Effect of Insulation and Static Time on the Uniformity of Aluminum Melt and Ingot Quality-4

The influence of insulation and static time on the quality of aluminum alloy ingots

The uniformity of the melt is a decisive prerequisite for the quality of the ingot. The differences in the state of the melt caused by different insulation and static periods will be directly transmitted to the three core dimensions of the ingot microstructure, mechanical properties, and defect state, forming differentiated characteristics of ingot quality. This article combines the actual production of DC casting and systematically analyzes the evolution law of ingot quality corresponding to each static interval.

1. Influence on the microstructure of ingots
When the settling time is insufficient, the problem of uneven melt composition and temperature is directly inherited into the ingot structure. Local element segregation in the melt can lead to differential grain growth in the ingot, with rapid grain growth in areas of element enrichment and delayed grain nucleation in areas of element depletion. Ultimately, a microstructure with uneven grain size and disordered morphology is formed, accompanied by a large number of intergranular segregation phases and coarse intermetallic compounds. The residual temperature gradient will cause significant differences in solidification rate in different regions of the ingot, with large differences in grain size between the surface and center, and extremely poor consistency in microstructure. In addition, the residual fine inclusions in the melt will serve as heterogeneous nucleation cores, disrupting the orderly growth pattern of grains and further exacerbating tissue disorder.

The molten state is balanced and stable under the optimal settling time, and the corresponding microstructure of the ingot reaches the optimal state. The uniform distribution of components ensures that the grain nucleation and growth rhythm are unified throughout the entire region, with small and uniform grain size, regular morphology, and no coarse grains or mixed crystal structures; A stable melt temperature ensures that the solidification process of the ingot is orderly and controllable, with clear and smooth grain boundaries and no obvious intergranular segregation phases; The high-purity melt effectively avoids the interference of inclusions and pores on grain growth, and the ingot structure is dense with very few defects. The uniformity and stability of the microstructure are greatly improved, providing a tissue foundation for excellent mechanical properties.

When the static time is excessive, problems such as melt aging, element burning, and secondary inclusions can lead to continuous deterioration of the ingot structure. The burning of vulnerable elements leads to insufficient content of strengthening phases in the alloy and a decrease in grain boundary strength; The fine inclusions generated by long-term static placement are dispersed at grain boundaries, which can sever the continuity of the matrix and form a large number of micropores; The decrease in melt fluidity leads to insufficient solidification and shrinkage, resulting in an increase in intergranular porosity defects. At the same time, the agglomeration of grain precursors causes the grains of the ingot to become abnormally coarse after solidification, resulting in a significant decrease in the density of the microstructure, a significant increase in the density of microscopic defects, and a complete destruction of the uniformity of the microstructure.

2. Influence on the mechanical properties of ingots
The stability and consistency of the mechanical properties of ingots depend entirely on the uniformity of the melt and the microstructure of the ingot. Ingots prepared with insufficient standing time exhibit significant mechanical properties variability due to uneven microstructure and numerous intergranular defects. The strength and plasticity of the coarse grain area are low, while the performance of the fine grain area is high. There is a significant difference in the longitudinal and transverse mechanical properties of the ingot. The fluctuation range of tensile strength and yield strength can reach 10%~15%, and the elongation rate attenuation is obvious, which cannot meet the performance consistency requirements of high-end processed products. Meanwhile, intergranular inclusions and micro pores can form stress concentration points, significantly reducing the fatigue resistance and impact toughness of the ingot.

The ingot prepared by the optimal static process has the best mechanical properties and extremely strong stability. Uniform and dense microstructure, defect free matrix structure, enable the tensile strength, yield strength, and elongation of the ingot to reach the standard optimal values, and the batch fluctuation amplitude is controlled within 3%. The uniformity of the organization eliminates weak areas of local performance, significantly reduces the anisotropy of the ingot, and greatly improves the consistency and reliability of mechanical properties. It is fully compatible with subsequent pressure processing processes such as rolling, extrusion, and forging, and can effectively avoid problems such as processing cracking and delamination.

Excessive static time can lead to continuous degradation of the mechanical properties of the ingot. Elemental burning causes a decrease in the strengthening effect of the alloy, resulting in a significant reduction in the strength and hardness of the matrix; The intergranular looseness and inclusion defects damage the continuity of the matrix, and the stress concentration effect is significant, resulting in a significant decrease in the plasticity and toughness of the ingot; The abnormal coarsening of grains further exacerbates performance degradation, resulting in the overall mechanical properties of the ingot not meeting the standards, and the batch scrap rate significantly increases. Related experiments have shown that after excessive standing of aluminum alloy melt, the tensile strength and elongation of the ingot after T6 heat treatment decrease by more than 15% and 20% respectively compared to the optimal working condition, and the service performance of the product is significantly degraded.

3. Impact on typical defects of ingots
The imbalance of insulation static duration is an important source factor that induces defects such as cracks, shrinkage porosity, porosity, and segregation in ingots. The types and severity of defects corresponding to different static intervals vary significantly. When the settling time is insufficient, residual gases and inclusions in the melt directly form porosity and inclusion defects in the ingot, mostly dispersed micro pores and point like inclusions; Uneven composition and temperature of the melt can cause stress imbalance and uneven shrinkage during solidification, which can easily lead to micro shrinkage porosity and intergranular cracks. At the same time, it exacerbates the segregation of the ingot composition, forming defects such as band segregation and center segregation. These types of defects are mostly hidden microscopic defects that are difficult to fully detect through conventional testing, and are prone to expansion and elongation during subsequent processing, leading to batch quality issues.

The optimal settling time can suppress the majority of ingot defects from the source. The melt is thoroughly purified to eliminate hidden dangers such as pores and inclusions. The uniform composition and temperature state ensure stable stress and sufficient shrinkage during the solidification process, without segregation, shrinkage porosity, or crack propagation conditions. The internal quality of the ingot is complete, the surface is smooth, and the defect occurrence rate is minimized. Industrial practice data shows that after adopting the optimal static process, the occurrence rate of micro defects in aluminum alloy ingots is reduced by more than 90%, and the yield rate is increased by about 10%.
Excessive static time can induce complex macroscopic and microscopic defects. The large amount of fine inclusions and redissolved gases generated by the secondary oxidation of the melt will form dense pores and inclusion defects; Elemental burning and secondary segregation exacerbate the uneven macroscopic composition of the ingot, inducing severe band segregation and center segregation; The deterioration of melt flowability and insufficient solidification shrinkage ability can lead to the large-scale growth of central shrinkage porosity and intergranular porosity. At the same time, the tissue stress caused by grain coarsening can easily induce the propagation of microcracks, ultimately forming a composite defect system of "pores+inclusions+shrinkage porosity+microcracks". In severe cases, it can directly lead to the scrapping of the ingot.