- What is laser drilling?
- What are the applications of laser drilling?
- How does the laser drill process work?
- What are Nd: YAG lasers?
- How does melt expulsion work?
- What are the factors affecting melt-vapor front and laser energy absorption?
What is drilling?
Drilling refers to a cutting method used to produce a hole having a round cross-section in solids. The standard drill bit is usually a rotary cutting tool made of tough metals. As an alternative to standard drilling technique using the drill bit, there are different advanced methodology has been invented nowadays to cope up with industrial demands.
What is laser drilling?
Laser drilling refers to the method of forming thru-holes, also known as “percussion drilled” holes or “popped” holes by using lasers. This is obtained by focusing high power pulsed laser beam repeatedly on a material like metal, alloy, etc.
The holes generated by this process can have a diameter as small as 0.002 inches or 50 micrometers. The diameter of holes can be increased according to requirement by moving the laser around the circumference of the percussion drilled hole. This method is termed as “trepanning”.
Applications of laser drilling:
- Laser drilling is a technique that is capable of producing high-aspect-ratio holes or holes having a depth-to-diameter ratio of much more than 10:1.
- High-aspect-ratio holes drilled with the help of lasers are used for a number of different uses such as the aerospace turbine-engine cooling holes, oil gallery of some engine blocks, printed circuit board micro-vias, and laser fusion components.
- Turbine engine manufacturers for aircraft power generation and propulsion highly prefer the use of lasers for drilling small (having a diameter of 0.3–1 mm, generally) cylindrical holes on the surface of sheet metal, cast, and machined components at a temperature of 15–90°. Laser drilling can produce holes at very shallow angles to the material surface having a rate of 0.3 to 3 holes per second. This has allowed the formation of new designs including film-cooling holes for reduced noise, enhanced fuel efficiency, and lower CO and NOx emissions.
- Developments in the process of laser use and control technologies have led to a considerable increase in the number of cooling holes present in turbine engines. The increased use of laser-drilled holes depends upon a number of parameters like hole quality and drilling speed.
How does the laser drill process work?
Cylindrical holes are drilled with lasers usually by the process of vaporization or ablation and melting of the given material. This happens due to the absorption of energy provided by a focused laser beam. Melting a material requires an energy supply of approximately 25% of that needed for vaporizing the equal volume of material. For this reason, the process of melting is often preferred over the vaporization process.
For laser drilling, both melting or vaporization can be used depending on several factors. Duration of laser pulse duration and energy are important determining factors. Vaporization or ablation is preferred when a Q-switched Nd:YAG laser is used for laser drilling. When a flash tube pumped Nd:YAG laser is utilized, the process of melt expulsion that creates a hole through melting the material is favored.
Generally, a Q-switched Nd:YAG laser comes with
- A material removal rate of a few micrometers/pulse,
- Peak power in the range of some ten to hundreds of MW/cm2.
- A pulse duration in the range of a few nanoseconds,
On the other hand, a flashlamp-pumped Nd:YAG laser usually comes with
- A pulse duration on the range of hundreds of microseconds to a few milliseconds,
- Peak power in the range of sub MW/cm2.
- A material removal rate of ten to hundreds of micrometers/pulse.
The ablation and melt expulsion can simultaneously exist for machining processes by each laser. Melt expulsion occurs due to the swift build-up of gaseous pressure or recoil force inside a cavity formed by evaporation. A molten layer should generate and the gradients of pressure affecting the surface because of vaporization should be large enough to cross the surface tension force barriers and eject the molten material from the hole, for melt expulsion to occur.
Both fine and coarse melt expulsion can be provided by a single system are known as the “best of both worlds”. Fine melt expulsion forms material features with great wall definition and small heat-affected zones. Whereas, coarse melt expulsion is capable of removing materials quickly without much quality precision and is used in percussion trepanning and drilling.
Peak temperature highly influences the recoil force function. The surface tension and recoil forces are equal when the value of TCr is equal to the critical temperature in case of liquid expulsion. For example, liquid expulsion from titanium can occur when the temperature exceeds 3780K at the center of the hole.
What are Nd: YAG lasers?
Nd: YAG (neodymium-doped yttrium aluminum garnet) is a crystal lasing medium crystal used commonly in solid-state lasers. The neodymium atoms Nd(III) are triply ionized and acts as the dopant. The Nd(III) dopant ion replaces about 1% of yttrium ions in the yttrium aluminum garnet (YAG) crystal structure. This is possible because the two ions are comparable in terms of size. Similar to red chromium ions in a ruby laser, neodymium Nd(III) ion is responsible for providing the lasing action in the Nd: YAG laser.
How does melt expulsion work?
Melt expulsion and melt layer flow are obtained by using the hydrodynamic equations after finding the vapor pressure. When the vapor pressure is supplied to the liquid free surface, melt expulsion takes place. This drives the melted material away in the radial direction. For obtaining very fine melt expulsion, the melt flow pattern especially the velocity of melt flow at the edge of the hole needs to be precisely calculated.
What are the factors affecting melt-vapor front and laser energy absorption?
Laser energy absorption coefficient mainly depends on:
• Wavelength of the laser used.
• Type and composition of the target material.
When the intensity of the laser is high and the duration of the pulse is short, the state variables (including ablation rate) of the material at the melt-vapor front are seen to experience discontinuous changes across the material layers.