The interaction between laser and materials involves many physical phenomena and characteristics. The next three articles will introduce the three key physical phenomena related to laser welding process in order to provide colleagues with a clearer understanding of the laser welding process: divided into laser absorption rate and changes in state, plasma and keyhole effect. This time, we will update the relationship between changes in state of laser and materials and absorption rate.
Changes in state of matter caused by the interaction between laser and materials
The laser processing of metal materials is mainly based on the thermal processing of photothermal effects. When laser irradiation is applied to the material surface, various changes will occur in the surface area of the material at different power densities. These changes include surface temperature rise, melting, vaporization, keyhole formation, and plasma generation. Moreover, the changes in the physical state of the material surface area greatly affect the material’s absorption of laser. With the increase of power density and action time, the metal material will undergo the following changes in state:
When the laser power density is low (<10 ^ 4w/cm ^ 2) and the irradiation time is short, the laser energy absorbed by the metal can only cause the temperature of the material to rise from the surface to the inside, but the solid phase remains unchanged. It is mainly used for part annealing and phase transformation hardening treatment, with tools, gears, and bearings being the majority;
With the increase of laser power density (10 ^ 4-10 ^ 6w/cm ^ 2) and the prolongation of irradiation time, the surface of the material gradually melts. As the input energy increases, the liquid-solid interface gradually moves towards the deep part of the material. This physical process is mainly used for surface remelting, alloying, cladding, and thermal conductivity welding of metals.
By further increasing the power density (>10 ^ 6w/cm ^ 2) and prolonging the laser action time, the material surface not only melts but also vaporizes, and the vaporized substances gather near the material surface and weakly ionize to form a plasma. This thin plasma helps the material absorb the laser; Under the pressure of vaporization and expansion, the liquid surface deforms and forms pits. This stage can be used for laser welding, usually in the splicing thermal conductivity welding of micro connections within 0.5mm.
By further increasing the power density (>10 ^ 7w/cm ^ 2) and prolonging the irradiation time, the material surface undergoes strong vaporization, forming a plasma with high ionization degree. This dense plasma has a shielding effect on the laser, greatly reducing the energy density of the laser incident into the material. At the same time, under a large vapor reaction force, small holes, commonly known as keyholes, are formed inside the melted metal, The existence of keyholes is beneficial for the material to absorb laser, and this stage can be used for laser deep fusion welding, cutting and drilling, impact hardening, etc.
Under different conditions, different wavelengths of laser irradiation on different metal materials will result in specific values of power density at each stage.
In terms of the absorption of laser by materials, the vaporization of materials is a boundary. When the material does not undergo vaporization, whether in the solid or liquid phase, its absorption of laser only changes slowly with the increase of surface temperature; Once the material vaporizes and forms plasma and keyholes, the material’s absorption of laser will suddenly change.
As shown in Figure 2, the absorption rate of laser on the material surface during laser welding varies with laser power density and material surface temperature. When the material is not melted, the absorption rate of the material to the laser slowly increases with the increase of the material surface temperature. When the power density is greater than (10 ^ 6w/cm ^ 2), the material vaporizes violently, forming a keyhole. The laser enters the keyhole for multiple reflections and absorption, resulting in a significant increase in the material’s absorption rate to the laser and a significant increase in the melting depth.
Absorption of Laser by Metal Materials – Wavelength
The above figure shows the relationship curve between the reflectivity, absorbance, and wavelength of commonly used metals at room temperature. In the infrared region, the absorption rate decreases and the reflectivity increases with the increase of wavelength. Most metals strongly reflect 10.6um (CO2) wavelength infrared light while weakly reflect 1.06um (1060nm) wavelength infrared light. Metal materials have higher absorption rates for short wavelength lasers, such as blue and green light.
Absorption of Laser by Metal Materials – Material Temperature and Laser Energy Density
Taking aluminum alloy as an example, when the material is solid, the laser absorption rate is around 5-7%, the liquid absorption rate is up to 25-35%, and it can reach over 90% in the keyhole state.
The absorption rate of the material to the laser increases with increasing temperature. The absorption rate of metal materials at room temperature is very low. When the temperature rises to near the melting point, its absorption rate can reach 40%~60%. If the temperature is close to the boiling point, its absorption rate can reach as high as 90%.
Absorption of Laser by Metal Materials – Surface Condition
The conventional absorption rate is measured using a smooth metal surface, but in practical applications of laser heating, it is usually necessary to increase the absorption rate of certain high reflection materials (aluminum, copper) to avoid false soldering caused by high reflection;
The following methods can be used:
1. Adopting appropriate surface pre-treatment processes to improve the reflectivity of laser: prototype oxidation, sandblasting, laser cleaning, nickel plating, tin plating, graphite coating, etc. can all improve the material’s absorption rate of laser;
The core is to increase the roughness of the material surface (which is conducive to multiple laser reflections and absorption), as well as to increase the coating material with high absorption rate. By absorbing laser energy and melting and volatilizing it through high absorption rate materials, laser heat is transmitted to the base material to improve the material absorption rate and reduce the virtual welding caused by high reflection phenomenon.
Post time: Nov-23-2023
