Vacuum Atomization

K.M. Kulkarni, Advanced Metalworking Practices, Inc.

Atomized nickel-base alloy powders are used primarily for hardfacing or aerospace components. Materials for hardfacing processes include various types of hardfacing processes, equipment, materials, and types of applications that are described in greater detail in the article "Welding and Hardfacing Powders" in this Volume. Atomized nickel-base powders are also described below with additional information in the article "Powder Metallurgy Superalloys."

Nickel-base hardfacing powders are produced by gas and water atomization. Unlike other alloy systems, nickelbase hardfacing powders are predominantly spherical in shape, even when produced by water atomization. Oxygen content is somewhat higher than gas-atomized powders, but is still below 1000 mL/m3 (1000 ppm). Generally, melting practice and control during each manufacturing step affect product quality more than the atomization process.

Most nickel-base hardfacing powders are of the Ni-Cr-B-Si type and are self-fluxing during deposition because of the presence of boron and silicon. These elements also influence the melting temperature range of any specific alloy; melting range is an important factor for hardfacing powders. Table 4 gives typical compositions of several common grades.

Table 4 Compositions and melting ranges of typical nickel-base hardfacing alloys

Nominal Composition

Melting range

Deposit

°C °F

hardness, HRC

1.5B-2.8Si-Ni

940-1260 1725-2300

19-24

1.7B-0.35C-7.5Cr-3.5Si-Ni

994-1152 1820-2105

35-42

2.4B-0.45C-11Cr-4Si-Ni

976-1063 1790-1945

49-52

The industrial use of Ni-B-Si and Ni-Cr-B-Si alloys for thermally sprayed and subsequently fused coatings developed beginning around 1955. During the period 1956 to 1960, most of these metal powders were produced by crushing processes; however, since about 1960 atomization from a melt became more widely utilized. Most of the atomization installations for these types of alloys utilized high-pressure water atomization as was also used for the production of powders.

The influence of cooling rate during atomization can have important effects on the metallurgical structure of the powders and on their behavior during thermal spraying. For example, there is a clear tendency toward the presence of unstable intermetallic nickel-boron compounds, such as Ni3B4 and Ni4B3, with increased atomization cooling rates (Ref 9). The quantity of these unstable phases has an influence on the self-fluxing reactions and on the wetting properties during fusing. This is due to the much higher velocity of the reaction of these unstable phases with the oxides present on the base metal surface. For these phases this velocity is estimated to be more than twice that for the more stable phase, NiB.

When chromium boride particles are present (i.e., from a lower atomization cooling rate), they have little influence on wetting during fusing. An increase in the content of stable phases (such as Cr2B or CrB) increases hardener stability and ductility because less boron is used for the self-fluxing reaction. The crack resistance during bending also is distinctly improved for coatings produced with lower-cooling-rate powders.

The final boron content of the fused coatings produced with rapidly cooled powders may be up to 0.2% less than for the coatings produced with slower-cooled powders. The overall finer structure produced with more rapidly cooled powders also provides improved resistance to wear in the final coating. These same features are expected to contribute to improved corrosion resistance.

Superalloy Powders (Adapted from Ref 10). In aerospace applications, several P/M superalloys have replaced forged turbine disk alloys. The P/M alloys are characterized by a high homogeneous concentration of both solid-solution strengthening elements and the 7'-forming elements aluminum and titanium. These factors would limit forgeability of conventionally cast and wrought alloys. Compositions are listed in Table 5.

Table 5 Chemical compositions of some nickel-base superalloys produced by powder metallurgy

Alloy

Composition,

%

C

Cr

Co

Mo

W

Ta

Nb

Hf

Al

Ti

V

B

Zr

Ni

IN-100

0.07

12.4

18.5

3.2

5.0

4.3

0.8

0.02

0.06

bal

LC Astroloy

0.023

15.1

17.0

5.2

4.0

3.5

0.024

<0.01

bal

Waspaloy

0.04

19.3

13.6

4.2

1.3

3.6

0.005

0.048

bal

NASA II B-7

0.12

8.9

9.1

2.0

7.6

10.1

1.0

3.4

0.7

0.5

0.023

0.080

bal

René 80

0.20

14.5

10.0

3.8

3.8

3.1

5.1

0.014

0.05

bal

Unitemp AF2-1DA

0.35

12.2

10.0

3.0

6.2

1.7

4.6

3.0

0.014

0.12

bal

MAR-M 200

0.15

9.0

10.0

12.0

1.0

5.0

2.0

0.015

0.05

bal

IN-713 LC

0.05

12.0

0.08

4.7

(2.0)

6.2

0.8

0.005

0.1

bal

IN-738

0.17

16.0

8.5

1.7

2.6

1.7

0.9

3.4

3.4

0.01

0.1

bal

IN-792 (PA 101)

0.12

12.4

9.0

1.9

3.8

3.9

3.1

4.5

0.02

0.10

bal

AF-115

0.045

10.9

15.0

2.8

5.7

1.7

0.7

3.8

3.7

0.016

0.05

bal

MERL 76

0.025

12.2

18.2

3.2

1.3

0.3

5.0

4.3

0.02

0.06

bal

René 95

0.08

12.8

8.1 3.6

3.6

3.6

. . . 3.6

2.6 .

. 0.01

0.053

bal

Modified MAR-M 432

0.14

15.4

19.6 . . .

2.9

0.7

1.9

0.7 3.1

3.5 .

. 0.02

0.05

bal

New alloys

RSR 103

. . . 15.0

. . . 8.4

bal

RSR 104

. . . 18.0

. . . 8.0

bal

RSR 143

. . . 14.0

6.0

. . . 6.0

bal

RSR 185

0.04

. . . 14.4

6.1

. . . 6.8

Source: Ref 11

Various means of producing superalloy powders are summarized in Table 6. Inert gas atomization is the most common technique. An ingot is first cast, typically by vacuum induction melting, in order to minimize oxygen and nitrogen contents. In some cases remelting may be carried out by electron beam heating, arc melting under argon, or plasma heating. Atomization is carried out by pouring master melt through a refractory orifice; a high-pressure inert gas stream (typically argon) breaks up the alloy into liquid droplets, which are solidified at a rate of about 102 K/s.

Table 6 Powder production methods

Step

Inert gas atomization1^"1

Soluble gas process

Rotating electrode process®

Plasma rotating electrode process

Centrifugal atomization with forced convective cooling (RSR)(c)

Melting 1

VIM; ceramic crucible

VIM; ceramic crucible

VIM, VAR, ESR

VIM, VAR, ESR

VIM; ceramic crucible

Melting 2

Argon arc

Plasma

Melt disintegration system/environment

Nozzle; argon stream

Expansion of dissolved hydrogen against vacuum and Ar + H2 mixture

Rotating consumable electrode; argon or helium

Rotating consumable electrode; argon

Rotating disk; forced helium convective cooling

Source: Ref 11

VIM, vacuum induction melting.

VAR, vacuum arc remelting; ESR, electroslag remelting. RSR, rapid solidification rate.

The spherical powder is collected at the outlet of the atomization chamber. The maximum particle diameter resulting from this process depends on the surface tension, 7, viscosity, '/, and density, P. of the melt, as well as the velocity, V, of the atomizing gas. The principal factor is gas velocity. Oxygen contents are of the order of 100 ppm, depending on particle size. Finer particle sizes are obtained by screening.

Another important powder production method, the soluble gas process, is based on the rapid expansion of gas-saturated molten metal, resulting in a fine spray of molten droplets that form as the dissolved gas, usually hydrogen, is suddenly released (Fig. 12, Ref 12). The droplets solidify at a rate of about 103 K/s, and the cooled powder is collected under vacuum in another chamber, which is sealed and backfilled with an inert gas. This method is capable of atomizing up to 1000 kg (2200 lb) of superalloy in one heat and produces spherical powder that can be made very fine. This method has been successfully employed for LC Astroloy, MERL 76, and IN-100.

Source: Ref 11

Vacuum Atomization
Fig. 12 Soluble gas atomization system for producing superalloy powder. Source: Ref 12

The third method of powder preparation is based on centrifugal atomization. The melt is accelerated and disintegrated by rotating under vacuum or in a protective atmosphere. One example of this method is the rotating electrode process (REP) used in the early production of IN-100 and René 95 powder. In this process, a bar of the desired composition, 15 to 75 mm (0.6 to 3 in.) in diameter, serves as a consumable electrode. The face of this positive electrode, which is rotated at high speed, is melted by a direct current electric arc between the consumable electrode and a stationary tungsten negative electrode. The process is carried out in helium. Centrifugal force causes spherical molten droplets to fly off the rotating electrode. These droplets freeze and are collected at the bottom of the tank, which is filled with helium or argon. A major advantage of this process is the elimination of ceramic inclusions and the lack of any increase in the gas content of the powder relative to the alloy electrode.

A variant on the REP process is the plasma rotating electrode process (PREP). Instead of a tungsten electrode, a plasma arc is used to melt the superalloy electrode surface. Cooling rates are higher, up to 105 K/s for IN-100 powder. On average, particle sizes are nearly twice as large in these processes as in gas atomization. More information on the REP process is described in the article "Rotating Electrode Process" in this Volume.

When solidification rates of powder exceed 105 K/s, the process is referred to as rapid solidification rate (RSR). For superalloys, the objective of the high rates is to obtain a microcrystalline alloy rather than an amorphous material. Apart from extremely fine grain size, such powders display nonequilibrium solubilities and very uniform compositions because of the very fine dendritic arm spacings resulting from rapid solidification. Both conventional superalloys and new alloys based on Ni-Al-Mo-X alloys, where X = tantalum or tungsten, have been prepared by RSR.

Rapid solidification processing can be done by centrifugal atomization with forced convective cooling. In this method, the alloy is vacuum induction melted in the upper part of a chamber. The chamber is then backfilled with helium, and the alloy is poured in a preheated tundish. The liquid is poured through the tundish nozzle onto a rotor that is spun at 24,000 rev/mm. The melt is accelerated to rim speed and then ejected longitudinally as droplets. Further atomization and cooling of the droplets is accomplished by the injection of helium gas through annular nozzles. Spherical powder in the 10 to 100 /'m (400 to 4000 / 'in.) diam range is produced; the cooling rate typically varies between 105 and 107 K/s, the higher rates being achieved with the smaller particles.

Rapidly solidified powders can also be prepared from melt spun ribbon that is pulverized after solidification. The ribbon is produced by pouring the melt through an orifice under pressure and impinging it on a rotating wheel of, for example, copper, that acts as a heat sink. Typical cooling rates are approximately 106 K/s, and ribbon thicknesses are less than 25 t-1 m (1000 /'in.). The ribbon must be mechanically pulverized, and this method is generally limited to small quantities of experimental alloys.

0 0

Post a comment