Pressure Infiltration Process Research Articles

182 Chemical Changes in the Cell Wall Composition of Calcium-infiltrated `Golden Delicious' Apples

`Golden Delicious' apples (Malus ×domestica Borkh.) were pressure-infiltrated after harvest with 0%, 1%, 2%, 3%, or 4% CaCl2 solutions (w/v) and the chemical composition of the cell wall of the cortical tissue 2 to 4 mm under the epidermis was studied. The mineral composition of the control cell wall (0% CaCl2) was not affected by the pressure infiltration process. In addition, no significant change was noticed in cell wall associated protein, uronic acid, total polysaccharides, or non-cellulosic neutral sugar contents except for xylose and rhamnose, which decreased (-25%) and increased (+20%), respectively. When apples were infiltrated with CaCl2, Ca content of the cell wall increased and maximum accumulation was achieved with a 2% CaCl2 solution. Calcium infiltration also induced a two-fold increase in Na, a 27% decrease in P, and a 40% decrease in protein content. These data suggest that chemical changes occurring after Ca infiltration are not related to pressure infiltration alone, but are mainly due to the Ca accumulation in the cell wall after pressure infiltration of CaCl2 solutions. Saturation of the available binding sites for Ca occurred in the cell wall when fruit were infiltrated with 2% CaCl2, as no further significant changes in the cell wall chemical composition was detected in fruits infiltrated with 3% or 4% CaCl2.

Simulation of metal-matrix composite isothermal infiltration processing

In pressure infiltration processing of metal-matrix composites, molten metal is injected under external pressure into a porous preform of the reinforcing phase and solidified, either during infiltration or after the mold is filled. If infiltration is isothermal, the physics of the process are similar to drainage phenomena encountered in soil mechanics. Using this similarity, a finite-element software is developed to simulate metal-matrix composite infiltration by adaptation of a code originally developed for soil mechanics. Solutions are given for isothermal infiltration of porous preforms by a molten metal under any increasing function of the applied pressure vs time, taking into account capillary phenomena. Experimental validation is performed using SAFFIL alumina fiber preforms infiltrated with an aluminum matrix in a series of isothermal infiltration experiments in unidirectional and axisymmetric configurations. Numerical and experimental data show good agreement, both in terms of infiltration kinetics and porosity distribution. The simulation tool can, thus, be applied for isothermal infiltration of complex geometries and boundary conditions and aid mold design and process parameter optimization.

Thermofluid Analysis of Entrance Section of a Continuous Pressure Infiltration Process for Fiber-Metal Matrix Composites

Thermofluid Analysis of Entrance Section of a Continuous Pressure Infiltration Process for Fiber-Metal Matrix Composites

Cellular structure controllable aluminium foams produced by high pressure infiltration process

Cellular structure controllable aluminium foams produced by high pressure infiltration process

Effect of spatial inhomogeneity in fiber packing on the strength variability of Al–matrix composites

Abstract Nonuniform packing of fibers in Al2O3 fiber-reinforced Al–matrix composite (AMC) arises due to the pressure infiltration processing technique used for producing these composites. The effect of spatial variation in fiber volume fraction, fL, on the strength variability of a Al–2Cu alloy matrix reinforced with 65 vol.% Nextel™–610 Al2O3 fibers was investigated. Measurements indicate a variance of ∼7% in the fL, much larger than the measured variance of ∼3% in the specimen-to-specimen fiber volume fraction, fG. An analytical model to account for the variance in fG and fL in predicting the Weibull modulus and the mean strength variation with the volume of AMCs is presented. This model incorporates the characteristic link concept and utilizes the theoretical predictions for their strength variability under local load sharing conditions. Comparisons of the model predictions with the experimental results of three- and four-point flexure, and tensile tests show excellent agreement.

Correlation of thermal models with microstructural effects in continuous MMC wire production

A recently developed continuous pressure infiltration process at Northeastern University has successfully produced completely infiltrated metal matrix composite (MMC) wires showing high mechanical strength. Thermal models are developed for two key regions (the entrance and middle orifices) of the high-pressure infiltration apparatus to identify heat transfer conditions that relate to process limitations and resulting MMC microstructure. For composites composed of 2024-aluminum alloy with alumina fiber reinforcement, experimental measurement of resulting grain size and solute segregation are compared for two processing velocities. At lower pulling velocity, the overall average matrix grain diameter (3.92 μm) is larger than that measured at higher pulling velocity (1.84 μm). Segregation is notably higher at lower pulling velocities. Predictions from thermal models correlate with microstructural observations, and are utilized to determine a maximum processing velocity for improved process control.

Fracture chemistry of δ-Al2O3 (Saffil) fibres in an MgLi matrix environment

Fracture surfaces of δ-Al2O3 (Saffil) fibres embedded in an Mg-8 wt% Li matrix by the pressure infiltration process were investigated by in situ Auger electron spectroscopy to study the chemistry of embrittlement of the former, resulting from a cross-section attack by the molten Mg-8 wt% Li matrix. The unaffected fibres failed transgranularly without any indications of silica being the crack-controlling flaws. Displacement reductive reactions between fibre constituents (δ-Al2O3, silica) and penetrating lithium and magnesium species produce the phases that disturb the structural coherency of fibres and provide the paths for the crack propagation. In the early fibre/matrix reaction stage (slightly affected fibres) there are elemental silicon and aluminium and, most likely, also Li2O that cause the intergranular fracture of fibres, whereas in the advanced reaction stage (strongly affected fibres), MgO is predominantly formed and the fracture propagates throughout the fibres via the MgO-enriched regions.

A new process for the fabrication of in-situ particle reinforced metalmatrix composites

The fabrication processes of metal matrix composites are very important for their application. There are many kinds of processes used, such as the squeeze casting process, the vacuum-high pressure infiltration process and the stirring casting process. In this article a new brief process for the fabrication of particle reinforced metal matrix composites will be introduced that has been used for silicon particulate reinforced Al-based composite and zinc-based composite. This process is the rheological compocasting process of a multicomponent system, in which a part of the raw materials mix with the rest where the latter are in the form of semi-solid moltens. The primary phases that are in the semi-solid molten material are left and are used as reinforcement phases. The tests show that the process is feasible, convenient and practical.

Infiltration and wetting of alumina participate preforms by aluminum and aluminum-magnesium alloys

The infiltration and wetting of alumina participates by Al and by Al-Mg alloys was studied through pressure infiltration experiments. In these experiments, a noninvasive capacitance technique was used to determine the infiltration front position as a function of time. An unsaturated slug flow model was used to interpret the infiltration results and determine capillary pressures characteristic of the infiltration process. The characteristic capillary pressures for Al, Al-2Mg, and Al-3Mg at 750 °C and Al-2Mg at 850 °C were not significantly different. Therefore, contrary to usual belief, Mg did not significantly aid the pressure infiltration process. At 750 °C, the maximum values of the contact angle calculated from these capillary pressures were 106 deg for Al and 105 deg for Al-2Mg and Al-3Mg. These contact angle values indicate substantial removal of the oxide layer on the surface of the liquid metal during the infiltration process. The small difference in the contact angles indicates that magnesium had little effect on the wetting of alumina by aluminum. The small effect of Mg on the wetting may be due to absence of reactive wetting at the infiltration speeds present in the experiments and to partial disruption of the oxide layer on the surface of the liquid metal during the infiltration process.

Compatibility between PAN-based carbon fibres and Mg8Li alloy during the pressure infiltration process

The interaction of PAN-based T800H type carbon fibres, which had been coated with (i) pyrocarbon and (ii) pyrocarbon+SiC protective layers, was studied by means of SEM, energy-dispersive X-ray analysis and Auger electron spectroscopy. The interaction occurred during the pressure infiltration process at temperatures from 898 to 908 K and at contact times of 4–30 s. As detected by AES measurements, both Mg and Li penetrated throughout the carbon fibres, and Li2C2 was formed without remarkable reaction zone occurrence, which led to carbon fibre degradation. The pyrocarbon layer (about 50 nm thick) demonstrated a good protective efficiency and the corresponding metal-matrix composite exhibited satisfying strength characteristics; nevertheless, the influence of processing variables (temperature, time) was obvious. On the other hand, the double-coated pyrocarbon+SiC carbon fibres were strongly affected by Li2C2 formation.

Discussion of a liquid metal pressure infiltration process to produce metal matrix composites

The two major processes presently used to manufacture metal matrix composites are solid-state diffusion bonding and squeeze casting. The former has shape limitations; it is suitable to produce only flat plates or sheets, and the manufacturing rate is low and expensive. The latter, although suitable for quantity production of complex shapes, requires heavy and expensive tooling. The described pressure infiltration process was developed at Massachusetts Institute of Technology. With this process, fiber, whisker or particulate pre-forms are placed in inexpensive throw-away containers, into which the molten matrix material is forced by gas pressure. Because the gas pressure on the pre-form container is quasi hydrostatic, its strength requirement is minimal, consequently pre-form containers, even those with complicated shapes, are easy to preprare. By virtue of the simplicity of the containers and the absence of the requirement of a heavy mechanical system to effect the infiltration of the liquid metal into the pre-forms, the process is suitable to produce bulk or final-shape composite parts inexpensively. As the processing parameters such as pre-form temperature, infiltrating metal temperature, infiltrating pressure and cooling rate, can be accurately controlled, the process is particularly suitable for research and limited production.