Laser polishing, as a new type of surface treatment technology, offers the advantages of non-contact, high precision, high efficiency, and minimal pollution. Continuous laser polishing is usually referred to as laser macro polishing. The object to be polished possesses a high-roughness surface with undulations in the range of 10-80 μm. In contrast, a pulse laser is normally used for polishing low-roughness surfaces. However, the surface morphology and formation mechanism of the molten pool after continuous laser polishing of low-roughness surfaces have not been studied in detail. In this paper, samples with a low-roughness surface are polished by a continuous laser. The surface morphology of the molten pool after continuous laser polishing, as well as the mechanisms of single-pass and multi-pass overlapping polishing, are studied, which provide an optimized laser scanning strategy for continuous laser polishing technology.

Low-roughness austenitic stainless steel samples with an original surface roughness of R_a=0.95 μm are used in this study. First, a metallographic microscope and a laser confocal microscope are used to analyze the size of the molten pool and the three-dimensional morphology of the molten pool surface after continuous laser polishing using different laser process parameters to identify the optimal continuous laser single-pass polishing process parameters. Second, on the basis of single-pass polishing, the effects of line spacing and multi-pass overlapping polishing on the surface morphology are studied, and an optimized continuous laser polishing scanning strategy is determined. Finally, the hardness and element content of the cross section after continuous laser polishing are analyzed to determine the influence of continuous laser polishing on the surface properties.

In this paper, continuous laser polishing of a low-roughness surface is studied (Fig. 1). Under the same laser energy density, with increasing laser power and scanning speed, the surface fluctuations of the molten pool gradually increase, and the surface fluctuation difference increases from 0.450 μm to 10.436 μm. The morphology of the molten pool exhibits a middle bulge on both sides of the depression (Fig. 6). As the line spacing increases from 0.01 mm to 0.08 mm, the probability of fluctuations in the non-overlapped area increases, and the surface fluctuation difference increases from 1.037 μm to 3.201 μm (Fig. 9). The method of “orthogonal scanning + non-overlapped area backfill scanning” is therefore proposed. First, the orthogonal scanning method is used to compensate for the fluctuations caused by the first laser polishing. Then, after the orthogonal scanning laser polishing, non-overlapped area backfill scanning is carried out. Using this method, the roughness can be reduced to R_a=0.048 μm (Figs. 11, 12, and 14). In addition, laser polishing can improve the work hardening of 316L stainless steel (Fig. 16) and restore the hardness value to 200 HV. There is no obvious influence on the surface element content after laser polishing (Fig. 18).

In this paper, a continuous laser is used to further polish low-roughness surfaces with high quality and precision. The influence of laser power and scanning speed on the surface morphology and size of single-pass laser polished molten pool is analyzed. The influence mechanism of the surface morphology and profile using double-pass lap polishing under different line spacings is analyzed. The scanning strategy using continuous laser polishing is then further optimized, and the influence of laser polishing on the surface properties of stainless steel is studied. The results demonstrate that with increasing laser power and scanning speed, the surface is prone to undulations, and the depth of the depressions on both sides of the single-pass laser polished surface is increased. The overall surface topography caused by continuous laser polishing is significantly influenced by the line spacing. When the line spacing is 0.02 mm, surface undulations after laser polishing are minimal. By optimizing the scanning strategy, the surface roughness is reduced to R_a=0.048 μm by orthogonal scanning and non-overlapped area backfilling scanning. This polishing strategy restores the surface hardness to 200 HV with no significant change in surface element content.