Week 10 - Powder Metallurgy 2

PIM vs Extrusion vs Slip Casting

PIM: Fine spherical powder and thermoplastic binder injected into hot mould and sintered. Best for small complext 3d Shapes.

Extrusion: Powder binder forced through a die, can only create constant cross sections and good for continuous lengths

Slip casting: Powder in water, coats outside, then poor slurry water out.

Density progression

Apparent density → Tap Density → Green density → Sintered density. Larger particles compact more easily, and higher green desnity correlates to higher sintered density.

Surface vs Bulk Transport

Surface transport: Source atoms are from particle surface, no change in particle centre to centre distance and no desnification. Only makes knecks and smooth geometry, usually for low T.

Bulk transport → Source of atoms are grain interior, drawing particles together, shrinking and densifying.

Three sintering stages

1. Initial: Neck formation via surface transport with no density change. Pores are open.

2. Intermediate: Grain boundary diffusion takes place, densifying and pores round off but still open.

3. Final: Pores close off, trap isolate gas/ Large shrinkage.

Compaction design rules

• Part must ejects straight along pressing direction

• Vertical holes, stepped hole are all allowed

• Side holes, undercuts and centre hags are therefore not allowed

Atmosphere selection

Need to be careful of what gas chosen, air only good for ceramics. H2 good for removing oxides but flammable and caused embrittlement, but cheap.Vaccum is best for reactive metals and desnity, however very expensive.

Binders & Viscocity

Binders - Use thermoplastic polymers, sometimes water. Can be up to 40% of the volume. Make sure powders are bound.

Viscocity - (Resistance to motion). Too high and powder doesnt hit mould as too thick, and too low and the powder and binder seperate.