Net-shaped preform

The first step in the manufacture of a CMC component is the production of a net-shape or near-net-shape preform from either ceramic particles or ceramic fibers. Particulate preforms can be produced by any of the conventional ceramic processing routes, such as uniaxial or isostatic pressing, injection molding, sediment casting, extrusion, or slip casting. A... 

Only the confined die pressing in punch-and-die machines is suitable for producing accurately shaped compacts with extreme demands on tolerance. Such requirements exist, for example, for dry dosage forms in the pharmaceutical industry and for near net-shape preforms in powder metallurgy. 

Turner MR, Rudd CD, Long AC, Middleton V, McGeehin P, Net-shape preform manufacture using automated fibre placement. Advanced Composites Letters, 4(4), 121-124, 1995. 

The described 3DNCOW approach is also suitable for producing some relatively complex net-shaped preforms, such as I, T, II, H, Y, as well as integral truss core and pile structures. Examples of such structural shapes designed and manufactured by 3TEX, Inc., can be found in Mohamed et al. (2001, 2003, 2005), Mohamed and Wetzel (2006), and Bogdanovich et al. (2008). Specific directions of those efforts have been determined by the practical industrial and military market needs and resulted in a broad spectrum of manufactured composite products. 

Schreiber, F., et al., 2011. 3D-hexagonal braiding possibilities in near-net shape preform production for lightweight and medical applications. In 18th International Conference on Composite Materials, Jeju. 

Preform binder activation and manufacture of 3D net shape preforms. 

A preform or a textile preform is a dry fiber structure that is then impregnated with a matrix and transferred into a consolidated fiber-reinforced plastic component. A preform can also be called a Net Shape preform if the preform is a reinforcing structure (depending on the component geometry). A preform can be a 3D structure (3-dimensional fiber orientation) as well as a 3D geometry (3-dimensional component geometry) and consists mostly of different single parts [2]. 

Liquid Composite Molding Process for Net-Shape Preforms... 

The net-shape preform manufacturing with a compact wall structure is an advantageous feature in terms of quick LCM processing. For closed-mold LCM, after placing the stitched 3D preforms inside the mold, the mold has to be closed without any further obstacles. The mold closure is very difficult if the preforms are of extra size. Net-shape preforms allow avoiding such inconvenience in the mold closing. The preform LCM process chain helps to correlate the preforming process with the LCM technique. 

Weft-knitted NCF can be produced as biaxial and multiaxial fabrics or as two- and three-dimensional near-net-shape textile structures. Warp and weft threads out of rovings are superposed and fixed together with loop systems. Due to the course-wise weft insertion the reinforcing threads remain noncrimped. Through the combination of weft insertion over the entire width or part of the width with or without support of a stitch-transferring device and by changing loop sizes on individual needles, three-dimensional near-net-shape preforms can be produced (Hufenbach, 2007). 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Fig. 1. A schematic illustration of CMC growth to net-shape using a directed metal oxidation process where the preform is formed by cold-pressing.

The potential of embossing flow field patterns in the preform materials has also been investigated. A two-sided proprietary flow field pattern has been successfully transferred to the carbon/carbon material yielding near-net-shape bipolar plates. Subsequent materials property testing has confirmed adequate material properties. 

Due to the inherent symmetry of as-produced textile fabrics, woven composite plates are orthotropic. Their stiffness can be represented by engineering constants Young s modules shear modules Gy and Poisson coefficients py, ij= 1,...,3. If the reinforcement is deformed during production of the composite, or if the preform is net shaped, or for some knitted performs, then the assumption of orthotropy does not necessarily apply and the full stiffness matrix has to be introduced. 

In RBSN, a silicon metal powder preform reacts with nitrogen gas to form silicon nitride. The reaction occurs by percolation of the gas phase into the open porosity of the silicon preform, while the large specific volume of the silicon nitride product substantially reduces the volume fraction of residual porosity (down to a lower limit of approximately 9%), The mechanical properties of RBSN are inferior to those of sintered silicon nitride powder products, but the reduced costs of machining the near-net-shape product is a very attractive advantage. 

Of the two most commonly used preform materials, one, alumina, is chemically inert during DMO while the other, silicon carbide, is reactive. We first examine the growth of a DMO composite into an alumina particulate, followed by that into SiC preforms, and finally the DMO infiltration of a suitably coated fibrous preform. Coatings of calcium sulfate with calcium silicate may be used to ensure that oxidative growth ceases at the external boundary of the preform, thereby ensuring near-net-shape fabrication. The mechanism by which these barriers poison the oxidation without impeding the flow of oxygen is not clear at this time. 

Fabrication by Liquid Silicon Infiltration (reaction bonding) (LSI) A leading candidate for use in industrial gas turbine engine is a SiC matrix composite named toughened Silcomp [175]. It is produced by melt infiltration of molten silicon into a porous preform containing carbon as well as BN-coated SiC fibers (e.g. Textron SCS - 6). The composites thus produced consist of a fully dense matrix of SiC + Si, reinforced with continuous SiC fibers. Moreover, the melt infiltration process is net shape and fast. Ultimate strength and strain at ultimate strength are 220 MPa and 0.8 /o, respectively at room temperature (LSI-SiC/SiC Si). 

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