Process Anatomy for High Aspect Ratio Micro-Hole Drilling with Short MicroSecond Pulses Using a CW Single-Mode Fiber

The objective of this paper is to establish a detailed process anatomy of a micro-hole drilling process using a CW single-mode 300 W fiber laser with short micro-second pulses. For the drilling process with a one micro-second laser pulse, the process started with drastic evaporation and concluded with melt ejection approximately 150 micro-seconds later. It was concluded that two distinct hole drilling mechanisms occurred, one as adiabatic evaporation by the high power initial spike of the laser beam and one as melt ejection by the low power laser energy deposition. The drilling process time line was established based on the measurements by several in-process sensors, such as photodiodes, a high-speed camera, and a spectrometer. The theoretical modeling zoomed in the early stage of the process to investigate the hole formation which could not be observed experimentally. Both experimental and theoretical results were then compared to determine the laser-material interaction mechanisms among three media involved in the process: laser, material, and vapour/plasma. Finally, a series of temporal anatomy diagrams, denoted as process anatomy, were presented to describe the entire drilling process, including process temperature, laser energy deposition, hole formation, and material removal mechanisms.


Introduction
Laser drilling is an important industrial process to produce various sizes of holes for critical applications, such as cooling holes in turbine components.Typical laser pulse durations for laser drilling are in the nano-, pico-and femto-second ranges.Ultra-short-pulse lasers operate in the femto-second (10 -15 sec) (fs) or pico-second (10 -12 sec) (ps) ranges to produce peak power density in the range of 10-1,000 GW/cm 2 .The holes produced with ultra-short pulses usually exhibit a clean finish because melting is not significant.However, the Material Removal Rate (MRR) is usually very low, for example, in the range of 10-200 nm/pulse for steels with a Ti: Sapphire femto-second laser (Bauerle, 2000).Breitling et al. (2004) presented fundamental aspects in machining of metals with short and ultra-short laser pulses.Mao et al. (2007) presented detailed high speed photography images of laser ablation mechanisms using short and ultra-short laser pulses.The micro-hole drilling process using nanosecond (10 -9 sec) (ns) pulses usually produces holes in metal with acceptable quality, but, in general, worse than those by ultra-short lasers because melting is more significant.The material removal rate is usually in the order of 1-10 μm/pulse.The power density of these lasers is in the range of GW/cm 2 .
Laser drilling using long pulses in the range of several hundred microseconds (10 -6 sec) to several milliseconds (10 -3 sec) rely on mostly on melting to remove material because their power densities typically are below 10 7 W/cm 2 .The hole quality is usually low and the hole diameter is large due to significant melting.For example, in the experiments presented in Low and Li (2002) and Low et al. (2004), a Q-switiched 400W Nd:YAG laser was used to produce a pulse at 1.0 ms, with pulse energy up to 7 J and peak power over 7 kW.Using multiple pulses and with O 2 as the assist gas, it could drill through a 2.5 mm stainless steel plate with a hole diameter approximately 600 μm.Similarly, Pandey et al. (2006), with a similar laser, conducted laser drilling with pulse durations ranging from 490 to 890 μs, pulse energy 0.5 J, and argon gas as the assist gas.Kayukov et al. (1998) applied a single pulse from an Nd:YAG laser for up to 40 J in energy and 20 ms in pulse duration, with 3-5 kW peak power, to drill blind holes in different metals.For chromium steel, they achieved 6.5 mm deep blind hole with a single pulse at 17.5 J with a power density below 10 6 W/cm 2 .The blind hole demonstrated a large opening similar to a nail head shape over 1 mm in diameter and an average waist diameter about 300 μm.
In summary, the laser drilling process can be evaporation dominated or melting dominated depending on the pulse duration.In comparison, the body of work regarding short microsecond laser drilling/ablation is small.It is expected that the evaporation and melting both play an important role in the short microsecond drilling process.It is the objective of this paper to establish the drilling mechanisms and the temporal characteristics of the short microsecond drilling process, to be depicted in a series of diagrams, denoted as process anatomy.
This paper is organized as follows.First, the time line of the short microsecond drilling process is established based on the measurements of several in-process sensors, including photo-diodes, a high speed camera, and a spectrometer.Theoretical modeling is then used to zoom in the early hole formation stage which cannot be observed experimentally.Both experimental and theoretical results are then compared to determine the laser-material interaction mechanisms.Finally, a series of temporal anatomy diagrams, denoted as process anatomy, are presented to describe the entire drilling process by a one micro-second laser pulse.The body of a manuscript opens with an introduction that presents the specific problem under study and describes the research strategy.Garnov et al. (2004) performed laser ablation using pulse durations between 150 ns and 4.5 μs on samples of stainless steel, aluminum, alumina ceramics, and graphite with a Q-switched Nd:YAG laser that used an optical fiber as part of the resonator to produce the desired pulse length.They reported an MRR at 11 μm/pulse with a power density of 20 MW/cm 2 and pulse duration of 4.5 μs for stainless steel.

Review of Short Microsecond Pulse Drilling
The advance of laser technology in the last decade has produced many high power lasers with very high beam quality.For example, Trippe et al. (2004) used a Q-switched Nd:YAG slab laser to produce pulses between 30 μs and 150 μs with power densities up to 220 MW/cm 2 .With a single pulse, they drilled holes in steel with diameters between 80 μm and 120 μm and depths from about 120 μm to 700 μm with argon gas.Their experimentally determined drilling speed was reported as 6 m/s.They also presented a dimensionless 2D model of free boundary problem and they computed the time for vaporization to be approximately 100 nanoseconds.Kreutz et al. (2007) applied this drilling technique to drill cooling holes in turbine components with multiple pulses (percussion drilling) combined with oxygen, helium, and argon assist gasses.Walther et al. (2008) combined the above Nd:YAG slab laser and the DPSS Nd:YAG laser to achieve very high aspect ratio percussion drilling (5 mm through holes with 170-180 μm waist diameter).Walther et al. (2008) utilized a flash lamp pumped Nd:YAG slab laser (FM015, LASAG) with a beam quality as M 2 = 2 and it can produce pulses with durations from 100 to 500 μs and a laser energy of 0.64 J.This excellent beam quality allows a focus spot size 45 μm in diameter, producing a power density of 220 MW/cm 2 , as compared with M 2 = 22-38, 600 μm spot size, and approximately 3 MW/cm 2 for the laser used in Low and Li (2002) and Voisey et al. (2002).Walther et al. (2008) also utilized a diode-pumped solid state (DPSS) Nd:YAG laser (Powergator 1064, Lambda Physik) which has a beam quality of M 2 = 1.7, with a spot size of 42 μm, 17 ns pulse, and 1.8 mJ pulse energy, producing a power density over 20 GW/cm 2 .
Most interestingly, the development of high power, single-mode fiber laser has established a new level of beam quality.For example, Harp et al. (2008) conducted micro-hole drilling with a 300 W, CW, Yb-doped fiber laser (YLR-300, IPG), which has a near perfect beam quality M 2 =1.04 and the beam can be focused down to 10 μm with a 100 mm lens.It can also be modulated to produce pulses from 1 μs to any length of pulse duration.The modulated pulses all contain an initial spike at 1,500 W, followed by a constant power at 300 W, to reach a peak power density of 1.9 GW/cm 2 and 380 MW/cm 2 at the steady state.In comparison with ns-, ps-, and fs-lasers, the peak power of this modulated fiber laser pulse is low but its excellent beam quality allows for tight focusing to reach very high power density.
The feasibility of laser drilling with such a CW laser via modulation control was demonstrated by Harp et al. (2008), in which pulses from 15-40 μs were used to produce holes with diameters from 40 to 100 μm and depths from 25 to 50 μm.This work indicated that, by using high beam quality lasers, micro-hole drilling with pulse durations in the short microsecond range (1-10 μs) has become feasible for applications which require moderate material removal depth per pulse, narrow opening (due to much smaller beam spot sizes), and moderate hole quality (due to moderate melt ejection).
In this paper, we focus on a single pulse drilling process of blind holes on a stainless steel substrate with pulse    1, we can conclude that a hot vapour/plasma over 16,000 K and as high as 71,000 K did exist in the first 1 μs of the process during the laser initial spike.Note that even at the highest temperature estimated in Table 1, the vapour/plasma was still optically thin with respect to the fiber laser wavelength of 1075 nm.Therefore, the study of Table 1 also allows us to conclude that the laser beam did not heat the vapour/plasma and nearly all laser energy was used to either evaporate or to melt the material.The vapour temperature measured for the steady state welding condition at 300 W indicated that a weaker vapour at temperature between 7,000 to 9,000 K could exist at the later stage of the drilling process.

Melt Ejection and the Expanding Vapour/Plasma
The existence of a high temperature vapour/plasma indicates that its pressure would be high as well.Because the micro-hole was covered by this high temperature and high pressure plasma, the melt is likely to be superheated in the early stage of the drilling process.As the vapour/plasma cooled down to weaker vapour (after at least 24 microseconds), the pressure also dropped.The drop of pressure could cause the superheated melt boil and to eject as fine droplets as seen in the high speed images of Figure 4.

The Hole Formation Simulation during the First One Microsecond
Figures 3 and 4 provide the timeline of this drilling process; however, the actual hole formation could not be observed directly.Therefore, computer simulation based on a 3D finite-element thermo-hydrodynamic model was used to study the hole formation due to evaporation by the initial laser spike.

3D Finite-Element Thermo-Hydrodynamic Model
This 3D finite-element based thermo-hydrodynamic model accounts for the effect of multiple reflections inside the cavity, the movement of the liquid-vapour interface in time, the effect of recoil pressure on the liquid-vapour interface and the mass loss by evaporation during the drilling process (Ohmura & Noguchi, 2010).
The model contained the continuity, Navier-Stokes and energy equations: where v was the velocity vector of the material in the molten state, ρ was the density of the material, p was the pressure, μ is the material viscosity, F comprised the body force vector for all the recoil pressure force and the surface tension force at work on an element of material, H was the enthalpy, k was the thermal conductivity, c p was the specific heat and w is the source term which was the heat input into the system by the laser.
The computational domain (Figure 5) was a 3D rectangular domain with the z axis parallel to the axis of propagation for the laser beam and the x-y plane parallel to the material surface.The bounds of the domain are: x(-42 m, +42 m), y(-42 m, +42 m), z(-180 m, +30 m).The plane z = 0 was considered the surface of the material and the point (0, 0, 0) was the center of the laser beam where it irradiated at the surface (see Figure 5, indicated by a blue dot).The whole domain is at the room temperature as the initial condition and the boundaries of the domain remain at the room temperature during the simulation.The laser beam was considered to be Gaussian in space.A triangular function was used to approximate the actual laser pulse shape (the red profile in Figure 1) so that only the effect of the initial spike was simulated.The conditions at the material boundaries were considered to be insulated with zero flow velocity.
The model used a volume of fluid (VOF) method for handling of the liquid-vapour interface by computing the volume fraction of liquid, F, in each element.The range of F was from 0 to 1, with a value of 0.5 indicating that the element was an interface element.As the interface position was calculated, the normal vectors to the surface were determined and used to compute the angle of reflection for the surface.Using the Fresnel equation, the reflection coefficient was determined for each surface and the amount of reflected energy was calculated.It was also assumed that, when the surface material was vaporized, the vaporized mass carried energy with it and traveled away from the surface.The amount of mass vaporized was calculated using an energy balance along the surface of the material.
where m v was the mass of the vaporized material, Q was the laser power absorbed at the surface area S and L v was the latent heat of vaporization.
As the vaporized mass traveled away from the surface, it generated a recoil pressure on the surface.To calculate the recoil pressure generated by the vaporization of the material, the recoil pressure, p r , was determined as the mass evaporated per unit area per unit time multiplied by the velocity of the vapour: where v T was the vapour velocity at the edge of the Knudsen layer and was defined by Anisimov et al. (1971) as one-quarter of the square mean velocity: where T d was the surface temperature and m a is the atomic mass of the vapour.This generated recoil pressure created a force on the surface of the material and acted against the surface tension force.These two forces acted as surface forces, whereas the momentum equation (Equation 2) required the input to be body forces.To convert the surface forces to body forces a continuum surface force (CSF) model was used (Brackbill, 1992).
The pressure-correction method for the flow velocity-pressure coupling used was the Simplified Marker-And-Cell (SMAC) method by Harlow and Amsden (1971).The algorithm for the model involved a series of steps.The first step was to calculate the normal vectors for the surface.Next, the surface forces were converted to body forces through the CSF method and used in the momentum equation, which was solved with the continuity equation through the SMAC method.The volume liquid fraction was then determined for the elements and the multiple reflections were calculated for the walls of the hole.The energy absorbed at the     The laser power fell to its steady state (300 W) at 2.5 μs.The vapour/plasma intensity continued to fall but at a slower rate than that of the laser power.The intensity of the vapour/plasma was still high enough to be registered by the photodiodes, which indicated that its temperature was still high, so was its pressure.The melt produced by the laser at the cavity wall was likely super-heated because it was covered by the high temperature and high pressure vapour/plasma.The hole continued to deepen via mild evaporation and melting.
Stage 5 (2.5 μs ~ 10 μs) (Process Anatomy Diagram #4): Laser power dropped below 200 W and continued to drop to zero after 10 μs.At 2.8 μs, plasma had turned into weaker vapour, not detected by the horizontal photo-diode.The laser energy deposited at lower power could still produce some spurts of vapour/plasma inside the micro-hole registered by the steep angle photodiode.This part of laser energy deposited at lower power most likely produced more melt than vapour.The hole continued to deepen mostly via melting.
Stage 6 (10 μs ~ 80 μs) (Process Anatomy Diagram #4): No laser energy was deposited in this stage, but hot vapour still covered the micro-hole.This intensity of this hot vapour was too low to be measured by the photodiodes but was high enough to saturate the CCD sensor of the high speed camera (first four frames of Figure 4. Therefore, the temperature of the vapour should be much lower than the vapour/plasma observed in Stages #1-5 to render its pressure lower.The drop vapour pressure likely would cause the superheated melt to boil and eject outward as seen in frames #2-4 of Figure 4 and the 4 th process anatomy diagram.The hole likely stopped deepening and was partially filled with melt, ejecting upward and out. Stage 7 (80 μs ~ 150 μs): Vapour was no longer visible after around 80 μs.The last melt ejection was observed around 150 μs, which was the end of the drilling process.Some melt was re-solidified around the hole as recast.

Conclusion
This paper presented a single-pulse micro-hole drilling technique using short micro-second pulses by modulating a CW single-mode 300 W fiber laser.The capability of this drilling technique was demonstrated and several in-process monitoring experiments, such as photodiode measurements of the vapour intensity, high-speed photography, and spectroscopic plasma measurements, were used to identify the laser/material interaction mechanisms.For the drilling process with a 1-μs laser pulse, the process started with drastic evaporation and concluded with melt ejection approximately 150 μs later.It is concluded that two distinct drilling mechanisms occurred, one as adiabatic evaporation by the high power initial spike and one as melt ejection by the low power energy deposition.The time line of this drilling process was established experimentally based on measurements by several in-process sensors, such as photodiodes, a high-speed camera, and a spectrometer.The theoretical modeling zoomed in the early stage of the process to investigate the hole formation which could not be observed experimentally.Both experimental and theoretical results were then compared to determine the laser-material interaction mechanisms among three media involved in the process: laser, material, and vapour/plasma.Finally, a series of temporal anatomy diagrams, denoted as process anatomy, were presented to describe the entire drilling process by a one micro-second laser pulse.This process anatomy summarizes the laser/material interaction mechanisms, hole formation, material removal mechanisms, and vapour/plasma characterization of this short micro-second laser drilling process.

Fig
Fig pla by the photo-di of the plasma ayed over 200 nt of