powder metallurgy (PM)

powder metallurgy

mixing powders and compacting and heating the resultant mixture to create new parts

process steps

1. powder productions

2. blend (get an alloy, include additives, binders etc)

3. compacting

4. sinter

5. finishing operation if needed

additives

other metals creating an alloy with powder

lubricants and binders

what is green strength?

the strength of the part before sintering

how do you increase green strength?

use better compaction methods, use additives (polymers and waxes) to bind

and lubricants that reduce wall friction to get better compaction

also - warm compaction

how does warm compaction affect green strength?

as you increase temp, increase ductility, better green strength out of parts

PM advantages

waste can be put back into procedure (almost no waste), good dim. tolerances (depending on sintering time), good surface finish, control of porosity, low operating costs, energy efficient, moderate to high production

PM disadvantages

less strong parts (depending on density), sintering time affects dimensions (because of shrinkage), usually restricted to small parts

particle shape and size affect what material property?

hardness

(can also affect compaction and sintering process)

more pressure and softer powder results in what?

higher density

how does isostatic pressure cause more uniform compaction?

eliminates die wall friction

cold isostatic pressing (CIP)

metal powder in flexible rubber mold, placed in pressure vessel and compressed (usually by water)

hot isostatic pressing (HIP)

placing powder in high temperature sheet metal container, pressurized with inert gas (or high temp fluid)

what are the benefits of HIP

parts have 100% density, good bonding between particles, uniform density with high strength and high toughness

metal injection molding (PIM powder injection molding)

injected molded parts, made of metal and polymers mixed together, are placed into oven to burn off plastic binding

advantages of injection molding over traditional compaction

can create thin-walled, complex shapes

good surface finish and dimensional tolerances

high production rate

disadvantages of injection molding over traditional compaction

hard to find very fine powders, high cost, parts are limited in size (very small)

roll compaction

powder fed into gap between 2 rollers, high speeds

powder extrusion

powder in metal container is hot extruded, used to make preforms (to be re-heated and forged later)

pressureless compaction

die is gravity fed with powder and directly sintered in the die

used for porous filters and low density parts

ceramic molds

molds filled with powder and placed in steel container, subjected to HIP (like investment casting

spray deposition

atomized metal that is sprayed onto a cooled pre-formed mold where it solidifies, can make continuous lengths

sintering

metal particles are heated to 70-90% of melting temperature for 10 min to 8 hours to allow bonding (fusion)

solid-state material transport

sintering mechanism involving diffusion

liquid-phase material transport

sintering mechanism where at each step, particles get closer together and bond

what happens when sintering time increases?

strength, ductility, density, and thermal/electrical conductivity all increase - and the part shrinks