**3. Laser surface melting (LSM)**

Laser surface melting is one of the surface alteration processes that the surface of the substrate is melted and rapidly solidified to form the fine microstructure and improving the mechanical properties without changing the bulk properties and without addition of any metallic elements. The piston, valve and sliding parts are made of magnesium alloys, which are used in the automobile components and energy saving material. The application and limitation of magnesium alloy is decided by properties. In order to improve the tribological and mechanical properties, the laser surface melting process is focused on magnesium alloy. In the conventional heat treatment of HSS materials are presented the retained austenite, which transforms into brittle martensite during service. But, the life of high-speed tool steel is increased by using LSM. The schematic view of LSM is shown in **Figure 7**. The LSM treatment are carried out using a 2 kW fiber laser with 1.06 μm wavelength, laser power of 1500 W, the laser scanning speed of 600 mm/min and the distance between the laser head, spot size of 3 mm, shielding gas pressure of 0.3 MPa and the specimen

**69**

**Figure 8.**

*Microhardness variation of magnesium alloy.*

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

surface of 12 cm are used in the LSM. The microhardness and corrosion resistance of magnesium alloy is also improved by using LSM with electromagnetic stirring [12]. In order to enhance the microhardness of melted substrate, the LSM process parameters effect of hardness of magnesium alloy is studied. The CW CO2 laser, beam diameter of 4 mm, argon gas of 6 l/min, speed varying from 100 to 400 mm/min and power varying from 1.5–3.0 kW are used in the process. The result showed that the melt depth of magnesium alloy is directly proportional to the laser power and inversely proportional to the scan speed. Laser surface melting enhances the microhardness of the melted zone by 2–3 times than the substrate [13]. The laser processed hardness of high speed tool steel and magnesium alloy is decreased from as-received substrate by increasing distance from the melting surface which is shown in **Figure 8**. This is due to the refined, solid solution strengthening and uniform microstructure. The LSM is also performed in electric contact material of Cu-50Cr.

ning speed of 6000–10,000 mm/min and argon gas are used in this process. From the analysis found that the microhardness and withstanding voltage of Cu-50Cr are significantly improved by using LSM [14]. The effects of LSM process parameters are affecting the microstructure and hardness of AZ31B magnesium alloy substrate. The result found that the grain size in the fused layer increases by increasing power. The schematic diagram of as-received magnesium alloy is shown in **Figure 9a**. The effects of different power on microstructure of layer fused layers are shown in **Figure 10b**–**e**. The Nd: YAG laser power varying from 1600 to 2200 W, laser beam scanning velocity of 900 mm/min, laser beam spot diameter of 4 mm, number of superimposed tracks of 9, overlap ratio of 15%, and argon flow rate of 25 mL/min are used in the process. The depth of the metal pool and grain size is increased by increasing the power. This is due to the grain growing freely in the higher metal pool depth compared to smaller metal pool depth. The reason for increasing the hardness and wear resistance are

to 107

W/cm2

, scan-

The 1 kW CW Nd: YAG laser, power density varying from 106

**Figure 7.** *Schematic view of laser surface melting.*

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

*Practical Applications of Laser Ablation*

**3. Laser surface melting (LSM)**

*Schematic of different zones of laser transformation hardening.*

**Figure 6.**

Laser surface melting is one of the surface alteration processes that the surface of the substrate is melted and rapidly solidified to form the fine microstructure and improving the mechanical properties without changing the bulk properties and without addition of any metallic elements. The piston, valve and sliding parts are made of magnesium alloys, which are used in the automobile components and energy saving material. The application and limitation of magnesium alloy is decided by properties. In order to improve the tribological and mechanical properties, the laser surface melting process is focused on magnesium alloy. In the conventional heat treatment of HSS materials are presented the retained austenite, which transforms into brittle martensite during service. But, the life of high-speed tool steel is increased by using LSM. The schematic view of LSM is shown in **Figure 7**. The LSM treatment are carried out using a 2 kW fiber laser with 1.06 μm wavelength, laser power of 1500 W, the laser scanning speed of 600 mm/min and the distance between the laser head, spot size of 3 mm, shielding gas pressure of 0.3 MPa and the specimen

**68**

**Figure 7.**

*Schematic view of laser surface melting.*

surface of 12 cm are used in the LSM. The microhardness and corrosion resistance of magnesium alloy is also improved by using LSM with electromagnetic stirring [12]. In order to enhance the microhardness of melted substrate, the LSM process parameters effect of hardness of magnesium alloy is studied. The CW CO2 laser, beam diameter of 4 mm, argon gas of 6 l/min, speed varying from 100 to 400 mm/min and power varying from 1.5–3.0 kW are used in the process. The result showed that the melt depth of magnesium alloy is directly proportional to the laser power and inversely proportional to the scan speed. Laser surface melting enhances the microhardness of the melted zone by 2–3 times than the substrate [13]. The laser processed hardness of high speed tool steel and magnesium alloy is decreased from as-received substrate by increasing distance from the melting surface which is shown in **Figure 8**. This is due to the refined, solid solution strengthening and uniform microstructure. The LSM is also performed in electric contact material of Cu-50Cr. The 1 kW CW Nd: YAG laser, power density varying from 106 to 107 W/cm2 , scanning speed of 6000–10,000 mm/min and argon gas are used in this process. From the analysis found that the microhardness and withstanding voltage of Cu-50Cr are significantly improved by using LSM [14]. The effects of LSM process parameters are affecting the microstructure and hardness of AZ31B magnesium alloy substrate. The result found that the grain size in the fused layer increases by increasing power. The schematic diagram of as-received magnesium alloy is shown in **Figure 9a**. The effects of different power on microstructure of layer fused layers are shown in **Figure 10b**–**e**. The Nd: YAG laser power varying from 1600 to 2200 W, laser beam scanning velocity of 900 mm/min, laser beam spot diameter of 4 mm, number of superimposed tracks of 9, overlap ratio of 15%, and argon flow rate of 25 mL/min are used in the process. The depth of the metal pool and grain size is increased by increasing the power. This is due to the grain growing freely in the higher metal pool depth compared to smaller metal pool depth. The reason for increasing the hardness and wear resistance are

**Figure 8.** *Microhardness variation of magnesium alloy.*

#### **Figure 9.**

*Schematic diagram of (a) As received AZ31B magnesium alloy, microstructure of laser fused layer of (b) laser melted at 1600 W, (c) laser melted at 1800 W, (d) laser melted at 2000 W.*

**71**

**Figure 11.**

*The effect of LSM on wear resistance of Hastelloy C-276.*

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

W/cm2

of 1.20 × 107

β- Mg17Al12 phase in the fused layer [15].

due to the grain refinement, high dislocation density and dispersive distribution of

The LSM method produced the higher surface roughness of AZ80 magnesium alloys compared to MB26 due to the variation in cooling rate. A nanosecond pulsed fiber laser with the wavelength of 1060 nm is used for the LSM process. The process parameters such as pulse duration, repetition rate, and spot size are 220 ns, 500 kHz, and 44 μm, respectively. Alloys are irradiated with a laser power density

overlapping. The higher microhardness was observed for MB26 than the AZ80 due to the higher melting layer thickness [16]. The LSM is also used to study the grain size, microhardness of hybrid composites. The laser power is varied from 1.8 to 2.0 kW, the laser beam diameter range is 4.72–6.07 mm, standoff distance range is 35–45 mm and a constant scan speed of 400 mm/s is maintained. Argon shielding gas is used during the laser melting process to prevent the oxidation. The study found that the LSM treated hybrid metal matrix composite has lower grain size compared to untreated composites due to rapid solidification after LSM. The LSM produces the higher hardness of composites compared to untreated composite [17]. The effect of different laser power on microhardness and wear of AISI M2 high speed steel is studied by using LSM. The Nd: YAG laser, stand of distance varying from 1 to 2 cm, power varying from 600 to 1800 W, argon gas of 0.5 bar, laser spot varying from 2 to 4 mm and speed varying from 50 to 100 cm/min are used in this process. The results found that the maximum hardened depth of 0.85 mm is achieved by using power of 1400 W. The wear resistance of tool steel is nearly equal to conventionally hardened work material and it is shown in **Figure 10**. The reason for LSM produces high wear resistance and high hardened surface is due to the fine dendrites with dissolved carbides [18]. The LSM is also used to improve the hardness and wear resistance of Hastelloy C-276. The CW CO2 laser with the parameters of 2 mm beam diameter, 0.6 MPa argon pressure, power varying from 1.25–1.75 kW, speed of 300 mm/min and interaction time of 400 ms are used in the work. The result found that the maximum hardness of 447 HV is achieved by using the power of 1.5 kW and scanning speed of 300 mm/min. The hardness is improved by 1.8 times compared to parent metal. The wear resistance of hastelloy is high in the sample laser treated at 1.5 kW of power and 300 mm/min speed and it is shown in **Figure 11**. This is due to the significant effect of grain refinement on hardness [19].

and at a scanning speed of 200 mm/s with 50% beam bath

**Figure 10.** *The effect different heat treatment on weight loss of AISI M2 tool steel.*

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

*Practical Applications of Laser Ablation*

**70**

**Figure 10.**

**Figure 9.**

*Schematic diagram of (a) As received AZ31B magnesium alloy, microstructure of laser fused layer of (b) laser* 

*melted at 1600 W, (c) laser melted at 1800 W, (d) laser melted at 2000 W.*

*The effect different heat treatment on weight loss of AISI M2 tool steel.*

due to the grain refinement, high dislocation density and dispersive distribution of β- Mg17Al12 phase in the fused layer [15].

The LSM method produced the higher surface roughness of AZ80 magnesium alloys compared to MB26 due to the variation in cooling rate. A nanosecond pulsed fiber laser with the wavelength of 1060 nm is used for the LSM process. The process parameters such as pulse duration, repetition rate, and spot size are 220 ns, 500 kHz, and 44 μm, respectively. Alloys are irradiated with a laser power density of 1.20 × 107 W/cm2 and at a scanning speed of 200 mm/s with 50% beam bath overlapping. The higher microhardness was observed for MB26 than the AZ80 due to the higher melting layer thickness [16]. The LSM is also used to study the grain size, microhardness of hybrid composites. The laser power is varied from 1.8 to 2.0 kW, the laser beam diameter range is 4.72–6.07 mm, standoff distance range is 35–45 mm and a constant scan speed of 400 mm/s is maintained. Argon shielding gas is used during the laser melting process to prevent the oxidation. The study found that the LSM treated hybrid metal matrix composite has lower grain size compared to untreated composites due to rapid solidification after LSM. The LSM produces the higher hardness of composites compared to untreated composite [17]. The effect of different laser power on microhardness and wear of AISI M2 high speed steel is studied by using LSM. The Nd: YAG laser, stand of distance varying from 1 to 2 cm, power varying from 600 to 1800 W, argon gas of 0.5 bar, laser spot varying from 2 to 4 mm and speed varying from 50 to 100 cm/min are used in this process. The results found that the maximum hardened depth of 0.85 mm is achieved by using power of 1400 W. The wear resistance of tool steel is nearly equal to conventionally hardened work material and it is shown in **Figure 10**. The reason for LSM produces high wear resistance and high hardened surface is due to the fine dendrites with dissolved carbides [18]. The LSM is also used to improve the hardness and wear resistance of Hastelloy C-276. The CW CO2 laser with the parameters of 2 mm beam diameter, 0.6 MPa argon pressure, power varying from 1.25–1.75 kW, speed of 300 mm/min and interaction time of 400 ms are used in the work. The result found that the maximum hardness of 447 HV is achieved by using the power of 1.5 kW and scanning speed of 300 mm/min. The hardness is improved by 1.8 times compared to parent metal. The wear resistance of hastelloy is high in the sample laser treated at 1.5 kW of power and 300 mm/min speed and it is shown in **Figure 11**. This is due to the significant effect of grain refinement on hardness [19].

**Figure 11.** *The effect of LSM on wear resistance of Hastelloy C-276.*

The laser surface melting is carried out on nodular cast iron (NCI) [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 melt the NCI surface. The microstructure of as received nodular cast iron showed with more ferrite and less pearlite as shown in **Figure 12a**. The γ-phase dendrites and an interdendritic carbide structure were observed in the laser treated region and it is shown in **Figure 12b**. The reason for forming dendrite in the laser treated region is due to the rapid heating and solidification. The needle shape interdendritic structure of Fe3C and M-phase is also observed due to the higher cooling rate. The convection is also the reason for forming of homogeneous dendritic. The small diameter of nodules is also observed in the bottom layer with partial dissolution of nodular graphite due to the heat treatment and self-quenching. The uneven martensite and dendrite phases are observed in the intermediate layer due to the rapid re-solidification of the melt pool. Finally, fine martensite is observed in the bottom region. Moreover, no cracks and no voids are observed in the processed depth.

The worn out surface of as received and laser melted surface is shown in **Figure 13a** and **b**. The LSM specimen wear track showed with smooth, minor grooves and delamination. The wear depth and pile-up of laser processed specimens are lesser than untreated specimens. The laser treated surfaces have

#### **Figure 12.**

**73**

**Figure 14.**

*Schematic view of laser surface alloying.*

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

samples showed less wear than substrate.

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

fine grooves resulting in improving the wear resistance of specimens due to the microstructure changes. The root causes for improving the wear resistance of laser processed materials are fine M-phase and retained γ-phase with Fe3C phase. The length of depth of hardness is increased by increasing the melted depth. The reasons are due to the precipitation hardening, residual stress by refinement of grains through rapid re-solidification. The 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 four time higher hardness due to the uniform grain structure. The partially melted zone shows the higher hardness due to the graphite nodules and fine ledeburite microstructure with the graphite interface. The wear loss is calculated for both the laser processed sample and untreated sample. The laser processed

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 13.** *Worn out surface of; (a) base metal, (b) laser melted specimen.*

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

*Practical Applications of Laser Ablation*

and no voids are observed in the processed depth.

The laser surface melting is carried out on nodular cast iron (NCI) [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 melt the NCI surface. The microstructure of as received nodular cast iron showed with more ferrite and less pearlite as shown in **Figure 12a**. The γ-phase dendrites and an interdendritic carbide structure were observed in the laser treated region and it is shown in **Figure 12b**. The reason for forming dendrite in the laser treated region is due to the rapid heating and solidification. The needle shape interdendritic structure of Fe3C and M-phase is also observed due to the higher cooling rate. The convection is also the reason for forming of homogeneous dendritic. The small diameter of nodules is also observed in the bottom layer with partial dissolution of nodular graphite due to the heat treatment and self-quenching. The uneven martensite and dendrite phases are observed in the intermediate layer due to the rapid re-solidification of the melt pool. Finally, fine martensite is observed in the bottom region. Moreover, no cracks

The worn out surface of as received and laser melted surface is shown in **Figure 13a** and **b**. The LSM specimen wear track showed with smooth, minor grooves and delamination. The wear depth and pile-up of laser processed specimens are lesser than untreated specimens. The laser treated surfaces have

*Microstructure of; (a) as-received nodular cast iron, and (b) laser surface melted nodular cast iron.*

**72**

**Figure 13.**

*Worn out surface of; (a) base metal, (b) laser melted specimen.*

**Figure 12.**

fine grooves resulting in improving the wear resistance of specimens due to the microstructure changes. The root causes for improving the wear resistance of laser processed materials are fine M-phase and retained γ-phase with Fe3C phase. The length of depth of hardness is increased by increasing the melted depth. The reasons are due to the precipitation hardening, residual stress by refinement of grains through rapid re-solidification. The 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 four time higher hardness due to the uniform grain structure. The partially melted zone shows the higher hardness due to the graphite nodules and fine ledeburite microstructure with the graphite interface. The wear loss is calculated for both the laser processed sample and untreated sample. The laser processed samples showed less wear than substrate.
