Laser milling has the advantageous characteristics of wide material adaptability, adjustable laser energy density, and no mechanical force, which make it suitable for the material removal processing of difficult-to-machine materials such as Ni-based superalloys. In order to expand the process system of laser processing and explore the material removal mechanism of novel laser milling methods to process nickel-based superalloys, the multi-beam coupled nanosecond laser milling of the DZ411 nickel-based superalloy was investigated in this work. The influence mechanisms of laser processing parameters on the surface quality and processing efficiency were analyzed, the results of which can provide technical support for the optimization of laser milling parameters.
The material used in this paper was the DZ411 nickel base superalloy. First, the laser milling of the Ni-based superalloy was carried out with the nanosecond pulse laser under different laser process parameters. Then, the surface morphologies of the machined nickel-based superalloy groove were observed using laser scanning confocal microscope (LSCM) and scanning electron microscopy (SEM), and the element distributions on the sample surface before and after laser milling were analyzed by EDS. Finally, the influence mechanisms of process parameters such as pulse frequency, scanning speed, scanning pitch, laser power, and scanning times on the milled surface morphology, surface roughness, milling efficiency, and element distribution were analyzed.
The surface roughness of the material after laser milling increased with scanning times, and the trend became slower; the laser milling efficiency tended to firstly increase, then decrease, and then slightly increase again with scanning times. The highest milling efficiency was achieved at the scanning times of 10 (Figure 3). As the number of scanning times increased, the size of the bumps and pits at the bottom of the face grooves increased and the shape gradually became irregular (Figures 4 and 5). The surface roughness decreased with the increase of scanning speed, and the milling efficiency tended to increase and then decrease with increasing scanning speed (Figure 7). As the scanning speed increased, the laser overlap along the scanning feed direction decreased, the machining depth decreased, and the size of the craters and bumps at the bottom of the face grooves decreased (Figures 8 and 9). With an increase in the scanning pitch, the surface roughness tended to firstly decrease and then increase, and the milling efficiency showed an overall trend with the increase of the scanning pitch (Figure 10). When the scanning pitch was less than 30 μm, a large amount of raised melt appeared at the bottom of the face groove, and the smaller the scanning pitch was, the larger was the size of the raised feature size. When the scanning pitch was 30 μm, the surface roughness was the smallest; when the scanning pitch was larger than 30 μm, more obvious ridge-like bumps were formed between two adjacent scan paths, which made the surface roughness higher (Figures 11 and 12). The surface roughness at the bottom of the face groove increased with increasing laser power, and the milling efficiency increased with increasing laser power and decreasing pulse frequency (Figure 13). At a pulse frequency of f=30 kHz and a power of P=5 W, the convex surface microstructure formed by laser radiation scanning on the material surface was larger than the original surface. When the power increased, the depth of the surface groove increased, and the bottom materials were further melted and vaporized to form the larger protrusion (Figures 14 and 15). The laser interacted with the substrate material to form metal oxides and intermetallic compounds such as Al2O3 (Figure 16).
In this work, a new type of multi-beam coupled laser was used in the machining process, and the material removal mechanism was analyzed. The effects of repeated scanning times, scanning speed, scanning pitch, laser power, and pulse frequency on surface morphology, bottom surface roughness, milling efficiency, and surface material element distribution were studied. The main mechanism behind material removal using a laser involves the laser irradiating on the surface of the material so that it melts and even vaporizes instantaneously. Partial melts leave the bottom of the groove through a jet force, and the partial melts condense at the bottom of the groove to form bulges. When the pulse frequency is high, the scanning speed is low, the laser power is high, and the scanning speed is low, the laser energy absorbed in the unit area at the bottom of the groove is large, and the ablation effect at the bottom of the groove is intense, resulting in a relatively high surface roughness at the bottom of the groove. When the scanning pitch is 30 μm, the surface roughness is the minimum. When the scanning times is 10 and the scanning speed is 100 mm/s, the milling efficiency is the highest, and it generally increases with the increase in the scanning pitch, the increase in the laser power, and the decrease in the pulse frequency. During the laser milling process, the material undergoes complex physical and chemical changes, and some metal oxides and intermetallic compounds are formed on the machined surface.
