Particulate Processing of Metals and Ceramics
Particulate Processing of Metals and Ceramics
Particulate Processing of Metals and Ceramics
Presented By D.Manesi
Contributed By : 2010 John Wiley & Sons, Inc. M. P. Groover, Fundamentals of Modern Manufacturing 4/e
WHY POWDER METALLURGY IS IMPORTANT PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining PM process wastes very little material - about 97% of the starting powders are converted to product PM parts can be made with a specified level of porosity, to produce porous metal parts
Examples: filters, oil-impregnated bearings and gears
Certain alloy combinations and cermets made by PM cannot be produced in other ways PM compares favorably to most casting processes in dimensional control PM production methods can be automated foreconomical production
Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing Variations in density throughout part may be a problem, especially for complex geometries
PM WORK MATERIALS
Largest tonnage of metals are alloys of iron, steel, and aluminum Other PM metals include copper, nickel, and refractory metals such as molybdenum and tungsten Metallic carbides such as tungsten carbide are often included within the scope of powder metallurgy
Figure 16.1 - A collection of powder metallurgy parts (courtesy of Dorst America, Inc.)
ENGINEERING POWDERS
A powder can be defined as a finely divided particulate solid Engineering powders include metals and ceramics Geometric features of engineering powders:
Particle size and distribution Particle shape and internal structure Surface area
Figure 16.3 - Several of the possible (ideal) particle shapes inpowder metallurgy
Figure 16.4 - Interparticle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel. Larger angles indicate greater interparticle friction
OBSERVATIONS
Smaller particle sizes generally show greater friction and steeper angles Spherical shapes have the lowest interpartical friction As shape deviates from spherical, friction between particles tends to increase
Bulk density - density of the powders in the loose state after pouring
Because of pores between particles, bulk density is less than true density
POROSITY
Ratio of the volume of the pores (empty spaces) in the powder to the bulk volume In principle, Porosity + Packing factor = 1.0 The issue is complicated by the possible existence of closed pores in some of the particles If internal pore volumes are included in above porosity, then equation is exact
Possible surface films include oxides, silica, adsorbed organic materials, and moisture
As a general rule, these films must be removed prior to shape processing
Figure 16.6 - Iron powders produced by decomposition of iron pentacarbonyl; particle sizes range from about 0.25 - 3.0 microns (10 to 125 -in) (photo courtesy of GAF Chemicals Corporation, Advanced Materials Division)
Figure 16.7 - Conventional powder metallurgy production sequence: (1) blending, (2) compacting, and (3) sintering; (a) shows the condition of the particles while (b) shows the operation and/or workpart during the sequence
Video Blending
COMPACTION
Application of high pressure to the powders to form them into the required shape The conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die The workpart after pressing is called a green compact, the word green meaning not yet fully processed The green strength of the part when pressed is adequate for handling but far less than after sintering
Figure 16.9 - Pressing in PM: (1) filling die cavity with powder by automatic feeder; (2) initial and (3) final positions of upper and lower punches during pressing, and (4) ejection of part
Figure 16.11 - A 450 kN (50-ton) hydraulic press for compaction of powder metallurgy components. This press has the capability to actuate multiple levels to produce complex PM part geometries (photo courtesy Dorst America, Inc.).
SINTERING
Heat treatment to bond the metallic particles, thereby increasing strength and hardness Usually carried out at between 70% and 90% of the metal's melting point (absolute scale) Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy Part shrinkage occurs during sintering due to pore size reduction
Figure 16.12 - Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions
Figure 16.13 - (a) Typical heat treatment cycle in sintering; and (b) schematic cross-section of a continuous sintering furnace
IMPREGNATION
The term used when oil or other fluid is permeated into the pores of a sintered PM part Common products are oil-impregnated bearings, gears, and similar components An alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part
INFILTRATION
An operation in which the pores of the PM part are filled with a molten metal The melting point of the filler metal must be below that of the PM part Involves heating the filler metal in contact with the sintered component so capillary action draws the filler into the pores The resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength
Elemental powders are also mixed with other metal powders to produce special alloys that are difficult to formulate by conventional methods
Example: tool steels
PM PRODUCTS
Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery parts Advantage of PM: parts can be made to near net shape or net shape
They require little or no additional shaping after PM processing
When produced in large quantities, gears and bearings are ideal for PM because:
The geometry is defined in two dimensions There is a need for porosity in the part to serve as a reservoir for lubricant
Figure 16.16 - Four classes of PM parts (side view shown; cross-section is circular): (a) Class I - simple thin shapes, pressed from one direction; (b) Class II - simple but thicker shapes require pressing from two directions; (c) Class III two levels of thickness, pressed from two directions; and (d) Class IV - multiple levels of thickness, pressed from two directions, with separate controls for each level
PM can be used to make parts out of unusual metals and alloys - materials that would be difficult if not impossible to produce by other means
Figure 16.17 - Part features to be avoided in PM: side holes and (b) side undercuts since part ejection is impossible
Figure 16.19 - Chamfers and corner radii are accomplished but certain rules should be observed: (a) avoid acute angles; (b) larger angles preferred for punch rigidity; (c) inside radius is desirable; (d) avoid full outside corner radius because punch is fragile at edge; (e) problem solved by combining radius and chamfer
The solidification processes for glass are covered in a different slide set The particulate processes for traditional and new ceramics as well as certain composite materials are covered in this slide set
For traditional ceramics, the powders are usually mixed with water to temporarily bind the particles together and achieve the proper consistency for shaping For new ceramics, substances other than water are used as binders during shaping After shaping, the green parts are fired (sintered), whose function is the same as in powder metallurgy:
To effect a solid state reaction which bonds the material into a hard solid mass
Figure 17.1 - Usual steps in traditional ceramics processing: (1) preparation of raw materials, (2) shaping, (3) drying, and (4) firing Part (a) shows the workpart during the sequence, while (b) shows the condition of the powders
Shaping processes for traditional ceramics require the starting material to be a plastic paste
This paste is comprised of fine ceramic powders mixed with water
The raw ceramic material usually occurs in nature as rocky lumps, and reduction to powder is the purpose of the preparation step in ceramics processing
COMMINUTION
Reducing particle size in ceramics processing by use of mechanical energy in various forms such as impact, compression, and attrition Comminution techniques are most effective on brittle materials such as cement, metallic ores, and brittle metals Two general types of comminution operations:
1. Crushing 2. Grinding
CRUSHING
Reduction of large lumps from the mine to smaller sizes for subsequent further reduction Several stages may be required (e.g., primary crushing, secondary crushing), the reduction ratio in each stage being in the range 3 to 6 Crushing of minerals is accomplished by compression against rigid surfaces or by impact against surfaces in a rigid constrained motion
Jaw Crusher Large jaw toggles back and forth to crush lumps against a hard, rigid surface
GRINDING
In the context of comminution, grinding refers to the operation of reducing the small pieces after crushing to a fine powder Accomplished by abrasion, impact, and compaction by hard media such as balls or rolls Examples of grinding include:
Ball mill Roller mill Impact grinding
Ball Mill Hard spheres mixed with stock are rotated inside a large cylindrical container; the mixture is carried up the container wall as it rotates, and then pulled back down by gravity for grinding action
Figure 17.3 - Mechanical methods of producing ceramic powders: (a) ball mill
Roller Mill Stock is compressed against a flat horizontal grinding table by rollers riding over the table surface
Figure 17.3 Mechanical methods of producing ceramic powders: (b) roller mill
SHAPING PROCESSES
Slip casting
The clay-water mixture is a slurry
Semi-dry pressing
The clay is moist but has low plasticity
Dry pressing
The clay is basically dry (less than 5% water) and has no plasticity
Figure 17.4 - Four categories of shaping processes used for traditional ceramics,
compared to water content and pressure required to form the clay
SLIP CASTING
A suspension of ceramic powders in water, called a slip, is poured into a porous plaster of paris mold so that water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface The slip composition is 25% to 40% water Two principal variations:
Drain casting - the mold is inverted to drain excess slip after a semi-solid layer has been formed, thus producing a hollow product Solid casting - to produce solid products, adequate time is allowed for entire body to become firm
Figure 17.5 - Sequence of steps in drain casting, a form of slip casting: (1) slip is poured into mold cavity, (2) water is absorbed into plaster mold to form a firm layer, (3) excess slip is poured out, and (4) part is removed from mold and trimmed
Mechanized methods generally use a mixture with less water so starting clay is stiffer
HAND MODELING
Creation of the ceramic product by manipulating the mass of plastic clay into the desired geometry Hand molding - similar only a mold or form is used to define portions of the part geometry Hand throwing on a potter's wheel is another refinement of handcraft methods
Potter's wheel = a round table that rotates on a vertical spindle, powered either by motor or foot-operated treadle Products of circular cross-section can be formed by throwing and shaping the clay, sometimes using a mold to provide the internal shape
Jiggering Similar to potter's wheel methods, but hand throwing is replaced by mechanized techniques
Figure 17.6 - Sequence in jiggering: (1) wet clay slug is placed on a convex mold; (2) batting; and (3) a jigger tool imparts the final product shape
PLASTIC PRESSING
Forming process in which a plastic clay slug is pressed between upper and lower molds contained in metal rings Molds are made of porous material such as gypsum, so when a vacuum is drawn on the backs of the mold halves, moisture is removed from the clay The mold sections are then opened, using positive air pressure to prevent sticking of the part in the mold Advantages: higher production rate than jiggering and not limited to radially symmetric parts
EXTRUSION
Compression of clay through a die orifice to produce long sections of uniform cross-section, which are then cut to required piece length Equipment utilizes a screw-type action to assist in mixing the clay and pushing it through die opening Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators Also used to make the starting clay slugs for other ceramics processing methods such as jiggering and plastic pressing
Semi-dry Pressing Uses high pressure to overcome the clays low plasticity and force it into a die cavity
Figure 17.7 - Semi-dry pressing: (1) depositing moist powder into die cavity, (2) pressing, and (3) opening the die sections and ejection
DRY PRESSING
Process sequence is similar to semi-dry pressing the main distinction is that the water content of the starting mix is typically below 5% Dies must be made of hardened tool steel or cemented carbide to reduce wear since dry clay is very abrasive No drying shrinkage occurs, so drying time is eliminated and good dimensional accuracy is achieved in the final product Typical products: bathroom tile, electrical insulators, refractory brick, and other simple geometries
Figure 17.8 - Volume of clay as a function of water content Relationship shown here is typical; it varies for different clay compositions
DRYING
The drying process occurs in two stages: Stage 1 - drying rate is rapid and constant as water evaporates from the surface into the surrounding air and water from the interior migrates by capillary action to the surface to replace it
This is when shrinkage occurs, with the risk of warping and cracking
Stage 2 - the moisture content has been reduced to where the ceramic grains are in contact
Little or no further shrinkage occurs
Figure 17.9 - Typical drying rate curve and associated volume reduction (drying shrinkage) for a ceramic body in drying Drying rate in the second stage of drying is depicted here as a straight line; the function is sometimes concave or convex
GLAZING
Application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance The usual processing sequence with glazed ware is:
1. Fire the piece once before glazing to harden the body of the piece 2. Apply the glaze 3. Fire the piece a second time to harden the glaze
While the sequence is nearly the same as for the traditional ceramics, the details are often quite different
And some of the traditional ceramics forming techniques are used to shape the new ceramics, such as: slip casting, extrusion, and dry pressing The processes described here are not normally associated with the forming of traditional ceramics, although several are associated with PM
PM press and sinter methods have been adapted to the new ceramic materials
HOT PRESSING
Similar to dry pressing except it is carried out at elevated temperatures so sintering of the product is accomplished simultaneously with pressing This eliminates the need for a separate firing step Higher densities and finer grain size are obtained, but die life is reduced by the hot abrasive particles against the die surfaces
2002 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 2/e
ISOSTATIC PRESSING
Uses hydrostatic pressure to compact the ceramic powders from all directions Avoids the problem of nonuniform density in the final product that is often observed in conventional uniaxial pressing Same process used in powder metallurgy
CEMENTED CARBIDES
A family of composite materials consisting of carbide ceramic particles imbedded in a metallic binder Classified as metal matrix composites because the metallic binder is the matrix which holds the bulk material together However, the carbide particles constitute the largest proportion of the composite material, normally between 80% and 95% by volume
Usual proportion of binder metal is 4% up to 20% Powders of carbide and binder metal are thoroughly mixed wet in a ball mill to form a homogeneous sludge The sludge is then dried in a vacuum or controlled atmosphere to prevent oxidation in preparation for compaction
Cobalt works best with WC, while nickel is better with TiC and Cr3C2
COMPACTION
Most common process is cold pressing, used for high production of cemented carbide parts such as cutting tool inserts
Dies must be oversized to account for shrinkage during sintering (shrinkage can be 20% or more) For high production, the dies are made with WC-Co liners to reduce wear For smaller quantities, large flat sections may be pressed and then cut into smaller pieces Other methods: isostatic pressing and hot pressing
SINTERING OF WC-CO
It is possible to sinter WC (and TiC) without a metal binder, but the resulting material is less than 100% of true density
The usual sintering temperatures for WC-Co are 1370-1425C (2500-2600F), which is below cobalt's melting point of 1495C (2716F) Thus, the pure binder metal does not melt at the sintering temperature
These mechanisms cause a rearrangement of the remaining WC particles into a closer packing, which results in significant densification and shrinkage of the WC-Co mass
As the liquid phase forms, it flows and wets the WC particles, further dissolving the solid Presence of molten metal also serves to remove gases from the internal regions of the compact
SECONDARY OPERATIONS
Subsequent processing is usually required after sintering to achieve adequate dimensional control of the cemented carbide parts Grinding with a diamond or other very hard abrasive wheel is the most common secondary operation performed for this purpose Other secondary operations include
Electric discharge machining Ultrasonic machining
The End
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