Rare earth elements are often added to the cast magnesium alloy to change the properties of the cast magnesium alloy, such as strength, creep resistance, heat resistance, corrosion resistance, and improve the ignition temperature in the melting process.
La is widely used in casting magnesium alloy. It is generally used as grain refiner and modifier to refine the grain, improve the structure and mechanical properties of magnesium alloy. Sometimes it is also used to improve the corrosion resistance and flame retardancy of magnesium alloy during smelting.
1. Application of lanthanum in AZ91 series cast magnesium alloy
La has an obvious effect on the microstructure and hardness of AZ91 casting alloy. The results show that the as cast grains of AZ91 alloy are refined and Mg17Al12 phase is reduced after adding La, most of La and aluminum combine to form acicular rare earth phase (al11la3) with high melting point and high thermal stability, so as to improve the distribution of grain boundary phase and the macro hardness of as cast. After solid solution + aging treatment, lamellar Mg17Al12 phase is precipitated at the grain boundary, and the hardness is further increased with the extension of aging time.
In order to improve the corrosion resistance of magnesium alloy, the effect of La conversion coating on the corrosion behavior of AZ91D magnesium alloy in 5% NaCl solution was studied by capacity method and electrochemical method. The results show that the pretreatment does not change the basic law of La conversion process, but it is helpful to the deposition of La oxide and improve the corrosion resistance of La conversion film of magnesium alloy. The corrosion resistance of La conversion film after 30 h immersion is similar to that of the traditional closed CR acid salt conversion film.
The effect of adding different content of CE or LA to AZ91 cast magnesium alloy on the microstructure and corrosion resistance of the alloy was studied. The results show that the addition of rare earth Ce to AZ91 magnesium alloy can improve the distribution of β phase, refine the grains, and produce a small amount of fine needle like intermetallic γ phase (mgalre). The addition of a small amount of rare earth Ce and La can significantly reduce the rate of hydrogen evolution of AZ91 magnesium alloy, make the self corrosion potential of AZ91 magnesium alloy negative shift, self corrosion current drop, and effectively improve the corrosion resistance of AZ91 magnesium alloy.
2. Application of lanthanum in AM60 series cast magnesium alloy
The effects of CE and La alloying on the structure and corrosion resistance of AM60 cast magnesium alloy were studied by epma-eds, field emission scanning electron microscopy and X-ray diffraction. The results show that the addition of CE and La can form rare earth rich γ phase (mgalre) in AM60 magnesium alloy, which can effectively inhibit hydrogen evolution, improve the corrosion resistance of magnesium alloy and improve the corrosion resistance of magnesium alloy in CL solution.
3. Effect of Lanthanum on ignition temperature of cast magnesium alloy
Rare earth elements can improve the flame retardancy of cast magnesium alloy, and increase with the addition of rare earth. This is mainly because rare earth, as a surface active element, is enriched on the surface of magnesium alloy, which hinders the contact between magnesium and oxygen. Recently, some researchers have studied how to increase the ignition temperature of magnesium alloy by adding La, CE and mixed rare earth. In order to improve the ignition point of magnesium alloy, the addition of Ca, be and other elements can increase the ignition point by about 200-250 ℃; the addition of Ca makes the grain of the alloy coarse and the mechanical properties poor, so that it loses the use value, but the addition of Ca, be and other elements at the same time, the addition of a proper amount of RE can reduce this adverse effect, so that the ignition point is increased by 250 ℃, and the mechanical properties are close to AZ91D alloy.
The mg-be-re rare earth cast magnesium alloy developed in China can increase the ignition point by 250 ℃, and its mechanical properties are equivalent to AZ91D. It is a kind of flame retardant magnesium alloy that can be used in practice.
The influence of La, CE and mischmetal addition on the ignition temperature of ZM5 magnesium alloy was studied. It was found that the different addition of La, CE and mischmetal can significantly increase the ignition temperature of ZM5 magnesium alloy (about 150 ℃), which is mainly due to the formation of protective film composed of MgO, Al2O3, (RE) 2O3 and Mg17Al12 on the liquid surface of ZM5 magnesium alloy, which plays a role of flame retardant. Zou Yongliang and others think that ZM5 has the best flame retardant effect when adding 0.12% mixed rare earth, which can raise the ignition temperature about 165.5 ℃, which may be a practical flame retardant magnesium alloy.