Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

doi:10.1016/j.jmst.2020.05.048

Abstract

Abstract:

Alloys with large solidification intervals are prone to issues from the disordered growth and defect formation; accordingly, finding ways to effectively optimize the microstructure, further to improve the mechanical properties is of great importance. To this end, we couple travelling magnetic fields with sequential solidification to continuously regulate the mushy zones of Al-Cu-based alloys with large solidification intervals. Moreover, we combine experiments with simulations to comprehensively analyze the mechanisms on the optimization of microstructure and properties. Our results indicate that only downward travelling magnetic fields coupled with sequential solidification can obtain the refined and uniform microstructure, and promote the growth of matrix phase α-Al along the direction of temperature gradient. Additionally, the secondary dendrites and precipitates are reduced, while the solute partition coefficient and solute solid-solubility are raised. Ultimately, downward travelling magnetic fields can increase the ultimate tensile strength, yield strength, elongation and hardness from 196.2 MPa, 101.2 MPa, 14.5 % and 85.1 kg mm^-2 without travelling magnetic fields to 224.1 MPa, 114.5 MPa, 17.1 % and 102.1 kg mm^-2, and improve the ductility of alloys. However, upward travelling magnetic fields have the adverse effects on microstructural evolution, and lead to a reduction in the performance and ductility. Our findings demonstrate that long-range directional circular flows generated by travelling magnetic fields directionally alter the transformation and redistribution of solutes and temperature, which finally influences the solidification behavior and performance. Overall, our research present not only an innovative method to optimize the microstructures and mechanical properties for alloys with large solidification intervals, but also a detailed mechanism of travelling magnetic fields on this optimization during the sequential solidification.

Key words: Large solidification intervals, Travelling magnetic fields, Sequential solidification, Mushy zones, Al-Cu-based alloys

Lei Luo, Liangshun Luo, Robert O. Ritchie, Yanqing Su, Binbin Wang, Liang Wang, Ruirun Chen, Jingjie Guo, Hengzhi Fu. Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification[J]. J. Mater. Sci. Technol., 2021, 61: 100-113.

Figures/Tables 17

Fig. 1. Schematic diagrams of experiment and the selection of samples. (a) Equipment for the coupling of TMF with sequential solidification. (b) The selection of experimental samples and simulated models.

Table 1 Related characteristics of the TMF generator and the parameters used in the calculations of TMF and flow fields [35,36].

Parameters	Symbol	Value
TMF inner diameter	D_i, mm	40
TMF outer diameter	D_o, mm	120
Number of windings	n	330
Current frequency	f, Hz	50
Phase sequence	-	Down-TMF: 0, 2π/3, 4π/3;
Phase sequence	-	Up-TMF: 4π/3, 2π/3, 0
Maximum electric current	I₀, A	24
Effective electric current	I_e, A	17
Temperature gradient	G_T, K mm^-1	2
Cooling rate of alloys	v_c, K s^-1	0.3
Thermal Conductivity	C_T, W mK^-1	236
Magnetic permeability	μ_Al, H m^-1	1
Viscosity coefficient	?, Pa·s	0.00125
Electrical conductance	σ, S m^-1	35.3e+6
Latent heat	L_m, kJ kg^-1	396.1

Table 2 Preparation process of different samples and actual chemical compositions measured by XPS.

No.	Al, wt. %	Cu, wt. %	I_e, A	TMF	v_d, μm·s^-1
1	Bal	4.99	17	Up-TMF	150
2	Bal	5.00	0	No-TMF	150
3	Bal	4.99	17	Down-TMF	150

Fig. 2. Images of each slice along the radial direction taken by 3D-CT. Higher brightness in the images indicate the regions with higher content of Cu. With respect to microstructure, the green represents the Cu-containing precipitates, while the black within green represents the primary dendritic α-Al. Circle I in (b) indicates the accumulation of Cu. Circle II in (b) indicates the deflection of the primary α-Al dendrites.

Fig. 3. Three-dimensional morphology of the precipitated phases taken by 3D-CT.

Fig. 4. The composition and content of precipitation phases. XRD maps of the (a) transverse and (b) longitudinal sections of the solidified samples. (c) The content statistics for the matrix phase α-Al and precipitation phase Al2Cu, as measured by EBSD.

Table 3 Related results of the crystal structural evolution on the (200)Al and (111)Al crystal planes derived from the XRD data.

Crystal face index		Samples
Crystal face index		No-TMF	Up-TMF	Down-TMF
(200)_Al	θ, degree	44.781	44.779	45.040
	a, nm	0.40444	0.40446	0.40222
	S_max, at. %	0.6019	0.5555	5.7407
(111)_Al	θ, degree	38.555	38.555	38.817
	a, nm	0.40410	0.40412	0.40149
	S_max, at. %	1.3889	1.3426	7.4306

Fig. 5. Microstructural energy spectrum analysis measured in cross-sections of different samples. The red circles in (a), (b) and (c) represent the energy spectrum points. (d) The variation in Pma, Pws and Pma/Pws, the corresponding values of each energy spectrum test point, which represent the concentration of Cu in the matrix, the concentration of Cu in the whole section and the relative solubility ratio, respectively.

Table 4 EDS results for Cu element in the whole sectiona and at each pointb (a “Whole section” refers to the weight percent of Cu in all areas of the microstructural images. b “Point” refers to the weight percent of Cu at each energy spectrum test point in the matrix phase α-Al.).

Processing	Cu wt.%
Processing	Whole section	Point 1	Point 2	Point 3	Point 4	Point 5
Down-TMF	4.95	2.87	2.87	2.81	2.80	2.86
No-TMF	4.97	2.43	2.66	2.60	2.46	2.47
Up-TMF	4.97	1.75	1.81	2.01	1.92	1.87

Fig. 6. Related statistical results of the as-cast microstructural morphologies. (a) The measuring method used to calculate the PDAS using Image-Pro software. The p1, p2 and p3 in (a) denote the PDAS value in the tangential, diagonal and radial directions, respectively. (b) Results of the measurements of the PDAS. (c) The dendrite sizes in the α-Al phase, as measured by EBSD. (d) Deviation angles for the growth direction of α-Al in the direction of temperature gradient, again measured by EBSD.

Fig. 7. Statistical results of the crystal orientation measurements of the α-Al phase determined by EBSD. The “a”, “b” and “c” stand for the results of Up-TMF, No-TMF and Down-TMF respectively. The numbers “1”, “2” and “3” denote the serial number of photos. (a1), (b1), and (c1) are the IQ images of EBSD maps. (a2), (b2), and (c2) are the corresponding <001> - inverse pole figures. (a3), (b3), and (c3) are the corresponding <001> - pole figures.

Fig. 8. Performance testing results. (a) Stress-strain curves. (b) Performance statistics. (c), (d) and (e) are scanning fractograph for No-TMF, Up-TMF and Down-TMF, respectively.

Fig. 9. The distribution of temperatures and melt flows at different times with the flow velocities after 0.6 s with No-TMF processing. The black arrows indicate the directions of the melt flow.

Fig. 10. The distribution of temperature and melt flows at different times with the flow velocities after 0.6 s with Up-TMF processing. The white arrows indicate the directions of the melt flow.

Fig. 11. The distribution of temperatures and melt flow at different times with the flow velocities after 0.6 s with Down-TMF processing. The white arrows indicate the directions of the melt flows.

Fig. 12. Schematic diagrams of solidification process under different conditions. (a) Solidification path models. (b) Phase diagram curves. Lines 1, Lines 2 and Lines 3 in.(b) are the equivalent phase diagram curves of No-TMF, Up-TMF and Down-TMF, respectively. (c) Distributions of solutes and temperatures at the solid-liquid interface. In (c), S refers to solid phase, L refers to liquid phase, T is the melting point, N is the position of solid-liquid interface, x is the position of the alloy melt distant from the solid-liquid interface and C0, CL, and CS are, respectively, the solute concentration of the initial, liquid and solid states.

Fig. 13. Schematic diagrams showing the mechanisms for different TMF processes coupled with sequential solidification. The purple circles are the solute enriched at the solidification front. The red squares are the latent heat generated at the solidification front. “HTZ” denotes a higher temperature zone. The purple and black arrows in (a) (b) and (c) represent melt flows.

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