Powders use cold welding to best advantage because they present large surface areas over which vacuum contact can occur. For instance, a 1 cm cube of metal comminuted into 240-100 mesh-sieved particles (60-149 μm) yields approximately 1.25×106 grains having a total surface area of 320 cm2. This powder, reassembled as a cube, would be about twice as big as before since half the volume consists of voids.
If a strong final product is desired, it is important to obtain minimum porosity (that is, high starting density) in the initial powder-formed mass. Minimum porosity results in less dimensional change upon compression of the workpiece as well as lower pressures, decreased temperatures, and less time to prepare a given part. Careful vibratory settling reduces porosity in monodiameter powders to less than 40%. A decrease in average grain size does not decrease porosity, although large increases in net grain area will enhance the contact welding effect and markedly improve the "green strength" of relatively uncompressed powder. In space applications cold welding in the forming stage may be adequate to produce usable hard parts, and molds may not even be required to hold the components for subsequent operations such as sintering.
Hard monodiameter spheres packed like cannonballs into body-centered arrays give a porosity of about 25%, significantly lower than the ultimate minimum of 35% for vibrated collections of monodiameter spheres. (The use of irregularly shaped particles produces even more porous powders.) Porosity further may be reduced by using a selected range of grain sizes, typically 3-6 carefully chosen gauges in most terrestrial applications. Theoretically. this should permit less than 4% porosity in the starting powder, but with binary or tertiary mixtures 15-20% is more the rule. Powders comprised of particles having a wide range of sizes, in theory can approach 0% porosity as the finest grains are introduced. But powder mixtures do not naturally pack to the closest configuration even if free movement is induced by vibration or shaking. Gravitational differential settling of the mixture tends to segregate grains in the compress, and some degree of cold welding occurs immediately upon formation of the powder compress which generates internal frictions that strongly impede further compaction. Considerable theoretical and practical analyses already exist to assist in understanding the packing of powders (Dexter and Tanner. 1973; Criswell, 1975; Powell. 1980a. 1980b; Shahinpoor, 1980; Spencer and Lewis, 1980, Visscher and Bolsterzi, 1973).
Powder metallurgy in zero-g airless space or on the Moon offers several potential advantages over similar applications on Earth. For example, cold-welding effects will be far more pronounced and dependable due to the absence of undesirable surface coatings. Gravitational settling in polydiameter powder mixtures can largely be avoided, permitting the use of broader ranges of grain sizes in the initial compact and correspondingly lower porosities. Finally. it should be possible to selectively coat particles with special films which artificially inhibit contact welding until the powder mixture is properly shaped. (The film is then removed by low heat or by chemical means, forming the powder in zero-g conditions without a mold.)
Moderate forces applied to a powder mass immediately cause grain rearrangements and superior packing. Specifically, pressures of 105 Pa (N/m2) decrease porosity by 1-4%; increasing the force to 107 Pa gains only an additional 1-2%. However, at still higher pressures or if heat is applied the distinct physical effects of particle deformation and mass flow become significant. Considerably greater force is required mechanically to close all remaining voids by plastic flow of the compressed metal.
4C.2 Sintering
Sintering is the increased adhesion between particles as they are heated. In most cases the density of a collection of grains increases as material flows into voids causing a decrease in overall size. Mass movements which occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation with diffusion. In the final stages metal atoms move along crystal boundaries to the walls of internal pores, redistributing mass from the internal bulk of the object and smoothening pore walls.
Most, if not all, metals may be sintered. This is especially true of pure metals produced in space which suffer no surface contamination. Many nonmetallic substances also sinter, such as glass, alumina, silica, magnesia, lime, beryllia, ferric oxide, and various organic polymers. The sintering properties of lunar materials have been examined in detail (Simonds, 1973). A great range of material properties can be obtained by sintering with subsequent reworking. Physical characteristics of various products can be altered by changing density, alloying. or heat treatments. For instance, the tensile strength En of sintered iron powders is insensitive to sintering time, alloying, or particle size in the original powder, but is dependent upon the density (D) of the final product according to En/E = (D/d)3.4, where E is Young's Modulus and d is the maximum density of iron.
Particular advantages of this powder technology include: (1) the possibility of very high purity for the starting materials and their great uniformity; (2) preservation of purity due to the restricted nature of subsequent fabrication steps; (3) stabilization of the details of repetitive operations by control of grain size in the input stages: (4) absence of stringering of segregated particles and inclusions as often occurs in melt processes: and (5) no deformation is required to produce directional elongation of grains (Clark, 1963). There exists a very large literature on sintering dissimilar materials for solid/solid phase compounds or solid/melt mixtures in the processing stage. As previously noted (and see below), any substance which can be melted may also be
