Manufacturing Technology: Powder Metallurgy

Vocabulary flashcards covering the principles, powder production methods, blending additives, compaction techniques, sintering stages, finishing operations, and economic trade-offs in powder metallurgy.

Powder Metallurgy

A manufacturing process where metal powders are compacted into a desired shape and sintered (heated without melting) to form a solid component.

Five Stages of Powder Metallurgy

The sequential manufacturing sequence consisting of: (1) Metal Powders production, (2) Blending with additives, (3) Compaction (cold or hot), (4) Sintering, and (5) Secondary & Finishing Operations.

Atomisation

A powder production technique where a stream of molten metal is broken up by jets of inert gas, air, or water; it produces $>80\%$ of all commercial metal powder.

Reduction (Powder Production)

A method of metal powder production that removes oxygen from metal oxides via a solid-state chemical reaction.

Electrolytic Deposition

A powder manufacturing process capable of producing powders with very high chemical purity, but at an expensive cost.

Thermal Decomposition

A powder production method that generates metal powders through the chemical breakdown of hydrides or carbonyls.

Comminution

A mechanical pulverisation or grinding process, typically performed in a ball mill, which can produce alloy powders from two or more metals.

Blending

The process of mixing particles of differing shapes, sizes, or metallic compositions to achieve homogenous powders and ensure part-to-part uniformity.

Lubricants (Blending Additives)

Additives introduced during the blending stage to reduce inter-particle and die friction, which extends die life.

Binders (Blending Additives)

Additives mixed with powders during blending to increase the green strength of the pressed component prior to sintering.

Green Compact

The pressed powder workpiece formed immediately after compaction that possesses only low green strength sufficient for careful handling.

Green Strength

The mechanical strength of a compacted powder component prior to sintering; it can be improved by increasing compaction pressure, adding binders, or careful die design.

Cold Isostatic Pressing (CIP)

A compaction method in which blended powder is sealed inside a flexible rubber mould (bag) and subjected to hydrostatic fluid pressure to produce a uniform density compact.

Hot Isostatic Pressing (HIP)

A compaction process using high melting point sheet metal dies and high-pressure inert gas to produce almost $100\%$ dense parts while eliminating the need for a separate sintering stage.

Powder Injection Moulding (PIM)

A forming process where fine metal powders blended with a polymer or wax-based binder are injected using a rotating and reciprocating screw to create accurate, complex parts at high production rates.

Sintering

The thermal process of holding green compacts at temperatures typically between $70\%$ and $90\%$ of their melting point in a protective atmosphere or vacuum to bond particles via solid-state diffusion.

Three Stages of Sintering

The three sequential zones of continuous sintering: (1) Burn-off (removing lubricants/binders under controlled temperature), (2) High-temperature sintering (solid-state diffusion bonding and shrinkage), and (3) Cooling (controlled to prevent cracking under a protective atmosphere).

Closed-Die Forging (PM Finishing)

A secondary finishing operation where a sintered workpiece is pressed inside a closed cavity between two dies to take its final form (e.g., coining).

Open-Die Forging (PM Finishing)

A secondary finishing operation where the sintered workpiece is deformed and shaped between two simple, often flat, dies.

Impregnating

A secondary finishing treatment unique to powder metallurgy where a porous component is immersed in a pressurised bath to introduce liquids or resins, such as oil for self-lubricating bearings, copper for graphite brushes, or Teflon.

Infiltrating

A finishing process where a molten metal or alloy with a lower melting point than the base part is drawn into open pores by capillary action, reducing porosity and improving mechanical properties.

Refractory Metals

A class of high melting point metals routinely fabricated using powder metallurgy, specifically including molybdenum, tungsten, niobium, tantalum, and rhenium.

Minimum Production Run (Powder Metallurgy)

The typical batch threshold required to justify the high tooling and equipment expenses of powder metallurgy, usually requiring runs of $>10{,}000\text{ parts}$.