High-Strength Aluminum P/M Alloys

Powder metallurgy (P/M) technology provides a useful means of fabricating net-shape components that enables machining to be minimized, thereby reducing costs. Aluminum P/M alloys can therefore compete with conventional aluminum casting alloys, as well as with other materials, for cost-critical applications. In addition, P/M technology can be used to refine microstructures compared with those made by conventional ingot metallurgy (I/M), which often results in improved mechanical and corrosion properties. Consequently, the usefulness of aluminum alloys for high-technology applications, such as those in aircraft and aerospace structures, is extended.

Aluminum alloys have numerous technical advantages that have enabled them to be one of the dominant structural material families of the 20th century. Aluminum has low density (2.71 g/cm³) compared with competitive metallic alloy systems. It also has good inherent corrosion resistance because of the continuous, protective oxide film that forms very quickly in the air, and good workability that enables aluminum and its alloys to be economically rolled, extruded, or forged into useful shapes.

Major alloying additions to aluminum such as copper, magnesium, zinc, and lithium - alone, or in various combinations - enable aluminum alloys to attain high strength. Designers of aircraft and aerospace systems generally like to use aluminum alloys because they are reliable, reasonably isotropic and low in cost compared to more exotic materials such as organic composites.

Aluminum alloys do have limitations compared with competitive materials. For example, Young`s modulus of aluminum (about 70 GPa, or 10 x 10⁶ psi) is significantly lower than that of ferrous alloys (about 210 GPa, or 30 x 10⁶ psi) and titanium alloys (about 112 GPa, or 16 x 10⁶ psi). This lower modulus is almost exactly offset by the density advantage of aluminum compared to iron- and titanium-base alloys. Nevertheless, designers could exploit higher-modulus aluminum alloys in many stiffness-critical applications.

Aluminum P/M Processing

There are several steps in aluminum P/M technology that can be combined in various ways, but they will be conveniently described in three general steps:

Powder can be made by various RS processes including atomization, splat quenching to form particulates, and melt spinning to form ribbon. Alternatively, powder can be made by non-RS processes such as by chemical reactions including precipitation or by machining bulk material.

Powder-processing operations are optional and include mechanical attrition (for example, ball milling) to modify powder shape and size or to introduce strengthening features, or comminution such as that used to cut melt-spun ribbon into powder flakes for subsequent handling.

Aluminum has a high affinity for moisture, and aluminum powders readily adsorb water. The elevated temperatures generally required to consolidate aluminum powder causes the water of hydration to react and form hydrogen, which can result in porosity in the final product, or under confined conditions can cause an explosion. Consequently aluminum powder must be degassed prior to consolidation. This is often performed immediately prior to consolidation at essentially the same temperature as that for consolidation to reduce fabrication costs. Consolidation may involve forming a billet that can be subsequently rolled, extruded, or forged conventionally or the powder may be consolidated during hot working directly to finished-product form.

Atomization is the most widely used process to produce aluminum powder. Aluminum is melted, alloyed, and sprayed through a nozzle to form a stream of very fine panicles that are rapidly cooled, most often by an expanding gas.

Splat cooling is a process that enables cooling rates even greater than those obtained in atomization. Aluminum is melted and alloyed, and liquid droplets are sprayed or dropped against a chilled surface of high thermal conductivity - for example, a copper wheel that is water cooled internally. The resultant splat particulate is removed from the rotating wheel to allow subsequent droplets to contact the bare, chilled surface. Cooling rates of 10⁵ K/s are typical, with rates up to 10⁹ K/s reported.

Melt-spinning techniques are somewhat similar to splat cooling. The molten aluminum alloy rapidly impinges a cooled, rotating wheel, producing rapidly solidified product that is often in ribbon form. The leading commercial melt-spinning process is the planar flow casting (PFC) process developed by M.C. Narisimhan and co-workers at Allied-Signal Inc. The liquid stream contacts a rotating wheel at a carefully controlled distance to form a thin, rapidly solidified ribbon and also to reduce oxidation. The ribbon could be used for specialty applications in its PFC form but is most often comminuted into flake powder for subsequent degassing and consolidation.

Mechanical Attrition Process

Mechanical attrition processes often involve ball milling in various machines and environments. Such processes can be used to control powder size and distribution to facilitate flow or subsequent consolidation, introduce strengthening features from powder surfaces, and enable intermetallics or ceramic particles to be finely dispersed. The two leading mechanical attrition processes today - mechanical alloying in the United States and reaction milling in Europe - are improvements on sinter-aluminium-pulver (SAP) technology developed by Irmann in Austria.

SAP Technology. In 1946, Irmann and co-workers were preparing rod specimens for spectrographic analysis by hot pressing mixtures of pure aluminum and other metal powders. They noticed the unexpectedly high hardness of the resulting rods. Mechanical property evaluations revealed that the hot-pressed material had strength approaching that of aluminum structural alloys. Based on microstructural evaluations, Irmann attributed the high strength of the hot-pressed compacts to the breakdown of the surface oxide film on the powder panicles during hot pressing. Irmann performed mechanical tests at elevated temperatures and showed that these alloys not only had surprisingly high elevated-temperature strength, but also retained much of their room-temperature strength after elevated-temperature exposure.

The mechanical alloying process is a high-energy ball-milling process that employs a stirred ball mill called an attritor, a shaken ball mill, or a conventional rotating ball mill. Elemental powders may be milled with aluminum powders to effect solid-solution strengthening or to disperse intermetallics. The dispersed oxides, carbides, and/or intermetalics create effective dislocation sources during the milling process and suppress dislocation annihilation during subsequent working operations, which result in greatly increased dislocation density.

Thus, mechanical alloying enables the effective superimposition of numerous strengthening mechanisms, including:

Consequently, high strength can be obtained without reliance on precipitation strengthening, which may introduce problems such as corrosion and SCC susceptibility, and propensity for planar slip. Nevertheless, the aforementioned five strengthening contributions can be augmented by precipitation strengthening as well as intermetailic dispersion strengthening.

Powder Degassing and Consolidation

Can Vacuum Degassing. This is perhaps the most widely used technique for aluminum degassing because it is relatively non-capital intensive. Powder is encapsulated in a can, usually aluminum alloys 3003 or 6061. A spacer is often useful to increase packing and to avoid safety problems when the can is welded shut. Packing densities are typically 60% of theoretical density when utilizing this method on mechanically alloyed powders. Care must be used to allow a clear path for evolved gases through the spacer to prevent pressure buildup and explosion.

Dipurative Degassing. Roberts and co-workers at Kaiser Aluminum & Chemical Corporation have developed an improved degassing method called "dipurative" degassing. In this technique, the vacuum-degassed ponder, which is often canned, is backfilled with a dipurative gas (that is, one that effectively removes water of hydration) such as extra-dry nitrogen, and then re-evacuated. Several backfills and evacuations can be performed resulting in lower hydrogen content. In addition, the degassing can often be performed at lower temperatures to reduce microstructural coarsening.

Vacuum Degassing in a Reusable Chamber. The cost of canning and de-canning adversely affects the competitiveness of aluminum P/M alloys. This cost can be alleviated somewhat by using a reusable chamber for vacuum hot pressing. The powder or CIPed compact can be placed in the chamber and vacuum degassed immediately prior to compaction in the same chamber. Alternatively, the powder can be "open tray" degassed, that is degassed in an unconfined fashion, prior to loading into the chamber.

Direct Powder Forming. One of the most cost-effective means of powder consolidation is direct powder forming. Degassed powder, or powder that has been manufactured with great care to avoid contact with ambient air, can be consolidated directly during the hot-forming operation.

Hot Isostatic Pressing (HIP). In HIP degassed and encapsulated powder is subjected to hydrostatic pressure in a HIP apparatus. Can vacuum degassing is often used as the precursor step to HIP. Furthermore, net-shape encapsulation of degassed powder can be used to produce certain near net-shape parts. Relatively high HIP pressures (~200 MPa) are often preferred. Unfortunately, the oxide layer on the powder particle surfaces is not sufficiently broken up for optimum mechanical properties. A subsequent hot-working operation that introduces shear-stress components is often necessary to improve ductility and toughness.

Rapid Omnidirectional Consolidation. Engineers at Kelsey-Hayes Company have developed a technique to use existing commercial forging equipment to consolidate powders in several alloy systems. Called rapid omnidirectional consolidation (ROC), it is a lower-cost alternative to HIP.