Transcript MetalPowders

Chapter 17
Processing of Metal Powders
Copyright Prentice-Hall
Parts Made by Powder-Metallurgy
(b)
(c)
(a)
(a) Examples of typical parts made by powder-metallurgy processes. (b) Upper trip lever for
a commercial sprinkler made by P/M. This part is made of an unleaded brass alloy; it replaces
a die-cast part with a 60% savings. (c) Main-bearing metal-powder caps for 3.8 and 3.1 liter
General Motors automotive engines. Source: (a) and (b) Reproduced with permission from
Success Stories on P/M Parts, 1998. Metal Powder Industries Federation, Princeton, New
Jersey, 1998. (c) Courtesy of Zenith Sintered Products, Inc., Milwaukee, Wisconsin.
Steps in Making Powder-Metallurgy Parts
Outline of processes and operations involved in making powder-metallurgy parts.
Particle Shapes in Metal Powders
Particle shapes in metal powders, and the processes by which they are
produced. Iron powders are produced by many of these processes.
Powder Particles
(a)
(b)
(a) Scanning-electron-microscopy photograph of iron-powder particles made by
atomization. (b) Nickel-based superalloy (Udimet 700) powder particles made by the
rotating electrode process. Source: Courtesy of P.G. Nash, Illinois Institute of
Technology, Chicago.
Methods of
Metal-Powder
Production by
Atomization
Figure Methods of metalpowder production by
atomization: (a) gas
atomization; (b) water
atomization; (c)
atomization with a rotating
consumable electrode; and
(d) centrifugal atomization
with a spinning disk or cup.
Mechanical Comminution to Obtain Fine Particles
Methods of mechanical comminution to obtain fine particles: (a) roll
crushing, (b) ball mill, and (c) hammer milling.
Mechanical Alloying
Mechanical alloying of nickel particles with dispersed smaller particles. As nickel particles are
flattened between the two balls, the second smaller phase is impresses into the nickel surface
and eventually is dispersed throughout the particle due to successive flattening, fracture, and
welding events.
Bowl Geometries in Blending
Metal Powders
(e)
(a) through (d) Some common bowl geometries for mixing or blending powders. (e) A
mixer suitable for blending metal powders. Since metal powders are abrasive, mixers
rely on the rotation or tumbling of enclosed geometries as opposed to using aggressive
agitators. Source: Courtesy of Gardner Mixers, Inc.
Compaction
(a) Compaction of metal powder to form a bushing. The pressed-powder part is called
green compact. (b) Typical tool and die set for compacting a spur gear. Source:
Courtesy of Metal Powder Industries Federation.
Density as a Function of
Pressure and the Effects of
Density on Other Properties
(a) Density of copper- and iron-powder
compacts as a function of compacting
pressure. Density greatly influences the
mechanical and physical properties of P/M
parts. (b) Effect of density on tensile strength,
elongation, and electrical conductivity of
copper powder. Source: (a) After F. V. Lenel,
(b) IACS: International Annealed Copper
Standard (for electrical conductivity).
Density Variation in Compacting Metal Powders
Density variation in compacting metal powders in various dies: (a) and (c) singleaction press; (b) and (d) double-action press. Note in (d) the greater uniformity of
density from pressing with two punches with separate movements when compared
with (c). (e) Pressure contours in compacted copper powder in a single-action press.
Source: After P. Duwez and L. Zwell.
Compacting Pressures for Various Powders
Cold Isostatic Pressing
Schematic diagram of cold isostatic pressing, as applied to forming a tube. The
powder is enclosed in a flexible container around a solid-core rod. Pressure is
applied isostatically to the assembly inside a high-pressure chamber. Source:
Reprinted with permission from R. M. German, Powder Metallurgy Science, Metal
Powder Industries Federation, Princeton, NJ; 1984.
Capabilities Available from P/M Operations
Capabilities, with respect to part size and shape complexity, available form various P/M
operations. P/F means powder forging. Source: Courtesy of Metal Powder Industries
Federation.
Hot Isostatic Pressing
Schematic illustration of hot isostatic pressing. The pressure and temperature
variation versus time are shown in the diagram.
Valve Lifter for Diesel Engines
A valve lifter for heavy-duty diesel engines produced form a hot-isostatic-pressed
carbide cap on a steel shaft. Source: Courtesy of Metal Powder Industries
Federation.
Powder Rolling
Schematic illustration of powder rolling.
Spray Deposition
Spray deposition (Osprey Process) in which molten metal is sprayed over
a rotating mandrel to produce seamless tubing and pipe.
Sintering Time and Temperature for Metals
Mechanisms for Sintering Metal Powders
Schematic illustration of two mechanisms for sintering metal powders: (a) solidstate material transport; and (b) vapor-phase material transport. R = particle
radius, r = neck radius, and p = neck-profile radius.
Mechanical Properties of P/M Materials
Comparison of Properties of Wrought and Equivalent P/M Metals
Mechanical Property Comparisons for Titanium Alloy
Design Considerations for P/M
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The shape of the compact must be kept as simple and uniform as possible.
Provision must be made for ejection of the green compact without damaging the
compact.
P/M parts should be made with the widest acceptable tolerances to maximize tool life.
Part walls should not be less than 1.5 mm thick; thinner walls can be achieved on
small parts; walls with length-to-thickness ratios above 8:1 are difficult to press.
Steps in parts can be produced if they are simple and their size doesn’t exceed 15%
of the overall part length.
Letters can be pressed if oriented perpendicular to the pressing direction. Raised
letters are more susceptible to damage in the green stage and prevent stacking.
Flanges or overhangs can be produced by a step in the die.
A true radius cannot be pressed; instead use a chamfer.
Dimensional tolerances are on the order of ±0.05 to 0.1 mm. Tolerances improve
significantly with additional operations such as sizing, machining and grinding.
Poor and Good
Designs of P/M
Parts
Examples of P/M parts showing
poor and good designs. Note
that sharp radii and reentry
corners should be avoided and
that threads and transverse
holes have to be produced
separately by additional
machining operations. Source:
Courtesy of Metal Powder
Industries Federation.
Design Features for Use with Unsupported
Flanges or Grooves
(a) Design features for use with unsupported flanges. (b) Design features for use with
grooves. Source: Courtesy of Metal Powder Industries Federation.
Use of Smooth Transitions in Molds
The use of smooth transitions in molds for powder-injection molding to ensure
uniform metal-powder distribution throughout a part.