Effect of Insulation and Static Time on the Uniformity of Aluminum Melt and Ingot Quality-3

Differential effects of different insulation and static durations on the uniformity of aluminum alloy melt

1. Insufficient settling time (<20 minutes): Significant non steady state characteristics of the melt
Insufficient insulation and settling time is one of the main causes of substandard melt quality in industrial production. After refining and stirring, the molten material is poured directly. The element diffusion, temperature balance, and impurity purification have not yet been completed, and the overall state is highly unstable with extremely poor uniformity. The specific defect characteristics are reflected in three aspects.
At the level of compositional uniformity, the diffusion of alloy elements is insufficient under short-term standing, and the concentration gradient inside the melt is not eliminated. There is enrichment of silicon, magnesium, copper and other elements in local areas, and the macroscopic compositional deviation can reach ± 0.05%~± 0.15%, forming grain boundary element segregation zones at the microscopic level. For commonly used deformed aluminum alloys such as 6061 and 6063, the magnesium silicon enriched areas are prone to the formation of coarse intermetallic compounds, while the alloying degree in the element depleted areas is insufficient, directly leading to inconsistent melt composition and posing hidden dangers for ingot composition segregation and mechanical property dispersion.
In terms of temperature uniformity, insufficient settling time cannot eliminate the temperature gradient inside the melt, and the temperature difference between the surface and center of the melt remains at 8-15 ℃. The overall temperature fluctuation of the melt is large and the stability is poor. The uneven temperature field leads to significant differences in viscosity and flowability in different regions of the melt. The low-temperature region has poor flowability and weak shrinkage ability, while the high-temperature region has high superheat and strong driving force for grain growth, resulting in uneven grain growth and chaotic solidification rhythm during the subsequent solidification process.
In terms of purity, insufficient standing time cannot meet the separation and escape requirements of fine inclusions and gases. The residual micro scale alumina inclusions and small slag particles after refining cannot fully settle and float, and the supersaturated hydrogen gas is difficult to completely escape. A large number of hidden impurities and bubbles are uniformly retained throughout the melt. Such minor defects cannot be removed through conventional slag and gas removal processes, and will directly accompany the melt into the casting process, becoming the core source of micro porosity and dispersed inclusion defects in ingots.
2. Optimal settling time (20~60min): The melt has the best overall uniformity
According to the differences in alloy grade, melt capacity, and furnace working conditions, a 20~60 minute insulation and static interval can achieve comprehensive balance optimization of aluminum alloy melt composition, temperature, and purity, which is the optimal process interval for industrial production. Different alloys have significant differences in their adaptation and subdivision intervals. Low alloy pure aluminum melt has fewer impurities and lower element diffusion resistance, and the optimal static range is 20-30 minutes; 6 series and 5 series conventional deformed aluminum alloys have complex compositions and many alloy elements, and the optimal static range is 30-45 minutes; ADC12 and other high silicon die cast aluminum alloys, as well as high alloying aluminum alloys, have a high content of refractory elements and are prone to inclusions retention. The optimal static range is 45-60 minutes.
Under this static interval, the uniformity of the melt is comprehensively improved. At the compositional level, sufficient diffusion time enables various alloy elements to fully migrate and fuse, with a concentration deviation of ± 0.03% for the entire region. Macroscopic and microscopic component segregation is basically eliminated, and the distribution of alloy components is uniform and stable, providing a core guarantee for the uniformity of ingot structure. At the temperature level, the melt achieves global temperature balance through sufficient heat conduction, with a temperature difference controlled within 3 ℃. The melt has stable superheat and consistent rheological properties, completely solving the problem of uneven solidification caused by temperature fluctuations.
At the level of purity, the gradient settling time can be adapted to the impurity and gas removal requirements of different alloys. Light oxide inclusions and heavy metal slag are fully floated and settled, and the content of inclusions inside the melt is significantly reduced; Fine hydrogen bubbles continue to accumulate and float, reducing the hydrogen content in the melt to the lowest level, achieving a steady-state high-purity purity of the melt. At the same time, the static duration can avoid the problem of secondary pollution of the melt. Under the inert protective atmosphere inside the furnace, the melt has no obvious secondary oxidation or element burning, perfectly balancing uniformity and purity. Industrial data shows that the purity qualification rate of aluminum alloy melt under the optimal static interval can reach over 99%, laying the foundation for high-quality ingot production.
3. Excessive static time (>60 minutes): Reverse deterioration of melt uniformity
The behavior of blindly extending the insulation and settling time in industrial production in pursuit of ultimate purity can lead to a reverse decline in the uniformity of the melt, continuous deterioration of metallurgical quality, and the longer the settling time, the more significant the deterioration effect, mainly reflected in three negative mechanisms.
One is the burning and compositional shift of alloy elements. Under high temperature and constant temperature static environment, volatile and easily oxidizable alloy elements such as magnesium and zinc will continue to undergo oxidation and burn damage. Prolonged static time can lead to a continuous decrease in the content of such elements, breaking the preset alloy ratio and causing the overall composition of the melt to shift, resulting in unqualified composition problems. Meanwhile, long-term standing will cause some refractory elements to continue to accumulate at the bottom of the furnace, forming secondary component stratification and inducing macroscopic component segregation again.
The second is secondary oxidation of the melt and regeneration of inclusions. Even under a protective atmosphere, prolonged high-temperature standing can still cause the surface of the melt to react with trace amounts of oxygen, continuously generating new fine alumina inclusions. This type of newly formed inclusion has a smaller particle size and stronger dispersibility, making it difficult to remove through conventional slag removal processes. It will be evenly distributed inside the melt, greatly increasing the density of inclusion defects and deteriorating the purity of the melt.
The third is gas re dissolution and melt aging. The solubility of hydrogen in aluminum alloy melt gradually increases with prolonged standing time. Prolonged standing can cause hydrogen atoms produced by the decomposition of moisture in the air to continue to integrate into the melt, resulting in the re emergence of supersaturated hydrogen content in the degassed melt and an increase in the risk of porosity defects. Meanwhile, long-term constant temperature standing can lead to atomic agglomeration inside the melt, forming a large number of grain precursors, resulting in a decrease in the rheological properties and flowability of the melt. During the solidification process, the grains are prone to abnormal coarsening, which can damage the solidification stability of the melt. Research has shown that after the aluminum alloy melt is left to stand for more than 8 hours, the modification effect of the melt significantly deteriorates, the porosity of the microstructure increases significantly, and the metallurgical quality deteriorates completely.