**3. Laser surface treatment processes**

Extensive laser surface treatment techniques are available. Laser surface treatment usually modifies the topography, phase composition, and microstructure of a substrate material to improve its surface properties. When a laser is incident on a substrate material, laser radiations are absorbed by conduction electrons near the surface region (in nm range). These excited electrons collide with lattice ions and rapidly produce heat. The heat produced in this thin layer is conducted to the bulk substrate. This causes swift heating of a layer of material, having a thickness greater than the characteristic radiation absorption depth. As soon as the laser irradiation is stopped, the substrate material cools due to heat transfer. **Figure 3** presents a block diagram of laser interaction with substrate.

These thermal cycles may possibly cause phase transformations, topography, and microstructural variations. The extent of these changes depends on the behavior and type of material irradiated, the maximum temperature attained, and heating and cooling rates experienced. All the above said factors depend on the laser power density and interaction time between laser and substrate material. Laser surface treatment techniques are differentiated on the basis of temperature observed at the surface due to irradiation. If surface temperature attained due to laser irradiance is less than the melting temperature of a material, solid state transformations can be observed. Such a system is observed in hardening, shock peening, and engraving. When the surface temperature obtained is greater than the melting temperature but lower than the vaporization temperature of the substrate material, melting of substrate surface takes place. This is the most widely used regime for surface modification. Techniques such as laser cladding, laser alloying, laser glazing, and selective laser melting fall under this regime. If the surface temperature is greater than the vaporization temperature of the substrate, vaporization of substrate surface takes place. This regime is used in laser machining techniques such as laser drilling, cutting, and contouring. **Figure 4** presents a block diagram of this classification.

### **3.1 Laser cladding**

Laser cladding technique is employed to produce coatings with enhanced surface properties or to repair surface defects of different components. LC employs high energy density of laser beams to melt and alloy the surface of substrate materials. Due to high energy density, most of the metals can be melted and alloyed. Usually when the dilution percent is less than 10%, LC is meaningful because low concentration of substrate is desired in LC. Thick to moderate layers of almost any material can be developed. **Figure 8(a)** shows a cross-sectional image of clad bead.

The material to be deposited on a substrate may be supplied using two techniques: preplaced powder deposition [21–25] or codeposition method [26–30].

**243**

**Figure 5.**

*Laser Surface Treatment*

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.91800*

the shielding gas and particle nature of laser.

*(a) Preplaced technique and (b) codeposition technique [33].*

*Classification of laser material processing techniques.*

These methods differ in the supply of clad material. In the first method as shown in **Figure 5(a)**, the powder to be clad is first mixed with certain adhesives (polyvinyl alcohol) to form slurry. This slurry is placed above the substrate as a uniform coat and allowed to dry and harden. This is done so that it can withstand the pressure of

In the second method as shown in **Figure 5(b)**, the powder to be clad is fed through the powder feeder nozzle onto the laser beam and subsequently on the molten pool. This powder feeding can be done at various angles through the laser beam. When the angle of feeding is zero degrees, it forms a coaxial feeding system. Some authors have also studied LMP using different types of nozzles [31, 32]. Generally, off axis, four stream and coaxial nozzles have been employed. **Figure6** shows the gas and particle flow patterns for various nozzles. Thus, cladding of substrate is possible using both the techniques.

*Engineering Steels and High Entropy-Alloys*

**3. Laser surface treatment processes**

diagram of laser interaction with substrate.

The type of process also affects the selection of laser. Generally surface alloying requires a large amount of heat to melt considerable amount of substrate surface. Thus high-energy lasers are required. Glazing and sintering usually employ low-

Extensive laser surface treatment techniques are available. Laser surface treatment usually modifies the topography, phase composition, and microstructure of a substrate material to improve its surface properties. When a laser is incident on a substrate material, laser radiations are absorbed by conduction electrons near the surface region (in nm range). These excited electrons collide with lattice ions and rapidly produce heat. The heat produced in this thin layer is conducted to the bulk substrate. This causes swift heating of a layer of material, having a thickness greater than the characteristic radiation absorption depth. As soon as the laser irradiation is stopped, the substrate material cools due to heat transfer. **Figure 3** presents a block

These thermal cycles may possibly cause phase transformations, topography,

Laser cladding technique is employed to produce coatings with enhanced surface properties or to repair surface defects of different components. LC employs high energy density of laser beams to melt and alloy the surface of substrate materials. Due to high energy density, most of the metals can be melted and alloyed. Usually when the dilution percent is less than 10%, LC is meaningful because low concentration of substrate is desired in LC. Thick to moderate layers of almost any material

can be developed. **Figure 8(a)** shows a cross-sectional image of clad bead.

The material to be deposited on a substrate may be supplied using two techniques: preplaced powder deposition [21–25] or codeposition method [26–30].

and microstructural variations. The extent of these changes depends on the behavior and type of material irradiated, the maximum temperature attained, and heating and cooling rates experienced. All the above said factors depend on the laser power density and interaction time between laser and substrate material. Laser surface treatment techniques are differentiated on the basis of temperature observed at the surface due to irradiation. If surface temperature attained due to laser irradiance is less than the melting temperature of a material, solid state transformations can be observed. Such a system is observed in hardening, shock peening, and engraving. When the surface temperature obtained is greater than the melting temperature but lower than the vaporization temperature of the substrate material, melting of substrate surface takes place. This is the most widely used regime for surface modification. Techniques such as laser cladding, laser alloying, laser glazing, and selective laser melting fall under this regime. If the surface temperature is greater than the vaporization temperature of the substrate, vaporization of substrate surface takes place. This regime is used in laser machining techniques such as laser drilling, cutting, and contouring. **Figure 4** presents a block diagram of this classification.

energy lasers, whereas cladding uses intermediate-energy lasers.

**242**

**Figure 3.**

*Laser material interactions.*

**3.1 Laser cladding**

These methods differ in the supply of clad material. In the first method as shown in **Figure 5(a)**, the powder to be clad is first mixed with certain adhesives (polyvinyl alcohol) to form slurry. This slurry is placed above the substrate as a uniform coat and allowed to dry and harden. This is done so that it can withstand the pressure of the shielding gas and particle nature of laser.

In the second method as shown in **Figure 5(b)**, the powder to be clad is fed through the powder feeder nozzle onto the laser beam and subsequently on the molten pool. This powder feeding can be done at various angles through the laser beam. When the angle of feeding is zero degrees, it forms a coaxial feeding system. Some authors have also studied LMP using different types of nozzles [31, 32]. Generally, off axis, four stream and coaxial nozzles have been employed. **Figure6** shows the gas and particle flow patterns for various nozzles. Thus, cladding of substrate is possible using both the techniques.

**Figure 5.** *(a) Preplaced technique and (b) codeposition technique [33].*

#### **Figure 6.**

*Gas and particle flow for (a) off focus, (b) four stream, and (c) coaxial nozzles [32].*

Through extensive review of articles, it was observed that beads produced using powder preplaced method are prone to more defects. This may be attributed to the presence of a bonding agent which vaporizes during laser beam interaction. Also, the height of clad can be manipulated in a codeposition method which is difficult in preplaced technique [34].

The type of microstructure formed depends on the temperature gradient (G) and solidification rate of crystal (R) [35–37]. High G/R ratio leads to planar structure, with decrease in G and increase in R columnar structure being achieved, and low G/R ratio leads to equiaxed dendritic structure. In LST high cooling rates (103 –108 K/s) can be achieved [38]. Hence in LST generally dendritic structure is visible. Change in structure can also be realized with change in mode of laser. It is observed that in continuous laser mode, columnar dendritic structure was formed which was oriented towards the center of clad bead, whereas in pulsed laser mode, stacks of dendrite were randomly oriented. This was due to cyclic melting and resolidification phases, leading to progressive change in the molten pool. **Figure 7** shows the LC using continuous (a) and pulsed (b) mode.

## **3.2 Laser surface alloying**

Laser surface alloying (LSA) is a similar process to LC but using high energy density. LSA sample is shown in **Figure 8(b)** [40]. It is observed that dilution percent is greater than 10%. Hence, no clear distinction up to a certain depth can be observed, and alloy bead has some proportions of substrate material. Usually, LC is employed in applications requiring entirely different properties at the surface and core, whereas LSA is employed in applications requiring change in properties for greater depth.

**245**

**Figure 9.**

*Steps in SLM technique [41].*

*Laser Surface Treatment*

**3.3 Selective laser melting**

**Figure 8.**

**3.4 Laser glazing**

Selective laser melting (SLM) utilizes laser energy to create 3D parts using a 3D CAD sketch of the part geometry to be produced. The 3D model is then broken to a 2D stack of layers which form the required geometry. These 2D layers are created by laser scanning over the cross-sectional area. This scanning of laser melts and bonds particles together to form a thin layer. Repeating this process, a subsequent layer may be produced and altogether bonded to previously produced layers. These formed stacks of 2D layers represent the final 3D required geometry. Selective laser sintering (SLS) is a similar process to SLM, but in SLS complete melting of powder does not take place. SLS uses low-power lasers for fabrication of 3D parts compared to SLM. Thus, the final products formed using SLS usually have high porosity and require impregnation of different materials. **Figure 9** presents the steps in SLM. Literatures suggests that SLM is successfully applied to aluminum and its alloys,

high-speed steels, nickel, and copper alloys. The main problems with SLM are porosity, cracking, oxide inclusion, and loss of alloying elements. Porosity may be reduced by proper selection of laser energy density for specific material. Cracking

Laser glazing (LG) is a surface melting method using a continuous high-energy

laser beam which traverses the surface of a substrate, generating a thin layer of melted material. After the solidification of this thin melted layer, the material's surface appears glassy; therefore this method is termed as laser glazing. Researchers have done LG to improve surface properties [42–44]. It employs low peak power;

can be reduced by decreasing the cooling and solidification rate.

*Cross-sectional images of (a) laser clad [30] and (b) laser surface alloyed bead [40].*

*DOI: http://dx.doi.org/10.5772/intechopen.91800*

**Figure 7.** *LC using (a) continuous mode and (b) pulsed mode [39].*

*Engineering Steels and High Entropy-Alloys*

preplaced technique [34].

**Figure 6.**

high cooling rates (103

pulsed (b) mode.

greater depth.

**3.2 Laser surface alloying**

–108

Through extensive review of articles, it was observed that beads produced using powder preplaced method are prone to more defects. This may be attributed to the presence of a bonding agent which vaporizes during laser beam interaction. Also, the height of clad can be manipulated in a codeposition method which is difficult in

The type of microstructure formed depends on the temperature gradient (G)

K/s) can be achieved [38]. Hence in LST generally

and solidification rate of crystal (R) [35–37]. High G/R ratio leads to planar structure, with decrease in G and increase in R columnar structure being achieved, and low G/R ratio leads to equiaxed dendritic structure. In LST

*Gas and particle flow for (a) off focus, (b) four stream, and (c) coaxial nozzles [32].*

dendritic structure is visible. Change in structure can also be realized with change in mode of laser. It is observed that in continuous laser mode, columnar dendritic structure was formed which was oriented towards the center of clad bead, whereas in pulsed laser mode, stacks of dendrite were randomly oriented. This was due to cyclic melting and resolidification phases, leading to progressive change in the molten pool. **Figure 7** shows the LC using continuous (a) and

Laser surface alloying (LSA) is a similar process to LC but using high energy density. LSA sample is shown in **Figure 8(b)** [40]. It is observed that dilution percent is greater than 10%. Hence, no clear distinction up to a certain depth can be observed, and alloy bead has some proportions of substrate material. Usually, LC is employed in applications requiring entirely different properties at the surface and core, whereas LSA is employed in applications requiring change in properties for

**244**

**Figure 7.**

*LC using (a) continuous mode and (b) pulsed mode [39].*

**Figure 8.** *Cross-sectional images of (a) laser clad [30] and (b) laser surface alloyed bead [40].*
