Powder Metallurgy616

While PM product forms do not currently play an important role in aluminum or titanium alloys, powder metallurgy is important for some of the superalloys. The majority of PM superalloy parts fall into one of two categories. In the first, spherical superalloy powders are produced, screened to remove oversize particles, blended to provide a uniform distribution, loaded into container, vacuum degassed and sealed, and then hot isostatic pressed (HIP) to provide a near net shape. This process is used with highly alloyed compositions that would crack during hot working if they were made using conventional ingot metallurgy. In the second, mechanically alloyed powders strengthened by ultrafine dispersions of yttria (Y2O3) are consolidated by hot extrusion and hot rolling. In this case, oxide dispersion strengthened (ODS) alloys are being fabricated for improved creep resistance.

6.4.1 Powder Metallurgy Forged Alloys16

As the alloy contents of conventional wrought forged superalloys increased, the forging of these alloys became more and more difficult. The use of PM techniques permits the attainment of fine grain sizes that are often superplastic during hot forging operations. In addition, since PM allows near net shape capabilities, fewer processing steps and better material utilization are obtainable. The main advantages of using PM for superalloys are: the ability to add more alloying elements for greater high temperature capability, more uniform composition and phase distribution, finer grain sizes, reduced carbide segregation and, in some cases, higher material yields. However, potential problems with the powder metallurgy process include increased residual gas contents, carbon contamination, ceramic inclusions and the formation of prior particle boundary oxides and/or carbon films.

Superalloy powders are made by gas atomization processes that produce spherical powders. The most common method is inert gas atomization (Fig. 6.10). A molten metal is poured through a refractory orifice. A high pressure argon gas stream breaks up the molten metal into liquid droplets which solidify on the fly at cooling rates of around 102° K/s. The spherical powder is collected at the outlet of the atomization chamber. The maximum particle diameter produced depends on the surface tension, viscosity and density of the melt, as well as the velocity

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Tundish

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Vacuum Late Addition ^--Charger

Me Cham

Vacuum Late Addition ^--Charger

Atomization Chamber

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