A void in a casting where solidification shrinkage could not be made up by melt. It tends to form in thick sections that solidify last and is prevented by the design of risers and the order of solidification.
Japanese name: 引け巣
Shrinkage cavity appears in 3 practice questions across 2 topics, including Materials processing and joining, Metals and alloys.
Between pouring the melt into the mold and reaching room temperature, a metal shrinks in three stages: liquid contraction while it cools as a liquid, solidification shrinkage as it changes from liquid to solid, and solid contraction as the solid cools to room temperature. The first two cause shrinkage cavities; if melt cannot be supplied to make up for them, voids remain. Solidification shrinkage is as large as about 7% for pure aluminum and several percent even for steel. Solid contraction, on the other hand, reduces the overall dimensions of the casting and is compensated by a shrinkage allowance that makes the mold dimensions larger. Shrinkage cavities and dimensional shrinkage differ in the stage that causes them and in how they are dealt with.
To prevent shrinkage cavities, the riser must solidify after every part of the casting, with solidification progressing in order toward the riser. A guide to solidification time is Chvorinov's rule t = B(V/A)², where V is the volume, A the surface area that contributes to cooling, and V/A is called the modulus. For example, a cube with 100 mm edges has a modulus of about 16.7 mm. Making the riser modulus about 1.2 times that of the casting, as experience suggests, gives a solidification time 1.2² ≈ 1.44 times longer, so melt remains until after the casting has solidified. Chills that speed cooling of thick sections and insulating or exothermic sleeves that keep the riser liquid longer are also used.
The form of shrinkage cavities depends on how the alloy solidifies. Pure metals and eutectic alloys, which solidify over a narrow temperature range, grow a solid shell from the outside inward, so a large cavity tends to gather at the center that solidifies last. Alloys with a wide freezing range have a broad region where solid and liquid coexist, melt can no longer reach between the dendrites, and fine cavities form dispersed through the casting (microshrinkage). Gray cast iron is notable in that the expansion when graphite forms at the end of solidification offsets the shrinkage, so smaller risers suffice and castings tend to have few shrinkage cavities.
Internal shrinkage cavities cannot be seen from outside, so they are examined by radiography and ultrasonic testing, with X-ray CT also used for precise evaluation. At the design stage, casting simulation predicts the order of solidification and where solidification ends, and the positions of risers and chills are decided accordingly. Shrinkage cavities impair pressure tightness and act as initiation sites for fatigue cracks, so they are controlled especially strictly in parts that carry pressure and in rotating parts. Internal cavities not connected to the surface can be closed by hot isostatic pressing (HIP), but those open to the surface cannot, so they are repaired by welding or the casting is rejected.
Answer: C. The volume shrinkage on going from liquid to solid cannot be compensated by melt from the surroundings
Most metals shrink by a few percent in volume on solidification (about 7% for pure aluminum). Solidification proceeds inward from the mold wall, so when a thick section that solidifies last becomes isolated, surrounded by solid, its shrinkage cannot be made up by melt, and a shrinkage cavity with a rough inner surface forms. Countermeasures are to make the riser solidify after the casting so that it can feed melt, and to use chills and similar means so that solidification proceeds progressively toward the riser. Release of dissolved gas produces gas porosity (blowholes) with smooth, round inner surfaces, a different defect. Restrained contraction after solidification produces cracks (hot tears or cold cracks), which differ from a shrinkage cavity, which is a void.
The inner surface of a shrinkage cavity is rough, with dendrite tips visible, and such cavities gather in the thick sections that solidify last. Gas pores form when dissolved gas comes out, so their inner surfaces are smooth and round and they appear throughout the casting. On radiographs, shrinkage cavities show as irregular shapes and pores as round shadows.
They arise at different stages. The shrinkage allowance is a dimensional margin that makes the mold larger to allow for contraction of the solid as it cools to room temperature after solidification. A shrinkage cavity is a void formed when the earlier shrinkage during liquid cooling and solidification is not made up by feeding melt, and it is prevented by designing the risers and the order of solidification.
So that the riser solidifies after the casting, its modulus (volume divided by cooling surface area) is made larger than that of the thick sections of the casting, often about 1.2 times or more by experience. Enough melt must also remain to make up the casting's shrinkage. Insulating or exothermic sleeves keep even small risers liquid for longer.
Solidification proceeds inward from the mold wall, so when a thick section that solidifies last becomes isolated, surrounded by solid, its shrinkage cannot be made up by melt, and a shrinkage cavity with a rough inner surface forms.
What is the main cause of a "shrinkage cavity," a void that…Because a riser (the feeder that supplies molten metal) cannot compensate for shrinkage cavities unless it solidifies after the casting, this rule is also the basis for designing risers with a larger V/A than the casting.
Chvorinov's rule gives the solidification time t of a casti…Small internal shrinkage cavities and pores are pressed shut and disappear, as long as they are not connected to the surface.
When carbon steel parts of the same shape are made by casti…Recrystallization Recovery Work hardening Carbon equivalent Extrusion Heat-affected zone Hot isostatic pressing Investment casting Sensitization Springback