Lecture 4 - Powder Metallurgy Flashcards
Process
Powdered materials
Blended
Compacted
Sintered
Advantages
No wasted materials
Semi-skilled labourers required
Final products require little/no finishing
Atomisation
Liquid-metal stream ejected through orifice
Stream broken up by jets of inert gas, air or water to create small particles
Size of molecules depends on temp, rate of flow, nozzle size and jet characteristics
Reduction
Gases such as H2 and CO used as reducing agents for metal oxides
Very fine metallic oxides reduced to metallic state
Powders produced spongy and porous, uniformly sized spherical or angular shapes
Electric Deposition
Uses aq solutions or fused salts
Powders produced very pure
Communition
Mechanical process involving crushing, milling or grinding
Small particles produced using ball mill
Why is blending required?
powder particles of different sizes and shapes need to be mixed to obtain uniformity
powders of different metals and other materials may be mixed and need to impart physical and mechanical properties
Compaction
Pressing blended powders into a shape using wither hydraulically or mechanically activated presses.
Performed at room temp
Also used to strengthen a part for further processes
Green powder
Pressed powder after compaction
Cold Isostatic Pressing
Metal powders placed in flexible rubber mould and pressurised hydrostatically in chamber
Hot Isostatic Pressing
Die made of high melting point metal
Inert gas used as pressurisation medium
Advantages HIP
produce compacts with 100% density
produce good metallurgical bonding among particles and good mechanical properties
Powder rolling
Powders fed through gap between rollers in two-high rolling mill and compacted into a continuous strip
Extrusion
Powders encased in metal die and extruded
Injection Moulding
Very fine metal powders blended with polymer or wax based binder then pressed
Moulded greens placed in low temp oven to remove binder
Sinter in furnace
Complex shapes can be produced and removed easily from dies
Sintering
Compressed metal powders heated in controlled atmosphere to temp below melting point but high enough to allow bonding of particles
Protective atmosphere to avoid oxidation
Bonds formed through diffusion as temp rises
Advantages to sintering
Increases strength, density, durability, and thermal and electrical conductivities
Disadvantages to sintering
Compact shrinks after sintering
Allowances must be made for shrinkage.
Different shrinkages depending on material
Forging
Form of finishing
Workpiece shaped by compressive forces applied through various dies and tools
Coining
Type of forging process
Slug coined into completely closed die cavity
Improves dimension, strength and surface finish
Infiltration
Slug of lower melting point placed against sintered part and assembly heated to temp sufficient to melt slug
Molten metal infiltrates pores by capillary action, resulting in pore-free part with good density and strength
Used to improve hardness and tensile strength
Pores filled to prevent moisture penetration that could cause corrosion
Plating
Coating process that imparts resistance to wear and corrosion, high electrical conductivity, better appearance and reflectivity and other desirable properties
Properties
Produced at near net shape - avoids scraps
Wide range of properties in products
Lower tensile strength than those wrought of similar composition
Economics
Eliminates secondary manufacturing and assembly operations.
Eliminates secondary machining costs
Initial cost is high
High production volume required to justify usage
Break-even at around 10,000+ units
