Laser Drilling Assisted with Jet Electrochemical Machining

Laser drilling is a noncontact, precise and reproducible technique that can be used to form small diameter and high-aspect ratio holes in a wide variety of materials. Laser drilling is most extensively used in the aerospace, aircraft, and automotive industries. The most important application of laser drilling in the aerospace industry is the drilling of a large number of closely spaced effusion holes with small diameter and high quality to improve the cooling capacity of turbine engine components. Drilling rates as high as 100 holes/s can be achieved in production environment by coordinating the workpiece motions with pulse period of pulsed laser source. Laser drilling does not pose substantial problems at high angles of incidences. Laser drilling is also well suited for the nonconducting substrates or metallic substrates coated with nonconducting materials where the electric discharge machining is limited. In addition, recently the laser drilling of composite materials such as multilayer carbon fiber composites for aircraft applications is attracting increasing interest due to potential advantages of rapid processing, absence of tool wear, and ability to drill high-aspect ratio holes at shallow angles to the surface.


Introduction
Laser drilling is a noncontact, precise and reproducible technique that can be used to form small diameter and high-aspect ratio holes in a wide variety of materials. Laser drilling is most extensively used in the aerospace, aircraft, and automotive industries. The most important application of laser drilling in the aerospace industry is the drilling of a large number of closely spaced effusion holes with small diameter and high quality to improve the cooling capacity of turbine engine components. Drilling rates as high as 100 holes/s can be achieved in production environment by coordinating the workpiece motions with pulse period of pulsed laser source. Laser drilling does not pose substantial problems at high angles of incidences. Laser drilling is also well suited for the nonconducting substrates or metallic substrates coated with nonconducting materials where the electric discharge machining is limited. In addition, recently the laser drilling of composite materials such as multilayer carbon fiber composites for aircraft applications is attracting increasing interest due to potential advantages of rapid processing, absence of tool wear, and ability to drill high-aspect ratio holes at shallow angles to the surface. The physical processes occurring during laser drilling are shown in Fig.1. The process can be split up into three stages. Initially a thin region of molten material is formed by absorption www.intechopen.com of laser energy at target surface. After some time the surface of this melt pool reaches vaporization temperature. The sudden expansion of the vapor evaporating from the surface eventually leads to the splashing stage when the melt pool is pushed radially out by the recoil pressure. Melt expulsion occurs when the pressure gradients on the surface of the molten material from the hole. On its way of escaping out some part of this molten material may re-solidify at the wall. These molten materials re-solidify adhering to the sidewall of the hole as a thin layer referred to as the 'recast layer'. Also, often spatter of molten material on the surface surrounding the entrance and exit of the hole will occur during the laser drilling process. The spatter and recast layer are defects of laser drilling and limit the application. Therefore, the elimination of these defects is one subject of intense research in laser machining.
This chapter contains a new typical hybrid process of laser drilling assisted with jet electrochemical machining (JECM-LD) for the minimization of recast and spatter.

Principle
JECM-LD combines two different sources of energy simultaneously: energy of photons (laser drilling) and energy of ions (ECM). The main aim of combining a jet electrolyte with laser beam is to obtain high process quality by reducing the recast layer and spatter produced in laser drilling. The jet electrolyte is aligned coaxially with a focused laser beam and creates a noncontact tool-electrode. The focused laser beam and the jet electrolyte are acting on the same surface of workpiece synchronously.

Measurement of laser attenuation in electrolyte
JECM-LD aligns a jet electrolyte coaxially with a laser beam. The laser beam needs to transmit in the jet electrolyte before being focused on the machining area. Electrolyte is a neutral salt solution, which is attenuable to laser energy by absorption and scattering. The property of laser attenuation in electrolyte is the key factor of JECM-LD.

Theoretical background
Electrolyte is a neutral salt solution with the solute of sodium salt and the solvent of pure water. The solution has a number of particles such as water molecules, metallic ions and suspended matter. These particles have significant effects on the laser attenuation in the electrolyte with absorption and scattering. The total attenuation coefficient () Where 0 () I  is a source of intensity and () x I  is the intensity transmitted a distance x in solution. The total attenuation coefficient is the sum of the absorption and scattering coefficients. In terms of the absorption coefficient ()   and the total scattering coefficient The absorption coefficient is correlative with the refraction of solution. The index of refraction () n  and the extinction coefficient () k  of solution are, respectively, the real and imaginary parts of its spectral complex refractive index is written as The absorption coefficient ()   is determined by () k  and the expression is given by The total scattering coefficient includes two parts of Rayleigh scattering coefficient () and Mie scattering coefficient () M   , which can be written as The scattering coefficient is determined by the radius of the particles in solution. A dimensionless parameter q ( 2/ qR    ) is used to be token of particle size. When 0.1 q  , the scattering is Rayleigh scattering, contrariwise, which is Mie scattering. The electrolyte of this work is an industrial reagent of sodium chloride or sodium nitrate. The electrolyte has different sizes (Radius: 5~10m) suspended particles result from undissolved matters involved in the industrial reagent. With the higher concentration, the solution has more suspended particles and the effect of scattering is more evident. So the laser attenuation in electrolyte is directly affected by the concentration of solution. The temperature of solution www.intechopen.com is also a factor to influence the attenuation coefficient. Here two parameters of C  and T  are used to represent the influence of the concentration and temperature, then Eq. (1) can be deduced a new expression 00 0

Measurement and results
A measurement of laser attenuation in electrolyte has been carried out based on the theoretical analysis mentioned above. The laser systems used in the measurement are a CW semiconductor green laser (wavelength: 532 nm) and a pulsed Nd:YAG laser (wavelength: During the measurement, the first step is measuring the source intensity of laser transmitted in the empty sample cell and the result is taken as I 0 , then measured the intensity I x transmitted a distance x in sample solution with different concentration and temperature. Finally, it is straightforward to invert Eq. (6) and find the attenuation coefficient ()   , the concentration influence coefficient C  and the temperature influence coefficient T  .  Pure water is most transparent to the visible laser, especially, to green-blue laser of wavelength in the range 470-560 nm. Absorption of infrared laser in pure water is evident. The absorption coefficients of green laser (532 nm) and infrared laser (1064 nm) are 0.0355 and 34.2438m -1 respectively. It is shown that the attenuation of green laser in the electrolyte is mainly due to scattering; however, infrared laser is absorbed in electrolyte compared with pure water. The infrared laser attenuation coefficients in various concentrations of electrolyte are higher than 34.5m 1. Thus the preferred laser in JECM-LD is the green laser or ultrashort infrared laser with high power density.

Measurement of mechanical effects during pulsed laser and metals interaction in neutral solution
JECM-LD can be added on purpose to gain better results: to avoid redeposition of debris, to cool the material, or to increase plasma pressure. The improvement of plasma pressure is a major physical phenomenon, which is the reason of mechanical effect during JECM-LD.

Principle of test on mechanical of laser in neutral solution
As shown in Fig.5, a test setup is comprised of optical parts and measuring parts. A pulsed Nd: YAG laser was used operating at two wavelengths of 1064nm and 532nm. The neutral solution is sodium nitrate solution and the concentration is 20%. The focal length of lens is 100mm. The thickness of metal target is 0.5mm. The metal target is fixed on the base in quartz glasses tank and forms a cantilever. A piezoelectric accelerometer is fixed on the free end of the cantilever to detect vibration acceleration signals. These signals are output on the oscilloscope through a charge amplifier.
When a pulsed laser is focused on axis position of the cantilever, the laser pulse will produce an instant pressure on the target. Then the cantilever makes a lateral vibration, as shown in Fig.6. The piezoelectric accelerometer is comprised of piezoelectric element, mass, preload spring, et al. It has the same vibration as the cantilever, and then the piezoelectric element is loaded with an inertia force of the mass. According to Newton's Second Law, the inertia force is written as, Where F is inertia force, a is acceleration.
According to vibration theory, the cantilever can be seen as a freedom vibration system with a single degree, the movement of free end can be describe as, initial phase angle. The acceleration of free end can be obtained by the second differential equation (8) Initial amplitude 0 A is the maximum deformation of the cantilever. According to mechanics of materials, 0 A should be written as, Where E is modulus of elasticity, I is moment of inertia. Substitution of equation (11) According to the piezoelectric effect, the charge generated by the piezoelectric element has a certain proportion to the acceleration of the free end, Where, q S is charger sensitivity. According to theory of circuit, the output of the charge amplifier is calculated as function of charge q with the following equation, Where, f C is feedback capacitance. Substitution of equation (13) and (14) into equation (15) yields, Therefore, the value of signal on oscilloscope can be used to calculate the impact force by the laser and metal interaction. www.intechopen.com much higher pressure plasma is formed through the interaction between the laser pulse and metal under neutral solution than in air. The mechanical effect during pulsed laser and stainless target interaction in neutral solution is more than three times higher than in air.

Measurement and results
It is also shown that the pressure of the target with infrared laser is decreased with the distance of laser propagation in neutral solution. However, the effect with green laser is stable in the laser transmission range of 20mm. The most probable reason for this is infrared laser is absorbed in neutral solution, but green laser is a case of low absorption. Therefore, the green laser is more suitable for JECM-LD.

Theoretical analysis
Theoretical analysis of JECM-LD is performed by changing machining conditions. There are two conditions in JECM-LD: One is an electrolyte-jet-guided focused pulsed laser with a pulse width of 0.2ms and a frequency of 5Hz. The other is only an electrolyte during the inter-pulse. Fig.9 presents the main idea for the theoretical analysis. Fig. 9. Schematic diagram of the energy to be used for JECM-LD As the laser pulse width constitutes only one percent of the inter-pulse and the electrical energy is much lower than laser energy in pulse width, the main process energy E is the heat energy E I of pulsed laser during the pulse width  . In contrast, the process energy E of the inter-pulse (1/f-) is the electrical energy E U of electrical charge flow. The processes comprising both laser drilling and jet electrochemical machining unceasingly proceeds during JECM-LD, so the theoretical model of JECM-LD should embrace the two kinds of reactions.

Mathematical model of electrolyte-jet-guided laser drilling
Since the electrolyte-jet-guided laser drilling is quite different from laser drilling in air, the following assumptions should be made pertinent to the effects of the electrolyte jet and the temperature dependent material properties: 1. The melting temperature of the material under study is the maximum temperature the work-piece can reach. 2. The material possesses constant thermal properties.

Energy
www.intechopen.com 3. The evaporated material does not interfere with the laser beam, and the scattering of laser radiation within the hole can be neglected.
Suppose that the top surface of the workpiece made of stainless plate is located at the plane z = 0 and the plate is exposed at time t = 0 to the electrolyte jet guided laser. Fig.10 schematically illustrates the cylindrical coordinate system.
Where  is the material density, c the heat capacity, t the machining time, k the heat conductivity and Q the laser heat source term. In the cylindrical coordinate system, Eq. (1) can be written into As the heat loss due to convection and radiation would take place at the top workpiece surface resulting from the cooling by the electrolyte jet, the boundary condition can be expressed by Where c h is the convection coefficient, a T the ambient temperature and  the Stephan-Boltzman constant.
The scraps of the workpiece would be carried away in molten state by the electrolyte jet whenever and wherever the material has reached the melting temperature. Thus, for this model, the isotherm line, which represents the melting temperature of the material, can be considered as the profile of the hole during electrolyte-jet-guided laser drilling.

Mathematical model of jet electrochemical machining
During the inter-pulse, with the ending of pulsed laser beam, the electrolyte jet creates a contactless electrode and has electrochemical reactions onto the workpiece-anode. In this way, the recast layers and spatters have been effectively removed by the electrochemical reactions.
Based on the cylindrical coordinate system of the model of electrolyte-jet-guided laser drilling, a two-dimensional model for JECM can be developed as follows: In order to model the shaping process with a free jet, the following assumptions should be made (see Fig.11).
1. The top boundary is the shape formed by the preceding electrolyte-jet-guided laser drilling. 2. The cross-section of the jet remains constant along the whole length of the jet. 3. The diameter of the electrolyte jet is assumed to be the same as that of the nozzle. 4. The jet is axisymmetric, thus the system can be described as 2D in cylindrical coordinates     The current density a i on the anode depends on the distribution of electric potential, u, in the electrolyte, and can be determined from the following Ohm's Law: Where  is the electrolyte electrical conductivity and a E the electric field intensity at anode given by a a Eu   Where u is the electrical potential across the gap.
In the case of JECM, the potential between the electrodes, u, can be described by With the inter-pulse ending, the reaction of JECM comes to a halt and the result of the model becomes the renewed boundary for the next pulse laser drilling. Thus the two parts of JECM-LD model alternatively work during machining.

Experimental apparatus
As shown in Fig.12, the JECM-LD apparatus consists of three systems, namely, a laser system, an electrolyte supplying system and a power unit. A stream of electrolyte pumped out of the reservoir flows into the jet cell and meantime joins with a coaxial focused laser beam to form a jet 0.5mm dia. The jet together with the beam strikes against one spot on the workpiece surface. By means of the pump, the pressure of the electrolyte jet can be adjusted within the range of 0-1.5Mpa.
A jet cell with an annular cavity is used to obtain a stable electrolyte jet and slim down the transmitting distance of laser in electrolyte. Fig.13 shows the construction of a jet cell.  Fig. 13. Structure of a jet cell

Experimental parameters
As a pulse Nd:YAG laser at second harmonic wavelength, it has parameters as follows: wavelength of 532nm; pulse length of 0.2ms; frequency of 5Hz and energy per pulse in the range of 0-300mJ. The workpiece is made of 321 stainless steel 0.5mm thick. Table1 lists the material properties.
A DC power unit with a working voltage in the range of 0-50V and current 5A is adopted in the experiments. The electrolyte from the reservoir on the manifold enters a plunger pump, where it is pumped out at a pressure of 1.5MPa. (m )  . The volume electrochemical equivalent of 321 stainless steel is 2.1×10 -9 3 m/ ( Am i n )  . The current efficiency is about 0.6. The distance between the nozzle and the workpiece surface is 0.8mm.

Simulation results
The model of JECM-LD includes two sub-models at different machining stages. One is the sub-model of electrolyte-jet-guided laser drilling and the other of jet electrochemical machining. With the machining stage changing, the simulation is computed with different sub models and the results are summarized step by step.  .14 illustrates the shape of the vertical cross-section along the hole-axis from the simulation and from the experiment, which is plotted by means of the three-dimensional profilemeter (MicroXAM, ADE, USA). The comparison of Fig.14 displays a good agreement between the simulated and experimental results in terms of both the hole depth and the shape of the cross-section. However, a noticeable difference exists between the bottom shapes of the simulated cross-section and the experimental one. The former contrasts markedly with the latter by its greater flatness. This might be attributed to the more serious attenuation of laser energy by the scattering and absorption of electrolyte jet.
Therefore, the feasibility of the two-dimensional model is evidenced by the satisfied compliance of the simulated results with the experimental ones.

Experimental results
In order to make comparison with the laser drilling in air, a millisecond second green laser was used for JECM-LD to work out penetrated holes. The energy per pulse was 200mJ, and the electrochemical machining voltage 40V. The optical microscope was used to examine the experimental results.
As can be seen from Fig.15, for the holes laser-drilled in air, the surfaces both at the entrance and the exit are all encircled by a large irregular area comprising spattering deposits and re-solidified molten layers. In contrast, from Fig.16, it is clear that few spatters could be unveiled on both sites around the hole drilled with JECM-LD. This may be chiefly ascribed to the high-speed electrolyte jet, which effectively cools the material to be processed and discharge scraps. Fig.16 also exhibits better surface quality and a smoother hole periphery, however, an obvious annular electrochemical overcut can be observed at the entrance surface.   Fig.15 and Fig.16. Unlike the laser drilling in air(see Fig.17(a)), neither spatters nor recast layers could be discovered on the periphery of the hole drilled with JECM-LD (see Fig.17 (b)). This might be attributed to the electrolyte jet, which rids the recast layers of the surface to be processed.
www.intechopen.com  The typical cross-sectional view of holes machined with JECM-LD and laser drilling in air are shown in Fig.18. It is shown that there is an obvious boundary between the recast and substrate in Fig.18(a). The thickest recast layer is about 55μm. It is confirmed that recast is an inherent defect associated with holes produced with laser drilling. As can be seen in Fig.18(b), neither boundary nor recast layer can be seen on the sidewalls of hole machined with JECM-LD. There are only few of limited left melt debris adhering to the sidewalls. With comparing the Fig.18(a) and Fig.18(b), the new hybrid process really reduce the recast more than 90% on line.
As the most popularized criterion for defining the cutting efficiency in laser drilling, Material Removal Rate (for short MRR) can be defined as    Fig.19, it can be seen that MRR increases with the energy per pulse rising. Since a portion of the laser beam has been absorbed and scattered by the electrolyte, the MRR of JECM-LD appears lower than the laser drilling in air, the efficiency of JECM-LD with millisecond green laser is about 70% of laser drilling in air. Fig.20 shows that MRR remains almost un-changed with the electrochemical machining volt-age increasing. This is because of the insignificant anodic dissolution provided that the voltage is low and the working distance long. It stands to reason that laser beam takes the www.intechopen.com chief responsibility for removing scraps from the processed surfaces during JECM-LD, and the jet electrochemical machining plays an auxiliary part in reducing the recast layers and spatters.

Conclusions
This work has developed and investigated a novel hybrid process of laser drilling assisted with jet electrochemical machining to improve the quality of laser-drilled holes. The main conclusions can be summarized as follows: 1. The attenuation of green laser in the electrolyte mainly is scattering and the infrared laser is absorbed in electrolyte. 2. Mechanical effect during pulsed laser and stainless target interaction in neutral solution is more than three times higher than in air. 3. Able to effectively reduce the recast layers and spatters, the JECM-LD has an annular electrochemical overcut left behind surrounding the entrance surface after the completion of the process. 4. The laser beam takes the chief responsibility to remove the scraps material during JECM-LD while the jet electrochemical machining serves the auxiliary function of eliminating the recast layers and spatters. 5. The efficiency of JECM-LD with millisecond green laser declines to about 70% of the laser drilling in air.
It is evidenced that the JECM-LD that combines the laser drilling and jet electrochemical machining is on the position to obtain high machining quality with reduced recast layers and spatters. Having great potentiality in eradicating the defects inherent in laser-drilled holes, the hybrid process for hole drilling is expected to become a versatile tool finding broader applications in aerospace and aircraft industries.