Spray Deposition

In spray deposition, finely divided molten metal droplets, produced by disintegration of a stream of molten metal using high-energy inert gases, impinge on a substrate before they completely solidify. This allows some of the characteristics of RS to be achieved in combination with a near-net-shape capability.

Dramatic enhancements in mechanical behavior can result because of the refinement in constituent particle size. An example of this is shown in Fig. 8 for 7075 aluminum. The spray deposition of titanium-base alloys also has been investigated only recently with much further development necessary.

45.0

42.5

0.2% yield strength Ksf 65 70 75 BO

0.2% yield strength Ksf 65 70 75 BO

45.0

42.5

—r

o\

—I-

--

-1-

Spra

yed o

K c

-

o

Cast

o

û

O

425 450 475 500 525 550 575 600 0-2% yield strength, MPg

o ra

425 450 475 500 525 550 575 600 0-2% yield strength, MPg

Fig. 8 Enhanced fracture toughness in spray-deposited Al-7075 Mechanical Alloying

Mechanical alloying (MA) is a process in which heavy working of powder particles results in intimate alloying by repeated welding and fracturing. This process has the same attributes as RS: extension of solubility limits, production of novel structures and refinement of the microstructure (down to the nanostructure range); additionally, it allows production of a dispersion of second-phase particles.

Aluminum. High-temperature alloys with titanium additions, low-density alloys with magnesium additions, and extra-low-density alloys with lithium additions have been developed through MA. The mechanical property combination that can be obtained in an MA aluminum-base alloy is shown in Table 6. The MA aluminum-lithium alloys exhibit minimal degradation of properties when stressed in the transverse direction and are characterized by excellent general corrosion resistance that is 100* better than that exhibited by the I/M alloy.

Table 6 Room-temperature mechanical properties of MA aluminum alloys (longitudinal)

Process

Alloy

Ultimate tensile strength, MPa

Yield strength, MPa

Elongation %

Fracture toughness (Klc), MPa V^

Density g/cm3

Mechanical alloying

Al-Li-Mg-O-C

510

450

10

45

2.55

Ingot mettalurgy

7075-T73

505

435

13

32

Magnesium. Mechanical alloying has been employed in the development of "supercorroding" alloys for submarine applications, such as a heat source in diver suits, and as a hydrogen gas generator.

Titanium. Mechanical alloying of titanium alloys has resulted in supersaturated solid solutions, metastable crystalline and glassy phases as well as nanometer-sized grain structures.

Titanium Aluminide Intermetallics. Work on MA of the titanium aluminides indicates that this is an interesting fabrication method for both the 0 2 and /'families of alloys, resulting in the formation of surprisingly stable nanosized grains, which are stable even after compaction by HIP (Ref 17, 18, 19). In dispersoid-strengthened material in comparison to RS alloys, MA alloys exhibit finer grain structures and absence of dispersoid-free zones near grain boundaries; further dispersoids do not coarsen significantly up to very high temperatures; however further work is needed to optimize the product.

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