increased density improves the mechanical properties of the P/F forgings significantly; meanwhile, the concomitant work hardening changes their tensile fracture modes and slows down their ductility rising. In this study, the P/F Fe–C–Cu forgings prepared at the optimized impact energy of 56.3J/cm3 have a high and uniform density and good mechanical properties. This study can provide theoretical guidance for the rational process design of powder forging.

Simulation of capillary infiltration into packing structures for the optimization of ceramic materials using the lattice Boltzmann method

ABSTRACTThis study uses the lattice Boltzmann method (LBM) to simulate in 2D the capillary infiltration into porous structures obtained from the packing of particles. The experimental problem motivating the work is the densification of carbon preforms by reactive melt infiltration. The aim is to determine the optimization principles for the manufacturing of high-performance ceramics. Simulations are performed for packings with varying structural properties. The results suggest that the observed slow infiltrations can be ascribed to interface dynamics. Pinning represents the primary factor retarding fluid penetration. The mechanism responsible for this phenomenon is analyzed in detail. When surface growth is allowed, it is found that the phenomenon of pinning becomes stronger. Systems trying to reproduce typical experimental conditions are also investigated. It turns out that the standard for accurate simulations is challenging. The primary obstacle to overcome for enhanced accuracy seems to be the over-occurrence of pinning.

Effects of Heating Rate and Sintering Temperature on Micro Forming of 316L Stainless Steel Powder Under Multifield Coupling

316L stainless steel powders with an average particle size of 20 μm were sintered at different heating rates under multi-field action in a Gleeble-1500D thermal simulation instrument. It was shown that both heating rate and sintering temperature directly affected the densification of sintering products, and the electric field not only provided the Joule heat for densification of preform, but also accelerated the diffusion of atoms, which leads to intensified sintering. Multi-field action includes the stress field, temperature field and electric field, and these fields work reciprocally rather than independently. The relative density of the sintered compact can be of the order of 88.9% at relatively low temperature (500°C) within relatively short sintering time (6 min).

Effects of Pressure during Preform Densification on SiC/SiC Composites

The nano-infiltration and transient eutectic-phase (NITE) method is one of the most attractive methods for fabrication of SiC and SiC/SiC composites. In the NITE method, preform densification is essential option for damage less near-net shaping technique. However, optimization of preform densification is insufficient yet. The objective of this study is to evaluate the effects of pressure during preform densification on SiC/SiC composites. The preform before preform densification has many pores in the inter-prepreg sheets. These pores were disappeared by preform densification. As the effects of pressure on preform, densification in the intra-fiber-bundle was improved due to increasing pressure. Flexural strength of the preforms with 1 MPa and 17 MPa indicated almost same value. The result suggested that increasing of pressure did not cause any change in fiber properties. In the effects of pressure on the composites, the composites with 17 MPa was exhibited improvement in bulk density and mechanical property, compared with that with 1 MPa.

NITE 法による SiC 成型挙動への SiC 粉末表面特性の影響

NITE 法による SiC 成型挙動への SiC 粉末表面特性の影響

Effects of Preform Densification on Near-Net Shaping of NITE-SiC/SiC Composites

Large volumetric shrinkage (~50 Vol%) occurs during ceramic matrix composites fabrication by hot-pressing due to infiltration and densification process of powder for matrix formation, resulting in unfortunately significant fiber-architecture and -strength damage. This study tries to explore damage-less near-net shaping technique by preform densification before hot-pressing in Nano-Infiltration and Transient Eutectic-phase (NITE) process. In particular, effects of preform densification for the damage-less near-net shaping, important influencing elements such as fiber-architecture, microstructural integrity and homogeneity, and composite's mechanical properties were evaluated using simple shaped (plate) and complex shaped composites. The preform densification demonstrated protective fiber-architecture in complex shaped composites and enhanced composites' density to 2.77/cm3 and ultimate bending strength to ∼200 MPa in simple shaped composites, owing to significantly reduced volumetric shrinkage.

Investigation of dynamic effects during cold upset-forging of sintered aluminium truncated conical preforms

The present paper deals with the upper bound analysis, experimental investigation and computer simulation of upset-forging of sintered aluminium truncated conical preforms, which considers heterogeneous deformation due to barreling of preform slant sides, composite die–workpiece interfacial friction conditions, preform densification along with compression and inertia effects. A generalized theoretical expression for average forging load and preform bulged profile based on the exponential velocity field and strain rate has been established. The upset-forging experiments were conducted using the sintered preforms prepared by compacting aluminium metal powder in the closed dies and various deformation characteristics were measured. Finally, the computer simulation of the upset-forging of the sintered aluminium truncated conical preforms was performed using DEFORM software and the deformation behavior velocity, effective strain and effective stress distribution were critically studied. The effect of die speed, i.e. dynamic effects on average die load has been investigated using energy and load factors. The DOE (Design of Experiment) and RSM (Response Surface Methodology) techniques were also used to investigate and determine interrelationships between important upset-forging characteristics and the results were presented graphically. During the present investigations, it was found that inertia energy dissipation and energy factor increases, where as die load and load factor decreases exponentially with die speed. It is expected that the present work will be useful for effective assessment of various deformation characteristics during processing of sintered materials, especially at high-speed.

プリフォーム高密度化処理によるSiC/SiC複合材料の微細組織および強度特性への影響

As an advanced liquid phase sintering method of SiC/SiC composites production, Nano-Infiltration and Transient Eutectic-phase (NITE) process has been intensively studied, where large volumetric shrinkage during sintering process is an issue to be solved. The objective of this work is to investigate the effects of preform densification on microstructure and mechanical properties of SiC/SiC composites fabricated by hot-press of preforms. The significant improvement seen in preform is the suppression of macro-pores at inter-/intra- fiber-bundles. Finally, this is contributing the reduction of volumetric shrinkage to suppress fiber damage and micro pore formation and eliminate macro pore formation after sintering. And, the composites fabricated presents microstructure homogeneity. The improvement in structure is reflected in high flexural strength. However, pseudo-ductility was worth than anticipated. The brittle behavior of the composites is interpreted to be caused by the damage of PyC interphase.

Manufacturing SiC‐Fiber‐Reinforced SiC Matrix Composites by Improved CVI/Slurry Infiltration/Polymer Impregnation and Pyrolysis

Two‐ and three‐dimensional SiCr/SiC composites have been prepared starting from Tyranno SA(tm) fiber preforms. Preform densification has been performed by a modified preceramic polymer impregnation and pyrolysis (PIP) process consisting of filling the preform large interbundle voids with SiC powder before the PIP process. This step was accomplished by low‐pressure infiltration of a SiC powder dilute slurry through the preform thickness. Specimens were further processed with polymer impregnation and pyrolysis to determine the effects on structural, thermal, and mechanical properties of the obtained composites. High‐temperature pyrolysis treatment, which promoted polymer derived SiC matrix crystallization, markedly increased thermal diffusivity.

04/01273 Mesophase pitches for 3D-carbon fibre preform densification: rheology and processability: Dumont, M. et al. Fuel, 2003, 82, (12), 1523–1529

04/01273 Mesophase pitches for 3D-carbon fibre preform densification: rheology and processability: Dumont, M. et al. Fuel, 2003, 82, (12), 1523–1529

Mesophase pitches for 3D-carbon fibre preform densification: rheology and processability ☆

Synthetic mesophase pitches, produced from naphthalene (ARA24, ARA24r), were examined in terms of fluidity and thermal stability in comparison with other mesophase, isotropic pitches, or mesophase pitch/softening molecules blends. Their interest for carbon/carbon (C/C) composites processing by liquid impregnation or vacuum transfer inside three-dimensional carbon preform is discussed. Viscosity versus reduced temperature plots is reported on the same graph to compare the studied pitches and to define a suitable C/C processing window. Methylnaphthalene-based pitch is a good candidate with viscosity and stability adapted to the processing temperature and high carbon yield.

Numerical simulation of isothermal chemical vapor infiltration process in fabrication of carbon-carbon composites by finite element method

The chemical vapor infiltration process in fabrication of carbon-carbon composites is highly inefficient and requires long processing time. These limitations add considerably to the cost of fabrication and restrict the application of this material. Efforts have been made to study the CVI process in fabrication of carbon-carbon composites by computer simulation and predict the process parameters, density, porosity, etc. According to the characteristics of CVI process, the basis principle of FEM and mass transport, the finite element model has been established. Incremental finite element equations and the elemental stiffness matrices have been derived for the first time. The finite element program developed by the authors has been used to simulate the ICVI process in fabrication of carbon-carbon composites. Computer color display of simulated results can express the densification and distributions of density and porosity in preform clearly. The influence of process parameters on the densification of preform has been analyzed. The numerically simulated and experimental results give a good agreement.

Kinetic Study of the Infiltration of Aluminum Alloys into TiC Preforms

A study of the infiltration process of several alloys (Al-2024, Al-6061, and Al-7075) into preforms of TiC was carried out. The preforms were sintered for one hour under argon at 1250,1350, and 1450°C with the aim of achieving different levels of densification. Using a thermogravimetric analyzer (TGA), infiltration profiles were obtained by continuously monitoring the weight change of preforms partially immersed in molten aluminum. Infiltration was carried out at different temperatures (ranging from 900 to 1200°C) under argon to evaluate infiltration kinetics. The infiltration rate of the aluminum into the preforms followed parabolic behavior and the activation energy changed according to the extent of preform densification. Average activation energies of 74, 99, and 138 k J mol−1 were obtained for the Al-7075, Al-6061, and Al-2024 alloys, respectively. Prior to infiltration, an incubation period was observed and its length depended on the temperature; lower the infiltration temperature, longer was the incubation period.

Interaction between a reactive preform and the surrounding gas-phase during CVI

An integrated modelling approach of Chemical Vapour Infiltration of a fibrous preform by methyltrichlorosilane, featuring coupled reactor fluid mechanics and porous medium gas transport has been developed. The coupling between transport/reaction phenomena inside and outside the preform is rather strong, both physically and numerically. An important counter-effect of the reactive preform on the surrounding gas-phase composition is evidenced; also, an appreciable dissymmetry of the densification rate inside the preform, even with negligible convective transport inside it . Such an approach brings new informations for understanding the physico-chemical phenomena of CVI and new guidelines for preform densification optimisation.

Vapor Grown Carbon Fiber Reinforced Aluminum Matrix Composites for Enhanced Thermal Conductivity

AbstractAluminum matrix composites reinforced with high thermal conductivity vapor grown carbon fiber (VGCF) were developed for improved thermal efficiencies in electronic devices. The carbon fiber was heat treated to increase its thermal conductivity. Various aluminum matrix composites were fabricated by the densification of fiber preforms using a pressure casting technique. Uniformity of the density was examined using optical microscopy. A scanning electron microscope equipped with a microprobe was utilized to examine the mechanical integrity of the composite. Mechanical properties, including tension, compression and flexural properties, were measured. While the results of the mechanical property measurements indicate moderate values, the composite exhibited remarkable thermal conductivity that reached 642 W/m.K, three times that of aluminum, at a fiber volume fraction of 36.5%, following closely the rule of mixture.

Multidimensional Modeling of Chemical Vapor Infiltration: Application to Isobaric CVI

Multidimensional (2D and 3D) chemical vapor infiltration models are formulated and used to simulate densification of isotropic or anisotropic preforms of various geometries using SiC deposition from methyltrichlorosilane. A generalized form of the dusty-gas model for mass transport in anisotropic porous structures is used as a flux model in the pore space, and the multidimensional model equations are solved using the Galerkin finite element method. Structures consisting of freely overlapping fibers parallel to a line (one-directional), parallel to a plane (two-directional), or without preferred orientation (three-directional) are employed to model the microstructure of the preforms. The obtained results show that the effects of preform geometry can lead to partial pressure and deposition profiles in the preform that are considerably different from those suggested by one-dimensional analyses of the chemical vapor infiltration process.