Powder Compaction of Metal

Question 1

Compressibility

Answer 1

Measure to which the powder will compress or densify upon application of external pressure
Influenced by hardness, particle shape, internal porosity, particle size distribution, presence of nonmetallic, solid lubricants

Question 2

Powder Compaction

Answer 2

Characterization of compressibility of a metallic powder

1. Green density
2. Graph of green density vs cold compaction pressure

Question 3

Compression ratio

Answer 3

Volume of powder prior compaction/volume of powder after compaction
For PM a lower compression ratio is better
-dimension of compaction tool is smaller
-amplitude of tool is lower
-dies get filled quicker
-wear is reduced

Question 4

Densification parameter

Answer 4

(rho g - rho a)/(rho th - rho a)
Allow comparison of a powder compressibility of different nature
Compressibility increases with increasing apparent density (large amount of densification occuring at low pressure)

Question 5

Green Strength

Answer 5

Mechanical strength of an unsintered powder compact
Important characteristic to determine the ability of a green compact to maintain its size and shape during handling prior to sintering

Question 6

What is green strength promoted by

Answer 6

Increasing:
 particle roughness
surface area
green density (compaction pressure)
Decreasing:
powder apparent density
particle surface oxidation and contamination
amount of interfering elements

Question 7

Characteristics of green compacts

Answer 7

1. Porosity

Question 8

Methods to evaluate the porosity of green compacts

Answer 8

• apparent density-total porosity
• liquid impregnation-open porosity
• Mercury prosimeter-open porosity
• image analysis- total porosity

Question 9

Porosity of green compacts

Answer 9

GC contains approx 10-20% of porosity

Pores all elongated the direction prependicular to the pressure

Question 10

Density of green compacts

Answer 10

Obtained by uniaxial pressing

Major problem is non uniform density - L/D ratio

Question 11

Effect of cold compaction pressure on green density

Answer 11

greater pressure -> greater green density
On average greater for soft powders
On average greater for isostatic vs uniaxial

Question 12

Effect of particle size on green density

Answer 12

Greater particle size leads to greater green density

Question 13

Effect of Sd/Sp on green density

Answer 13

Decreases with greater Sd/Sp, on average greater with lubricant
Also on average greater with double motion

Question 14

Effect of compaction speed on green density

Answer 14

Increased compaction speed leads to lower green density

Question 15

Mechanical strength of green compacts

Answer 15

Obtained by uniaxial pressing

Question 16

Effect of particle size on GC strength

Answer 16

Increased particle size leads to decreasing green strength

Question 17

Effect of compaction pressure on GC strength

Answer 17

Increased compaction pressure leads to increased GC strength

Greater on average for finer powders

Question 18

Effect of applied pressure duration on GC strength

Answer 18

Increased applied pressure duration leads to increased GC strength

Question 19

Effect of lubricant on GC strength

Answer 19

Strength increases, then peaks, then decreases

Question 20

Press Sinter route

Answer 20

pressing operation

Question 21

Pressing operation

Answer 21

filling of the die with required powder quantity
pressing to obtain green density and part thickness
Withdrawal of the upper punch from the compact
Ejection

Question 22

Factors affecting tooling design

Answer 22

Fill
Flow
Apparent density and fill ratio
Compaction Pressure
Dimensional changes

Question 23

Fill

Answer 23

Amount of powder taken into the tool prior to compaction 
function of flow, apparent density, part configuration, and tool design
(height of filling capacity/compact height) = (green density/apparent density)

Question 24

Flow

Answer 24

Adequate flow is required (lubricant)

Question 25

Apparent density and fill ratio:

Answer 25

tooling must be the designed to provide enough fill to produce the part of desired thickness, ratio of apparent density to green density is used to calculate the fill depth

Question 26

Compaction pressure

Answer 26

limitation in tool strength (maxi 1000 MPa)

Question 27

Dimensional changes

Answer 27

Green parts undergo dimensional changes during sintering, this factor must be included in tool design

Question 28

Three causes of green cracks

Answer 28

rough surface finish in the tooling, possibly from wear
improper tool motion
tensile stresses during ejection

Question 29

Delamination

Answer 29

if green strength of a compact is low, ejection stress is still high

Question 30

What happens when neighbouring region is compressed?

Answer 30

Compression flow passes near an already densified area, creating a crack along the interface

Question 31

What can differential strain due to difference in tool length do?

Answer 31

May leave part unsupported during the ejection