Effect of Insulation and Static Time on the Uniformity of Aluminum Melt and Ingot Quality-2

1. Composition homogenization: Mechanism of element diffusion and concentration gradient elimination During the alloying process of aluminum alloy melting, alloy elements such as copper, magnesium, silicon, zinc, etc. are prone to form concentration gradients in local areas of the melt due to differences in melting point, density, and diffusion rate, resulting in high concentration enriched areas and low concentration depleted areas, leading to initial component segregation. During the insulation and static stage, a constant temperature environment provides stable thermodynamic conditions for the diffusion of alloy elements. According to Fick's diffusion law, alloy atoms in high-temperature melts will spontaneously migrate from high concentration regions to low concentration regions. As the static time increases, the concentration gradient of each element inside the melt gradually weakens until global equilibrium is reached.

At the same time, after the melting and stirring are completed, there is a brief residual fluid turbulence inside the melt. Short term standing can eliminate the difference in fluid flow velocity and avoid dynamic component stratification. For high alloy aluminum alloys, the diffusion rate of refractory elements is slow, and the required settling time for uniform composition is longer; For low-alloy and pure aluminum melts, the diffusion resistance of elements is low and the homogenization rate is faster. Reasonable settling time can completely eliminate macroscopic component segregation and microscopic concentration unevenness of the melt, ensure the consistency of the overall composition of the subsequent ingot, and avoid defects such as macroscopic segregation and banded structure of the ingot from the source.

2. Temperature homogenization: In the process of melting and refining, there are significant temperature gradients in the furnace wall, melt surface, and melt center due to the balance mechanism of heat conduction and temperature gradient. The contact between the melt surface and the furnace wall dissipates heat quickly and the temperature is low, while the heat concentration and temperature in the central area are high, and the local temperature difference can reach 10-25 ℃. Uneven temperature can lead to regional differences in melt viscosity and flowability, directly disrupting the orderliness of the solidification process. During the insulation and static process, the constant temperature control system of the furnace continuously compensates for heat and achieves temperature balance through internal heat conduction of the melt.

In the initial stage of settling, the temperature difference between the melt is large and the heat conduction rate is fast; As the settling time increases, the overall temperature gradually becomes consistent, and the overall temperature stability of the melt is greatly improved. Adequate settling time can eliminate radial and longitudinal temperature gradients of the melt, ensuring uniform and constant temperature of the melt entering the crystallizer, avoiding problems such as uneven solidification rate, stress concentration, and unbalanced shrinkage caused by temperature fluctuations, and laying the foundation for uniform solidification of the ingot.

2.3 Homogenization of Purity: Mechanism of Inclusion Settlement and Gas Release
After refining the aluminum alloy melt, there are still a small amount of fine alumina inclusions, slag particles, and supersaturated hydrogen gas remaining, which are the core hidden dangers that induce porosity, inclusions, and micro shrinkage in the ingot. Thermal insulation and settling is a key process for self purification of molten materials, which relies on a dual mechanism of gravity settling and buoyancy sorting. Metal inclusions and heavy slag with a density greater than that of aluminum melt will slowly settle to the bottom of the furnace under the action of gravity; Lightweight oxide inclusions and fine slag with a density lower than that of the melt will float upwards to the surface of the melt, forming floating slag for subsequent removal; The supersaturated hydrogen gas inside the melt will accumulate and form tiny bubbles, which will continue to float and escape from the surface of the melt.

The above purification processes require sufficient time guarantee. Insufficient standing time can lead to fine inclusions and microbubbles remaining inside the melt, which cannot be completely removed; Moderate standing can achieve sufficient separation of impurities and gases in the melt, achieve uniformity of overall purity, and effectively reduce the occurrence rate of hidden defects in the ingot.