Lecture 3: Powder Metallurgy

Pressing

compression is accomplished in a press-type machine using tools designed specifically for the part to be manufactured

sintering

the heat treatment is performed at a temperature below the melting point of the metal

particle size

refers to the dimensions of the individual powders

mesh count

refer to the number of openings per linear mm of screen

classification

the procedure of separating the powders by size

angle of repose

A common measure of interparticle friction which is the angle formed by a pile of powders as they are poured from a narrow funnel

what does packing characteristics depend on

two density measures- true density and bulk density

true density

density of the true volume of the material, this is the density when the powders are melted into a solid mass

bulk density

density of the powders in the loose state after pouring, which includes the effect of pores between particles, because of the pores bulk density is less than true density

packing factor

bulk density divided by the true density

porosity

defined as the ratio of the volume of the pores (empty spaces) in the powder to the bulk volume

atomization

this method involves the conversion of molten metal into a spray of droplets that solidify into powders

gas atomization

in which a high velocity gas stream (air or inert gas) is utilized to atomize the liquid metal

rotating disk method

the liquid metal stream pours onto a rapidly rotating disk that sprays the metal in all directions to produce powders

blending

when powders of the same chemical composition but possible different particle sizes are intermingled, different particle sizes are often blended to reduce porosity

mixing

refers to powders of different chemistries being combined

what other ingredients are added to the metallic powders during the blending or mixing step

lubricants, binders, deflocculants

lubricants

such as stearates of zinc and aluminum, in small amounts to reduce friction between particles and at the die wall during compaction

binders

which are required in some cases to achieve adequate strength in the pressed but unsintered parts

deflocculants

which inhibit agglomeration of powders for better flow characteristics during subsequent processing.

compaction

high pressure is applied to powders to form them into the required shape. The conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die

green compact

the word green meaning not yet fully processed

green density

As a result of pressing, the density of the part, called the green density, is much greater than the starting bulk density

green strength

the part when pressed is adequate for handling but far less than that achieved after sintering

sintering

heat treatment operating performed on the compact to bond its metallic particles

solid state sintering

used for this conventional sintering because the metal remains un-melted at these treatment temperatures