Casting - Part 1 Notes

Fundamentals of Metal Casting

• In casting, molten metal flows into a mold and solidifies.
• It's an ancient shaping process, dating back 6000 years.
• Basic steps:
  • Melt the metal.
  • Pour into a mold.
  • Cool and solidify.
• Shape casting produces complex geometries close to the final shape.
• Versatile due to various methods.

Capabilities and Advantages of Casting

• Creates complex geometries, including internal and external shapes.
• Some processes achieve net shape, requiring no further operations.
• Can produce very large parts (over 100 tons).
• Applicable to any metal that can be liquefied.
• Some methods are suited to mass production.

Disadvantages of Casting

• Poor mechanical properties.
• Porosity.
• Poor dimensional accuracy.
• Poor surface finish.
• Safety hazards.
• Environmental problems.

Overview of Casting Technology

• Casting is performed in a foundry.
• A foundry is a factory for making molds, melting metal, casting, and cleaning.
• Workers are called foundrymen.

Casting Processes

• The mold cavity determines the cast part's shape.
• Cavity size is oversized to account for metal shrinkage during solidification and cooling.
• Mold material varies (sand, plaster, ceramic, metal).
• Casting processes are classified by mold type.

Open vs. Closed Molds

• Open mold: liquid metal is poured into an open cavity.
• Closed mold: uses a gating system for molten metal to flow into the cavity.
• Closed molds are more common.

Types of Mold Casting

• Expendable mold casting: the mold is destroyed to remove the casting.
• Permanent mold casting: the mold can be reused.

Expendable Mold Casting

• Mold is destroyed to remove the casting.
• Made of sand, plaster, or similar materials with binders.
• Sand casting is a prime example, using sand molds.
• Intricate geometries are possible.

Permanent Mold Casting

• Molds are reusable.
• Made of metal to withstand high temperatures.
• Consists of two or more sections that can be opened.
• Die casting is a common process.
• Part shapes are limited by the need to open the mold.
• Economical for high production.

Sand-Casting Molds

• Sand casting is the most important casting process.
• The mold has two halves: cope (upper) and drag (bottom).
• Contained in a flask, also divided into two halves.
• The two halves separate at the parting line.

Mold Cavity

• Formed by a pattern made of wood, metal, or plastic shaped like the part.
• Sand is packed around the pattern in the cope and drag.
• The pattern is oversized to allow for metal shrinkage.
• The sand is moist and contains a binder.
• The cavity provides the external surfaces of the cast part.

Core

• Cores define internal surfaces.
• Placed inside the mold cavity.
• Made of sand, metal, plaster, or ceramics.

Gating System

• Channels for molten metal to flow into the cavity.
• Consists of a downsprue (metal entry), a runner (leads to cavity), and a pouring cup (minimizes splash).

Riser

• A reservoir of liquid metal to compensate for shrinkage during solidification.
• Must freeze after the main casting.

Air vents

• Evacuate air and gases from the mold cavity.
• Sand casting uses the natural porosity of the sand.
• Permanent molds have vent holes.

Heating the Metal

• Metal must be heated above its melting point for pouring.
• Heating furnaces are used.
• Required heat energy includes:
  • Heat to raise temperature to the melting point.
  • Heat of fusion (solid to liquid).
  • Heat to raise molten metal to pouring temperature.
• Expressed as: $H = V \rho [C<em>s(T</em>m - T<em>o) + H</em>f + C<em>l(T</em>p - T_m)]$
  • H = Total heat required (J).
  • $\rho$ = Density (g/cm3).
  • $C_s$ = Specific heat of solid metal (J/g-°C).
  • $T_m$ = Melting temperature (°C).
  • $T_o$ = Starting temperature (°C).
  • $H_f$ = Heat of fusion (J/g).
  • $C_l$ = Specific heat of liquid metal (J/g-°C).
  • $T_p$ = Pouring temperature (°C).
  • V = Volume of metal (cm3).
• The equation is of conceptual, but limited computational value.

Factors complicating the equation

• Specific heat varies with temperature.
• Metal's specific heat differs in solid and liquid states.
• Alloys melt over a temperature range, not a single point.
• Property values are not readily available.
• Significant heat losses occur during heating.

Pouring the Molten Metal

• Critical step: introducing molten metal into the mold.
• Metal must flow into all regions before solidifying.
• Factors include pouring temperature, pouring rate, and turbulence.

Pouring Temperature

• Temperature of molten metal when introduced into the mold.
• Superheat: the difference between solidification and pouring temperatures.

Pouring Rate

• Volumetric rate at which metal is poured.
• Too slow: metal chills and freezes before filling.
• Too fast: turbulence becomes a problem.

Turbulence

• Erratic variations in velocity.
• Agitated and irregular flow.
• Avoid during pouring because it:
  • Accelerates formation of metal oxides.
  • Aggravates mold erosion (wearing away of mold surfaces).

Engineering Analysis of Pouring

• Flow velocity: $v = \sqrt{2gh}$
  • v = Velocity (cm/s).
  • g = Gravity (981 cm/s2).
  • h = Sprue height (cm).
• Volume flow rate: $Q = v<em>1A</em>1 = v<em>2A</em>2$
  • Q = Volumetric flow rate (cm3/s).
  • v = Velocity (cm/s).
  • A = Cross-sectional area (cm2).
• Mold filling time: $t = V/Q$
  • t = Filling time (s).
  • V = Mold cavity volume (cm3).
  • Q = Volumetric flow rate (cm3/s).
• Computed filling time is a minimum, as it ignores friction losses.

Fluidity

• The ability of a metal to flow into and fill the mold before freezing.
• Inverse of viscosity.
• Assessed using standard testing methods, like the spiral mold test.
• A longer cast spiral indicates greater fluidity.

Factors Affecting Fluidity

• Pouring temperature relative to melting point.
• Metal composition.
• Viscosity of liquid metal.
• Heat transfer to surroundings.

Pouring Temperature

• Higher temperature increases time in the liquid state.
• Can aggravate oxide formation, gas porosity, and penetration into sand grains.

Composition

• Best fluidity with metals that freeze at a constant temperature (pure metals and eutectic alloys).
• Solidification over a temperature range reduces fluidity.

Heat of Fusion

• Higher heat of fusion tends to increase measured fluidity.

Solidification

• Transformation of molten metal back into the solid state.
• Differs for pure elements and alloys.

Pure Metals

• Solidify at a constant temperature (freezing point = melting point).
• Takes time (local solidification time).
• Latent heat of fusion is released.
• Total solidification time: time between pouring and complete solidification.

Cooling Action

• A thin skin forms at the mold wall.
• Freezing proceeds inward.
• Rate depends on heat transfer and thermal properties.
• Fine, randomly oriented grains in the skin.

Grain Formation

• Grains grow inwardly as needles or spines (dendritic growth).
• Lateral branches form.
• Treelike structures are filled in.
• Coarse, columnar grains aligned toward the center.

Most Alloys

• Freeze over a temperature range (liquidus to solidus).

Freezing Start

• A thin skin forms at the mold wall.
• Dendrites grow away from the walls.
• Advancing zone with liquid and solid metal coexist (mushy zone).
• Liquid islands solidify as temperature drops to solidus.
• Dendrite composition favors higher melting point metal.
• Internal coaxial grains can form.

Composition Imbalance

• Segregation of elements: microscopic and macroscopic.
• Microscopic: chemical composition varies within each grain.
• Macroscopic: chemical composition varies throughout the casting.

Eutectic Alloys

• Solidus and liquidus are at the same temperature.
• Solidification occurs at a constant temperature.
• Example: Lead-tin alloy with 61.9% tin and 38.1% lead has a melting point of 183°C.

Solidification Time

• Time required for casting to solidify after pouring.
• Dependent on size and shape, described by Chvorinov’s rule: $T<em>{TS} = C</em>m (V/A)^n$
  • $T_{TS}$ = Total solidification time (min).
  • $C_m$ = Mold constant (min/cm2).
  • V = Volume of casting (cm3).
  • A = Surface area of casting (cm2).
  • n = Exponent (usually 2).

Implications of Chvorinov's Rule

• A higher volume-to-surface area ratio cools and solidifies more slowly.
• Used in riser design: the riser must remain liquid longer than the casting to feed molten metal.

Shrinkage

• Occurs in three steps:
  1. Liquid contraction during cooling.
  2. Contraction during the phase change (liquid to solid).
  3. Thermal contraction of the solidified casting.

Steps of Shrinkage

• Liquid contraction reduces height.
• Solidification shrinkage further reduces height and creates a void in the center.
• Thermal contraction occurs after solidification.

Solidification vs Graphitization

• Solidification shrinkage occurs in nearly all metals because the solid phase has a higher density than the liquid phase.
• Cast iron with high carbon content is an exception due to graphitization, which results in expansion.

Compensation for Shrinkage

• In sand casting, liquid metal is supplied by risers.
• In die casting, molten metal is applied under pressure.
• Pattern-makers use oversized mold cavities using pattern shrinkage allowance.
• Shrink rules with elongated scales are used to create the patterns and molds.

Directional Solidification

• Regions most distant from the liquid metal supply freeze first.
• Solidification progresses toward the riser(s).
• Molten metal is available from the risers to prevent shrinkage voids.
• Chvorinov’s rule is observed in design.

Encouraging Directional Solidification

• Use chills (internal or external heat sinks).
• Internal chills: small metal parts inside the cavity.
• External chills: metal inserts in the mold walls.

Importance of Proper Freezing

• Avoid premature solidification in sections nearest the riser.
• The passageway between the riser and the main cavity must be designed to avoid early freezing.

Riser Design

• Used in sand-casting molds to feed liquid metal during freezing, compensating for solidification shrinkage.
• The riser must remain molten until after the casting solidifies.
• Chvorinov’s rule is used to compute the riser size.
• The riser represents waste metal to be remelted.

Forms of Risers

• Side riser: attached to the side of the casting.
• Top riser: connected to the top surface of the casting.
• Open riser: exposed to the outside, promoting faster solidification.
• Blind riser: entirely enclosed within the mold.