27Entrapped gas expansion

The solubility in metals of inert gases like argon is very low. Powder metallurgy techniques have been developed to manufacture materials with a dispersion of small pores containing an inert gas at a high pressure. When these

METAL DEPOSITION ON CELLULAR PREFORM

a) Vapor deposition of Nickel b) Burnout polymer

■ Open cell polymer foam polymer ligaments

■ Open cell polymer foam polymer ligaments

"Thermally decomposable Nickel containing gas

Nickel deposit

Ni(CO)4

"Thermally decomposable Nickel containing gas

Nickel deposit

Ni(CO)4

Hollow Nickel ligaments Heater b) Burnout polymer

Hollow Nickel ligaments Heater

c) Sinter

(Ligament densification)

Collapsed (dense) ligaments

Collapsed (dense) ligaments

Vacuum pump

Figure 2.6 Schematic illustration of the CVD process used to create open-cell nickel foams (INCO process)

Vacuum pump

Figure 2.6 Schematic illustration of the CVD process used to create open-cell nickel foams (INCO process)

materials are subsequently heated, the pore pressure increases and the pores expand by creep of the surrounding metal (Figure 2.7). This process has been used by Boeing to create low-density core (LDC) Ti-6A1-4V sandwich panels with pore fractions up to 50%.

In the process Ti-6A1-4V powder is sealed in a canister of the same alloy. The canister is evacuated to remove any oxygen (which embrittles titanium) and then backfilled with between 3 to 5 atmospheres (0.3-0.5MPa) of argon. The canister is then sealed and consolidated to a high relative density (0.9-0.98) by HIPing causing an eight-fold increase in void pressure. This is too low to cause expansion of Ti-6AI-4V at room temperature. The number of pores present in the consolidated sample is relatively low (it is comparable to the number of powder particles in the original compact), so a rolling step is introduced to refine the structure and create a more uniform distribution of small pores. In titanium alloys, rolling at 900-940°C results in void flattening and elongation in the rolling direction. As the voids flatten, void faces come into contact and diffusion bond, creating strings of smaller gas-filled pores. Cross-rolling improves the uniformity of their distribution. Various cold sheet forming processes can then be used to shape the as-rolled plates.

The final step in the process sequence is expansion by heating at 900° C for 20-30 hours. The high temperature raises the internal pore pressure by the ratio of the absolute temperature of the furnace to that of the ambient (about a factor of four) i.e. to between 10 and 16 MPa, causing creep dilation and a reduction in the overall density of the sample.

This process results in shaped Ti-alloy sandwich construction components with a core containing a closed-cell void fraction of up to 0.5 and a void size of 10-300 |im. While it shares most of the same process steps as P/M manufacturing, and the cost of the inert gas is minor, HIPing and multipass hot cross-rolling of titanium can be expensive. This process is therefore likely to result in materials that are more costly to manufacture than P/M alloys.

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