Effect of Rare Earth on As-cast Microstructure and Properties of 3X04 Aluminum Alloy

3X04 (3004,3104) aluminum alloy belonging to Al-Mn-Mg-based aluminum alloy rust-proof, the alloy has excellent overall performance of high strength, corrosion resistance and formability, etc., currently used in canning sheet. At present, there is still a big gap between China's domestic cans and plates, and more than 95% still need to be imported. There are two main constraints: first, the rolling process is backward (especially hot rolling), and second, the material purity is low. The internal organization is poor. The current research is mostly concentrated on the former and neglects the importance of systematic research to improve the intrinsic metallurgical quality of aluminum. Traditionally, the TlTiB intermediate alloy is used for the refinement of aluminum alloy grains, but the TiB _{2 is} easy to aggregate and precipitate and loses its effect. It has a poor anti-attenuation performance. AlTiBRE master alloy is a new type of refiner developed in recent years, but the role of rare earth in 3X04 aluminum alloy is rarely reported. Based on the addition of AlTiB master alloy to refine 3X04 alloy, the effect of rare earth on its microstructure and properties was studied by adding trace rare earth elements.

    I. Experimental design and method

The experiment uses an Al-10RE master alloy containing Ce-rich mixed rare earth. The energy spectrum analysis is shown in Figure 1. The alloy is smelted in a graphite crucible resistance furnace. Al-10Fe, Al-10Mn, Al-12Si are added to the alloying elements. Al-10RE master alloy, the remaining elements are added in pure metal form. The smelting temperature is 740-780 ° C, covered with aluminum alloy special covering agent, C ₂ Cl ₆ degassing, iron casting, casting temperature about 720 ° C, mold temperature preheating about 200 ° C. The size of the ingot is Ф20mm×160mm. The ingot is homogenized and aging in a 4kW box-type resistance furnace. The furnace is heated at 600°C for 20h, then quenched into room temperature water. The tensile specimen is processed according to GB6397-86 (Ф10mm) ×50mm), mechanical performance test on SANS SHT5350 microcomputer-controlled electro-hydraulic servo universal testing machine (loading speed 500N/S), each numbering test was repeated three times to take the average value, and the ingot structure was observed by Philips XL30 scanning electron microscope. With a fracture fracture morphology. The DTA curve of the alloy was measured by a differential thermal analysis instrument (DSC), and the phase transition temperature was investigated. The sample was heated to 850 ° C at 10 ° C·min ^-1 .

Figure 1 Energy spectrum analysis of Al-10RE alloy

    Second, the results and analysis

(1) Effect of rare earth on as-cast microstructure of alloy

Figure 2 shows the as-cast microstructure of 3X04 aluminum alloy with different rare earth contents. It can be seen that the rare earth-free as-cast microstructure is coarse or thick and elongated. As the content of rare earth increases, the microstructure changes, and the large bones The eutectic disappears substantially, and the thick long strip-shaped compound becomes a small strip. When the rare earth content is 0.2% (mass fraction, the same below), the effect is optimal, and the rare earth content is further increased to 0.3%, the second phase is gradually increased, and more spherical phase appears in the alloy, when the rare earth content is 0.4%. The second phase compound is obviously increased, and its shape is also changed from long strip to short thick bone, and the spheroid phase is further increased. Figure 3 shows the energy spectrum analysis of the corresponding regions in the different compositions, and the results show that the mixed rare earth When the content is less than 0.2%, the rare earth element can enter the rich (FeMn) phase to form a complex compound of (Al, Fe, Mn, Mg, Si, RE), and no RE is detected in α-Al, indicating that the rare earth content is higher. When low, most of the RE elements are enriched in the second phase between the dendrites and significantly change the morphology of the intercrystalline compound. When the RE content exceeds 0.3%, a spherical (Al, Mg, Si, Cu, RE) complex rare earth compound is formed. When the RE content reached 0.4%, the energy spectrum analysis showed that the bone-like eutectic at this time contained no rare earth elements, and it was found that almost all of the rare earth elements formed spherical compounds.

Fig. 2 Effect of different rare earth contents on as-cast microstructure of 3X04 aluminum alloy (BSE)

Figure 3 Figure 2 area corresponding energy spectrum analysis

From Al-La, Al-Ce binary phase diagram, La, Ce, etc. have a solid solubility of up to 0.05% in α-Al, a partition coefficient k ₀ <<1, in the formation and growth of α-Al crystal nucleus In the middle, due to the redistribution of solute, the rare earth is easily enriched in the liquid phase of the crystallization front; on the other hand, the La, Ce atom radius is 0.187, 0.182 nm, which is greater than the aluminum atom radius of 0.143 nm, in order to keep the system free energy minimum. The rare earth atoms do not enter the α-Al lattice and can only be enriched in the grain boundaries where the atoms are arranged irregularly. In addition, since rare earth is a kind of surface active element and hinders the probability of atoms such as Fe and Si entering α-Al, the concentration gradient of Fe and Si in the liquid phase of the interface front is increased. This effect not only causes the components to be too cold, but also increases the growth tendency of dendrites, increases the chance of dendritic neck fracture, and enhances the kinetic nucleation of the α-Al phase, thereby refining the dendrites for further study in 3X04. The influence of the addition of rare earth on the supercooling of the alloy was tested by differential thermal analysis. The solid and liquidus temperatures of the alloy were obtained as shown in Fig. 4.

Figure 4 DTA curve of rare earth and rare earth content 3X04 aluminum alloy

It can be seen that the solidus temperature of the 3X04 alloy without any rare earth is 643.1 °C, the liquidus temperature is 698.1 °C, and the solid-liquid temperature difference is 55 °C. The solidus temperature of the alloy increases slightly after adding 0.2% rare earth element. It is 645.3 ° C, and the liquidus temperature rises significantly, which is 747.7 ° C, and the solid-liquid temperature difference is 102.4 ° C. It can be seen that the addition of trace rare earth elements increases the crystallization temperature interval and the composition is too cold. Thereby, the branching process is intensified, the crystallization mode of cell dendrite growth is severe, and the growth of dendrites is more developed, the secondary dendrites are increased, and finally the dendrite spacing is reduced. However, the excessive addition of RE will cause a large amount of spherical rare earth compounds (Al, Mg, Si, Cu, RE) to be formed first in the alloy, which reduces the enrichment of the rare earth at the solid/liquid interface, resulting in the coarsening of the alloy structure.

(II) Effect of rare earth on mechanical properties of alloys

Figure 5 shows the mechanical properties of alloys with different rare earth contents after aging at 600 °C for 20 h. It can be seen that the tensile strength of the alloy increases with the addition of mixed rare earth. When the rare earth content is 0.2%, σ _b reaches the maximum value. Compared with the case where no rare earth is added, the strength is increased by 28 MPa, and the increase is 11.3%. When the rare earth content is more than 0.2%, the strength is decreased, even lower than that of the alloy without added rare earth. In addition, with the increase of the amount of rare earth added, the plasticity of the alloy is also obviously improved. When the rare earth content is 0.2%, the elongation of the alloy is increased from 23% of the rare earth to 25%, and the increase rate is 8.7%. When the RE content is > 0.1%, the plasticity of the alloy deteriorates. Fig. 6 shows the fracture structure of different rare earth contents. It can be seen that when the rare earth content is 0.1%, the dimples are fine and uniform, and no strip-shaped tearing ribs are observed. It indicates that the precipitated phase is fine and uniform at this time. When the rare earth content is 0.3%, the dimple becomes shallow and thick, and stripe tearing ridge occurs. At this time, the material strength and plasticity are the worst.

Figure 5 Effect of rare earth on mechanical properties

Figure 6: Tensile fracture morphology of 3X04 aluminum alloy

The above test results show that when the rare earth is added in an appropriate amount, the dendrites can be refined to improve the mechanical properties of the material, and the composition of the phase in the alloy is changed due to the interaction of the rare earth element with the alloying elements and impurity elements in the alloy. When the content is 0.2%, the precipitated phase is small and strip-shaped as shown in Fig. 2(b). After the aging, the rare earth phase which is dispersed and distributed plays a role of dispersion strengthening, which improves the tensile strength of the alloy, increases the rare earth content, and coarsens the rare earth phase transition. After aging, it appears in the form of large refractory substances, which tend to cause stress concentration at the grain boundary, which in turn reduces the strength and plasticity of the alloy.

    Third, the conclusion

(1) In the 3X04 aluminum alloy, when the rare earth content is less than 0.2%, the rare earth element can enter the rich (FeMn) phase to form a complex compound of (Al, Fe, Mn, Mg, Si, RE), and the refining effect is best. As the rare earth element increases, a spherical (Al, Mg, Si, Cu, RE) complex rare earth compound is formed, and the refining effect is deteriorated.

(2) DTA analysis showed that the solid-liquid temperature difference of the alloy after adding 0.2% rare earth element increased from 55 °C to 102.4 °C without time, and it was found that the addition of trace rare earth elements caused a large component to be too cold.

(3) Mechanical properties tests show that the alloy has the highest tensile strength and elongation when the rare earth content is 0.2%.