Chapter 10

Question 1

Explain the Powder Metallurgy production sequence (2)

Answer 1

Pressing: Compressed to a green compact using punch and die.

Sintering: Green compacts are heated to bond into a hard, rigid mass below melting point.

Question 2

Main benefit of Powder Metallurgy (2)

Answer 2

The process is net shape or near net shape at 97% and very porous.

Question 3

Why is Powder Metallurgy important (4)

Answer 3

Certain metals are difficult to shape .

Certain composites are are only possible thru powder metallurgy

Favorable compared to casting since dimensional control

Economic production because of automated production methods

Question 4

Disadvantages of Powder Metallurgy (4)

Answer 4

High tooling and equip cost as well as expensive metallic powders

Metallic powders degrade over time and are a fire hazard

Metal powders do not flow laterally in the die during pressing

Variations in density

Question 5

What are the work materials used in Powder Metallurgy (

Answer 5

Alloys of iron, steel, and aluminum. Copper, nickel, and refractory metals .

Question 6

What metals can be turned into powders? And how? (3)

Answer 6

All metals.

Atomization
Chemical
Electrolytic

Question 7

How does atomization work?

Answer 7

High velocity inert gas thru an expansion nozzle siphoning molten metal and spraying it into container

Question 8

Explain the conventional powder metallurgy production sequence (3)

Answer 8

Blending - Same chemistry but can be different sizes. Mixing refers to combining different chemistries together.

Compacting - Pressing (opposing punches squeeze powders in a die). After, called green compact since not processed.

Sintering - Temp at .7% to .9% of Tm. Purpose to reduce surface energy. Partial shrinking due to pore size reduction.

Question 9

Go into the specifics of blending

Answer 9

Same chemistry. Then lubricant and binder are added to reduce friction and defects. When sintering, lubricant and binder are removed since it is very porous it is an easy escape.

Note that we use different particle size and material to add strength.

Question 10

What is isostatic?

Answer 10

Compaction and sintering are done at the same time.

Question 11

What are some secondary operations performed on sintered part? (4) And why? (3)

Answer 11

Repressing - Pressing in closed die to increase density and improve properties

Sizing - Pressing to improve dimensional accuracy

Coining - Pressing details into its surface

Machining - Geometric features such as threads and side holes. Not done during pressing.

Increase density, improve accuracy, or accomplish additional shaping

Question 12

What is impregnation?

Answer 12

When oil, other fluids or polymer resins permeate into a sintered powder metallurgy part then solidify to create a pressure tight part.

Question 13

What is infiltration? What is the result?

Answer 13

When the a part’s pores are filled with a molten metal.

Melting point must be below that of the PM part.

Increase uniform density. Improve toughness and strength.

Question 14

Name the three other alternatives to Pressing and Sintering

Answer 14

Isostatic Pressing - Hydraulic pressure is applied from all directions

Powder Injection Molding - Starting polymer has 50 to 80% powder content. Polymer is removed and PM part is sintered

Hot pressing - Combined pressing and sintering

Question 15

Name the three common elemental powders

Answer 15

Iron, aluminum, and copper.

Question 16

Name four classes of PM parts

Answer 16

Class I Simple thin shapes

Class II Simple but thicker

Class III Two Levels of thickness

Class IV Multiple levels of thickness

Question 17

Class I

What is the minimum quantity? Maximum porosity?

Answer 17

10.000 units

50%

Question 18

Class II

Requirements for part.

Answer 18

Must have vertical sides. Must not interfere with ejection.

Question 19

Class III

What is possible? Minimum thickness b/w holes and outside wall?

Answer 19

Chamfers and corner radii are possible.

1.5 mm. Hole diameter must also be 1.5 mm

Question 20

Guidelines for Chamfer and corner radii? (4)

Answer 20

Avoid acute angles 45 min.

Use larger angles for punch rigidity

Avoid full outside corner radius since punch fragile at edge. Flat 45 degree edge.

Combine radius and chamfer

Question 21

Difference b/w lubricant and binders (3)

Answer 21

Lubricants such as graphite and stearic acid improve flow and compressibility. Decrease green strength.

Binders reduce flow and compressibility. Increase green strength.

Question 22

Correlation b/w friction and particle (3)

Answer 22

Smaller particle greater friction.

Spherical shapes least friction

Easier flow, low friction

Question 23

Classification of metallic powders

Answer 23

Elemental - particle -> pure element

Pre-alloyed - particle -> alloy

Question 24

Why titanium expensive? (4)

Properties? (4)

Answer 24

High extraction cost

High alloying and ingot casting cost

high processing cost

Typical 10% utilisation

45% lighter than steel
4x corrosion resistant
High strength
Can be welded, forged, machined, etc