The influence of melting temperature does not exist in isolation, but works together with the insulation time to determine the final quality of the melt. Simply controlling the maximum temperature while ignoring the high temperature residence time cannot achieve stable melt quality.
The dual effect of insulation duration on melt quality
The core function of insulation in the melting process is to ensure uniform temperature of the melt, sufficient diffusion of alloy elements, and sufficient time for inclusions and bubbles to float and separate. Extending the insulation time appropriately within a reasonable temperature and duration range is beneficial for improving the uniformity of components and the cleanliness of the melt.
But the longer the insulation, the better. When the insulation time exceeds the critical value, the quality of the melt will continue to deteriorate: on the one hand, prolonged insulation will allow the oxidation reaction to continue, causing the accumulation of oxidation inclusions and increasing aluminum loss; On the other hand, if the melt comes into contact with the furnace gas for a long time, the hydrogen content will continue to increase, especially in humid environments. After 2 hours of insulation, the hydrogen content can increase by more than 30% compared to the end of insulation. In addition, prolonged high-temperature insulation can lead to continuous burning and volatilization of alloy elements, resulting in compositional deviations.
For most aluminum alloys, the optimal static insulation time after refining is 15-30 minutes. If it is less than 10 minutes, the slag and bubbles will not float sufficiently, and if it exceeds 40 minutes, the quality of the melt will begin to decline, resulting in the abnormal phenomenon of "more defects after standing for a longer time".
2. The principle of coordinated matching between temperature and insulation duration
The core principle of coordinated control of temperature and insulation duration is short stay at high temperature and long insulation at low temperature. Through the reasonable matching of the two, the negative impact of high temperature is minimized while ensuring uniform composition and smooth production.
Firstly, strictly control the residence time during the high temperature stage. In order to improve efficiency during the melting stage, the temperature can be set near the upper limit of the range. However, after the furnace material is completely melted and the composition adjustment is completed, it must be immediately cooled to the refining temperature to reduce the total residence time of the melt in the high-temperature zone. It is strictly prohibited to wait at high temperatures for a long time after melting is completed, which is the core reason for high aluminum loss and excessive hydrogen content in many workshops.
Secondly, the insulation can be appropriately extended in the medium and low temperature range. When the melt temperature is controlled at the lower limit of the process range and the oxidation and hydrogen absorption rates are low, the static time after stirring can be appropriately extended to allow the slag and bubbles to fully float up, which is actually beneficial for improving the quality of the melt. For example, when a 5-series alloy is left to stand at 710 ℃ for 30 minutes, its melt cleanliness is much higher than when it is left to stand at 740 ℃ for 10 minutes.
Thirdly, control the total heat exposure time. The cumulative effect of "temperature x time", i.e. the total heat exposure, has an impact on the quality of the melt. If the short-term high temperature shock rapidly cools down, its harm may be less than that of long-term medium temperature insulation. Therefore, production scheduling should minimize the waiting time of the melt in the furnace as much as possible, achieve the goal of "refining and casting as soon as it is melted", and shorten the total duration of the entire process.
3. Segmented temperature control throughout the entire melting process
To achieve precise temperature time coordinated control, the entire smelting process needs to be divided into different stages and differentiated temperature and duration control should be implemented.
3.1 Melting period
After the loading is completed, the temperature starts to rise, with the goal of quickly melting the furnace material and shortening the high-temperature oxidation time of solid metals. The temperature is controlled at the upper limit of the melting temperature range, and the first stirring is carried out when the furnace material is melted by 70%~80%, breaking the local high temperature zone and avoiding overheating of the furnace bottom melt. The total melting time is controlled according to the furnace capacity. A 10 ton melting furnace is usually controlled within 2-3 hours to avoid prolonged heating.
3.2 Composition adjustment period
After the furnace charge is completely melted, immediately cool it down to the composition adjustment temperature (usually 10-20 ℃ lower than the melting temperature), take samples for spectral analysis, and add alloying elements based on the results. Stir evenly when adding, then keep warm for 5-10 minutes until the ingredients are uniform, and take another sample for confirmation. High temperature is strictly prohibited during this stage to avoid significant burning of newly added alloy elements.
3.3 Refining period
Adjust to the refining temperature of the corresponding alloy, with temperature fluctuations controlled within ± 10 ℃, and refine strictly according to the required duration of the process. Immediately start cooling after refining, reducing the temperature of the melt to the casting temperature range without high temperature retention.
3.4 Static casting period
Keep at room temperature for 15-30 minutes, during which frequent stirring and uncovering are strictly prohibited to reduce contact between the melt and air. After settling, transfer to casting as soon as possible to avoid quality degradation caused by prolonged insulation waiting.
In addition, it is necessary to regularly calibrate the furnace thermocouple and detect the actual temperature of the melt to avoid the hidden problem of "normal display temperature but actual melt overheating" caused by thermocouple deviation. At the same time, ensure uniform heating of the furnace body and avoid local area temperatures far exceeding the average temperature, resulting in hidden overheating.