Powder Metallurgy Overview

Powder Metallurgy

• Powder metallurgy is a metal forming and fabrication technique that competes with casting, forging, and machining.
• It involves three major stages:
  • Dividing a primary material into small particles (powder)
  • Injecting the powder into a mold or passing it through a die to create a green structure (a structure similar to the final product's shape but not yet fully solidified or strong).
  • Forming the final part by applying pressure, high temperatures, and long-setting times (during which the part self-welds).

Powder Metallurgy Applications

• Solid parts like gears, cams, bushings, and rotors.
• Porous parts like Oilite bushings and filter elements.
• Taps and dies for cutting threads.
• Tungsten carbide ball-point pen balls.
• Piston rings and brake pads.
• Surgical instruments and implants.

Powder Metallurgy Advantages/Disadvantages

• Advantages:
  • Competition with forging and casting.
  • Wide variety of net and near-net shapes.
  • Controlled porosity or nearly full density.
  • Useful for refractory alloys.
  • High production rates.
  • Wide range of material properties and specialty alloys.
• Disadvantages:
  • Metal powder sometimes dangerously explosive.
  • Specialized metal powder can be expensive.
  • Density inconsistency can occur in the finished product.
  • Expensive tooling and equipment.
  • Large parts not feasible.
  • Parts are not as strong as forgings (but some P/M parts can be forged).

Typical Sequence for Powder Part Production

1. Production of powder (elemental particles or pre-alloyed particles).
2. Blending of the powder, especially elemental powders. This step may include the addition of binder particles for metal injection molding.
3. Compaction (consolidation) to form a compact (green compact).
4. Sintering (heating at a controlled, sub-melting temperature) to bind, densify, and strengthen the compact.
5. Finishing operations (re-pressing, forging, sealing, plating, etc.).

Powder Production Methods

• Atomization: Molten metal sprayed into a jet of water or argon.
• Reduction: Chemical removal of oxygen from metal oxides.
• Pulverizing: Crushing metal in a ball mill.

Consolidation by Pressing

• Similar to forging.
• Typically done at room temperature.
• High pressures are needed.
• Density increase by varying particle sizes and applying high pressure.

Density & Mechanical Properties

• Density of the powder product significantly influences its mechanical properties.
• Compacting pressure affects the density.
• Higher density results in better mechanical properties.

Isostatic Pressing

• Compaction by applying pressure in all directions.
• More uniform compaction.
• Improved mechanical properties.
• Reduced density variations.

Cold Isostatic Pressing (CIP)

• Uses pressurized fluid (typically water).
• Followed by sintering.
• Powder part isolated from the pressurized fluid using a mold.

Hot Isostatic Pressing (HIP)

• Uses pressurized fluid (typically argon).
• Combines compaction and sintering.
• High pressure and temperature for improved density and properties.
• Heating provided by resistive heating coils within a pressure vessel.

Sintering

• Heating the green compact at a controlled temperature in a controlled atmosphere.
• Sintering temperature is below the melting point of the metal.
• Diffusion occurs during sintering to produce bonding and densification.

Powder Metal Injection Molding (MIM)

• Suitable for small, complex parts.
• Metal powder mixed with binders (polymers or wax).
• Powder mixture injected into a mold.
• Steps include debinding or debinding, followed by sintering.
• Advantages include high production rate, close tolerances, and complex shapes.

Powder Metallurgy Definitions

• Atomization: Liquid metal stream, broken by inert gas, air, or water jets.

Mechanical & Physical Properties (depending on Density)

• Mechanical properties rely on their density to increase metal particle contact.
• Improved cohesion results in greater strength, ductility, elastic modulus, and toughness.