Plasma Rotating Electrode Process

The plasma-rotating electrode process (PREP) is a centrifugal atomization process for making titanium prealloyed powder developed by Starmet (Ref 22). In this process, a helium plasma is used to melt the end of a rapidly rotating bar and molten droplets are spun off and solidify in flight in a helium atmosphere (see the article "Rotating Electrode Process" in this Volume). Melting and atomization are conducted in a circular stainless steel tank ---2440 mm in diameter maintained at a positive pressure of helium. The electrodes are prealloyed bars nominally 60 to 65 mm in diameter and are spun at speeds up to 15,000 rpm.

The PREP powder (Fig. 15) is spherical, has good flow characteristics, and a tap density of —65% of theoretical density. The size of the powder particles depends on the alloy, the electrode diameter, and the rotational speed at the time of atomization. Typical sizes for Ti-6A1-4V powders are between 100 and 300 /'m with a d50 of —175 /'m (Ref 23).

Gas Atomized Metal Powder

Fig. 15 Ti-6Al-4V powder made by the plasma-rotating electrode process

Compared to gas atomized powder, PREP powder is relatively coarse. Various process modifications have been investigated to produce finer powder. The use of a large diameter disk electrode rather than a bar electrode has been reported to decrease the mean particle size by more than 40% and to reduce cost of input material (Ref 23). In another process modification, the metal droplets were quenched into liquid argon. The result was a major reduction in powder particle size (Ref 23). In yet another modification, gas atomization was coupled with the rotating-electrode process (REP), as shown in Fig. 16. The REP powder process is similar to PREP, except that a tungsten arc is used instead of a plasma to melt the end of the rotating electrode (Ref 22). As shown in Fig. 17, the yield of -44 /' m titanium powder increased from less than 1% with regular REP to greater than 20% with gas atomization assisted REP.

Atomization Rotating Electrode Method
Fig. 16 Conceptual schematic of gas-assisted, rotating-electrode process. Source: Ref 23

100 AO

60 40

20 10"

100 AO

60 40

• REP alone □ P rnP^

■ 1.75 rriPa 1.40 rnPa o 1.05 mPa a 0,70 rnPa

/

5 10 20 30 SO Percent less than

70 SO SO 9S

Fig. 17 Titanium powder size distributions produced by the rotating-electrode process (REP) assisted by gas atomization at various pressures. Source Ref 23

An engine mount support (Fig. 18) made from PREP Ti-6Al-4V powder has been qualified for use on the F/A-18A Hornet fighter aircraft (Ref 25). Table 8 gives mechanical property data used to qualify the hot isostatically pressed near-net shape. The PREP powder is also used to produce a porous coating on hip prostheses for bone ingrowth (Fig. 19).

Table 8 Room temperature static mechanical properties of P/M Ti-6Al-4V engine mount supports in the mill annealed condition

Property

Minimum specification

Minimum result

Maximum result

Average result

Number of tests

0.2% yield strength, MPa (ksi) _

827 (120)

861 (125)

916 (133)

888 (129)

34

Ultimate strength, MPa (ksi) _

896 (130)

923 (134)

978 (142)

958 (139)

34

Elongation, %

10

12

16

14

16

Reduction of area, %

20

37

42

39

16

Compressive yield strength, MPa (ksi)

868 (126)

888 (129)

992 (144)

944 (137)

13

Ultimate shear strength, MPa (ksi)

523 (76)

613 (89)

640 (93)

627 (91)

8

Bearing yield strength, edge distance/pin diameter, e/D = 1.5, MPa (ksi)

1123 (163)

1260 (183)

1351 (191)

1316 (191)

8

Ultimate bearing strength, MPa (ksi)

1315 (191)

1557 (226)

1610 (238)

1598 (232)

8

Hot Isostatic Pressing Process
Fig. 18 Near-net engine mount support made by using PREP powder and hot isostatic pressing. Source: Ref 24
Rotating Electrode Process

Fig. 19 Hip prostheses coated with PREP powder. Courtesy of Starmet

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