Powder Metallurgy Flashcards
What type of product would be considered to be a prospect
for powder metallurgy manufacture?
Powder metallurgy processes involve blending of powders, pressing of the powders to
a desired shape and sintering.
Since powders are used, the composition of the part can be varied over a wide range and
so a wide range of part properties can be produced.
By using a powder component that can be removed after the part is formed permeable
parts can be made.
Since powders are compacted in relatively high precision tooling, accurate parts can be
produced.
Compaction may require high pressure and in such a case compaction forces will be large
and parts will be small.
Sintering enables part characteristics to be controlled by controlling the sintering process.
In general, small, high precision parts that need to have carefully controlled
microstructure are candidates for production using powder metallurgy techniques.
What were some of the earliest powder metallurgy products?
Some of the earliest mass-produced powder metallurgy products included coins and
medallions, platinum ingots, and tungsten wires. These were followed by carbide cutting
tool tips, nonferrous bushings, self-lubricating bearings, and metallic filters.
What are some of the newest technologies in powder
metallurgy?
Automotive applications currently account for nearly 75% of P/M production. Other
major markets include: household appliances, recreational equipment, hand tools,
hardware items, business machines, industrial motors, and hydraulics.
Which metal family currently dominates the powder metallurgy market?
Iron and low alloy steels are used in about 85% of powder metallurgy production. The
large amount of this metal family previously (and currently) used and the workability and
experience with working it led to the early and continued development of powder
metallurgy processes using it.
What are the four basic steps that are usually involved in
making products by powder metallurgy?
The powder metallurgy process normally consists of four steps: powder manufacture,
mixing or blending, compacting, and sintering .
What are some of the important properties and characteristics of metal powders to be used in powder metallurgy?
Some important properties and characteristics of metal powders are: chemistry and
purity, particle size, size distribution, particle shape, and the surface texture of the
particles .
What is the most common method of producing metal
powders?
The most common means of producing metal powders is by melt atomization where
molten metal is fragmented into small droplets and the droplets solidify into particles of
metal. Any material that can be melted can be atomized and the resulting particles retain
the chemistry of the parent material.
What are some of the other techniques that can be employed
to produce particulate material?
Other techniques of powder manufacture include chemical reduction of particulate
compounds, electrolytic deposition from solutions or fused salts, pulverization or
grinding of brittle materials (comminution), thermal decomposition of hydrides or
carbonyls, precipitation from solution, and condensation of metal vapors.
Which of the powder manufacturing processes are likely to
be restricted to the production of elemental (unalloyed)
metal particles?
Powder production processes based on processes in which elemental forms of material
are produced and exist will be practically useful (not overly complicated, time
consuming, energy intensive, easily controllable) only for producing elemental powder.
For example, as chemical reduction, thermal decomposition and condensation processes
occur different elements are obtained at different stages (time, temperature or
composition) and so elemental powders are the logical product to be produced using such
processes.
Why is powder metallurgy a key process in producing products from amorphous or rapidly solidified material?
The production of amorphous and rapidly solidified powders requires large energy
density (energy per unit volume of material produced) and so with reasonable energy
levels only small particles can be produced. That is, for fixed energy input the
requirement of high energy density means that only small volume products can be
produced. To make useful products these small particle, powder, raw materials have to be
combined and powder metallurgy techniques accomplish this consolidation task
effectively and efficiently.
Why is flow rate an important powder characterization
property?
To make a powder metal product powder in placed in a die, pressed and then sintered.
To describe the ability of the powder to flow into the die and into various, small, sections
of the die cavity and to be uniformly distributed in the die, quantitative measures of
powder flow are useful. Flow rate tests provide such a powder behavior measure in flow
rate.
What is apparent density, and how is it related to the final
density of a P/M product?
Apparent density is the density of the loose powder to which there has been no
application of external pressure. Final density is measured after compaction and sintering
and is typically about twice the value of the apparent density.
What is green strength, and why is it important to the manufacture of high-quality P/M products?
Green strength refers to the strength of the powder metallurgy material after pressing,
but before sintering. Good green strength is required to maintain smooth surfaces, sharp
corners, and intricate details during ejection from the compacting die or tooling and
subsequent transfer to the sintering operation .
What are some of the objectives of powder mixing or
blending?
Mixing or blending is performed to combine various grades or sizes of powders or
powders of different compositions, or add lubricants or binders to the powder.
How does the addition of a lubricant affect compressibility?
Green strength?
The addition of a lubricant improves the flow characteristics and compressibility of
the powder at the expense of reduced green strength.
How might the use of a graphite lubricant be fundamentally
different from the use of wax or stearates?
While lubricants such as wax or stearates can be removed by vaporization, the
graphite remains to become an integral part of the final product. In the production of
steel products, the amount of graphite lubricant is controlled so it will produce the desired carbon content in the final material when it is dissolved in the iron powder.
What types of composite materials can be produced through powder metallurgy?
Composites of compatible (easily bonded in sintering) materials should be easy to
produce by powder metallurgy techniques.
Since there is a large amount of mechanical working of the powders used in powder
metallurgy processes, and since surface areas are high energy regions on material bodies,
and since powder surface area is large, the opportunity for producing composites of
usually incompatible materials exists. That is the combination of mechanical working and
high surface energy may make powder sintering possible and an effective way to create
composite materials. Examples are composites composed of an immiscible material
dispersed in a matrix and combinations of metals and nonmetals.
What are some of the objectives of the compaction
operation?
The goal of the compacting operation is to compress and densify the loose powder
into a desired shape. Uniform high density is desired and the product should possess
adequate green strength .
Describe the movement of powder particles during uniaxial compaction.
During compaction, the powder particles move primarily in the direction of the
applied force. The powder does not flow like a liquid, but simply moves in the direction
of pressing until an equal and opposing force is generated through either friction between
the particles and die surfaces or by resistance from the bottom punch.
For what conditions might a double-action pressing be more
attractive than compaction with a single, moving punch?
When pressing with rigid punches, the maximum density occurs adjacent to the punch
and diminishes as one moves away. With increased thickness, it is almost impossible to
produce uniform, high density throughout the compact. By using two opposing punches,
a more-uniform density can be obtained in thicker pieces.
How is the density of a P/M product typically reported?
The final density of a P/M product can be reported as either an absolute density in
units of weight per volume, or as a percentage of the theoretical density, where the
difference between this number and 100% is the amount of void space still present in the
product.
Describe the four classes of conventional powder metallurgy
products.
Conventional powder metallurgy products fall into the classes of
1. Porous or permeable products that are design and produced to small pores or voids that
can be filled with another material or to function as porous media as do filters.
2. Complex shaped parts with dimensional and geometric tolerance and surface finish
requirements that require only light finishing when produced by powder metallurgy. Such
parts can be produced in powder metallurgy dies and so avoid complex machining
operations.
3. Product made from high melting point or difficult to machine materials. The
mechanical action in compaction and high surface energy of powders make consolidation
by sintering practical and avoids difficult machining tasks.
4. Products made from composite materials. Composites can be made from compatible,
easily sintered materials and even from some incompatible materials.
The use of the concepts of better properties and economic advantage (items 5 and 6 in the
text) are useful for describing the advantages of the powder metallurgy processes, but not
for describing types of products.
What is isostatic compaction? For what product shapes
might it be preferred?
Isostatic compaction is the process in which the powder is exposed to uniform
compacting pressure on all surfaces, and is usually achieved by encapsulating the powder
in a flexible mold and immersing it in a pressurized gas or liquid. The process is
generally employed on complex shapes that would be difficult to compact by the faster,
more traditional techniques.
What is the benefit of warm compaction?
Warm, elevated temperature, compaction produces more uniform compaction and
improved as-compacted and after processing properties, primarily strength. As
temperature is increased powder strength decreases. During warm compaction increased
deformation of the powder and increased mechanical action makes for more uniform
density and increased bonding due to disruption of surface layers on the powder particles.
The increased, more intimate contact between powder particles also increases the
material consolidation effects of sintering.
