**4. Laser surface alloying (LSA)**

Laser surface alloying is a material processing technique that utilizes the focused laser sources and produces the high power density to melt the metal coating and a portion of the underlying substrate. The schematic view of laser surface alloying is shown in **Figure 14**. The schematic diagram of shape and dimensions of laser surface alloyed zone is shown in **Figure 15**. Here, W = width, T = thickness, B = build-up and D = melted depth. Aluminum alloys are widely used in automobile and aerospace applications due to the availability and low cost, ductility, good strength-to-weight ratio and lightweight. These alloys have low hardness and poor tribological properties which leads to wear problem. Hence, the additional protection is required to enhance the wear resistance properties to localized areas. So, LSA can be used to improve the surface properties of aluminum alloys, titanium alloys, magnesium alloys, copper alloys and nickel-copper alloys. The laser alloyed component properties are depending upon the selection of alloy material, composition and elemental surface distribution. These factors are affecting the microstructural

**Figure 14.** *Schematic view of laser surface alloying.*

development in the alloyed surface. The ceramic alloys, carbide, oxide and boride (SiC, WC, TiO2, TiB2 and TiC) are widely used as the coating material on aluminum alloys due to the low density, high hardness, good wear, high melting temperature and corrosion resistance. The hybrid ceramics, a component coating on aluminum produces better wear resistance than the single ceramics component coating. Titanium is added to the carbon resulting in forming TiC to improve the surface properties and by the same to prevent the formation of Al4C3 carbides. A study on FeCoCrAlCuNix high entropy alloy coating on pure copper is carried out using LSA to evaluate the microhardness and wear. The laser power 1.7 kW, laser spot diameter 1.2 mm, scanning speed 2.0–3.0 mm/s, argon as shielding gas and flow rate 12 L/ min are used in this process.

**Figure 16a** shows the microstructure of HEA FeCoCrAlCuNix. The HEA coating have high density, little holes and adequate metallurgical bonds to substrate. It is noticed that the dilution ratio of the tested HEA coating is higher than 20%. Typical dendrite and interdendrite structures are clearly observed in Ni05 and Ni10 HEAs (**Figure 16b** and **c**), while only one phase was observed for Ni15 HEA (**Figure 16d**). Compared to hardness of copper, coated copper produces higher hardness and it is shown in **Figure 17** [21]. The effect of addition of Ni–Cr–Si–B alloy to brass substrate was studied through LSA. The 2 kW CW Nd-YAG laser with a spot diameter of 3 mm, the laser power density varied between 141 and 212 W/mm, while the scanning speed is kept constant at 5 mm/s. Argon with a flow rate of 15 l/min is used as the shielding gas to prevent the oxidation. Laser surfacing is achieved by overlapping of adjacent tracks, with an overlapping ratio of 50%.The hardness of the modified layers increased slightly from the surface to a maximum and sharply fell to the value of the substrate at the interface between the treated layer and the substrate. The increases in hardness observed for the modified layer is attributed to the formation of hard borides [22]. The effects of addition of SiC and TiO2 to aluminum alloy are studied by continuous mode CO2 laser. The CO2 laser with the parameters of 1.7 kW, scan speed of 400 mm/min, standoff distance of 40 mm and laser beam diameter of 7.4 mm are used for SiC alloying. The CO2 laser with the parameters of 1.8 kW, scan speed of 300 mm/min, standoff distance of 30 mm and laser beam diameter of 5.8 mm are used for TiO2 alloying. The result found that the ceramic nature of SiC

**75**

**Figure 17.**

**Figure 16.**

and TiO2 improved microhardness of alloyed zone from 30 HV0.3 substrate material

*Microstructure images of (a) FeCoCrAlCuNix HEA coatings on cross sectional view, (b) high magnifications* 

A study on the effect of addition of WC + Co + NiCr to AISI 304 stainless steel through Nd: YAG laser. The 5 kW Nd: YAG with beam diameter of 4 mm, power varying from 1 to 3 kW, scan speed from 0.005–0.1 m/s and argon gas of 5 L/min are

to 180 HV0.3 with SiC and 220 HV0.3 with TiO2 [23].

*Microhardness of FeCoCrAlCuNix HEA coatings.*

*image of Ni05 HEA (c), Ni10 HEA (d) and Ni15 HEA.*

*Laser Surface Modification of Materials DOI: http://dx.doi.org/10.5772/intechopen.94439* *Laser Surface Modification of Materials DOI: http://dx.doi.org/10.5772/intechopen.94439*

#### **Figure 16.**

*Practical Applications of Laser Ablation*

min are used in this process.

**Figure 15.**

development in the alloyed surface. The ceramic alloys, carbide, oxide and boride (SiC, WC, TiO2, TiB2 and TiC) are widely used as the coating material on aluminum alloys due to the low density, high hardness, good wear, high melting temperature and corrosion resistance. The hybrid ceramics, a component coating on aluminum produces better wear resistance than the single ceramics component coating. Titanium is added to the carbon resulting in forming TiC to improve the surface properties and by the same to prevent the formation of Al4C3 carbides. A study on FeCoCrAlCuNix high entropy alloy coating on pure copper is carried out using LSA to evaluate the microhardness and wear. The laser power 1.7 kW, laser spot diameter 1.2 mm, scanning speed 2.0–3.0 mm/s, argon as shielding gas and flow rate 12 L/

*Schematic diagram of shape and dimensions of laser surface alloyed zone.*

**Figure 16a** shows the microstructure of HEA FeCoCrAlCuNix. The HEA coating have high density, little holes and adequate metallurgical bonds to substrate. It is noticed that the dilution ratio of the tested HEA coating is higher than 20%. Typical dendrite and interdendrite structures are clearly observed in Ni05 and Ni10 HEAs (**Figure 16b** and **c**), while only one phase was observed for Ni15 HEA (**Figure 16d**). Compared to hardness of copper, coated copper produces higher hardness and it is shown in **Figure 17** [21]. The effect of addition of Ni–Cr–Si–B alloy to brass substrate was studied through LSA. The 2 kW CW Nd-YAG laser with a spot diameter of 3 mm, the laser power density varied between 141 and 212 W/mm, while the scanning speed is kept constant at 5 mm/s. Argon with a flow rate of 15 l/min is used as the shielding gas to prevent the oxidation. Laser surfacing is achieved by overlapping of adjacent tracks, with an overlapping ratio of 50%.The hardness of the modified layers increased slightly from the surface to a maximum and sharply fell to the value of the substrate at the interface between the treated layer and the substrate. The increases in hardness observed for the modified layer is attributed to the formation of hard borides [22]. The effects of addition of SiC and TiO2 to aluminum alloy are studied by continuous mode CO2 laser. The CO2 laser with the parameters of 1.7 kW, scan speed of 400 mm/min, standoff distance of 40 mm and laser beam diameter of 7.4 mm are used for SiC alloying. The CO2 laser with the parameters of 1.8 kW, scan speed of 300 mm/min, standoff distance of 30 mm and laser beam diameter of 5.8 mm are used for TiO2 alloying. The result found that the ceramic nature of SiC

**74**

*Microstructure images of (a) FeCoCrAlCuNix HEA coatings on cross sectional view, (b) high magnifications image of Ni05 HEA (c), Ni10 HEA (d) and Ni15 HEA.*

#### **Figure 17.**

*Microhardness of FeCoCrAlCuNix HEA coatings.*

and TiO2 improved microhardness of alloyed zone from 30 HV0.3 substrate material to 180 HV0.3 with SiC and 220 HV0.3 with TiO2 [23].

A study on the effect of addition of WC + Co + NiCr to AISI 304 stainless steel through Nd: YAG laser. The 5 kW Nd: YAG with beam diameter of 4 mm, power varying from 1 to 3 kW, scan speed from 0.005–0.1 m/s and argon gas of 5 L/min are used in the alloying process. The experimental result found that the LSA has been performed to form a defect free and uniform alloy zone. Compared to hardness of substrate, laser alloying produces the higher hardness due to the grain refinement [24].

The laser surface alloying is carried out on nodular cast iron by adding Ni-20%Cr alloy [20]. The laser parameters, power of 1.5 kW, scan speed of 600 mm/min, overlapping of 30% and defocus of 15 mm and argon gas are used to alloying the NCI surface. The microstructure of the laser alloyed specimen, worn out surface of substrate and laser alloyed specimen is shown in **Figure 18a**–**c** respectively. The ledeburite and pre-eutectic austenite are observed in the LSA surface. In addition, γ-phase (austenite) to M-phase (martensite) is transformation observed. The laser alloyed surface has produced the defect free and fine microstructure. The γ-phase has a higher percentage of Ni than cementite, whereas the Fe3C phase has Cr more and Ni less element. Hence, the presence of Fe3C on the laser-alloyed surface is rich in Cr and the γ-phase was supported through the solid solution of both alloy powders of Ni and Cr. The rapid solidification is the reason for obtaining the fine microstructure in the laser alloyed surface. The laser processed worn out surfaces have severe plastic deformation, wear track, delamination, grooves and adhesive particles. The NiCr alloying is also observed by using the LSA. The length of depth of hardness is increased by increasing the melted depth. The reasons are due to the refinement of grains through rapid re-solidification. The rate of cooling rate and thermal gradient also support the refinement of grains resulting in increased the hardness of the laser treated zone. Compared to hardness of substrate material, the laser processed depth has 2.62 time higher hardness due to the uniform grain structure. The wear loss is calculated for laser processed sample and untreated

**Figure 18.**

*Microstructure of LSA specimen (a), worn out surface of substrate (b), and worn out substrate of LSA (c).*

**77**

**Figure 19.**

*Schematic of laser cladding process.*

*Laser Surface Modification of Materials DOI: http://dx.doi.org/10.5772/intechopen.94439*

due to the improved hardness.

**5. Laser cladding (LC)**

sample. The laser processed samples are produced lesser wear rate than substrate

Laser cladding is similar to arc welding. The laser is used to melt the clad material coated on the substrate. The powder, wire and strip form of clad materials are commercially available to perform by different laser processes. The major benefits of LC have low porosity, good surface uniformity and low dilution. The clad materials have rapid quench and cooling down after deposition resulted in a fine grained microstructure. The laser is used to deposit clad material on substrate through the interaction of powder with laser. The substrate permits the melt pool to solidify and form the solid track. The schematic of laser cladding process is shown in **Figure 19**. Compared to other different surface processing used to enhance the wear and corrosion resistance of substrate, LC is an attractive alternative method. This is due to the intrinsic properties of laser radiation. The LC benefits are high input energy, low distortion, and minimum dilutions observed between the substrate, processing flexibility and cladding on small areas. The LC can be used in surface alloys and composites in order to achieve the required properties. The LC produces desired properties are obtained by varying the process parameters such as laser beam power density, laser beam

The laser solution strengthening, laser surface alloying and laser cladding have highly correlation to corrosion and erosion resistance. The laser solution strengthening and laser surface alloying are used to improve the erosion and corrosion resistance of old components without changing their sizes whereas laser cladding is used to repair wasted components by restoring their size. The high entropy alloy of CoCrFeNiNbx is coated to a pure titanium sheet by using laser cladding to study the hardness of the material. The laser cladding parameters such as power of 100 W, scanning speed of 8 mm/s, defocusing amount of +2 mm, pulse duration of 5 ms,

diameter at the workpiece surface and laser beam travel speed.

sample. The laser processed samples are produced lesser wear rate than substrate due to the improved hardness.
