Shrinkage in metal casting describes internal cavities or surface sink marks that form because molten metal contracts by roughly 1–6 % during solidification. Without properly placed risers to supply extra liquid metal, the solidifying metal cannot fill the volume loss, leaving voids that weaken the finished part.
What Is Shrinkage in Metal Casting
Shrinkage in casting refers to the volumetric contraction that occurs when molten metal cools and solidifies, resulting in a reduction in volume of the solid metal. Because metals are generally less dense in liquid form than in solid form, all cast metals experience shrinkage upon solidification. If this natural contraction is not properly compensated (for example, by supplying additional molten metal during solidification), voids or cavities can form within the casting.
These shrinkage-related voids are a common casting defect class, distinct from gas porosity. Notably, shrinkage voids tend to have irregular, jagged or angular shapes (due to the pulling apart of the solidifying dendritic network), whereas gas porosity holes are typically smooth and rounded.
The Three Stages of Shrinkage
Stage 1: Liquid Shrinkage
Liquid shrinkage begins immediately after pouring molten metal into the mold. The metal contracts as it cools from its pouring temperature to its liquidus temperature (the point where solidification starts). During this stage, the metal remains completely liquid and can flow freely to compensate for volume changes.
Stage 2: Solidification Shrinkage
Solidification shrinkage occurs as the metal transforms from liquid to solid at the freezing temperature. This stage causes the most significant volume reduction because atoms pack more tightly in the solid crystal structure. The metal cannot flow to fill voids once it solidifies, making this the most critical stage for defect formation.
Stage 3: Solid-State Contraction
Solid-state contraction happens after the metal completely solidifies and continues cooling to room temperature. The solid metal contracts uniformly in all directions during this stage. This contraction is generally predictable and causes overall dimensional changes rather than internal defects.
Types of Shrinkage Defects
Open Shrinkage
Open shrinkage defects connect to the atmosphere or the casting surface. These defects are visible during inspection and often appear at the top surfaces of castings. Two main types of open shrinkage exist:
- Pipes: Deep, funnel-shaped cavities that extend from the casting surface into the interior. These form when liquid metal cannot feed the shrinking areas below.
- Caved Surfaces: Shallow depressions on the casting surface caused by insufficient liquid metal to compensate for solidification shrinkage. These appear as dish-shaped indentations.
Closed Shrinkage
Closed shrinkage defects form entirely within the casting without connecting to any surface. These internal voids are harder to detect and require X-ray or ultrasonic testing. Three types of closed shrinkage commonly occur:
- Macro-Shrinkage: Large cavities visible to the naked eye when the casting is sectioned. These typically form in heavy sections or at hot spots.
- Centerline Cavities: Linear shrinkage voids that form along the central axis of cylindrical or symmetrical castings. These develop when solidification progresses uniformly from all sides.
- Micro-Shrinkage: Tiny voids dispersed throughout the casting, often between dendrite arms. These microscopic defects reduce mechanical properties even when individually small.
The Influence of Solidification Modes on Shrinkage Behavior
Eutectic Solidification
Eutectic solidification occurs at a single temperature where liquid transforms directly to solid without a mushy zone.
Eutectic alloys tend to form pipe-type shrinkage defects rather than dispersed porosity. The clear solidification front allows better feeding of liquid metal to compensate for shrinkage.
Directional Solidification
Directional solidification progresses from one end of the casting to the other in a controlled manner. This mode creates a temperature gradient that keeps liquid metal available to feed shrinking regions. Properly designed castings use directional solidification to push shrinkage into risers rather than the casting itself.
Equiaxed (Mushy) Solidification
Equiaxed solidification creates a wide mushy zone where liquid and solid coexist throughout a temperature range. Aluminum alloys and many steels solidify this way. The dispersed solidification pattern makes liquid metal feeding difficult.
This mode typically produces micro-shrinkage and dispersed porosity rather than large cavities. The interconnected network of solid dendrites blocks liquid flow paths early in solidification.
Cause of Shrinkage
The Influence of Casting Geometry
Casting geometry directly determines where and how shrinkage defects form. Certain geometric features create conditions that promote shrinkage:
- Hot Spots: Thick sections that cool slower than surrounding areas create isolated liquid pools. These regions cannot receive liquid feed after surrounding metal solidifies, resulting in shrinkage cavities.
- Section Thickness Variation: Abrupt changes from thick to thin sections disrupt uniform solidification. The thick sections solidify last and often develop shrinkage defects at the transition zones.
Alloy Composition & Freezing Characteristics
The specific metal alloy and its solidification behavior strongly influence shrinkage defect formation:
- Volumetric Shrinkage: Different alloys contract by different amounts. Pure aluminum shrinks 6.6% while adding 12% silicon reduces shrinkage to 3.8%.
- Solidification (Freezing) Range: Alloys with wide freezing ranges (difference between liquidus and solidus temperatures) are more prone to micro-shrinkage. Pure metals and eutectic alloys with narrow ranges form cleaner shrinkage patterns.
- Alloying Elements: Specific elements affect shrinkage behavior. Silicon in aluminum reduces shrinkage, while sulfur in steel can increase hot tearing tendencies.
Process Parameters
How the metal is poured and cooled significantly impacts shrinkage defect formation:
- Pouring Temperature: Higher pouring temperatures increase total shrinkage from liquid cooling. Excessive superheat also slows solidification, giving more time for shrinkage cavities to develop.
- Pouring Rate: Fast pouring maintains temperature but can cause turbulence. Slow pouring allows premature solidification that blocks feeding paths.
- Cooling Rate: Rapid cooling reduces the time for liquid feeding but can increase thermal gradients. Slow cooling allows more complete feeding but may create larger grain structures.
Mold and Riser Design
Proper mold system design prevents shrinkage through:
- Mold Design: Proper placement of ingates and runners promotes favorable solidification patterns. Bottom filling reduces turbulence, while proper venting prevents gas entrapment that compounds shrinkage issues.
- Riser Design: Risers must solidify after the casting sections they feed. The riser volume should be 1.2 to 2 times the shrinkage volume of the fed section. Proper neck dimensions ensure continuous liquid flow while facilitating riser removal.
Cooling Rate and Chills
Controlled cooling manages solidification patterns through:
- Cooling Rate: A faster cooling rate (achieved by using chills – heat sinks placed in the mold, or using metal molds in permanent mold casting) will make the metal freeze quicker and more uniformly, which can reduce the size of shrinkage cavities or eliminate them by forcing directional solidification toward feeders。
- Cooling Chills: Chills effectively shift hot spots by extracting heat at strategic locations, encouraging those areas to freeze earlier (and thus be fed from still-molten regions elsewhere). Conversely, very slow cooling (as in insulating molds or very large castings) can allow significant segregation of liquid and promote large shrinkage voids.
