Appendix 4C Review Of Powder Metallurgy
Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. First, the primary material is physically powdered - divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure very near the true dimensions of the object ultimately to be manufactured. Finally, the end part is formed by applying pressure, high temperature, long setting times (during which self-welding occurs), or any combination thereof. Powder metallurgy technologies may be utilized by minimum initial support facilities to prepare a widening inventory of additional manufacturing techniques, and offer the possibility of creating "seed factories" able to grow into more complex production facilities which can generate many special products in space. The following sections review the basics of powder metallurgy (Jones, 1960).
The history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. (Jones, 1960). In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
A much wider range of products can be obtained using powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminum/iron powders poses major problems (Sheasby, 1979). Other substances that are especially reactive with atmospheric oxygen, such as tin (Makhlouf et at, 1979), are sinterable in special atmospheres or with temporary coatings. Such materials may be manipulated far more extensively in controlled environments in space.
In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion forming, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic (Kahn, 1980), and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.
4C.1 Cold Welding
Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the workpieces, or any combination of the above. Research has shown that even for very smooth metals, only the high points of each surface, called "asperites," touch the opposing piece. Perhaps as little as a few thousandths of a percent of the total surface is involved. However, these small areas of taction develop powerful molecular connections - electron microscope investigations of contact points reveal that an actual welding of the two surfaces takes place after which it is impossible to discern the former asperitic interface. If the original surfaces are sufficiently smooth the metallic forces between them eventually draw the two pieces completely together and eliminate even the macroscopic interface.
Exposure to oxygen or certain other reactive compounds produces surface layers which reduce or completely eliminate the cold welding effect. This is especially true if, say, a metal oxide has mechanical properties similar to those of the parent element (or softer), in which case surface deformations do not crack the oxide film. Fortunately, the extremely low concentrations of contaminating gases in free space (less than 10-14 torr is achievable) should produce minimal coating, so cold welding effects can persist on fresh metal surfaces for very long periods. Contact welding promises a convenient and powerful capability for producing complex objects from metallic powders in space with a minimum of support equipment.
