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Q: |
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What affects the mechanical
properties of ferrous P/M materials? |
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A: |
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Density: Mechanical properties improve as the
amount of porosity is reduced. Tensile strength is almost a linear function of density.
Ductility, toughness, and fatigue strength are even more dependent on density; they
increase significantly at low levels of porosity.
Composition: Different alloying methods ( Admixing,
partially-alloying, prealloying, and (hybrid mixes) are used to achieve the desired
chemistry of a material. The composition and alloying method determine the hardenability
for a given consolidation and sintering route. Constituents not in solution do not
contribute to hardenability.
Microstructure: The microstructure depends on the
chemistry of the powder, the alloying method used, the sintering time and temperature, the
sintering atmosphere, and the cooling rate after sintering. |
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Why is the apparent density so
much lower in sponge products than atomized steel products? |
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Apparent density measurements are an indicator of
how close particles pack together when not in a stressed mode. Sponge iron powder exhibits
a lower apparent density (AD) than steel atomized powder due the different particle shapes
each type of material possesses.
To
produce sponge iron, magnetite ore is directly reduced at elevated temperatures. The
resulting powder contains internal porosity which is not present in steel atomized powder.
So, the sponge particles to pack together with more unfilled space than the atomized
particles, resulting in a lower apparent density. |
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Why is the apparent density of a
powder mix or premix so important? |
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A: |
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The apparent density of a powder or premix is used
by part designers to design tooling that will produce parts with the desired dimensions.
If the apparent density of the material changes, problems may result in terms of meeting
part specifications, particularly with a fixed fill tooling or shelf die. Also, the
apparent density is an indication of the green strength of the material, in general, as
green strength increases with decreasing apparent density. |
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Q: |
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What is ‘apparent’
hardness? |
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Apparent hardness is a macrohardness (hardness
testing in the macro scale). Due to the fact the P/M parts are composites of metal and
pores, the apparent hardness values obtained, since taken on the macro scale, is the
hardness of the composite. Hardness is a measure of the resistance of a metal to permanent
(plastic) deformation. |
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What is the effect of density on
Young’s Modulus? |
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Young’s Modulus increases linearly as a
function of density in the range of 6.5 to 7.4 g/cm3 from 15 to 25.
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What is Poisson’s ratio for
ferrous P/M materials? |
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The value for Poisson’s Ratio of ferrous P/M
materials may be taken as 0.27+0.02. Poisson’s Ratio is a weak function of
density. |
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What are the coefficients of
thermal expansion of P/M materials? |
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A: |
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The thermal expansion of a P/M material is a weak
function of density. A rough estimate can be obtained by taking the cube root of the
relative density of the P/M part and multiplying that value by the bulk thermal expansion
coefficient. |
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What is meant by the expression
‘pore-free’ density of a green compact? |
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A: |
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Pore-free density is the density of a green
compact if ALL of the porosity could be removed. Pore-free density is dependent on the
density and percentage of each premix constituent addition. Graphite and lubricant have
significant negative effects on pore-free density. The pore-free density of a premix can
be calculated from the premix composition:
Density of Various
Materials as Measured by Pycnometry
Material |
Density
(g/cm3) |
Ancorsteel
1000B |
7.84 |
Ancorsteel
4600V |
7.84 |
Distaloy
AE |
7.90 |
Copper
(atomized) |
8.05 |
Nickel
(Inco 123) |
8.85 |
Graphite |
2.30 |
Lubricants |
0.90 to
1.15 |
Pore-Free Density
Example
Ancorsteel 1000B + 2 w/o Ni + 0.6 w/o graphite + 0.6 w/o lubricant |
Additive |
Mass% |
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Density |
Volume |
Ancorsteel
1000B |
96.80 |
÷ |
7.84 |
12.35 |
Nickel |
2.00 |
÷ |
8.85 |
0.23 |
Graphite |
0.60 |
÷ |
2.30 |
0.26 |
Lubricant |
0.60 |
÷ |
1.00 |
0.60 |
Total Mass
(g) = 100.00 Total Volume (cm3) = 13.44
Pore-Free Density = Mass ÷ Volume = 100.00 ÷ 13.44 = 7.44 g/cm3 98%
Pore-Free Density* = 7.29 g/cm3
*98% Pore-Free Density is
what is seen typically in practice |