14.7 Powder Metallurgy

What Powder Metallurgy Is For

Powder metallurgy shapes components from metal powders without ever fully melting them. It exists because it can do four things no other process can:

1. Produce refractory metals such as tungsten, molybdenum and tantalum, whose melting points are too high for practical casting.
2. Combine materials that are mutually insoluble or have widely different melting points — tungsten carbide in a cobalt binder, copper-graphite brushes.
3. Produce controlled porosity, which is a defect in any other process but is the entire point of a self-lubricating bearing or a filter.
4. Make large numbers of small, complex parts to close tolerance with almost no material waste — utilisation typically exceeds 95%, against 50 to 60% for machining from bar.

Powder Production

Powder Characterisation

Powder behaviour is governed by properties that must be controlled:

The key trade-off: spherical particles flow well but bond poorly; irregular particles bond well but flow poorly. Commercial powders are engineered to balance the two.

Blending and Mixing

Powders are blended with:

• Alloying additions, so that a homogeneous alloy forms by diffusion during sintering.
• Lubricants such as zinc stearate, typically 0.5 to 1.5%, to reduce die wall friction and ease ejection. The lubricant is burned off in the early stage of sintering.
• Binders, where green strength must be higher than compaction alone provides.

Compaction

Powder is pressed in a rigid die at 100 to 900 MPa depending on the material. The result is a green compact — strong enough to handle but not yet a solid metal.

The central difficulty is density variation. Friction between the powder and the die wall means pressure is not transmitted uniformly, so density falls with distance from the punch face. Remedies:

• Double-action pressing, with punches moving from both ends, halves the maximum distance from a punch face.
• Lubricants reduce wall friction.
• Limiting the length-to-diameter ratio, generally to less than about 3 to 1.

Alternative consolidation routes

Sintering

Sintering is the heart of the process. The green compact is heated in a controlled atmosphere to a temperature typically 70 to 80% of the absolute melting temperature of the principal constituent — below the melting point, so no general melting occurs.

Mechanism

Bonding occurs by solid-state diffusion. Atoms migrate across the points of contact between particles, driven by the reduction of surface free energy. Three stages are conventionally distinguished:

1. Neck formation at the contact points between adjacent particles.
2. Neck growth and pore rounding, with densification and shrinkage.
3. Pore closure and grain growth, as isolated pores shrink and disappear.

Consequences

• The compact shrinks, typically by a few percent, so the die must be oversized by the anticipated shrinkage. Predicting shrinkage accurately is what makes close tolerances achievable.
• Strength, ductility, electrical conductivity and thermal conductivity all rise substantially.
• Residual porosity remains unless full density is forced by HIP or by a secondary operation.

Atmospheres

Air is never used, because oxidation would prevent metallic bonding entirely.

Liquid phase sintering

Where one constituent melts and the other does not, the liquid wets the solid particles and draws them together by capillary action, giving rapid densification. This is how cemented carbide is made: tungsten carbide particles held in a cobalt binder that melts and flows around them. The result — extreme hardness from the carbide, adequate toughness from the binder — cannot be made by any melting route, because the carbide would dissolve or decompose.

Secondary Operations

Applications

Advantages and Limitations

Advantages. Very high material utilisation; close tolerances with little or no machining; controlled porosity; production of alloys and composites otherwise impossible; excellent reproducibility in high volume; unskilled operation once tooling exists.

Limitations. High tooling cost makes it uneconomic at low volumes; part size is limited by available press capacity; the shape must be ejectable from a rigid die, so undercuts, re-entrant angles and cross holes are excluded; residual porosity reduces strength, ductility and fatigue life unless removed; and thin sections are difficult to fill uniformly.

The economic point deserves emphasis. Powder metallurgy is a mass production process. For a handful of parts it will always lose to machining; for hundreds of thousands, it usually wins decisively.