Powder Compaction of Metal Flashcards
Compressibility
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
Powder Compaction
Characterization of compressibility of a metallic powder
- Green density
- Graph of green density vs cold compaction pressure
Compression ratio
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
Densification parameter
(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)
Green Strength
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
What is green strength promoted by
Increasing: particle roughness surface area green density (compaction pressure) Decreasing: powder apparent density particle surface oxidation and contamination amount of interfering elements
Characteristics of green compacts
- Porosity
Methods to evaluate the porosity of green compacts
- apparent density-total porosity
- liquid impregnation-open porosity
- Mercury prosimeter-open porosity
- image analysis- total porosity
Porosity of green compacts
GC contains approx 10-20% of porosity
Pores all elongated the direction prependicular to the pressure
Density of green compacts
Obtained by uniaxial pressing
Major problem is non uniform density - L/D ratio
Effect of cold compaction pressure on green density
greater pressure -> greater green density
On average greater for soft powders
On average greater for isostatic vs uniaxial
Effect of particle size on green density
Greater particle size leads to greater green density
Effect of Sd/Sp on green density
Decreases with greater Sd/Sp, on average greater with lubricant
Also on average greater with double motion
Effect of compaction speed on green density
Increased compaction speed leads to lower green density
Mechanical strength of green compacts
Obtained by uniaxial pressing
Effect of particle size on GC strength
Increased particle size leads to decreasing green strength
Effect of compaction pressure on GC strength
Increased compaction pressure leads to increased GC strength
Greater on average for finer powders
Effect of applied pressure duration on GC strength
Increased applied pressure duration leads to increased GC strength
Effect of lubricant on GC strength
Strength increases, then peaks, then decreases
Press Sinter route
pressing operation
Pressing operation
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
Factors affecting tooling design
Fill Flow Apparent density and fill ratio Compaction Pressure Dimensional changes
Fill
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)
Flow
Adequate flow is required (lubricant)
Apparent density and fill ratio:
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
Compaction pressure
limitation in tool strength (maxi 1000 MPa)
Dimensional changes
Green parts undergo dimensional changes during sintering, this factor must be included in tool design
Three causes of green cracks
rough surface finish in the tooling, possibly from wear
improper tool motion
tensile stresses during ejection
Delamination
if green strength of a compact is low, ejection stress is still high
What happens when neighbouring region is compressed?
Compression flow passes near an already densified area, creating a crack along the interface
What can differential strain due to difference in tool length do?
May leave part unsupported during the ejection
