Karteikarten
powder processing
Karten in diesem Stapel (17)
- 1Describe powder processingpowder is formed under pressure to make a green part, which is then heated to allow the powder particles to stick together.
A simple process
can be applied to several materials - 2Name the stages of power metallurgy (PM)– powder production
– (blending)
– compaction
– sintering
– finishing - 3Name the three commercial routes to Powder ProductionAtomisation (most common)
Chemical Reduction
Electrolysis - 4What is atomisation– Most common route
– Molten material is forced through a small orifice to produce a thin
stream of fluid material
– The stream is then hit by a jet of water or air (gas or water
atomisation)
– The stream disintegrates, and the droplets cool to form powder
particles
– Particle size controlled by jet velocity
– Powders are often spherical in geometry - 5What is chemical reduction– Metallic compounds are reduced to metal powders – Use of reducing agents such as hydrogen or carbon
monoxide
– Frequently used in the production of iron powders - 6What is electrolysis– Source of metal is anode in electrolytic cell or uses aqueous solution
– Metal powder of high purity deposited on cathode - 7For which characteristics are powders often screened ?Size
Shape
Size distribution - 8Why blend the powders?– to produce a mixture of two materials if an alloy is to be produced
– to add lubricants or binders to improve the powder processing properties
– to generate a uniform powder size distribution throughout the batch - 9What does compacting do?Compresses the powder
within a die
The density of the green
part depends on the pressure applied
(must be uniform)
The strength and modulus
are density dependent - 10Describe SinteringThe green part is heated
Tightly controlled atmosphere to prevent oxidation
bonding between adjacent powder particles may occur by diffusion (solid state sintering)
Liquid phase sintering normally occurs where two
metals are involved,
for metals sintering time <60 min
normally associated with shrinkage and
residual porosity - 11name three finishing operations– Coining or sizing to increase the density of the component
– Infiltration with a lower melting point metal to remove porosity
– Impregnation with oil for lubrication - 12Powder properties and design rules• Particle size and size distribution-affects packing and how well the powder “flows”
• Particleshape-affects how well the powder flows, the surface area available in sintering, and packing
• Minimum wall thickness of compact about2mm
• Fillets and draft angles necessary to make compact removal easy
• Large overhangs can cause problems in the green stage when compact is weak - 13Advantages of Powder Processing• Good surface finish and close tolerances
• Net shape or near net shape so little subsequent machining and high material utilisation
• Can be used with materials which can’t be easily processed any other way e.g. ceramic - 14Disadvantages of Powder Processing• Powder and processing equipment expensive
• Tooling can be expensive (depends on powder properties)
• Porosity means inferior mechanical properties - 15Cold isostatic pressing– Compaction at room temperature
• Pressure applied by water or oil
• Gives uniform density
BUT difficult to achieve and needs post pressing finising - 16Hot Isostatic Pressing– Compaction at high temperature and pressure
• Pressure applied by gaseous media such as argon
• Pressing and sintering occur in one step
• Expensive route – aerospace and medical industries - 17Powder Injection Moulding/Extrusion• Where powder has excellent flow characteristics, it can be
injection moulded to form a green part
• Allows very complicated parts to be made
• material is
processed below its melt temperature
Informationen
- Beschreibung
- Bachelor Menelaos (Semester 1) Karteikarten am powder processing, erstellt von athanasia.kanell am 11/05/2016.
- Erstellt von
- athanasia.kanell
- Aufrufe
- 0
- Bewertung