Flashcards
Powder Metallurgy
Cards in this deck (19)
- 1Powder MetallurgyRaw material in powder form compacted into die then sintered
Large range of powder compositions
Produces parts to net dimensions
Used when:
High melting temperature
Too hard to machine
Large quantities needed
Controllable porosity - 2SinteredHeated without melting
- 3Size and Shape Distribution AffectSpherical powders with wide size distribution give highest packing density but poor strength
Irregular shaped powders give a poor packing density but good strength - 4Mixing and BlendingMix powders
Mechanical alloying
- Powders put into ball mill
- Fracture
- Welding occurs between different powder particles
Diffusion Bonding
- Powder mixture heated to yield a sintered cake
- Cake ground to produce powder - 5Powder AdditivesLubricants can be used to improve flow of powder
Binders can be used to improve green strength
Sintering aids - 6Powder Consolidation Die PressingPowder cold compacted in a die to produce a green compact
Single / Multiple presses can be used
Green density depends on compact pressure
Wall friction and interparticle friction oppose compaction
Pressure tapers off rapidly from punch/powder interface
Multiple punches with separate actions ensure a more uniform green density - 7Powder Consolidation Isostatic PressingPowder is put into a flexible walled mould and surrounded by pressurised water
Relatively expensive - 8Powder consolidation RollingPowder fed into roll gap in a two high rolling mill and compacted to form a continuous shape
- 9Powder Consolidation PressurelessDie filled with powder
Powder sintered in die
Produces porous parts - 10Powder Consolidation Metal Injection MouldingVery fine metal powder blended with polymer / wax binder
Injected into die to form small complex parts
Binder burnt off - 11Powder Consolitdation Spray DepositionPowder used directly after atomisation
Sprayed onto cool, rotating, preform mould
Produces tubes - 12Sintering Property ChangesReduction in pore volume and hence shrinkage
Volume reduces hence density increases
High sintered density gives good mechanical properties
Better dimensional accuracy by minimising volume changes - 13PorosityNot an issue when making parts which absorb lubricant or filters
To improve porosity:
Cold Restriking - Forging sintered product
- Resintered
Heat Treatment - Improved hardness and strength
Infiltration with metal
Machining to tighter tolerance - 14Improving PropertiesCombing consolidation and sintering
High p applied at sintering T and causes rearrangement of particles
- Induces plasticity and creep
- Augments effects of surface tension as driving force of diffusion
- Accelerates sintering
Protective atmosphere
- Prevents oxidisation
Methods include:
- Hot pressing in graphite / ceramic dies
- Hot rolling & extrusion
- Hot forging of sintered compact
- Hot isostatic pressing - 15Design LimitationsPowder has to fill die and be uniformly compressed
Has to be a shape which can be released from die - 16AdvantagesVirtually unlimited choice of alloys
Uniform fine microstructures
Controlled porosity
Very economical for large quantities
Reliable through close control of dimensions and physical properties
Very good material utilisation - 17DisadvantagesLimited part size and complexity
High cost of powder
High cost of tooling
Not as strong as wrought - 18Mechanical AlloyingPowders of two or more pure metals mixed in a ball mill
Under the impact of the hard balls the powders repeatedly fracture and weld together forming alloy powders - 19Elemental PowdersMixed to form alloys
Most common and preferred method
Better compressibility than pre-alloyed powders
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- Flashcards on Powder Metallurgy, created by tom8390 on 09/01/2015.
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- tom8390
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