Sand casting is one of the most versatile and widely used metal casting processes. Its key advantage is its ability to cast a enormous range of metals and alloys, from common iron to high-performance superalloys. The choice of material depends on the required properties of the final part, such as strength, weight, corrosion resistance, machinability, and cost.
The materials can be broadly categorized into the following groups:
- Ferrous Alloys (Iron-Based)
Ferrous alloys are the most commonly sand-cast materials due to their excellent mechanical properties and relatively low cost.
Gray Iron (Grey Cast Iron):
Characteristics: Excellent fluidity, good machinability, high compressive strength, and outstanding vibration damping capacity. It is brittle due to the graphite flakes in its microstructure.
Common Applications: Engine blocks, cylinder heads, brake discs, machine tool bases, pipes, and cookware.
Ductile Iron (Nodular Cast Iron or Spheroidal Graphite Iron – S.G. Iron):
Characteristics: The graphite is in a spherical nodule form (due to magnesium or cerium treatment), which gives it significantly higher ductility, toughness, and tensile strength than gray iron while retaining good castability.
Common Applications: Crankshafts, gears, heavy-duty bearings, automotive components, and water pipes.
White Iron:
Characteristics: Extremely hard and wear-resistant due to its carbide structure, but also very brittle. It is often used in a composite form with ductile iron (e.g., chilled iron rolls).
Common Applications: Lifter bars in mills, pump liners, crusher parts, and wear plates.
Malleable Iron:
Characteristics: Heat-treated white iron where the carbides decompose into irregularly shaped graphite nodules (temper carbon). It offers good ductility and shock resistance.
Common Applications: Railroad fittings, hardware, hand tools, and agricultural equipment.
Carbon Steel and Alloy Steel:
Characteristics: Offer the highest strength and toughness among ferrous castings. They can be heat-treated to achieve a wide range of properties. However, they have higher melting points and poorer fluidity than cast irons, making them more challenging to cast.
Common Applications: High-stress components like gears, valves, pump casings, mining equipment, and military hardware.
- Non-Ferrous Alloys
Non-ferrous alloys are chosen for their lightweight, corrosion resistance, or special thermal and electrical properties.
Aluminum Alloys:
Characteristics: Lightweight, good corrosion resistance, high thermal and electrical conductivity, and excellent machinability. They have good castability but a high shrinkage rate.
Common Alloys: A356, A319, 356, 380.
Common Applications: Automotive intake manifolds and wheels, aerospace components, electronic housings, and marine fittings.
Copper-Based Alloys (Bronzes and Brasses):
Characteristics: Excellent corrosion resistance (especially to seawater), good wear resistance, and aesthetic appeal. They are often used for bearings and bushings.
Common Types:
Tin Bronze: Classic bronze, good bearing properties.
Aluminum Bronze: High strength and exceptional wear/corrosion resistance.
Silicon Bronze: Good strength and corrosion resistance.
Brass (Copper-Zinc): Good machinability and acoustic properties.
Common Applications: Ship propellers, bearings, bushings, valves, pumps, and architectural hardware.
Magnesium Alloys:
Characteristics: The lightest of all structural metals. They have excellent strength-to-weight ratios and good machinability but are highly susceptible to corrosion if not properly coated or alloyed.
Common Applications: Aerospace components, racing engine blocks, laptop housings, and power tools.
Zinc Alloys:
Characteristics: Have high ductility and impact strength, excellent as-cast surface finish, and are easy to plate. They are typically die-cast, but certain sand casting is done for larger prototypes or specialized parts.
Common Applications: Automotive fittings, decorative hardware, and prototypes.
Nickel-Based Superalloys:
Characteristics: Retain their strength at extremely high temperatures and offer outstanding corrosion and oxidation resistance. They are very expensive and difficult to cast due to their high melting points.
Common Applications: Turbine blades and vanes for jet engines, chemical processing equipment, and exhaust systems.
Tin-Based Alloys (Pewter):
Characteristics: Very low melting point, excellent fluidity, and non-toxic. Easy to cast with minimal equipment.
Common Applications: Decorative items, tableware, and souvenirs.
- Other Specialty Alloys
Cobalt-Based Alloys: Similar to nickel superalloys, used for high-temperature and wear-resistant applications.
Titanium Alloys: Exceptionally high strength-to-weight ratio and superb corrosion resistance. They are extremely challenging to sand cast due to their high reactivity with oxygen and nitrogen, requiring a vacuum or inert gas environment (e.g., vacuum arc melting), which makes the process very expensive.
Applications: Aerospace, biomedical implants, and high-performance automotive components.
Factors Influencing Material Choice for Sand Casting:
Mechanical Requirements: Strength, hardness, ductility, and impact resistance.
Weight: Critical for aerospace and automotive applications (favors Al and Mg).
Corrosion/Heat Resistance: Required for marine, chemical, and engine applications (favors stainless steel, bronze, Ni-alloys).
Machinability and Weldability: Ease of post-casting processing.
Cost: Includes both the raw material cost and the cost associated with casting it (e.g., melting temperature, fluidity).
Castability: How easily the molten metal flows to fill the mold, its shrinkage rate, and its tendency to form defects.
In summary, sand casting is compatible with almost any metal or alloy that can be melted and poured. Its material selection is virtually unlimited, making it the foundation of the metal casting industry.
