Surface modification of aluminum alloys with carbon nanotubes by laser surface melting

Surface & Coatings Technology 377 (2019) 124930

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Surface modification of aluminum alloys with carbon nanotubes by laser surface melting

T

L.A. Ardila-Rodrígueza, , B.R.C. Menezesb, L.A. Pereirac, R.J. Takahashia, A.C. Oliveiraa, D.N. Travessaa ⁎

a

Institute of Science and Technology, Federal University of São Paulo (UNIFESP), São José dos Campos, SP 12231-280, Brazil Division of Fundamental Science, Technological Institute of Aeronautics (ITA), São José dos Campos, SP 12228-900, Brazil c National Institute for Space Research – INPE, São José dos Campos, SP 12227-010, Brazil b

ARTICLE INFO

ABSTRACT

Keywords: MWCNT Aluminum alloy Laser surface melting

Carbon nanotubes (CNTs) could be an excellent reinforcement for metal matrix composites, specifically for composites with aluminum or aluminum alloy matrix. Surface modification to improve hardness and other material properties has been performed by laser surface melting (LSM) process, where the laser beam melts the substrate together with alloying elements or reinforcing phase additives. In this work, Multiwalled CNTs (MWCNTs) were mixed by the electrostatic adsorption process with aluminum powder and the resulting mixed powder was laser melted on the surface of a 6061-aluminum alloy substrate. As a result, a modified substrate surface has been obtained from the Al/MWCNT – substrate co-melting, dilution and re-solidification processes. This modified layer was obtained by different LSM parameters and were characterized by Optical (OM) and Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Field Emission Gun Scanning Electron Microscopy (FEG-SEM), X-Ray Diffraction (XRD) and Raman Spectroscopy (RS). The resulting mechanical behavior was evaluated by Vickers microhardness tests. The results showed that the presence of MWCNT in the aluminum powder improves the laser energy absorption, leading to the formation of a deeper modified layer, with segregation of Si particles homogeneously dispersed and improving the hardness. The formation of Al4C3 was not observed, evidencing that the MWCNTs did not react with the molten Al matrix during the LSM process.

1. Introduction

role for the future of the aerospace industry [4]. In the development of metal matrix composites (MMC), literature concerning the use of CNTs has reported lower mechanical properties than the expected from the classical mixtures rule. This has been principally ascribed to the difficulties in the dispersion of the CNTs in the metal matrix that reduces their expected performance as phase reinforcement [5–7]. In this sense, LSM represents an interesting process to complete the consolidation of such type of metal matrix composites [8]. The high heating and cooling rates, of the order of 103–108 K/s, and the molten metal dynamics produced by the laser beam, can help in the homogeneous dispersion of the reinforcement phase in a fine grain matrix structure. By changing the laser thermal input (laser beam power and scanning speed), parts with differentiating microstructure and hardness could be obtained [9]. LSM has been used to improve the surface properties such as hardness, wear resistance, and corrosion resistance of several alloys. For the obtainment of coatings, this technique consists in creating in

Laser surface melting (LSM) has been of great interest in the last years because of its capacity to improve both the morphology and structure of the materials. Bulk processing and surface modification, such as surface re-melting, local compositional changes and/or phase transformation, among many other techniques, can be performed with precise control. The high energy density reached by the laser beam, concentrated in a very small area, enables the efficient use of the energy to modify the material with short exposure times. Carbon nanotubes (CNTs) show very interesting properties, such as high elastic modulus (between 1 and 1.8 TPa) and high mechanical strength (between 50 and 150 GPa), associated with a low density (1.0 and 1.5 g/cm3) and high aspect ratio [1–3]. The use of CNTs as the reinforcement phase in metal matrix composites is a very promising strategy to develop high-performance materials for the transport industry [3,4]. Specifically, it is believed that CNTs will have a significant

⁎

Corresponding author. E-mail address: [email protected] (L.A. Ardila-Rodríguez).

https://doi.org/10.1016/j.surfcoat.2019.124930 Received 11 March 2019; Received in revised form 5 July 2019; Accepted 24 August 2019 Available online 24 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 377 (2019) 124930

L.A. Ardila-Rodríguez, et al.

situ layers of composites, metals or alloys, depositing and/or melting the desired materials over the surface of a substrate using a laser beam. The literature has reported the use of the LSM to produce composite layers with improved hardness and wear resistance on aluminum and aluminum alloy substrates. These obtained composites includes TiC and SiC [10], WC with NieAl [11], NieTi [12], TiC and TiB2 [13], Al2O3 [6] and TiO2 [14]. Surface alloying has also been used to increase the corrosion resistance of commercial Al-1100 alloy, saturating its surface with Mo and producing a metallurgical laser alloyed region [15]. LSM also improved the pitting corrosion resistance of the friction stir weld of AA2024-T351 alloy by refining its microstructure and redistributing the alloying elements with the laser beam [16]. Concerning the production of MMC, some interesting works have been reported with the use of LSM technique. Hwang et al. [17] obtained CNTs/Ni composites, and their results demonstrated that CNTs remain unchanged when in contact with the molten nickel matrix. Yuang et al. [9] studied the properties of the multiwalled CNTs (MWCNTs)/magnesium alloy (AZ91D) composite and obtained fully dense and uniform microstructures, grain refinement and higher hardness when compared to monolithic AZ91D. They also found that good adhesion between MWCNTs and Mg matrix was obtained, without damages to the structure of the nanotubes. Chang et al. [18] prepared a fully dense CNTs/Ti composite by direct metal laser sintering, showing uniform distribution of CNTs with a significantly enhanced hardness on the surface. Also, Chen et al. [19] fabricated IN718/NiPCNT composites using the laser melting process. The results showed outstanding tensile strength values of the composites. Some difficulties during LSM of aluminum and aluminum alloys have been reported, regarding their high oxidation potential [20] and high surface reflectivity leading to a low energy absorption [21]. These characteristics can result in several defects, such as porosity and cracks, and the need for higher heat (energy) input [22,23]. Consequently, the obtainment of CNTs reinforced aluminum matrix composites by LSM has not been deeply investigated. Zhao et al. [24] fabricated carbon/ AlSi10Mg composites by selective laser melting and found that CNTs were decomposed because of their low thermal stability under a higher heat input condition. Another study was performed by Hu et.al [25] who fabricated graphene-Al composites. Their results proved that graphene sheets were still present in the composite after LSM, increasing the hardness when compared to pure aluminum. In the present work, acid-treated MWCNTs were mixed with aluminum powder by the electrostatic adsorption method, in order to obtain a well dispersed mixed powder. A thin bed of this mixed powder was sedimented on the surface of the 6061-aluminum alloy, and different LSM parameters were used to melt this deposit over the substrate. The co-melting of a thin substrate layer during the passage of the laser beam resulted in a modified substrate surface layer. It was found that the presence of MWCNTs previously dispersed in the aluminum powder increased the laser energy absorption and intensified the Marangoni convection, leading to a presumed uniform dispersion of the MWCNTs in the modified substrate layer. No evidence of Al4C3 formation was found, and the resulting layer increased the hardness of the substrate.

and a length > 1 μm, were used as the starting reinforcement materials. Raw MWCNTs were previously acid treated in 1: 3 (v:v) nitric acid and sulfuric acid at room temperature under magnetic stirring for 6 h. 2.2. Preparation of Al/MWCNT powders The mixing process was performed following the method presented in our previous work [27], where the acid-treated MWCNTs were added to 450 mL of deionized water and then sonicated for 30 min for full dispersion in aqueous suspension. In a beaker containing 25 mL of deionized water and 25 mL of absolute ethyl alcohol, the Al powder was added. Both Al and MWCNTs suspensions were mixed with each other and stirred at 400 rpm for 1 h so that the electrostatic adsorption occurred. The mass of Al and reinforcing MWCNTs was calculated to obtain a final composite containing 2% wt. of the reinforcement phase. The final mixture was filtered, rinsed with absolute ethyl alcohol, and dried at 60 °C for 24 h. 2.3. Al/MWCNT powder mixture melting by LSM For the Al/MWCNT powder mixture melting by LSM, the solubilized 6061 aluminum alloy substrate surface was activated by gridding with #600 SiC paper and cleaned with acetone under sonication. A mixed powder bed was obtained over the substrate using a filtration system, using a 10 μm grid membrane. The substrate was placed at the bottom of the filtration system, over the filtration membrane. Afterward, a definite mass of Al/MWCNT was thoroughly mixed with pure ethyl alcohol and completely poured into the filtration system. Then, the ethyl alcohol based suspension slowly passed through the membrane by gravity action only, leaving the Al/MWCNT powder sedimented over the substrate surface, as shown in Fig. 1a. Once the ethyl alcohol was completely filtrated, the substrate with a single and homogeneous deposited powder bed was dried 80 °C for 30 min prior to LSM. The obtained powder thickness was of 95 ± 16 μm, measured with digital microscopy. A control sample with pure aluminum powder was also processed under the same conditions as the Al/MWCNT powder sample, for comparison purposes. LSM was performed using a Synrad CO2 laser with maximum power of 100 W, wavelength of 10.6 μm, spot diameter of 160 μm and beam quality M2 < 1.2. A Raylase scan head system with a maximum scanning speed of 3 m/s and controlled by the WinLase Professional software was used for the LSM process. A Laser beam power of 100 W with 50% of superposition between longitudinal tracks was maintained, and scanning speeds were varied: 2, 5, 10 and 15 mm/s. An argon gas supply device, shown in Fig. 1b, was developed to feed homogeneously the protective gas on the surface of the powder bed during the LSM, in order to minimize the metal oxidation. The system basically allows the inlet of the argon jet at a pressure of 2 bar into an internal chamber that decelerates the flow and distributes homogenously the gas over the sample through several holes. Thus, it was possible to process the samples with a protective atmosphere, without the risk of powder bed particles being ejected from the substrate surface. Fig. 2 shows the overall flow of the Al/MWCNT composite single layer consolidation over the aluminum alloy substrate by the LSM technique.

2. Experimental procedure

2.4. Materials and modified surface layer characterization

2.1. Materials

The pristine MWCNT and the mixed Al/MWCNT powders were characterized by X-ray diffraction using a Rigaku X-ray diffractometer, model Ultima IV, with Cu-Kα radiation (λ = 1.54178 Å), voltage of 40 kV and 30 mA current, fast detector, step of 0.01° and angular speed of 2°/min. X-Ray analysis was also performed on consolidated composite layers. The structure of the MWCNT, along the several processing steps, was analyzed using Raman spectroscopy with a 532 nm laser Horiba LabRAM microscope; the spectra were collected using three accumulations during 30 s between 500 and 2000 cm−1. The MWCNT

Aluminum powder Grade 123 with 99.5% purity was kindly provided by ALCOA Aluminum Brazil, with a particle size distribution of d10 = 12 μm, d50 = 25 μm and d90 = 52 μm measured by the fabricant using Microtrac Laser Light Scattering [26], and Baytubes® C 150 P multi-walled carbon nanotubes (MWCNTs), produced by Bayer Materials Science-Germany by CVD process with purity level greater than or equal to 95%, were employed in the present work. The MWCNTs, having internal and external diameters of 4 and 13 nm, respectively, 2

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Fig. 1. (a) Powder sedimentation system to obtain a homogeneous powder bed and (b) argon protective gas supplying device employed for LSM.

morphology was analyzed using a Tescan model Mira 3 Field Emission Gun Scanning Electron Microscope (FEG-SEM). The mixed powder, laser processed layer morphology and the modified layer cross-section microstructure were observed by Scanning Electron Microscopy (SEM) in an FEI Inspect S50 microscope. For metallographic examinations, the cross-sectioned samples were grinded with abrasive paper (SiC grits of 240, 400, 600 and 1200, respectively), polished to a mirror finish (1.0 μm Al2O3 solution and colloidal SiO2 suspension) and chemically pre-etched for 10 s with 3% vol. HF in H2O solution. The final etching

was performed using Weck's reagent (100 mL H2O, 4 g KMnO4 and 1 g NaOH) for 5 s. The samples were analyzed by SEM, Energy-dispersive Xray spectroscopy (EDS) and Optical microscopy (Zeiss AxioCam optical microscope). The hardness of the modified surface was evaluated using a Vickers microhardness test. The hardness test conditions employed were 10 gf of load during 10 s of dwell time, using a digital microhardness tester FutureTech, FM8000 model. At least ten measurements were done, and the average value was reported.

Fig. 2. Fabrication procedures for laser surface melting of Al/MWCNT composite layers.

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Fig. 3. FEG-SEM images of (a) pure aluminum powder (b) MWCNT electrostatically adsorbed on aluminum powder.

3. Results and discussion

G Band

D Band

Raman Intensity (a.u)

3.1. Al/MWCNT mixture powder morphology Fig. 3 presents the overall aspect of Al powder observed by SEM. It shows also the aspect of the mixed Al/MWCNT powder after the electrostatic adsorption procedure. The Al particles, before the MWCNT adsorption, are observed in Fig. 3a. They exhibit an irregular morphology. Fig. 3b shows the acid treated MWCNTs adsorbed on the Al surface. The aluminum powder is spontaneously ionized in aqueous suspension (positively charged Al+3) and the MWCNTs containing functional groups, after acid treating, develop negative surface charges. When mixed in aqueous suspension electrostatic interactions between them occur, leading to the spontaneous and homogeneous adsorption of MWCNTs onto the Al particles [28]. In Fig. 3b, few agglomerates of MWCNTs are still seen in the inset of the figure. The agglomerates could be attributed to irregular morphology of the Al powders, as their surface area limits the adsorption amount of MWCNTs due to large curvature dimension. [29]. The use of aluminum flakes obtained by milling conventional aluminum powders could improve the MWCNT dispersion, as their high specific surface area increases the adsorption capacity of the particles [28]. On the other hand, the flake powder geometry is less efficient to obtain a homogeneous powder bed over the substrate by the suspension sedimentation process. The powder dispersion by electrostatic attraction is considered nondestructive process when regarding the MWCNT integrity. To evaluate the structural disorder degree of the MWCNTs after mixing with Al powder, as well as their interaction at the electronic level, Raman spectroscopy was employed, and the obtained spectra are presented in Fig. 4. It is well known that the relative intensity of the bands D and G of carbon-based materials in a Raman spectrum indicates the structural disorder level. In the case of the MWCNT, they are related to their graphitic nature associated with the tangential stretching mode of the C]C bond [30,31]. The ID/IG ratio is a simple and relatively precise tool to evaluate eventual damages that the MWCNTs can suffer during the overall composite processing. It is expected to increase when defects are created on the structure of the nanotubes. The ID/IG calculated ratios of all the Raman spectra obtained are summarized in Table 1. An increase is observed after the acid treatment process, due to the introduction of functional groups in the external wall of the MWCNTs. Then, when the MWCNTs are mixed with aluminum powder, by the electrostatic adsorption method, a further increase is also evidenced, which can be due to the use of an ultrasound probe during the mixing process [32]. Also, as evidenced in the Raman spectrum of electrostatically adsorbed powders (Fig. 4), there is an upshift of the G band, which means that an electronic interaction

MWCNT

Al/MWCNT

Al/MWCNT SLM

1200

1300

1400

1500

1600

1700

1800

-1

Raman Shift (cm ) Fig. 4. Raman Spectra of MWCNT, Al/MWCNT, and Al/MWCNT Laser Surface Melted with 2 mm/s of scanning speed (LSM). Table 1 ID/IG ratio and localization of G band from the Raman spectra of raw MWCNT, acid treated MWCNT, the electrostatically adsorbed powders, and the LSM coating with 2 mm/s of scanning speed. Sample

D Band

G Band

ID/IG

Raw MWCNT MWCNT Al/MWCNT Al/MWCNT LSM

1337 1337 1347 1349

1574 1574 1584 1596

1.24 1.36 1.42 0.72

between the MWCNTs and aluminum particles has been obtained [28]. Details of the Raman spectra of LSM processed samples are provided in the next section when discussing the characteristics of the composite layer formed. The Al/MWCNT mixed powder was also evaluated by XRD (Fig. 5a). The resulting patterns show only aluminum peaks (JCPDS No.00-0040787), as the weight percentage of the MWCNT present is below the detection limit of the XRD technique. 3.2. Al/MWCNT surface modified layer The XRD analysis of the surface modified layers obtained by LSM indicates that, besides the use of Ar protective gas, surface oxidation at some extension occurred during the process. The phases present in the 4

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3.3. MWCNT integrity during LSM process Many studies on aluminum/MWCNTs composites reports the interfacial reaction of aluminum and carbon during processing routes such as plasma spraying [34], spark plasma sintering [35], or friction stir processing [36,37]. In the present work, the formation of Al4C3 that usually confirms the reaction at larger extensions, degrading the MWCNT, was evidenced neither by Raman Spectroscopy (Fig. 4) nor by XRD (Fig. 5). The Al4C3 phase is stable up to 1000 °C, being decomposed above 1400 °C [38]. Consequently, it can be supposed that the high temperatures reached during LSM in the present work could be high enough to decompose any eventual Al4C3 phase formed. On the other hand, it can be also supposed that, due to the high cooling rates inherent to LSM, there is not enough time for the Al/MWCNT reaction and Al4C3 is not substantially formed [24]. Raman Spectroscopy was also helpful in evaluating the level of structural disorder of the MWCNTs after the LSM and comparing to their initial state. The Raman Spectrum shown in Fig. 4 reveals the presence of the typical carbon bands. However, the D-band is broader in comparison to the spectra in the same figure. Table 1 presents a comparison of the ID/IG ratio and the position of the G band before and after the LSM. It can be observed that after LSM the G-band has been shifted to higher values, and the ID/IG ratio has been reduced. One possible reason for the G-band upshifting is that high compression stresses have been imposed by the aluminum matrix to the MWCNT during solidification contraction of the metal [39]. However, due to the high heat input, this behavior could be better explained by a possible graphitization of the nanotubes, or the formation of other carbon nano-allotropes under the heat effect [19]. The greater the G band displacement, the higher the graphitization degree of MWCNTs [9,17]. This transformation also leads to the observed reduction of ID/IG ratio, with peak bands broadening. Consequently, in the present work, despite the presence of typical carbon bands in the LSM layer spectrum, it is not possible to prove without further investigation if the MWCNTs are still present in the consolidated composite layer at their original form, or if they were partially or totally transformed into other carbon allotropic forms [19,24]. 3.4. Surface morphology of the layers processed by LSM Fig. 6 presents the surface morphology of the pure Al and Al/ MWCNT layers obtained over the 6061-aluminum alloy substrate by LSM process. The images show that when the scanning speed employed is low (2 mm/s), a homogeneous deposit to substrate mixture is obtained. On the other hand, the increase of the scanning speed results in the formation of droplets of molten metal with poor wettability over the substrate. Another important observation in Fig. 6 is that the deposits of Al/MWCNT composite material are continuous, without droplets, especially at lower speeds. The formation of droplets or balls in the laser processed surface has been called balling effect [40,41]. Their morphology depends on the processing parameters, such as laser power and scanning speed [42]. Other factors such as powder bed thickness [43] and the surface oxidation [20] have some influence on the balling effect as well. When processed by LSM, the powder bed reaches a peak temperature that leads to the liquid phase formation by the melting of the powder particles. The amount of liquid phase formed depends on the amount of energy absorbed by the material, which is controlled by the laser power, scanning speed, and by the optical properties of the powder. If the scanning speed is high or the laser power is low, the incident energy will be low, leading to the formation of a high viscosity liquid. A high viscosity liquid, along with high surface tension during solidification, will promote the formation of individual balls [41]. Thus, most of the incident energy will be consumed by the material to form the balls, driving less energy to the substrate and reducing the remelting depth. The final result is a discontinuous deposit layer weakly

Fig. 5. X-Ray diffraction patterns of Al and Al/MWCNT LSM samples. In (b) the diffractogram is shown in detail in 2θ between 20 and 46° to highlight the Al2O3 and Al2.4Mg0.4O4 diffraction peaks for Al and Al/MWCNT processed with 2 and 5 mm/s. The “M” letter in the patterns corresponds to Al/MWCNT samples and “Al” to pure Aluminum samples. The number indicates the scanning speed used during the sample processing 2, 5, 10 and 15 mm/s.

spectra of Fig. 5 were Al (PDF No. 01-089-2837), Al2O3 (PDF No. 01075-0782) and a small quantity of Al2.4Mg0.4O4 (PDF No. 01-0870345). In Fig. 5a it is possible to observe that, as the scanning speed increases from 2 to 15 mm/s, the intensity of the Al2O3 peaks is reduced. In addition, when comparing to the layer obtained with pure aluminum, the Al2O3 peaks are more intense in the composite layer, see Fig. 5b for details. Aluminum is very reactive with oxygen, and the formation of oxide films in Al LSM processed samples have been widely reported [20,33]. In the present work, the laser interaction time necessary to promote Al melting is high (low scanning speed) due to the limited power of the laser source employed (100 W). Under such conditions, the gas protective system employed was not completely efficient to prevent oxidation, and the lower the scanning speed, the higher the intensity of Al2O3 peaks. Furthermore, as also observed by Yuan et al. [9] and Zhao et al. [24], the presence of MWCNT in the Al powder bed increases the laser absorption, increasing the heat input and contributing to oxidation, as evidenced by the higher intensity of the Al2O3 peaks in the Al/MWCNT composite layers. 5

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Fig. 6. Surface morphology of the pure Al and Al/MWCNT LSM layers.

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Fig. 7. Cross-sectioned LSM layers with measured thickness.

Fig. 8. (a) Microstructure and identified zones in the Al/MWCNT individual track processed with 2 mm/s. (b) larger magnifications of (a). Backscattered Electrons Images showing segregations (d) larger magnifications of (c).

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Fig. 9. EDS punctual analysis done inside and outside the segregations of Al/MWCNT sample processed with 2 mm/s of scanning speed.

Fig. 10. Epitaxial growth and orientation of the samples after LSM. Sample of Al/MWCNT processed (a) 10 mm/s (b) 2 mm/s. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

bonded to the substrate [43]. This explains the increased balling presence in the samples processed with higher scanning speeds and also confirms that the presence of MWCNT increases the laser energy absorption of the powder bed [9,24]. Louvis et al. [20] showed that the balling presence can be also due to the presence of surface oxides, but in that case, an ytterbium fibre laser beam (wavelength of 1.06 μm) has been used. Tolochko et.al [44], showed that the energy absorption of oxides in 1.06 μm wavelength lasers is only 3%, while the absorption of 10.6 μm CO2 lasers is 96%. In the present work, a CO2 laser has been used, and the presence of oxides can even increase the overall energy absorption.

order to obtain a metallurgically bonded layer. This results in the dilution of the powder bed into the substrate, which means a change in its chemical composition [45]. Powder dilution increases with the laser/ powder interaction time, as well as with the absorbed energy during LSM [45]. LSM is usually performed by subsequent passes of laser tracks that are usually overlapped to form a homogenous modified layer. The pre-heating effect caused by the preceding track increases the temperature during the subsequent tracks, and as a consequence, increases the molten pool dimensions [46]. As the width of the molten pool increases, it extends further into the subsequent tracks. Consequently, the previous track, which already contains some substrate material, mixes with the current track and changes its chemical composition. Dilution also increases with the number of tracks [15]. Fig. 7 presents the melting depth obtained by LSM with different scanning speeds. It is clear that, as the energy absorption increases (at lower

3.5. Cross-sectioned morphology of the layers processed by LSM LSM involves the addition of molten material on a substrate, in 8

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processed AlSi10Mg alloy and found that magnesium is preferentially oxidized. Louvis et al. [20] also studied the fumes emitted during laser processing of 6061-aluminum alloy, observing the presence of aluminum magnesium oxides formed by the evaporation of surface oxides from the molten pool. This agrees with the present work since the X-ray diffraction pattern showed the presence of a low percent of Al2.4Mg0.4O4 on the deposit layers. Thus, due to the high dilution degree obtained in the present work, both magnesium and silicon were conducted to the molten pool, with the magnesium being vaporized and/or oxidized in form Al2.4Mg0.4O4. The silicon, in turn, could not diffuse during the solidification, remaining entrapped in the α-Al contours [53]. Therefore, the segregation observed in the processed layers seems to be related to a Si rich phase rather than the expected Mg2Si compound [51,54]. Other important characteristic of materials processed by LSM is the preferred orientations and epitaxial growth. Fig. 10a and b present the optical micrographs of the cross-section of the modified layers obtained in the present study. Grain growth in the solidified layer is based on the substrate grains (indicated in the images with yellow lines), as a result of the epitaxial growth [55]. In the competitive epitaxial growth, only the grains with the easy direction of growth, 〈100〉 for the FCC aluminum are formed parallel to the direction of the maximum thermal gradient [56]. Fig. 10a shows the growth of the columnar grains approximately normal to the processing direction (yellow dotted lines) when high laser scanning speeds are employed. When lower scanning speeds are employed, columnar grains are curved according to the heat source or laser scanning direction [50], shown in Fig. 10b.

Vickers Microhardness (HV10 gf)

70

Al Al/MWCNT

60 50 40 30 20 10 0 2

5

10

15

Scanning Speed (mm/s) Fig. 11. Vickers microhardness of the LSM layers. The dashed line indicates the hardness of the substrate.

speeds or in presence of the MWCNT), deeper is the Al/MWCNT to substrate interaction and higher is the dilution [47]. 3.6. The microstructure of the LSM processed layers

3.7. Vickers microhardness

In general, the microstructure developed during the surface remelting is influenced by the solidification rate. LSM is characterized by high heating and cooling rates, leading to a fine microstructure. Fig. 8 presents in detail the microstructure of one of the tracks formed on the Al/MWCNT layer processed at 2 mm/s. Four different zones are present: (i) an internal zone (or core) characterized by equiaxial cells; (ii) a deeper zone near to the boundary of the molten pool, with cells elongated in the direction of the thermal gradient [48,49] (indicated by arrow in Fig. 8b); (iii) the boundary of the molten pool or the heat affected zone [43] and, a little further down, (iv) the base metal or substrate. In the zone (i), the presence of segregation is observed. This segregation is observed as brighter points in the images obtained with SEM using backscattered electrons detector (Fig. 8c and d), and are present in all samples, but more evident in the Al/MWCNT deposits. The more energy absorbed (in presence of the MWCNT), the greater the deposit/ substrate interaction depth and, therefore, the greater is the dilution and the amount of alloying elements from the substrate in the molten pool. Furthermore, as the energy absorbed is higher, higher is the temperature of the molten pool and slower the solidification. Under these conditions, the probability that the atoms of the solute be locked in the solid decreases, and the segregation is greater [50]. In the 6061-aluminum alloy, the main alloying elements are magnesium (Mg) and silicon (Si). To verify the composition of the segregated phase, punctual EDS analysis was performed (Spectrum 23), as well as in the surrounding matrix (Spectrum 25), see Fig. 9. In the matrix (Spectrum 25), a very low quantity of both magnesium and silicon elements are seen, while the segregated phase in Spectrum 23 is enriched in Si. This is somewhat contradictory, since the 6061- aluminum alloy composition presents between 0.8–1.2 wt% of Mg and 0.4–0.8 wt% of Si [51] that easily forms the Mg2Si compound. During melting and solidification of the deposit layer and substrate, some elements are susceptible to vaporization if the energy supplied by LSM is of the order of magnitude of their vaporization heat. In the present study, the most volatile element is magnesium [52]. Simonelli et al. [33] performed compositional analysis on scattered particles from laser

With the microhardness characterization of the substrate modified layers, the effect of the LSM parameters on the microstructure, as well as the effect of the MWCNT on the strength of the substrate modified layer are expected to be observed. In order to better understand the hardness to microstructure dependence, several micro-indentations in different areas of the deposited layer were performed. The average hardness value of the 6061-aluminum alloy substrate is 37.8 ± 2.5 HV. The results in Fig. 11 show that the samples processed with 2 mm/s present higher hardness values, reaching 48.5 ± 7.2 HV for the sample processed with pure aluminum and 54 ± 5.4 HV for the Al/MWCNT sample. Thus, LSM of pure aluminum increases the hardness by about 28% and Al/MWCNT by up to about 43%, when compared to the 6061aluminum alloy substrate. These results can be ascribed to the microstructural changes observed in the LSM processed samples, including the diffusion of alloying elements from the substrate to the molten pool as a result of the dilution and to a clear contribution from the MWCNT presence to the hardness of the substrate modified layer. These effects seem to overcompensate the expected softening of the substrate surface due to the decreasing in the relative percentage of alloying elements due to the introduction of pure aluminum in the substrate surface. The variation in the chemical composition through the obtained layer [43] and the resulting segregation of Si that forms a dispersed second phase seems to cause the most significant hardening effect. On the other hand, as the Raman spectroscopy results suggest that the MWCNTs are probably transformed into other C allotropic phases, the mechanism by which they contribute to the hardness of the layer is uncertain. Even a hardening effect by the dispersion of oxides, in this case, cannot be discarded. Although the dispersion in the hardness values measured is high, it seems that, as the heat input decreases (increasing the laser scanning speed), less dilution of the melting pool and poor MWCNT dispersion leads to both: the decrease of the hardness of the modified substrate layer and a smaller contribution of the MWCNT to this resulting hardness.

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4. Conclusions

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In the present work, MWCNTs were acid treated and their dispersion by electrostatic adsorption process on the surface of aluminum particles was confirmed. The pure aluminum and Al/MWCNT powders were deposited as a powder bed by the sedimentation process on the 6061aluminum alloy substrate for further melting using LSM process with different laser scanning speeds. XRD analysis showed the presence of oxides formed in the modified substrate layers obtained, being more evident as the process heat input increases (lower laser scanning speeds or higher powder energy absorption in presence of the MWCNT). The surface and cross-section morphologies of the layers obtained confirm that the absorption of the laser energy increases in the powder bed containing MWCNT. The maximum hardness obtained for the deposited layer was 43% higher than the substrate, being significantly dependent on the heat input. Diffusion of Si from the substrate to the solidified molten pool seems to be the main cause of the deposit layer hardening, although a small contribution from the dispersion of oxides or any carbon allotropic form present in the deposited layer cannot be ruled out. No Al4C3 formation was evidenced, but Raman spectroscopy analysis suggested the partial loss of the tubular structure of the MWCNTs due to the high temperature developed during the laser processing. Acknowledgments The authors are thankful to the Associated Laboratory for Sensors and Materials (LAS-INPE – Brazil) for the FEG-SEM images, to the Technological Institute of Aeronautics - ITA for the Raman analyses, to the Institute for Advanced Studies-IEAv for the microhardness test. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001, CNPq (Process no. 443395/2014-4) and FAPESP (Process no. 2015/18235-0). References [1] E.T. Thostenson, Z. Ren, T.-W. Chou, Advances in the science and technology of carbon nanotubes and their composites: a review, Compos. Sci. Technol. 61 (13) (2001) 1899–1912. [2] M. Paradise, T. Goswami, Carbon nanotubes - production and industrial applications, Mater. Des. 28 (5) (2007) 1477–1489. [3] S.R. Bakshi, D. Lahiri, A. Agarwal, Carbon nanotube reinforced metal matrix composites - a review, Int. Mater. Rev. 55 (1) (2010) 41–64. [4] O. Gohardani, M.C. Elola, C. Elizetxea, Potential and prospective implementation of carbon nanotubes on next generation aircraft and space vehicles: a review of current and expected applications in aerospace sciences, Prog. Aerosp. Sci. 70 (2014) 42–68. [5] Z. Hu, et al., Fabricating graphene-titanium composites by laser sintering PVA bonding graphene titanium coating: microstructure and mechanical properties, Compos. Part B Eng 134 (2018) 133–140. [6] J. Jue, D. Gu, K. Chang, D. Dai, Microstructure evolution and mechanical properties of Al-Al2O3composites fabricated by selective laser melting, Powder Technol. 310 (2017) 80–91. [7] D.D. Phuong, et al., Effects of carbon nanotube content and annealing temperature on the hardness of CNT reinforced aluminum nanocomposites processed by the high pressure torsion technique, J. Alloys Compd. 613 (2014) 68–73. [8] E.O. Olakanmi, R.F. Cochrane, K.W. Dalgarno, A review on selective laser sintering/ melting (SLS/SLM) of aluminium alloy powders: processing, microstructure, and properties, Prog. Mater. Sci. 74 (2015) 401–477. [9] X. Yuan, S. Huang, Microstructural characterization of MWCNTs/magnesium alloy composites fabricated by powder compact laser sintering, J. Alloys Compd. 620 (2015) 80–86. [10] H.C. Man, S. Zhang, F.T. Cheng, T.M. Yue, In situ synthesis of TiC reinforced surface MMC on Al6061 by laser surface alloying, Scr. Mater. 46 (3) (2002) 229–234. [11] H.C. Man, Y.Q. Yang, W.B. Lee, Laser induced reaction synthesis of TiC+WC reinforced metal matrix composites coatings on Al 6061, Surf. Coatings Technol 185 (1) (2004) 74–80. [12] H.C. Man, S. Zhang, F.T. Cheng, Improving the wear resistance of AA 6061 by laser surface alloying with NiTi, Mater. Lett. 61 (19–20) (2007) 4058–4061. [13] D. Ravnikar, N.B. Dahotre, J. Grum, Laser coating of aluminum alloy en AW 6082T651 with TiB2and TiC: microstructure and mechanical properties, Appl. Surf. Sci. 282 (2013) 914–922.

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