Material Phase Changes Under Laser Action

Upon absorption by the processed material, the laser is converted into thermal energy based on the photothermal effect. Under different laser radiation illuminances, the surface state of the material will undergo different changes, including temperature rise, melting, vaporization and formation of pores, and plasma generation. The changes in the physical state of the material surface in turn greatly affect the material’s absorption of the laser.

Figure 1-6 shows several phase changes on the metal surface under the action of lasers with different radiation illuminances.

Figure 1-6 Several phase changes on the metal material surface under laser action

a) Solid-state heating
b) Surface melting
c) Surface vaporization, and formation of pores and plasma
d) Formation of laser-blocking plasma

Solid-state heating

When the radiation illuminance is relatively low (<104W/cm2), the energy absorbed by the metal from the laser can only cause a rise in the surface temperature of the material while maintaining the solid phase unchanged. This is mainly used for surface annealing and phase hardening of parts (Fig. 1-6a).

Surface Melting

When the radiation illuminance increases to the range of 104-105W/cm2, the material surface will melt, and the depth of the melt pool will increase with the increase in radiation illuminance and the length of irradiation time. This is mainly used for surface remelting, alloying, surfacing, and conduction welding of metals (Fig.1-6b).

Surface Vaporization, and Formation of Pores and Plasma

When the radiation illuminance reaches the range of 106W/cm2, the material surface vaporizes under the laser action. Under the effect of vapor recoil pressure, the liquid surface forms pores by sinking downwards, and the metal vapor absorbs the energy of the subsequent laser and ionizes to produce photo-induced plasma. At this stage, the density of the plasma is still relatively low, and its presence can enhance the material’s absorption of the laser.

This is used for deep penetration laser welding (or laser keyhole welding), which can obtain the highest efficiency and ideal weld seam formation (Fig. 1-6c).

Formation of Laser-Blocking Plasma

When the radiation illuminance reaches the range of 106~107W/cm2, the material surface vaporizes intensely, forming a high-density plasma. This has a significant absorption, refraction, and scattering effect on the laser beam, reducing the proportion of laser power entering the pores, so the melt depth cannot increase proportionally with the increase in laser power density.

However, due to the radiant heating of the plasma on the workpiece surface, the heated range at the outlet of the pore expands. This stage is used for deep penetration laser welding, which will result in a goblet-shaped weld seam formation (Fig. 1-6d).

Formation of Periodic Oscillating Plasma

When the radiation illuminance further increases to the range of greater than 107W/cm2, both the temperature and electron number density of the photo-induced plasma are very high, causing the laser radiation on the workpiece to be completely blocked temporarily.

This interrupts the vaporization and ionization process on the workpiece surface, triggering periodic oscillations of the plasma. Laser welding becomes very unstable and must be avoided. These issues will be discussed in the following section on “Laser-Assisted Absorption Waves”.

The above ranges of radiation illuminance are roughly divided based on steel materials as the heating objects. For lasers of different wavelengths, different metal materials, and different process conditions, the specific values of radiation illuminance in each stage will vary.

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