Investigation on efficiency and quality for ultrashort pulsed laser ablation of nickel-based single crystal alloy DD6

Abstract

The high processing efficiency and quality are constant pursuits for modern manufacturing industry. This paper investigated the ablation efficiency and quality of ultrashort pulsed laser ablation of DD6 single crystal alloy based on experiments and theoretical analysis. The experimental results showed that the ablation rate increases with the increase of laser fluence and with the decrease of scanning speed and scanning width, while the ablation efficiency decreases with the increase of laser fluence. A relatively flat and low melted zone ablation surface could be obtained by employing a low laser fluence and high scanning speed. The influence of laser parameters on the ablation diameter, equivalent energy density, and heat accumulation effect was analyzed based on the theory of laser ablation and heat conduction. The theoretical analysis revealed the material removal transforms from plasma or vaporization removal to melt ejection with the pulse energy increases and the scanning speed decreases, which can explain the formation mechanism of surface morphology very well. In addition, the scanning strategy of high efficiency and quality was proposed based on the theoretical analysis and experimental results.

Appendix

The residual heat coefficient η_heat depends on the material and laser properties, including the pulse energy, pulse duration, and laser energy distribution, according to Weber et al. [23]. In this work, the reverse method is used to calculate the residual heat coefficient η_heat as follows:

• 1) 1) For multiple-pulse ablation process, the temperature increase T_sum(t) resulted from heat accumulation by Nt pulse can be calculated by Eq. (9). So the heat accumulation temperature after Np pulses can be given by:

$$ {T}_{\mathrm{HA}}(Np)=\frac{2{Q}_{\mathrm{heat}\cdotp }}{\rho c\sqrt{{\left(4\pi \alpha t\right)}^3}}\ \sum \limits_{N=1}^{N= Np}\frac{1}{\sqrt{N}} $$

(12)

Therefore, the heat accumulation temperature T_v of after N_p pulses is approximately expressed as:

$$ {T}_{\mathrm{v}}={T}_{\mathrm{HA}}(Np)+{T}_{\mathrm{m}} $$

(13)

where T_m is the experiment ambient temperature (25°C). And the quantitative relationship of the heat accumulation temperature T_v and residual heat Q_heat can be obtained.

• 2) Then, the line scanning experiments are carried out with the same experiment device. The scanning speed is constant at 100mm/s (corresponding the effective pulse number at 72), and the laser pulse energy is from 72.7–127.3μJ. The ablation morphology shown in Fig. 13 is observed by SEM. It can be seen that the ablation morphology transforms from non-heat melt ablation (Fig. 13a and b) to heat melt ablation (Fig. 13c and d). Especially for the pulse energy of 109.3μJ, the critical melting state is observed on ablation surface (Fig. 13c). Therefore, it could be inferred that the scanning speed of 100mm/s and pulse energy of 109.1μJ is the critical transform condition. In this moment, the heat accumulation temperature reaches the melting point (1350°C) of DD6 superalloy.

  Fig. 13.

  

  Ablation topography by line scanning with different pulse energy. a 72.7μJ, b 90.4μJ, c 109.1μJ, d 127.3μJ

• 3) Therefore, the heat accumulation temperature T_HA (72) that can be determined by Eq. 13 is about 1325°C. Thereafter, the residual heat Q_heat and the residual heat coefficient η_heat are 43.5μJ and 0.398 calculated by Eq. 12 and Eq. 8, respectively.

However, the heat accumulation coefficient is affected by many factors as earlier mentioned. In this paper, the approximate of residual heat coefficient is used for subsequent analysis.