Treatment of aluminum alloy liquid

Grain refinement can improve thermal cracking resistance, reduce the harmful effects of pores (making pressure-resistant castings) and redistribute shrinkage cavities in aluminum alloys. The grain size of the cast alloy depends on the number of crystal nuclei in the molten metal and the cooling rate at the beginning of solidification. The faster the cooling rate, the smaller the grain size.

Adding certain elements to the aluminum alloy liquid can provide nuclei for growing grains. Titanium, especially when acting simultaneously with boron, has a strong nucleation effect and is the most commonly used grain refiner. Titanium can also be added separately as a master alloy in an amount of 0.02~0.15%, but its grain refining effect decays within 40 minutes. Adding boron and titanium together can produce finer grains and reduce decay.

Titanium and boron can be added as a flux or as a master alloy. In the primary aluminum industry, the benefits of continuously adding TiBAl master alloy rods are well known. Most foundries use small melting furnaces and holding furnaces, so they are not suitable for continuous addition, so they need to be added in batches. Although the addition of master alloy has the advantages of precision and control, it is still widely used due to its convenient flux method and low cost.

The content of silicon in the alloy affects the effect of Ti and B on grain refinement. The higher the silicon content of the casting alloy, the greater the amount of grain refiner required to be added. The typical addition amount is as follows.

The following table shows the NUCLEANT series of grain refinement block products. Part of the salt in this type of product will absorb moisture, and it may absorb some moisture from the atmosphere when exposed to the atmosphere, which may increase the hydrogen content in the alloy. Therefore, it is recommended to first refine and then degas.

When using the rotating degassing equipment FDU, first put the self-sinking NUCLEANT sheet in the molten metal and then put the rotor into the molten metal. At this time, the grain refinement and degassing are performed simultaneously.

Grain refinement of hypereutectic alloys

Aluminum-silicon alloys with a silicon content of more than 12% are used for their wear resistance. The uniform distribution of primary silicon in the casting is important to maintain stable and consistent casting performance. Due to the large crystallization temperature range, the primary silicon particles may grow up and float. Large silicon particles are harmful to castability, machinability and mechanical properties, so the structure needs to be refined.

Hypereutectic alloys can add 0.003~0.015% of phosphorus for grain refinement (modification) treatment, and the formed aluminum phosphide can provide nucleation points for primary silicon, ensuring that there is fine grain dispersion in the eutectic matrix.

Grain refinement using master alloy

A master alloy containing titanium and boron in a ratio of 5:1 has the best effect. This master alloy is supplied as a rod material cut to length and weighing 200 grams, and it can quickly and completely melt into the molten metal. When the transfer bag is placed in the rotating degassing position, add an appropriate amount of titanium-boron rods to the molten metal. The rod material melts quickly, usually when the rotor is immersed in the molten metal, and then the grains are refined and removed. Qi goes on simultaneously. Any increase in hydrogen due to the addition of the master alloy can be removed immediately by the FDU rotary degasser. No special temperature is required for degassing, and the addition amount is as per the above table.