Results and Discussion Laser polishing with the optimal combination of process parameters reduces the surface roughness of 304 stainless steel by approximately 67%. After laser polishing, the mass fraction of C and Cr in the polished layer decreases by 2.4% and 3.2%, respectively [Figs. 8(b), 8(d), 9(b), and 9(d)], while the average grain size decreases by 0.37 μm (Fig. 13). The polished surface generates the residual tensile stress of approximately (139.6±68.7) MPa (Table 4). The hardness test reveals that the surface microhardness of 304 stainless steel decreases by approximately 15% (Table 5). This is primarily due to the decrease in the C content, which reduces the amount of carbides on the surface of the material. Electrochemical tests reveal that the corrosion current density increases by 10.978 μA/cm2 after laser polishing (Table 6), primarily because the reduction in the Cr content weakens the passivation ability of stainless steel. Concurrently, grain refinement and residual tensile stress provide a rapid diffusion path for corrosive Cl-.
As a new technology for laser surface treatment, laser polishing has the advantages of being a no-contact and non-polluting approach with high controllability. Laser polishing is accompanied by rapid heating and cooling of the shallow metal layer in the area being polished. Changes in polishing layer performance and their causes must be multifaceted. Few studies have been conducted on the performance changes and mechanisms associated with the pulse laser polishing process. Moreover, the variation trend of element content has not been considered while analyzing the mechanism of polishing layer performance in previous studies. In this study, an orthogonal experiment of laser polishing of AISI304 stainless steel is conducted; the correlation among various process parameters is analyzed, and the optimal combination of process parameters for achieving high polishing performance is determined. In addition, the properties of the surface before and after laser polishing are evaluated, and the mechanism of the properties of the polishing layer is analyzed based on grain size, residual stress, and variations in the element content. We expect that our findings will serve as a reference for improving metal surface quality using laser-polishing technology in future.
In this study, the laser power, repetition frequency, defocus distance, and scanning speed of the pulse laser are considered as the influencing factors, and surface roughness is used as the evaluation index to design an orthogonal laser polishing experiment. The polishing route has a “bow” shape [Fig. 3(b)], with a 0.03 mm gap between the filling lines. The three-dimensional (3D) morphology of the surface is observed using an optical microscope, and the surface roughness is measured using a probe roughness instrument. The surface microhardness values of the samples are measured using a Vickers hardness tester. In addition, electrochemical polarization tests are conducted in an electrochemical workstation to detect the corrosion resistance before and after laser polishing. The polished layer is subsequently analyzed using scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and residual stress detection analyses.
In the orthogonal experiment of the pulsed laser polishing of AISI304 stainless steel, the laser power, repetition frequency, defocus distance, and scanning speed are considered as the influencing factors, and the surface roughness is used as the evaluation index. The optimal combination of laser polishing process parameters has been obtained. Before and after laser polishing, the changes and mechanisms of the surface properties have been systematically analyzed. The following conclusions are drawn. 1) According to the range analysis of the orthogonal experimental results, the defocus distance, laser power, repetition frequency, and scanning speed have the highest impact on the laser polishing effect. The optimized combination of process parameters is as follows: the laser power is 12 W; the defocus distance is 2 mm; the repetition frequency is 70 kHz; the scanning speed is 1750 mm/s, the energy density of a single laser spot is 2.82 J/cm2; the overlap rate of the transverse spot is 72%; the overlap rate of the longitudinal spot is 66%; the surface roughness of the sample can be rapidly changed from Ra=0.5724 μm to Ra=0.1903 μm. 2) Following laser polishing, the microhardness of the sample surface decreases by approximately 15%; the self-corrosion current density increases from 2.858 μA/cm2 to 13.836 μA/cm2; the mass fractions of C and Cr in the polished layer decrease by approximately 2.4% and 3.2%, respectively, and the average grain size is refined by approximately 0.37 μm. The polished surface has a residual tensile stress of approximately (139.6±68.7)MPa, and laser polishing enhances the conversion of γ to α. 3) Considering the analysis results, the decrease in the C element content reduces the amount of carbides on the surface of the material, thereby decreasing the surface microhardness of 304 stainless steel. The decrease in the Cr content in the polishing layer causes the newly formed oxide film on the surface of the sample to be less dense than that on the original surface. Simultaneously, the continuous action of residual tensile stress causes cracks in the newly formed oxide film, and grain refinement increases the grain boundaries, providing a rapid diffusion path for corrosive Cl-. Consequently, the corrosion resistance of 304 stainless steel is reduced.
