Laser surface treatment of aluminum composite: surface characteristics

1 Introduction

Metal matrix composites are widely used in industry because of their low density, high toughness, and resistance to harsh environments such as high temperature and corrosion. In general, when metal matrix composites are produced from a mixture of metallic materials and hard particles, such as SiC, TiC, WC, and B₄C, voids are formed around the hard particles due to wettability of the particles during hot pressing [1]. Surface defects such as voids and irregular textures limit the practical applications of the metal matrix composites in harsh environments. Control melting at the surface using high-energy beams minimizes the defect sites and, possibly, further improves the tribological properties and corrosion resistance of the surface. A high-power laser can be considered as one of the high-energy beams, which can be effectively used for surface processing of engineering materials [2–4]. Laser control melting has several advantages over the other melting methods such as plasma arc and plasma torch, since it provides fast processing time, precision of operation, and local treatment. Although laser control melting has several advantages, it generates excessive thermal stresses in the treated region because of high cooling rates at the treated surface [5]. This modifies the microstructure at the laser-treated surface and alters the mechanical properties and corrosion resistance of the surface. Consequently, investigation into laser-controlled melting of metal matrix composites and corrosion resistance of the resulting surface becomes essential.

Aluminum-based composites composed of aluminum and hard particles have low density and high toughness, and therefore they are widely used in industry [6]. The surface properties of the composites can be improved through laser-controlled melting. Considerable research studies have been carried out to examine laser treatment of aluminum-based composites. Aluminum metal matrix composites through direct metal laser deposition were studied by Waldera and Kalita [7]. They showed that the carbide aluminum metal matrix composites demonstrated good interfacial bonding, and improved modulus and hardness after the laser deposition process. Laser deposition of ceramic composite coatings on aluminum alloys was investigated by Kadolkar et al. [8]. They indicated that the micro-stresses in the TiC particulate and aluminum matrix phases within the coatings were independent of the amount of de-bonding in the composite coating. The microstructure and strength of a laser-treated sub-micron particulate-reinforced aluminum matrix composite were examined by Liu and Niu [9]. They demonstrated that the laser pulse frequency directly affected the reinforcement segregation and the reinforcement distribution in the laser-treated region. The wear resistance of a laser-alloyed aluminum-WC composite was studied by Staia et al. [10]. The findings revealed that the incorporation of WC hard particles in the aluminum-based alloy was not beneficial from the point of view of wear resistance. Metal matrix composite layer formation on an aluminum alloy using a laser surface alloying process was studied by Tomida et al. [11]. They showed that the hardness of a laser-treated layer increased with increasing copper content and the volume fraction of the TiC particle.

Aluminum alloys have high resistance to corrosion; however, when treated by a laser, the electrochemical response of the surface alters. Although laser treatment improves the surface hardness and mechanical properties of the treated layer [3, 12–14], the corrosion response of the laser-treated surface requires further improvement. Considerable research studies were carried out to examine laser-treated aluminum alloys and the electrochemical response of the treated surfaces. The microstructure and corrosion behavior of laser-cladded aluminum-based composite coatings was studied by He et al. [15]. They showed that the corrosion resistance of laser cladding coatings was greatly increased as compared with the uncladded substrate surface, which was mainly due to the existence of hard particles such as TiC, and the corrosion resistance was improved simultaneously with the increase of TiC content in the coating. The corrosion resistance of the Al/SiC composite by excimer laser gas alloying was investigated by Mei and Yue [16]. They demonstrated that the presence of SiC particles influences significantly the corrosion rate at the laser-treated surface. The multiple laser aluminum-based metal matrix composite and corrosion resistance of the treated surface were examined by Popoola et al. [17]. They indicated that multi-track laser processing improved the corrosion resistance of the surface as compared to single-track laser processing. The morphology and corrosion characteristics of a laser-treated aluminum-silicon surface were examined by Serbinski et al. [18]. They observed that laser treatment improved the corrosion resistance of the alloy considerably in 0.01 m sulfuric acid solution. The erosion and corrosion behavior of a laser-treated aluminum-based metal matrix was investigated by Man et al. [19]. The findings revealed that a decrease in pitting resistance took place at the surface after the laser treatment process. YAG laser treatment of an aluminum composite surface and the corrosion resistance of the resulting surface were examined by Hu et al. [20]. They indicated that the reduction of the reinforcement Al18B4O33 whisker and intermetallic CuAl₂ on the surface of the laser-treated composite along with the formation of a homogeneous and defect-free microstructure in the laser-modified layer resulted in the improvement of the corrosion resistance of the laser-treated composite. The corrosion behavior of a laser clad aluminum bronze surface was investigated by Xu et al. [21]. They demonstrated that the laser treatment improved the corrosion resistance of the surface significantly. The corrosion resistance of a laser-cladded composite surface was studied by Yue et al. [22]. The findings revealed that the corrosion potential and the breakdown potential of the composite surface were significantly increased after laser cladding. Microstructural characterization and corrosion behavior of a laser-cladded composite surface was examined by Bakkar et al. [23]. They showed that the corrosion current of the clad coating was around two orders of magnitude lower than that of the untreated composite surface.

Although laser treatment of aluminum alloys was studied earlier [24, 25], the corrosion characteristics of a laser-treated aluminum-based composite with the presence of hard particles such as SiC are left for the future study. Therefore, in the present study, the corrosion resistance of a laser-treated aluminum-based composite is investigated. The composite material is composed of aluminum and 15% SiC particles, and it is produced through sintering and by hot pressing. Morphological and metallurgical changes in the surface region of the alloy are examined before and after the corrosion tests using the analytical tools such as optical, electron, X-ray diffraction, and energy dispersive spectroscopy. A micro-tribometer is used to evaluate the friction coefficient of the laser-treated surface. Electrochemical tests are carried out in 0.5 m NaCl solution at room temperature. The tests are repeated several times to ensure the reproducibility of the corrosion data, and DC105 corrosion software is used to analyze the Tafel region.

2 Materials and methods

In line with the previous study [26], workpieces were formed from high-purity aluminum mixed with 15% commercial-grade SiC powders. The workpieces were in a circular pellet form of 25.4 mm diameter and 3 mm thickness. Aluminum powders had particle size in the order of 200 μm and SiC powders had a median size of 2 μm. To prepare the pellets, B₄C and aluminum powders were mixed in isopropyl alcohol. The slurry was then ultrasonically shaken to achieve homogeneity for 30 min. The pellets were compacted through sintering, hot pressing, and hot isostatic pressing. They were cold-pressed at 60 MPa, provided that the higher pressures resulted in undesirable striations in the compacts, and they were sintered in a vacuum furnace at a temperature within the range of 450°C and a pressure of 200 MPa in an environment pressure of 5×10^-2 Pa for 30 min. Hot isostatic pressing was performed in accordance with the maximum temperature (1100°C) for 30-min hold at 200 MPa argon environments. Heating and cooling rates of 50°C/min were used at 200 MPa. The pellets were furnace-cooled under vacuum conditions.

The CO₂ laser (LC-ALPHAIII) delivering a nominal output power of 2 kW was used to irradiate the workpiece surface. The nominal focal length of the focusing lens was 127 mm, and the diameter of the laser beam focused at the workpiece surface was ∼0.3 mm. Nitrogen gas used as the assisting gas was applied co-axially with the laser beam using a conical nozzle, and the laser treatment was repeated several times using different laser parameters. The laser output power level was selected to avoid excessive melting/surface evaporation or the shallow treatment layer. It should be noted that several initial experiments were conducted to select the laser scanning speed and the laser output power level. In this case, lowering laser scanning speed or increasing laser output power caused excessive melting and large-scale surface evaporation at the workpiece. Alternatively, lowering the laser output power level or increasing scanning speed resulted in a considerably shallow treated depth. The interpass time for laser scanning tracks is 12 s. Therefore, laser parameters resulting in controlled melting of the surface with a minimum of surface defects, such as very small cavities without crack networks, were selected and the laser treatment conditions are given in Table 1.

Material characterization of the laser-treated surfaces was conducted using an optical microscope, scanning electron microscope (SEM), energy dispersive spectroscope (EDS), and X-ray diffractometer (XRD). A Jeol 6460 SEM was used for SEM examinations and a Bruker D8 advanced XRD using CuKα radiation was used for XRD analysis. Typical settings of the XRD were 40 kV and 30 mA with the scanning angle (2θ) ranging from 20° to 80°. The surface roughness measurement of the laser-melted surfaces was performed using an Agilent 5100 AFM in the contact mode. The tip was made of silicon nitride probes (r=20–60 nm) with a manufacturer-specified force constant, k, of 0.12 N/m.

A microphotonics digital microhardness tester (MP-100TC) was used to determine the microhardness at the surface. The standard test method for Vickers indentation hardness of advanced ceramics (ASTM C1327-99) was adopted. The microhardness was measured at the surface of the workpiece after the laser treatment process, and the measurement was repeated five times at each location to determine the consistency of the results.

A linear micro-scratch tester (MCTX-S/N: 01-04300) was used to determine the wear characteristics and friction coefficient of the laser-treated and untreated surfaces. The equipment was set at a contact load ranging from 0.03 to 5 N. The scanning speed was 5 mm/min with a loading rate of 5 N/s, and the total length for the scratch tests was 5 mm.

Corrosion tests were carried out in a three-electrode cell, which composed of a specimen as a working electrode, a Pt wire as a counter electrode, and a saturated calomel reference electrode (SCE). The specimens were degreased in benzene, cleaned ultrasonically, and subsequently washed with distilled water prior to electrochemical tests. The investigations were carried out with an exposed working electrode area of 0.05 cm² in 0.5 m NaCl solution at room temperature in a PCI4/750 Gamry potentiostat (Gamry Instruments, USA) and repeated several times to ensure the reproducibility of the data. The DC105 corrosion software was used to analyze the Tafel region, while potentiodynamic polarization experiments were performed at a scan rate of 0.5 mV/s.

3 Results and discussion

Laser treatment of an aluminum-based composite surface is carried out and the corrosion resistance of the surface is evaluated. An aluminum-based composite is produced through hot pressing of aluminum and 15% SiC particle mixture.

3.1 Morphological and metallurgical changes in a laser-treated layer

Figure 1 shows the SEM micrograph of a laser-treated surface prior to the corrosion tests. Laser-treated surfaces consist of equally spaced regular scan patterns (Figure 1A). These patterns are formed by the melted spots due to laser repetitive pulse irradiation at the surface. The laser pulses irradiate the workpiece surface at 1000 Hz frequency and consecutive pulses melt the surface by forming a continuous melt zone along a scanning track because of overlapping of the pulses (Figure 1B). The overlapping ratio of the laser pulses is estimated as 70% at the surface. The laser-treated surface is free from large-scale asperities, including melt overflow across the laser scanning tracks, cracks, and large-size cavities. Since the laser-irradiated pulse has a Gaussian intensity distribution at the surface, pulse intensity remains high at the irradiated spot center. This, in turn, causes local evaporation at the surface while forming a small-scale cavity in the central region of the irradiated spot. In the region next to the small cavity, surface melting occurs and melt flow from this region toward the cavity center modifies the cavity size and the texture of the surface. Consequently, surface cavities are replaced with small-size surface undulations at the surface. However, the surface roughness measurements show that the surface roughness is in the order of 1.2 μm and the undulation texture at the surface contributes to the surface roughness, provided that the surface roughness is small. Moreover, continuous melting along the laser scanning tracks modifies the cooling rates at the surface. In this case, heat conducted from the recently formed laser scanning tracks to the previously formed tracks generates the self-annealing effect on the previously formed tracks. This reduces the temperature gradient below the surface and lowers the thermal strain and stress levels in the surface region. Consequently, stress-induced cracking is not observed at the laser-treated surface. In addition, thermal expansion coefficients of SiC and aluminum are different, which causes micro-stresses due to contraction during the solidification cycle in the surface region. It should be noted that the melting temperature of SiC is higher than that of aluminum, and most of SiC particles remain in solid phases during laser-controlled melting. Laser scanning tracks and heat transfer from the lately formed tracks to the early formed tracks modify the cooling rates while influencing the micro-stress levels around the hard particles. The close examination of the surface indicates that the surface is free from micro-cracks. An untreated workpiece surface is composed of whiskers such as structures and SiC particles (Figure 1C). Since the melting temperature of SiC is high, few scattered partially imbedded SiC particles are observed (Figure 1D); however, they are randomly distributed at the surface.

Figure 1

Laser-treated surface: (A) regular laser scanning tracks, (B) overlapping of laser-irradiated spots, (C) untreated surface, and (D) partially imbedded SiC particles.

Figure 2 shows the SEM micrographs of the cross-section of the laser-treated layer. The laser-treated layer extends almost 40 μm below the surface and it is free from large-scale cracks and voids (Figure 2A). A dense layer, consisting of fine grains and hard particles, is formed in the surface region of the laser-treated layer (Figure 2B). Although the cooling rate is high at the surface, no micro-crack or crack-network is observed in the dense layer. In addition, no voids and micro-cracks are observed around the hard particles. This is associated with the self-annealing effect of the lately formed laser scanning tracks on the initially formed tracks. As the depth below the surface increases, columnar-like stacked structures are observed because of gradual cooling of the solid phase in this region (Figure 2C). As the depth below the surface increases further, a heat-affected zone is formed, and due to the low thermal diffusivity of the substrate material, the extension of the heat-affected zone into the solid bulk is suppressed and grain sizes become large (Figure 5D). Figure 3 shows X-ray diffractograms for the laser-treated surface. SiC peaks are evident from the diffractogram for laser-treated and untreated workpieces, which is associated with the partially and fully imbedded hard particles in the dense layer. However, use of nitrogen at high pressure causes formation of an AlN compound at the surface, which is visible from the X-ray diffractogram. EDS data obtained for the laser-treated surface are given in Table 2, which shows that the elemental composition of the laser-treated surface remains almost uniform. Although quantification of the lighter elements, such as nitrogen, using EDS involves a large error, the presence of nitrogen is evident from the data in Table 2, which indicates the formation of nitride compounds at the surface during the laser treatment process.

Figure 2

SEM micrographs of the cross-section of the laser-treated workpiece: (A) laser-treated layer, (B) dense layer, (C) compact structure, and (D) heat-affected zone.

Figure 3

X-ray diffractogram of the laser-treated surface.

Table 2

EDS data for the laser-treated surface (wt%).

Spectrum	Si	Al
Spectrum 1	14.2	Balance
Spectrum 2	92.3	Balance
Spectrum 2	15.2	Balance

3.1.1 Microhardness and scratch characteristics

Table 3 shows the microhardness of the laser-treated and untreated surfaces. Laser treatment results in an increased microhardness at the surface, which is about 50% higher than the hardness of the base material. Microhardness enhancement is related to the formation of a dense layer consisting of fine grains due to the high cooling rates, micros-stresses formed at the vicinity of the surface because of the mismatch between the thermal expansion coefficients of aluminum and the hard particles. In addition, formation of the nitride phase is also responsible for the attainment of the high hardness at the laser-treated surface. Figure 4 shows the friction coefficient of the laser-treated and untreated surfaces obtained from scratch tests, while Figure 5 shows the scar mark resulting from the scratch tests. Laser treatment lowers the surface friction coefficient, which is due to the low surface roughness and high hardness of the laser-treated surface. Moreover, hard particles at the surface contribute to the attainment of low friction coefficient at the surface. The optical image of the scratched surface demonstrates that the scar size remains almost uniform along the tested surface and the depth of the scar marks is shallow, which is more pronounced for the laser-treated surface (Figure 5B).

Table 3

Corrosion data treated and untreated alloys in 0.5 m NaCl solution at room temperature.

	EcorrVSCE (V)	icorr (A/cm2)	i at –0.62 VSCE (A/cm2)	Corrosion rate (mpy)
Laser treated	–0.66	9×10-9	1.3×10-5	0.81
Untreated	–0.67	8×10-9	5.5×10-5	23

Figure 4

Friction coefficient for laser-treated and untreated workpiece surfaces: (A) laser-treated surface and (B) untreated surface.

Figure 5

Scratch marks at the workpiece surface: (A) untreated surface and (B) laser-treated surface.

3.2 Electrochemical tests

Figure 6 shows the results of the potentiodynamic polarization response of laser-treated and untreated samples in 0.5 m NaCl solution at room temperature. In general, the laser-treated workpiece surface improves the corrosion resistance slightly as compared to that of the bare (untreated) surface. The treated surface undergoes fast solidification at high cooling rates and demonstrates the advantages of refinement and reduced microsegregation; therefore, the treated surface is expected to have improved corrosion resistance compared to that of the bare surface. It should be noted that during the solidification of the laser-treated layer, the grain boundaries can be imperfectly conformed due to the deformation at the atomic level, mainly on the Al-rich phase side of the interface. Since the cooling rates are non-uniform in the surface region, localized deformation can take place in the microstructure, which influences the corrosion resistance at the treated surface in consistent with the previous studies [27]. On the other hand, aluminum nitride formation enhances the corrosion resistance at the surface in line with the previous findings [28]. Therefore, the possible explanation for the improvement of corrosion resistance of the laser-treated surface is due to the passive film including nitride compound, formation and the microstructural refinement at the surface during the treatment process. In addition, coarser dendritic structures yield higher corrosion resistance than finer dendritic structures, which is associated with the morphology of the interdendritic eutectic mixture [29, 30]. Moreover, the corrosion potential (E_corr) of all specimens is found to be in the range of –0.66 to –0.67 V_SCE, while the passive corrosion current density (i_p) is found to be the least for the laser-treated specimen, i.e., it is 1.3×10^-5 A, as compared to 5.5×10^-5 A for the untreated specimen. The same trend is observed for the corrosion rate calculated by Tafel analysis; it is found to be the least for the laser-treated specimen, which is 0.81 mils per year (mpy), and 2.3 mpy for the untreated specimen.

Figure 6

Potentiodynamic polarization response of treated and untreated alloys in 0.5 m NaCl solution at room temperature.

Figure 7 shows the SEM micrographs of laser-treated and untreated workpiece surfaces after the electrochemical tests. The pit sites are visible at the surfaces of all workpieces. In general, the pits are small and shallow for the laser-treated surfaces while indicating the presence of the passive layer due to oxide and nitride compounds formed at the surface during the laser treatment process, since aluminum nitride is highly resistant to the corrosion attack [31]. The pit sites is formed because of the volume expansion (Figures 7A and B) while indicating the inter-granular attack is the governing process for the pitting. Although a corrosion product is present in some pit sites, no secondary pitting is observed at the surface. This behavior is associated with the passivation of the pit site once it is formed, which in turn results in shallow pit sites. However, locally scattered few deep pit sites are observed at the surface. This is attributed to the surface texture peaks, which may initiate crevice-like corrosion at the laser-treated surface. Nevertheless, the pit site sizes are small.

Figure 7

SEM micrographs of laser-treated and untreated surfaces: (A) laser-treated surface and localized pit site, and (B) shallow pit site at the untreated surface.

4 Conclusion

Laser treatment of an aluminum-based composite is carried out to improve the surface properties. The workpiece consists of 15% SiC particles and pure aluminum, and it is produced through a hot pressing process. Analytical tools including optical and scanning electron microscopes, energy dispersive spectroscopy, X-ray diffraction, a micro-tribometer, and a potentiostat are used to assess the morphological, metallurgical, mechanical, and electrochemical characteristics of the resulting surfaces. It is found that a laser-treated surface is free from large-scale cracks and cavities. Regular laser scan tracks formed at the surface modify the cooling rates. In this case, heat conduction from the recently formed laser scan tracks generates a self-annealing effect on the early formed tracks. The presence of partially imbedded hard particles (SiC) is evident at the laser-treated surface, which is attributed to the high melting temperature of the hard particles. Although the thermal expansion coefficients of SiC and aluminum are different, micro-cracks or voids are observed around the hard particles due to volume shrinkage and micros-stress formation. This behavior is also attributed to the self-annealing effect of the lately formed laser scanning tracks. The small cavities are formed at the workpiece surface because of the local evaporation, which are modified by the melt flow from the cavity vicinity toward the cavity center. This lowers the roughness at the laser-treated surface. A dense layer, consisting of small grains and hard particles, is formed at the laser-treated surface. As the depth below the surface increases, columnar-like stacked structures are formed due to the relatively lower cooling rates as compared to that at the surface. Large-size grains are formed in the heat-affected region. Laser treatment increased the surface hardness, which is associated with the fine grains formed at the surface. The friction coefficient of the laser-treated surface improves significantly and the starched depth becomes shallower for the laser-treated surface than that for the untreated surface. The laser treatment modifies the electrochemical properties at the surface, in which case, the corrosion resistance of the treated surface improves.