The amount of strontium modified is 0.02%~0.06%, the upper limit is taken for sand casting, and the lower limit is taken for the metal type. Strontium is often added in the form of Al-Sr master alloy containing 3%, or added in the form of SrF2+SrCl2 mixed salt form is added.
After the deterioration, it cannot be refined with chlorine salt to avoid the formation of SrCl2, and the strontium deterioration effect is lost. Only argon can be used for refining.
The boiling point of strontium is 1380℃, which is much higher than that of sodium, and it is not easy to burn. The effective time for the deterioration of sand casting can reach 6~8h, so it is called long-acting modifier.
Pure strontium is extremely active. It is strongly oxidized to white powder SrO in the atmosphere. It will decompose hydrogen when it meets water. It must be stored in kerosene or paraffin oil. It should be cleaned with acetone before use and immediately put into the aluminum liquid. The disadvantage of strontium deterioration is SrH exists in strontium, it is not easy to remove hydrogen, and it is easy to produce mold reaction, often forming pinholes in castings.
Al-Sr master alloy deterioration is suitable for occasions with long pouring time.
The Al-Sr master alloy modification treatment is generally carried out after refining. The melting point of the modifier should preferably be between the modification temperature and the pouring temperature. It is in a liquid state during the modification treatment, which is conducive to the progress of the reaction, and has become a very thick slag during pouring, which is conveniently stripped off, can not become slag.
The higher the deterioration temperature, the faster the reaction speed between the molten aluminum and the deterioration agent, and the shorter the required deterioration time. The specific time is determined by experiments.
