Microstructure and mechanical properties of TiC nanoparticles reinforced 7075 aluminium alloy fabricated by oscillating laser-arc hybrid additive manufacturing

In this work, TiC nanoparticles (TiCps) reinforced 7075 aluminium alloy was fabricated via an oscillating laser-arc hybrid additive manufacturing process. To show the beneficial effect of TiCps addition on microstructure and mechanical properties, a conventional 7075 aluminium alloy was also manufactured for comparison. Upon the addition of TiCps, remarkable grain refinement was achieved with a decrease in average grain size from 111.8 to 12.5 µm, and grain boundary segregation was alleviated. It was also found that many secondary phases precipitated at the grain interior in the deposit with TiCps. Owing to the refined microstructure and favourable precipitation, as-deposited 7075 aluminium alloy with TiCps showed enhanced tensile properties of ultimate tensile strengths of 346 and 355 MPa along scanning and build directions.


Introduction
Aluminium alloys are important structural materials in aerospace and other industries owing to their desirable properties of low density, high specific strength and superior corrosion resistance [1,2]. As a typical ultra-high-strength aluminium alloy, the 7075 (Al-Zn-Mg-Cu series) aluminium alloy is widely utilised in modern constructions of aircraft, spacecraft and satellites [3]. Large-scale aluminium alloy components made of 7075 aluminium alloy, such as fuselages and wing ribs, are still routinely manufactured by casting, forging, and welding processes [4], which result in the long manufacturing cycle, low material utilisation efficiency and high cost [5]. Wire-based additive manufacturing enables the production of large structures with reduced material waste [6].
Several attempts have been made to fabricate Al-Zn-Mg-Cu alloy parts using wire-based AM processes. Xu et al. [7] found that grains tend to transform from coarse grains at inner layers to fine grains at inter-layers during wire and arc additive manufacturing (WAAM) of the Al-Zn-Mg-Cu alloy. They also reported the uneven microstructure resulted in anisotropy of tensile strength (300 MPa along the scanning direction and 250 MPa along the build direction). Wang et al. [8] explored wire-based laser metal deposition of the 7075 aluminium alloy. They demonstrated that large columnar grains were aligned along the build direction. Dong et al. [9] conducted a study of microstructure and mechanical properties of WAAMed 7055 aluminium alloy and observed the surfaces of coarse columnar grains, continuous brittle secondary phases and pore defects in the fracture surfaces of tensile test samples, which are suggested as the main factors that hindered the mechanical properties. Dong et al. [10] further analysed the influence of interlayer temperature on microstructure characteristics. They stated that higher interlayer temperature contributed to a lower aspect ratio of columnar grains and faster dynamic precipitation progress. From the aforementioned results, the uneven microstructure is a critical issue that affects the mechanical properties of the wirebased AMed 7075 aluminium alloy. Therefore, refining and homogenising microstructure are necessary for fabricating structurally sound 7075 aluminium alloys with desirable mechanical properties.
Recently, a novel approach of introducing ceramic particles to the solidification processing of aluminium alloy has drawn attention to refining the microstructure [11][12][13][14]. And several attempts were also made in powder-based AM owing to the easiness of adding ceramic particles. Fan et al. [15] conducted laser powder bed fusion (LPBF) of TiC particles enhanced 2024 aluminium alloy. They found that coarse columnar grains were transformed into fine equiaxed grains. Li et al. [16] stated that sub-micron grains generated and nano-scale Si precipitated in LPBFed AlSi10Mg alloy with the addition of TiB 2 nanoparticles. In wire WAAM, efforts were also made to introduce ceramic particles to refine microstructure and enhance mechanical properties. Yuan et al. [17,18] used the surface coating method to add TiN particles in WAAM of the Al-Zn-Mg-Cu alloy. They found that coarse columnar grains were transformed into fine equiaxed grains. Jin et al. [4,19] also investigated the effect of adding ceramic particles in WAAM of the 2219 aluminium alloy via surface coating. However, it is unfeasible to build large components using surface coating during the waiting time of inter-layer, so it cannot be utilised for practical applications. Thus, the incorporation of TiC ps particles from wire feedstock into the material helps to achieve a uniform structure.
In this work, TiC nanoparticles (TiC ps ) reinforced 7075 aluminium (7075 Al + TiC ps ) filler wire was used as a feedstock to fabricate thin-wall deposits. The oscillating laser-arc hybrid additive manufacturing (O-LHAM) process was chosen due to its ability to stir additional TiC ps and promote a uniform distribution through the oscillation of the beam. The microstructure, secondary phases characteristics and mechanical properties of the 7075 Al + TiC ps deposit were symmetrically analysed. To show the beneficial effect of TiC ps addition, a 7075 Al deposit without TiC ps was also fabricated for comparison. This work can provide a pathway to refine microstructure and enhance mechanical properties in wire-based AM of the aluminium alloy.

Materials and experimental procedures
The commercial TiC ps reinforced 7075 aluminium alloy wire (7075 + TiC ps ) was used as a feedstock, which was fabricated via molten salt-assisted solidification nanoprocessing [20]. For comparison, the conventional 7075 aluminium alloy wire was also employed in this study. The SEM images of the two types of wires are shown in Figure S1. The diametres of these two types of wires were both 1.2 mm. The main chemical compositions of substrate and wires were detected using inductively coupled plasma spectrometre (ICP) in the Analysis Test and Computing Center of Harbin Institute of Technology, as listed in Table S1. According to the measured Ti content in 7075 + TiC ps wire, the content of TiC ps is derived as c. 1.875 wt-%. The substrate used in this work is 7075 aluminium alloy with dimensions of 200 mm × 100 mm × 15 mm. Before experiments, the surface of the substrate was mechanically cleaned and then wiped with acetone to remove the oxide film. Figure 1 shows the schematic diagram of the O-LHAM system used in this work, which predominantly consists of a fibre laser (YLS-10000, IPG), a laser processing head (D50 Wobble, IPG), a CMT welder (TPS3200, Fronius) and a 6-axis industrial robot (KR60, KUKA). The wavelength of the laser is 1070 nm and the beam diametre at the focal position is 0.4 mm. Conventional CMT mode was used in the experiment. The welding torch was perpendicular to the surface of the substrate. The oscillating laser processing head was ahead of the welding torch at a 45°angle. The 7075 Al and 7075 Al + TiC ps deposits were fabricated via the O-LHAM process using a unidirectional deposition strategy. Argon was used as a shielding gas in the experiment. All the used parameters are listed in Table S2. Scanning speed and oscillating parameters were selected according to our previous work [21]. Figure S2(a) shows the sampling locations of metallographic and tensile test samples. Metallographic samples were cut along the cross-section and then were ground and polished to a mirror-like plane according to standard procedures, i.e. ground by 180, 400, 1000, 2000 and 3000 grit abrasive papers and then polished using the diamond compounds with diametres of 1.0 and 0.25 μm on the disk polishing machine. The samples were etched by Keller's reagent (2.5 ml HNO 3 + 1.5 mL HCl + 1.0 mL HF + 95 mL H 2 O) for macro morphology observation by an optical microscope (OM). A scanning electron microscope (SEM, SU5000, Hitachi) equipped with an energy-dispersive X-ray spectrometre (EDS) was used to analyse microstructure and secondary phases morphologies. Electron back-scattered diffraction (EBSD) samples were ground, mechanically polished and electropolished to analyse average grain size and texture characteristics via the above SEM with an Aztec EBSD system. Channel 5 software was used to process EBSD data. The microhardness of the deposits was tested by a Vickers microhardness tester at a load of 0.49 N and a loading time of 10 s. The tensile test was conducted using a universal testing machine at a tensile rate of 1 mm min −1 . The detailed dimensions of tensile specimens are provided in Figure S2(b). Figure 2 shows the OM images of cross sections of 7075 Al and 7075 Al + TiC ps deposits fabricated by oscillating laser-arc hybrid additive manufacturing. The two deposits showed significantly different microstructures. In the 7075 Al deposit, the microstructure exhibited a typical hierarchical characteristic, as presented in Figure 2(a). The inner-layer zone (Figure 2(b)) was composed of coarse columnar grains, whose growth directions were perpendicular to the boundary of the molten pool because grains tend to grow along the direction of the maximum temperature gradient. The inter-layer zone in Figure 2(c) consists of coarse equiaxed grains. Pores more frequently occurred in the inter-layer zone. It could be due to the significant difference in hydrogen solubility between liquid and solid aluminium [22]. In 7075 Al + TiC ps deposit, the microstructure was refined significantly. Fine equiaxed grains were observed in both inner-layer (Figure 2(e)) and inter-layer zones (Figure 2(f)). Coarse columnar grains were transformed into fine equiaxed grains owing to the heterogeneous nucleation generated by TiC ps .

Grain morphology
In order to show the beneficial effect of adding TiC ps , the microstructures of 7075 Al and 7075 Al + TiC ps deposits were further examined using EBSD. The EBSD test was carried out at the region where the field of the microscope contained at least one inner-layer and one inter-layer. Figure 3(a) shows the microstructure distribution of the 7075 Al deposit. Compared with the 7075 Al deposit, the microstructure of the 7075 Al + TiC ps deposit was remarkably refined. As shown in Figure 3(e), equiaxed grains were evenly distributed in the deposit. The boundaries between inner-layer zones and inter-layer zones could not be observed in Figure 3   7075 Al + TiC ps deposit also proved the grain size was more stable and the microstructure was more homogeneous. Figure 3(c,g) shows the grain boundary angle distributions of the two deposits. Low-angle grain boundaries (LAGBs) indicate the grain boundary angle is smaller than 15°, while the high-angle grain boundaries (HAGBs) represent that is larger than 15°. The proportion of HAGBs for 7075 Al deposit is 67.4%, while that for 7075 Al + TiC ps is 81.0%. TiC ps could act as the heterogeneous nucleation sites and promote columnar to equiaxed grain transition. The equalisation effect generated by TiC ps contributed to more random crystal orientation, which motivated the transformation of LAGBs to HAGBs. Figure 3(d,h) shows the texture characteristics of 7075 Al and 7075 Al + TiC ps deposits, respectively. The 7075 Al deposit had the maximum orientation density of 14.62 on the (001) basal plane due to the orientated growth of columnar grains, while the texture feature on other low exponential basal planes was not obvious. Orientation density decreased to 1.46 on the (001) basal plane as a result of the promoted columnar to equiaxed transition induced by TiC ps , indicating a splendid grain refinement and homogenisation effect.

Secondary phases characteristics
For additively manufactured aluminium alloy, morphologies and distribution of secondary phases play a vital role in determining mechanical properties and fracture behaviours. Hence, SEM was used to characterise the secondary phases of the two deposits. Figure  4(a) shows an SEM image of columnar grains in the inner-layer zone for the 7075 Al deposit. The enlarged SEM image of secondary phases on grain boundaries is provided in Figure 4(b). Dendrite and discontinuous banded phases were observed in columnar grain boundaries, while short rod-like phases and granular phases existed in the grains. Figure 4(a1-a4) shows the EDS mappings corresponding to Figure 4(a). The secondary phases were Zn, Mg, and Cu-rich phases. The precipitation sequence of 7075 aluminium alloy is α Al → GP zones → η'-MgZn 2 → η-MgZn 2 [23]. In fact, in the η-MgZn 2 phase, the Zn atoms can be replaced by Cu and Al atoms easily to form a continuous solid solution without influencing the lattice structure. Therefore, the η phase almost always contains Cu and Al in the Al-Zn-Mg-Cu aluminium alloy, which is written as the η-Mg(Zn, Cu, Al) 2 phase. EDS point analysis was carried out at grain boundaries and the interior of grains to identify the specific type of secondary phases. Corresponding results are listed in Table S3. Dendrite and banded phases at columnar grain boundaries (P1 and P2) have high Al content, and the atomic ratio of (Zn + Cu) to Mg was less than 2. When taking the Al content into consideration, the atomic ratio of (Zn + Cu + part of Al) to Mg could reach 2. Therefore, considering the surplus Al atoms, dendrite and banded phases at columnar grain boundaries were α Al +η-Mg(Zn, Cu, Al) 2 eutectics. Short rod-like phases were broken secondary dendrite arms, so their phase composition was consistent with dendrites at grain boundaries. Granular phases in columnar grains (P3 and P4) had similar atomic ratio features with dendrite and banded phases at grain boundaries. High Al content was attributed to a larger reacting area of the electron beam in SEM than that of a single granular phase, so excess Al atoms in α Al were detected and contained in EDS results. These granular phases were identified as η phases instead of α+η eutectics which was consistent with the results in the literature [9,24,25]. Figure 4(c) shows an SEM image of equiaxed grains in the inter-layer zone of the 7075 Al deposit. The enlarged SEM image of secondary phases on grain boundaries is provided in Figure 4(d). The atomic ratio feature of secondary phases in equiaxed grains had no obvious difference with columnar grains. Secondary phases of equiaxed grains also consisted of α Al +η-Mg(Zn, Cu, Al) 2 eutectics at grain boundaries (P7 and P8). Meanwhile, the granular η-Mg(Zn, Cu, Al) 2 phase was found in the grains as well (P5 and P6). Moreover, the equiaxed grain boundaries were more continuous than columnar grain boundaries. During the solidification process of the molten pool, the solubility of alloying elements (Zn, Mg and Cu) in α Al reduced with the decrease in temperature. As a consequence, alloying elements were repelled to the liquid phase, i.e. mainly the top region of the inner-layer zone, which was transformed into the inter-layer zone after the deposition of the next layer. Thus, the concentrations of alloying elements were higher in inter-layer zones than in inner-layer zones, contributing to the more beneficial condition of nucleation and growth of secondary phases, which eventually promoted the formation of continuous equiaxed grain boundaries. Figure 5(a,b) shows SEM images of equiaxed grains in the 7075 Al + TiC ps deposit. There were no obvious boundaries between inter-layer zones and innerlayer zones. As shown in Figure 5(a), fine equiaxed grains were evenly distributed in the deposit. The thickness of grain boundaries decreased significantly upon the addition of TiC ps , which led to the alleviation of grain boundary segregation. The EDS results are shown in Figure 5(a1-a5) and summarised in Table S4. The blocky phases at the junctions of equiaxed grains (P9 and P10) had relatively high Ti content, which was identified as TiC ps [4]. It is found that the grain boundaries (P11 and P12) consisted of non-ignorable Mg, Zn and Cu elements. Taking part of the Al content into account, the atomic ratio of (Zn + Cu + Al) to Mg could reach 2. Thus, grain boundaries were α Al +η-Mg(Zn, Cu, Al) 2 eutectics. Compared with the deposit without TiC ps , a large number of fine needle-like phases (seen in the area that the arrows point to in Figure  5(b)) precipitated at the interior of equiaxed grains in the deposit. These fine needle-like phases were η'-Mg(Zn, Cu, Al) 2 as described by Wang et al. [26] and Fu et al. [27]. From the results of the 7075 Al deposits in Figure 4(b,d), no obvious η' phase could be observed. The η' phase and α-Al matrix have a completely coherent interface which results in a much stronger interfacial bonding [28]. Since the η' phase is the dominant strengthening phase in the Al-Zn-Mg-Cu alloy, the precipitation of the η phase could increase the microhardness and strength of the 7075 Al + TiC ps deposit. TiC ps introduced nucleation undercooling during the solidification of the aluminium alloy molten pool, which was two orders of magnitude lower than constitutional supercooling. Hence, nucleation undercooling counteracted the constitutional supercooling, and then promoted the columnar to equiaxed transformation and alleviated grain boundary segregation [29][30][31]. The alleviation of grain boundary segregation meant that the concentrations of alloying elements increased at the interior of grains. Adequate concentrations of alloying elements provided the driving force of secondary phase precipitation, so the precipitation of the η phase was motivated.
The distribution mapping of Ti can indicate the distribution of TiC ps in the deposit (Figure 5(a5)). TiC ps were evenly dispersed at the interior of equiaxed grains, which promoted the multi-site nucleation and grain refinement of α-Al. Evenly dispersed TiC ps were ascribed to the enhancement of the fluidity of the molten pool induced by the stirring effect of the oscillating laser [32]. A few TiC ps could be observed at grain boundaries, which played the role of restricting the growth of grains and hindered the grain boundary migration. which is higher than that of 7075 Al deposits from 90 to 130 HV. The higher hardness of 7075 Al + TiC ps was attributed to refine equiaxed grains and the hardening effect of η phase distributed in the Al matrix. Meanwhile, the microhardness curve of the 7075 Al deposit showed relatively larger fluctuations, which were attributed to uneven microstructures with layer characteristics. Higher microhardness values were located in inter-layer zones due to the hardening effect generated by finer equiaxed grains and higher concentrations of alloying elements. In contrast, the value of microhardness of the 7075 Al + TiC ps deposit was much more stable due to the refined and homogeneous microstructure. In order to quantitatively characterise the difference in microhardness, the average values and standard deviations of microhardness are calculated and exhibited in Figure 6(b). The average microhardness and standard deviations of the 7075 Al deposit were 112.7 and 7.49 HV, respectively. Upon the addition of TiC ps , the average microhardness of the deposit increased to 143.4 HV, and the standard deviation decreased to 3.46 HV. The average microhardness increased by 27.2% and the standard deviation decreased by 53.8%, respectively.  Al + TiC ps deposits. For the 7075 Al deposit, UTS along the scanning direction and build direction were 210 and 247 MPa, respectively, and EL were 3.1% and 5.9%, respectively. Upon the introduction of TiC ps , the tensile properties of the deposit were significantly improved. UTS increased to 346 and 355 MPa along with scanning and build directions, respectively. For the EL, it increased to 7.9% and 10.3%, respectively. Compared with the 7075 Al deposit, UTS increased by 64.8% and 43.7% and EL increased by 154.8% and 74.6% along the above two directions. The difference between UTS and EL in the two directions was attributed to the anisotropic microstructure. For the tensile specimens along the scanning direction, the tensile force was nearly perpendicular to the grain boundaries of columnar grains. Cracks easily formed and propagated along the brittle eutectics at columnar grain boundaries, which led to lower mechanical properties than the specimens along the build direction. The tensile properties enhancement of the deposits was attributed to grain refinement, dispersion strengthening of TiC ps and precipitation strengthening of η phases [33,34].

Mechanical properties
Fracture surface analysis was conducted to reveal the fracture patterns of the tensile specimens. Figure  7 shows the fracture surface morphologies of the deposits. In the 7075 Al deposit, many exposed external surfaces of columnar grains existed on the fracture surface along the scanning direction, suggesting that the cracks dominantly propagated along columnar grain boundaries under tensile stress. In Figure 8(a), secondary cracks were non-visible around the fracture path, suggesting the fracture was rapid and the deformation of specimens was small. Moreover, large-size (c. 300-500 μm) pores were distributed regularly on the fracture surface along the scanning direction, indicating the region was one single deposited layer. These features suggested that the fracture pattern of the 7075 Al deposit along the scanning direction was an intergranular brittle fracture. In terms of the build direction, explicit crack propagation paths and numerous smaller pores (c. 30-180 μm) are exhibited on the fracture surface. The fracture surface presented typical coarse granular morphology. The initiation and propagation of cracks were similar to the scanning direction ( Figure  8(b)). The intergranular brittle fracture was the fracture pattern of 7075 Al deposit along the build direction.
For the 7075 Al + TiC ps deposit, the fracture surfaces of both the scanning direction and build direction were characterised by densely distributed fine dimples, indicating a ductile transgranular fracture. Cracks formed at grain boundaries, while TiC ps and fine spot-like η phase could hinder the crack propagation (Figure 8(c,d)). Evenly well-dispersed TiC ps in the α-Al matrix played a role in pinning the dislocations. Hence, dislocation glide needed more energy to bypass TiC ps and cut through the η phase and then mechanical properties were enhanced remarkably.

Conclusions
In this work, TiC nanoparticles reinforced 7075 aluminium alloy was fabricated using a novel process of oscillating laser-arc hybrid additive manufacturing. The microstructure, secondary phases characteristics and mechanical properties of the 7075 Al + iC ps deposit were symmetrically investigated with the comparison of the 7075 Al deposit. The following conclusions can be drawn: (1) Due to the multi-site nucleation effect generated by TiC ps , fine equiaxed grains were evenly distributed in the 7075 Al + TiC ps deposit, while the 7075 Al deposit consisted of layer microstructures of coarse columnar grains and equiaxed grains. The 7075 Al + TiC ps deposit showed reduced texture tendency, especially on the (100) basal plane, and an average grain size of 12.5 μm, which was significantly lower than that of 118 μm of the 7075 Al deposit. (2) Upon the addition of TiC ps , dendrite and thickbanded eutectics at grain boundaries were eliminated and grain boundary segregation was alleviated. A large number of fine needle-like η phases that acted as dominant strengthening phases were precipitated at the interior of grains in the 7075 Al + TiC ps deposit. (3) The 7075 Al + TiC ps deposit showed an ultimate tensile strength of 346 MPa along the scanning direction and 355 MPa along the build direction, respectively, while those for 7075 Al were 210 and 247 MPa, respectively. Owing to the introduction of TiC ps , the elongation also increased from 3.1% to 7.9% and 5.9% to 10.3% along the two directions, respectively. The enhanced mechanical properties were attributed to grain refinement, dispersion strengthening and precipitation strengthening.

Disclosure statement
No potential conflict of interest was reported by the author(s).