Handout 5: Powder Processing Flashcards
What is powder processing?
Material in fine powder form (particle size) 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. Occasionally (in the case of some ductile metals) the compacted powder is used as source material for further processing (e.g. extrusion, forging etc).
Powder processing normally involves cold compaction followed by a high temperature sintering stage, in which heat is applied with or without pressure.
What are reasons to use powder processing?
- High melting-point materials can be formed to final shape: ceramics (e.g. alumina, silicon nitride, zirconia); cermets (e.g. tungsten carbide/cobalt); metals,polymers (e.g. PTFE, ultra-high MW polyethylene).
- Near net shape, good surface finished, minimal final machining. Can be used to make metal gears, connecting rods etc.
- Material Wastage is low (below 3% while cast/forged parts 60%)
- Achieves good dispersion of phases e.g. for reactive materials or of materials which cannot easily be mixed in the molten state.
- Avoids segregation effects which occur during casting (e.g. when densities of phases are very different).
- Porosity can be controlled, either to achieve low porosity (essentially ‘fully dense’) or high porosity (up to 50%) for porous (‘self-lubricating’) bearings or filters.
- Relatively cheap for large production runs (e.g. >104 ): materials costs are reduced by 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.
What are the effects of different particle shapes?(round or irregular)
- Irregular particles experience more deformation than rounded particles during pressing. The high pressure at particle contact points leads to heavy plastic deformation,interlocking and local cold welding which increases their adhesion, leading to stronger ‘green’ components.
- rounded particles flow more easily than irregular particles and can pack together more uniformly and densely than irregular particles. For highest packing density we use mainly rounded particles with a range of particle sizes (small ones to fill up the gaps between big ones). Controlled low packing density leading to high porosity can be achieved by using a single particle size.
What additives are added to powders and how do they aid?
- 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.
- 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 allows:
- Reduced friction, more uniform product density produced.
- part is ejected without cracking and die life is increased.
What is a ‘green compact’?
- The powder is pressed into a shaped mould/die at high pressure.
- 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).
Why do ceramic green parts require more machining than metal?
- For ceramic materials the linear dimensions of the green compact will be 15-20% greater than those of the finished part because of the remaining porosity (up to 50%).
- With metals the powder particles themselves deform during compaction and the green compact has a similar density to the final product (up to 95% of bulk metal); there is little or no shrinkage on sintering.
Why does the homogeneity of the compact determine the mechanical properties of the final product?
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.
How is a more uniform density distribution achieved in powder processing using punches?
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?
What is its advantage over typical uniaxial pressing?
The powder is contained in a rubber mould (n.b. no heating is used at this point), and pressure is applied by external fluid or gas.
- Uniaxial (i.e. 1-D) pressing is restricted to simple shapes. For more complex shapes (especially for ceramics) isostatic pressing is needed to form a green compact with a uniform density distribution.
- More expensive than uniaxial pressing, and dimensional accuracy low (products are often machined before sintering). However, the need for lubricants/binders is reduced or removed.
What is the driving force in sintering?
- Driving force for sintering: diffusion along composition gradients and reduction in particle surface area.
- Particles bond together, and interfaces between particles become grain boundaries. (Vapour and/or liquid phase transport can also occur.)
What mechanisms occur when the material is sintered?
- Mechanism: atoms diffuse to fill the pores.
- Diffusion occurs along different paths:
- bulk (through the lattice), dislocation core, surface (along surface of unsintered particles), grain boundary (along the boundaries between the particles, once fused). Total rate is the sum of all of these, though one mechanism tends to be dominant at a particular temperature and particle or pore size.
What is the equation for sintering rate?
What are the four stages of sintering?
- Plastically deformed contacts (in metals)
- Cohesion of particles by formation of bridges. Porosity is interconnected.
- Particles grow competitively by diffusion across interparticle interfaces, leading to grain growth
- Porosity becomes spherical and shrinks. Porosity becomes discrete.
Why is the sintering rate inversely proportional to (particle size)3?
Driving force from surface energy, diffusion distances.
Fine compacts sinter faster, and as grains grow, the rate falls.
Explain hot isostatic pressing (HIPing)
- Similar to cold isostatic pressing, but powder is canned in a metal container to provide shape and subjected simultaneously to high temperature (e.g. 2000ºC for SiC) as well as high hydrostatic pressure (using gas, generally argon).
- HiPing allows very low final porosity. The short process time (minutes, rather than the hours needed for conventional sintering) reduces grain growth problems.
- However, the high gas pressure (typically 10 MPa = 100 bar) mean that the process is expensive (large pressure-vessels, operating at high T).
- HIPing provides components with good mechanical properties (low porosity), but dimensional accuracy is low.
- The improvement in ductility and tensile strength can be very significant
Explain the process for Metal Injection Moulding (MIM)/Powder Injection Moulding (PIM).
- Uses conventional polymer injection moulding technology with a blended polymer-metal or polymer-ceramic feedstock to produce the initial green compacts.
- These are therefore highly-filled polymer-metal or polymerceramic composites. Typical MIM process uses very fine metal powder (1 – 20 µm) with specially designed polymer binders.
- The volume fraction of binder is 30-50%. Binder removal (debinding) is a critical stage involving:
- heating the green compact – taking several hours, or
- chemical decomposition by e.g. gaseous nitric acid – quicker, or • dissolving the binder with a solvent The debinding process converts the ‘green’ part (as-moulded, containing the polymer binder) to a ‘brown’ part (with no binder, but still not sintered, so containing high porosity) which is then sintered.
- During sintering, the part typically shrinks 50% by volume (15-20% linearly). Care is therefore needed to retain shapes while dimensions change dramatically.
