The surface roughness Ra near the ablation pit can objectively reflect the micro geometric characteristics of the ablated material surface. The smaller the RA, the smaller the amplitude of the peak valley and the smoother the surface. The wire length of the roughness measuring instrument is 0.1 mm, and the roughness of the area around the ablation pit in figure (a) is measured, and the results are shown in the figure.
Figure (a) shows the roughness ofwithout laser energy. The average value of RA is 0.250 μ m, and there is no ablative material. Figure (b) shows the roughness measured under the effect of energy density of 31 MJ / cm2, and the average value of RA is 0.357 μ M. at this time, the surface temperature of gear material reaches the melting temperature. According to the waveform in the figure, the roughness value of gear material crater fluctuates greatly, which is mainly due to the molten substance accumulated around the ablation pit; figure (c) shows that the energy density is 67 MJ / cm2 The average Ra value is 0.271 μ M. at this time, the tooth surface temperature reaches the gasification temperature, and there is less accumulation near the ablation pit, which can maintain good roughness; figure (d) shows the roughness measured under the energy density of 105 MJ / cm2, and the average Ra value is 0.26 At this time, the tooth surface temperature is much higher than the gasification temperature, and there is no residue near the ablation pit, which is close to the initial roughness value.
It can be found that femtosecond laser micro machining has little effect on the surface roughness of the material. When the laser energy density reaches 67 MJ / cm2, that is, when the temperature of the gear surface is greater than the gasification temperature, the surface roughness of the gear surface can be guaranteed.
The ablation mechanism of 20CrMnTi in femtosecond laser micro machining ofmaterial is studied. Considering the dynamic absorption effect of material and the coupling effect of photon electron lattice system, the energy required for the critical temperature of ablating gear material is 67 MJ / cm2, and the thickness of material removed by single pulse laser is 30.15 With the increase of laser energy density, the tooth surface temperature and ablation pit depth increase. The experimental results show that when the equilibrium temperature of electron and lattice reaches the gasification temperature of the material, the crater and wall of the ablation pit are relatively flat, which can maintain the good morphology and roughness of the tooth surface. According to the ablation characteristics of gear material, adjusting the appropriate energy density can effectively remove the machining allowance of tooth surface, which can provide reference for improving the machining quality of spiral .