Rapidly Solidified Aluminum Alloys

Among the specific contributions and potentials of rapid solidifications are: increased solid solubility, minimization of segregation, highly refined grain size, modification or elimination of segregation phases, possibility of glass formation and production of new metastable microcrystalline structures.

Corresponding improvements have been achieved in mechanical, corrosive, magnetic and other properties, higher ultimate tensile (UTS) and yield strengths, with better elongations, better pitting and general corrosive behavior due to ultrafine grain sizes, decreased segregation and refinement of phases, increased fatigue life, higher strength-to-weight ratios and many others.

Rapid solidification processing opens new horizons for alloy development. This applies to modification of commercial products and to new compositions which can lead to unusual structures and superior properties. Aluminum alloys have been extensively investigated in terms of the response to rapid solidification. For aviation and space programs, higher specific modulus and strength values without loss of toughness, improved fatigue and improved corrosion resistance are of interest.

Appropriate market targets are the high-strength series 2XXX and 7XXX or substrates thereof. Currently, high-technology processes are exclusively aimed at the aerospace industries, even though substantial potential exists in land transportation and other segments.

The 2XXX lithium-containing alloys, by virtue of their low density, high specific elastic modulus, and ability to achieve high strength via simple precipitation hardening, offer great potential for weight savings in many aerospace applications. These alloys also feature projected costs lower than those of alternatives such as titanium alloys or advanced composites.

In general, however, alloys of the Al-Li-X family do not always exhibit commercially acceptable ductility. To help circumvent these problems, advanced powder metallurgy technologies were investigated. After initial encouraging results, this route was abandoned, and the leading producer, Alcoa, is now seriously considering conventional ingot metallurgy. The efforts for improvement of ingot metallurgy technology are based on: new casting technologies, addition of new alloying elements, thermo-mechanical treatments, and tighter processing control.

Improvements in ingot metallurgy alloys, however, are reaching a point of economic unfeasibility because of restrictions on alloying, and because of the limited improvements that can result from enhanced purity or additional thermo-mechanical treatments. Moreover, the overall improvements in properties are not as great as those obtainable by RS technology. Commercial use of rapidly solidified powders in the automotive industry is primarily restrained by cost.

In the 2XXX lithium-containing alloys, rapid solidification powder metallurgy technology allows addition of 1-2 wt% lithium with much less segregation than is possible in ingot metallurgy. This results in enhanced specific properties such as elastic modulus and yield strength particulars.

The improved properties of advanced aluminum alloys derive from the nature of the particulate from which they are produced and the alloy production method used. In general, very small grain sizes, extended solubility or supersaturation of solute elements, fine dispersion of precipitates, dispersoids, and insoluble particles are the key factors leading to improved mechanical and anticorrosive properties.

Therefore, these advanced products have great potential not only in aeronautical, military, and space application but also in industry. So far, however, sufficient actual in-service data have not been accumulated. Thus, near-team opportunities are limited to the aerospace industry where major payoffs can be realized primarily through weight reduction.

There exists a large potential for the utilization of advanced aluminum in the automotive market. The expected superior high-temperature properties of similar alloys modified with elements such as Fe and Ce could lead to increased piston life in automotive engines. The largest market share would be realized if high-strength aluminum alloys could be incorporated into the body of the automobile as a substitute for steel. Substantial interest will develop if weight savings greater than 60 percent could be realized at a comparable functional price. Alloy bumpers and truck axles would be an ideal application because of increased fatigue life.

Reciprocating machine parts as in the textile industry also possess potential. The desired benefits include increased fatigue resistance and stiffness. In the chemical process industries, there is a potential for tube and piping applications as a result of superior corrosion resistance and elevated temperature stability obtainable with rapidly solidified aluminum alloys.

Currently, only two rapidly solidified Al alloys are beyond the experimental state. The 7090 and 7091 alloy powders are commercially produced by several companies. Alcoa relatively recently announced the first production quantity commercial orders for wrought aluminum powder metallurgy parts made from alloy X7090. The parts, a main landing gear support beam and a landing gear door actuator, are being forged for the Boeing 757 aircraft. The alloy contains 2.0-3.0% Mg, 7.3-8.7% Zn, 1.0-1.3% Co, and rest of balance is Al.

Molten, fully-alloyed aluminum is air atomized into powered and rapidly solidified. The resulting powder particles are isostatically compacted and encapsulated into an aluminum canister. Then, the compact is heated, and the gases are evacuated from the canister. The heated canister and compact are the pressed into a billet to 100 percent density. The billet is then ready for fabrication using conventional techniques such as forging and extruding.

From the marketing viewpoint, development efforts have been essentially limited to aerospace applications where price elasticity is relatively low. Here, however, market potential is limited. Another factor is the intermaterial competition with respect to fiber-reinforced, polymer base composites.

Further market introduction will only be achieved after the barriers hindering further rapid solidified technology commercial development are overcome. Some of these barriers are:

• Lack of exposure and experience
• Inadequate industry standards and the lack of reliable, repeatable, nondestructive tests for powder metallurgy parts
• Historically bad reputation of powder metallurgy parts as brittle, porous, and very low-strength.
• The rate of technological change, which can inhibit major capital investment. Manufacturers are afraid to invest heavily in new technology, only to have it change overnight.