Variations of the improved characteristics on microstructure and mechanical properties of Al–Cu alloy fabricated by magnetic field assisted wire-arc additive manufacturing after heat-treated

In this investigation, Al–Cu alloy was used for wire-arc additive manufacturing experiment with and without assisted longitudinal magnetic field (ALMF) introduced, subsequently, heat treatment was conducted, aiming to investigate the variations of the improved characteristics brought by ALMF on microstructure and mechanical properties after heat-treated. The results indicated that the improved pores condition brought by ALMF was greatly weakened after heat-treated, while the promotion for the columnar to equiaxed transition and the improvement to precipitation remained. As for mechanical properties, the improvement of the uniform distribution of microhardness by the ALMF deteriorated. Moreover, the optimisation brought by ALMF on reducing the anisotropy of tensile strength was greatly weakened, and the improvement of elongation disappeared.


Introduction
Wire-arc additive manufacturing (WAAM) is a manufacturing method that uses the arc as the heat source to melt the wire, deposits it layer by layer under the preset command, and directly produces metal components [1].WAAM technology has the advantages of low cost and high deposition efficiency, which is known as sustainable green manufacturing technology [2][3][4].
Since the twenty-first century, the WAAM has been rapidly developed and advanced, with many relatively mature cases in various fields, for example, it has successfully manufactured multidirectional pipeline joints [5], landing gear rib wall structures [6], and crane structure [7] and all served well in their fields.Nevertheless, there are still some tough issues to be addressed, including low surface forming accuracy, segregation, and internal pores, especially for aluminium alloys, the huge difference in the solubility of hydrogen in solid and liquid aluminium causes hydrogen pores to be generated extremely easily, also with a large anisotropy in mechanical properties [8][9][10][11].Therefore, how to improve the microstructure and mechanical properties has always been the focus of research in the field of WAAM.
There have been some methods to achieve this point, including synchronous milling [12], synchronous interlayer rolling [13], synchronous interlayer hammering [14], heat treatment [15], as well as the introduction of CONTACT Yanhong Wei nuaadw@126.comThis article has been corrected with minor changes.These changes do not impact the academic content of the article.Supplemental data for this article can be accessed here.https://doi.org/10.1080/13621718.2023.2236358 the special energy field to assist WAAM [16], etc.And the introduction of the magnetic field is one kind of special energy field-assisted method.
External magnetic fields have been utilised to assist WAAM in recent years, with much positive feedback received.Zhou et al. [17] reported that the introduction of an appropriate external transverse magnetic field could improve the condition of arc plasma in the WAAM process.Qv et al. [18] took an external alternating magnetic field to assist micro-plasma arc-directed energy deposition to control the position of the liquid bridge during droplet transition, further control heat input and dynamically control the metal transfer.Zhao et al. [19] reported that the introduction of an external magnetic field during WAAM of Al-Mg alloy promoted convection and thus broke up dendrites and precipitates under the effect of electromagnetic stirring.
The above literature shows that magnetic fields can improve the morphology and energy state of the arc as well as the melt-drop transition, and microscopically, electromagnetic stirring can effectively break dendrites and improve precipitate distribution.As Derekar KS et al. [20] reported, the mode of the arc, heat input, and arc morphology have a huge impact on hydrogen pore production in WAAM-fabricated compositions of aluminium alloys.Meanwhile, Al-Cu alloys are prone to hydrogen pores between and at the ends of columnar dendrites during solidification [21], and the precipitates in aluminium alloys, such as Al 2 Cu, Al 7 Cu 2 Fe, and Al 2 CuMg particles are prone to form heterogeneous nucleates of hydrogen micropores [22,23].Therefore, the introduction of the magnetic field would theoretically affect the pore conditions in the deposits, and it has been verified in the Al-Mg alloy [19,24].
Meanwhile, heat treatment is also a common way performed after WAAM to improve the properties [25].For Al-Cu alloys, the precipitation of θ phase during aging is the main strengthening mechanism.Zhou et al. [26] reported that during the T6 treatment of WAAMfabricated 2219 aluminium alloy, the supersaturation of copper in the aluminium matrix affects the precipitation strengthening when the solution temperature was lower than 540°C, and the yield strength improved with the increase in solution temperature.Li et al. [27] compared the microstructure and properties of thermomechanically and aging 2219 alloy and found that the thermomechanical treatment resulted in a finer and more uniform distribution of θ and θ ' phases evolving from the G.P.zone, which facilitated the hindrance of dislocation movement and thus resulted in higher strength.Wang et al. [28] confirmed that the strength of 2219 alloy during aging is closely related to the content of θ phase.Meanwhile, Gu et al. [29] mentioned that the deposit of 2319 alloy is prone to higher porosity during heat treatment due to the dissolution of the reticulated θ phase to produce tiny spaces.And the magnetic field may improve the precipitation conditions and further affect the heat treatment process.
Currently, the magnetic field-assisted WAAMfabricated aluminium alloys were mostly Al-Si or Al-Mg, and few of the Al-Cu alloys.The effect of magnetic field on the porosity and θ phase conditions of Al-Cu alloy, and whether the effect and characteristics with an assisted magnetic field introduced in the WAAM process would be strengthened, retained, weakened, or even disappeared during the heat treatment stage is still lack of research.
Accordingly, a typical ER2319 Al-Cu alloy is selected in this investigation.Using the assisted longitudinal magnetic field (ALMF) in WAAM, then the deposits were treated by solid solution and artificial aging [30], so as to investigate the influence of ALMF and heat treatment on the microstructure and properties of the deposits of ER2319 alloy.

Experimental materials
In this investigation, a 2A12 aluminium alloy substrate with a size of 300 mm × 200 mm × 20 mm was selected, on which thin-walled parts were deposited by melt inert-gas (MIG) welding utilising ER2319 welding wire with a diameter of 1.2 mm.Besides, the chemical compositions of the two are shown in Table 1.

Experimental equipment and methods
The experiment is carried out on WAAM integrated platform equipped with ALMF.The integrated platform is illustrated in Figure 1(a), and the schematic diagram of the experiment process is illustrated in Figure 1(b).The WAAM integrated platform is composed of a Panasonic TA-1400 six-axis welding robot, a Panasonic TRANS-30 welding power source, a YD-ABD35 robot control cabinet, a water tank, a wire feeding mechanism, a gas cylinder with shielding gas, and a workbench.WAAM experiment with or without ALMF was conducted on this platform, and the deposition parameters and other details for the WAAM experiment were exhibited in the supplementary document.
The sampling diagram and the dimensions of the tensile samples are shown in Figure 1(c).Three tensile samples were respectively taken along the deposition direction and vertical deposition direction for the tensile text, besides, two samples were respectively taken from the middle for microstructure observation and hardness test.In order to compare with heat-treated deposits more scientifically, repeat taking the same number of tensile samples at the same position, and cut the hardness sample and metallographic sample into two equal parts, then take these samples for subsequent heat treatment.
Figure 1(d) illustrates the heat treatment curve, specifically, the heat treatment parameters were solution at 540°C for 50 min, and then water quenching immediately.The next step is to age at 175°C for 20 h, then air cooling.Finally, four groups of texting samples were obtained, which were under four states, respectively, without ALMF and not heat-treated, with ALMF but not heat-treated, without ALMF but heat-treated, and with ALMF and heat-treated.
The tensile test was carried out on a universal stretching machine, then the fracture morphology was observed by scanning electron microscope (SEM).The microhardness was measured by a Vickers microhardness tester, and the loading force, holding time, and spacing at each point were 200 g, 15 s, and 5 mm, specifically.Meanwhile, after mechanical grinding and polishing of the metallographic sample, the condition of the macroscopic pores was observed by low magnification optical microscopy (OM), and the precipitates were observed by SEM.Moreover, the samples  were corroded by the Kroll reagent, subsequently, the microstructure was observed by OM.

Pores condition analysis
Firstly, the macroscopic morphology of the deposit was photographed.One of the macro characteristics under the condition without ALMF and not heat-treated is exhibited in Figure 2(a).Capturing the middle parts of the deposits under four conditions by a low magnification OM, as shown in Figure 2(b)-(e).Where numerous pores were presented in the deposits and mainly distributed near the fusion line, primarily due to there being great supercooling nearby, causing grains to refine, resulting in much more grain boundaries, lattice gaps, and vacancies, besides, the strengthening phase precipitated a lot along the grain boundaries, these places provided a hotbed for pores adhesion and heterogeneous nucleation [22,29].Thus, a large number of pores were produced here.
In order to analyse the condition of the pores accurately, the number and area of the pores at the top, middle, and bottom of the deposits were counted.The ratio of pores area to cross-section area (Ratio PC ) was calculated by Equation ( 1), where n is the total number of pores in the cross-section, s_average is the average area of the pores, and S is the area of the cross-section.
As shown in Figure 2(f), the number and Ratio PC of pores under the four conditions were counted by binarisation of the pores photos.Figure 2(g) presents the statistical results, it indicated that the condition of the pores is worse in the middle and top than in the bottom under the four conditions, which may be influenced by the thermal cycle in WAAM.Besides, the number and Ratio PC of pores in the three parts of the deposits both became larger after heat-treated, but no matter whether heat-treated or not, the number and Ratio PC of pores are both lower with ALMF introduced.The results prove the optimisation of pores conditions by ALMF and the destruction of pores conditions by heat treatment.
The optimisation of pores condition was mainly owing to the effect of electromagnetic stirring from ALMF.On the one hand, it broke dendrites and precipitates (see next section for details), which weakened the condition of pores nucleation.On the other hand, the effect of electromagnetic stirring on molten metal promoted the overflow of pores, so the introduction of ALMF improved the pores condition overall, this can be demonstrated by the inner wall morphology of the pores in Figure 3(a-d).Comparing the pores morphologies of Figure 3(a) and Figure 3(b), as well as Figure 3(c,d) even after heat-treated, which indicated that the inner wall of the pore is smoother when without ALMF, while the inner wall of the pore became more irregular and rougher with ALMF introduced.At the early stage of solidification, the viscosity and tension of the molten metal are small, the force that hinders the formation of pores in all directions is small, and the pores are easy to expand, so the inner wall of the pores produced at this time is smooth.However, the opposite is true for the later stage of solidification, thus the inner wall of the pore became more irregular and rougher.This proves that the pores produced at the early stage of solidification were retained without ALMF introduced, while with ALMF, the pores produced at this time overflowed, and only the pores produced at the late stage of solidification remained.
Nevertheless, the pores condition deteriorated significantly after heat treatment.This is because the generation of micropores in Al-Cu alloy during heat treatment is mainly controlled by the hydrogen micropore precipitation mechanism and it is closely related to the dissolution of the precipitates [31].As shown in Figure 3(e-h) which are under heat-treated, tiny spaces were produced where the precipitates dissolved, simultaneously, hydrogen atoms were expelled from the Al matrix, and these spaces provided storage for hydrogen atoms, thereby forming secondary micropores.Figure 3(i) illustrates this process.Attributed to the introduction of ALMF promoted the spillage of hydrogen atoms, and improved the distribution of the precipitates (See next section for details), thus providing a good foundation for pores condition of the deposit before heat treatment, the condition of the pores after heat treatment was still superior to that without ALMF, but significantly worse than that before.

Microstructure under four conditions
Since the middle area of the deposit is the most representative, the microstructure of the middle area of the deposits in four states was observed respectively, and more parts of the metallographic photos are detailed in the supplementary materials.
The grain dendrite spacing near the fusion line was larger and the equiaxed degree was higher with ALMF introduced comparing with Figure 4(a,b).As demonstrated in Figure 4(e), because the electromagnetic stirring has broken part of the dendrite arms, some even disappeared, the dendrite distribution became sparse, and the broken dendrites acted as a new heterogeneous nucleation core to promote the CET transition.Meanwhile, the electromagnetic stirring reduced the temperature gradient of the molten pool's centre and edge, contributing to promoting the CET transition of the dendrites [[19[].According to Figure 4(c,d), although the grains grow significantly after heat-treated, the area of the equiaxed crystal layer near the fusion line with ALMF introduced was still thicker than that without ALMF introduced.Moreover, as marked in yellow in Figure 4(c,d), where adjacent pores were moving to each other and attempting to combine into larger pores.This was operated by the Oswald ripening mechanism during heat-treated [32], which resulted in the increase of large pores after heat treatment.
Furthermore, the morphology and distribution characteristics of the precipitates in the middle region of the thin-walled deposits under four conditions were observed by backscattered electron (BSE), as shown in Figure 5(a)-(f).Energy Dispersive Spectrometer (EDS) analysis was performed on the precipitates and matrix, and the results were exhibited in Figure 5(h).It can be determined that the dark gray part was the matrix phase α-Al, and the precipitates in light gray included the Al 2 Cu-θ phase and a small amount of iron-rich phase.
By comparing Figure 5(a,b), the shape of the precipitates changed from a long strip to a short strip after the introduction of ALMF.Further, the average length and average spacing of the precipitates have been calculated.The average length and spacing of the precipitations in Figure 5(a) were 71.3 and 48.3 μm, these of the precipitates in Figure 5(b) were 54.4 and 30.6 μm.Equation 2 is the modified Orowan equation proposed by Zhu AW et al. [33] Where r_p is the strengthening stress, which can reflect the strengthening effect of the precipitated particles.The G, b, r_0 are the shear modulus, the magnitude of the Burgers vector, and the inner cut-off radius for the calculation of the dislocation line tension, respectively, and considering them as constants due to the same matrix.Besides, l and d are the average length and spacing of the precipitated particles.
In this investigation, when the ALMF was introduced, the value of d/l increased from 1.47 to 1.77, the lnl/d increased from 0.09 to 0.13, meanwhile, ln (0.1/r_0)/d increased with d decreased.Therefore, the r_p enhanced overall, proving the ALMF improved the morphology and distribution of the precipitates, which will further enhance the strength of the WAAMfabricated composition.By contrast, there was no significant difference in the morphology of the precipitates after heat treatment.In Figure 5(c)-(f), a large number of precipitates have dissolved into the matrix during the solution stage, and the precipitates all precipitated in the form of tiny particles and short rod-like after aging.However, the segregation of the precipitates in the deposit with ALMF introduced was better than that without ALMF introduced.Figure 5(i) simply illustrates the homogenous process, mainly attributed to ALMF's electromagnetic stirring promoting the convection of molten metal further to promote uniform distribution of elements.

Mechanical properties analysis
Figure 6(a) shows the Vickers microhardness of the deposits under four conditions, whose average values were 78.5 HV, 83.4 HV, 126.7 HV, and 133.1 HV, respectively.After the introduction of ALMF, whether heat treated or not, the average hardness of the deposit was slightly higher than that of the deposit without ALMF, which mainly owing to the optimisation of the morphology and distribution of the precipitates by ALMF introduced.Furthermore, by analysing the hardness fluctuation amplitude under four conditions, the standard deviations were 9.01, 6.11, 9.24, and 7.62, respectively.It indicated that the microhardness distribution uniformity of the deposits with ALMF was better.Not only due to the optimisation of the precipitation but also because the CET transition enhanced the hardness uniformity.Since the mechanical anisotropy of equiaxed crystals is smaller than that of columnar crystals.Although the microhardness after heat treatment was improved, the fluctuation became larger.Generally, defects such as micropores and segregation are often concentrated at the interlayer [34], which makes the microstructure and mechanical properties of the interlayer and the intralayer uneven, resulting in hardness fluctuation.These defects were not improved or even worsened after heat treatment, so the mechanical properties' unevenness between them was intensified.
Figure 6(b) shows the engineering stress-strain curves, and Figure 6(c) shows tensile properties.It can be concluded from them that after the introduction of ALMF, both transverse and longitudinal tensile strengths were improved.And the tensile strength was further improved after heat treatment, but the anisotropy between transverse and longitudinal tensile strength increased too.Moreover, the elongation enhancement got previously by the introduction of ALMF disappeared after heat treatment.
According to the longitudinal fracture surface morphology of Figure 6(d)-(g), except for dimples marked with a yellow box, which were the characteristics of ductile fracture, a large number of pores were distributed in the fracture, and pores could reduce the force area of the section.Meanwhile, the brittle and hard θ phases were distributed in the dimples.Since the elastic-plastic deformation ability of θ phase and Al matrix are completely different, their deformation degree is uncoordinated, and when the mismatched deformation accumulates beyond the critical value, θ phase will be separate from the matrix to form cavities, and once damage occurs, the cracks will rapidly expand along these θ phases distributed at the grain boundaries [35].Whether heat treated or not, the pores and precipitation conditions with ALMF were superior to that of those without ALMF, so the tensile performance was better with ALMF introduced.
Figure 6(h) illustrates the distribution of precipitates and pores conditions under four processes, numerous pores and the precipitates distributed near the fusion line formed weak zones.For the longitudinal tensile samples, the weak zones were perpendicular to the force direction, the pores and the precipitates would provide the resources for crack initiation and propagation.Therefore, the longitudinal tensile strength and elongation were lower than that of the transverse, resulting in anisotropy of the tensile strength.
After heat treatment, the strength of the matrix was strengthened by the solution treatment, but the pores condition and segregation of the interlayer deteriorated more seriously than that in the intralayer, so the anisotropy of tensile strength further [35].
Meanwhile, the deterioration of the pores condition also led to the rapid fracture of the tensile samples after entering the yield stage, so the elongation decreased.The difference in the elongation of the deposits with or without ALMF introduced also disappeared, the transverse and longitudinal elongation was only about 5% under the four conditions.

Conclusions
In this investigation, the ALMF was introduced in the WAAM process of ER2319 aluminium alloy and subsequently heat-treated.The ALMF had an improved effect on ER2319 deposition, and the improved characteristics were partially retained after heat-treated.Specific findings are as follows.
(1) The introduction of ALMF improved the pores conditions in the deposit, while the heat treatment aggravated the pores conditions.Both the number of pores and the Ratio PC decreased after the introduction of ALMF, while both indicators became larger again after heat treatment.(2) Even though the grains grow significantly after heat treatment, the CET transition of partial grains induced by the introduction of ALMF was retained after heat treatment.(3) After heat treatment, the improvement of precipitation distribution brought by ALMF was still retained, but the optimisation of morphology disappeared.(4) After heat treatment, both microhardness and tensile strength were significantly improved.However, due to the great influence of porosity on tensile properties and the significant deterioration of pores condition after heat treatment, the effect of introducing ALMF on improving the elongation and reducing the anisotropy of transverse and longitudinal tensile strength was greatly weakened.

Figure 1 .
Figure 1.Experimental process and principle.(a) WAAM integrated platform; (b) schematic diagram of WAAM with ALMF introduced; (c) heat treatment process curve; (d) sampling diagram of test samples.

Figure 2 .
Figure 2. (a) macroscopic characteristics of as-deposited; (b)-(e) pores conditions under four processes; (f) image binarisation; (g) number and the ratio of pores area to the cross-section area of pores under four conditions.

Figure 3 .
Figure 3. (a)-(d) morphology of the inner wall of the pores; (e)-(h) tiny spaces produced after heat treatment; (i) the pores evolution diagram.

Figure 4 .
Figure 4. (a)-(d) the middle microstructure under four conditions; (e) schematic diagram of dendrite broken and growth of Al-Cu alloy with ALMF introduced.