Microstructure evolution and mechanical behavior of Al–Li alloy fabricated by laser melting deposition technique

Journal Pre-proof Microstructure evolution and mechanical behavior of Al–Li alloy fabricated by laser melting deposition technique Shikun Jiao, Xu Cheng, Shuxin Shen, Xin Wang, Bei He, Dong Liu, Huaming Wang PII:

S0925-8388(19)34371-3

DOI:

https://doi.org/10.1016/j.jallcom.2019.153125

Reference:

JALCOM 153125

To appear in:

Journal of Alloys and Compounds

Received Date: 2 September 2019 Revised Date:

17 November 2019

Accepted Date: 20 November 2019

Please cite this article as: S. Jiao, X. Cheng, S. Shen, X. Wang, B. He, D. Liu, H. Wang, Microstructure evolution and mechanical behavior of Al–Li alloy fabricated by laser melting deposition technique, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153125. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

CRediT author statement Shikun Jiao: Conceptualization, Methodology, Formal analysis, Writing Original Draft, Writing Review & Editing. Xu Cheng：Data Curation, Validation, Writing Review & Editing. Shuxin Shen：Visualization, Data Curation. Xin Wang: Resources, Investigation. Bei He: Resources, Data Curation. Dong Liu: Writing Review & Editing, Project administration, Supervision. Huaming Wang: Funding acquisition, Supervision.

Microstructure evolution and mechanical behavior of Al-Li alloy fabricated by laser melting deposition technique

Shikun Jiao1,3, Xu Cheng1,2, Shuxin Shen1,2, Xin Wang1,2, Bei He1,2,Dong Liu1,2,*, Huaming Wang1,2

1. National Engineering Laboratory of Additive Manufacturing for Large Metallic Components, Beihang University, School of Materials Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100083, China 2. Beijing Engineering Technological Research Center on Laser Direct Manufacturing for Large Critical Metallic Components, 37 Xueyuan Road, Beijing 100083, China 3. Beijing Xinghang Electro-mechanical Equipment Co., Ltd, NO.9 Dongwangzuo North RD, Fengtai District, Beijing 100074, China

*Corresponding author: Dong Liu E-mail:

Liu:[email protected], Tel:+86 82339697

E-mail address of the others: Shikun Jiao: [email protected] Xu Cheng: [email protected] Shuxin Shen: [email protected] Xin Wang: [email protected] Bei He: [email protected] Dong Liu: [email protected] Huaming Wang: [email protected]

Abstract This paper systematically studies the microstructure evolution, tensile property and microhardness of Aluminum-lithium (Al-Li) alloy with dimension of 80 mm×30 mm×80 mm manufactured by laser melting deposition (LMD) technology. Results indicate that the microstructures of the as-deposited Al-Li alloy are variable from the top to the bottom of the plate. At the top region, a fully dendritic structure is observed with underdeveloped secondary arms. And the copper-rich precipitates with irregular stripe morphology appear in the inter-dendritic areas. Attributing to the thermal cycle during laser deposition process, as distance increasing away from the top, the number of irregular stripe phases reduces with the massive precipitation of needle-like TB phases (soft and brittle). At the bottom of the plate, dendritics of as-deposited Al-Li alloy obviously coarsened, TB phases fully decorate the entire grains with a few residual copper-rich precipitates. Attributing to the different microstructures, the micro-hardness firstly increases from 121 HV to 134 HV at a distance of 30 mm, and then decreases to about 98 HV at the bottom of the deposited Al-Li alloy plate. The ultimate tensile strength of the as-deposited Al-Li alloy (longitudinal direction, at the middle-bottom part of the plate) is comparable to the hot-rolled Al-Li alloy (244 MPa). However, the yield strength and elongation of the as-deposited Al-Li alloy are relatively poor due to the embrittlement of copper-rich phases as well as TB phase. Key words: Laser melting deposition, Aluminum-lithium alloy, Microstructure evolution, TB phase, Microhardness, Tensile property

1 Introduction Al-Li alloy has good combination of low density, high elastic modulus, enhanced specific strength and rigidity, excellent fracture toughness and fatigue crack growth resistance[1-3]. In general, around 1wt.% Li addition could provide approximately 3% decreasing in density and 6% increasing in Young’s elastic modulus of Aluminum alloy[1]. Thus, Al-Li alloy is considered as a promising candidate material using in the next generation aerospace application. Casting technology is one of the most common methods to fabricate Al-Li alloy components. However, the high chemical activity of lithium may cause serious oxidation and even explosion when directly contacting with oxygen or water vapor[4, 5]. In addition, due to slow cooling rates and improper selection of processing parameters, the as-casted Al alloy always presents coarsened-grains with randomly distribution of defects such as gas pores and shrinkage holes[5, 6]. Laser melting deposition manufacturing (LMD) is one of a rapid prototyping manufacturing methods[7]. During the process of LMD, laser is utilized to create a melt pool at the surface of a substrate within an inert atmosphere accomplished by delivering coaxial powders from powder nozzles. Near-net-shape metal components are consequently built in a layer-by-layer mode by melting and solidification via computer numerical control system(CNC)[7-10]. Because of advantages of no mold, few material wastes, short production cycle and great designing flexibility, LMD is fast developed and has been successfully apply to titanium alloy[11-13], ultra-high strength steel[14] and superalloy[15, 16]. However, compared with aforesaid alloys, the fabrication of aluminum alloys have many intrinsic problems[6,

17]

such as high reflectivity, low laser energy

absorption and poor oxidation resistance, which raise the difficulties of production Al based components. The most reported Al alloys, preparation by additive manufacturing (AM) process, are Al-Si based alloys[18-20]. For example, J Li[20] successfully fabricated a Al-5Si-Cu-Mg alloy component using the LMD technique. Compared with Al-Si alloy, Li element in Al alloy has high chemical activity and low

melting temperature, which easily evaporate during LMD process. Besides, high cooling rates and unstable thermal cycles can cause unpredictable microstructure thus [21-23]

influence mechanical property of alloy

. For example L Cui

[22]

suggested that

during fast cooling, some un-wanted phases of δ′ appeared in fusion line when welding of Al-Li alloy. Hence, many efforts are required to put forward to fabricate Al-Li alloy using LMD technique. In this study, by adjusting the working parameters and powder compositions, a third generation Al-Li alloy (based on 2A97 alloy) is fabricated by LMD technique. The microstructure evolution and mechanical behavior of LMDed Al-Li alloy are carefully analyzed and which will offer some fundamental knowledge for the subsequent heat treatment.

2 Methods 2.1 Powder material An Al-Li alloy (ρ=2.64 g/cm3) was designed on the basis of 2A97 alloy (a third generation Al-Li alloy[24, 25]) with less lithium but more Cu element addition in order to obtain lower anisotropy and improve weldability than traditional Al-Li alloy[23]. Al-Li alloy powders used in this study were produced by plasma rotating electrode process (PERP) with size around 150 mesh. The surface and cross-section morphology of these powders are presented in Fig.1, and the chemical composition is listed in Table 1.

Fig.1. (a) Surface morphology of as-received Al-Li alloy powders; (b) cross-section morphology of Al-Li alloy powders.

2.2 Laser melting deposition manufacturing process Deposition experiment was carried out using a LMD-V system in Beihang University with the processing parameters are listed in Table 2. A plate with dimension of 80 mm×30 mm×80 mm (Fig.2) was fabricated on a pure Al plate substrate and the chemical compositions of as-deposited Al-Li alloy are also listed in Table1. Table.1. Chemical compositions of powders and as-deposited Al-Li alloy (wt. %). Cu

Li

Mg

Zn

Mn

Zr

Be

Ti

Fe

Si

Al

Al-Li Powder

4.40

1.38

0.45

0.50

0.26

0.10

-

-

0.076

0.043

Bal.

As-deposited Al-Li alloy

4.41

1.32

0.48

0.36

0.28

0.11

-

-

0.080

0.034

Bal.

2A97

2.03.2

0.82.3

0.250.5

0.171.0

0.20.6

0.08- 0.0001- 0.0001- <0.15 0.2 0.1 0.1

<0.15

Table.2. Experiment parameters of LMD process.

Parameters Laser power P (W) Laser beam diameter D (mm) Scanning speed Vs (mm/min) Powder feeding rate Vf (g/min) Atmosphere condition (ppm) Scanning strategies Powder size Thickness of deposition layer (mm)

Values 4000-6000 6-7 600-900 8-12 O2≤50；H2O≤50 snakelike 150 mesh 2

Fig.2. (a) Schematic illustration of the LMDed Al-Li alloy thick plate, scanning strategy and deposition direction, (b) as-deposited plate.

Bal.

2.3 Microstructure characterization Microstructures of the longitudinal direction (YOZ section) of as-deposited samples were investigated. Metallographic specimens with dimension of 10mm× 10mm×10mm were cut at different position by wire-electrode discharge cutting machine, which were then ground by SiC abrasive paper and mechanically polished using diamond polishing paste. After that, the polished samples were etched using Keller’s reagent which consisted of 1.0mL HF, 2.5mL HNO3, 1.5mL HCl and 95 mL H2O. Optical microscopy (OM, Leica-DM 4000M), scanning electronic microscopy (SEM, JSM 6010 and CS 3400) equipped with an energy dispersive spectrometer (EDS) were used to observe the microstructure and fracture surfaces of alloy. Identification of precipitations were conducted using a transmission electron microscopy (TEM, JEOL-2100). Before the test, TEM foils of 3 mm in diameter were mechanically polished to around 200µm and then ion-milled. Composition of AL-Li alloy was identified using the x-ray diffraction machine (XRD, Rigaku D/MAX 2500). And XRD test was conducted with Cu Kα radiation, scanning rates 6°/min, voltage 40 kV and current 150 mA. 2.4 Mechanical test The average Vickers microhardness of the sample (YOZ section) was measured using a load of 100 gf for a dwell time of 10 seconds. The average microhardness at particular position was calculated from seven indents. Room-temperature tensile properties were measured using an AG-250 KNIS universe tensile testing machine with an extension rate of 1 mm/min. Specimens with dimensions of φ6 mm× 47 mm were machined along deposition direction (Fig.2 (a)). In order to minimize the measurement errors, three specimens were tested under same condition. 2.5 Heat treatment Heat treatment were taken to investigate the precipitation behavior of need-like TB phase. The temperature of solution treatments was identified by the results of DSC (differential scanning calorimeter) around 520℃. Samples were firstly heat treated to the temperature of 520℃ for 2 h. Then they were cooled to room temperature by

quenching in water. After that, they were re-heated to the temperatures of 360℃, 400℃, 435℃, 460℃, 500℃ for 5h respectively to study the precipitation of need-like TB phase.

[26]

3. Results 3.1 Microstructural features and phase identification

Fig.3. Solidification microstructures of the as-deposited Al-Li alloy (a) overview of sample, (b, e) top region, (c, f) middle region and (d, g) bottom region.

The as-deposited grain morphologies and microstructures of Al-Li plate along the deposition direction are illustration in Fig.3. Uni-directional aligned columnar grains are observed epitaxial growth from substrate with variation of microstructures (Fig.3 (b-g)). At the top region (Fig.3 (b, e)), the microstructure shows a fully dendritic structure with underdeveloped secondary dendritic arms, and the primary arm spacing ranges from 27.8µm to 40.8µm. Some precipitations with irregular

B

morphology appear along the inter-dendritic areas. At the middle region (Fig.3 (c, f)), the dendritic structure gradually disappears, and some needle-like phases with length around 7µm appear at the inter-dendritic areas. As distance increasing away from the top, the irregular inter-dendritic phases reduce further with the massive precipitation of needle-like phases until the entire grains are fully decorated (Fig.3 (d, g)). Backscattered electron microcopy with EDS analysis shown in Fig.4 indicates that these irregular phases at grain boundaries are mainly consisted of a light grey phase (spectrum 1 and 2) and a dark grey (spectrum 3) phase. EDX analysis point out that the inter-dendritic intermetallics are rich in copper. The light grey phase containing high amount of Cu (~33 at.%) with Al: Cu around 2:1, which probably the θ′(Al2Cu) phases or R phase (Al5CuLi3) phases[27, 28]. The dark grey phases with high [29, 30]

amount of Cu and Mg suggested probably to be S´phase

. However, due to the

light atomic weight, Li element is hardly to be detected in EDS test[31].

Fig.4. EDS analysis at the top position of the as-deposited Al-Li alloy plate (at.%) .

TEM bright field images and corresponding SAED patterns are further carried out to identify the needle-like phases in alloy, as can be seen from Fig.5. The diffraction patterns corresponding to the matrix and needle-like phases are obtained along [100]α and [110]α zone axis. The needle-like phases have a face-centered cubic structure with lattice constant a=0.583nm, which is identified as TB (Al7Cu4Li) phase. The orientation relationship of TB phase and α-Al matrix follows: (100)TB//(110)Al, (001)TB//(001)Al[31]. Besides, another needle-like phase with much thinner width and a hexagonal closed-packed structure with lattice constant of a=0.496nm and c=0.935nm is also found in the alloy matrix, which is confirmed as T1(Al2CuLi) phase. The

—

——

[30]

crystallographic orientation relationship of T1 and α-Al matrix is (1120)T1//(211)Al . And T1 phase is one of the critical strengthening phase in Al-Li alloy.

Fig.5. TEM images and SAD pattern at bottom region of deposited Al-Li alloy. (a) Bright Field image of needle-like TB phase, taken along [100]α, (b) SAD patterns view along [110]α zone axis. (c) Bright Field image of needle-like T1 phase, (d) SAD patterns view along [100]α zone axis.

To further identify the phases, XRD patterns at top-middle and bottom-middle region of the as-deposited plate were conducted. As it can be seen in Fig.6, phases at top-middle position (black curve) mainly is consisted of α-Al matrix. Whereas for bottom-middle position (red curve), phases of TB(Al7Cu4Li) and T1(Al2CuLi) appear with the increase peak intensity. The different types of phases are attributed to thermal cycles during laser deposition process.

Fig.6. XRD analysis of the LMDed Al-Li alloy at different position. the disappearance of (220)α and (311)α peaks at top-middle position may due to the different cutting direction.

3.2 Mechanical property

Fig.7. Microhardness from top to bottom region of the as-deposited Al-Li alloy.

Microhardness distribution of the as-deposited Al-Li alloy along the vertical section (YOZ section) are shown in Fig.7. The microhardness increases gradually from 121 HV from the top of the plate to 134 HV at a distance of 30 mm, and then

decreases to about 98 HV at the bottom of the plate. The variations of micro-hardness of alloy is closely related to the precipitations such as TB phase, T1 and Copper-rich phases which could also influence the tensile property of alloy. The

room-temperature

tensile

properties

of

the

as-deposited

alloy

(bottom-middle position) are illustration in Table 3. The ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) are 244 MPa, 168 MPa and 5.0% respectively. Compared with the hot rolled 2A97 alloy[32], the ultimate strength is comparable but the yield strength and elongation are poor. Fracture surface of LMDed Al-Li alloy is shown in Fig.8. Necking is observed, and some small dimples are found on the fracture surface which indicate the failure mode of the alloy belongs to the ductile fractures. Table.3. Tensile properties of LMDed Al-Li alloy.

Sample As-deposited Al-Li alloy (L direction)

UTS, (MPa)

YS, (MPa)

El, (%)

244±16

165±3

5.0±1.6

Hot-rolled 2A97

282

261

18.2

Fig.8. Fracture surface of LMDed Al-Li alloy (a) low magnification of fracture surface, (b) high magnification of fracture surface, (c) low magnification of fracture section, and (d) high magnification of fracture subsurface.

4. Discussion 4.1 Rapid solidification behavior of Al-Li alloy during LMD process In this study, the as-deposited Al-Li alloy shows an obvious dendritic microstructure with underdeveloped secondary arms at top region (Fig.3 (b)), which is significantly different from traditional casting process. It is due to the directional distribution of temperature gradient with a super-quick cooling rate of around 105-106 K/s

[20]

[31]

. The same microstructure can also be observed in arc welding of Al-Li alloy .

In Fig.3(b-d), copper-rich phases appear at the inter-dendritic areas, which is related to equilibrium solute atomic partition coefficient of elements in Al alloy. As is well known, the equilibrium solute atomic partition coefficient of Cu is low (Keq Cu=0.15) in Al alloy[33]. According to Sheil law[31], during LMD process, Cu atom can be continuously discharged into low melting point liquid through the front edge of the solid-liquid interface. Therefore, the enrichment of Cu atoms at the inter-dendritic regions will promote the formation copper-rich precipitates. 4.2 Effect of thermal cycle on microstructure and hardness of alloy during LMD process As mentioned in Fig.3, the morphologies of LMDed Al-Li alloy from top to bottom exhibit different characteristics. It is expected that a significant amount of solid solution and aging can occur during thermal cycling. At initial stage of deposition, high cooling rate as well as directional temperature gradient encourages the formation of dendrites with under-developed secondary dendritic arms (Fig.3 (b)). With continued deposition, thermal cycling and thermal accumulations allow copper-rich phase to dissolve, as shown in Fig.9. And thermal cycling also promotes the formation of TB phase in grains, as shown in Fig.3(e, f).

Fig.9. Dissolution of Copper-rich precipitations at different positions, (a) 6mm from the top, and (b) 12mm from the top.

As numbers of thermal cycling increase with decrease of peak temperature for every cycle, massive precipitation of TB phase are observed. In order to further identify the precipitation temperature of TB phase, the as-deposited Al-Li samples were first solution-heated at 520℃ for 2 h, and then re-heated at different temperatures of 360℃, 400℃, 435℃, 460℃, 500℃ for 5h respectively. Fig.10 shows the microstructures of re-heated samples. As it can be seen from TEM work in Fig.11, TB phases appear in alloy when heat-treated at 360℃. As the temperatures increase from 400℃ to 460℃, the number of TB phase decreases with the obvious coarsening. And TB phases will not precipitate when the aging temperature reaches to 500℃.

Fig.10. Microstructures at X temperature aging for 5h after solution-heated at 520℃ for 2h： ： ((a) X=RT; (b) X=360℃; (c) X=400℃; (d) 435℃; (e) 460℃; (f) 500℃).

Fig.11. TEM images of the Al-Li alloy after aging at 360℃for 5h. (a) bright filed; (b) SAD patterns view along [100]α zone axis.

The whole microstructure evolution of Al-Li alloy during LMD process with the change of microhardness is sketched and can be seen in Fig 12. At initial stage of deposition, large quantities of Cu-rich phases formed along the inter-dendritic area (Fig.12 I). With continued deposition, heat cycling and thermal accumulations allow Cu-rich phase to dissolve and the dissolution of Copper-rich phases will enhance the microhardness of alloy due to the solid-solution effect (Fig.12 stage I to II)

[34]

. And the

microhardness reaches to the maximum value of 134 HV at the distance 16 mm from the top of the plate. When the resident temperature of thermal cycles fall down to the TB phase precipitation temperature from 500℃ to around 360℃, TB phases appear from the inter-dendritic regions (Fig.12 III). TB phase has been conformed a high temperature aging production of Al-Li alloy, and which has been proved as a soft but [34]

brittle phase . As a result, the massive transformation of TB phases lead to the decrease of micro-hardness with the lowest hardness achieved at distance around 72mm (Fig.12 stage III to IV). When the resident temperature declines to the formation temperature of T1 phase (Fig.12 Ⅴ)[26, 34]. A few numbers of T1 phase precipitate out, but due to the limited diffusion rate of atoms at such temperatures, only a small quantity of T1 phases appear with slightly increase of hardness. (Fig.12 stage IV to V).

Fig.12. Sketched phase evolution of a Al-Li alloy during laser deposition process.

Mechanical properties of the as-deposited Al-Li alloy at the middle-bottom region show poor strength and elongation. The loss of tensile properties for the as-deposited Al-Li alloys are mainly caused by the formation of cracks at the inter-dendritic areas. Al-Li alloys posses inherent characteristics like large solidification temperature range, high coefficient of thermal expansion, high shrinkage stresses, and tendency to form low-melting constituents, and to the formation of eutectics. Under the non-equilibrium solidification condition during LMD, low melting Cu elements are segregation to the inter-dendritic areas, which promote the formation of Copper-rich precipitates. Because high coefficient of thermal expansion of Al-Li alloy leads to high stress and strain[21,

30, 35]

. The

solidification cracking will generate from inter-denderitic areas and lead to the final rupture of the alloy which can be seen in Fig.11. Besides, missive transformed TB phase with soft and brittle characteristics can act as the crack initiation sites during tensile test and further reduce the tensile property. Hence, further study based on parameters of LMD process as well as heat treatment is required to improve the mechanical property of LMDed Al-Li alloy.

5. Conclusion In this investigation, an Al-Li alloy plate is firstly prepared by LMD technology. The main conclusions are summarized as follows: The microstructure of the as-deposited Al-Li alloy in upper position of the plate has rapid solidification characteristics. It shows fully dendritic structure with underdeveloped

secondary

arms.

Copper-rich

precipitations

appear

at

the

inter-dendritic region. The thermal cycle during subsequent deposition leads to the dissolution of Copper-rich precipitations as well as the precipitation of TB and T1 phases. The micro-hardness increases gradually from 121 HV from the top of the plate to 134 HV at a distance of 30 mm, and then decreases to about 98 HV at the bottom of the deposited Al-Li plate because of the different microstructures causing by thermal cycling during LMD process. The ultimate tensile strength of the LMDed Al-Li alloy (longitudinal direction) is comparable to the hot-rolled Al-Li alloy (244 MPa), but the yield strength and elongation are poor due to the generation of microcracks causing by low melting Copper-rich phases at inter-dendritic regions as well as soft and brittle TB phase.

ACKNOWLEDGMENTS The authors would like to thank for the financial support provided by the Beijing municipal science and technology plan (Granted Number: D151100001515002)

Reference [1] RJ Rioja, J Liu, The Evolution of Al-Li Base Products for Aerospace and Space Applications, Metall. Mater. Trans. A. 43A (2012) 3325-3337. https://doi.org/10.1007/s11661-012-1155-z. [2] T Dursun, C Soutis, Recent developments in advanced aircraft aluminium alloys, Mater. Des. 56 (2014) 862-871. https://doi.org/10.1016/j.matdes.2013.12.002. [3] VI Elagin, VV Zakharov Modern Al-Li alloys and prospects of their development, Mater. Sci.Heat Treat. 55 (2013) 184-190. https://doi.org/10.1007/s11041-013-9603-y. [4] N Akhtar, W Akhtar, SJ Wu, Melting and casting of lithium containing aluminium alloys, Int. J. Cast Met. Res. 28 (2015) 1-8. https://doi.org/10.1179/1743133614Y.0000000134. [5] N Nayan, SVSN Murty, AK Jha, B Pant, SC Sharma, KM George, GVS Sastry, Processing and characterization of Al–Cu–Li alloy AA2195 undergoing scale up production through the vacuum induction

melting

technique,

Mater.

Sci.

Eng.,

A.

576

(2013)

21-28.

https://doi.org/10.1016/j.msea.2013.03.054. [6] EO Olakanmi, RF Cochrane, KW Dalgarno, A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties, Process Mater. Sci. 74 (2015) 401-477. https://doi.org/10.1016/j.pmatsci.2015.03.002. [7] T DebRoy, HL Wei, JS Zuback, T Mukherjee, Additive manufacturing of metallic components Process,

structure

and

properties,

Process

Mater.

Sci.

92(2018)

112-224.

https://doi.org/10.1016/j.pmatsci.2017.10.001. [8] A Yadollahi, N Shamsaei, SM Thompson, DW.Seely, Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316L stainless steel, Mater. Sci. Eng., A. 644 (2015) 171-183. https://doi.org/10.1016/j.msea.2015.07.056. [9] T Wang, YY Zhu, SQ Zhang, HB Tang, HM Wang, Grain morphology evolution behavior of titanium alloy components during laser melting deposition additive manufacturing, J. Alloys Compd. 632 (2015) 505-513. https://doi.org/10.1016/j.jallcom.2015.01.256. [10] SM Thompson, L Bian, N Shamsaei, A Yadollahi, An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics, Adv. Manuf. 8 (2015) 36-62. https://doi.org/10.1016/j.addma.2015.07.001. [11] C Qiu, GA Ravi, C Dance, A Ranson, S Dilworth, MM.Attallah, Fabrication of large Ti–6Al–4V structures

by

direct

laser

deposition,

J.

Alloys

Compd.

629

(2015)

351-361.

https://doi.org/10.1016/j.jallcom.2014.12.234. [12] GC Li, J Li, XJ Tian, X Cheng, B He, HM Wang, Microstructure and properties of a novel titanium alloy Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si manufactured by laser additive manufacturing, Mater. Sci. Eng., A. 684 (2017) 233-238. https://doi.org/10.1016/j.msea.2016.11.084. [13] X Wu, J Liang, J Mei, C Mitchell, PS Goodwin, W Voice, Microstructures of laser-deposited Ti–6Al–4V, Mater. Des. 25 (2004) 137-144. https://doi.org/10.1016/j.matdes.2003.09.009. [14] X Ran, D Liu, A Li, HM Wang, H Tang, X Cheng, Microstructure characterization and mechanical behavior of laser additive manufactured ultrahigh-strength AerMet100 steel, Mater. Sci. Eng., A. 663 (2016) 69-77. https://doi.org/10.1016/j.msea.2016.03.051. [15] J Jones, M Whittaker, R Buckingham, R Johnston, M Bache, D Clark, Microstructural characterisation of a nickel alloy processed via blown powder direct laser deposition (DLD), Mater. Des. 117 (2017) 47-57. https://doi.org/10.1016/j.matdes.2016.12.062.

[16] MY Shen, XJ Tian, D Liu, HB Tang, X Cheng, Microstructure and fracture behavior of TiC particles reinforced Inconel 625 composites prepared by laser additive manufacturing, J. Alloys Compd. 734 (2018) 188-195. https://doi.org/10.1016/j.jallcom.2017.10.280. [17] H Rao, S Giet, K Yang, X Wu, CHJ Davies, The influence of processing parameters on aluminium alloy A357 manufactured by Selective Laser Melting, Mater. Des. 109 (2016) 334-346. https://doi.org/10.1016/j.matdes.2016.07.009. [18] C Yan, L Hao, A Hussein, P Young, J Huang, W Zhu, Microstructure and mechanical properties of aluminium alloy cellular lattice structures manufactured by direct metal laser sintering, Mater. Sci. Eng., A. 628 (2015) 238-246. https://doi.org/10.1016/j.msea.2015.01.063. [19] XJ Wang, LC Zhang, MH Fang, TB Sercombe, The effect of atmosphere on the structure and properties of a selective laser melted Al–12Si alloy, Mater. Sci. Eng., A. 597 (2014) 370-375. https://doi.org/10.1016/j.msea.2014.01.012. [20] J Li, X Cheng, D Liu, SQ Zhang, Z Li, B He, HM Wang, Phase evolution of a heat-treatable aluminum alloy during laser additive manufacturing, Mater. Lett. 214 (2017) 56-59. https://doi.org/10.1016/j.matlet.2017.11.111. [21] R Xiao, X Zhang, Problems and issues in laser beam welding of aluminum–lithium alloys, J. Mater. Process. 16 (2014 ) 166-175. https://doi.org/10.1016/j.jmapro.2013.10.005. [22] L Cui, X Li, D He, L Chen, S Gong, Effect of Nd:YAG laser welding on microstructure and hardness

of

an

Al–Li

based

alloy,

Mate.

Charact.

71

(2012)

95-102.

https://doi.org/10.1016/j.matchar.2012.06.011 [23] MS Domack, KM Taminge, M Begley, Metallurgical Mechanisms Controlling Mechanical Properties of Aluminium Alloy 2219 Produced by Electron Beam Freeform Fabrication, Mater. Sci. Forum.

519-521

(2006)

1291-1296.

https://doi.org/10.4028/www.scientific.net/MSF.519-521.1291. [24] B Fu, G Qin, X Meng, Y Ji, Y Zou, Z Lei , Microstructure and mechanical properties of newly developed aluminum-lithium alloy 2A97 welded by fiber laser, Mater. Sci. Eng., A. 617 (2014) 1-11. https://doi.org/10.1016/j.msea.2014.08.038 [25] GAO Chong, L Yang, J Yu, MA Yue, Effect of thermo-mechanical treatment process on microstructure and mechanical properties of 2A97 Al-Li alloy, Trans. Nonferrous Met. Soc. China. 24 (2014) 2196–2202. https://doi.org/10.1016/S1003-6326(14)63332-X. [26] PS Chen, BN Bhat, Time-Temperature-Precipitation Behavior in An AI-Cu-Li Alloy 2195, Technical Report, NASA Marshall Space Flight Center, 2002. [27] KH Hou, WA Baeslack, Effect of solute segregation on the weld fusion zone microstructure in CO2 laser beam and gas tungsten arc welds in Al-Li-Cu alloy 2195, J. Mater. Sci. Lett. 15(1996) 208-213). [28] X Zhang, B Liu, X Zhou, J Wang, T Hashimoto, C Luo, Z Sun, Z Tang, F Lu, Laser welding introduced segregation and its influence on the corrosion behaviour of Al-Cu-Li alloy, Corros. Sci. 135 (2018) 177-191. https://doi.org/10.1016/j.corsci.2018.02.044. [29] X Zhang, T Huang, W Yang, R Xiao, Z Liu, L Li, Microstructure and mechanical properties of laser beam-welded AA2060 Al-Li alloy, J. Mater. Process.Technol. 237 (2016) 301-308. https://doi.org/10.1016/j.jmatprotec.2016.06.021. [30] NE Prasad, A Gokhale, RJH Wanhill, Aluminum-Lithium Alloys: processing, properties, and applications, Butterworth-Heinemann, 2013. [31] DW Heard, R Gauvin, M Brochu, Microstructural investigation of controlled short circuiting gas

metal arc welding deposited aluminium–lithium alloy, Can. Metall. Q, 53 (2014) 416-422. https://doi.org/10.1179/1879139514Y.0000000146. [32] GAO Chong, L Yang, J Yu, MA Yue, Effect of thermo-mechanical treatment process on microstructure and mechanical properties of 2A97 Al-Li alloy, Trans. Nonferrous Met. Soc. China. 24 (2014) 2196–2202. https://doi.org/10.1016/S1003-6326(14)63332-X. [33] JY Bai, CL Fan, CL Yang, BL Dong, Effects of thermal cycles on microstructure evolution of 2219-Al during GTA-additive manufacturing, Int. J. Adv. Manuf. Technol, 87(2016) 2615-2623. https://doi.org/10.1007/s00170-016-8633-1. [34] Chen, D.L. and M.C. Chaturvedi, Effects of welding and weld heat-affected zone simulation on the microstructure and mechanical behavior of a 2195 aluminum-lithium alloy, Metall. Mater. Trans. A. 32 (2001) 2729-2741. https://doi.org/10.1007/s11661-001-1025-6 [35] X Cao, C. Poon, Research and Progress in Laser Welding of Wrought Aluminum Alloys. II. Metallurgical Microstructures, Defects, and Mechanical Properties, Mater. Manuf. Processes. 18 (2003) 23-49,DOI: 10.1081/AMP-120017587

Figure captions Fig.1. (a) Surface morphology of as-received Al-Li alloy powders; (b) cross-section morphology of Al-Li alloy powders. Fig.2. (a) Schematic illustration of the LMDed Al-Li alloy thick plate, scanning strategy and deposition direction, (b) as-deposited plate.

Fig.3. Solidification microstructures of the as-deposited Al-Li alloy (a) overview of sample, (b, e) top region; (c, f) middle region and (d, g) bottom region.

Fig.4. EDS analysis at the top position of the as-deposited Al-Li alloy plate (at.%) .

Fig.5. TEM images and SAD pattern at bottom region of deposited Al-Li alloy. (a) Bright Field image of needle-like TB phase, taken along [100]α, (b) SAD patterns view along [110]α zone axis. (c) Bright Field image of needle-like T1 phase, (d) SAD patterns view along [100]α zone axis.

Fig.6. XRD analysis of the LMDed Al-Li alloy at different position. the disappearance of (220)α and (311)α peaks at top-middle position may due to the different cutting direction.

Fig.7. Microhardness from top to bottom region of the as-deposited Al-Li alloy.

Fig.8. Fracture surface of LMDed Al-Li alloy (a) low magnification of fracture surface, (b) high magnification of fracture surface, (c) low magnification of fracture section, and (d) high magnification of fracture subsurface.

Fig.9. Dissolution of Copper-rich precipitations at different positions, (a) 6mm from the top, and (b) 12mm from the top.

Fig.10. Microstructures at X temperature aging for 5h after solution-heated at 520℃ for 2h：((a) X=RT; (b) X=360℃; (c) X=400℃; (d) 435℃; (e) 460℃; (f) 500℃).

Fig.11. TEM images of the Al-Li alloy after aging at 360℃ for 5h. (a) bright filed; (b) SAD patterns view along [100]α zone axis.

Fig.12. Sketched phase evolution of a Al-Li alloy during laser deposition process.

Table captions Table.1.Chemical compositions of powders and as-deposited Al-Li alloy (wt. %).

Table.2. Experiment parameters of LMD process.

Table.3. Tensile properties of LMDed Al-Li alloy.

Highlight：


A third generation Al-Li alloy (based on 2A97 alloy) is successfully fabricated by using LMD technique.




Precipitation behavior of needle-like TB phase is well studied.




The different microstructure lead to the change of microhardness form top to bottom of as-deposited Al-Li alloy.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

We declare that we have no commercial or associative interest that represents a conflict of interest in connection with the work submitted.