PROCESSING FOR METALS
In this part, basic particulate processing, in which starting materials are metal powders, is discussed. The unit deals with characterization of metal powders, powder metallurgy (Fig. II.1) processes used in shaping these materials, pressing and sintering techniques, and the economics of the powder metallurgy process.
Fig. II.1 An outline of the particle processing described in Part II.
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POWDER METALLURGY
4.2 Characteristics of Metal Powders
4.3 Production of Metallic Powder
4.4 Powder Manufacturing Processes of Metal Parts
4.5 Powder Metallurgy Materials
4.6 Design Considerations in Powder Metallurgy
4.7 Economics of Powder Metallurgy
Powder metallurgy, or P/M, is a process for forming metal parts by heating compacted metal powders to just below their melting points. The heating treatment is called sintering. Although the modern field of powder metallurgy dates to the early 19th century, over the past quarter century, it has become widely recognized as a superior way of producing high quality parts for a variety of important applications.
Powder metallurgy actually comprises several different technologies for fabricating semidense and fully dense components. The conventional P/M process, referred to as press-and-sinter, has been used to produce many complex parts such as the planetary carrier, helical gears and blades, piston rings, connecting rods, cams, brake pads, surgical implants, and many other parts for aerospace, nuclear, and industrial applications.
P/M’s popularity is due to a number of attributes: a) the advantages that the process offers over other metal-forming technologies such as forging and metal casting, b) its advantages in material utilization, c) shape complexity, d) near-net-shape dimensional control, and others. P/M’s benefits add up to cost effectiveness, shape and material flexibility, application versatility, and part-to-part uniformity for improved product quality.
Advantages that make powder metallurgy an important commercial technology include the following:
•eliminates or minimizes machining by producing parts at, or close to, final dimensions;
•eliminates or minimizes scrap losses by typically using more than 97% of the starting raw material in the finished part;
•permits a wide variety of alloy systems;
•produces good surface finishes and tolerances of ±0.13 mm (±0.005 in.);
•provides materials that may be heat-treated for increased strength or increased wear resistance;
•provides controlled porosity for self-lubrication or filtration;
•facilitates manufacture of complex or unique shapes that would be impractical or impossible with other metalworking processes;
•is suited to moderate-to-high-volume component production requirements;
•offers long-term performance reliability in critical applications;
•is cost effective.
There are some disadvantages associated with P/M processing. These include:
•high tooling and equipment cost;
•expense of metallic powder;
•difficulties with storing and handing metal powders (degradation of the metal over time and fire hazards with particular metals).
Most parts weigh less than 2.5 kg (5.5 lb), although parts weighing as much as 50 kg (110 lb) can be produced in conventional powder metallurgy equipment. While many of the early powder metallurgy parts, such as bushings and bearings, were very simple shapes, today’s sophisticated powder metallurgy process produces components with complex contours and multiple levels and does so quite economically.
4.2 CHARACTERISTICS OF METAL POWDERS
A powder is defined as a finely divided solid, smaller than 1000 µm (0.039 in.) in its maximum dimension. In most cases the powders will be metallic, although in many instances they are combined with other materials such as ceramics or polymers. Powders exhibit behavior that is intermediate between that of a solid and a liquid. Powders will flow under gravity to fill containers or die cavities, so in this sense they behave like liquids. They are compressible like a gas. But the compression of a metal powder is essentially irreversible, like the plastic deformation of a metal. Thus, a metal powder is easily shaped, but it has the desirable behavior of a solid after it is processed.
When one deal with powders, the properties of both the individual particles and the collective (bulk) properties of the powder must be considered. The properties of single particles include size, shape, and microstructure, which can be determined by optical or scanning electron microscopic observations. In order to characterize a bulk powder, it is necessary to be able to determine at least the following properties:
Basic chemical composition. The minimum percentage of the base metal or the percentages of main elements in case of metal alloy powders.
Impurities. The percentage of impurities.
Particle size distribution (see next section).
Apparent density. The weight per unit volume of a simply poured metal powder, which is always less than the density of the metal itself. It is measured by letting the powder drop freely through a funnel to fill a 25 cm3 (1.52 in.3) cylindrical container. The ratio between mass and volume, that is, the apparent density, is provided through leveling and weighing and is expressed in kg/m3. The apparent density depends on a series of factors, the more important of which are the metal’s true density, powder shape and structure, particle size distribution, corrosion resistance, etc.).
Flowability. To assess the speed, standardized funnels with varying calibrated openings are used. A certain amount of powder is poured in the funnel and the flow time is recorded.
4.2.1 Particle Size Measurement and Distribution
A particle is defined as the smallest unit of a powder. The particles of many metal powders are 25 to 200 µm (0.00098 to 0.0078 in.) in size.
Describing
