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23 Sintered materials A wide range of useful metallic materials is made from powder by the process known as sintering. Such materials are commonly referred to as sintered materials, and individual engineering components made by the process are known as sintered parts, sintered components, or PM parts-PM or PM/M being the acronym for powder metallurgy. 23.1 The PM Process This process consists of compacting the powder by means of pressure in a die or mould of prescribed shape; the compact thus formed must have sufficient green strength to allow it to be removed from the die and handled without fracture. This is an important property. After removal from the die, the compact is heated, normally in a protective atmosphere or vacuum, to a temperature usually below the melting point, such that the particles weld together and densify markedly increasing the strength. This is the step known as sintering. In some cases, especially when the sintered compact is to be subsequently rolled or extruded, isostatic compaction in a flexible mould is used. An exception to the compaction process is the production of filter elements from spherical powders, often of bronze, by what is known as loose powder sintering, in which powder is poured into a mould of the appropriate shape and sintered in the mould. Increasingly, sintered billets are being made for subsequent mechanical working such as hot extrusion or rolling. These processes yield wrought material to which the name sintered parts does not apply, but the bulk of PM products are items made individually to the final shape and size. 23.2 The Products The main classes of PM products are: 1. Engineering components, i.e. parts for machinery made by directly pressing in rigid dies of the required shape, and sintering with either no or only minor further shaping. This is the largest and most familiar class of PM product. 2. Refractory metals. These often have melting points that are inconveniently high and/or are difficult or impossible to work in the cast state because they are brittle. This group includes tungsten, molybdenum, tantalum, and related metals. 3. Intentionally porous materials for use as filters or as oil-retaining (self-lubricating) bearings. 4. Composites. These may consist of two or more metals that are insoluble in each other in the soiid state, but the name is generally used to signify a metal in which is dispersed one or more non-metallic ingredients such as refractory oxide, carbide, or other compound that is insoluble in the metal. These are referred to as metal matrix composites. A very important member of this group, the hardmetals, is the subject of a later section. High-duty alloys in wrought form having mechanical properties superior to those of the corresponding material made from ingot. In this category must be included certain composite materials having fine dispersions of oxide or the like designed to improve the strength, especially a: elevated temperatures. In some other cases the principal reason for making wrought products from powder is that higher yields of usable material are possible. This applies to metals such as titanium alloys the scrap from which cannot readily be re-used. 5. 6. Magnetic materials. This veryimportant class ofsintered material is dealt within Chapter 20. 23-1 2S2 Sintered materials 23.3 Manufacture and Properties of Powders 23.3.1 Powder manufacture There is a large number of possible ways of making metal powders, the principal ones used on a commercial scale for powders for PM being: - Solid state reduction of a compound of the metal, commonly the oxide. This often results in a loosely caked sponge which is converted into powder by milling. - Thermal decomposition of a compound of the metal. This also may yield a sponge. - Electrolysis. By suitable choice of the various parameters certain metals can be deposited in a spongey or powder form. Such powders are often dendritic. In the case of iron, a dense deposit is formed which is reduced to powder by comminution. - Atomization, in which molten metal is disintegrated into small drops which are caused or allowed to solidify out of contact with each other and with any solid surface. There are many ways in which the molten droplets may be produced, the most common being to allow a stream of molten metal to fall vertically from a tundish into a chamber where it is broken up by jets of high-pressure liquid or gas. Another process now gaining in importance is centrifugal atomization, in which droplets are flung from a rapidly rotating pool of molten metal or from a disk on to which a thin stream of metal impinges. In another process the end of a rotating bar is progressively melted and if the heating is by laser beam or plasma arc the process can be carried out in near vacuum, thus substantially eliminating the risk of contamination. This is called REP (Rotating Electrode Process) and if a plasma arc is used PREP (Plasma REP). An important feature of atomization is that it can produce homogeneous fully pre-alloyed powders. Table 23.1 lists the powder production processes used for many of the metals used in PM. - Rapid solidification. This means cooling molten metal at a very high rate such that on solidification an amorphous or other non-equilibrium structure results. In the case of certain aluminium alloys, for example, it is possible to hold in solution significantly higher percentages of alloying elements (see Table 23.2). If a rapidly solidified alloy is consolidated by a powder metallurgical process that does not involve heating to a temperature high enough to induced recrystallization, wrought material with substantially improved mechanical properties may result. The processes used mostly involve projecting molten metal to form a very thin film on a rotating water cooled cylinder to produce flake or ribbon which is crushed to powder. - Mechanical alloying. Another way of achieving higher strength and especially retention of that strength at elevated temperatures is to include in the metal matrix a finely dispersed insoluble Table 23.1 The most widely used method is put first. PRODUCTION PROCESSES FOR METAL POWDERS Aluminium Beryllium Brass Bronze Cobalt Copper Iron: unalloyed Iron: cast Molybdenum Nickel Nickel alloys inc. superalloys Platinum Silver Silver alloys Steels Tin Titanium Tungsten Gas atomization (usually air), Comminution Comminution, electrolysis Atomization-water or gas Water atomization for irregular powder, Air atomization for spherical powder Oxide reduction Water atomization, electrolysis. (Both are in general use) Oxide reduction, water atomisation, electrolysis Comminution Oxide reduction-by hydrogen Thermal decomposition of the carbonyl, reduction of a salt in solution Atomization-inert gas, centrifugal Thermal decomposition of a salt Precipitation from solution of a salt, usually by an organic reducing agent Atomization Atomization-usually with water Air atomization Reduction of the chloride by magnesium, Thermal decomposition of the hydride Reduction of the oxide-by hydrogen Note: Powders of practically all metals can he produced by atomization-the table lists the processes common in present day commercial practice. Manufacture and properties o f powders 2S3 Table 2 3 . 2 EXTENDED SOLUBILITY IN ALUMINIUM VIA RAPID SOLIDIFICATION Additiue Maximum equilibrium solubility, at. % Observed maximum by rapid quenching CU Mn Si 2.5 (at 821 K) 0.7 (at 923 K) 1.6 (at 850 K) 18 9 i6 phase, commonly a stable oxide. Oxide Dispersion Strengthened (ODS) materials are increasingly of interest in aerospace technology, and they are of necessity made by PM. In general the improvement is greater the finer the particles of the dispersed phase, the mechanism being dislocation locking as in precipitation hardening. An approach near to ideal is mechanical alloying, in which a metal powder is mixed with a fine oxide powder and the mixture milled for several hours or even days usually in an attritor, during which process the oxide is beaten into the metal, the particles of which are repeatedly flattened, broken up, and reflattened while at the same time the oxide particles are broken down into progressively smaller fragments, typically less than 100 nm. Mechanically alloyed powders are generally consolidated and converted into wrought shapes. 23.3.2 Properties of metal powders and how they are measured Chemical composition Chemical composition is the main factor that determines the properties of the finished material. One feature that is specific to powders is the oxide layer on the surface of the particles. This has an important influence of the green strength of compacts but more especially on the sintering process which depends on the formation of true metallurgical bonds between adjacent particles. In the case of metals with easily reducible oxides the level of surface oxide is determined by heating a weighed sample in hydrogen under specified conditions, cooling in hydrogen, and weighing. The reduction in weight is commonly called the loss in hydrogen or hydrogen loss. The procedure gives a misleading figure if volatile material is present, such as organic additions (pressing lubricants), moisture, or volatile metals such as zinc. The process is not applicable to metals with very stable oxides that are not reducible in this way, nor to alloys containing a high percentage of metals such as aluminium, titanium, chromium, and stainless steels. Table 23.3 gives the specified reduction conditions for some common metals. Particle size and size distribution The meaning of size is complicated by the enormous variety of shape of particles which makes quantitative assessment difficult. Volume, maximum linear dimension, average dimension, cross TabIe 23.3 CONDITIONS FOR DETERMINATION OF LOSS IN HYDROGEN Metal ,powder Reduction temperature “C Reducarion time min Tin bronze Cobalt Copper Copper lead' and leaded bronze' Iron Alloyed steel Lead' Mo!ybdenum Nickel Tin Tungsten 775*15 1050+20 875* 15 600 * 10 i150k20 1150+20 550+ 10 1 100 * 20 1050*20 550k 10 1150k20 30 60 30 10 60 60 30 60 60 30 60 ' Results should be interpreted with caution 23-4 Sintered materials sectional area are different measurements which can be made. A considerable range of techniques is or has been used, including among others microscopical examination, elutriation, sedimentation, light obscuring, and light scattering, but for most PM powders the process normally used is sieving through wire mesh screens. The former designation of screen size by the number of strends of wire per unit length has been superseded by specifying the aperture size in terms of the largest sphere that will pass through it. However the older designation is still used and Table 23.4 shows the relationship between the various standards. The less equiaxed the particles the less meaningful will be the result of a sieve test. For example in the extreme case of an acicular powder, the length rather than the diameter is likely to be the controlling dimension. Nevertheless, meaningful and useful results are obtained, especially when different batches of nominally similar powder are being compared. For very fine, so called subsieve size powders other methods have to be employed. In common use are the Fisher subsieve sizer which is based on the permeability to gas of a bed of powder, and the BET method which is based on gas absorption. This measures in effect the total surface area. Apparent density (AD) This most important property is in a general sense the same as bulk density, i.e. it is the mass of a unit volume of powder, but the method and apparatus used to determine it need to be carefully specified if consistent results are to be obtained. The test normally specified-IS0 3923/1-is to allow the powder to emerge from a funnel through a specified orifice into a cylindrical cup of specified dimensions which overflows. The surplus is carefully removed by drawing a straight edge across the top of the cup, the powder in the cup then being weighed. The apparatus used is specified in IS0 3923/1 and illustrated in Figure 23.1. (The same apparatus is also used to determine the flow properties of powders.) AD is of importance in several ways but especially in that it determines the amount, i.e. mass, of powder in the die before compaction. Flowfactor In the commercial production of compacts, powder is fed automatically into the die cavity and must, therefore, flow readily. The flowability is measured by timing the flow of 50g of powder from a funnel of specified geometry and the result expressed in seconds. The procedure is laid down in IS0 4490, the apparatus being generally referred to as the Hall flowmeter after its designer (Figure 23.1). The properties of some widely used PM grade powders are given in Tables 23.5 and 23.6 2 3 . 4 Properties of Powder Compacts Apart from dimensions which are, of course, of major importance in the manufacture of individual PM parts, the relevant properties of compacts are density and strength. Green density as it is called has a marked influence on green strength and also largely determines the final, i.e. sintered, density. It is determined by Archimedes principle. Green strength is the term used to indicate the strength of a compact and two standard methods are in general use for measuring it. 1. A three point bend test on a standard compact of rectangular cross section gives the Transverse Rupture Strength-IS0 3995. 2. A compression test on the diameter of a ring shaped compact gives the Radial Crushing Strength (K factortIS0 2739. This test applies especially to cylindrical bearings. The k factor is given by the formula: (K = F(D - e)/Le2 N/mm2, whereF = breaking load in Newtons, L = the length in mm, D = the external diameter in mm, E = the wall thickness in mm. Compressibility (compactibility) For economic as well as technical reasons it is desirable to use as low a pressure as possible to achieve the required compact density, therefore the compressibility of the powder is of considerable importance. This factor is usually expressed either as the density obtained with a given compacting pressure (see Table 23.6) or is shown as a curve relating density to pressure. Compressibility as defined in this way is influenced by the AD of the powder and by the actual strength of the particles. Thus an alloy powder will have a lower compressibility than that of a pure metal. For this reason it is often preferred to use mixtures of elemental powders rather than pre-alloyed powders of the composition ultimately required. 23.5 Sintering Most PM parts (other than hardmetals) are sintered in continuous furnaces normally having three zones: 1. A pre-heating or de-waxing zone in which the lubricant is driven off. Smtermg 23-5 125 106 - Tabk 125 120 120 22 125 2 1 - 100 - 150 140 90 75 63 53 45 37 I 3.15 t 2.80 2 . 5 0 2.40 2.00 1 . 6 8 1 . 4 0 1 . 2 0 %: 300 250 250 200 212 160 t 180 150 + 125 100 c 106 qi 50 53 45 37 23-