Blog

Causes and Countermeasures of Aluminum Alloy Casting Cracks

Cracks are a defect that we must consider and avoid in various castings. Due to the different properties of casting materials and the varying designs of casting cavities, we can control the occurrence of cracks by adding various alloy primary colors, or improving casting structures, casting methods, sprues, etc,

Cracks in alloy castings can be classified into two types: hot cracks and cold cracks. Hot cracks are also known as high-temperature cracks, while cold cracks are also known as room temperature cracks. Cold cracking is a type of crack that occurs in a solid state after complete solidification due to deformation caused by the shape of the casting, and is generally rare.  

Alloy solidification is a process of crystal nucleation and growth. In the liquid phase region, due to the lack of contact between crystals, stress will not be generated, so cracks will not occur. When the alloy solidification process enters the quasi solid phase zone, dendrites come into contact, interpenetrate, and contract with each other, resulting in internal stress. When the stress generated between solid phases exceeds the tensile strength, it will crack and produce thermal cracks. These cracks cannot be replenished by the liquid phase and will oxidize, resulting in a lack of metallic luster. The shape of hot cracks is tortuous and irregular, often occurring along grain boundaries, which is a characteristic of hot cracks.

According to the position of the crack in the casting, it can be divided into external cracks and internal cracks. External cracks generally occur from irregular areas, sharp corners, and areas with large differences in cross-sectional thickness and stress concentration on the surface of castings. They extend from the surface to the interior, with a wider surface and narrower interior. At first, a solidified shell was formed on the mold wall, which cracked due to weak points (a single crack). Liquid replenishment, upon cooling, crystallizes into small grains. Due to shrinkage, stress is generated and cracks (secondary cracks) are formed. Without liquid replenishment, cracks remain on the surface of the casting.

Internal cracks occur inside the casting, in the quasi solid phase zone, due to the contact between dendrites, they intersect and generate stress, resulting in cracks. When there is no liquid replenishment, the cracks remain. The surface of the internal crack is not smooth, often bifurcates, rarely oxidizes, and generally does not extend to the surface of the casting.  

The magnitude of the tendency for alloy hot cracking depends on the properties of the alloy, including its high-temperature mechanical properties, flowability, and shrinkage size. Alloys with good high-temperature mechanical properties have strong tensile strength and are not easily broken; Alloys with good flowability, even if cracks occur, will have liquid replenishment to maintain their integrity. Alloys with large shrinkage generate high stress and have a greater tendency to crack. Al Cu and Al Mg alloys, due to their wide quasi solid phase region and poorer flowability compared to Al Si alloys, generally have a greater tendency towards hot cracking than Al Si alloys.

That is to say, solid solution type alloys have a greater tendency to heat up, while eutectic type alloys have a smaller tendency to hot crack. However, whether the same alloy casting produces hot cracks also depends on external conditions, such as mold resistance, casting structure, melt treatment, and pouring process. Therefore, in practical work, measures such as increasing the retreat of the mold, improving the mold structure, setting up cold iron, insulation riser, and refining the alloy structure are often taken to avoid or reduce the occurrence of hot cracks. The following table shows the casting properties of common aluminum alloys.