1、 Core difference (distinguish at a glance)

-Appearance: Internal voids, holes, honeycomb like looseness, surface mostly smooth pits, inner wall shiny or sponge like; Mostly concentrated at thick walls, tendon roots, corners, and wall thickness convergence points.
-Property: Volume shrinkage defect, caused by insufficient shrinkage compensation due to the decrease in volume during metal solidification.
-Hazards: Air and oil leakage, poor airtightness, decreased strength, and scrapped exposed pores after processing.

2. Shrinkage cracking (hot cracking/shrinkage cracking)

-Appearance: Linear and strip-shaped cracks with obvious cracking gaps, tortuous patterns, and mostly distributed along grain boundaries; Often located at the boundary between thickness and thinness, sharp corners, sudden wall thickness changes, and concave corners.
-Property: The solidification shrinkage stress exceeds the strength of the alloy and is pulled apart.
-Harm: Force fracture, leakage, assembly cracking, are fatal defects.

Root cause

Root cause of shrinkage

The alloy undergoes volume shrinkage from liquid to solid state, and in the later stage of solidification, no liquid metal can be replenished, resulting in voids/looseness.

1. Product structure: Local excessive wall thickness, thick wall aggregation, too thick ribs, uneven meat thickness.
2. Mold factors: - Unreasonable gate position, final solidification at thick walls;
-Uneven mold temperature and localized overheating;
-Overflow groove, insufficient exhaust, and poor compensation channel.
3. Process parameters: - Insufficient die-casting pressure and holding pressure, unable to force shrinkage;
-The pouring temperature is too high and the mold temperature is too high, resulting in slow solidification and large shrinkage;
-Slow/fast switching is unreasonable, and the temperature of cavity filling is uneven.
4. Material: The alloy contains impurities, high gas content, and is prone to forming loose shrinkage cavities during solidification.

Root cause of shrinkage cracking

During the solidification process of castings, shrinkage is hindered and internal tensile stress is generated, which is greater than the high-temperature tensile strength of the alloy and directly causes tensile cracking.

1. Product structure: - Sharp corners without rounded corners, sudden wall thickness changes;
-The connection between the tendon and the body is too thick and the constraint is large, causing shrinkage to be forcibly pulled by the mold.
2. Mold factors: - The demolding slope is too small, the surface of the mold cavity is rough, and the mold is stuck and pulled during shrinkage;
-The local temperature difference of the mold is large, and the solidification sequence is unreasonable, which mutually restricts and shrinks;
-The arrangement of the ejector pins is unreasonable, resulting in uneven force and tearing during ejection.
3. Process parameters: - High pouring temperature/mold temperature, coarse grain size, low high-temperature strength;
-The cooling rate is too fast or uneven, resulting in concentrated shrinkage stress;
-Premature mold opening and ejection result in incomplete solidification and insufficient strength of the castings, leading to tensile cracking.
4. Alloy material: The alloy has high thermal brittleness and is prone to thermal cracking due to excessive impurities (such as iron and zinc).

Quick distinction mnemonic

Shrinkage is a cavity, with round honeycomb and thick walls;
Shrinkage is a crack, a sharp angle variable diameter stress pull.
Shrinkage and shrinkage are compensated, and shrinkage cracks are pulled.