Powder Metallurgy Review Questions

Question 1

importance of PM

Answer 1

• Mass produced to create net-shape and near net-shape
• Very little waste of materials
• PM parts have a specific level of porosity
• Can made shapes that are difficult for other methods
• certain metal alloy combos and cermets can be formed
• Tolerances around +/- 0.13mm
• PM production methpods can be automated for economical production

Question 2

disadvantage of PM

Answer 2

• tooling and equipment cost are high
• metallic powders expensive
• difficulties in storing and handling powders
• limitations in part geometries. powders dont flow laterally
• variations in metal density throughout part may be a problem

Question 3

What are principle methods used to produce PM powders

Answer 3

atomization

electrolytic

chemical

Question 4

what are three basic steps used to produce PM

Answer 4

1. blending and mixing of powders
2. compaction
3. sintering

Question 5

what is technical difference between blending and mixing in PM

Answer 5

blending - when powders of same chemical composition but different particle sizes are intermingled

mixing - when powders of different chemistries are combined

Question 6

what are some ingredients usually added to metallic powders during blending and/or mixing

Answer 6

1. lubricants
2. binders
3. delocculants

Question 7

what is meant by green compact

Answer 7

Pressed but not yet sintered

Question 8

describe what happens to individual particles during compaction

Answer 8

Particles are repacked into a more efficient arrangement; followed by deformation of particles as pressure increases

Question 9

what are 3 steps in sintering cycle of PM

Answer 9

1. preheat - lubricants/binders burned off
2. sintering
3. cool down

Question 10

What are some reasons why a controlled atmosphere furnace is desirable in sintering

Answer 10

1. protect against oxidation
2. provide reducing atmosphere to remove existing oxides
3. provide carburizing atmosphere
4. remove lubricants and binders from pressing

Question 11

What is the difference between impregnation and infiltratrion in PM

Answer 11

impregnation - oil or other fluid is permeated into pores of sintered PM part

infiltration - molten metal is permeated through pore of sintered part

Question 12

How is isostatic pressing distinguished from conventional pressing and sintering in PM

Answer 12

isostatic pressing - hydrostatic pressure applied to all sides of the mold

conventional pressing - uniaxial pressure

Question 13

describe liquid phase sintering

Answer 13

two powders are mixed together and heated at a temp higher than the lowest melt temp of the two powders but not the melt temp of the other. melted metal wets the solid particles, creating a dense structure with bonding between the metals upon solidification

Question 14

what are 2 basic classes of metal powders as far as chemistry is concerned

Answer 14

Elemental - pure metal and used when high purity is needed

Pre-Alloyed - each particle is an alloy composed of desired chemical compositions

Question 15

why is PM Technology so well suited to production of gears and bearings

Answer 15

• tolerances
• near net shape
• mass production
• little to no waste material

(1) geometries lend themselves to PM Pressing
(2) porosity allows impregnation