How to measure and reduce the gas content in aluminum liquid

How to measure and reduce the gas content in aluminum liquid
When the gas content of aluminum liquid is higher than 0.2ml/100g, the risk of shrinkage and porosity increases significantly, and the use of a hydrogen analyzer combined with rotating degassing is a conventional control method.
Criteria for determining gas content
The gas content of aluminum alloy melt is often expressed in ml/100g. The control targets at the casting site are mostly below 0.15-0.2ml/100g.
Exceeding 0.25ml/100g, the porosity tendency of the casting significantly increases, and the airtight components are particularly sensitive.
The lower the value, the better. Excessive degassing will increase costs and also take away some alloying elements.
Method for measuring hydrogen
The commonly used methods are the vacuum solidification method (Straube Pfeiffer) and the hydrogen analyzer method. The former is intuitive, while the latter is quantifiable.
Take a small cup of aluminum liquid using the vacuum solidification method, solidify it under reduced pressure, and observe the pores on the cross-section of the sample. Easy to operate, but biased towards qualitative.
Hydrogen meters are mostly based on electrochemical or circulating gas principles, with fast readings and suitable for quick judgment in front of the furnace.
Degassing process
Rotating degassing is the mainstream. The graphite rotor is immersed in aluminum liquid, and argon or nitrogen gas is introduced to remove hydrogen with bubbles.
The rotor speed is usually 300-500rpm, the gas flow rate is 10-25L/min, and the processing time is 5-15 minutes.
Control the processing temperature between 700-730 ℃. The temperature is too low, the viscosity of the aluminum liquid is high, and the degassing efficiency decreases.
Standing still and covering
After degassing, let it stand for 10-20 minutes to allow the inclusion to float up. Insufficient settling time may cause inclusions to enter the casting.
Liquid level covering agent can reduce secondary inhalation. The amount of covering agent is calculated based on the area of the furnace mouth, and a thin layer is sufficient.
The shorter the opening time of the furnace door, the better. Every time it is turned on, moisture is introduced, increasing the hydrogen content.
Control of raw materials
A high proportion of waste will bring in moisture and oil stains. It is recommended to control the recycled material at 30% -50% and perform preheating and dehumidification.
Preheating aluminum ingots and recycled materials to above 200 ℃ before entering the furnace can significantly reduce the amount of moisture introduced.
Xinfeng Mold suggests that customers include the gas content in the process control point for the wheel hub mold project, and monitor it in parallel with mold temperature and pressure.
Corresponding to defects
The pores are circular in shape, with smooth inner walls, and are mostly distributed on the surface of the casting. Unlike the form of shrinkage, shrinkage is sponge like.
Pores and shrinkage often occur simultaneously, forming "gas shrinkage pores". The difficulty of interpreting such defects is greater.
After the decrease in gas content, the number of pores decreases but shrinkage may still occur. The two should be governed separately.
The hydrogen analyzer is calibrated before each use, and the aging of the probe can cause the readings to be systematically low.
After degassing, do not let the aluminum liquid sit for a long time. If it exceeds 40 minutes, it will inhale again.
The degassing record should clearly state the gas flow rate and processing time, and only indicate that the degassing has been carried out and the effect cannot be traced.
Frequently Asked Questions
Question: What is the gas content controlled to?
Answer: It is recommended to use 0.15-0.2ml/100g or less.
Question: How to measure gas content?
Answer: Vacuum solidification method or hydrogen analyzer, quick judgment in front of the furnace.
Q: How to determine the parameters for rotary degassing?
Answer: Speed 300-500rpm, gas 10-25L/min.
Q: Do I need to let it sit still after degassing?
Answer: Yes, let it stand for 10-20 minutes to let the mixture float up.
Question: What is the proportion of recycled materials?
Answer: It is recommended to use 30% -50% preheating and dehumidification.
Question: How to distinguish between pores and shrinkage?
Answer: The pores are round and smooth, with a sponge like shrinkage.
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