28 Hollow sphere structures

Several approaches have recently emerged for synthesizing hollow metal spheres. One exploits the observation that inert gas atomization often results in a small fraction (1-5%) of large-diameter (0.3-1 mm) hollow metal alloy spheres with relative densities as low as 0.1. These hollow particles can then be sorted by flotation methods, and consolidated by HIPing, by vacuum sintering, or by liquid-phase sintering. Liquid-phase sintering may be the preferred approach for some alloys since it avoids the compressive distortions of the thin-walled hollow powder particles that results from the HIPing process and avoids the prolonged high-temperature treatments required to achieve strong particle-particle bonds by vacuum sintering methods. Porous nickel

ENTRAPPED GAS EXPANSION Process Steps a) Powder/ Can preparation

Ti - 6Al - 4V can a) Powder/ Can preparation

Powder _ packing i) density D0

Evacuate and backfill with Argon gas to pressure p0

Powder _ packing i) density D0

Evacuate and backfill with Argon gas to pressure p0

b) HIP Consolidation

Isolated pressurized voids t p(t), T(t)

Isolated pressurized voids

Final relative density 0.85 - 0.95

c) Hot rolling

Internal gas pressure

Internal gas pressure

Thinning of facesheet

Changes in pore shape, matrix microstructure

Thinning of facesheet

Changes in pore shape, matrix microstructure d) Expansion heat treatment (900°C, 4 - 48 hrs.)

Sandwich I panel |

Facesheet

Sandwich I panel |

111

M

O o o°

o Co

Figure 2.7 Process steps used to manufacture titanium alloy sandwich panels with highly porous closed-cell cores superalloys and Ti-6Al-4V with relative densities of 0.06 can be produced in the laboratory using this approach. The development of controled hollow powder atomization techniques may enable economical fabrication of low-density alloy structures via this route.

In an alternative method, hollow spheres are formed from a slurry composed of a decomposable precursor such as TiH2, together with organic binders and solvents (Figure 2.8). The spheres are hardened by evaporation during their

HOLLOW SPHERICAL POWDER SYNTHESIS

a) Slurry cast of hollow spheres

Gas Gas a needle

TiH2

& organic binder & solvent

Gas Gas a needle

TiH2

& organic binder & solvent

Hollow O "green" O spheres q

b) Hollow sphere metallization

Heat to evaporate solvent and binder and decompose TiH2

b) Hollow sphere metallization

Heat to evaporate solvent and binder and decompose TiH2

Figure 2.8 The Georgia Tech route for creating hollow metal spheres and their consolidation to create a foam with open- and closed-cell porosity

flight in a tall drop tower, heated to drive off the solvents and to volatilize the binder. A final heat treatment decomposes the metal hydride leaving hollow metal spheres. The approach, developed at Georgia Tech, can be applied to many materials, and is not limited to hydrides. As an example, an oxide mixture such as Fe2O3 plus Cr2O3 can be reduced to create a stainless steel.

In a third method developed at IFAM, Bremen, polystyrene spheres are coated with a metal slurry and sintered, giving hollow metal spheres of high uniformity. The consolidation of hollow spheres gives a structure with a mixture of open and closed porosity. The ratio of the two types of porosity and the overall relative density can be tailored by varying the starting relative density of the hollow spheres and the extent of densification during consolidation. Overall relative densities as low as 0.05 are feasible with a pore size in the range 100 |im to several millimetres.

0 0

Post a comment