Casting = pour molten metal into a mold cavity; on solidification it takes the cavity shape. Advantages: complex shapes with internal cavities, very large parts, hard-to-process materials, near-net shape, competitive cost, all metals castable.

Key considerations: molten-metal flow into the cavity, solidification & cooling, and the mold material (sets cooling rate).

Solidification

Pure metals solidify at a constant temperature (Al 660°C, Fe 1537°C, W 3410°C). A fine equiaxed skin forms at the cool mold wall; grains grow opposite to heat flow into columnar grains; farther in they become coarse equiaxed.

Alloys solidify over a range (liquidus → solidus) with a mushy zone of dendrites + liquid. Eutectics solidify with a plane front (like pure metals).

• Slow cooling (~10² K/s) → coarse dendrites; faster (~10⁴ K/s) → finer arm spacing; 10⁶–10⁸ K/s → amorphous.
• Finer grains → ↑strength & ductility, ↓microporosity, ↓hot tearing.

Structure–Property Notes

• Microsegregation (cored dendrites): solute-rich surface vs core.
• Macrosegregation: composition differs across the whole casting.
• Gravity segregation: denser compounds sink.
• Convection / agitation / vibration → finer grains.

Fluid Flow

Molten metal: pouring basin → sprue (vertical) → runners → gate → cavity. Risers feed shrinkage and must solidify after the casting. Gating must avoid premature cooling, turbulence, and gas entrapment.

Bernoulli (energy conservation):

$h + \frac{p}{\rho g} + \frac{v^2}{2g} = \text{constant}$

$h_1 + \frac{p_1}{\rho g} + \frac{v_1^2}{2g} = h_2 + \frac{p_2}{\rho g} + \frac{v_2^2}{2g} + f$

where $h$ = elevation, $p$ = pressure, $v$ = velocity, $\rho$ = density, $g$ = gravity, $f$ = friction loss.

Continuity (mass conservation, incompressible):

$Q = A_1 v_1 = A_2 v_2$

Sprue design. With negligible friction and equal top/bottom pressure, taking location 0 at the still basin surface, $v_1 = \sqrt{2g(h_0 - h_1)}$ and $v_2 = sqrt{2g(h_0 - h_2)}$, so

$\frac{A_1}{A_2} = \frac{v_2}{v_1} = \sqrt{\frac{h_0 - h_2}{h_0 - h_1}} \equiv \sqrt{\frac{H_2}{H_1}}$

The sprue must taper down (smaller area toward the bottom), or air gets entrapped in the metal.

Reynolds number (inertia / viscous):

$Re = \frac{\rho v D}{\eta}$

• $Re < 2000$ → laminar
• $2000 < Re < 20000$ → laminar + turbulent (harmless)
• $Re > 20000$ → severe turbulence (air entrapment)

Fluidity & Castability

Fluidity = ability to fill the cavity.

• Molten-metal factors: ↑viscosity → ↓fluidity; ↑surface tension → ↓fluidity; inclusions ↑viscosity; shorter freezing range → higher fluidity.
• Casting factors: mold design; mold thermal conductivity & roughness (↑ → ↓fluidity — a dilemma, since high conductivity also speeds cooling); slower pouring → ↓fluidity.

Solidification Time — Chvorinov’s Rule

$t = C\left(\frac{V}{A}\right)^2$

$C$ depends on mold material and temperature. A cube cools faster than a sphere of equal volume (more surface area). Skin is thinner at internal angles (slower cooling there).

Defects & Porosity

Defects: metallic projections (flash, swells), cavities (blowholes, pinholes, shrinkage), discontinuities (cracks, hot/cold tearing, cold shuts), defective surfaces (misruns), wrong dimensions, inclusions.

Porosity (shrinkage and/or gas): reduce with chills (speed local solidification) and by degassing (inert-gas flushing or vacuum pouring). Pores lower toughness, strength, $E$, and conductivity.