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
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—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.
|
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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