Mechanism of Crack Formation in 7075 Aluminum Alloy

1. From the perspective of physical metallurgy, the fundamental conditions for the formation of solidification cracks (hot cracks) are mainly twofold: first, the alloy must have a relatively wide solidification temperature range, resulting in a prolonged solid–liquid coexistence stage during the final phase of solidification; second, the tensile stress generated by solidification shrinkage must exceed the bonding strength of the residual liquid film at grain boundaries.

2. According to classical criteria for evaluating hot cracking susceptibility in aluminum alloys, the total alloying element content is a key indicator determining crack sensitivity. Industrial research data show that the hot cracking susceptibility of 7000-series aluminum alloys increases with the total content of Zn + Mg + Cu. In 7075 alloy, the combined content of these three elements reaches approximately 8%–10%, making it a highly concentrated alloy system and the fundamental reason for its strong crack susceptibility.

For comparison, the total alloying element content in 1xxx and 3xxx series aluminum alloys is generally below 1.5%, resulting in extremely low crack sensitivity. 6xxx series alloys typically contain less than 2.5% alloying elements and exhibit only slight susceptibility. In contrast, 7075 alloy contains 8%–10% Zn + Mg + Cu, far exceeding the safe range. The combination of high alloy concentration, segregation effects, enrichment of low-melting eutectic phases, high solidification shrinkage stress, and a wide solidification temperature range makes 7075 one of the most hot-crack-sensitive grades among commercial aluminum alloys.

3. The alloy composition provides the intrinsic basis for the high crack susceptibility of 7075 aluminum alloy. However, the direct causes of crack initiation, propagation, and formation are the evolution of microstructure, stress development, and liquid film fracture during non-equilibrium solidification. Industrial semi-continuous casting of aluminum alloys occurs under rapid non-equilibrium solidification conditions, which differ significantly from theoretical equilibrium solidification. This process amplifies the negative effects caused by alloy composition and drives the entire process of crack incubation, initiation, and propagation.

4. When 7075 aluminum alloy melt enters the casting mold, the surface melt undergoes rapid undercooling and preferentially nucleates to form fine equiaxed grains. Meanwhile, the core region cools more slowly and experiences a stronger temperature gradient. Under the influence of the highly alloyed composition, coarse columnar dendrites preferentially grow in the center.

Unlike ordinary aluminum alloys with fine and relatively uniform equiaxed grain structures, the dendrite arms formed in 7075 alloy are coarser, with wider interdendritic regions. Columnar grains grow directionally along the heat flow direction and remain relatively parallel to each other. The grains lack interlocking structures and mechanical constraints, resulting in relatively straight and continuous grain boundaries.

This microstructural feature creates a critical defect condition: tensile stresses generated during solidification shrinkage cannot be effectively relieved through grain rotation, deformation, or grain boundary displacement. Instead, plastic deformation becomes concentrated at straight grain boundaries and interdendritic regions, providing favorable conditions for crack initiation.

Meanwhile, rapid non-equilibrium solidification causes severe solute redistribution. Zn, Mg, and Cu atoms cannot dissolve uniformly into the aluminum matrix and are continuously rejected by the advancing solid–liquid interface, accumulating in interdendritic regions and grain boundaries. This segregation provides the foundation for subsequent precipitation of large amounts of low-melting eutectic phases.

5. Ordinary aluminum alloys generally possess narrower solidification ranges, shorter solid–liquid coexistence periods, and only a small amount of dispersed residual liquid at grain boundaries. In contrast, 7075 alloy has an extremely wide solidification range of approximately 40–55°C, causing the alloy to remain in a semi-solid state with around 80% solid phase and 20% liquid phase for an extended period.

As solidification proceeds, the aluminum matrix crystallizes first and forms a rigid dendritic skeleton. Meanwhile, high concentrations of Zn, Mg, and Cu enriched in interdendritic regions and grain boundaries continue to form large amounts of low-melting multi-component eutectic phases.

These eutectic phases have significantly lower melting points than the aluminum matrix. Even after the matrix has fully solidified, they may remain in a liquid or semi-liquid state, forming continuous and fragile liquid film networks along grain boundaries and between dendrites.

This liquid film represents a typical mechanical weak zone. At elevated temperatures, it has almost no strength or ductility and relies mainly on liquid surface tension to maintain grain bonding. As a result, the casting structure becomes a combination of a strong solid dendritic skeleton and weak liquid grain-boundary films. Once shrinkage stress develops, these liquid films become the primary sites of failure.

6. During the middle and late stages of solidification, the casting temperature continues to decrease. The solid dendritic skeleton undergoes thermal contraction, while the grain-boundary liquid films cannot contract synchronously. This mismatch generates strong high-temperature tensile stresses between adjacent grains.

The extremely high solidification shrinkage of 7075 alloy causes interdendritic tensile stresses to accumulate rapidly. When the tensile stress exceeds the bonding strength of the grain-boundary liquid film, the continuous brittle liquid films fracture first, generating large numbers of micron-scale and nanoscale cracks at interdendritic regions and triple junctions.

Compared with other aluminum alloys, crack initiation in 7075 alloy is characterized by a wider distribution range, higher crack density, and stronger randomness. In conventional alloys, dispersed liquid phases and lower stress levels allow liquid feeding and backfilling to compensate for shrinkage. However, in 7075 alloy, continuous liquid films, severe stress concentration, and poor liquid feeding capability prevent effective healing. Once cracks initiate, they cannot be repaired.

7. During the final stage of solidification, the entire casting continues to cool. The matrix becomes completely solidified, plasticity decreases further, and the residual liquid phase at grain boundaries is gradually exhausted.

The initially dispersed interdendritic microcracks rapidly connect, extend, and grow under the combined effects of solid-state shrinkage stress and casting mechanical stress. The coarse columnar grains and straight grain boundaries in 7075 alloy provide little resistance to crack propagation, allowing microcracks to quickly develop into millimeter-scale or even centimeter-scale macroscopic hot cracks.

At the same time, local compositional segregation produces heterogeneous microstructures and localized stress concentration, further accelerating crack growth. Ultimately, typical intergranular solidification hot cracks are formed.

8. After the casting completely cools to room temperature, the grain structure becomes fully stabilized, and crack surfaces are completely separated from any liquid-phase protection. These cracks cannot be eliminated through subsequent processing.

Unlike defects such as porosity and shrinkage cavities, which may be partially improved by heat treatment, solidification hot cracks in 7075 alloy are irreversible structural defects characterized by intergranular fracture caused by the coupling of microstructure and stress. They directly result in rejection of the ingot and prevent its use in high-performance structural applications.

9. In comparison, the solidification temperature range of 3003 aluminum–manganese alloy is approximately 20°C, while that of 6061 aluminum–magnesium–silicon alloy is around 25°C. Their solid–liquid coexistence periods are short, and continuous grain-boundary liquid films rarely form.

By contrast, 7075 alloy has a solidification range as wide as 45–55°C, with a solid–liquid coexistence time more than twice that of conventional alloys. The extended existence of liquid films greatly enlarges the crack incubation window.

Conventional aluminum alloys generally contain fewer grain-boundary eutectic phases, and no continuous brittle liquid-film network develops. A small amount of residual liquid can flow freely during the final stage of solidification to compensate for shrinkage gaps.

However, in 7075 alloy, grain boundaries are filled with continuous low-melting brittle eutectic phases. The liquid phase has poor fluidity and almost no plasticity, making feeding and compensation difficult. Under stress concentration, cracking becomes inevitable.

Meanwhile, conventional aluminum alloys usually have solidification shrinkage below 5%, resulting in relatively low internal stresses and weak grain-to-grain tensile forces. In contrast, 7075 alloy exhibits shrinkage exceeding 7%, generating high and rapidly accumulating shrinkage stresses that easily surpass grain-boundary strength.

Conventional aluminum alloys tend to form fine equiaxed grains with tortuous grain boundaries, effectively hindering crack propagation. 7075 alloy, however, tends to form coarse columnar grains with straight and continuous grain boundaries, allowing cracks to propagate rapidly.

Therefore, 7075 aluminum alloy exhibits far higher solidification crack susceptibility than conventional aluminum alloys, which is the fundamental reason for its demanding casting process requirements and relatively low yield rate. Its cracks typically appear in network-like or dendritic patterns, with significantly more cracks occurring in the core region than near the surface. This distribution is consistent with the slower cooling rate, longer solidification duration, and stronger stress concentration in the core region.

10. Considering these characteristics, the production of 7075 aluminum alloy requires strict control of casting temperature, casting speed, and cooling water flow. Low-superheat and slow-speed casting processes should be adopted. Alloy composition must be precisely controlled, impurity elements strictly limited, and melt purification and grain refinement processes carefully optimized to prevent segregation.

Only through comprehensive control of composition, melt quality, solidification conditions, and casting parameters can stable continuous casting production and high-quality 7075 aluminum alloy products be achieved.