Читать книгу Metal Shaping Processes - Vukota Boljanovic - Страница 14
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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 a three-dimensional particle is often a more complex matter than it first appears. For simplicity, it is convenient to describe particle size in terms of one single number. The sphere is the only shape that can be described by one dimension, its diameter (D). However, as a particle is rarely a perfect sphere, there are many different techniques that have been devised for determining particle size distribution; for a metal powder, screening techniques and laser diffraction have become the preferred choices.
a) Screening Method
The most common method used for measured particle size is the use of screens (sieves) of different mesh sizes. The term mesh count is used to refer to the number of openings per linear inch of screen.
The basic principle of sieving techniques is as follows. A representative sample of a known weight of particles is passed through a set of sieves of known mesh sizes. The sieves are arranged in downwardly decreasing mesh diameters. The higher the mesh size number, the smaller is the opening in the screen. For example, a mesh size No. 200 has an opening of 74 µm, size No. 100 has an opening of 149 µm, size No. 10 has an opening of 2.00 mm. The sieves are mechanically vibrated for a fixed period of time. The weight of particles retained on each sieve is measured and converted into a percentage of the total sample. This method is quick and sufficiently accurate for most purposes.
b) Laser Diffraction Method
Laser diffraction, alternatively referred to as low angle laser light scattering (LALLS), can be used for the nondestructive analysis of wet or dry samples, with particles in the size range of 0.02 to 2000 µm; this method has inherent advantages which make it preferable to other options for many different materials.
Laser diffraction-based particle size analysis relies on the fact that particles passing through a laser beam will scatter light at an angle that is directly related to the particles’ size. As particle size decreases, the observed scattering angle increases logarithmically. Scattering intensity is also dependent on particle size, diminishing with particle volume. Large particles, therefore, scatter light at narrow angles with high intensity, whereas small particles scatter light at wider angles but with low intensity (Fig. 4.1).
A typical system consists of: a laser, to provide a source of coherent, intense light of fixed wavelength; a series of detectors to measure the light pattern produced over a wide range of angles; and some kind of sample presentation system to ensure that the material being tested passes through the laser beam as a homogeneous stream of particles in a known, reproducible state of dispersion. The dynamic range of the measurement is directly related to the angular range of the scattering measurement, with modern instruments making measurements from around 0.02 degree to beyond 140 degrees (Fig. 4.2). The wavelength of light used for the measurements is also important, with smaller wavelengths (e.g., blue light sources) providing improved sensitivity to sub-micron particles.
Fig. 4.1 Light scattering patterns observing for: a) large particle; b) small particle.
Fig. 4.2 Typical laser diffraction instrument layout.
In laser diffraction, particle size distributions are calculated by comparing a sample’s scattering pattern with an appropriate optical model. Traditionally, two different models are used: the Fraunhofer approximation and the Mie theory.
The Fraunhofer approximation was used in early diffraction instruments. It assumes that the particles being measured are opaque and scatter light at narrow angles. As a result, it is only applicable to large particles and will give an incorrect assessment of fine-particle fractions.
The Mie theory provides a more rigorous solution for the calculation of particle size distributions from light scattering data. It predicts scattering intensities for all particles, small or large, transparent or opaque. The Mie theory allows for primary scattering from the surface of the particle, with the intensity predicted by the refractive index difference between the particle and the dispersion medium. It also predicts the secondary scattering caused by light refraction within the particle. This is especially important for particles below 50 µm in diameter, as stated in the international standard for laser diffraction measurements.
