# Construction of a Cutting-Tool Wear Prediction Model through Ensemble Learning
## 4 Results and Discussion
### Machined Surface Morphology  
After completing the milling experiments, the test samples were adequately cleaned and placed on a tool microscope with a 20\(\times\) objective lens to observe the upper edge of the machined surface morphology. This section focuses on comparing the surface morphology under different process parameters along with rotary ultrasonic-assisted and hybrid-assisted methods in the second-phase of the experiment. Figure 15 shows the machined surface morphology comparisons for some specific combinations of process parameters and assisted milling methods. It can be observed that the machined surface morphology of the hybrid-assisted method performs better than the rotary ultrasonic-assisted method alone under the same process parameters. The rotary ultrasonic assistance method can reduce the contact area in the cutting zone and create a vacuum region between the cutting tool and the workpiece material, aiding chip evacuation and reducing cutting forces and temperature. This is beneficial because the high hardness and strength of SKD11 may lead to surface edge fracture or damage, especially at high feed rates. It can be observed that under high feed rates and large radial depths of cut, defects in the surface morphology are more pronounced. Under the same cutting process parameters, hybrid-assisted milling has a better surface morphology. This assistance combines the advantages of rotary ultrasonic assistance with the local softening of SKD11 by the laser  
Figure 14: Cutting force comparison between hybrid assistance and rotary assistance for process parameter combinations in an L9 orthogonal array.  
spot, providing better machinability. The maximum locally measured laser preheating temperature in this experiment is approximately 620 \({}^{\circ}\)C. According to reference [30], maintaining surface temperatures between 600 \({}^{\circ}\)C and 750 \({}^{\circ}\)C on SKD11 workpieces results in better machinability. Therefore, compared to rotary ultrasonic assistance alone, it can reduce the occurrence of surface edge defects. At a feed rate of 2000 mm/min, the surface edge of rotary ultrasonic assistance shows significant fracture and damage. In addition, with an increase in radial depth of cut, the defects in the surface morphology become more pronounced. At a feed rate of 1500 mm/min, a similar trend is observed. When the feed rate is reduced to 1000 mm/min, defects in the surface morphology become more apparent, with an increase in radial depth of cut. However, compared to feed rates of 2000 mm/min and 1500 mm/min, the defects are significantly reduced, and the integrity of the surface morphology is better. Increasing the feed rate leads to an increase in the number of cutting-tool edges engagement in the workpiece material per minute, enhancing cutting forces accordingly. Excessive cutting forces may cause vibration, resulting in more severe defects in the surface morphology. From Figure 15, it is evident that hybrid-assisted milling provides significantly better surface morphology integrity than rotary ultrasonic assistance alone. Although high feed rates and deeper radial depths of cut may affect the laser irradiation preheating time and preheating depth on SKD11, the hybrid assistance method achieves localized preheating and softening, resulting in a more complete surface morphology as compared to rotary ultrasonic assistance.