Laser cutting extends far beyond simply dividing materials; it encompasses a spectrum of techniques that significantly broaden the capabilities of laser machines. Numerous cutting methods open up diverse possibilities for precision and versatility in material processing. Laser cutting machines can be utilized to attain intricate and customized results, with the choice of method tailored to the specific properties of the material and the desired outcome. There are four main methods of laser cutting:
Sublimation or Vaporization Laser Cutting
Sublimation is a phase change wherein a solid substance undergoes a direct transition into a gaseous state without an intermediary liquid phase. A high amount of energy is applied to the material in a relatively short time. This causes a direct phase change of the material from solid to gaseous states, while minimizing melting. The cutting procedure involves the formation of an initial keyhole or kerf, where increased absorptivity accelerates the rapid vaporization of the material. The absorbed energy leads to quicker heating of the material, facilitating rapid vaporization or conversion of the material into a gaseous state.
The rapid vaporization generates a high-pressure material vapor, contributing to the erosion of the kerf's walls and ejecting materials from the cut, thereby expanding and deepening the incision. This method is suitable for cutting plastics, textiles, wood, paper, and foam, as they necessitate minimal energy for vaporization.
Fusion or Melting Laser Cutting
Melting is a process where the laser beam heats the material, resulting in its liquefaction. This cutting process involves a laser source emitting a high-powered beam that melts or vaporizes the material. A gas jet, typically using nitrogen or air, blows away the molten debris to clear the cutting path. The assisting gases, inert or non-reactive like helium, argon, and nitrogen, assist in the cutting solely through mechanical methods. This process is coordinated and managed by a control system, while a workpiece holding system ensures stability and precise positioning during cutting. As the material transforms into a liquid state, a gas jet expelled from the coaxial nozzle along with the laser beam ejects the molten substance from the cut. This collaborative effort enables cleaner, more accurate cuts in materials prone to debris or re-solidification.
In comparison with sublimation, melting requires less energy to achieve. The energy required is about a tenth of the sublimating laser cuts. This method is employed for cutting non-oxidizing or active metals like stainless steel, titanium, and aluminum alloys.
Reactive Cutting (Oxygen-assisted Cutting, Flame Cutting)
Reactive laser cutting also known as laser flame cutting utilizes gas-induced oxidation to cut through sheet metals such as thick steel, carbon steel, and titanium sheets, etc. This process resembles welding through an oxygen torch, differing only in using a laser for heating instead of an oxygen-fed flame. The process begins by applying a laser beam to melt the material. Upon the melting of material, a flow of oxygen gas comes out of the coaxial nozzle, initiating a reaction with the molten metal. This exothermic reaction between the metal and oxygen liberates heat. The liberated heat facilitates the material's further melting, constituting approximately 60% of the total energy required for the cutting process. The expulsion of molten metal oxides is achieved through the pressure exerted by the oxygen jet.
This method reduces the laser beam's energy demand and speeds up cutting compared to inert gases. However, its reliance on a chemical reaction results in residual molten metal oxide forming along the cut's edge. Consequently, the quality of cuts produced is inferior to those achieved with inert gases.
The reactive laser cutting is employed to cut carbon steel above 1 mm in thickness. It is particularly effective for severing very thick steel plates while minimizing the need for high laser power. This method is also used to cut titanium steels, and other easily oxidized metals.
Thermal Stress Fracture Laser Cutting
Thermal stress fracture also known as fracture-controlled cutting is a unique type of laser cutting process that involves the use of hot, high-powered lasers to induce fractures in hard, brittle materials like glass and ceramics. The brittle properties of certain materials tend to develop cracks upon laser exposure. When the laser heats up the material, it introduces a small kerf at depths of about one-third the thickness of the material. As the laser beam passes through the material, the region experiences a gradual cooling, resulting in the development of thermal stresses.
Some methodologies involve the utilization of coolants to aid in the generation of these thermal stresses. Upon reaching critical failure thresholds, these induced stresses lead to crack propagation, causing material separation. This kerf can spread to other areas of the metal depending on where the laser is applied.
The movement of the laser beam coordinates the precise separation in this technique. It often necessitates less power than laser vaporization while achieving improved cutting speeds. Typically, localized heating occurs below the glass transition temperature as part of this process.
The limitation of thermal stress laser cutting lies in its exclusive suitability for brittle materials. Materials with greater strength and resistance to thermal stress remain unaffected by this process, thereby narrowing its potential applications.
CO2 lasers are commonly employed for this purpose as their utilization of infrared light at a wavelength of 10.6 µm is well-suited for cutting most nonmetallic materials. However, not all materials can be cut by a single laser type due to variations in light absorption at different wavelengths. Thermal stress fractures are extensively utilized in the cutting of brittle materials such as ceramics and glass.
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