**2. Laser – assisted materials surface treatment requirements**

Figure 2 shows general regimes of various laser surface treating parameters for both pulsed and continuous wave lasers. Short pulses (ns to fs) with high peak power densities are desirable for laser shock processing and ablation applications. In general, longer pulses (μ<sup>s</sup> to ms) or continuous wave lasers are preferred for melting and heating processes (Nagarathnam & Taminger, 2001).

Laser chemical vapor deposition and laser surface transformation hardening require lower densities and interaction times as compared to processes involving meting and vaporization.

Laser Surface Treatments of Aluminum Alloys 117

interdiffuse into the substrate. As soon as the laser pulse is finished the resolidification

Laser surface alloying may induce an extreme heating/cooling rate of 104-1011k/s, thermal gradient of 105-108 k/m and solidification velocity as high as 30m/s (Draper & Poate, 1985). Due to the high cooling rates, solid state diffusion can be neglected and homogeneous and fine solidification microstructures can be achieved with a wide variety of surface compositions without the limitations of conventional processes, for instance, to extend solid solutions and to obtain metastable structures or even metallic glasses (Damborenea, 1998). Laser surface alloying of Al- alloys by different alloying elements and different techniques was investigated by several researchers. In most of their reports, it was shown that the structure of the zones of laser alloyed depends on the properties of the treated and alloying materials and on the dispersity of the alloying particles, the power of the laser radiation, and the duration of the irradiation (Aleksandrov, 2002). Figure 3 shows the structure of Al-alloy D16 saturated with NiO2 and NbSi2 particles. The saturation with NiO2 is provided by convective mass transfer, which is confirmed by the vortex-like appearance of the structure of the molten zone (figure 3a). The well–manifested heterophase (figure 3b) in the surface layer is provided by the mechanism of penetration of particles of NbSi2 into the molten pool.

NiO2

Fig. 3. Structure of the molten zone after laser alloying of Al-alloy D16 with NiO2 (a) and

NbSi2 (b). x200. (Aleksandrov, 2002)

NbSi2

process begins from the liguid/solid interface towards the surface.

Fig. 1. Various laser surface treatment methods

Fig. 2. Laser power density, specific energy and interaction times for various laser processing regimes (Nagarathnam & Taminger, 2001)

### **3. Laser surface alloying of aluminium alloys**

Laser surface alloying (LSA) involves tailoring the surface microstructure and composition by rapid melting, intermixing and solidification of a pre/co deposited surface layer with apart of the underlying substrate (Majumdar & Manna, 2002 ). Also in this treatment, a shallow layer at the surface of the material is melted by the laser beam which becomes efficiently coupled to the surface, while alloying elements are added simultaneously to give a local composition having the desired surface properties on solidification (Renk et al., 1998). When alloying elements are added to the melted pool then they will start to

Laser Surface Treatment

Solid state processing Ageing

10 10 10 10 10 -8 -6 -4 -2 Log(Interaction Time), D/V

Fig. 2. Laser power density, specific energy and interaction times for various laser processing

Laser surface alloying (LSA) involves tailoring the surface microstructure and composition by rapid melting, intermixing and solidification of a pre/co deposited surface layer with apart of the underlying substrate (Majumdar & Manna, 2002 ). Also in this treatment, a shallow layer at the surface of the material is melted by the laser beam which becomes efficiently coupled to the surface, while alloying elements are added simultaneously to give a local composition having the desired surface properties on solidification (Renk et al., 1998). When alloying elements are added to the melted pool then they will start to

LCVD

● ●

●

Cutting Glazing VAPORIZATION

Welding

Stereolithography

● ●

Melting

●

Cladding

●

LPVD

MELTING

●

Remelting

Alloying Hardfacing

Shock

Glazing

Transformation hardening

VAPORISING

MELTING

Liquid state processing

HEATING

HEATING

Shock hardening

●

Transformation hardening

Drilling

Fig. 1. Various laser surface treatment methods

●

●

10

8

10

Log(Power Density), P/D , W/mm

2

4

10

regimes (Nagarathnam & Taminger, 2001)

**3. Laser surface alloying of aluminium alloys** 

interdiffuse into the substrate. As soon as the laser pulse is finished the resolidification process begins from the liguid/solid interface towards the surface.

Laser surface alloying may induce an extreme heating/cooling rate of 104-1011k/s, thermal gradient of 105-108 k/m and solidification velocity as high as 30m/s (Draper & Poate, 1985). Due to the high cooling rates, solid state diffusion can be neglected and homogeneous and fine solidification microstructures can be achieved with a wide variety of surface compositions without the limitations of conventional processes, for instance, to extend solid solutions and to obtain metastable structures or even metallic glasses (Damborenea, 1998).

Laser surface alloying of Al- alloys by different alloying elements and different techniques was investigated by several researchers. In most of their reports, it was shown that the structure of the zones of laser alloyed depends on the properties of the treated and alloying materials and on the dispersity of the alloying particles, the power of the laser radiation, and the duration of the irradiation (Aleksandrov, 2002). Figure 3 shows the structure of Al-alloy D16 saturated with NiO2 and NbSi2 particles. The saturation with NiO2 is provided by convective mass transfer, which is confirmed by the vortex-like appearance of the structure of the molten zone (figure 3a). The well–manifested heterophase (figure 3b) in the surface layer is provided by the mechanism of penetration of particles of NbSi2 into the molten pool.

Fig. 3. Structure of the molten zone after laser alloying of Al-alloy D16 with NiO2 (a) and NbSi2 (b). x200. (Aleksandrov, 2002)

Laser Surface Treatments of Aluminum Alloys 119

They suggested that the influence of convection in liquid homogenization could be characterized by the surface tension number, S, which relates thermocapillarity-induced

( )

*<sup>d</sup> qd dT <sup>s</sup> u k*

μ

Where (dσ/dT) is the temperature coefficient of the surface tension, q is the net heat flow from the laser beam, d is diameter of the laser beam, μ is the viscosity, u0 is the scanning speed of the laser beam, and k is the thermal conductivity. When S is low (S≤45000), convection is negligible. Due to the short lifetime of the melt pool, mass transport will be insufficient for melt homogenization. When S is high, convection plays a dominant role in transport phenomena in the melt pool. In general for metals, the convection speed is several orders of magnitude higher than the scanning speed, leading to rapid homogenization. However, for liquid Al the temperature coefficient of the surface tension (dσ/dT) is relatively low (-0.155×10-3 Nm-1k-1), and therefore S will be low. Consequently, in some cases insufficient homogenization of the melt pool is to be expected for laser surface alloying of Al-alloys. For example, this was happened for recently mentioned Almeida's research that is shown in figure 4 (Almeida et al., 2001). A further difficulty arises when the alloying elements react with the melt pool material to form insoluble high melting temperature phases, such as intermetallic compounds. In the matter the diffusion phenomena is

The dissolution kinetics was theoretically analysed by Costa and Vilar (Costa & Vilar, 1996) using a spherical geometry and dropping the quasi-steady–state approximation. Figure 5 shows the results of evaluation of intermetallic layer thickness of Al3Nb as a function of time. This result is reported by Almeida and co-workers (Almeida et al., 2001). They calculated the dissolution time of Nb particles with a diameter of 100μm. These particles takes about 22s to transform to Al3Nb, a time much longer than the interaction time used

Fig. 5. Thickness of intermetallic layer and size of particle for any laser interaction time

σ

0

<sup>=</sup> (1)

convection velocity and laser beam scanning speed, and is give by:

responsible to control of dissolution kinetics.

(0.24s) in their research.

(Almeida et al., 2001)

The structure of the laser surface alloyed of Al with Nb is shown in figure 4 (Almeida et al., 2001). A strong segregation of Nb in structure leading to the formation of a zone of resolidified Al solid solution and a zone with a high Nb concentration, consists of dendrites of Nb-free α-Al solid solution and undissolved particles of Nb (figure 4a), that some of these particles can be surrounded by a layer consisting of Al3Nb dendrites in an α- Al matrix (figure 4b) showing incipient dissolution and partial redistribution of Nb due to convective flow. It is necessary to mentioned that the temperature and convective mass transport must be sufficient to allow for the complete homogenization of the alloyed layers. This, it can be seen in figure 4 that the temperature and convective mass transport were not sufficient to allow for the complete homogenization of the material (Almeida et al., 2001).

Mazumder (Majumdar & Manna, 2002) studied mass transport in melt pools using a numerical model and concluded that the extremely fast homogenization frequently observed in laser surface alloying can only be explained by the intense Marangoni convection caused by the high temperature gradients within the melt pool (Almeida et al., 2001), with diffusion having only a minor role.

Fig. 4. (a) Structure of the bottom layer (A). (b) Undissolved Nb particle surrounded by a layer of Al3Nb dendrites and α-Al (Almeida et al., 2001)

The structure of the laser surface alloyed of Al with Nb is shown in figure 4 (Almeida et al., 2001). A strong segregation of Nb in structure leading to the formation of a zone of resolidified Al solid solution and a zone with a high Nb concentration, consists of dendrites of Nb-free α-Al solid solution and undissolved particles of Nb (figure 4a), that some of these particles can be surrounded by a layer consisting of Al3Nb dendrites in an α- Al matrix (figure 4b) showing incipient dissolution and partial redistribution of Nb due to convective flow. It is necessary to mentioned that the temperature and convective mass transport must be sufficient to allow for the complete homogenization of the alloyed layers. This, it can be seen in figure 4 that the temperature and convective mass transport were not sufficient to

Mazumder (Majumdar & Manna, 2002) studied mass transport in melt pools using a numerical model and concluded that the extremely fast homogenization frequently observed in laser surface alloying can only be explained by the intense Marangoni convection caused by the high temperature gradients within the melt pool (Almeida et al.,

Fig. 4. (a) Structure of the bottom layer (A). (b) Undissolved Nb particle surrounded by a

layer of Al3Nb dendrites and α-Al (Almeida et al., 2001)

allow for the complete homogenization of the material (Almeida et al., 2001).

2001), with diffusion having only a minor role.

They suggested that the influence of convection in liquid homogenization could be characterized by the surface tension number, S, which relates thermocapillarity-induced convection velocity and laser beam scanning speed, and is give by:

$$s = \frac{\left(d\sigma \Big/\_{dT}\right)\eta d}{\mu u\_0 k} \tag{1}$$

Where (dσ/dT) is the temperature coefficient of the surface tension, q is the net heat flow from the laser beam, d is diameter of the laser beam, μ is the viscosity, u0 is the scanning speed of the laser beam, and k is the thermal conductivity. When S is low (S≤45000), convection is negligible. Due to the short lifetime of the melt pool, mass transport will be insufficient for melt homogenization. When S is high, convection plays a dominant role in transport phenomena in the melt pool. In general for metals, the convection speed is several orders of magnitude higher than the scanning speed, leading to rapid homogenization. However, for liquid Al the temperature coefficient of the surface tension (dσ/dT) is relatively low (-0.155×10-3 Nm-1k-1), and therefore S will be low. Consequently, in some cases insufficient homogenization of the melt pool is to be expected for laser surface alloying of Al-alloys. For example, this was happened for recently mentioned Almeida's research that is shown in figure 4 (Almeida et al., 2001). A further difficulty arises when the alloying elements react with the melt pool material to form insoluble high melting temperature phases, such as intermetallic compounds. In the matter the diffusion phenomena is responsible to control of dissolution kinetics.

The dissolution kinetics was theoretically analysed by Costa and Vilar (Costa & Vilar, 1996) using a spherical geometry and dropping the quasi-steady–state approximation. Figure 5 shows the results of evaluation of intermetallic layer thickness of Al3Nb as a function of time. This result is reported by Almeida and co-workers (Almeida et al., 2001). They calculated the dissolution time of Nb particles with a diameter of 100μm. These particles takes about 22s to transform to Al3Nb, a time much longer than the interaction time used (0.24s) in their research.

Fig. 5. Thickness of intermetallic layer and size of particle for any laser interaction time (Almeida et al., 2001)

Laser Surface Treatments of Aluminum Alloys 121

Fig. 6. The micro structural aspects of the laser alloyed layer (Gingu et al., 1999)

Fig. 7. The adherence aspect of the layer at the material support (Gingu et al., 1999)

 and *<sup>T</sup> Z* ∂

*X* ∂ ∂

temperature gradient *<sup>T</sup>*

pool. In the case of low gradients *<sup>T</sup>*

the form (Aleksandrov, 2002):

*X* ∂

A mathematical modeling of laser surface alloying with solid particles is established by Aleksandrov (Aleksandrov, 2002). According to this model, the presence of a transverse

effective force of gravity (with allowance for the buoyancy force). The higher the difference in the densities of the alloying and Al alloys, the more effective the immersion of the particles. The equations of the motion of a single particle in Cartesian coordinates X, Z have

<sup>∂</sup> makes the particles move to the peripheral part of the molten

<sup>∂</sup> the particles simply drown in the field of the

In order to obtain significant particle dissolution, the temperature of the melt pool must be higher than the melting point of the intermetallic compounds.

Since convection-driven homogenization is negligible, and the melting point of alloying elements is higher than the melting temperature of Al, the latter starts to solidify before the alloying particles dissolve, and a layer consists of the starting material will be formed. In general, this happened in the bottom layers. In the upper layers, due to the higher temperature of the melt, the alloying particles dissolve in the liquid Al.

The microstructure of alloyed layer depends on solidification rate (R) and the temperature gradient at the solid-liquid interface (G) which in turn depend on heat and mass transfer in the system (Almeida et al., 2001). A simple relation exists between the local solidification rate (R) and the scanning speed gives by fallow equation:

$$\mathbf{R} = \mathbf{V}\_{\\$} \cos \theta \tag{2}$$

where θ is the angle between the normal to the solid-liquid interface and the scanning direction. The solidification rate increases with decreasing depth from 0 at the bottom of the melt pool (cosθ=0 at this point) to a value that remains lower that the scanning speed, because cos θ<1. Conversely, the thermal gradient G is higher at the bottom of the melt pool and decreases as depth decreases. Both solidification parameters vary rapidly during the first stages of solidification (near the bottom of the melt pool) to reach a value that remains approximately constant during most of the solidification process. Consequently, the microstructure in most of the re-solidified layer can be characterized by a single set of solidification parameters and should not change significantly.

Sometimes, in laser surface alloying the microstructure of alloyed layer appears as a texture. The texture effect increases with increasing solidification speed. The origin of this texture can be understood by considering the solidification mechanism in laser surface alloying and the variation of the shape of the melt pool as a function of scanning speed.

Gingo and et. al (Gingu et al., 1999) produced Al/SiCp composite by laser surface alloying. Figure 6 presents the microstructural aspect of the alloyed layer produced at the surface of an AA413 alloy. There is an obvious difference between the base microstructure of the Al alloy, which is the classic eutectic AlSi12, characterised by dendrites grains dispose randomly in the eutectic mass (zone 1), and the very fine granulated microstructure of the alloyed layer (zone2).

In this process, depending on the processing parameters, it is possible to use or not use an adhesive layer at the material support. As can be seen in figure 7, in this case there is a perfect adherence of the alloyed layer at the AA413 support; this phenomenon can be explained by the perfect compatibility of the matrix of Al- alloy (Gingu et al., 1999).

In laser surface alloying of Al with Nb as an alloying element, the dendritic structure was observed by Almeida et. al (Almeida et al., 2001), showing that Al3Nb grows with a dendritic solid/liquid interface. In this type of growth, there is a preferential growth direction usually a low index crystallographic direction. During the initial stages of solidification competition between neighboring dendrites with different orientations occurs, and those with the preferential growth direction nearest to the heat flow direction will be favored, leading to preferential orientation, and eventually to a strong texture. When the scanning speed of surface is increased the shape of melt pool increasingly elongated from semi-hemispherical. Also, when the scanning speed is low the heat flow direction changes progressively from the fusion line to the surface, leading to a variety of preferential growth directions of columnar grains (Almeida et al., 2001).

In order to obtain significant particle dissolution, the temperature of the melt pool must be

Since convection-driven homogenization is negligible, and the melting point of alloying elements is higher than the melting temperature of Al, the latter starts to solidify before the alloying particles dissolve, and a layer consists of the starting material will be formed. In general, this happened in the bottom layers. In the upper layers, due to the higher

The microstructure of alloyed layer depends on solidification rate (R) and the temperature gradient at the solid-liquid interface (G) which in turn depend on heat and mass transfer in the system (Almeida et al., 2001). A simple relation exists between the local solidification

 R=VS cos θ (2) where θ is the angle between the normal to the solid-liquid interface and the scanning direction. The solidification rate increases with decreasing depth from 0 at the bottom of the melt pool (cosθ=0 at this point) to a value that remains lower that the scanning speed, because cos θ<1. Conversely, the thermal gradient G is higher at the bottom of the melt pool and decreases as depth decreases. Both solidification parameters vary rapidly during the first stages of solidification (near the bottom of the melt pool) to reach a value that remains approximately constant during most of the solidification process. Consequently, the microstructure in most of the re-solidified layer can be characterized by a single set of

Sometimes, in laser surface alloying the microstructure of alloyed layer appears as a texture. The texture effect increases with increasing solidification speed. The origin of this texture can be understood by considering the solidification mechanism in laser surface alloying and

Gingo and et. al (Gingu et al., 1999) produced Al/SiCp composite by laser surface alloying. Figure 6 presents the microstructural aspect of the alloyed layer produced at the surface of an AA413 alloy. There is an obvious difference between the base microstructure of the Al alloy, which is the classic eutectic AlSi12, characterised by dendrites grains dispose randomly in the eutectic mass (zone 1), and the very fine granulated microstructure of the

In this process, depending on the processing parameters, it is possible to use or not use an adhesive layer at the material support. As can be seen in figure 7, in this case there is a perfect adherence of the alloyed layer at the AA413 support; this phenomenon can be

In laser surface alloying of Al with Nb as an alloying element, the dendritic structure was observed by Almeida et. al (Almeida et al., 2001), showing that Al3Nb grows with a dendritic solid/liquid interface. In this type of growth, there is a preferential growth direction usually a low index crystallographic direction. During the initial stages of solidification competition between neighboring dendrites with different orientations occurs, and those with the preferential growth direction nearest to the heat flow direction will be favored, leading to preferential orientation, and eventually to a strong texture. When the scanning speed of surface is increased the shape of melt pool increasingly elongated from semi-hemispherical. Also, when the scanning speed is low the heat flow direction changes progressively from the fusion line to the surface, leading to a variety of preferential growth

explained by the perfect compatibility of the matrix of Al- alloy (Gingu et al., 1999).

higher than the melting point of the intermetallic compounds.

rate (R) and the scanning speed gives by fallow equation:

solidification parameters and should not change significantly.

directions of columnar grains (Almeida et al., 2001).

alloyed layer (zone2).

the variation of the shape of the melt pool as a function of scanning speed.

temperature of the melt, the alloying particles dissolve in the liquid Al.

Fig. 6. The micro structural aspects of the laser alloyed layer (Gingu et al., 1999)

Fig. 7. The adherence aspect of the layer at the material support (Gingu et al., 1999)

A mathematical modeling of laser surface alloying with solid particles is established by Aleksandrov (Aleksandrov, 2002). According to this model, the presence of a transverse temperature gradient *<sup>T</sup> X* ∂ <sup>∂</sup> makes the particles move to the peripheral part of the molten pool. In the case of low gradients *<sup>T</sup> X* ∂ ∂ and *<sup>T</sup> Z* ∂ <sup>∂</sup> the particles simply drown in the field of the effective force of gravity (with allowance for the buoyancy force). The higher the difference in the densities of the alloying and Al alloys, the more effective the immersion of the particles. The equations of the motion of a single particle in Cartesian coordinates X, Z have the form (Aleksandrov, 2002):

Laser Surface Treatments of Aluminum Alloys 123

conglomerates of high-hardness particles. The use of lubricating materials improves the service properties of various friction pairs (Aleksandrov, 2002). Similar results are reported by Tomlinson and co- workers (Tomlinson & Bransden, 1996). They studied the effect of laser alloying of metallic elements such as Si, Ni, Fe, Cu, Mn, Cr, Co, Mo and Ti on hypoeutectic cast Al-Si alloys using a pre-placed coating method, and found an improvement in the wear resistance of aluminum. Senthile selvan and co-workers (Senthil Selvan et al., 2000), reported that when laser alloying of Al with Ni was carried out at the highest scan speed of 1.1 m min-1, the hardness increased to 800-900 Hv with negligible fluctuations in the hardness behavior. This may be attributed to a uniform LAC with well distributed intermetallic phases. While, at a slightly increased scan speed, the hardness increased from 300 to 800 Hv, but with large fluctuations, which can be attributed to the

Fig. 9. Dependence of the wear intensity of aluminium alloy on the specific load in a wear test laser treatment at P=1kw, ν=12.5 mm/sec, 1)Initial state, 2) after LHT 3,4,5) after alloying

The homogeneous distribution of hard intermetallic phases in Al matrix can prevent adhesion and abrasive wear during fretting. Yongqing Fu (Yongqing et al., 1998), reported that after a large number of fretting cycles, the rate of fretting wear depth decreases, which means that the wear volume loss is probably caused by an increase in fretting area rather by wear along the depth. This phenomenon is probably caused by the formation and compaction of fretting oxide debris, which can reduce the wear along the fretting depth. Laser surface alloying can decreases the fretting wear volume by a factor of three and decreases the coefficient of friction, probably due to the hardening effect of oxide debris which can prevent adhesion and abrasive wear during fretting, therefore, it can offers an

effective means of preventing fretting wear (figure 11). The (Yongqing et al., 1998).

with Ni, NbSi2, Cr, respectively (Aleksandrov, 2002)

homogeneous alloyed layer (figure 10).

$$\frac{4}{3}\pi p R^2 \stackrel{\cdots}{\mathbf{x}} = 2\alpha\_0 \frac{\partial T}{\partial \mathbf{x}} R - 6\pi \eta R \stackrel{\cdots}{\mathbf{x}}\tag{3}$$

$$\frac{4}{3}\pi p R^3 \stackrel{\cdots}{z} = 2\alpha\_0 \frac{\partial T}{\partial z} R + \frac{4}{3}\pi \left(P - P\_1\right) R^3 \mathcal{g} - 6\pi \eta R \stackrel{\cdots}{z} \tag{4}$$

where ρ is the density of the particle, R is the radius of the particle, ρ1 is the density of the Al alloy, η is the viscosity of the Al alloy, . .. *x x*, are the transverse velocity and acceleration of the particle respectively, and . .. *z z*, are the vertical velocity and acceleration of the particles, respectively.

The mechanism of the infusion and velocity of particles in melt pool affects on the formation of heterogeneous or homogenous structure in alloyed layer. The corresponding qualitative dependence ν0(R) is plotted in figure 8.

Fig. 8. Dependence of the initial velocity of the particles ν0 on their radius R at a fixed surface density (ρi) of the energy of laser radiation (P1<P2<P3<P4<P5) (Aleksandrov, 2002)

The order of magnitude of the initial velocity of the particles νo needed for the alloying is determined by the time τ of the action of laser radiation and the depth of the molten region <sup>H</sup> (νo= *<sup>H</sup>* τ ). Particles with a size ranging between R2 and R3 may acquire the requisite values νo.

Under actual conditions and fixed energy and time of the laser action, the dependences νo(R) corresponds to a certain domain (hatched in figure 8).

Aleksandrove (Aleksandrov, 2002) also studied the wear resistance of laser surface alloyed layer of Al alloy with Ni, NbSi2 and Cr. The results of wear tests are presented in figure 9.

It can be seen that wear resistance of the hardened surface of aluminum alloy layer is much higher (by factor of 4-5) that the initial one or the one provided by LHT. Also, the friction coefficient tests show that laser surface alloying of Al alloys with Cr, NbSi2 and Ni decreases the friction coefficient of the friction surface by about a factor of 3-4, which makes it possible to vary it by changing the filling factor of the surface and the filling of the alloyed zone with

 α

*T*

∂

*z*

3

πα

alloy, η is the viscosity of the Al alloy, . ..

dependence ν0(R) is plotted in figure 8.

particle respectively, and . ..

respectively.

<sup>H</sup> (νo= *<sup>H</sup>* τ

π

3 3

.. . <sup>2</sup> 0 <sup>4</sup> 2 6

( ) .. . 3 3 0 1 4 4 2 6

*pR z R P P R g Rz*

 π

where ρ is the density of the particle, R is the radius of the particle, ρ1 is the density of the Al

The mechanism of the infusion and velocity of particles in melt pool affects on the formation of heterogeneous or homogenous structure in alloyed layer. The corresponding qualitative

Fig. 8. Dependence of the initial velocity of the particles ν0 on their radius R at a fixed surface density (ρi) of the energy of laser radiation (P1<P2<P3<P4<P5) (Aleksandrov, 2002)

The order of magnitude of the initial velocity of the particles νo needed for the alloying is determined by the time τ of the action of laser radiation and the depth of the molten region

Under actual conditions and fixed energy and time of the laser action, the dependences

Aleksandrove (Aleksandrov, 2002) also studied the wear resistance of laser surface alloyed layer of Al alloy with Ni, NbSi2 and Cr. The results of wear tests are presented in figure 9. It can be seen that wear resistance of the hardened surface of aluminum alloy layer is much higher (by factor of 4-5) that the initial one or the one provided by LHT. Also, the friction coefficient tests show that laser surface alloying of Al alloys with Cr, NbSi2 and Ni decreases the friction coefficient of the friction surface by about a factor of 3-4, which makes it possible to vary it by changing the filling factor of the surface and the filling of the alloyed zone with

νo(R) corresponds to a certain domain (hatched in figure 8).

). Particles with a size ranging between R2 and R3 may acquire the requisite values νo.

= +− −

 πη

<sup>∂</sup> = − <sup>∂</sup> (3)

πη

∂ (4)

*x x*, are the transverse velocity and acceleration of the

*z z*, are the vertical velocity and acceleration of the particles,

*T pR x R Rx x*

conglomerates of high-hardness particles. The use of lubricating materials improves the service properties of various friction pairs (Aleksandrov, 2002). Similar results are reported by Tomlinson and co- workers (Tomlinson & Bransden, 1996). They studied the effect of laser alloying of metallic elements such as Si, Ni, Fe, Cu, Mn, Cr, Co, Mo and Ti on hypoeutectic cast Al-Si alloys using a pre-placed coating method, and found an improvement in the wear resistance of aluminum. Senthile selvan and co-workers (Senthil Selvan et al., 2000), reported that when laser alloying of Al with Ni was carried out at the highest scan speed of 1.1 m min-1, the hardness increased to 800-900 Hv with negligible fluctuations in the hardness behavior. This may be attributed to a uniform LAC with well distributed intermetallic phases. While, at a slightly increased scan speed, the hardness increased from 300 to 800 Hv, but with large fluctuations, which can be attributed to the homogeneous alloyed layer (figure 10).

Fig. 9. Dependence of the wear intensity of aluminium alloy on the specific load in a wear test laser treatment at P=1kw, ν=12.5 mm/sec, 1)Initial state, 2) after LHT 3,4,5) after alloying with Ni, NbSi2, Cr, respectively (Aleksandrov, 2002)

The homogeneous distribution of hard intermetallic phases in Al matrix can prevent adhesion and abrasive wear during fretting. Yongqing Fu (Yongqing et al., 1998), reported that after a large number of fretting cycles, the rate of fretting wear depth decreases, which means that the wear volume loss is probably caused by an increase in fretting area rather by wear along the depth. This phenomenon is probably caused by the formation and compaction of fretting oxide debris, which can reduce the wear along the fretting depth. Laser surface alloying can decreases the fretting wear volume by a factor of three and decreases the coefficient of friction, probably due to the hardening effect of oxide debris which can prevent adhesion and abrasive wear during fretting, therefore, it can offers an effective means of preventing fretting wear (figure 11). The (Yongqing et al., 1998).

Laser Surface Treatments of Aluminum Alloys 125

(c) Fig. 10. Microhardness profiles of laser- alloyed Cp- Al with Ni at powers of a) 1.1, b) 1.3

 (a) The maximum fretting wear depth (b) Fretting wear volume Fig. 11. The maximum fretting wear depth and retting volume for the Al6061 and lasertreated Al- alloy (normal load of 2N, amplitude of 50μm, frequency of 50Hz under

and c) 1.5 kw at different scan speeds (Senthil Selvan et al., 2001)

unlubricated conditions) (Yongqing et al., 1998)

(a)

(b)

Fig. 10. Microhardness profiles of laser- alloyed Cp- Al with Ni at powers of a) 1.1, b) 1.3 and c) 1.5 kw at different scan speeds (Senthil Selvan et al., 2001)

Fig. 11. The maximum fretting wear depth and retting volume for the Al6061 and lasertreated Al- alloy (normal load of 2N, amplitude of 50μm, frequency of 50Hz under unlubricated conditions) (Yongqing et al., 1998)

Laser Surface Treatments of Aluminum Alloys 127

Laser surface alloying of Al with Fe is studied by Tomida and Nakata (Tomida & Nakata, 2003). They reported that the hardness of laser alloyed layer increases with increasing Fe content as shown in (Figure 12). However, cracking occurred in the alloyed layer with higher hardness than Hv600, because the brittle 1ump-like Fe2Al5 compound was produced in these layers. The wear resistance of the alloyed layer improved with increasing the hardness due to the formation of the fine Fe rich intermetallic compounds. This result is

Laser surface melting (LSM) is a well established technology applied to many materials for

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

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

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

The diagram shown in figure 15 associated the microstructural evolution with the

hardening, reducing porosity and increasing wear and corrosion resistance.

shown in figure 13.

al., 2003).

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

important factor for laser surface melting process.

microstructure of melted zone can be changed.

solid/liquid front velocity (Aparecida Pinto et al., 2003).

Fig. 12. Relation between Fe content and hardness of laser alloyed layer on aluminum (Tomida & Nakata, 2003)

Fig. 13. Relation between surface hardness of laser alloyed layer and abrasive wear behavior. (Tomida & Nakata, 2003)

Fig. 12. Relation between Fe content and hardness of laser alloyed layer on aluminum

Fig. 13. Relation between surface hardness of laser alloyed layer and abrasive wear

(Tomida & Nakata, 2003)

behavior. (Tomida & Nakata, 2003)

Laser surface alloying of Al with Fe is studied by Tomida and Nakata (Tomida & Nakata, 2003). They reported that the hardness of laser alloyed layer increases with increasing Fe content as shown in (Figure 12). However, cracking occurred in the alloyed layer with higher hardness than Hv600, because the brittle 1ump-like Fe2Al5 compound was produced in these layers. The wear resistance of the alloyed layer improved with increasing the hardness due to the formation of the fine Fe rich intermetallic compounds. This result is shown in figure 13.
