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Laser Cutting

New to laser cutting? Consider this.

According to ISQ Directory:

“Chapter 1: What is Laser Cutting?

“Laser cutting is a non-traditional machining method that uses an intensely focused, coherent stream of light called lasers to cut through the material. This is a type of subtractive machining process where the material is continuously removed during the cutting process. This is done through either vaporization, melting, chemical ablation, or controlled crack propagation. The laser optics is digitally controlled by a CNC (Computer Numerical Control) making the process suitable for drilling holes as small as 5 microns. Moreover, the process does not produce residual stresses on the material allowing the cutting of fragile and brittle materials.

“Laser drilling is a type of laser machining process that is done by several methods, including single-shot drilling, percussion drilling, trepanning, and helical drilling. Single-shot and percussion laser drilling produce holes at a higher rate than the other two processes. Trepanning and helical drilling, on the other hand, produce more accurate, higher quality holes.

“Aside from the accuracy of the process, there are other advantages offered by laser cutting. Since there are no cutting tools used, the non-contact nature of lasers produces no tool wear issues. High strength, brittle materials such as diamond tools and refractory ceramics. The first production laser cutting was introduced in 1965 and was used to drill holes in diamond dies. Laser cutting technology was then used for cutting high strength alloys and metals such as titanium for aerospace applications. Its range of applications also covers the cutting of polymers, semiconductors, gems, and other metallic alloys.

Chapter 2: Laser Cutting Theory and Working Principle

“Laser stands for “light amplification by stimulated emission of radiation”. Aside from the cutting applications of lasers, they can also be used for joining, heat treating, inspection, and free form manufacturing. Lasers used for laser cutting differ from other machining processes since it requires higher power densities but with shorter interaction times.

“Lasers are produced by generating light from a high-intensity light source inside a reflective laser cavity. The laser cavity contains a laser rod where the radiation is generated. The light source is used to stimulate the laser rod which is composed of atoms of a lasing media that absorbs certain wavelengths of light from the light source. From physics, it is known that light is composed of small bundles of energy called photons. As photons strike the atoms of the lasing media, the atoms become energized. When another photon strikes the energized atom, the atom gives off two more photons with the same wavelength, direction, and phase. This is called stimulated emission. The new photons further stimulate other energized atoms producing more photons, causing a cascade of excitations. Two parallel mirrors are located on both ends of the laser rod. Photons moving perpendicular to these mirrors stay within the laser rod. One mirror is partially transmissive, enabling the partial escape of light from the cavity. This escaping stream of coherent, monochromatic light is the laser beam used to cut the material. Another set of mirrors or fiber-optics direct light into a lens. This lens focuses the light into the material.

“There are three main types of lasers used for cutting. These are CO2, Nd-YAG (Neodymium Yttrium-Aluminum-Garnet) lasers, and fiber-optic lasers. They differ on the base material used to generate the laser beam.

CO2 Lasers
“This type has a gas discharge lasing medium filled with 10 – 20% carbon dioxide, 10 – 20% nitrogen, traces of hydrogen and xenon, and helium for the balance. Instead of light, laser pumping is done by discharging an electrical current. When the electrical discharge passes through the lasing medium, nitrogen molecules become excited, bringing it to a higher energy level. Unlike what was described before, these excited nitrogen molecules do not lose their energy by photon emission. Rather, it transfers its vibrational mode energy to CO2 molecules. This process goes on continuously until most of the CO2 molecules are at the metastable state. The CO2 molecules then emit infrared light at either 10.6 µm or 9.6 µm which bring them to lower energy levels. The resonating mirrors are designed to reflect the emitted photons on those wavelengths. One mirror is a partially reflecting mirror allowing the release of the infrared beam that is used for cutting the material. After releasing infrared light, the CO2 molecules then return to the ground state by transferring its remaining energy to the doped helium atoms. The cold helium atoms then become hot which is cooled by the cooling system of the laser. The efficiency of a CO2 laser is around 30% which is higher than other lasers.

Crystal (Ruby, Nd, and Nd-YAG) Lasers
“Unlike the CO2 laser, this type is a solid-state laser that uses a synthetic crystal as a lasing medium. The most popular is the YAG (Y3Al5O12) crystal doped with 1% ionized neodymium (Nd3+). In this crystal, the Nd ions replace the Y ions in the crystal structure. The length of the rod is about 10 cm with a diameter of 6 to 9 cm. The ends of the YAG rod is polished and coated by highly reflective materials acting as the resonator system.

“Laser pumping is achieved by krypton flashlamps or laser diodes. This laser pumping excites the Nd ions into higher energy levels. After a short while, the excited Nd ions move into a lower, more stable state, without emitting photons. This process goes on until the medium is populated with excited Nd ions. From its metastable state, the Nd ions release infrared light with a wavelength of 1064 nm.

Fiber-optic Laser
“Fiber-optic lasers are one of the newer types which use fiber-optics as the lasing medium instead of gases (CO2 lasers) and crystals (Nd-YAG lasers). Since it uses fiber-optics, fiber lasers are solid-state lasers that operate the same way as crystal lasers. The optic fiber is doped with elements such as erbium and ytterbium. Erbium generates light at the 1528 to 1620 nm range. Ytterbium, on the other hand, produces light with a wavelength of 1030 nm, 1064 nm, and 1080 nm.

“It is known that as light travels through a fiber-optic, it remains inside with minimal energy losses. This makes fiber-optics more stable as compared with other types that require it to be aligned accurately.

Chapter 3: Methods of Laser Cutting
“The previous chapter discussed the different types of lasers according to how the laser beam is formed using different types of lasing pumps and lasing media. Next will be the methods of laser cutting—how the small bits of materials are removed to produce a cut. There are four main methods of laser cutting: sublimating, melting, reacting, and thermal stress fracturing.

Sublimating or Vaporizing
“Sublimation is a type of phase change from a solid state to a gaseous state, with no intermediate liquid phase. This is the same process of how dry ice turns into a vapor without becoming a liquid. The material quickly absorbs energy in which there is no chance for melting to occur. The same principle is applied to laser cutting wherein a high amount of energy is imparted into the material in a relatively short time that causes direct phase change of the material from solid to gaseous states, with as little melting as possible.

“The cut begins by creating an initial keyhole or kerf. In the kerf, there is more absorptivity which causes the material to vaporize more quickly. This sudden vaporization creates a material vapor with high pressure that further erodes the walls of the kerf while ejecting materials from the cut. This deepens and enlarges the hole or cut made.

“This process is suitable for cutting plastics, textiles, wood, paper, and foam which requires only small amounts of energy to be vaporized.

Melting
“In comparison with sublimation, melting requires less energy to achieve. The energy required is about a tenth of the sublimating laser cuts. In this process, the laser beam heats the material which causes it to melt. As the material melts, a jet of gas from the coaxial nozzle with the laser beam expels the material from the cut. The assist gases used are inert or non-reacting (e.g., helium, argon, and nitrogen) which only aids the cutting through mechanical means.

“Because of its low energy requirement, it is used for cutting non-oxidizing or active metals such as stainless steel, titanium, and aluminum alloys.

Reactive Laser Cutting
“In this process, a reactive gas is used to generate more heat by reacting with the material. The process begins by melting the material with a laser beam. As the material melts, a stream of oxygen gas comes out of the coaxial nozzle which then reacts with the molten metal. The reaction between the metal and oxygen is an exothermic process which means heat is released from the reaction. This heat assists the melting of the material which is about 60% of the total energy required to cut the material. The molten metal oxides are expelled by the pressure of the oxygen jet.

“Aside from the lower energy required from the laser beam, cutting speeds using reactive gases are faster than laser cutting with inert gases. However, since this process relies on chemical reaction, the molten metal oxide which is not expelled by the oxygen jet forms along the edge of the cut. This produces low-quality cuts than using inert gases.

“This process is used to cut thick carbon steels, titanium steels, and other easily oxidized metals.

Thermal Stress Fracture
“This process involves introducing a small kerf at depths of about one third the thickness of the material using a laser. The laser is then used to induce localized stresses. This is achieved by heating a small spot which creates compressive forces around it. After passing the laser beam, the area slightly cools creating thermal stresses. In some designs, coolants are used to assist in the generation of thermal stress. When these induced stresses reach failure levels, a crack is propagated that causes separation.

“The movement of the laser beam directs this separation in a controlled manner. This method usually requires less power than laser vaporization with better cutting speeds. Localized heating is normally carried out below the glass transition temperature.

“CO2 lasers are widely used for this application since infrared light with a wavelength of 10.6 µm is ideal for cutting most nonmetals. However, not all materials can be cut by one type of laser since different materials absorb light at different wavelengths. Thermal stress fracture is widely used to cut brittle materials such as ceramics and glass.

Stealth Dicing
“This is a laser cutting technology originally developed by Hamamatsu Photonics which is used in cutting semiconductor wafers and parts of microelectromechanical systems or MEMS. In this type of cutting, the initial kerf is created at an internal point within the material. Stealth dicing is a dry cutting process where the cut produced is clean with no molten deposits.

Chapter 4: Laser Drilling Techniques
“There are different ways to create a hole using a laser. These are classified according to the movement of the laser beam relative to the workpiece. Each technique has its advantages and disadvantages.

Single-shot Laser Drilling
“In this type of laser drilling, a single laser pulse with high energy is used to create a hole. This single beam laser is focused on a single location until the material melts layer by layer. The melting process is done efficiently and in a short amount of time which makes this process desirable in producing multiple holes quickly.

Percussion Laser Drilling
“In percussion drilling, the laser beam diameter is the same as the hole diameter. Comparing it with single-shot drilling, instead of using a single laser pulse, successive low-energy pulses are used to remove material. These repeating pulses eventually penetrate the material which takes about 4 to 20 pulses depending on the depth of the material and laser beam properties. This process is also completed quickly which makes it effective in working with thick materials and producing multiple holes in a short amount of time.

Trepan Laser Drilling
“In trepan laser drilling, the laser beam spot size is significantly smaller than the hole size. When an initial hole is made, the laser beam is then traversed around the hole, expanding the drilled hole size into the desired diameter. This is done to drill large holes efficiently than single-shot and percussion drilling. Trepan drilling is slower but can produce holes with better metallurgy and geometry.

Helical Laser Drilling
“Like trepan drilling, this type uses a moving laser beam to drill through a material. However, it does not require an initial hole. In this method, the laser beam is rotated relative to the workpiece. The laser beam’s rotation is similar to that of a conventional drill bit. Rotation is achieved by a spinning dove prism or other optic systems rotated by a high-speed motor. The quality of the hole produced is comparable to holes made by trepan drilling.

Chapter 5: Laser Cutting Machine Configurations
“The previous chapter discussed different laser drilling techniques. Next will be the different configurations of laser cutting systems. These are also classified according to the way the laser beam moves relative to the workpiece.

Moving Material Configuration
“In this setup, the laser cutter is stationary while the material surface is moving. Since no movement from the laser is required, the optics system is simpler than other configurations. However, this is slower than other methods and is usually limited to cutting flat materials.

Flying Optics System
“This setup is the opposite of the moving material. Flying optics involves a stationary material and a movable laser cutter. Since the laser is moving constantly, the laser beam length must be adjusted constantly as well because of the divergence of the laser beam. Greater divergence produces a poorer quality of cut. To mitigate this, re-collimating optics and adaptive mirror control are used. This setup is the fastest among the three since the movement of the mirrors is easier to control.

Hybrid System
“In the hybrid system, the material moves on one axis while the optics move on the other axis. This setup combines both advantages and disadvantages of the previous two setups. One advantage of this system from the flying optics is that hybrid systems provide a more constant beam path which reduces power losses.

Chapter 6: Laser Marking
“Laser marking is the process of creating marks using lasers by cutting the surface of the workpiece at a shallow depth or by inducing chemical changes through burning, melting, ablation, polymerization, and so forth. Like laser cutting and laser drilling, laser marking has the advantage of being a non-contact process. Issues of tool wear and unwanted work hardening on the surface of the workpiece are eliminated. Moreover, laser marking does not use inks which is an advantage over traditional printing. Different types of laser marking processes are summarized below.

Surface Removal:
“This process involves removing specific regions of the layer of coating previously applied on the surface of the workpiece. The workpiece has a different contrast from the coating which makes the regions removed significantly visible. Materials for this type of laser marking are special films and coated metals.

Engraving:
“This is a type of laser marking where the surface is cut at the desired depth. The cut is made usually by laser vaporization process. The main advantage of this method is that it can be done at high speeds.

Thermal Bonding:
“This is done by fusing additional pigmented materials such as glass powders or crushed metal oxides on the surface of the workpiece. The materials are fused by the heat applied by the laser.

Annealing:
“This process involves heating specific regions using a laser. The heat applied by the laser causes the metal to oxidize producing different colors such as black, yellow, red, and green.

Carbonizing:
“In this process, plastic bonds between polymers are broken, releasing hydrogen and oxygen and producing a darker color. This process is done on plastics and organic materials.

Foaming:
“This is usually done on plastics where the color pigments and carbon are destroyed and vaporized resulting in foaming. Foaming process is done on dark-colored materials that need to have lighter colored markings.

Staining:
“This process induces chemical reactions on the surface of the workpiece where the products of the reaction have different colors.

Chapter 7: Advantages and Disadvantages of Laser Cutting
“Laser drilling is widely used in industries such as aerospace, automotive, electronics, and tool machining. Below are the main advantages of using lasers for drilling.

Non-contact Technique
“As mentioned earlier, since the laser drilling process has no cutting tools involved, there is no issue of tool wear or damage. In conventional drilling, drill bits can become dull making the cutting slower which also produces more heat. This can distort the material and change its mechanical properties due to heating.

Precision and Accuracy
“Since laser beams produced can be focused, this allows precise drilling of small holes that cannot be achieved by conventional drilling. The hole depth can be controlled even for micro-scale holes. Moreover, the process is digitally controlled by CNC methods. All parameters can be automatically controlled producing consistent and repeatable results.

Minimal Burrs Produced
“Secondary processes such as deburring are required in the manufacturing of precision parts to remove surface irregularities, metal spurs, raised edges, slags, and dross. Even the most accurate fabrication techniques such as laser cutting technology tends to develop dross or thermal burrs. In comparison with conventional cutting, however, laser-cut parts still have superior edge quality. This effectively lessens the cost of secondary processes particularly deburring which can be as high as 30% of the operating costs.

High Aspect Ratio
“This means very deep holes with small diameters can be drilled without issues. Drilling these kinds of holes using conventional drills causes the tool to heat up, wobble, and break due to torsional stress. Using a laser creates no frictional resistance and is only limited by the laser generator and the optical systems used.

Suitability for Difficult Materials
“Lasers can cut and drill different types of materials that are difficult for conventional machining. Lasers can cut high strength metals such as titanium and steel superalloys. Aside from these high strength metals, because of its ability to do controlled fracture, laser cutting is used for cutting crystals, ceramics, and even diamonds.

Fast Drilling Speeds
“Since there is no required tool positioning against the workpiece, drilling speeds only depend on the configuration of the optical system and the movement of the cutting head. Moreover, the complexity of the profile to be cut has minimal effect on the incremental cost to operate the machine.

No Residual Stress
“Since most of the molten material is blown off by the assist gas, there are no residual stresses present along the drilled edges. This results in a clean, mechanically stable cut.

“Despite these advantages, the current technology of laser drilling cannot completely replace conventional methods. Below are the main reasons.

High Investment Cost
“Laser cutting machines can reach prices twice as much as waterjet and plasma cutters. The investment’s rate of return may not be sufficient to produce any economic advantage.

High Expertise Required for Operation and Maintenance
“Operating a laser cutting machine requires a specialist that has good technical background because of the range of operating parameters involved. Also, for CO2 and crystal lasers, once it becomes misaligned, an expert is needed to bring it back to its operating condition.

Highly Precise Robotic Systems Required
“Highly precise movements are required in laser cutting, especially in applications in the order of microns. Two factors can affect the movement of the laser beam. One is the accuracy of the control system and drivers. The control system must be able to process and send precise signals to the high-resolution driver to finely position the laser beam. The other factor is the dimensional accuracy of the laser cutting parts. Linear guides, lead screws, and other parts of the transmission system must accurately mate together. This can be achieved by deburring the laser cutting parts.

Metal Thickness Limitations
“The depth of cut depends on many parameters, but the most significant is power. For the same power rating, plasma cutters can cut deeper than lasers. Common industrial laser systems of greater than 1kW can cut carbon steel up to 13 mm in thickness.

Conclusion:

  • “Laser cutting is a non-traditional machining method that uses an intensely focused, coherent stream of light called a laser to cut through the material. Laser drilling, on the other hand, is another type of laser machining process that produces a hole through the workpiece achieved by different techniques.
  • “A laser beam is generated by using a high-intensity light source or electrical discharge device to excite atoms or molecules inside a lensing medium. This lensing medium produces cascading excitations which result in the production of photons. The photons are then resonated and partially released. The released photons become the laser cutting beam.
  • “Lensing media used for laser cutting are CO2, crystals, and fiber-optics.
  • “There are four main methods to produce a cut or hole. These are sublimating, melting, reacting, and thermal stress fracturing. Each of these methods has its application.
  • “Laser drilling can be done by single-shot, percussion, trepanning, and helical drilling. Single-shot and percussion laser drilling produce holes at a higher rate than the other two processes. Trepanning and helical drilling, on the other hand, produce more accurate and higher quality holes.
  • “Laser cutting machines can be classified according to the movement of the laser relative to the workpiece. These are moving material, flying optics, and hybrid systems.”

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