**4. Laser surface remelting of aluminium alloy**

Laser surface melting (LSM) is a well established technology applied to many materials for hardening, reducing porosity and increasing wear and corrosion resistance.

LSM is a versatile and promising technique that can be used to modify the surface properties of a material without affecting its bulk property (Yue et al., 2004; Rams et al., 2007). The modifying in the surface properties of the material is due to rapid melting followed by rapid solidification. The intimate contact between the melt and the solid substrate causes a very fast heat extraction during solidification resulting in very high cooling rates of the order of 105 to 108 k/s. The high cooling rates to which this surface layer is submitted result in the formation of different microstructures from bulk metal leading to improved surface properties (Aparecida Pinto et al., 2003). Materials processed via rapid solidification tend to show advantages of refined microstructure, reduced microsegregation, extensive solid solubility and formation of metastable phases (Munitz, 1985; Zimmermann et al., 1989). It is generally accepted that the improvement in corrosion performance is due to refinement/homogenisation of microstructure and dissolusion/redistribution of precipitates or inclusions, which result from rapid solidification (Chong et al., 2003). This was considered to be due to the presence of the compact oxide layers on top of the lasermelted zone. The layers mainly consisted of structures α-Al2O3, which is a homogeneous and chemically stable phase and serves as an effective barrier to protect the matrix against corrosion attacks. In untreated surfaces of Al alloys the microsegregation in relatively thin surface layer plays an important role in initiating pitting in the inhomogeneous structures. The schematic of the laser surface melting process is shown in figure 14 (Aparecida Pinto et al., 2003).

Some industrial laser sources such as CO2, Nd:YAG, excimer and high power diode lasers were applied to surface melting of aluminium alloys. Since aluminum alloy have no solid phase transformation, if the surface of aluminium alloys should not be melted, the surface cannot be strengthened. In view of the basic physical properties of aluminium alloy, such as large specific heat, high heat conductivity and high reflectivity to laser power density than that for ferrous alloy (Wong et al., 1997). The controlling of laser parameters is very important factor for laser surface melting process.

Because the properties of a material depend largely on its microstructure, controlled formation of such microstructures is essential to develop new materials with desired properties (Aparecida Pinto et al., 2003). Laser parameters such as laser power density, interaction time and scan speed affect on solidification behaviour and thus the microstructure of melted zone can be changed.

The diagram shown in figure 15 associated the microstructural evolution with the solid/liquid front velocity (Aparecida Pinto et al., 2003).

Laser Surface Treatments of Aluminum Alloys 129

Taking a longitudinal section through the centerline of the laser track, the speed of the solid/liquid front (Vs) is correlated to the beam speed (Vb) by (Aparecida Pinto et al., 2003):

Vs = Vb cos φ (5)

This equation describes that Vs varies from zero at the bottom of the moltem pool to a

Pinto et. al. (Aparecida Pinto et al., 2003) investigated the microstructure of Al-Cu alloy after laser surface melting. The influence of Vb on the microstructure is shown in figure 17. The lower beam speed of 500 mm/min has permitted a more extensive cellular zone to be formed and a later transition from a cellular to a dendritic structure when compared with

Fig. 17. Solidification morphology transitions in the molten pool. (a) p = 1 kw, v = 500

From the result of hardness tests, Pinto reported that the mean hardness increase from 75 Hv in the unmelted zone to 160 Hv in the cellular structure. In contrast, a higher value of 210 Hv was measured in the dendritic structure due to the fineness of the microstructure

mm/min; (b) p = 1 kw, v = 800 mm/min (Aparecida Pinto et al., 2003)

(Aparecida Pinto et al., 2003).

where φ is the angle between Vs and Vb vectors that shown in figure 16.

approaching the value of Vb at the top of the molten pool.

the structure developed under a speed of 800 mm/min.

Fig. 14. Schematic illustration of the laser surface melting process (Aparecida Pinto et al., 2003)

Fig. 15. Microstructure variation according to the solid/liquid front velocity (Aparecida Pinto et al., 2003)

Fig. 16. Schematic representation of the relationship between solidification speed and laser beam speed (Aparecida Pinto et al., 2003)

Fig. 14. Schematic illustration of the laser surface melting process (Aparecida Pinto et al., 2003)

Fig. 15. Microstructure variation according to the solid/liquid front velocity (Aparecida

Fig. 16. Schematic representation of the relationship between solidification speed and laser

Pinto et al., 2003)

beam speed (Aparecida Pinto et al., 2003)

Taking a longitudinal section through the centerline of the laser track, the speed of the solid/liquid front (Vs) is correlated to the beam speed (Vb) by (Aparecida Pinto et al., 2003):

$$\mathbf{V}\_s = \mathbf{V}\_b \cos \phi \tag{5}$$

where φ is the angle between Vs and Vb vectors that shown in figure 16.

This equation describes that Vs varies from zero at the bottom of the moltem pool to a approaching the value of Vb at the top of the molten pool.

Pinto et. al. (Aparecida Pinto et al., 2003) investigated the microstructure of Al-Cu alloy after laser surface melting. The influence of Vb on the microstructure is shown in figure 17. The lower beam speed of 500 mm/min has permitted a more extensive cellular zone to be formed and a later transition from a cellular to a dendritic structure when compared with the structure developed under a speed of 800 mm/min.

Fig. 17. Solidification morphology transitions in the molten pool. (a) p = 1 kw, v = 500 mm/min; (b) p = 1 kw, v = 800 mm/min (Aparecida Pinto et al., 2003)

From the result of hardness tests, Pinto reported that the mean hardness increase from 75 Hv in the unmelted zone to 160 Hv in the cellular structure. In contrast, a higher value of 210 Hv was measured in the dendritic structure due to the fineness of the microstructure (Aparecida Pinto et al., 2003).

Leech (Leech, 1989) studied the laser surface melting of Al-Si alloys as a function of the beam interaction time τ, that determined by following equation:

$$
\pi = \frac{1}{V} \tag{6}
$$

Laser Surface Treatments of Aluminum Alloys 131

Fig. 20. SEM micrograph of the region of lamellar in Al-Si alloy at a traverse speed of 413

Fig. 21. Schematic phase diagram illustrating the micro structural–undercooling relations

thereby corresponded to the zone of couple growth (Leech, 1989).

An interpretation of the microstructures involves reference to the phase-under cooling diagram shown in figure 21. After laser melting, a rapid extraction of heat from the liquid adjacent to the substrate will produce direct cooling into the α+ eutectic region, the resulting nucleation and growth of α columnar dendrites causing a rejection of silicon into the remaining melt. As the silicon content of the melt increased and with rise in temperature due to latent heat, it is proposed that the composition-cooling line moved to the right into the eutectic-coupled zone. The formation of the lamellar region in the laser melt zone

mm/min (Leech, 1989)

during quenching (Leech, 1989)

Where l is the beam diameter and V is the scanning velocity. The microstructure features in the laser-melted zone consisted of a highly refined dendritic growth at beam traverse speed of 100 mm/min. Within the structure there is a progressive change in dendrite morphology from a planar melt-substrate interface (figure 18) at the maximum melt depth, through a region of oriented columnar dendrite growth (figure 19), to a central region which at more rapid scan rates comprised a fine, filamentary eutectic (figure 20).

Fig. 18. SEM micrograph showing the melt-substrate interface in the Al-Si alloy (beam traverse speed, 100 mm/min (Leech, 1989)

Fig. 19. SEM micrograph showing the columnar dendritic region in the melted zone in Al-Si-W-Ni alloy. (traverse speed, 100 mm/min) (Leech, 1989)

Leech (Leech, 1989) studied the laser surface melting of Al-Si alloys as a function of the

*l V* τ

Where l is the beam diameter and V is the scanning velocity. The microstructure features in the laser-melted zone consisted of a highly refined dendritic growth at beam traverse speed of 100 mm/min. Within the structure there is a progressive change in dendrite morphology from a planar melt-substrate interface (figure 18) at the maximum melt depth, through a region of oriented columnar dendrite growth (figure 19), to a central region which at more

Fig. 18. SEM micrograph showing the melt-substrate interface in the Al-Si alloy (beam

Fig. 19. SEM micrograph showing the columnar dendritic region in the melted zone in Al-Si-

= (6)

beam interaction time τ, that determined by following equation:

rapid scan rates comprised a fine, filamentary eutectic (figure 20).

traverse speed, 100 mm/min (Leech, 1989)

W-Ni alloy. (traverse speed, 100 mm/min) (Leech, 1989)

Fig. 20. SEM micrograph of the region of lamellar in Al-Si alloy at a traverse speed of 413 mm/min (Leech, 1989)

An interpretation of the microstructures involves reference to the phase-under cooling diagram shown in figure 21. After laser melting, a rapid extraction of heat from the liquid adjacent to the substrate will produce direct cooling into the α+ eutectic region, the resulting nucleation and growth of α columnar dendrites causing a rejection of silicon into the remaining melt. As the silicon content of the melt increased and with rise in temperature due to latent heat, it is proposed that the composition-cooling line moved to the right into the eutectic-coupled zone. The formation of the lamellar region in the laser melt zone thereby corresponded to the zone of couple growth (Leech, 1989).

Fig. 21. Schematic phase diagram illustrating the micro structural–undercooling relations during quenching (Leech, 1989)

Laser Surface Treatments of Aluminum Alloys 133

The microhardness variation with distance from the melt surface after laser surface melting of Al-Si-Ni alloy and Al-Si alloy is shown in figure 22 (Leech, 1989). Apart from the differences between the alloys in molt zone depth, the curves also illustrate the higher hardness attained throughout the resolidified region of Al-Si-Cu-Ni alloy that in the Al-Si alloy. Leech (Leech, 1989) also reported that micro hardness of laser surface melted layer of

Increasing quenching rates, may promote the formation of finer dispersions of copper and

Corrosion resistance is an important matter in aluminum alloys. There are several methods of surface engineering to improving the corrosion behavior of aluminum alloys, that everyone has advantages and disadvantages. Laser surface melting is one of those techniques. There have been a number of studies of the influence of LSM on the corrosion properties of aluminum alloys, and the results achieved have been ambiguous with respect to the benefits of LSM. In some cases, it is severally accepted that laser surface melting can be used for improving the localized corrosion resistance of aluminum alloys as a result of homogenization and refinement of microstructures, and phase transformations. For example, Chong et. al (Chong et al., 2003) studied the corrosion behavior of Al-2014 alloy in T6 and T451 conditions after laser surface melting. After the corrosion tests, they found a large number of pits, randomly distributed on the surface of as-received Al2014 alloy in two conditions (figure 24a). In this instances although Al2014 alloy in both tempers consisted of similar types of intermetallic particles, the copper content in the aluminum matrix for T6 is lower than that for T451. In the NaCl electrolyte, Al2Cu, and Al-Cu-Mn-Fe-Si particles tend to be cathodic to the matrix (Chong et al., 2003), and pits are likely to initiate and grow in the copper-depleted zone around these particles (Guillaumin & Mankowski, 1999). Mg2Si particles are anodic to the aluminum matrix, and have a tendency to dissolve and leaving cavities. Figure 24b shows that after LSM, pits formed on the laser-melted surfaces are larger but shallower that in the as- received alloy, with a semi-continuous network, consisting of copper-rich precipitates, remaining within the pits, indicating their cathodic nature. It is proposed that the concentrations of solid solution alloy elements, (particularly copper in Al 2014), are key factors influencing pitting corrosion. Such increase of copper content in the Al 2014 matrix can reduce the potential difference between the Al2Cu phases and the aluminum matrix, thereby reducing the driving force of pitting corrosion. The reduction in population or the elimination of Mg2Si particles which are anodic to aluminum matrix may

Al-Si-Cu-Ni and Al-Si alloys is dependent on laser scan rate (figure 23).

(a) (b)

T6 alloy (Chong et al., 2003)

Fig. 24. Pit morphology of (a) as- received Al 2014- T6 alloy and (b) laser- melted Al 2014-

nickel-bearing intermetallic particles (Leech, 1989).

Fig. 22. Hardness profiles taken across the laser-melted regions in the Al-13.6%Si-2.23%Cul.94%Ni (●) and Al-13.0%Si (○) alloys at a scan rate of 10 mm s- 1 (Leech, 1989)

Fig. 23. Micro hardness of the melted zone in the Al-Si-Cu-Ni (•) and Al-Si (○) alloys, plotted as a function of the beam scan rate (Leech, 1989)

Fig. 22. Hardness profiles taken across the laser-melted regions in the Al-13.6%Si-2.23%Cu-

•

○

Fig. 23. Micro hardness of the melted zone in the Al-Si-Cu-Ni (•) and Al-Si (○) alloys, plotted

○

○ ○

10 100 1000

BEAM SCAN RATE (mm. sec. ) -1

•

○ ○

• • •

○ ○

• • •

l.94%Ni (●) and Al-13.0%Si (○) alloys at a scan rate of 10 mm s- 1 (Leech, 1989)

HARDNESS (HV)

•

○

as a function of the beam scan rate (Leech, 1989)

•

○ ○

•

The microhardness variation with distance from the melt surface after laser surface melting of Al-Si-Ni alloy and Al-Si alloy is shown in figure 22 (Leech, 1989). Apart from the differences between the alloys in molt zone depth, the curves also illustrate the higher hardness attained throughout the resolidified region of Al-Si-Cu-Ni alloy that in the Al-Si alloy. Leech (Leech, 1989) also reported that micro hardness of laser surface melted layer of Al-Si-Cu-Ni and Al-Si alloys is dependent on laser scan rate (figure 23).

Increasing quenching rates, may promote the formation of finer dispersions of copper and nickel-bearing intermetallic particles (Leech, 1989).

Corrosion resistance is an important matter in aluminum alloys. There are several methods of surface engineering to improving the corrosion behavior of aluminum alloys, that everyone has advantages and disadvantages. Laser surface melting is one of those techniques. There have been a number of studies of the influence of LSM on the corrosion properties of aluminum alloys, and the results achieved have been ambiguous with respect to the benefits of LSM. In some cases, it is severally accepted that laser surface melting can be used for improving the localized corrosion resistance of aluminum alloys as a result of homogenization and refinement of microstructures, and phase transformations. For example, Chong et. al (Chong et al., 2003) studied the corrosion behavior of Al-2014 alloy in T6 and T451 conditions after laser surface melting. After the corrosion tests, they found a large number of pits, randomly distributed on the surface of as-received Al2014 alloy in two conditions (figure 24a). In this instances although Al2014 alloy in both tempers consisted of similar types of intermetallic particles, the copper content in the aluminum matrix for T6 is lower than that for T451. In the NaCl electrolyte, Al2Cu, and Al-Cu-Mn-Fe-Si particles tend to be cathodic to the matrix (Chong et al., 2003), and pits are likely to initiate and grow in the copper-depleted zone around these particles (Guillaumin & Mankowski, 1999). Mg2Si particles are anodic to the aluminum matrix, and have a tendency to dissolve and leaving cavities. Figure 24b shows that after LSM, pits formed on the laser-melted surfaces are larger but shallower that in the as- received alloy, with a semi-continuous network, consisting of copper-rich precipitates, remaining within the pits, indicating their cathodic nature. It is proposed that the concentrations of solid solution alloy elements, (particularly copper in Al 2014), are key factors influencing pitting corrosion. Such increase of copper content in the Al 2014 matrix can reduce the potential difference between the Al2Cu phases and the aluminum matrix, thereby reducing the driving force of pitting corrosion. The reduction in population or the elimination of Mg2Si particles which are anodic to aluminum matrix may

Fig. 24. Pit morphology of (a) as- received Al 2014- T6 alloy and (b) laser- melted Al 2014- T6 alloy (Chong et al., 2003)

Laser Surface Treatments of Aluminum Alloys 135

further improve the behavior by reducing cavities due to the dissolved particles. Regarding influences of preferred orientation, the literature (Guillaumin & Mankowski, 1999) indicates that the pitting potential of aluminum increases in order of pit {} {} {} <sup>001</sup> pit <sup>011</sup> pit <sup>111</sup> (E ) (E ) (E ) ≥ > , however, the presence of alloyed copper in solid solution reduces the dependence of Epit on surface orientation. Thus, the preferred orientation of α-Al along [200] direction in lasermelted alloy does not appear to play a significant role in the improved pitting behavior. High- strength aluminum alloys (HSAL) are high susceptible to various forms of corrosion, particularly in the presence of chloride-containing media. Thus, these alloys are very susceptible to pitting corrosion fatigue, and the degradation of HSAL by this phenomenon is a matter of major concern, particularly as many structural parts are inaccessible for inspection and cannot be monitored, thus hiding the defects of corrosion as they approach a critical for fatigue. The improvement in pitting corrosion fatigue behavior of HSAL alloys after LSM is reported by Xu and co-workers (Xu et al., 2006). Figure 25 shows the results of impedance measurements of unmelted and surface melted Al 6013 alloy. These results are displayed in

the form of Nyquist plots as a function of immersion time up to a period of 6 days.

with immersion time (figure 25a).

table 1.

et al., 2006)

The spectra suggest that for untreated and laser-treated Al alloy, corrosion pitting has occurred to various degrees at different times during the immersion test. This is evidenced by the presence of start of the immersion test, with the diameter of the arches decreasing

As for the laser- treated Al 6013 alloy, a compressed capacitive loop with a small diffusion tail at the low-frequency range was seen at the first hour of the test, and a second loop emerged after 1 day of immersion (figure 25b). Figure 26 shows the equivalent circuit of EIS plots to interpret the electrochemical behavior of untreated and laser-treated Al 6013 alloy. The equivalent circuit component values as a function of immersion time are listed in

Fig. 26. Equivalent circuits for the untreated and laser-treated Al 6013 alloy (Xu et al., 2006)

Table 1. Calculated values of the equivalent circuit components of the impedance plots (Xu

Fig. 25. Nyquist plots of the (a) untreated specimen, (b) laser, treated Al-6013 alloy (Xu et al., 2006)

untreated 1h untreated 1d untreated 3d untreated 6d

air treated 1h air treated 1d air treated 3d air treated 6d

0 10 20 30 Z'/k Ω cm<sup>2</sup>




0

0 10 20 30

Z'/k Ω cm 0 100 200 300 400 500

Fig. 25. Nyquist plots of the (a) untreated specimen, (b) laser, treated Al-6013 alloy (Xu et al.,

2



Z''/k Ω cm2


0






0

2006)

Z''/k Ω cm2

further improve the behavior by reducing cavities due to the dissolved particles. Regarding influences of preferred orientation, the literature (Guillaumin & Mankowski, 1999) indicates that the pitting potential of aluminum increases in order of pit {} {} {} <sup>001</sup> pit <sup>011</sup> pit <sup>111</sup> (E ) (E ) (E ) ≥ > ,

however, the presence of alloyed copper in solid solution reduces the dependence of Epit on surface orientation. Thus, the preferred orientation of α-Al along [200] direction in lasermelted alloy does not appear to play a significant role in the improved pitting behavior.

High- strength aluminum alloys (HSAL) are high susceptible to various forms of corrosion, particularly in the presence of chloride-containing media. Thus, these alloys are very susceptible to pitting corrosion fatigue, and the degradation of HSAL by this phenomenon is a matter of major concern, particularly as many structural parts are inaccessible for inspection and cannot be monitored, thus hiding the defects of corrosion as they approach a critical for fatigue. The improvement in pitting corrosion fatigue behavior of HSAL alloys after LSM is reported by Xu and co-workers (Xu et al., 2006). Figure 25 shows the results of impedance measurements of unmelted and surface melted Al 6013 alloy. These results are displayed in the form of Nyquist plots as a function of immersion time up to a period of 6 days.

The spectra suggest that for untreated and laser-treated Al alloy, corrosion pitting has occurred to various degrees at different times during the immersion test. This is evidenced by the presence of start of the immersion test, with the diameter of the arches decreasing with immersion time (figure 25a).

As for the laser- treated Al 6013 alloy, a compressed capacitive loop with a small diffusion tail at the low-frequency range was seen at the first hour of the test, and a second loop emerged after 1 day of immersion (figure 25b). Figure 26 shows the equivalent circuit of EIS plots to interpret the electrochemical behavior of untreated and laser-treated Al 6013 alloy. The equivalent circuit component values as a function of immersion time are listed in table 1.

Fig. 26. Equivalent circuits for the untreated and laser-treated Al 6013 alloy (Xu et al., 2006)


Table 1. Calculated values of the equivalent circuit components of the impedance plots (Xu et al., 2006)

Laser Surface Treatments of Aluminum Alloys 137

Aluminium-based metal matrix composites (Al-MMCs) have high strength, hardness and wear resistance, and find application in various industrial sectors, such as automotive and aerospace industries (Anandkumar et al,. 2007). The major drawbacks of these materials are their high coat and complex production methods compared to conventional alloys, but for many applications, like rapid tooling, the bulk stress levels are compatible with the use of high-strength Al alloy, the required wear resistance being achieved by coating the component with a high wear resistance materials such as a ceramic-reinforced Al-matrix composite (Anandkumar et al,. 2007). Aluminium alloys have been cladded with ceramics such as SiC, B4C, TiC due to their high hardness and thermal stability and various other metallic materials such as Ti, B, Ni etc. to enhance their surface properties (Anandkumar et al,. 2007). These ceramic reinforcement particles have a low reflectivity; therefore they absorb a considerable amount of laser energy (Anandkumar et al., 2009) and may reach very high temperatures, which will lead to intense reactions between the reinforcement and the liquid metal or to particle dissolution in the melt pool. The tendency of reactivity of reinforcement particles with depends on their temperature, which depends on the

interaction time between the particles and the laser beam (Anandkumar et al., 2009).

0°, because the particles trajectory through the laser beam is longer.

In this case, the velocity of injected powder is an important factor that affects on the interaction time and particles temperature. The temperature variation of injected powder particles is calculated by several researchers using mathematical modeling. Huang et al. (Huang et al., 2005) calculated the beam attenuation and particle temperature variation due to the interaction of an off-axis powder stream with a laser beam on the basis of Lambert-Beer law and Mie's theory. They found that the temperature of injected powder particles increases with decreasing the angle between the powder jet and the laser beam from 45 to

Also, a mathematical model for calculation of particles temperature under laser beam irradiation is established by jouvard and co-workers (Jouvard et al., 1997). Figure 28 shows

an off-axis blown powder laser cladding process diagram used for jouvard model.

Fig. 28. Diagram of laser beam-powder stream interaction (Anandkumar et al., 2009)

**5. Laser surface cladding of aluminium alloys** 

The magnitude CPEp (constant phase element), a measure of the capacitance at the surface of laser- melted alloy is much less than that of the as-received alloy, especially up to the immersion time of 3 days. This indicates that less ion adsorption has occurred at the surface of the laser melted alloy. This confirms the good corrosion resistance of the layers containing laser-formed aluminum oxide in reducing the rate of electro chemical reaction at the lasermeted surface. Xu (Xu et al., 2006) reported that the corrosion fatigue life of the laser-surface melted Al 6013 alloy is two times longer than that of the as-received Al alloy (figure 27). Also, the corrosion current for the laser-surface melted Al 6013 alloy is considerably lower than that for the as-received Al 6013 alloy. The improvement in the pitting corrosion of the laser-surface- melted Al alloy. An increase in the corrosion resistance of Al- Si alloys after laser surface melting in both 10% H2SO4 and 10% HNO3 solutions is observed by Wong and co-workers (Wong & Liang, 1997). Also, they reported that, in the 10% HCl and 5% NaCl solutions laser melting has little effect on the corrosion resistance of Al-Si alloys. Because the Cl ions destroy the Al2O3 film completely. In the case of 5% NaCl solution, NaAlO2 is formed and the protective oxide film Al2O3 is again destroyed, which intensifies the corrosion of the aluminum alloys (Yongqing et al., 1998).

Corrosion resistance of laser surface melted Al 2024 alloy is investigated by Li and coworkers (Li et al., 1996). Free corrosion in naturally aerated chloride electrolyte solution revealed a change in the mechanism of corrosion for the LSM alloy. A small number of large pits, initiated in the α-Al cells and/or dendrites, are found at random over the surface. In contrast, for the as-received alloy where pitting is initiated at Al2CuMg precipitates, corrosion took the form of intergranular corrosion.

Fig. 27. Test of the fatigue life of the untreated and laser- treated Al 6013 alloy in a 3.5% NaCl solution at a potential of – 675 mv (Xu et al., 2006)

The magnitude CPEp (constant phase element), a measure of the capacitance at the surface of laser- melted alloy is much less than that of the as-received alloy, especially up to the immersion time of 3 days. This indicates that less ion adsorption has occurred at the surface of the laser melted alloy. This confirms the good corrosion resistance of the layers containing laser-formed aluminum oxide in reducing the rate of electro chemical reaction at the lasermeted surface. Xu (Xu et al., 2006) reported that the corrosion fatigue life of the laser-surface melted Al 6013 alloy is two times longer than that of the as-received Al alloy (figure 27). Also, the corrosion current for the laser-surface melted Al 6013 alloy is considerably lower than that for the as-received Al 6013 alloy. The improvement in the pitting corrosion of the laser-surface- melted Al alloy. An increase in the corrosion resistance of Al- Si alloys after laser surface melting in both 10% H2SO4 and 10% HNO3 solutions is observed by Wong and co-workers (Wong & Liang, 1997). Also, they reported that, in the 10% HCl and 5% NaCl solutions laser melting has little effect on the corrosion resistance of Al-Si alloys. Because the Cl ions destroy the Al2O3 film completely. In the case of 5% NaCl solution, NaAlO2 is formed and the protective oxide film Al2O3 is again destroyed, which intensifies the

Corrosion resistance of laser surface melted Al 2024 alloy is investigated by Li and coworkers (Li et al., 1996). Free corrosion in naturally aerated chloride electrolyte solution revealed a change in the mechanism of corrosion for the LSM alloy. A small number of large pits, initiated in the α-Al cells and/or dendrites, are found at random over the surface. In contrast, for the as-received alloy where pitting is initiated at Al2CuMg precipitates,

Fig. 27. Test of the fatigue life of the untreated and laser- treated Al 6013 alloy in a 3.5% NaCl

corrosion of the aluminum alloys (Yongqing et al., 1998).

corrosion took the form of intergranular corrosion.

solution at a potential of – 675 mv (Xu et al., 2006)
