低粗糙度表面的连续激光抛光形貌演化机制研究

Morphology Evolution Mechanism of Low⁃Roughness Surface Polished by Continuous Laser

采用连续光纤激光器在氩气环境下对粗糙度为R_(a)=0.95μm的低粗糙表面进行激光抛光实验,通过光学显微镜和激光共聚焦显微镜对单道激光抛光熔池截面、抛光前后三维表面形貌及表面轮廓进行分析。研究结果显示:在其他参数不变的条件下,随着扫描速度的提升,单道连续激光抛光熔池两侧的咬边现象逐渐加重。随着激光功率的提高,由于单道抛光熔池不稳定,熔池表面的起伏现象逐渐加剧。当扫描线间距为0.02 mm时,连续激光多道搭接之间未重熔区域具有较低表面起伏值,粗糙度可降低至R_(a)=0.45μm。然而,激光单向扫描抛光后表面产生的波纹起伏会阻止粗糙度的进一步降低,采用正交扫描和未搭接区域回填扫描的策略,可对激光单向扫描抛光产生的波纹起伏进行重熔正交补偿,使粗糙度值由R_(a)=0.45μm进一步降低至R_(a)=0.048μm,较原始粗糙度下降95%。

Objective 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 10‒80μm.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.Methods Low-roughness austenitic stainless steel samples with an original surface roughness of R_(a)=0.95μm 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 threedimensional 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.Results and Discussions 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 0.450μm to 10.436μm.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 1.037μm to 3.201μm(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,nonoverlapped area backfill scanning is carried out.Using this method,the roughness can be reduced to R_(a)=0.048μm(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).Conclusions 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)=0.048μm 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.