Keywords

Nanosecond pulsed Nd:YAG laser beam; laser-matter interaction; static underwater laser beam cutting; Inconel 625 superalloy; height of water column

7.1 Introduction

Diverse application of the laser in various fields of science, technology, art and entertainment makes it an invaluable tool for mankind. The invention of the laser has made a very significant contribution to the advancement of science and technology for new materials and process techniques to meet global competition. The laser is a device that emits electromagnetic radiation through the process of stimulated emission. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, CO₂ laser, fiber laser, diode laser and excimer lasers are widely used for various material processing applications in industries. Nd:YAG laser is one of the most commonly used lasers for drilling, marking, cutting and welding applications. The lasing medium is Neodymium (Nd³⁺) doped Yattrium Aluminum Garnet (YAG) crystal which is pumped by light source such as arc lamp or flash lamp. The wavelength of the Nd:YAG laser radiation is 1.06 μm which lies in the near-infrared region of the electromagnetic spectrum (Küper & Stuke, 1992).

7.2 Laser as a machine tool

Laser can be considered as one of the nonconventional machining processes because of their precision of operation, low cost, localized processing, flexibility and high speed of operation. Laser beam machining is the machining process involving a beam of laser as machine tool. It is a thermal process where a laser beam is used as a heat source to remove material without mechanical engagement with workpiece. The main properties of laser radiation are high spatial and temporal coherence along with high collimation, high monochromaticity, high brightness and minimal divergence which results in producing a very high intensity laser beam. A wide range of materials can be processed by laser due to its large irradiances (up to 10²¹ W/cm²) at the surface, with high focusability which is enough to evaporate any material or even to start a nuclear fusion reaction (Küper & Stuke, 1992). In the laser beam machining process, material removal is not dependent on mechanical, physical or electrical properties of the workpiece but on thermo-optical properties of the material (Davim, 2013) (Fig. 7.1).

The beam wavelength has a great impact on absorptivity of laser light on a workpiece surface, which is relatively important for laser material processing. A discrete spatial profile, termed as transverse electromagnetic mode, is exhibited by a cross-section of laser beam. Generally fundamental TEM₀₀ mode is used for laser beam machining operations, having Gaussian spatial distribution. The output of laser can either be constant amplitude or periodic, commonly known as continuous wave (CW) mode and pulsed beam mode respectively. Normal pulsing, Q-switching and mode locking are the various pulsing modes during pulsed laser operation. A high energy density is achieved in short pulse duration laser operation after threshold energy is reached. Pulse duration and pulse repetition rate are the most influencing process parameters for pulsed laser machining (Ready, 1997). During laser beam machining one of two geometrics are generally used i.e. either a moving target or a moving beam.

7.3 Laser material interaction

Reflection, refraction, absorption, scattering and transmission are the physical phenomenans that take place when the electromagnetic wave (laser beam) interacts with a workpiece surface. Linear and nonlinear absorption are the most important phenomena of the laser matter interaction. Fig. 7.2 shows a schematic of interaction of a laser beam with work material.

Absorption at irradiate surface not only depends on the wavelength of the laser radiation but also on other factors such as incident angle, surface roughness and temperature of the solid/machine zone. The absorption coefficient of material is proportional to electrical resistivity whereas the absorption by target substrate increases with decrease in wavelength. The probability of nonlinear absorption increases significantly in proportion to growing laser intensity. This aforesaid relationship is the reason behind multiphoton excitation process at constant laser fluence for a shorter laser–material interaction time. Absorption of laser energy can be described as interaction between electrons and nuclei lattice of target material. A force is exerted by an optoelectric field of incident electromagnetic radiation by which the nuclei lattice and electrons get vibrational motion. Thus the absorbed radiation results in the excess energy of the electrons, such as kinetic energy of the free electrons, excitation energy of the bound electrons (Von, 1987). Excess energy of the electrons is damped by collision with vibrating lattice of nuclei and some incident laser energy is transferred to the lattice leads to generation of heat. By Beer-Lambert law (Steen, 1991), the absorption of laser radiation in the target substrate is generally expressed as,

$I\left(Z\right)=I_0{e}^{-\mu z}$

where I₀ is the incident intensity, I(z) is the intensity at depth z and μ is the absorption coefficient. The intensity of the laser radiation gets faded along with the depth inside the material. The length over which the significant reduction of intensity of the laser radiation takes place is known as attenuation length (L) and expressed as reciprocal of the absorption coefficient (μ) of target substrate (Welch & Gardner, 2002),

$L=\frac{1}{\mu }$

Generally for opaque material, the absorptivity can be defined as a fraction of incident radiation, absorbed at normal incidence which can be expressed (Duley, 1983) as,

$A=1-R$

where, A is absorptivity of irradiate substrate and R is the reflectivity of irradiate substrate.

Various physical effects in the material include heating, melting and vaporization of the substrate has occurred, depending on the temperature distributions in the material. Thermal energy is transported through the substrate by thermal conduction, convection and radiation from the surface of the substrate. Phase transformations such as surface melting and evaporation are taking place by sufficiently high laser intensity which is generally referred as melting (I_m) and vaporization (I_v) threshold intensity. During laser irradiation, the melting starts at a certain temperature when laser intensity reaches melting threshold energy and creates a solid–liquid interface at the surface. The solid–liquid interface moves away from the surface along the depth with increase in laser energy density and a melt pool is formed. The depth of melting is decreased during pulsed off time (Allmen, 1983). The depth of melting (Z max) reaches maximum when the laser irradiates surface temperature reaches at boiling point. Evaporation takes place with a further increase in laser power density and/or pulse on time, resulting in material removal from irradiate region. The liquid–vapor interface has to move inside the material with continuous laser irradiation after the vaporization is initiated. Depth of vaporization (Steen, 1991), mass of material removed per unit time and velocity of the liquid–vapor interface can be calculated by

$\stackrel{·}{m}=V_{\mathrm{s}}\rho$

where $\stackrel{·}{m}$ the mass of material removed per unit time, V_s is the velocity of the liquid–vapor interface and ρ is the density.

$d=V_{\mathrm{s}}{t}_{\mathrm{p}}$

where, $t_{\mathrm{p}}$ is the pulse time and d is the depth of vaporization.

$V_{\mathrm{s}}=\frac{H}{\rho \left(c{T}_{\mathrm{b}}+L_{\mathrm{v}}\right)}$

where, H is the absorbed laser power, T_b is the boiling temperature at surface, and L_v is the latent heat of vaporization. Evaporation-induced recoil pressure exceeds the highest possible value of surface tension pressure during laser beam processing, i.e., drilling, cutting, marking, welding, cladding, etc. During laser material processing, recoil pressure plays an important role in melt expulsion from machining zone (Steen, 1991). At high laser irradiance (I≥109 W/cm²) the vapor or the ambient gas becomes ionized due to the interactions between the resulting vapor and the incident laser beam, termed as plasma. Plasma plume forms a shield over the machining area and reduces the energy available to the workpiece. When the incident laser energy on the target surface is sufficiently large to exceed the boiling temperature, rapid vaporization starts. In the photo-thermal ablation process, material is removed by thermal stresses and surface vaporization. Whereas in photo-chemical ablation, material removal takes place by molecular fragmentation without significant thermal damage by the energy of the incident photon causes the direct bond breaking of the molecular chains in the organic materials (Bäuerle, 2000). The physical phenomena of material removal mechanism are shown in Fig. 7.3.

Utilization of the laser in various manufacturing process is increasing rapidly. The applications of lasers have been demonstrated in many casting, forming, joining, and machining processes. The laser beam machining process is widely used in industries. Laser beam cutting, drilling and welding are the most common applications in automotive, medical and aircraft industries.

7.4 Laser beam cutting

Nowadays the laser beam cutting technique is the most established laser material processing technology to obtain desired geometry. It is the two-dimensional laser beam machining technique where the motion of either focused beam and/or the workpiece are relative to each other. Advantages of laser beam cutting can be divided into two categories (Powell, 1993), i.e., process characteristics and cut quality. The schematic diagram of laser beam cutting is illustrated in Fig. 7.4.

7.5 Underwater laser beam machining

Undesired quality aspects, i.e., kerf taper, dross formation, HAZ, recast layer, burr formation, difficulties in debris removal and desired surface morphology during laser cutting are sometimes totally removed or partially corrected by irradiating the workpiece material at submerged condition.

7.5.3 Development of different types of liquid-assisted laser beam machining

Current research on different types of liquid-assisted laser beam machining, i.e., laser ablation in submerged condition, underwater waterjet guided laser beam machining, underwater gas-assisted laser beam machining, chemical-assisted laser beam machining is carried out by researchers. Some researchers have found that laser beam cutting in chemical solution produces better surface quality than processing in pure water (Datta, Romankiw, & Vigliotti, 1987). Electrochemical dissolution/localized breakdown is enhanced by laser beam in neutral salt solution. Optical absorptivity of workpiece substrate increases in water due to higher refractive index of water (Kim & Lee, 2001).

7.5.3.1 Laser beam cutting in submerged condition

In many underwater laser beam cutting techniques, the laser ablation at submerged condition can be said to be the simplest process to cool the workpiece by water during the cutting induced by laser. A schematic view is illustrated in Fig. 7.5. In this technique layer or thin film of stagnant water creates better natural convection than air, which results in reduction in temperature gradient and thermal stress on the top surface of workpiece substrate, thereby reducing the possibility of subsurface crack formation, results in micro crack free machined zone with smaller heat affected zone.

7.5.3.2 Underwater assist gas jet/waterjet assisted laser beam cutting

In this process a high pressure assist gas or waterjet coaxially flows along with high intensity laser beam from a specially designed nozzle for impingement on the top surface of submerged workpiece. Sometime an off axial waterjet is impinged on the irradiation spot by a specially designed delivery nozzle. A schematic view is illustrated in Fig. 7.6. Removal of molten pool from the bottom of the kerf is easier in this process (Mullick et al., 2013). Sometimes a waterjet is directed into the cutting region to increase the thermal efficiency of the laser beam in underwater conditions. Here high intensity laser beam is generally used. The direction of the waterjet in the fusion region results in desired cut quality with higher cutting speed (Owaki, Uehara, & Tsuchiya, 1999).

7.5.3.3 Molten salt-jet-guided/chemical laser beam

In this process, instead of a waterjet a molten salt jet or chemical is used as assisted medium which is directed in fusion zone. Molten salt jet/chemical is used for laser reactive cutting due to higher heat capacity and higher oxygen content by molten salt, which facilitates the removal of the material rapidly. In that instance, high temperature chemical etching of the workpiece surface also takes place, which increases the rate of material removal.

7.5.3.4 Water jet following the laser beam

In this process, the purpose of the waterjet is to confine the assist gas in order to increase the assist gas pressure in kerf which results in improvement in melt removal. This process is suitable for both the high and low intensity laser beam due to absence of the waterjet in the fusion zone (Stephen, 2011). Schematic diagram of this process given below in Fig. 7.7.

7.5.3.5 Laser beam cutting of opaque material at partially submerged condition

In this process during cut melt redeposition on the cutting front at backside is reduced or absent. Liquid prevents the adhesion of debris and dross at the backside of the workpiece.

7.5.3.6 Laser beam cutting of transparent material at partially submerged condition

Because of low absorption of the light, inorganic optical materials are difficult to machine with conventional laser cutting techniques. Desired microstructuring can be achieved by this technique. The damage and the optical breakdown threshold at a glass plate in contact with water were 2.5 times higher compared to those in air. Debris is removed efficiently by liquid motion, gravity force and debris dissolution. In this process, the etching threshold value is comparatively less than in air, and etching occurs well below the optical damage threshold of the materials. Absence of plasma shielding results in increased laser cutting efficiency (Kawaguchi, Sato, and Narazaki, 2005). Schematic diagram of this process is shown in Fig. 7.8.

7.5.3.7 Hybrid waterjet laser cutting

In this process a defocused laser beam travels along the workpiece followed by a high pressure waterjet. Here the waterjet flows off axially along the laser beam but enters in the fusion zone. In this process, the laser in defocused position is used to heated up the soft solid state and the material is removed by the impact of the following high pressure waterjet. For that reason, the amount of thermal damage on the workpiece surface and sub-surface regions is minimized. A specially designed mechanism is attached to the workpiece nozzle to change its inclination angle with the laser beam and standoff distance from the workpiece. During this process, compressive stress is generated on the laser-induced heated zone. Forced convection due to waterjet flow generates the quenching effect to the heated region, resulting in formation of tensile stress on the top surface of the workpiece. That tensile stress is the reason behind propagation of cracks leading to removal of material from workpiece specimen with flow of water as microchips (Kalyanasundaram, Shrotriya, & Pal, 2010).

7.6 Pulsed IR laser ablation of Inconel 625 superalloy at submerged condition: A case study

A parametric study on laser beam cutting of Inconel 625 superalloy at submerged condition has been carried out. The effect of the height of water column along with the other laser beam cutting parameters such as lamp current, pulse frequency, pulse width and cutting speed, on kerf width, HAZ width and depth of cut is investigated. Height of the water column is varied from 1 mm to 5 mm to see the effect on machining responses. In this study lamp current is consider as an function of average power or working power. Design of experiment based on response surface methodology (RSM) has been implemented here to carry out the experiments.

7.6.1 Experimental setup

Inconel 625 with 80 mm×80 mm×0.9 mm dimension has been considered as workpiece material for experimental purposes. In this study water is used as assisted medium at stagnant condition. A pulsed Nd:YAG laser-based CNC machining system (SLT-SP- 2000), manufactured by M/s Sahajanand Laser Technology, India, is used for the experimental study. The CNC controller consists of x–y–z axis and a controlling unit. One stepper motor is attached to each of the axes and connected to the controlling unit. A CCD camera is attached over the CNC z-axis controller unit which, along with the CCTV monitor, is used for viewing the location of the workpiece and also for checking the proper focusing condition of surface of the workpiece. The schematic diagram of the underwater pulsed Nd:YAG laser-based CNC machining system is shown in Fig. 7.9.

The workpiece holding unit for underwater laser beam cutting at static condition has been designed and fabricated at the workshop. Transparent PMMA sheets with different thicknesses are used for fabrication of the workpiece holding device. This specially designed workpiece holding unit is placed over the CNC controlled work table. Image of this unit is shown in Fig. 7.10. In this experimental procedure a water film from 1 to 5 mm is used to carry out the experiments. For the study, a laser beam comes from outside, first interacts with water film then travels a distance through the water to strike the workpiece substrate. The workpiece is held in submerged condition by pouring water externally. The height of the water column over the workpiece is maintained precisely by placing the slip gauge over the workpiece material. From the dimension of the slip gauge, the height of the water column over the surface of the workpiece material is controlled as required. A steady state of water column is maintained during the experiment. De-ionized water at room temperature (20–22°C) has been used as water medium because of its nonreactive nature to workpiece material even at elevated temperatures and easy availability. The bubbles accumulated near the fusion region are removed from the machining zone by a thin stick after each experimental run, followed by a few minute wait for stabilization of stagnant water. Jammer is used for precisely fixed the workpiece with proper alignment. The schematic diagram of the underwater pulsed Nd:YAG laser-based CNC machining system is shown in Fig. 7.10.

Experiments have been carried out according to the central composite rotatable second-order design based on RSM. Response surface modeling is used to establish the mathematical relationship between the response, y_u and the various machining parameters, with the eventual objective of determining the optimum operating conditions for the system. The ranges of these controllable process variables are selected on the basis of trial experiments conducted by using a one factor at a time approach.

Usually, a second order polynomial equation is used in RSM,

$y_{\mathrm{u}}={\beta }_0+\sum _{j=1}^k{\beta }_j{x}_j+\sum _{j=1}^k{\beta }_{jj}{x}_j^2+\sum _{i<j}\sum _{j=2}^k{\beta }_{ij}{x}_i{x}_j$ (7.1)

(7.1)

Here y_u is the corresponding response, x_i is the coded value of the ith machining parameter, k is the number of machining parameters and β_i, β_ii, β_ij are the second-order regression coefficients. Ranges of all controllable input process variables for underwater laser machining, i.e., lamp current, pulse frequency, pulse width, cutting speed, and height of water level as selected are listed in Table 7.1.

Table 7.1

Process parameters levels

Process parameter and symbol	Unit	Value of α	Levels
			Lowest (−1)	Highest (+1)
Lamp current (X1)	Amp	2.00	22	26
Pulse frequency (X2)	kHz		4	8
Pulse width (X3)	%		4	8
Cutting speed (X4)	mm/sec		1.5	2.5
Height of water level (X5)	mm		2	4

Images of cut region have been captured by Olympus (Model STM 6-F10-3; No. OH13697) optical measuring microscope at 20× magnification. Here heat affected zone (HAZ) is measured by measure the area of change in colour adjacent to machined zone. Then the machining responses are evaluated by image analysis software. Calculation of HAZ width is governed by the formula given below,

$HAZ\mathrm{width}(mm)=\frac{HA{Z}_{\mathrm{Top}}-Kerf{\mathrm{width}}_{\mathrm{Top}}}{2}$ (7.2)

(7.2)

Machining responses are measured at five different positions along the kerf for all experiments and an average is taken for further analysis to reduce the measurement error. Experimental results are listed in Table 7.2.

Table 7.2

Experimental results

Lamp current (Amp)	Pulse frequency (kHZ)	Pulse width (%)	Cutting speed (mm/sec)	Height of water level (mm)	Kerf width (mm)	Depth of cut (mm)	Heat affected zone (HAZ; mm)
22	4	4	1.5	2	0.1346	0.0271	0.06582
26	4	4	1.5	2	0.1836	0.0243	0.07205
22	8	4	1.5	2	0.0933	0.0274	0.05652
26	8	4	1.5	2	0.1039	0.0230	0.07403
22	4	8	1.5	2	0.1141	0.0272	0.06717
26	4	8	1.5	2	0.1886	0.0275	0.07583
22	8	8	1.5	2	0.0860	0.0274	0.06133
26	8	8	1.5	2	0.0963	0.0257	0.07983
22	4	4	2.5	2	0.1193	0.0315	0.06440
26	4	4	2.5	2	0.1571	0.0255	0.07280
22	8	4	2.5	2	0.0912	0.0232	0.04652
26	8	4	2.5	2	0.0910	0.0189	0.06787
22	4	8	2.5	2	0.1052	0.0274	0.06833
26	4	8	2.5	2	0.1640	0.0280	0.07682
22	8	8	2.5	2	0.0843	0.0198	0.05060
26	8	8	2.5	2	0.0854	0.0216	0.07402
22	4	4	1.5	4	0.1355	0.0220	0.05582
26	4	4	1.5	4	0.2294	0.0204	0.06347
22	8	4	1.5	4	0.0941	0.0233	0.05132
26	8	4	1.5	4	0.1156	0.0225	0.07473
22	4	8	1.5	4	0.1477	0.0243	0.05177
26	4	8	1.5	4	0.1997	0.0241	0.06120
22	8	8	1.5	4	0.0866	0.0229	0.05150
26	8	8	1.5	4	0.0949	0.0248	0.07607
22	4	4	2.5	4	0.1301	0.0265	0.05097
26	4	4	2.5	4	0.2176	0.0231	0.06157
22	8	4	2.5	4	0.0919	0.0187	0.03750
26	8	4	2.5	4	0.0942	0.0188	0.06297
22	4	8	2.5	4	0.1270	0.0257	0.04867
26	4	8	2.5	4	0.2043	0.0268	0.06110
22	8	8	2.5	4	0.0756	0.0161	0.04193
26	8	8	2.5	4	0.0824	0.0213	0.06718
20	6	6	2	3	0.0945	0.0225	0.04162
28	6	6	2	3	0.1526	0.0222	0.07380
24	2	6	2	3	0.1837	0.0307	0.06198
24	10	6	2	3	0.0707	0.0231	0.05858
24	6	2	2	3	0.1255	0.0189	0.06557
24	6	10	2	3	0.1291	0.0196	0.06850
24	6	6	1	3	0.1358	0.0299	0.06588
24	6	6	3	3	0.1378	0.0255	0.05267
24	6	6	2	1	0.1188	0.0309	0.07733
24	6	6	2	5	0.1400	0.0254	0.05488
24	6	6	2	3	0.1428	0.0286	0.06233
24	6	6	2	3	0.1349	0.0287	0.06232
24	6	6	2	3	0.1311	0.0289	0.06472
24	6	6	2	3	0.1359	0.0278	0.06167
24	6	6	2	3	0.1289	0.0301	0.06405
24	6	6	2	3	0.1371	0.0270	0.06360
24	6	6	2	3	0.1238	0.0301	0.06412
24	6	6	2	3	0.1277	0.0279	0.06410
24	6	6	2	3	0.1264	0.0291	0.06292
24	6	6	2	3	0.1328	0.0281	0.06130

7.6.2 Development of mathematical model

Response surface modeling (central composite design) is used to establish the mathematical relationship between the response and variable process parameters. The second order polynomial equations are given below:

$\begin{array}{l}\mathrm{Kerf}\text{width}=-0.925+0.0605X_1+0.0862X_2+0.0082X_3+0.0031X_4\hfill \\ -0.0052X_5-0.000645X_1*X_1-0.000417X_2*X_2-0.000413X_3*X_3+\hfill \\ 0.00291X_4*X_4-0.00112X_5*X_5-0.003672X_1*X_2-0.000105X_1*X_3-\hfill \\ 0.00152X_1*X_4+0.001680X_1*X_5-0.000213X_2*X_3+0.00106X_2*X_4\hfill \\ -0.003453X_2*X_5+0.00037X_3*X_4-0.000627X_3*X_5+0.00141X_4*X_5\hfill \end{array}$ (7.3)

(7.3)

$\begin{array}{l}\mathrm{Depth}\mathrm{of}\mathrm{cut}=-0.1572+0.01547X_1+0.00207X_2+0.00202X_3+0.01059X_4\hfill \\ -0.00984X_5-0.000391X_1*X_1-0.000107X_2*X_2-0.000585X_3*X_3-\hfill \\ 0.000897X_4*X_4-0.000114X_5*X_5+0.000076X_1*X_2+0.000251X_1*X_3+\hfill \\ 0.000138X_1*X_4+0.000293X_1*X_5-0.000054X_2*X_3-0.001755X_2*X_4+\hfill \\ 0.000109X_2*X_5-0.000417X_3*X_4+0.000109X_3*X_5+0.000404X_4*X_5\hfill \end{array}$ (7.4)

(7.4)

$\begin{array}{l}\mathrm{HAZ}\mathrm{width}=0.63058+0.007893X_1-0.001267X_2+0.001022X_3-0.002791X_4\hfill \\ -0.005027X_5-0.001270X_1^2-0.000626X_2^2+0.001061X_3^2-0.000879X_4^2+\hfill \\ 0.000830X_5^2+0.003362X_1*X_2+0.000316X_1*X_3+0.000607X_1*X_4+\hfill \\ 0.000820X_1*X_5+0.000844X_2*X_3-0.002134X_2*X_4+0.001911X_2*X_5+\hfill \\ 0.000410X_3*X_4-0.01027X_3*X_5-0.000711X_4*X_5\hfill \end{array}$ (7.5)

(7.5)

7.6.3 ANOVA analysis

Analysis of variance (ANOVA) and subsequent f- and p-value tests have been carried out on machining responses, i.e., kerf width, depth of cut, and HAZ width to test the adequacy of the corresponding developed mathematical models. From the Table 7.3 it is observed that p value of the model for all the machining responses is less than 0.05 (i.e., α=0.05, or at 95% confidence level) indicating a statistical significance. From the p value it is also observed that all the machining parameters are significant individually along with their square terms and 2-way interaction terms, except the square terms for kerf width. From the f value it is observed that lamp current is the most dominating factor for HAZ width but moderate in terms of kerf width and lesser dominant for depth of cut. Pulse frequency has the most dominant factor both the kerf width and depth of cut but a lesser dominant for HAZ width. Pulse width is the least dominant for all the responses. Cutting speed is more dominant factor depth of cut and HAZ width but lesser dominant for kerf width. Height of water level or column shows moderate dominancy for all the responses in which it has most dominant for HAZ width followed by depth of cut and kerf width. Values of other adequacy measures R-sq, adjusted R-sq, and predicted R-sq are in reasonable agreement and are close to 100%, which indicate adequacy of the model. Microscopic view of HAZ width is given in Fig. 7.11. A varying HAZ width along the kerf shown in microscopic view.

Table 7.3

ANOVA analysis of machining responses

Machining responses		Kerf width		Depth of cut		HAZ width
Source	DF	f value	p value	f value	p value	f value	p value
Model	20	45.47	0.000	37.49	0.000	196.8	0.00
Linear	5	156.67	0.000	52.05	0.000	641.99	0.00
Lamp current	1	166.32	0.000	5.98	0.020	2040.49	0.00
Pulse frequency	1	576.06	0.000	139.23	0.000	52.53	0.00
Pulse width	1	5.89	0.021	6.91	0.013	34.19	0.00
Cutting speed	1	10.69	0.003	24.47	0.000	255.17	0.00
Height of water level	1	24.40	0.000	83.65	0.000	827.58	0.00
Square	5	1.32	0.282	63.39	0.000	24.22	0.00
2-Way interaction	10	11.95	0.000	17.25	0.000	60.49	0.00
Error	31
Lack of fit	22	2.85	0.053	2.85	0.603	0.85	0.645
Pure error	9
Total	51

Machining responses	Value of S	R-sq	R-sq(adjusted)	R-sq(predicted)
Kerf width	0.0086769	96.70%	94.58%	88.71%
Depth of cut	0.0009534	96.03%	93.47%	88.72%
HAZ width	0.0011052	99.22%	98.71%	97.72%

7.6.4 Effects of different process parameters on machining responses

7.6.4.1 Effect of different process parameters on kerf width

From the Fig 7.12 it is observed that kerf width marginally increases with an increase in height of water level, then decreases with further increase in height of water level. With increase in water column bubbles form and their collapsing is moved away from fusion zone results in increase in material removal. But further increase in water column results in increase of travel distance for laser beam to interact with work substrate. For that absorption of laser energy by water increase, results in decrease in material removal. With increase in lamp current more laser power generates, which travels through the thin water column to interact with the workpiece which may result in more material being removed from fusion zone due to hydrodynamic mechanism.

7.6.4.2 Effect of different process parameters on depth of cut

Here from Fig. 7.13 it is observed that depth of cut decreases with the increase in water level height, and decreases with increase in cutting speed. Increase in cutting speed means less interaction time of the laser beam with the workpiece per pulse. For that reason less penetration may be achieved, resulting in decrease in depth of cut with increase in cutting speed. In underwater machining, laser energy is absorbed by water resulting in a lesser amount of laser fluence to interact with top surface of workpiece which also produces vapor bubbles and superheated water. These vapor bubbles sometimes may scatter the laser beam from its irradiation point, which may be the reason behind the decrease in depth of cut with increase in height of water level.

7.6.4.3 Effect of different process parameters on HAZ width

At lower laser pulse frequency, more laser power is generated than at higher pulse frequency, which could account for the amount of material removal. During underwater laser beam machining, settling time of the debris material removed from cut zone is much longer and the scattering and absorption of thermal energy of laser beam is greater. For that reason during laser machining at submerged condition, HAZ width is decreased with increase in pulse frequency. With change in height of water column, the refractive index changes linearly result in less heat input to the top surface of the workpiece. For that reason HAZ width may be decreased with increase in height of water column (Fig. 7.14).

From Fig. 7.15 it is observed that, with increase in height of water column, the laser energy reaching the workpiece surface is reduced; this may be due to the absorption of laser energy by ionization of water and laser beam scattering by water vapor formed in the laser–material–water interaction zone which results in a lesser amount of HAZ width along the kerf. Higher energy density of laser beam causes by increase in lamp current, resulting in more heat input to the fusion zone to increase the HAZ width.

Conclusion

In this chapter a overview on Nano second pulsed laser beam machining has been discussed. Illustration of laser matter interaction and material removal of short pulsed laser beam machining is given. Difficulties of pulsed laser ablation is discussed. Remedy of this difficulties in form of laser machining at different environments, specially in underwater is illustrated. Developments of laser material interaction in submerged condition is exemplified.

Laser beam micromachining of Inconel 625 superalloy at submerged condition has been experimentally studied here. Center composite design-based DOE technique is used here to carry out the experiments. The effect of height of a water column along with other controllable process parameters, i.e., lamp current, pulse frequency, pulse width and cutting speed on kerf width, depth of cut, and HAZ width are investigated successfully. According to result of ANOVA during experimental study, carried out within the selected range, it has to be seemed that height of water column has a great effect on all the machining responses. Lamp current which is used here as an function of average power is the most influencing process variable whereas pulse width is the least affecting factor for all the machining responses. HAZ width, kerf width and depth of cut is decreased with increase in height of water.

Acknowledgment

The authors acknowledge University Grants commission (UGC), New Delhi, for financial support under the scheme of Rajiv Gandhi National Fellowship Programme and CAS Ph-IV (UGC) programme of Production Engineering Department, Jadavpur university, Kolkata, INDIA for technical equipment support.