**2. Laser surface hardening (LSH)**

The laser surface hardening is defined as the heat energy from the laser beam which is directly impacted to the finished component surface for improving the wear resistance. The component life is increases without affecting the bulk material. During the hardening process, the surface layer is heated up to hardening temperature under the short period of time. The quenching is a necessary process to achieve the hard martensite phase in the heated surface. Thereby, the component surfaces are hardened by laser and achieve the high wear resistant surface with desired bulk properties. The components such as gear teeth, gears, shafts, camshafts, axles, cylinder liners, valve guides and exhaust valves showed with higher

**63**

**Figure 1.**

*Schematic of I-section beam used in rail.*

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

stresses due to laser surface hardening. The type of work materials, cast iron, die steel and medium-carbon steel are also required the laser surface hardening for better performance. The mass-production industries, automobile components and electronic parts are performed the laser hardening on the component surfaces [1]. The desired component performances are mainly depending upon the selection of laser process parameters such as power, scanning speed, pressure, beam shape and material properties. Now-a-days, in order to improve the surface quality of components, the number of surface treatment are commercially available to obtain the unique material properties. For example, the I-section rail (railway) is fabricated by hot rolled processes which have non-uniform properties in the flange and web. The I-section beam is shown in **Figure 1**. The flanges have been designed to withstand high stress whereas the web designed to withstand the least stress. The flange thickness is greater than the web thickness and stress developed in the I-section is within the allowable limit. The point is the different cross section of flange and web has produced the non-uniform properties. Hence, the laser surface hardening is

required for achieving the uniform properties over the flange and web.

and 625 HV0.1 respectively. The laser quenched sample has 0.4 mm3

which is lesser than the conventional quenched sample of 0.6 mm3

at 500 m sliding distance [3]. The wear and microhardness studies are performed on 40CrNiMoA steel by using laser quenching and high-frequency quenching.

/N-m wear rate

/N-m wear rate

The laser surface transformation hardening process is performed to obtain the required depth and width for steel material. The accurate parts are made of medium carbon steels which require the laser surface hardening. The small and complex components are easily surface hardened by laser. This is because of the high rate of cooling effects to increase the hardness rate in the quenching process [2]. Therefore, LSH is a better process compared to flame and induction hardening processes. The quenching process is suddenly reducing the work material temperature by using water, oil or air to get certain material properties through the phase transformation. Therefore, a comparative study is made between the laser quenching and conventional quenching on steel to study the hardness and wear rate. The conventional quenching and tempering is carried out by using the temperature of 1198 K for 4.5 h and temperature of 523 K for 4 h respectively. The air, 10 kW CW diode laser, 3.5 mm spot diameter and 168 mm/s linear speed are used in the laser treatment. The laser quenched and conventional quenched sample for 25 μm distance from the surface, the produced hardness is 600 HV0.1

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

*Practical Applications of Laser Ablation*

is possible through LSM by avoiding the carbides formation during subsequent homogenization and sensitizing treatment. The laser surface alloying (LSA) is defined as the high heat energy used to melt the metal coating through laser and a portion of underlying substrate. This technique is used to form highly resistant gradient layers on the metal surface. The major benefit of this technique is sudden heating followed by cooling and the surface properties are improved. The laser cladding (LC) is a coating method that the surface melting and new material layer formation by addition of material are simultaneously processed in the substrate at the same time by using the laser power. The desired surface properties are achieved after solidification. The large component surface properties are easily increased by using LC. The complete metallurgical bond is necessary between the melting of substrate and forming of a new material layer at the interface. The laser surface texturing (LST) is defined as the process in which the change of material surface properties by modifying its texture and roughness. The laser beam is used to create the micro patterns on the surface by laser ablation. The micro patterns are created on the surface in various shapes such as dimples, grooves and free forms with

precise dimension. This process is mostly used in biomedical applications.

The different types of laser have different abilities to perform the process on materials. All the lasers are producing the heat energy and the laser beam wavelength is majorly affecting the performance of materials. Generally, the total laser heat energy is supplied to work material in which can be divided into two ways such as the fraction of heat energy is observed by work material and remaining heat energy is reflected to the environment. This happens during the surface hardening by laser. The supply of heat energy to polished metal surface components is depending upon the heat absorbability of work material and wavelength of irradiation. Generally, the short wavelength has higher absorptivity. Hence, the Nd: YAG laser (λ = 1.064 μm) has produced the higher absorbing ability beam to work material than the CO2 laser (λ = 10.6 μm) for surface hardening of steel. In order to increase the CO2 laser absorbility (high wavelength) to work material, the coating or painting is required in the work material prior to the CO2 laser surface hardening. Therefore, the Nd: YAG laser surface hardening better than CO2 laser surface hardening because the Nd: YAG laser has short wavelength and produces a high absorbing rate to work material. The Nd: YAG laser produces heat energy to work material which is transferred through fiber cable whereas CO2 laser is impossible. The inert gases, helium, neon and argon are used to eliminate the atmospheric contamination. In order to reduce the wavelength of a laser, an excimer laser is developed with very short wavelength. This laser can be used to micromachining on medical parts. In this chapter, laser surface hardening, laser surface melting, laser surface alloying, laser surface cladding and laser surface texturing have been discussed to improve the microstructure, hardness and wear resistance of mechanical components.

The laser surface hardening is defined as the heat energy from the laser beam which is directly impacted to the finished component surface for improving the wear resistance. The component life is increases without affecting the bulk material. During the hardening process, the surface layer is heated up to hardening temperature under the short period of time. The quenching is a necessary process to achieve the hard martensite phase in the heated surface. Thereby, the component surfaces are hardened by laser and achieve the high wear resistant surface with desired bulk properties. The components such as gear teeth, gears, shafts, camshafts, axles, cylinder liners, valve guides and exhaust valves showed with higher

**62**

**2. Laser surface hardening (LSH)**

stresses due to laser surface hardening. The type of work materials, cast iron, die steel and medium-carbon steel are also required the laser surface hardening for better performance. The mass-production industries, automobile components and electronic parts are performed the laser hardening on the component surfaces [1]. The desired component performances are mainly depending upon the selection of laser process parameters such as power, scanning speed, pressure, beam shape and material properties. Now-a-days, in order to improve the surface quality of components, the number of surface treatment are commercially available to obtain the unique material properties. For example, the I-section rail (railway) is fabricated by hot rolled processes which have non-uniform properties in the flange and web. The I-section beam is shown in **Figure 1**. The flanges have been designed to withstand high stress whereas the web designed to withstand the least stress. The flange thickness is greater than the web thickness and stress developed in the I-section is within the allowable limit. The point is the different cross section of flange and web has produced the non-uniform properties. Hence, the laser surface hardening is required for achieving the uniform properties over the flange and web.

The laser surface transformation hardening process is performed to obtain the required depth and width for steel material. The accurate parts are made of medium carbon steels which require the laser surface hardening. The small and complex components are easily surface hardened by laser. This is because of the high rate of cooling effects to increase the hardness rate in the quenching process [2]. Therefore, LSH is a better process compared to flame and induction hardening processes. The quenching process is suddenly reducing the work material temperature by using water, oil or air to get certain material properties through the phase transformation. Therefore, a comparative study is made between the laser quenching and conventional quenching on steel to study the hardness and wear rate. The conventional quenching and tempering is carried out by using the temperature of 1198 K for 4.5 h and temperature of 523 K for 4 h respectively. The air, 10 kW CW diode laser, 3.5 mm spot diameter and 168 mm/s linear speed are used in the laser treatment. The laser quenched and conventional quenched sample for 25 μm distance from the surface, the produced hardness is 600 HV0.1 and 625 HV0.1 respectively. The laser quenched sample has 0.4 mm3 /N-m wear rate which is lesser than the conventional quenched sample of 0.6 mm3 /N-m wear rate at 500 m sliding distance [3]. The wear and microhardness studies are performed on 40CrNiMoA steel by using laser quenching and high-frequency quenching.

**Figure 1.**

*Schematic of I-section beam used in rail.*

A 2 kW CW CO2 laser, 1400 W laser power, 35 mm/s traverse speed, 60 degree incident angle, black organic absorbent coating, 0.9 m3 /h gas flow rate and 10 mm defocusing distance are used in the laser treatment. The hardness of the quenched groove surface reached 750 HV and is substantially higher than that resulting from the high-frequency quenching method. The results of wear testing showed that the wear resistance of laser quenched specimens is 1.3 times higher than that of a highfrequency quenching specimen [4]. Comparisons were made between the gray cast iron (GCI), laser hardened quench-tempered GCI and conventional austempered GCI specimens based on the hardness and wear loss. The air, CW Nd: YAG laser, 2 mm laser spot, 22 mm defocused distance, 2 mm/s scanning speed, 6 Hz frequency, 120 A current and 8 ms pulse duration is used for laser hardening. The hardness of the laser hardened zone with ledeburitic structure is approximately 68 HRC. The quenching-tempered GCI specimen showed higher wear resistance than untreated GCI specimen [5].

The advantage of laser surface hardening is listed below


The disadvantage of laser surface hardening is listed below


The performance of the components such as hardness and wear resistance of work materials are mainly focused in the laser surface hardening. This is depending upon the material type, material properties, and types of processing on materials. The desired properties of work materials are obtained through proper selection of laser surface treatment and optimization. In order to improve the durability of mechanical components namely gears, engine valve, brake drums and camshaft are highly needed the LSH. The induction hardening is one of the surface hardening process which is shown in **Figure 2**. It is performed to achieve the uniform

**65**

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

compared to laser surface hardening.

*Schematic of induction hardening.*

skin effect.

**Figure 2.**

microstructure and good wear resistance which is higher implementation cost

*d*

In this induction hardening, the depth of hardening is mainly depending upon the resistivity (ρ), frequency (v) and magnetic permeability (μ). The work material is placed inside the coil and supplies the high frequency. The surface is hardened by

> *v* ρ

<sup>=</sup> (1)

µ

The laser surface hardening can be performed on the components either partially or fully depending upon the application of the components. Specifically, the load bearing component is subjected to high surface wear. Hence, the laser surface hardening is required on the load bearing component surface. Therefore, the load bearing component is hardened by laser, the surface has produced a high hardenability and fine microstructure [6]. The service life of crankshaft and camshaft are made on EN18 steel in which properties are improved by diode laser surface hardening with beam diameter of 3 mm, velocity of 1 m/min and power of 1.5 kW. The argon gas is used as shield gas [7]. The advantages of induction hardening are localized areas heat treated, minimal surface decarburization, surface oxidation, slight deformation, improved fatigue strength and low operating cost. The disadvantages of induction hardening are high capital investment. The advantages of laser hardening are described as non-hardenable steels are surface hardened, higher hardness obtained than conventional hardening, eliminating dimensional distortion, no protective atmosphere required and very long and irregular shapes easily hardened. The disadvantages of laser hardening are high initial and working cost and difficult to harden the high alloy steel. The schematic diagram of substrate and laser processed materials are shown in **Figure 3(a)** and **(b)**. The parent substrate has coarse and uneven equiaxed grains. The laser processed work material showed the hardened depth varying from top surface to 200 μm depth. The depth of hardening increases with grain size increases from finer to coarser. The curved surface is formed at top surface due to the low scanning speed produces more evaporation in the laser melted surface. The laser process parameters, power of 1.5 kW, beam diameter of 3 mm, scan speed of 1 m/min and interaction time of 0.18 s are used to obtain the desired hardness. The Nd: YAG laser and argon gas with flow rate of 20 L/min is used in the laser surface hardening. The hardness decreases from 955 HV to

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

*Practical Applications of Laser Ablation*

untreated GCI specimen [5].

layer.

complex parts.

• High initial capital cost

• Skilled operators are needed

• Radiation protection is required

• Material hardness and wear

tional surface heat treatment.

A 2 kW CW CO2 laser, 1400 W laser power, 35 mm/s traverse speed, 60 degree

defocusing distance are used in the laser treatment. The hardness of the quenched groove surface reached 750 HV and is substantially higher than that resulting from the high-frequency quenching method. The results of wear testing showed that the wear resistance of laser quenched specimens is 1.3 times higher than that of a highfrequency quenching specimen [4]. Comparisons were made between the gray cast iron (GCI), laser hardened quench-tempered GCI and conventional austempered GCI specimens based on the hardness and wear loss. The air, CW Nd: YAG laser, 2 mm laser spot, 22 mm defocused distance, 2 mm/s scanning speed, 6 Hz frequency, 120 A current and 8 ms pulse duration is used for laser hardening. The hardness of the laser hardened zone with ledeburitic structure is approximately 68 HRC. The quenching-tempered GCI specimen showed higher wear resistance than

• The lower level of heat energy is used to work material compared to conven-

• The input laser energy is controlled by varying the process parameters such as power, scanning speed, defocus, different shapes of lenses and mirrors.

• The hardened surface is obtained through self-quenching of the heated surface

• The work material is made under the hardening and quenching process result-

• The beam guidance is automatically controlled over the work material.

• The surface heat treatment is specifically performed on small parts and

The performance of the components such as hardness and wear resistance of work materials are mainly focused in the laser surface hardening. This is depending upon the material type, material properties, and types of processing on materials. The desired properties of work materials are obtained through proper selection of laser surface treatment and optimization. In order to improve the durability of mechanical components namely gears, engine valve, brake drums and camshaft are highly needed the LSH. The induction hardening is one of the surface hardening process which is shown in **Figure 2**. It is performed to achieve the uniform

/h gas flow rate and 10 mm

incident angle, black organic absorbent coating, 0.9 m3

The advantage of laser surface hardening is listed below

ing in cleaning of work material is not required.

The disadvantage of laser surface hardening is listed below

• Surface preparations are required in difficult areas.

**64**

**Figure 2.** *Schematic of induction hardening.*

microstructure and good wear resistance which is higher implementation cost compared to laser surface hardening.

In this induction hardening, the depth of hardening is mainly depending upon the resistivity (ρ), frequency (v) and magnetic permeability (μ). The work material is placed inside the coil and supplies the high frequency. The surface is hardened by skin effect.

$$d = \sqrt{\frac{\rho}{\mu \upsilon}}\tag{1}$$

The laser surface hardening can be performed on the components either partially or fully depending upon the application of the components. Specifically, the load bearing component is subjected to high surface wear. Hence, the laser surface hardening is required on the load bearing component surface. Therefore, the load bearing component is hardened by laser, the surface has produced a high hardenability and fine microstructure [6]. The service life of crankshaft and camshaft are made on EN18 steel in which properties are improved by diode laser surface hardening with beam diameter of 3 mm, velocity of 1 m/min and power of 1.5 kW. The argon gas is used as shield gas [7]. The advantages of induction hardening are localized areas heat treated, minimal surface decarburization, surface oxidation, slight deformation, improved fatigue strength and low operating cost. The disadvantages of induction hardening are high capital investment. The advantages of laser hardening are described as non-hardenable steels are surface hardened, higher hardness obtained than conventional hardening, eliminating dimensional distortion, no protective atmosphere required and very long and irregular shapes easily hardened. The disadvantages of laser hardening are high initial and working cost and difficult to harden the high alloy steel. The schematic diagram of substrate and laser processed materials are shown in **Figure 3(a)** and **(b)**. The parent substrate has coarse and uneven equiaxed grains. The laser processed work material showed the hardened depth varying from top surface to 200 μm depth. The depth of hardening increases with grain size increases from finer to coarser. The curved surface is formed at top surface due to the low scanning speed produces more evaporation in the laser melted surface. The laser process parameters, power of 1.5 kW, beam diameter of 3 mm, scan speed of 1 m/min and interaction time of 0.18 s are used to obtain the desired hardness. The Nd: YAG laser and argon gas with flow rate of 20 L/min is used in the laser surface hardening. The hardness decreases from 955 HV to

#### **Figure 3.**

*(a) Schematic of; (a) as received tool steel microstructure, (b) laser surface hardened tool steel with modified structure.*

236 HV which is obtained by varying distance from top to 200-micron depth and it is shown in **Figure 4**. This is due to the grain size refinement [8]. The 5 kW CW CO2 laser, power ranging from 1.1–2.5 kW, traverse speed ranging from 6 to 15 mm/s and spot size of 6.3 mm, 2.27 mm, 4.63 mm and 1.2 mm are used to harden the various carbon steel. The argon gas is used as shielding gas. The traverse speed has mostly affecting the hardness. The carbon percentage increases, the average hardness value also increases. The C-45 steel has produced higher hardness. The hardness of the material was improved by minimizing the diameter of spot size [9]. Further, conventional type laser surface treatment is performed on large surface areas and irregular hardness was observed on the machined component. In order to overcome irregular hardness, a laser overlapping method is used in the laser transformation hardening which is presented in **Figure 5**.

After the laser treatment, the laser hardened zones are divided into three sections such as hardened zone, transition zone and heat affected zone which is shown in **Figure 6**. A study on the effect of process parameters on surface hardness splined shafts is performed by using laser surface hardening. The fiber laser, power varying from 1900 to 2500 W, scanning speed varying from 2 to 6 mm/s, rotation speed varying from 1500 to 2500 rpm, the flank tilt angle of spline tooth varying from 15 to 20 and tooth depth of spline shaft varying from 2.5–3.5 are used in the laser

**67**

**Figure 5.**

*Schematic of laser transformation hardening.*

**Figure 4.**

hardening of spline shaft. The result found that the maximum hardness is observed by using the power of 2500 W, scanning speed of 2 mm/s, rotational speed of 2500 rpm, the flank tilt angle of spline tooth of 20° and tooth depth of spline shaft of 3.5 mm [10]. An investigation on the underwater hardening of AISI 1055 steel is carried out using lasers. A 250 W CW Ytterbium based fiber laser, focal length of 300 mm, defocus distance of 10 mm and traverse speed varying from 1 to 100 mm/s are used in the laser surface hardening. The result found that the higher surface roughness is obtained in the underwater welding compared to conventional laser

hardening due to the additional cooling effect in the underwater [11].

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

*Microhardness variation from top surface to substrate through LSH.*

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

#### **Figure 4.**

*Practical Applications of Laser Ablation*

236 HV which is obtained by varying distance from top to 200-micron depth and it is shown in **Figure 4**. This is due to the grain size refinement [8]. The 5 kW CW CO2 laser, power ranging from 1.1–2.5 kW, traverse speed ranging from 6 to 15 mm/s and spot size of 6.3 mm, 2.27 mm, 4.63 mm and 1.2 mm are used to harden the various carbon steel. The argon gas is used as shielding gas. The traverse speed has mostly affecting the hardness. The carbon percentage increases, the average hardness value also increases. The C-45 steel has produced higher hardness. The hardness of the material was improved by minimizing the diameter of spot size [9]. Further, conventional type laser surface treatment is performed on large surface areas and irregular hardness was observed on the machined component. In order to overcome irregular hardness, a laser overlapping method is used in the laser transformation

*(a) Schematic of; (a) as received tool steel microstructure, (b) laser surface hardened tool steel with modified* 

After the laser treatment, the laser hardened zones are divided into three sections such as hardened zone, transition zone and heat affected zone which is shown in **Figure 6**. A study on the effect of process parameters on surface hardness splined shafts is performed by using laser surface hardening. The fiber laser, power varying from 1900 to 2500 W, scanning speed varying from 2 to 6 mm/s, rotation speed varying from 1500 to 2500 rpm, the flank tilt angle of spline tooth varying from 15 to 20 and tooth depth of spline shaft varying from 2.5–3.5 are used in the laser

**66**

**Figure 3.**

*structure.*

hardening which is presented in **Figure 5**.

*Microhardness variation from top surface to substrate through LSH.*

**Figure 5.** *Schematic of laser transformation hardening.*

hardening of spline shaft. The result found that the maximum hardness is observed by using the power of 2500 W, scanning speed of 2 mm/s, rotational speed of 2500 rpm, the flank tilt angle of spline tooth of 20° and tooth depth of spline shaft of 3.5 mm [10]. An investigation on the underwater hardening of AISI 1055 steel is carried out using lasers. A 250 W CW Ytterbium based fiber laser, focal length of 300 mm, defocus distance of 10 mm and traverse speed varying from 1 to 100 mm/s are used in the laser surface hardening. The result found that the higher surface roughness is obtained in the underwater welding compared to conventional laser hardening due to the additional cooling effect in the underwater [11].

**Figure 6.** *Schematic of different zones of laser transformation hardening.*
