Introduction
In the aluminum billet casting process, surface quality directly determines the stability of subsequent processing and product grade. Common surface defects include cold shuts, scratches, segregation nodules, and oxide scale. These problems not only spoil the appearance but also accelerate die wear during extrusion, thereby shortening die life.
These defects are further worsened during surface treatments like anodizing or spraying. Causing noticeable color differences, streaks, or uneven surfaces. This not only undermines the final product quality but also reduces customer acceptance.
Therefore, aluminum billet surface quality is not just a "face-saving project". But a comprehensive reflection of the stability of the casting process and the condition of the equipment. A stable surface means a uniform solidified shell, a reasonable thermal gradient, and good lubrication.
This article will systematically analyze the core technical points for improving the surface integrity of aluminum billets.
It will cover five key dimensions: lubrication control, secondary cooling water distribution, metal level control. Matching of casting temperature and speed and compensation during the casting start and finish processes.
Key Keywords: Oil-air lubrication, graphite content, oil film uniformity
In semi-continuous casting, the lubrication system directly impacts the friction between molten aluminum and the crystallizer's inner wall. Insufficient lubrication or uneven oil film can easily cause surface defects like scratches, aluminum adhesion, and even periodic ripples.
Key control measures include:
* Selecting casting oil viscosity suitable for the bar diameter and alloy type
* Precisely adjusting the oil-air lubrication ratio to ensure stable oil supply
* Regularly cleaning the crystallizer lubrication grooves and vents to prevent blockage
* Controlling graphite content to avoid deposition affecting oil film uniformity
A good lubrication system helps form a uniform, stable solidification shell when molten aluminum starts to solidify. This reduces friction fluctuations, thereby significantly reducing scratches and aluminum adhesion. This results in a smoother and more continuous aluminum bar surface.
II. Optimizing Secondary Cooling Water Distribution: Avoiding Surface Defects Caused by Thermal Stress
Key Keywords: Water flow rate density, cooling uniformity, soft water usage
The secondary cooling zone is key to controlling the thermal stress and solidification rate of the aluminum bar surface. Uneven water flow distribution or water pressure fluctuations can easily lead to localized excessively rapid or slow cooling. This can cause cold shuts, periodic ripples, and even surface microcracks.
Optimization measures include:
Calibrating the crystallizer water hole angle to prevent direct water flow impact on the primary shell;
Adjusting water flow density to ensure uniform circumferential cooling;
Using a soft water system to reduce scaling in the water holes;
Regularly checking the spray holes to prevent blockage or flow deviation.
When cooling is uniform and stable, the surface temperature gradient of the aluminum rod is more reasonable. This effectively reduces thermal stress concentration and prevents the formation of cold shuts and microcracks. It also improves overall surface uniformity.
III. Precise Metal Level Control: Reducing Segregation and Oxide InclusionsKeywords: Liquid level fluctuation, flow control system, PLC feedback
Metal level fluctuation is a major cause of segregation bands and oxide entrapment on aluminum rod surfaces. When the liquid level changes frequently, the thickness of the primary solidified shell becomes unstable. This easily leads to the formation of bright and dark streaks or localized segregation.
Key Control Measures:
* Real-time monitoring using laser or eddy current level sensors
* Establishing a PLC closed-loop control system to stabilize the liquid level
* Controlling liquid level fluctuations within ±1mm
* Optimizing the response speed of the stopper rod or flow control system
A stable molten metal level ensures smooth solidification interface progression, reducing oxide inclusion entrapment. It can also eliminate surface bright and dark segregation bands. Resulting in a more uniform and delicate aluminum rod surface.
IV. Reasonable Matching of Casting Temperature and Speed: Controlling Solidification Structure Stability
Core Keywords: Liquidus temperature, casting speed, thermal gradient
The match between casting temperature and speed determines the solidification structure and surface quality of aluminum rods. If the temperature is too high, the solidified shell is thin and prone to hot cracking. If the temperature is too low, cold shuts and surface roughness may occur.
Optimization Control Recommendations:
* Control the casting temperature within the range of 50–70℃ above the liquidus line
* Adjust the temperature window according to the alloy type
* Optimize the casting speed according to the rod diameter
* Find a balance between production volume and surface quality
* A reasonable matching of temperature and speed can effectively control the thickness of the reverse segregation layer. It can also reduce surface cracks and cold shuts, making the aluminum rod surface more stable and uniform.
V. Process Compensation in the Casting Start and Finishing Stages: Reducing the Proportion of Head and Tail Scrap
Key Keywords: Starting Block Condition, Ingot Head Sealing, Finishing Pressure Relief
The casting start and finishing stages are usually the areas where surface defects are most concentrated. If the starting block temperature is insufficient or the seal is poor, bottom cold shuts are likely to occur. If the finishing stage is not properly controlled, top cracks or shrinkage cavities may form.
Key Control Measures:
Control the ingot head preheating temperature to ensure stable initial solidification.
Check the condition of the graphite ring and sealing ring to prevent aluminum leakage.
Slowly increase the casting speed during the casting start stage to stabilize the liquid level and cooling.
Gradually reduce the casting speed and water volume during the finishing stage to release stress.
By optimizing the casting start and finishing processes, the length of head and tail scrap can be significantly shortened. It can also increase the effective aluminum rod proportion while reducing bottom cold shuts and top cracks.
Conclusion
Improving the surface quality of aluminum rods does not depend on a single parameter. Instead, it results from the synergistic effect of multiple factors. These factors include lubrication, cooling, liquid level, temperature, and the casting process itself. From fluid stability to heat conduction control and friction condition optimization, each aspect directly impacts the final surface integrity.
To achieve a yield improvement from 70% to 98%, companies need to establish a standardized process system. This system should include regularly checking the crystallizer's wear condition, optimizing the lubrication system maintenance process, and building a complete casting parameter database. Long-term consistent production of high-quality aluminum rod surfaces relies on continuous data accumulation and process optimization.