With the advancement of science and technology in the aerospace sector, the micro-hole laser processing tools have become increasingly important. Compared to traditional machining and electric spark processing of micro-plates, laser drilling has high processing efficiency, high speed, low cost, and wide application range, and a lot of high sensitive holes can be processed. An ultrashort laser pulse has gradually become the preferred one for high-quality micro-hole processing due to its high processing accuracy, but the existing studies have found that an ultrashort pulse laser still has defects such as recast layers, micro-cracks, and slags, and at the same time, the processing efficiency is far less than that for a traditional long laser pulse. Magnetic field assisted laser drilling can further reduce the defects of ultrashort laser processing to improve the micro-hole morphology, increase the etching depth, and reduce the hole taper. In order to further improve the geometry for laser drilling, improve the etching depth, and reduce the hole taper, the 304 stainless steel is used as the test material and the magnetic field rotation assisted laser drilling and machining is adopted.

This paper discusses the mechanism and experimental study of magnetic field rotation assisted laser drilling of 304 stainless steel. First, the effect of magnetic field rotation on the quality of microporous processing is studied. The variations of micro-hole etching depth, the surface spatter, the hole-wall geometry, and the oxygen content of inner wall are investigated under the action of different magnetic field rotation speeds. The microporous surface splatter and the inner-wall morphology are observed by the scanning electron microscope. The inner-wall removal amount is described with surface splatter, and the inner-wall quality is described with inner wall smooth flatness. Then, the inner-wall oxygen content is measured by the energy spectrometer, and it can reflect the inner-wall slag and the recast layer of the micro-holes. The inner-wall regional scanning and line scanning are studied at different magnetic field rotation speeds, which can infer the variation of inner-wall oxygen content (inner-wall slag and recast layer) with magnetic field rotation. The micro-hole etching depth is measured by the ultra-depth-of-field microscope, the micro-hole etching depth is quantified, and the etching depths under different rotation speeds for 5 s and 10 s are compared. Finally, the effect of magnetic field rotation on the micro-hole taper is studied under different single pulse energies. It can be evaluated from the micro-hole inlet size, outlet size, and inner-wall taper.

After applying a rotating magnetic field, the Lorenz force generated by the rotating magnetic field accelerates the motion of the charged particles in plasma and simultaneously the molten metal has a certain agitation and splash, so the inner-wall melt distribution is more uniform and the removal uniformity of the through-hole is further improved (Fig. 5). When the rotating magnetic field is applied, the surface splash is increased. The surface splatter is increased more significantly as the magnetic field rotation speed is increased to 250 r/min (Fig. 6). The application of the rotating magnetic field can accelerate the flow of the inner-wall molten metal. Therefore the inner-wall morphologies of the recast layers are smoother, and it can also accelerate the discharge of the slags in the micro-hole inlet and outlet. The oxygen content reflects the residues of inner-wall slags, indicating that the micro-hole slags are reduced after applying a rotating magnetic field (Fig. 7). As the magnetic field rotation speed increases, the micro-hole etching depth also increases. When the magnetic field rotation speed is 350 r/min and the processing time is 10 s, the increase of etching depth reaches the maximum of 24.7 μm (Figs. 8, 9, and 10). Compared those with and without a rotating magnetic field when the single pulse energy becomes large, the diameter of the micro-hole inlet under the rotating magnetic field is significantly reduced and the outlet diameter is slightly increased, so the micro-hole taper is reduced more significantly under a larger energy. When the single pulse energy of the laser is 85 μJ, the micro-hole taper difference between the two processes is the largest, and the micro-hole taper under a rotating magnetic field is reduced by 1.17° compared with that by laser direct processing (Figs. 11 and 12).

The variations of micro-hole etching depth, the surface spatter, the hole-wall geometry, and the inner-wall oxygen content are investigated under the action of magnetic fields with different rotation speeds. The effect of single pulse energy on the micro-hole taper is studied with and without the action of a rotating magnetic field. Experiments show that with the increase of magnetic field rotation speed, the micro-hole etching depth increases, the surface splashing becomes more pronounced, and the oxygen content further decreases. The introduction of a rotating magnetic field makes the geometric shape of the hole-wall smooth and can effectively reduce the micro-hole taper. The higher the energy of a single pulse, the more significant the decrease in micro-hole taper.