Powder metallurgy (PM) makes finished metal parts by compacting metal powder in a die and then heating the compact below its melting point until the particles fuse. No molten metal, no machining of bulk stock — the part comes out near-net-shape with the geometry pressed directly into it. PM dominates production of small, complex, high-volume parts: oil pump gears, automatic transmission synchronizer hubs, shock absorber valving, power tool gears, and the self-lubricating bronze bushings in your washing machine.

The four-step process:

• Powder blending: Mix elemental or pre-alloyed powders (iron, copper, nickel, graphite) with a lubricant like zinc stearate to reduce die wall friction.
• Compaction: Press the powder in a rigid die at 400–800 MPa. The result is a "green" part — held together only by mechanical interlocking and cold welding. Strong enough to handle, weak enough to crumble if dropped.
• Sintering: Heat the green part in a controlled atmosphere (typically endothermic gas or hydrogen) to 70–90% of the absolute melting point. For iron, that's about 1120 °C. Atoms diffuse across particle boundaries, necks grow between particles, and porosity shrinks. The part gains its real strength here.
• Secondary operations: Sizing (re-pressing for tighter tolerances), oil impregnation (for self-lubricating bearings), steam treatment, or selective machining of features that can't be pressed.

Why engineers choose PM: Material utilization is over 95% — almost no scrap, versus 40–60% for machining. Complex shapes with internal features, splines, and gear teeth come out of the die finished. Controlled porosity is a feature, not a defect: PM bronze bushings hold 20–30% oil by volume and run for the life of the appliance with no relubrication.

The fundamental limit — density: A pressed-and-sintered part typically reaches 85–92% of full theoretical density. Those pores reduce tensile strength roughly linearly with density. Rule of thumb: a PM steel part at 7.0 g/cm³ (about 89% dense) has roughly 70% the tensile strength of the same wrought steel at 7.85 g/cm³. For higher density, engineers use double-press/double-sinter, powder forging, or hot isostatic pressing (HIP) — at a cost premium.

Geometric constraints: Because powder must flow into the die and eject cleanly, PM parts can't have undercuts, side holes, or threads perpendicular to the pressing axis. Walls thinner than 1.5 mm don't fill reliably. Designers think in terms of "what can be pressed top-down" — anything else has to be machined after sintering.