Spray forming processes

Figure 1.5. (a) Schematic of spray forming process (b) a pilot-plant scale facility for spray forming at University of Bremen, Germany. Top top view. Bottom side view (Courtesy of Prof. Dr.-Ing. Klaus Bauckhage at University of Bremen, Germany.)... 

Figure 1.6. Size ranges of droplets/particles found in nature and generated by atomization of normal liquids and melts in aerosol spray, spray combustion, powder production, and spray forming processes.

Figure 1.7. Shapes of solidified droplets (particles) generated in powder production and spray forming processes, (a) Spherical shape gas-atomized gold alloy particles (b) near-spherical and dendritic shapes water-atomized bronze particles (c) irregular and porous (spongiform) shapes water-atomized zinc particles (d) irregular aggregates water-atomized copper particles (Cour. tesy of Atomizing Systems Ltd., UK.)...

Numerous atomization techniques have evolved for the production of metal/alloy powders or as a step in spray forming processes. Atomization of melts may be achieved by a variety of means such as aerodynamic, hydrodynamic, mechanical, ultrasonic, electrostatic, electromagnetic, or pressure effect, or a combination of some of these effects. Some of the atomization techniques have been extensively developed and applied to commercial productions, including (a) two-fluid atomization using gas, water, or oil (i.e., gas atomization, water atomization, oil atomization), (b) vacuum atomization, and (c) rotating electrode atomization. Two-fluid atomization... 

In some spray forming processes, the atomization gas exits the atomizer nozzle(s) at sonic or supersonic velocities. The gas may expand... 

The modeling results of the spray stage provide input data and initial conditions for the modeling of droplet-substrate interactions in the ensuing deposition stage of the spray forming process. 

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). 

Other uses for coal-tar pitch include production as a binder for foundry cores, as a sealant for dry batteries, and in the manufacture of clay pigeons. PeUeted pitch used as the binder in foundry cores is a hard pitch suppHed as spherical granules which are formed by a spray-cooling process. Clay pigeons consist of disks molded from a mixture of hard pitch and a mineral filler such as clay or limestone dust. 

Hampden-Smith and co-workers have prepared [Zn(SEt)Et]10 (Figure 30) by the insertion of sulfur into the Zn—C bond of diethylzinc. 7 Although this decameric thiolate possesses an arrangement of zinc and sulfur atoms similar to that found in wurtzite, pyrolysis of the material at 250 °C led to predominantly cubic ZnS. Cubic ZnS was also formed when the precursor is used in a spray CVD process. 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

Spray-forming, of particulate metal-matrix composites, 16 175 Spray functions, 23 174t Spray impact, 23 197 measurements, 23 194-195 Spraying processes, for sodium carbonate peroxohydrate, 18 412 Spraying techniques, in fluidized-bed encapsulation, 11 541-542 Spray instrumentation, 23 192-195 Spray irrigation... 

For example, amorphous clarithromycin was prepared by grind and spray-drying processes, and XRPD was used to follow changes in crystallinity upon exposure to elevated temperature and relative humidity [59]. Exposure of either substance to a 40°C/82% RH environment for seven days led to the formation of the crystalline form, but the spray-dried material yielded more crystalline product than did the ground material. This finding, when supported with thermal analysis studies, led to the conclusion that the amorphous substances produced by the different processing methods were not equivalent. 

Droplet Formation in Gas Atomization. Experimental and modeling studiesl160 161 169] 318] 319] 321]- 325] have shown that gas atomization of liquid metals in spray forming and powder metallurgy processes may take place in two primary modes, i.e., liquid jet-ligament breakup and liquid film-sheet breakup. 

It should be noted that the dynamic conditions of droplet impact processes discussed above cover a large range of the actual conditions in many industrial processes, such as spray forming, thermal spray, spray combustion, spray cooling, and aircraft flight. Under these conditions, the spreading behavior of droplets on a flat surface is essentially governed by inertia and viscous effects (Fig. 

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. 

---