## 3 Results and Discussion
### Influences of the Processing Parameters on Cutting Quality
#### 3.2.1 Surface Topography  
As shown in Figure 9, different surface topographies were observed, which can be classified into three types: molten globule topography, striation topography, and molten pool topography. Molten globule topography has many globules on the cutting surface; striation topography is a regular topography in the fiber laser cutting process; and molten pool topography has irregular pools on the surface. The repetition rate and pulse width have similar influences on surface topography; by increasing repetition rate (or pulse width), molten globule topography, striation topography, and molten pool topography appear successively. Figure 8: Topography of the oxygen sample processed with electrochemical polishing. Figure 6: Kerf geometry and cross formation of the cuts before the acid pickling process: (**a**) oxygen cut; and (**b**) argon cut. Figure 7: Kerf cross-sections of the cuts with the acid pickling process: (**a**) oxygen cut; and (**b**) argon cut. The pulse's wave shape is rectangle, and average power \(P_{\text{ave}}\) can be calculated using Equation (2). The applied laser energy per cutting length (\(E_{\text{pcl}}\)) is an important parameter that determines the heat input and mass of molten melt in the kerf section. \(E_{\text{pcl}}\) can be calculated using Equation (3). \[P_{\text{ave}}=P_{\text{pk}}\times\delta=P_{\text{pk}}\times\tau\times f \tag{2}\]  
\[E_{\text{pcl}}=\frac{P_{\text{ave}}}{\
u}=\frac{P_{\text{pk}}\times\tau\times f }{\
u} \tag{3}\]  
\(P_{\text{pk}}\) represents the peak power controlled by the power modulated fiber laser, \(\delta\) represents the duty ratio, \(\tau\) represents the pulse width, \(f\) represents the laser frequency, and \(\
u\) represents the cutting speed. With a low repetition rate (or pulse width), the applied laser energy per cutting length is low, and the mass of the melt is limited. Most of the melt is ejected from the cutting kerf, accompanied with a small amount of melt being turned into globules, induced by the cooling and ejecting effect of the assisting gas. With intermediate \(E_{\text{pcl}}\), an appropriate amount of metal is molten and regular striation topography is formed because of the combined effects of high repetition rate, pulses, and assisting gas. With high repetition rate (or pulse width), an excess of heat is concentrated in the base material, an excessive mass of metal is molten, and then the melt with good fluidity is ejected, and thereby forming pool topography with the ejecting effect of the assisting gas. Based on Equation (3), \(P_{\text{pk}}\) has the same impact on \(E_{\text{pcl}}\) as \(\tau\) and \(f\). However, different phenomena were observed in the cutting surface topography. As shown in Figure 9c, only striation topography was observed. In the given process parameter range, with increasing \(P_{\text{pk}}\), the striations became coarser and no molten globule topography or molten pool topography appeared. It could then be concluded that, although \(P_{\text{pk}}\) has the same impact on \(E_{\text{pcl}}\) as \(\tau\) and \(f\), it has slighter influences on the surface topography compared to \(\tau\) and \(f\). \(\tau\) and \(f\) not only determine the value of \(E_{\text{pcl}}\), but also the heating  
Figure 9: The influences of process parameters on surface topography: (**a**) repetition rate; (**b**) pulse width; (**c**) power; (**d**) cutting speed. and thermal conduction process. The heating and thermal conduction processes have underlying and complex impacts on cutting quality. \(\
u\) has more complex impacts on cutting quality, which not only influence the valve of \(E_{\text{pcl}}\), but also the cooling and ejecting effects. As shown in Figure 9d, with both low and high values for \(\
u\), globules could be observed on the cutting surface. With a low \(\
u\), \(E_{\text{pcl}}\) is high, but the cooling effect is strengthened, and a topography combined with globules and pools was observed. Increasing \(\
u\) will decrease \(E_{\text{pcl}}\) and weaken the ejecting and cooling effects.