Process Control Agents

A process control agent (PCA) is added to the powder mixture during milling, especially when the powder mix involves a substantial fraction of a ductile component. The PCAs are mostly organic compounds, which act as surface-active agents.

The PCA adsorbs on the surface of the powder particles and minimizes cold welding between powder particles and thereby inhibits agglomeration. The surface-active agents adsorbed on particle surfaces interfere with cold welding and lower the surface tension of the solid material. Because the energy required for milling is equal to the product of new surface area generated times the surface tension, a reduction in surface tension results in the use of shorter milling times and/or finer powders.

A wide range of PCAs have been used in practice. These are mostly organic compounds used at a level of — 1 to 4 wt% of the total powder charge and include stearic acid [CH3(CH2)i6COOH], hexane (C6Hi4), oxalic acid [(COOH)2 • 2H2O], methanol, ethanol, acetone, isopropyl alcohol, heptane, Nopcowax-22DSP, octane, toluene, trichlorotrifluoroethane, DDAA (diodecyl dimethyl ammomonium acetate), silicone grease, etc. Graphite, alumina, aluminum nitrate, and sodium chloride have also been used as PCAs. The majority of these compounds decompose during milling and interact with the powder and form compounds, which get incorporated in the form of dispersions inside the powder particles during milling. Thus, hydrocarbons containing hydrogen and carbon and carbohydrates containing hydrogen, carbon, and oxygen are likely to introduce carbon and/or oxygen into the powder particles, resulting in the formation of carbides and oxides that are uniformly dispersed in the matrix. These are not necessarily harmful to the alloy system because they can contribute to dispersion strengthening of the material. The hydrogen subsequently escapes as a gas or is absorbed into the metal lattice on heating or sintering.

Minimization or avoidance of cold welding of powder particles among themselves and to the grinding tools can also be achieved by other means. For example, the presence of air in the milling container acts like a PCA and prevents cold welding of the powders. Milling of the powders at very low temperatures (e.g., in liquid nitrogen) referred to as cryomilling also has been shown to minimize welding, probably due to the increased brittleness of the powder particles at such low temperatures. Metal powders (with face-centered cubic structure) milled in a hydrogen atmosphere have been found to become brittle and not stick to themselves or the container.

Some metals, such as aluminum, nickel, and copper, react with certain alcohols during milling to form complex metallo-organic compounds. For example, aluminum reacts with isopropyl alcohol. Other metals such as titanium and zirconium can react explosively with chlorinated fluids such as carbon tetrachloride. Chlorinated fluids should never be used with reactive metals. Reactive metals such as titanium and zirconium, when milled in presence of air, can pick up substantial amounts of oxygen and nitrogen and cause phase changes, including formation of new phases (Ref 12).

Reactive milling (milling of metal powders in the presence of reactive solids/liquids/gases enabling a chemical reaction to take place) has also been employed to synthesize metal oxides, carbides, and nitrides (Ref 13, 14, 15). Thus, milling of titanium in a nitrogen atmosphere has produced titanium nitride; several other nitrides have also been produced in a similar way. Milling of tungsten with carbon (graphite) has produced tungsten carbide. Milling of aluminum with carbon (or by use of a PCA containing carbon) produced aluminum carbide (Al4C3), which gets dispersed inside the aluminum matrix and improves the performance of the alloy. The carbide in this case formed only partially during milling; subsequent heat treatment was required to complete the reaction of carbon with aluminum. However, in other cases, the reaction can take place entirely during milling, only after heat treatment, or in part during both the stages.

The choice of a PCA for milling depends on the nature of the powder being milled and the purity of the final product desired. Use of a PCA normally introduces some impurities into the powder (see the section "Powder Contamination" at the end of this article). Thus, to produce a high-purity alloy, use of a PCA should be avoided. It should also be realized that there is no universal PCA. One would have to decide on a PCA looking at the possible interaction between the metal powder being milled and the components in the PCA.

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