Transcript Chapter 10 Powder Methods of Change of Form

Powder Methods of
Change of Form
Chapter 10
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Powder Methods of Change of Form
Competencies
 List the advantages of, and the products made
from Powder Metal
 List and describe the order of operations in
Powder Metallurgy
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Powder Methods of Change of Form
Power metallurgy - the process of compacting
metal powders in suitable dies and sintering
them.
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Net shape parts of fairly complex shape can be
produced economically
Competitive with casting, forging and machining
Good dimensional accuracy and size; from balls for
ball point pens to parts weighing 200 lbs.
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• Parts such as
• Self lubricating bearings impregnated with oil
• Break pads with embedded ceramic fibers
• Machine tool cutting instruments; cermets
(ceramic-metals) higher heat absorption
• Commonly made out of iron, copper, aluminum, tin,
and nickel
Order of operation: Powder production, blending,
compacting, sintering, finishing
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Powder Preparation
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Virtually any metal can be made into powder form.
There are three principal methods by which
metallic powders are commercially produced, each
of which involves energy input to increase the
surface area of the metal.
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Atomization – involves the conversion of molten
metal into a spray of droplets that solidify into
powder. It is the most versatile and popular
methods for producing metal powders today
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Gas atomization – in which a high velocity gas
stream is utilized to atomize the liquid metal.
Water atomization – a high-velocity water stream
is used instead of air.
Chemical
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Electrolytic – an electrolytic cell is set up in which the
source of the desired metal is the anode. The anode
is slowly dissolved under an applied voltage,
transported through the electrolyte, and deposited on
the cathode. The deposit is removed, washed, and
dried to yield a metallic powder of very high purity.
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In addition, mechanical methods are occasionally
used to reduce powder sizes; however, these
methods are much more commonly associated with
ceramic powder production.
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Powder Methods of Change of Form
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Comminution, a term used for the techniques for
reducing particles size in ceramics processing,
deliver mechanical energy in various forms. Two
general types of communition operations are
distinguished: crushing and grinding
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Crushing – the 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)
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Powder Methods of Change of Form
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Grinding – refers to the operation of reducing the small
pieces after crushing to a fine powder. Grinding is
accomplished by abrasion and impact of the crushed
mineral by the free motion of unconnected hard media
such as balls, pebbles, or rods.
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Ball Mill – hard spheres mixed with the stock to be
comminuted are rotated inside a large cylindrical container.
Roller mill – stock is compressed against a flat horizontal
grinding table by rollers riding over the table surface
Impact grinding – particles of stock are thrown against a
hard flat surface, either in high velocity air stream or in a
high-speed slurry. The impact fractures the pieces into
smaller particles.
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2. Blending
Mixing several powders of different sizes, a dry
lubricant or an antioxidant for uniform compaction.
(done carefully to avoid explosions)
• Can add lubricants- do not stick to mold walls
• Can add binders- so green strength is adequate
• Can add sintering aids- acceleration of densification
upon heating
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3. Compaction
Bringing the materials into required shape . The
workpart after pressing is called a green compact,
the word green meaning not yet fully processed.
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Briquetting - Compression of powder in the die cavity
from both the top and the bottom.
Roller Compaction - compacted between two rollers
to produce sheet or plate stock.
Extrusion Compacting - powder packed into a mild
steel tube, then forced through a die.
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4. Sintering
Process of heating compressed powdered metals
to within 70 – 90 % of its melting point. Often called
(solid-state sintering, or solid-phase sintering)
because the metal remains unmelted
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Used for materials such as ceramics and cermets
that cannot be melted and cast by other methods.
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Sintering involves mass transport to create the necks
and transform them into grain boundaries. The
principal mechanism by which this occurs is
diffusion; other possible mechanisms include plastic
flow.
Sintering – the heat treatment consists of three steps
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Preheat, in which lubricants and binders are burned off
Sinter
Cool down
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5. Finishing of Sintered Parts
(Secondary Operations)
Densification and Sizing
• Repressing – the part is squeezed in a closed die to
increase density and improve physical properties.
• Sizing - forcing the part through a finish die to
provide dimensional accuracy
• Coining – pressing details into its surface
• Machining
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Infiltration and Impregnation - Because powder-formed
parts can be very porous, other materials can be
placed in the voids to enhance the properties of the
product.
• Prevents moisture penetration
• Lowers frictional properties
• Infiltration – pores are filled with a molten metal
• Impregnation - Impregnating the sintered part with
oil to create a “self lubricating” bearing.
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Advantages
• Wide range of mech. & phys. properties
• Parts made from high melting point metals
• High production rates on relatively complex
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parts
Good dimensional control
Impregnating and infiltration
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Limitations
• Size of parts, complexity of shapes of parts
• High cost of powdered metal compared to
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other materials
High tooling cost for small production runs
Lower strength and ductility than forging
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