Chapter 12 - Powder Metallurgy

What is Powder Metallurgy (PM)?

PM is a metal processing technology in which parts are produced from metallic powders.

What are the two main steps in the PM production sequence?

Pressing (compressing powders into a green compact) and sintering (heating green compacts to bond particles).

Why is Powder Metallurgy important?

It allows for mass production of parts to net shape, reduces material waste, and enables the creation of porous metal parts.

What are some common applications of PM parts?

Filters, oil impregnated bearings, gears, and other machinery parts.

What are the limitations of Powder Metallurgy?

High tooling and equipment costs, the expense of metallic powders, and issues with part geometry.

How is particle size typically measured in PM?

Using screens of different mesh sizes where mesh count refers to the number of openings per linear inch of screen.

What is the packing factor in PM?

It is the ratio of bulk density to true density, typically ranging from 0.5 to 0.7.

What is the role of lubricants in powder metallurgy?

Lubricants reduce interparticle friction and facilitate powder flow during pressing.

What does the term 'green compact' refer to in PM?

The 'green compact' is the workpart after pressing, which is not yet fully processed.

What is sintering in Powder Metallurgy?

A heating process that bonds metallic particles, increasing strength and hardness without reaching melting point.

What is the typical temperature range for sintering?

Sintering is usually performed at 70% to 90% of the metal's melting point.

What are the two main categories of PM infiltration?

Impregnation (using oil or fluid in the pores) and infiltration (filling pores with molten metal).

What is a characteristic of PM that can be beneficial for certain applications?

PM technology allows for controlled levels of porosity in parts.

What major factors affect particle flow in PM?

Interparticle friction, particle shape, and particle size influence the ability of a powder to flow and pack tightly.

What does the term 'elemental powders' mean in PM?

Elemental powders are pure metals in particulate form used in powder metallurgy.

What is the difference between true density and bulk density?

True density is the density of the material if melted, while bulk density is the density of the powders in the loose state.

What are the four classes of PM parts defined by the Metal Powder Industries Federation?

Class I: Simple thin shapes, Class II: Simple thicker shapes, Class III: Two levels of thickness, Class IV: Multiple thickness levels.

What are some alternative techniques for pressing and sintering in PM?

Isostatic pressing, powder injection molding, and hot pressing.