Power Metallurgy

Question 1

What is powder metallurgy?

Answer 1

The process of producing parts by mixing elemental or alloy powders and compacting them in a die.

The shapes are then sintered or heated in a furnace to bond the powders metalurgically.

Question 2

What are the basic steps of PM?

Answer 2

1. Blending/mixing
2. Compacting
3. Sintering

Question 3

What are the powders made of?

Answer 3

There is a wide range of powders available such as pure metals, alloys, and even non-metalltic materials.

Most metal powders (~80%) are made by atomization, but can also be made by chemical reduction, electrolysis, and mechanical methods.

Question 4

Atomization

Answer 4

A liquid metal stream is sprayed through a small opening and broken up by jects of inert gas or water.

Question 5

Blending

Answer 5

When powders of the same chemistry but different particle sizes are intermingled.

This can help reduce porosity.

Question 6

Mixing

Answer 6

When powders of different chemistries are combined. This is most often done to create specific physical and mechanical properties.

Question 7

Why would lubricants be mixed in to the powders?

Answer 7

To reduce the friction between the die wall and the powder to improve flow.

Potentially to also reduce the chance of explosion.

Question 8

Powder compaction

Answer 8

Once in the die, the powder undergoes hydraulic or mechanical pressurization that ranges from 70-800 MPa.

Other compaction processes can include rolling, extrusion, and injection molding.

Question 9

Green compact

Answer 9

The product after it has been compressed, but before sintering.

It is strong enough to keep its shape for further processing.

Question 10

Cold isostatic pressing (CIP)

Answer 10

Is when the compaction process is completed at room temperature.

Question 11

Advantages of CIP

Answer 11

More uniform density, less expensive tooling, and greater application for shorter production runs

Question 12

Disadvantages of CIP

Answer 12

High dimensional accuracy is hard to achieve and requires post-processing to do so. This can be done before or after sintering.

Question 13

Hot isostatic pressing (HIP)

Answer 13

Is completed at high temperatures and pressures using and inert gas as the compression medium.

This process compresses and sinters in the same step.

Question 14

Advantages of HIP

Answer 14

Produces a high density and mechanically strong part.

Question 15

Disadvantages of HIP

Answer 15

High tooling cost. Is only found in the aerospace industry.

Question 16

Sintering

Answer 16

The green compacts are heated in a furnace with a controlled atmosphere (inert gas) to prevent oxidization and improve surface finish.

The sintering temp is arounf 70-90% of the melting temperature.

Particles are bonded through diffusion mechanisms to reduce surface energy.

Shrinkage occurs due to pore size reduction.

Question 17

What are the secondary steps of PM?

Answer 17

Coining and sizing, forging, impregnation, infiltration, heat treatment, machining, and surface treatment.

Question 18

Coining

Answer 18

A pressworking operation to press details onto the surface.

Is typically completed on all parts to improve strength and surface finish.

Question 19

Sizing

Answer 19

The pressing of a sintered part.

Is typically completed on all parts to increase dimensional accuracy.

Question 20

Forging (cold, hot, or impact)

Answer 20

Is typically completed on highly stressed structural parts to obtain its final shape and dimensions, a good surface finish, uniform/fine grains, and superior mechanical properties.

Question 21

Impregnation (oil, grease, resin)

Answer 21

An operation where the pores of a PM part are permiated with oils (etc.).

Is typically completed on bearings and bushings to make them self lubricating. It can also seal parts from corrosion and prepare them for plating.

Question 22

Infiltration (low melting point metals)

Answer 22

An operation wehre the pores of a PM part are filled with molten metal.

Is typically completed on structural parts to improve strength and ductility and to seal the part from corrosion. Its can also prepare parts for plating.

Question 23

Heat treatment

Answer 23

Typically completed on ferrous structural parts to improve strength and hardness.

Question 24

Machining

Answer 24

Typically completed on all parts to obtain the final dimension and finish.

Question 25

Surface treatment

Answer 25

Typically completed on all parts of improve finish, appearance, corrosion resistance, etc.

Question 26

Porosity in PM

Answer 26

Reduces density, tensile stiffness, and strength.

It can be used for self-lubricating parts and filters.

Question 27

Advantages of PM

Answer 27

1. Is a near net shape or net shape process.
2. Reduces machining and waste of materials.
3. Cost effective for moderate to high volume production

Question 28

Disadvantages of PM

Answer 28

1. Lower mechanical properties than wrought materials due to porosity
2. Limited accurate data of mechanical properties due to difference of dies in test coupons

Question 29

Applications of PM

Answer 29

1. Porous or permeable products (bearings)
2. Complex shapes that would require significant machining (gears, cams, etc.)
3. Products made from materials with high melting points (tungsten carbide cutting tools)
4. Products requiring properties of two or more metals/non-metals (graphite and copper in motor brushes)
5. HIP of aerospace products

Question 30

Die punch configurations

Answer 30

Single punch, opposite punches, multi-punches.

See pg. 6 for diagram.

Question 31

Part design considerations

Answer 31

Simple and uniform works best. Avoid sharp changes in contour and wall thicknesses.

The ejection of the green compact mist be considered.

Threads can not be obtained through the compaction process.

See pg. 7-14

Question 32

Micro-powder Injection Molding

Answer 32

The smaller the particles, the better the PIM process.

Particles in the nm scale can be used and works with elements/alloys that can’t be easily produced with traditional microfabrication tools.