Microstructure and Hardness of Al-Cu-Li Alloy Fabricated by Arc Additive Manufacturing Based on In Situ Metallurgy of Molten Pool
LI Kangjie, SUN Zeyu, HE Bei(), TIAN Xiangjun
National Engineering Laboratory of Additive Manufacturing for Large Metallic Structures, Beihang University, Beijing 100191, China
Cite this article:
LI Kangjie, SUN Zeyu, HE Bei, TIAN Xiangjun. Microstructure and Hardness of Al-Cu-Li Alloy Fabricated by Arc Additive Manufacturing Based on In Situ Metallurgy of Molten Pool. Acta Metall Sin, 2024, 60(5): 661-669.
Al-Li alloy has become an optional material for load-bearing components in aerospace because of its low density, high specific strength, and good fatigue performance. Currently, the widely used casting process to fabricate large Al-Li alloy structural parts has the issues of active and highly toxic Li elements, high equipment cost, long production cycle, limited forming size, and low material utilization. Wire and arc additive manufacturing technology uses an arc heat source to melt the raw materials, mostly prealloyed wires, and directly deposits the materials layer-by-layer by controlling the required components using a computer. It has the technical advantages of a short processing cycle, high material utilization, and a large frame formation, providing a new possibility for forming large Al-Li alloy components. Currently, the prealloyed welding wire is usually used as a raw material for arc additive manufacturing, but it is challenging to make high-performance Al-Li alloy wire and Li is strongly ablated under a high-temperature heat source. In situ metallurgy with an arc melt pool has prepared Al-Li alloys with good internal quality and superior performance potential while reducing manufacturing costs. Therefore, exploring the controllable addition of Li elements during the deposition process is necessary. Herein, the Al-Cu-Li alloy sample was successfully fabricated using a multimaterial arc melting deposition technology combining Al-Li alloy powder and 2219 Al-Cu alloy wire. The grain morphology, phase composition, and hardness of the as-built alloy sample were further analyzed. The as-built Al-Cu-Li alloy sample comprises fine equiaxed grains of 10-20 μm with semi-continuous reticular eutectic θ (Al2Cu) phases at the grain boundaries. TB (Al7Cu4Li) and T1 (Al2CuLi) phases can be observed near the grain boundaries under the influence of thermal cycling. T1 phases with significant strengthening effects can be observed in the middle and bottom of the sample. The number density of the T1 phase is higher in the bottom part compared to the middle, but the size of the T1 phase is relatively larger because the bottom of the sample near the substrate experienced more thermal cycling. The maximum hardness of the as-built Al-Li sample is 126.7 HV0.1, slightly higher than that of the other wire and arc additive manufactured using 2219 Al-Cu alloys, mainly owing to the fine equiaxed grains and the T1 phases formed via thermal cycling.
Table 1 Chemical compositions of 2219 wire and substrate
Fig.1 Schematic (a) and macroscopy (b) of MIG-arc deposited process for Al-Cu-Li alloy sample (MIG—melt inert gas)
Fig.2 OM images of the as-built Al-Cu-Li alloy sample at top (a), middle (b), and bottom (c) positions along the deposited direction, respectively; and schematic of heterogeneous nucleation of grains (d)
Fig.3 SEM images of the as-built Al-Cu-Li alloy sample at top (a), middle (b), and bottom (c) positions along the deposited direction, respectively; and EDS point result in Fig.3a (d)
Fig.4 EDS point analysis lattice diagram (a) and fitting results of Scheil formula (b) (Cs—solute content (mass fraction, %), fs—solid fraction, —practical distribution coefficient of Cu)
Fig.5 XRD (a) and partial enlarged (b, c) spectra of the as-built Al-Cu-Li alloy sample along the deposited direction
Fig.6 TEM images (a-c) and corresponding SAED patterns (d-f) of the as-built Al-Cu-Li alloy sample at top (a, d), middle (b, e), and bottom (c, f) positions along the deposited direction, respectively
Fig.7 HRTEM images and corresponding fast Fourier transformation (FFT) patterns (insets) of T1 phase in arc deposited Al-Cu-Li sample at middle (a-c) and bottom (d-f) positions along the deposited direction, respectively
Fig.8 Vickers hardness distribution of the as-built Al-Cu-Li sample
Process
Material
State
Vickers hardness / HV0.1
Ref.
MIG
2219 + Al-Li powder
As-build
126.7 ± 2.1
This work
TIG
2219 + TiC
As-build
97.5 ± 2.5
[11]
TIG
2219
As-build
89 ± 4
[24]
Laser + TIG
2219
As-build
92.6 ± 3.7
[25]
Table 2 Average Vickers hardnesses of the alloy samples under various processes[11,24,25]
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