20.2 Foundry Materials, Casting, Solidification & Defects

Foundry work produces complex metal shapes by melting an alloy, delivering it into a prepared mold and controlling solidification. Mining applications include crusher components, pump parts, liners, wheels, gears and wear-resistant castings. Process control must deliver geometry, soundness, chemistry and mechanical properties—not simply fill a cavity.

Pattern, Mold and Core

A pattern is the model used to form the mold cavity. It incorporates:

• shrinkage allowance for contraction;
• machining allowance for surfaces finished later;
• draft for pattern withdrawal;
• distortion allowance where shape may warp; and
• core prints to locate cores.

A core forms an internal hole or passage. Green-sand molds use sand, binder, water and additives; chemically bonded molds and permanent molds suit other sizes and quality requirements. Mold strength, permeability, collapsibility, refractoriness and moisture must be controlled.

Gating and Feeding

The pouring basin, sprue, runner, gates and filters deliver metal. The design should fill the mold before premature freezing while limiting turbulence, air aspiration, oxide entrainment and mold erosion. A tapered sprue helps prevent aspiration as the stream accelerates.

A riser or feeder supplies liquid metal to compensate for volumetric contraction during solidification. It must solidify after the casting region it feeds. Chills can accelerate local solidification and promote directional freezing toward the riser.

Solidification Time

Chvorinov's rule relates solidification time to volume-to-surface-area ratio:

$t_s=C_m\left(\frac{V}{A}\right)^n$

where $C_m$ depends on mold and metal and $n$ is often near 2. A riser with larger modulus $V/A$ than the casting remains liquid longer. Geometry can create isolated hot spots that form shrinkage even when total riser volume seems adequate.

Production Sequence

1. pattern and core design;
2. mold and core preparation;
3. charge selection and furnace melting;
4. chemistry adjustment, deoxidation or inoculation;
5. temperature measurement and slag removal;
6. controlled pouring;
7. solidification and cooling;
8. shakeout and core removal;
9. gate/riser removal, cleaning and finishing;
10. heat treatment where required; and
11. dimensional, nondestructive and mechanical inspection.

Charge materials must be identified and dry. Alloy chemistry can drift through oxidation, contamination or wrong scrap. Excess superheat increases energy, oxidation, gas pickup and mold reaction; inadequate temperature causes misrun or cold shut.

Defects and Causes

Do not repair a recurring defect by welding every casting without identifying cause. Use defect maps, process data and metallography.

Structure and Properties

Cooling rate affects grain size and phase distribution. Fast cooling generally refines structure, while thick sections cool slowly. In cast iron, composition, cooling and inoculation influence graphite form and matrix. Heat treatment can relieve stress or modify hardness and toughness. Confirm properties with hardness, tensile or impact testing and appropriate metallography.

Nondestructive testing includes visual, dye penetrant, magnetic particle, ultrasonic and radiographic methods according to material and defect orientation. No single method finds every defect.

Foundry Safety

Molten metal contacting water can cause explosive steam expansion. Keep charge, tools, ladles and floors dry. Inspect furnace lining, lifting gear, ladles and trunnions; control exclusion zones and travel paths. Provide heat and face protection, ventilation for metal and binder fumes, dust control, spill containment, emergency response and trained signals.

Worked Modulus Comparison

If a casting section has modulus $V/A=1.5$ cm and a riser has 2.0 cm, with common $C_m$ and $n=2$, the time ratio is $(2.0/1.5)^2=1.78$. The riser should remain molten about 1.78 times longer, though neck design and heat loss still require engineering review.

Casting Yield

Casting yield is sound casting mass divided by total poured metal, including gates and risers. Improving yield reduces remelt energy, but shrinking a riser without solidification analysis can create cavities. Optimize gating and feeding while preserving directional solidification and quality; recycled returns still require energy and can introduce chemistry or inclusion risk.