Powder Metallurgy

Question 1

Powder Production - Describe the comminution process

Answer 1

A mechanical process that involves crushing, milling or grinding.
Small particles are produced using a ball mill - rotating hollow cylinder partly filled with steel or white cast iron balls
(Most widely used)

Question 2

What are the methods of compaction?

Answer 2

Cold Isostatic Pressing
Hot Isostatic Pressing
Powder Rolling
Extrusion
Injection Moulding

Question 3

What is the main disadvantage of PM processing?

Answer 3

Initial cost for punches, dies, and equipment is high so a high production volume is required to justify usage. Break-even point is usually around 10,000+ units for the process to be economically viable

Question 4

Blending - Describe the mechanical alloying process

Answer 4

Powders of 2 or more pure metals are mixed in a ball mill

The powders fracture and intermingle with one another (diffusion), forming alloy powders

Question 5

Blending - How can a homogeneous mix of powders be obtained?

Answer 5

Selecting good particle size distribution

Using suitable lubricant

Question 6

What is powder metallurgy also known as?

Answer 6

Sintering

Sinter metal

Question 7

What is the volume of a sintered compact equal to?

Answer 7

(Volume of green compact)x(1 - linear shrinkage)^3

Density other way round

Question 8

Finishing - Describe the forging process

Answer 8

A workpiece is shaped by compressive forces applied through various dies and tools.
It is used to shape sintered compacts to the desired shapes

Question 9

Compaction - Describe the Powder Rolling process

Answer 9

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

Question 10

Compaction - What are the advantages of using Hot Isostatic Pressing

Answer 10

Produces compacts with 100% density

Produces good metallurgical bonding among particles, and good mechanical properties

Question 11

Compaction - Describe the Extrusion process

Answer 11

Powders are encased in a metal die and extruded

Question 12

Compaction - What is the core rod used for?

Answer 12

Central features

Question 13

Compaction - Describe the Cold Isotatic Pressing method

Answer 13

Metal powders are placed in a flexible rubber mould and then pressurised hydrostatically in a chamber

Question 14

Why is blending required?

Answer 14

Powders may have different sizes and shapes (they must be mixed to obtain uniformity)
Powders of different metals and other materials may be mixed (to impart special physical and mechanical properties and characteristics to the PM products)

Question 15

What is the formula for linear shrinkage?

Answer 15

change in length of a compact after sintered / original length of compact

Question 16

What are the methods of powder production?

Answer 16

Atomisation
Reduction
Electrolytic Deposition (Electroplating)
Comminution

Question 17

What are the factors that determine the powder production method to be used?

Answer 17

Economic, purity issue (also including particle size), desired physical or chemical characteristics of a compact

Question 18

What is the main advantage of PM processing?

Answer 18

Eliminates many secondary manufacturing and assembly operations, also eliminates many secondary machining costs

Question 19

Powder Production - Describe the reduction process

Answer 19

Gases such as hydrogen and carbon monoxide are used as reducing agents for metal oxides
Very fine metallic oxides are reduced to the metallic state
Powders produced are spongy and porous, with uniformly sized spherical or angular shapes

Question 20

Finishing - Describe the infiltration process

Answer 20

A process whereby a slug of lower MP metal is placed against the sintered part and the assembly is heated to a temperature sufficient to melt the slug
Molten metal infiltrates the pores by capillary action, resulting in a pore-free part with good density and shape
Used to improve the hardness and tensile strength of PM products
Pores within products are filled to prevent moisture penetration that could cause corrosion

Question 21

Compaction - Describe the Injection Moulding process

Answer 21

Very fine metal powders are blended with polymer or wax-based binder and then pressed
Moulded greens are placed in a low temperature oven to remove the binder and then sintered in a furnace

Question 22

Powder Production - Describe the electrolytic deposition (electroplating) process

Answer 22

Uses either aqueous solutions or fused salts
Powders produced are very pure
Electric circuit, deposit the metal we want onto a cathode and then remove the powder from the cathode

Question 23

Compaction - Describe the Hot Isostatic Pressing method

Answer 23

A die is made of a high MP metal

Inert gas is used as the pressurisation medium

Question 24

Compaction - What is the pressed powder called and what does the density depend on?

Answer 24

Green compact

Density depends on the pressure applied and size distribution of particles

Question 25

What does finishing do?

Answer 25

Further improves properties of sintered PM products

Question 26

Why is powder metallurgy widely used?

Answer 26

No material is wasted Only semi-skilled labourers are required Final products require little or no finishing

Question 27

What are the methods of finishing?

Answer 27

Forging (Including open die forging) Coining Infiltration Plating

Question 28

What properties does sintering increase?

Answer 28

Strength, density, durability and thermal and electrical conductivities of the compacts However, compact shrinks after sintering so allowances should be made. Levels of shrinkage depend on material composition, density/porosity etc

Question 29

Describe the basic process for powder metallurgy

Answer 29

``` Powder Production Blending Compaction Sintering Finishing operations ```

Question 30

Compaction - What is the main advantage of the Injection Moulding process

Answer 30

Complex shapes can be produced and removed easily from dies

Question 31

What are the applications of powder metallurgy?

Answer 31

Gears, cams, bushes, cutting tools, porous products (e.g. filters, oil-impregnated bearings) and piston rings

Question 32

Powder Production - Describe the atomisation process

Answer 32

A liquid-metal stream is ejected through a small orifice The stream is broken up by jets of inert gas, air, or water to create small particles The size of the particles depends on the temperature of the metal, rate of flow, nozzle size, and jet characteristics

Question 33

Finishing - Describe the plating process

Answer 33

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

Question 34

Why is compaction needed?

Answer 34

Obtain the required shape, density, and particle-to-particle contact Strengthen a part for further processes

Question 35

Finishing - Describe the coining process

Answer 35

A type of forging process where the slug is coined in a completely closed die cavity Used to improve dimension, strength, and surface finish of PM products

Question 36

Describe Sintering

Answer 36

A process whereby compressed metal powders are heated in controlled atmosphere to a temperature below their MP, but high enough to allow bonding of individual particles Protective atmosphere is employed to prevent oxidation, and reduce the probability of future surface oxidation

Question 37

Outline the properties of PM products

Answer 37

Produced at near net shape to eliminate scraps Can be produced with a wide range or properties from low density, high porosity with tensile strength as low as 70MPa to a high density with tensile strength of 1250MPa Most mechanical properties are dependent on density Lower tensile strength than those wrought of similar composition

Question 38

Describe the compaction process

Answer 38

The process of pressing the blended powders into a shape using either hydraulically or mechanically activated presses Normally performed at room temperature