Aerospace Applications

Spherical metal powders made by REP or by gas atomization are not well suited for cold pressing into green compacts. Therefore, spherical powders tend to be used in specialized applications where consolidation is achieved by hot isostatic pressing (HIP), or other high-temperature processing in which interparticle voids are more readily closed. These particles flow well into complex mold shapes and can be tapped to a reproducible density of packing to provide fully dense parts that closely approach the dimensions of the finished component (Ref 9).

Much research has been performed on near-net shape compaction for military airframe parts in titanium alloys. An example of a complex shape that can be made from titanium alloy powders pressed to near-net shape is shown in Fig. 10. Overseas research has resulted in as hot isostatically pressed PREP superalloy powder being used in both land- and air-based gas turbines (Ref 10, 11). This use of powders can offer both economic and metallurgical advantages. Near-net-shape technology ensures more efficient material utilization, which is important for high-cost materials such as titanium. Additionally, the metallurgical benefits include improved homogeneity and control of microstructure to achieve enhanced mechanical properties (Ref 12).

Fig. 10 Gas turbine engine compressor rotor made from hot isostatically pressed plasma rotating electrode processed Ti-6Al-4V powder

Because REP prevents contact of the melted alloy with any container material, it provides a decided advantage over other methods of making contamination-free spherical particles. This process generates powder with cleanliness and composition approximating that of the precursor electrode. Molten titanium is extremely aggressive and reacts with all container materials; consequently, REP is ideally suited for production of clean titanium alloy powders in commercial quantities. Typical characteristics for Ti-6Al-4V powder made by REP are shown in Table 1.

Table 1 Typical Ti-6Al-4V REP powder compositions and properties

Element

Composition, wt%

Aluminum

5.50-6.75

Vanadium

3.50-4.50

Oxygen(a)

0.13-0.20

Oxygen(b)

0.05-0.13

Iron

0.30 max

Carbon

0.10 max

Nitrogen

0.05 max

Hydrogen

0.0125 max

Tungsten

<10 ppm

Other

0.4 max

Titanium Bal

Screen analysis, i-'m

% retained

500

0

354

1

250

5.5

177

43

125

SS

88

9.4

63

S

44

0.1

<44

0

Other properties

Median particle size

(</m), t-1 m

175

Particle size range, ! ■' m

50-500

Bulk density (60% theoretical), g/cm3

2.65

Tap density (65% theoretical), g/cm3

2.90

Flow rate, s/50 g

24-S2

Surface rate, m2/g

(a) Standard grade.

(b) ELI grade (extra-low-interstitial, 0.13% oxygen max).

Tensile tests conducted on hot isostatically pressed Ti-6Al-4V compacted from samples of rotating electrode processed powder indicate excellent properties, as shown in Table 2. Subsequent improvements in the process, which included plasma torch melting, have provided plasma rotating electrode processed Ti-6Al-4V P/M compacts with fatigue properties that are comparable or superior to those for cast and wrought materials. More recent work has demonstrated that plasma rotating electrode processed Ti-6Al-4V compacts generally have a narrower fatigue distribution than other forms. This allows design engineers to work with minimum property values in advanced aerospace structures. Careful control of cleanliness is critical to obtain this type of performance.

Table 2 Typical tensile properties of hot isostatically pressed Ti-6Al-4V rotating electrode processed powder

Orientation

Tensile strength

0.2% offset

Elongation

Reduction

yield strength

(4D), %

in area,%

MPa

ksi

MPa

ksi

L

9SS.4

1S6.1

S50.S

12S.4

20.0

37.0

9S6.S

1S5.S

S6S.1

125.9

1S.0

37.4

T

950.S

1S7.9

S6S.S

125.2

1S.0

40.2

9S6.S

1S5.S

S4S.S

12S.1

1S.0

35.6

S

9S2.9

1S5.S

S4S.S

122.S

2S.0

42.2

AMS 4928-H

941.9

1S6.6

S4S.S

12S.1

20.0

39.1

S96.4(a)

1S0(a)

S27.4(a)

120(a)

10(a)

25(a)

Note: Consolidated material made by HIP at 950 °C (1750 °F) for 10 h at 100 MPa (15 ksi). Vacuum annealed for 10 h at 700 °C (1300 °F). Hydrogen after vacuum annealing equals 0.0057%.

Minimum.

The nonmetallic inclusions that occurred in early developmental work were associated with the initiation of fatigue failures in compacted pieces made from these materials. This problem has been eliminated by ensuring that the overall powder-making and handling process is designed so that foreign particles are excluded. As atomized powder is removed from the plasma rotating electrode machine under inert gas cover in closed collection vessels and is subsequently passed through a processing tower located within a class 100 clean room. This assembly sieves and passes particles whose sizes lie within desired upper and lower limits, and extracts a representative sample from a given lot of powder prior to loading into appropriate containers. The entire powder processing tower can be evacuated and backfilled with inert gas. The overall system is shown schematically in Fig. 11.

Fig. 11 Ultraclean powder generation and clean-room processing

Ti-6Al-4V powder is also being used for the production of continuously reinforced metal-matrix composites. In one approach the powder is mixed with organic binders and cast as a thin tape. The tapes are then alternatively laid up with layers of continuous silicon carbide fibers and hot pressed to form a sheet. An alternative technology involves the plasma spraying of the titanium alloy powder directly onto the fibers to produce reinforced foils.

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