How to avoid hot cracking in aluminum alloy castings?

Hot cracks are cracks that form in castings at elevated temperatures during the late solidification phase or after solidification. Hot cracks have an oxidized appearance and lack metallic luster. They originate and propagate along grain boundaries, with a tortuous, irregular shape. Hot cracks often occur inside sharp corners in hot zones and at the intersection of thick and thin sections, coexisting with porosity. Hot cracking is a major cause of casting rejection. To avoid hot cracking, consider the following:

 

 

1. Select appropriate materials. Chemical composition is the primary factor determining an alloy's hot cracking tendency. When conditions permit, choose alloys with a low hot cracking tendency. Generally speaking, alloys with a low content of the eutectic component (the last component to solidify) have a higher hot cracking tendency, while alloys with a high content of eutectic have a lower hot cracking tendency.

 

2. Control the solidification sequence. For cylindrical castings, since stress is distributed along the circumference of the cylinder, the cylinder can solidify simultaneously, evenly distributing stress across the circumference and avoiding stress concentration in the last-solidified area, which can lead to excessive stress and thermal cracking. For general castings, stress-prone areas can be solidified first, followed by other areas, maintaining sequential solidification to reduce stress.

 

3. Grain refinement. Grain refinement enhances the alloy's strength and effectively resists cracking. Grain refinement also reduces dendrites, facilitating shrinkage compensation and preventing cracking.

 

4. Controlling the casting temperature. A lower casting temperature results in less shrinkage and stress, but too low a temperature can lead to other solidification defects.

 

5. Controlling the mold temperature. Mold temperature essentially affects the alloy's solidification rate. Slower solidification results in less stress, but too slow a temperature can result in coarser grains and a greater risk of solidification defects.

 

6. Increase the yield of the mold and core. This is easy to achieve with sand casting, but difficult with metal mold casting.