C. Key Steps in PM Processing
1. Manufacture of Metal Powders
Atomization is the process used commercially to produce the largest tonnage of metal powders. In water and gas atomization (Figures 2-1 and 2-2, respectively) the raw material is melted then the liquid metal is broken into individual particles. To accomplish this, the melt stock, in the form of elemental, multi-element metallic alloys, and/or high quality scrap, is melted in an induction, arc, or other type of furnace. After the bath is molten and homogenous, it is transferred to a tundish which is a reservoir used to supply a constant, controlled flow of metal into the atomizing chamber. As the metal stream exits the tundish, it is struck by a high velocity stream of the atomizing medium (water, air, or an inert gas). The molten metal stream is disintegrated into fine droplets which solidify during their fall through the atomizing tank. Particles are collected at the bottom of the tank. Alternatively, centrifugal force can be used to break up the liquid as it is removed from the periphery of a rotating electrode or spinning disk/cup (Figure 2-3).
Figure 2-1: Water Atomization Process: Source "Powder Metallurgy Science" Second Edition, R.M. German, MPIF.
Figure 2-3: Centrifugal Atomization by the Rotating Electrode Process: Source "Powder Metallurgy Science" Second Edition, R.M. German, MPIF.
Additional alloying can be performed in the liquid metal bath after the original charge has become molten. Also, the bath can be protected from oxidation by maintaining an inert gas atmosphere as a cover over the liquid metal. Alternatively, the top of the furnace can be enclosed in a vacuum chamber. The furnace type and degree of protection are determined by the chemical composition of the bath and the tendency of the metal to oxidize.
Mechanical Comminution methods, such as milling, lathe turning, and chipping, comprise the second powder manufacturing group. Milling (Figure 3) is the primary method for reducing the size of large particles and particle agglomerates. Ball, hammer, vibratory, attrition, and tumbler mills are some of the commercially available comminuting devices. During milling, forces act on the feed metal to modify the resultant particles. Impact, attrition, shear, and compression all influence powder particle size and shape. Lathe turning is a technique used for materials such as magnesium for creating coarse particles from billets. These particles are reduced in size subsequently by milling or grinding.
Figure 3: Particle Size Reduction by Jar Milling-Schematic: Source "Powder Metallurgy Science" Second Edition, R.M. German, MPIF.
Chemical methods constitute the final manufacturing group. Included are the production of metal powders by the reduction of metallic oxides, precipitation from solution (hydrometallurgy), and thermal decomposition (carbonyl).
Materials used for subsequent oxide reduction are iron ore (magnetite), mill scale, and metallic materials oxidized for oxide reduction. In the case of iron ore, a refractory tube is filled with a combination of iron ore and a mix consisting of coal, coke, and limestone. The tube is then passed through a kiln at ~1200°C. The mix decomposes, producing a reducing atmosphere inside the tube and the magnetite ore is converted to metallic Fe.
Mill scale and oxidized metallic products are annealed to reduce both the oxygen and carbon contents. FeO, Fe2O3, or Fe3O4, are reduced in the presence of a reducing atmosphere. In addition, the carbon within the particles is removed via the formation of CO and CO2.
Hydrometallurgical manufacturing and thermal decomposition comprise alternative chemical methods. Precipitation of a metal from a solution can be accomplished by using electrolysis, cementation, or chemical reduction. This is done either from a solution containing an ore, or by means of precipitation of a metal hydroxide followed by heating which results in decomposition and reduction.
Electolytic deposition is often categorized as a fourth mode of powder fabrication; here we include it as a chemical method. It involves the precipitation of a metallic element at the cathode of an electrolytic cell (Figure 4). The most common application is in the production of copper powder.
Figure 4: Electrolytic Cell Operation for Deposition of Powder-Schematic: Source "Powder Metallurgy Science" Second Edition, R.M. German, MPIF.
These manufacturing techniques result in powders with different characteristics and appearance, for use in specific applications (Figure 5). Water atomization usually produces irregularly shaped particles free of internal porosity, whereas the shape of gas atomized particles is spherical, also without internal porosity. Metal powders produced by oxide reduction are irregular in shape, have a large surface area, and usually contain a substantial amount of internal porosity. Particles fabricated by milling or other mechanical methods exhibit a spectrum of shapes, depending on the relative ductility or brittleness of the feed material. Ductile powders are generally flat with a high aspect ratio whereas brittle particles can be angular and regularly shaped. The milling of agglomerated particles can cause the agglomerates to break up, sometimes with little effect on the shape of the individual particles. Powder particles produced chemically can have shapes ranging from spherical to angular. Electrolytic powders are of high purity with a dendritic morphology.
Figure 5: Representative Metal Powders: (a) Chemical; Sponge Iron-Reduced Ore; (b) Electolytic: Copper; (c) Mechanical: Milled Aluminum Powder Containing Disperoids (17); (d) Water Atomization : Iron; (e) Gas Atomization: Nickel-Base Hardfacing Alloy.: Source "Atomization - The Production Of Metal Powders" A. Lawley, MPIF.
Production of prealloyed powders is possible with the atomization process. The chemical composition of the feed material and alloy additions to the molten bath allow for the formulation of an almost unlimited combination of alloy compositions.