Lecture 4 - Powder Metallurgy

Question 1

Process

Answer 1

Powdered materials
Blended
Compacted
Sintered

Question 2

Advantages

Answer 2

No wasted materials
Semi-skilled labourers required
Final products require little/no finishing

Question 3

Atomisation

Answer 3

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

Question 4

Reduction

Answer 4

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

Question 5

Electric Deposition

Answer 5

Uses aq solutions or fused salts

Powders produced very pure

Question 6

Communition

Answer 6

Mechanical process involving crushing, milling or grinding

Small particles produced using ball mill

Question 7

Why is blending required?

Answer 7

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

Question 8

Compaction

Answer 8

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

Question 9

Green powder

Answer 9

Pressed powder after compaction

Question 10

Cold Isostatic Pressing

Answer 10

Metal powders placed in flexible rubber mould and pressurised hydrostatically in chamber

Question 11

Hot Isostatic Pressing

Answer 11

Die made of high melting point metal

Inert gas used as pressurisation medium

Question 12

Advantages HIP

Answer 12

produce compacts with 100% density

produce good metallurgical bonding among particles and good mechanical properties

Question 13

Powder rolling

Answer 13

Powders fed through gap between rollers in two-high rolling mill and compacted into a continuous strip

Question 14

Extrusion

Answer 14

Powders encased in metal die and extruded

Question 15

Injection Moulding

Answer 15

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

Question 16

Sintering

Answer 16

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

Question 17

Advantages to sintering

Answer 17

Increases strength, density, durability, and thermal and electrical conductivities

Question 18

Disadvantages to sintering

Answer 18

Compact shrinks after sintering
Allowances must be made for shrinkage.
Different shrinkages depending on material

Question 19

Forging

Answer 19

Form of finishing

Workpiece shaped by compressive forces applied through various dies and tools

Question 20

Coining

Answer 20

Type of forging process
Slug coined into completely closed die cavity
Improves dimension, strength and surface finish

Question 21

Infiltration

Answer 21

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

Question 22

Plating

Answer 22

Coating process that imparts resistance to wear and corrosion, high electrical conductivity, better appearance and reflectivity and other desirable properties

Question 23

Properties

Answer 23

Produced at near net shape - avoids scraps
Wide range of properties in products
Lower tensile strength than those wrought of similar composition

Question 24

Economics

Answer 24

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