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 gasses 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 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, reacting with the molten metal. The reaction between the metal and oxygen is an exothermic process which means heat is released. This heat assists in 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 gasses are faster than laser cutting with inert gasses. However, since this process relies on a chemical reaction, the molten metal oxide that is not expelled by the oxygen jet forms along the edge of the cut. This produces low-quality cuts than using inert gasses.
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 fractures are widely used to cut brittle materials such as ceramics and glass.
Another newer method that utilizes principles of thermal stress fracture is 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.