Powder processing Flashcards
What is powder processing?
Material in fine powder form is pressed into the required shape and then heated to bond the particles together by interdiffusion to form components, which generally require very little further processing
Powder processing normally involves cold compaction followed by a high temperature sintering stage, in which heat is applied with or without pressure.
Why use a powder route?
High melting-point materials can be formed to final shape:
near net shape forming process, with reasonably close tolerances and good surface finish
Material wastage is low typically <3%
Achieves good dispersion of phases e.g. for reactive materials or for materials which cannot easily be mixed in molten state
Avoids segregation effects which occur during casting
Porosity can be controlled
Realtively cheap for large production runs, mateial costs low due to lack of wastage but plant and die costs can be high
What is atomization?
An important process for metals is atomization, in which high pressure jets of water (water atomization) or gas (gas atomization) are directed at a stream of molten metal, causing it to break up into droplets and solidify.
Water atomization (fast quenching in a high heat-capacity medium) leads to irregularly shaped particles; gas atomized particles tend to be more spherical.
For very reactive materials, inert gas or fluid may be used.
Why is particle shape important?
i_rregular particles_ experience more deformation in pressing which increases their adhesion, leading to stronger ‘green’ components than rounded particles.
For metals this is because high pressure at particle contact points leads to heavy plastic deformation, interlocking and local cold welding.
rounded particles flow more easily than irregular particles and can pack together more uniformly and densely than irregular particles.
For highest packing density can use mainly rounded particles with a range of particle sizes (small ones to fill up the gaps between big ones).
With a single size, can achieve controlled high porosity.
Explain Powder blending?
The basic metal or ceramic powder is blended with various additives.
Steel components are made from iron powder mixed with carbon (graphite – up to 1%) and often copper (1 – 4%). The copper strengthens the steel and prevents shrinkage during sintering.
. Non-ferrous alloys (e.g. bronzes, titanium alloys, aluminium alloys) can also be blended as powders.
Ceramic powders usually incorporate additives as sintering aids and to inhibit grain growth
The mix also includes a lubricant/binder (e.g. stearic acid, zinc stearate) which performs several functions
: Powder flows more easily, so fills mould better
Die friction is reduced, so more uniform product density is achieved, part is ejected without cracking and die life is increased.
Explain the Compaction process
The powder is pressed into a shaped mould/die at high pressure:
. Presses need capacities (maximum force) up to hundreds of tonnes force (several MN) to achieve these pressures.
The press capacity usually limits the size of the part that can be formed (typ. <~1 kg steel).
The component is now called a ‘green compact’ with the correct shape, but little strength (typically 10-20 MPa).
Since it is so weak it must be handled carefully, but it is very easy to machine if necessary ( ‘green machining’ to produce features which cannot be produced by pressing – e.g. transverse holes, slots).
For ceramic materils the green compact is around 15-20% greater volume compared to finished part (due to large porosity)
for metals there is little differnce
Explain Homogeniety of the compact
The mechanical properties of the final product (after sintering) depend critically on the homogeneity of the compact.
If a compact contains a range of densities, each region will contract to a different extent on sintering.
This means that not only will it have different mechanical properties in different regions but even more important the product will contain internal stresses.
This causes problems in both metals and ceramics.
Explain the impact the die/punch design has
Design of the die/punch is very important. Friction between the powder and the die can lead to a nonuniform density distribution which results in strength variation.
More uniform distribution is achieved by
better lubrication between powder and die
multiple punches, e.g. with punches moving at both top and bottom of die. Get highest compaction close to moving punch, so average compaction increased and compaction variation is reduced
What is cold isostatic pressing?
for more complex shapes
Forms uniform density distribution
powder contained in rubber mould and pressure apllied by external fluid or gas
reduces need for lubricants and binders
Outline sintering and explain driving force and mechanism
Continuous process
Furnace atmoshphere is closely contolled (prevent oxidation)
Green compact is heated to a temp well above 0.5Tm so rapid diffusion can occur
_Driving force: _Diffusion along composition gradients and reduction in particle surface area
Mechanism: Atoms diffuse to fill pores, diffusion occurs along different paths:
- bulk
- Dislocation core
- surface
- Grain boundary
Total rate is sum of all these (though 1 tends to be more dominate at particular temp and particle size)
What are the stages of sintering
- plastically deformed contacts (in metals)
- cohesion of particles by formation of bridges. Porosity interconnected.
- particles grow competitively by diffusion across inter-particle interfaces, leading to grain growth
- porosity becomes spherical and shrinks. Porosity becomes discrete.
Rate inversely related to (particle size)3 : Fine compacts sinter faster, and as grains grow, the rate falls.
Very difficult to get 100% densification by sintering alone.
Pore size and spacing directly related to original particle size.
Many advantages in keeping sintering times as short as possible
How can porosity be completly removed in sintering?
For metals, if the sintering process is followed by mechanical working (e.g. hot forging, extrusion etc.) then the porosity can be completely removed and maximum mechanical properties are obtained. The process of forging and sintering can be combined into a single process (‘sinter forging’).
What is liquid phase sintering?
Sintering can be speeded up dramatically if a liquid phase is present which can be drawn (by capillary action) into the spaces between the particles.
Example: sintering of alumina + 1% MgO which reacts to form a low meltingpoint glass which bonds the alumina grains together (e.g. for sparkplug insulator). A disadvantage is that the high-temperature strength of the material is reduced.
What is Hot isostatic pressing?
Similar to cold isostatic pressing, but powder is canned in a metal container to provide shape and simultaneously subjected to high temperature (e.g. 2000 ºC for SiC) as well as high hydrostatic pressure
-very low final porosity
short process time reduces grain growth problems
Expensive, dimensional acccuract low
High improvement in ductility and tensile strength
