Laser drilling
Encyclopedia
Manufacturers of turbine engines for aircraft propulsion and for power generation have benefited from the productivity of laser
s for drilling small (0.3–1 mm diameter typical) cylindrical holes at 15-90º to the surface in cast, sheet metal
and machined components. Their ability to drill holes at shallow angles to the surface at rates of between 0.3 to 3 holes per second has enabled new designs incorporating film-cooling holes for improved fuel efficiency
, reduced noise, and lower NOx and CO emissions.
Incremental improvements in laser process and control technologies have led to substantial increases in the number of cooling holes used in turbine engines. Fundamental to these improvements and increased use of laser drilled holes is an understanding of the relationship between process parameters and hole quality and drilling
speed.
and vaporization
(also referred to as ‘ablation
’) of the workpiece material through absorption of energy from a focused laser beam.
The energy required to remove material by melting is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is generally favored.
Whether melting or vaporization is more dominant in a laser drilling process depends on many factors, with laser pulse duration and energy playing an important role. Generally speaking, ablation dominates when a Q-switched Nd:YAG laser is used. On the other hand, melt expulsion, the means by which a hole is created through melting the material, dominates when a flashtube
pumped Nd:YAG laser is used. A Q-switched Nd:YAG laser normally has pulse duration in the order of nanosecond
s, peak power on the order of ten to hundreds of MW/cm2, and a material removal rate of a few micrometre
s per pulse. A flash lamp pumped Nd:YAG laser normally has a pulse duration on the order of hundreds of microsecond
s to a millisecond
, peak power in the order of sub MW/cm2, and material removal rate of ten to hundreds of micrometer
s per pulse. For machining processes by each laser, ablation and melt expulsion typically coexist.
Melt expulsion arises as a result of the rapid build-up of gas pressure (recoil force) within a cavity created by evaporation
. For melt expulsion to occur, a molten layer must form and the pressure gradient
s acting on the surface due to vaporization must be sufficiently large to overcome surface tension
forces and expel the molten material from the hole (Figure 1).
The “best of both worlds” is a single system capable of both “fine” and “coarse” melt expulsion. “Fine” melt expulsion produces features with excellent wall definition and small heat-affected zone
while “coarse” melt expulsion, such as used in percussion drilling and trepanning, removes material quickly.
The recoil force is a strong function of the peak temperature
. The value of Tcr for which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion. For instance, liquid expulsion from titanium
can take place when the temperature at the center of the hole exceeds 3780 K.
In early work (Körner, et al., 1996), the proportion of material removed by melt expulsion was found to increase as intensity increased. More recent work (Voisey, et al., 2000) shows that the fraction of the material removed by melt expulsion, referred to as melt ejection fraction (MEF), drops when laser energy further increases. The initial increase in melt expulsion on raising the beam power has been tentatively attributed to an increase in the pressure and pressure gradient generated within the hole by vaporization.
It is generally agreed that a better finish can be achieved if the melt is ejected in “fine” droplets. Figure 2 shows droplet size distribution of a Ni–based superalloy
drilled by a Nd:YAG laser under; 2.5 J / 0.5 ms, 1.4 J / 0.5 ms and 2.5 J / 0.9 ms pulse energy / duration. Generally speaking, droplet size decreases with increasing pulse intensity. This is due to the increased vaporization rate and thus a thinner molten layer. For the longer pulse duration, the greater total energy input helps form a thicker molten layer and results in the expulsion of correspondingly larger droplets.
hole drilling and the drilling process is transient. Kar and Mazumder (1990) extended the model to 2-D, but melt expulsion was not explicitly considered. A more rigorous treatment of melt expulsion has been presented by Ganesh, et al. (1997), which is a 2-D transient generalized model to incorporate solid, fluid, temperature, and pressure during laser drilling, but it is computationally demanding. Yao, et al. (2001) developed a 2-D transient model, in which a Knudsen layer is considered at the melt-vapor front, and the model is suited for shorter pulse and high peak power laser ablation.
(1)
where
is the absorbed laser intensity, β is the laser absorption coefficient depending on laser wavelength
and target material, and I(t) describes temporal input laser intensity including pulse width, repetition rate, and pulse temporal shape. k is the heat conductivity
, T is the temperature, z and r are distances along axial and radial directions, p is density
, v the velocity
, Lv the latent heat of vaporization. The subscripts l, v and i denote liquid phase, vapor phase and vapor-liquid interface, respectively.
If the laser intensity is high and pulse duration is short, the so-called Knudsen layer
is assumed to exist at the melt-vapor front where the state variables undergo discontinuous changes across the layer. By considering the discontinuity across the Knudsen layer, Yao, et al. (2001) (Figure 3) simulated the surface recess velocity Vv distribution, along the radial direction at different times, which indicates the material ablation rate is changing significantly across the Knudsen layer.
pv, the melt layer flow and melt expulsion can be modeled using hydrodynamic equations (Ganesh et al.,1997). As seen in Figure 4, melt expulsion occurs when the vapor pressure is applied on the liquid free surface which in turn pushes the melt away in the radial direction. In order to achieve fine melt expulsion, the melt flow pattern needs to be predicted very precisely, especially the melt flow velocity at the hole’s edge. Thus, a 2-D axisymmetric transient model is used and accordingly the momentum
and continuity equations used.
Ganesh’s model for melt ejection is comprehensive and can be used for different stages of the hole drilling process. However, the calculation is very time consuming and Solana, et al. (2001), presented a simplified time dependent model that assumes that the melt expulsion velocity is only along the hole wall, and can give results with a minimum computational effort.
As shown in Figure 5, the liquid will move upwards with velocity u as a consequence of the pressure gradient along the vertical walls, which is given in turn by the difference between the ablation pressure and the surface tension divided by the penetration depth x.
Assuming that the drilling front is moving at a constant velocity, the following linear equation
of liquid motion on the vertical wall is a good approximation to model the melt expulsion after the initial stage of drilling.
(2)
where p is the melt density, μ is the viscosity
of the liquid, P(t)=(ΔP(t)/x(t)) is the pressure gradient along the liquid layer, ΔP(t) is the difference between the vapor pressure Pv and the surface tension
.
Grad and Mozina (1998) further demonstrated the effect of pulse shapes. A 12 ns spike was added at the beginning, middle, and the end of a 5 ms pulse. When the 12 ns spike was added to the beginning of the long laser pulse, where no melt had been produced, no significant effect on removal was observed. On the other hand, when the spike was added at the middle and the end of the long pulse, the improvement of the drilling efficiency
was 80 and 90%, respectively. The effect of inter-pulse shaping has also been investigated. Low and Li (2001) showed that a pulse train of linearly increasing magnitude had a significant effect on expulsion processes.
and experimental methods to better understand and control the laser drilling process. The result is higher quality and more productive processes that in turn lead to better end products such as more fuel efficient and cleaner aircraft
and power generating turbine engines.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s for drilling small (0.3–1 mm diameter typical) cylindrical holes at 15-90º to the surface in cast, sheet metal
Sheet metal
Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Countless everyday objects are constructed of the material...
and machined components. Their ability to drill holes at shallow angles to the surface at rates of between 0.3 to 3 holes per second has enabled new designs incorporating film-cooling holes for improved fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
, reduced noise, and lower NOx and CO emissions.
Incremental improvements in laser process and control technologies have led to substantial increases in the number of cooling holes used in turbine engines. Fundamental to these improvements and increased use of laser drilled holes is an understanding of the relationship between process parameters and hole quality and drilling
Drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...
speed.
Theory
Following is a summary of technical insights about the laser drilling process and the relationship between process parameters and hole quality and drilling speed.Physical phenomena
Laser drilling of cylindrical holes in turbine engine components generally occurs through meltingMelting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
and vaporization
Vaporization
Vaporization of an element or compound is a phase transition from the liquid or solid phase to gas phase. There are three types of vaporization: evaporation, boiling and sublimation....
(also referred to as ‘ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
’) of the workpiece material through absorption of energy from a focused laser beam.
The energy required to remove material by melting is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is generally favored.
Whether melting or vaporization is more dominant in a laser drilling process depends on many factors, with laser pulse duration and energy playing an important role. Generally speaking, ablation dominates when a Q-switched Nd:YAG laser is used. On the other hand, melt expulsion, the means by which a hole is created through melting the material, dominates when a flashtube
Flashtube
A flashtube, also called a flashlamp, is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end and are filled with a gas that, when triggered, ionizes...
pumped Nd:YAG laser is used. A Q-switched Nd:YAG laser normally has pulse duration in the order of nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s, peak power on the order of ten to hundreds of MW/cm2, and a material removal rate of a few micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s per pulse. A flash lamp pumped Nd:YAG laser normally has a pulse duration on the order of hundreds of microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s to a millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
, peak power in the order of sub MW/cm2, and material removal rate of ten to hundreds of micrometer
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
s per pulse. For machining processes by each laser, ablation and melt expulsion typically coexist.
Melt expulsion arises as a result of the rapid build-up of gas pressure (recoil force) within a cavity created by evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
. For melt expulsion to occur, a molten layer must form and the pressure gradient
Pressure gradient
In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...
s acting on the surface due to vaporization must be sufficiently large to overcome surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
forces and expel the molten material from the hole (Figure 1).
The “best of both worlds” is a single system capable of both “fine” and “coarse” melt expulsion. “Fine” melt expulsion produces features with excellent wall definition and small heat-affected zone
Heat-affected zone
The heat-affected zone is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding...
while “coarse” melt expulsion, such as used in percussion drilling and trepanning, removes material quickly.
The recoil force is a strong function of the peak temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. The value of Tcr for which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion. For instance, liquid expulsion from titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
can take place when the temperature at the center of the hole exceeds 3780 K.
In early work (Körner, et al., 1996), the proportion of material removed by melt expulsion was found to increase as intensity increased. More recent work (Voisey, et al., 2000) shows that the fraction of the material removed by melt expulsion, referred to as melt ejection fraction (MEF), drops when laser energy further increases. The initial increase in melt expulsion on raising the beam power has been tentatively attributed to an increase in the pressure and pressure gradient generated within the hole by vaporization.
It is generally agreed that a better finish can be achieved if the melt is ejected in “fine” droplets. Figure 2 shows droplet size distribution of a Ni–based superalloy
Superalloy
A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
drilled by a Nd:YAG laser under; 2.5 J / 0.5 ms, 1.4 J / 0.5 ms and 2.5 J / 0.9 ms pulse energy / duration. Generally speaking, droplet size decreases with increasing pulse intensity. This is due to the increased vaporization rate and thus a thinner molten layer. For the longer pulse duration, the greater total energy input helps form a thicker molten layer and results in the expulsion of correspondingly larger droplets.
Previous Models
Chan and Mazumder (1987) developed a 1-D steady state model to incorporate liquid expulsion consideration but the 1-D assumption is not suited for high aspect ratioAspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
hole drilling and the drilling process is transient. Kar and Mazumder (1990) extended the model to 2-D, but melt expulsion was not explicitly considered. A more rigorous treatment of melt expulsion has been presented by Ganesh, et al. (1997), which is a 2-D transient generalized model to incorporate solid, fluid, temperature, and pressure during laser drilling, but it is computationally demanding. Yao, et al. (2001) developed a 2-D transient model, in which a Knudsen layer is considered at the melt-vapor front, and the model is suited for shorter pulse and high peak power laser ablation.
Laser energy absorption and melt-vapor front
At the melt-vapor front, the Stefan boundary condition is normally applied to describe the laser energy absorption (Kar and Mazumda, 1990; Yao, et al., 2001). (1)where
is the absorbed laser intensity, β is the laser absorption coefficient depending on laser wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
and target material, and I(t) describes temporal input laser intensity including pulse width, repetition rate, and pulse temporal shape. k is the heat conductivity
Conductivity
Conductivity may refer to:*Electrical conductivity, a measure of a material's ability to conduct an electric current*Conductivity , also the specific conductance, is a measurement of the electrical conductance per unit distance in an electrolytic or aqueous solution*Ionic conductivity, a measure of...
, T is the temperature, z and r are distances along axial and radial directions, p is density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, v the velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
, Lv the latent heat of vaporization. The subscripts l, v and i denote liquid phase, vapor phase and vapor-liquid interface, respectively.
If the laser intensity is high and pulse duration is short, the so-called Knudsen layer
Knudsen layer
The Knudsen layer, also known as evaporation layer, is the thin layer between liquid and vapour. It is named after Danish physicist Martin Knudsen .-Definition:...
is assumed to exist at the melt-vapor front where the state variables undergo discontinuous changes across the layer. By considering the discontinuity across the Knudsen layer, Yao, et al. (2001) (Figure 3) simulated the surface recess velocity Vv distribution, along the radial direction at different times, which indicates the material ablation rate is changing significantly across the Knudsen layer.
Melt expulsion
After obtaining the vapor pressureVapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
pv, the melt layer flow and melt expulsion can be modeled using hydrodynamic equations (Ganesh et al.,1997). As seen in Figure 4, melt expulsion occurs when the vapor pressure is applied on the liquid free surface which in turn pushes the melt away in the radial direction. In order to achieve fine melt expulsion, the melt flow pattern needs to be predicted very precisely, especially the melt flow velocity at the hole’s edge. Thus, a 2-D axisymmetric transient model is used and accordingly the momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
and continuity equations used.
Ganesh’s model for melt ejection is comprehensive and can be used for different stages of the hole drilling process. However, the calculation is very time consuming and Solana, et al. (2001), presented a simplified time dependent model that assumes that the melt expulsion velocity is only along the hole wall, and can give results with a minimum computational effort.
As shown in Figure 5, the liquid will move upwards with velocity u as a consequence of the pressure gradient along the vertical walls, which is given in turn by the difference between the ablation pressure and the surface tension divided by the penetration depth x.
Assuming that the drilling front is moving at a constant velocity, the following linear equation
Linear equation
A linear equation is an algebraic equation in which each term is either a constant or the product of a constant and a single variable....
of liquid motion on the vertical wall is a good approximation to model the melt expulsion after the initial stage of drilling.
(2)where p is the melt density, μ is the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of the liquid, P(t)=(ΔP(t)/x(t)) is the pressure gradient along the liquid layer, ΔP(t) is the difference between the vapor pressure Pv and the surface tension
.Pulse shape effect
Roos (1980) showed that a 200 µs train consisting of 0.5 µs pulses produced superior results for drilling metals than a 200 µs flat shaped pulse. Anisimov, et al. (1984) discovered that process efficiency improved by accelerating the melt during the pulse.Grad and Mozina (1998) further demonstrated the effect of pulse shapes. A 12 ns spike was added at the beginning, middle, and the end of a 5 ms pulse. When the 12 ns spike was added to the beginning of the long laser pulse, where no melt had been produced, no significant effect on removal was observed. On the other hand, when the spike was added at the middle and the end of the long pulse, the improvement of the drilling efficiency
Efficiency
Efficiency in general describes the extent to which time or effort is well used for the intended task or purpose. It is often used with the specific purpose of relaying the capability of a specific application of effort to produce a specific outcome effectively with a minimum amount or quantity of...
was 80 and 90%, respectively. The effect of inter-pulse shaping has also been investigated. Low and Li (2001) showed that a pulse train of linearly increasing magnitude had a significant effect on expulsion processes.
Conclusion
Manufacturers are applying results of process modelingProcess modeling
The term process model is used in various contexts. For example, in business process modeling the enterprise process model is often referred to as the business process model. Process models are core concepts in the discipline of process engineering....
and experimental methods to better understand and control the laser drilling process. The result is higher quality and more productive processes that in turn lead to better end products such as more fuel efficient and cleaner aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
and power generating turbine engines.