Net shaping

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. 

The shaping of these fine, submicrometer powders into complex components and their subsequent consoHdation into dense ceramic parts of ideally zero porosity is a major technological challenge. The parts formed need to be consoHdated to near-net shape because Si N machining requires expensive diamond grinding. Additionally, Si N dissociates at or near the typical densiftcation temperatures used in the fabrication of stmctural ceramics and, therefore, special measures have to be taken to preserve the compositional integrity of the material. 

Metal injection mol ding (MIM) holds great promise for producing complex shapes in large quantities. Spray forming, a single-step gas atomization and deposition process, produces near-net shape products. In this process droplets of molten metal are coUected and soHdifted onto a substrate. Potential appHcations include tool steel end mills, superalloy tubes, and aerospace turbine disks (6,7). 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

The seven processing steps shown schematically in Figure 1 are involved to various degrees in making sol—gel-derived siUca monoliths by methods 1, 2, and 3. The emphasis herein is primarily on net-shape sol—gel-derived siUca monoliths made by the alkoxide process (method 3) prepared under ambient pressures. 

Near Net Shape Casting in Minimills, Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec, Canada, 1995. 

Sand Casting. Sand casting is the most popular method of molding for cast copper alloys. It is the least expensive near net shape process, and pattern costs are low. Dimensional tolerance capabiUties vary widely and are the least accurate when compared with other methods of casting. 

For rayon fiber based eomposites (Seetions 3 and 4) the fiber and powdered resins were mixed in a water slurry in approximately equal parts by mass. The isotropie piteh earbon fiber eomposites (Seetion 5) were manufaetured with less binder, typically a 4 1 mass ratio of fiber to binder being utilized. The slurry was transferred to a molding tank and the water drawn through a porous sereen under vacuum. In previous studies [2] it was established that a head of water must be maintained over the mold screen in order to prevent the formation of large voids, and thus to assure uniform properties. The fabrieation proeess allows the manufaeture of slab or tubular forms. In the latter case, the cylinders were molded over a perforated tubular mandrel covered with a fine mesh or screen. Moreover, it is possible to mold eontoured plates, and tubes, to near net shape via this synthesis route. 

In contrast, with composite materials, the materials utilization factor is rarely higher than 1.2 to 1.3. That is, only a maximum of 20-30% of the material is wasted with composite structures. Whereas obviously with a materials utilization factor for some metal parts of 15-25, the waste is 1500-2500% Those are not individually typical numbers, but are the worst cases in both situations, i.e., for metals and composite materiais. For metals, there are many, many operations for which the waste factor is very iow. And for composite materials there are also many situations where the waste factor is much lower than 20-30%. The point is that the worst-case situations are totaliy different for these two kinds of materials based on the way objects are inherently created with the two different types of materials. Composite materials are built up until the limits of the desired geometry are reached. At that point, the layup operation simpiy ceases. Composite materials and structures are fabricated in as ciose to the final configuration as possible, i.e., so-calied near-net shape. 

Spray Deposition Net or Near-Net Shape Manufacturing - Spray Forming - Spray Casting - Spray Rolling... 

As for normal liquids, modeling of droplet processes of melts provides tremendous opportunities to improve the understanding of the fundamental phenomena and underlying physics in the processes. It also provides basic guidelines for optimization and on-line control of the processes. This section is devoted to a comprehensive review of process models, computational methods, and numerical modeling results of the droplet processes of melts. The emphasis of this section will be placed on the droplet processes in spray atomization for metal powder production, and spray forming for near-net shape materials synthesis and manufacturing. Details of these processes have been described in Ref. 3. 

A review of alloy and process development of the TiAl alloy has been published by Wu (2006). The main conclusions are that TiAl development is now relatively mature the major requirement is the promotion of low cost processing coupled with the supply of high-quality raw material the future application of net shape powder process routes could be cost effective both for complex large components and for some small components. Special attention is dedicated in this review to alloys based on Ti-(45—46)Al-(5-8)Nb (at.%) and Ti-48Al-2Cr-2Nb (at.%). 

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