High cost-effective refining solution for casting aluminum alloys - AlLa3B1.5 grain refiner

AlLa3B1.5 is a high-efficiency aluminum grain refiner suitable for Al-Si casting alloys, which has a strong ability to refine α-Al in Al-Si casting alloy, and can strengthen the modification effect of Al-Sr alloys on β-Si phase to a certain extent. Its heterogeneous nucleation core LaB₆ is a stable compound composed of rare earth elements La and B elements, with high melting point (2715 ° C), high hardness, low thermal expansion rate, and stable chemical properties. It can significantly enhance the wear resistance and hardness of Al-Si casting alloys, improve fluidity, and enhance mechanical properties.

The refining mechanisms of AlLa3B1.5: Primary aluminum can nucleate and grow directly on the surface of LaB₆ particles. In this nucleation mode, the degree of mismatch between the two phases and the interface state significantly affect the refinement behavior. If the primary aluminum solubilizes the elements with larger lattice constants, such as Mg, Sr, and Y, it will increase the lattice constant of primary aluminum compared with pure aluminum, resulting in a smaller mismatch between LaB₆ and Al, and the smaller interfacial energy between LaB₆ and Al during nucleation. So that the nucleation undercooling required by LaB₆ will be lower, and the refining ability of LaB₆ will be stronger under certain other conditions. Adding elements with small lattice constants, such as Cu, Fe, Zn, will bring the opposite influence.

     However, the nature of this interaction has not prevent AlLa3B1.5 being used successfully in plant trials besides of Al-Si casting alloys, even on the alloys with high Cu content. It has been proved that, on 211Z alloy (a kind of AlCu5Mn alloy, similar with ZL204A/ZL205A), AlLa3B1.5 can optimize the electric conductivity and improve the mechanical prolerties, even though its refinging ability shows not so good as its effects on the Al-Si alloys.

The difference between AlLa3B1.5 and Al-Ti-B grain refiner (AlTi5B1):

1. The refinement mechanism is different:

The nucleation core of the AlLa3B1.5 grain refiner, LaB₆, has a good crystallographic orientation relationship with Al, so that primary aluminum can directly nucleate and grow on the surface of LaB₆ particles. The effective nucleation cores of AlTi5B1 are TiAl₃ and TiB₂. TiB₂ cannot directly nucleate with Al. It can effectively nucleate with Al, thereby to refine the grains, only if its wettability with α-Al has been improved under its peritectic reaction with TiAl₃.

2. On the high Si content alloys, Al-Ti-B grain refiners will be poisoned to decrease the refining ability. But AlLa3B1.5 grain refiner will not:

The free Ti dissolved by TiAl₃ in the Al-Ti-B grain refiner will form TiSi₂ and Ti₅Si₃ with Si, and coat the surface of TiB₂, so that TiB₂ loses its nucleating effect; at the same time, TiAl₃ in AlTi5B1 will also react directly with Si to form TixAlySiz compound, to affect the refinement. However, there is no similar poisoning phenomenon in AlLa3B1.5 grain refiner.

3. AlLa3B1.5 grain refiners will affect the modification of Al-Sr alloys, but AlLa3B1.5 grain refiner will strengthen the modification of Al-Sr alloys:

When Al-Sr alloy is used for modification treatment, AlB₂ in AlTi5B1 will form SrB compound with Al-Sr alloy, thereby weakening the modification effect of Al-Sr alloy. The LaB₆ in the AlLa3B1.5 grain refiner itself has a certain modification effect on β-Si phase, and will not react with the Al-Sr alloy, which will further strengthen the modification effect of the Al-Sr alloy or reduce the use amount of the Al-Sr alloy.

4. AlLa3B1.5 grain refiner is more beneficial for melt purification:

Under the premise of the same amount of addition, the number of TiB₂ particles in the AlTi5B1 grain refiner are nearly twice that of LaB₆ in the AlLa3B1.5 grain refiner; and the nucleation rate of LaB₆ in the Al-Si alloys is higher, so the redundant particles in the AlLa3B1.5 grain refiner are fewer, less risk of carrying inclusions and gases, and more conducive to melt purification.

5. AlLa3B1.5 grain refiner has longer refining duration than ordinary grain refiners:

Because TiB₂ is easily aggregated and precipitated, the refining effect of the Al-Ti-B grain refiners decline obviously in about 30-60 minutes, which affects the mechanical properties and related quality tability of the products casted by the latter part of the aluminum melt ladle. While in the AlLa3B1.5 grain refiner, LaB₆ does not aggregate and precipitate in the Al-Si alloy; and its refining effect can last for 5 hours without decay, which can further optimize the casting production process and stabilize the product quality.

6. AlLa3B1.5 grain refiner can reduce costs and increase efficiency:

Compared with AlLa3B1.5 grain refiners, the use of AlLa3B1. grain refiner can greatly reduce the use amount of the grain refiners and modifiers in per ton aluminum melt, on the basis of maintaining the same mechanical properties. From the perspective of material cost alone, considerable cost savings can be achieved. At the same time, the Al-Si alloy refined by the AlLa3B1.5 grain refiner has a higher degree of spheroidization of the grains, which is conducive to the further improvement of the product processing performance and mechanical performance stability.

On the other hand, when the related product performance needs to be improved, AlLa3B1.5 grain refiner can obviously bring more feasibility.