Effect of La on microstructure, mechanical properties and fracture behavior of Al/Mg bimetallic interface manufactured by compound casting

doi:10.1016/j.jmst.2021.08.011

Abstract

Abstract:

To improve the Al/Mg bimetallic interface, La was added into the Al/Mg bimetallic interface manufactured by a compound casting process. The effect of La addition on the microstructure, mechanical properties and fracture behavior of the Al/Mg bimetallic interface and the formation mechanism of the interface were studied in detail. Al₁₁La₃, Al₈Mn₄La, Al₂₀Ti₂La, and other rare earth precipitates (RE precipitates) preferentially precipitated at the interface with La addition, while the number of the Al₁₁La₃ and Al₈Mn₄La located in eutectic structure area (E area) gradually increased and aggregated in the interface with the increase of the La content. Besides, the matrix structure in different areas of the Al/Mg interface changed in different degrees, and the eutectic structure and primary γ (Mg₁₇Al₁₂) dendrites in the E area were refined, but the intermetallic compounds area (IMC area) had no obvious change. With the addition of the La, the interface was strengthened under the comprehensive effect of refinement strengthening and precipitation strengthening from the E area. When the La content increased to 1.0%, the shear strength of the Al/Mg bimetal reached the maximum of 51.54 MPa, which was 30.95% higher than the group without La addition. However, with the further increase of the La content, the large area aggregation of the Al₁₁La₃ and Al₈Mn₄La occurred in the interface, leading to the separation of the matrix structure of the E area and the decrease of the shear strength of the Al/Mg interface.

Key words: Compound casting, Al-Mg intermetallic compounds, Rare earth La, Al/Mg interface, Microstructure, Mechanical properties

Zheng Zhang, Wenming Jiang, Guangyu Li, Junlong Wang, Feng Guan, Guoliang Jie, Zitian Fan. Effect of La on microstructure, mechanical properties and fracture behavior of Al/Mg bimetallic interface manufactured by compound casting[J]. J. Mater. Sci. Technol., 2022, 105: 214-225.

Figures/Tables 21

Table 1. Chemical compositions of the experimental alloys (wt.%).

	Mg	Al	Zn	Mn	Si	Fe	Ti
A356	0.44	Bal.	—	—	6.81	0.21	0.02
AZ91D	Bal.	9.08	0.62	0.23	—	—	—

Fig. 1. Schematic diagram of the LFCC process principle.

Fig. 2. Schematic diagram of the shear strength testing.

Fig. 3. Microstructures of the Al/Mg interface in group A (Without La addition): (a) Overall morphology of the interface; (b) Low magnification SEM image of the eutectic layer included in E area; (c) High magnification SEM image of the eutectic layer; (d) Low magnification SEM image of the primary γ (Mg17Al12) layer included in E area and the E-IMC area; (e) High magnification SEM image of the primary γ layer; (f) Low magnification SEM image of the γ layer and β (Al3Mg2) layer included in IMC area; (g) The enlargement of area Ⅰ in the (f); (h) The enlargement of area Ⅱ in the (f).

Table 2. The EDS analysis results of the corresponding points in group A.

Point no.	Element compositions (at.%)						Possible component
Point no.	Mg	Al	Mn	Si	Fe	Ti	Possible component
1	63.15	36.85	—	—	—	—	γ
2	81.55	18.45	—	—	—	—	δ-Mg
3	60.41	39.59	—	—	—	—	γ
4	—	83.28	—	7.53	9.19	—	θ
5	—	86.93	—	—	13.07	—	Al₁₃Fe₄
6	47.09	52.91	—	—	—	—	γ
7	61.35	7.23	—	31.41	—	—	Mg₂Si
8	38.69	61.31	—	—	—	—	β
9	—	82.67	—	—	—	17.33	Al₃Ti

Fig. 4. The morphologies of the Al/Mg interfaces in group L1, L2 and L3: (a) Group L1 (With 0.7% La); (b) Group L2 (With 1.0% La); (c) Group L3 (With 1.3% La).

Fig. 5. Microstructures of the Al/Mg interfaces in group L1, L2 and L3: (1a-1f) Group L1, (1a) and (1b) The E area and the E-IMC area, (1c) The enlargement of rectangular area in Fig. 5(1b), (1d) The IMC area, (1e) and (1f) The enlargement of area Ⅰ and Ⅱ in Fig. 5(1d); (2a-2f) Group L2, (2a) and (2b) The E area and the E-IMC area, (2c) The enlargement of rectangular area in Fig. 5(2b), (2d) The IMC area, (2e) and (2f) The enlargement of area Ⅲ and Ⅳ in Fig. 5(2d); (3a-3f) Group L3, (3a) and (3b) The E area and the E-IMC area, (3c) The enlargement of rectangular area in Fig. 5(3b), (3d) The IMC area, (3e) and (3f) The enlargement of area Ⅴ and Ⅵ in Fig. 5(3d).

Table 3. EDS analysis results of the corresponding points in group L1, L2 and L3.

Point no.	Element compositions (at.%)							Possible component
Point no.	Mg	Al	La	Mn	Si	Fe	Ti	Possible component
1	—	70.83	4.79	—	7.58	16.80	—	θ
2	—	71.22	6.42	22.36	—	—	—	Al₈Mn₄La
3	—	77.68	22.32	—	—	—	—	Al₁₁La₃
4	—	89.59	3.67	—	—	—	6.74	Al₂₀Ti₂La
5	—	80.32	—	—	—	19.68	—	Al₁₃Fe₄
6	—	72.43	3.79	—	8.53	15.25	—	θ
7	—	80.56	19.44					Al₁₁La₃
8	73.35	6.18	20.47		—	—	—	Al₈Mn₄La
9		88.29	4.58	—	—	—	7.13	Al₂₀Ti₂La
10	—	78.80	—	—	—	21.20	—	Al₁₃Fe₄
11	—	69.20	4.58	—	7.63	18.37	—	θ
12	—	79.65	20.35	—	—	—	—	Al₁₁La₃
13	—	69.41	7.18	23.41	—	—	—	Al₈Mn₄La
14	—	87.17	4.86	—	—	—	7.97	Al₂₀Ti₂La
15	—	82.19	—	—	—	17.81	—	Al₁₃Fe₄

Fig. 6. Elements distributions of the Al/Mg interface in group A: (a) The morphology of the interface; (b)-(g) Distributions of different elements.

Fig. 7. Elements distributions of the interface with La addition: (a) The morphology of the interface; (b)-(h) Distributions of different elements; (i) The Enlargement of E area in Fig.7(a); (j) The distribution of La in (i).

Fig. 8. Comparison of elements diffusion distance of the Al/Mg bonding interfaces with and without La addition.

Fig. 9. TEM results of the RE precipitates in the E area with La addition: (a) Bright field image of the phase interface of Al11La3 and γ: region A stands for Al11La3, region B stands for γ, region C stands for the phase interface; (b) Bright field image of the phase interface of Al8Mn4La and γ: region D stands for Al8Mn4La, region E stands for γ, region F stands for the phase interface; (c)-(e) SAED results of Al11La3, γ and Al8Mn4La; (f) and (g) SAED result and high resolution image of the Al11La3 and γ phase interface; (h) and (i) SAED result and high resolution image of the Al8Mn4La and γ phase interface.

Fig. 10. Shear test results of the Al/Mg bimetals obtained with different La additions: A stands for the group without La addition; L1 stands for the group with 0.7% La addition; L2 stands for the group with 1.0% La addition; L3 stands for the group with 1.3% La addition.

Fig. 11. Fracture morphologies of the shear samples with different La additions: (a, b) The fracture of group A; (c) The enlargement of rectangular area in (b); (d, e) The fracture of group L1; (f) The enlargement of rectangular area in (e); (g, h) The fracture of group L2; (i) The enlargement of rectangular area in (h); (j, k) The fracture of group L3; (l) The enlargement of rectangular area in (k).

Fig. 12. Hardness distributions of the Al/Mg interfaces with different La additions: Each region in the Al/Mg interface of group A (without La addition) is indicated by black dotted arrow, and Each area in the Al/Mg interfaces with La addition is indicated by red dotted arrow.

Fig. 13. Optical micrographs of the hardness indentations with different La additions: (a) Group A: point 1 corresponding to point 1 on hardness curve of group A in Fig. 12; (b) Group L1; (c) Group L2: point 2 corresponding to point 2 on hardness curve of group L2 in Fig. 12; (d) Group L3.

Fig. 14. Formation mechanism of the Al/Mg interface with La addition: (a) The formation of the chilling area and the melting area; (b) The preferential precipitation of the RE precipitates, and the initial formation and expansion of E area, E-IMC area and IMC area; (c) The precipitation and formation of matrix structure of each layer (From left to right: eutectic and secondary γ, primary γ, γ (Mg17Al12), β (Al3Mg2)).

Fig. 15. The schematic diagram of vacancy diffusion mechanism of La in the δ-Mg lattice.

Fig. 16. Schematic diagram of La promoting short-circuit diffusion of Si, Fe and Ti along the α-Al grain boundaries (Al side).

Fig. 17. Average grain sizes of the eutectic structure and the primary γ dendrites.

Fig. 18. Fracture distribution diagram of the Al/Mg bimetal with different La additions.

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