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Effect of water-guided laser machining technology on micro-morphology of 316L stainless steel
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摘要
水导激光加工是一项利用水光纤将激光引导到材料加工表面的新颖加工技术，具有几乎无微裂纹、热影响区小、无污染、重熔层少、加工精度高和光束平行等优点。为研究不同水导激光加工工艺参数对微观形貌的影响，探索水导激光与物质的相互作用机理。本文采用自主研发的水导激光加工系统对316L不锈钢薄片试件进行切槽和打孔实验；使用Zeiss Vert.A1金相显微镜观察加工试件的二维形貌；使用Leica DVM6超景深显微镜和Bruke Contour Elite I白光干涉仪观察试件的三维微观形貌。实验结果表明：无论是对试件进行切槽还是打孔实验，均会在加工区域产生一定宽度的沉积层，且沉积层的大小不随加工时间和加工次数变化，其宽度约为13.5 µm；通过观察试件加工区域的二维形貌，发现打孔试件的dr和切槽试件的wl也不随加工试件和加工次数变化；通过观察切槽试件加工区域的三维形貌，其截面呈倒梯形。
Abstract
Water-jet guided laser (WJGL) machining is a novel processing technology using water beam fibers to guide the laser to machine the work-piece surface. This processing technology has the advantage of almost no micro-cracks, small heat-affected zone, pollution-free, less recast layer, high processing accuracy, parallel cuffing, etc. This work aims to investigate the effect of different WGLM parameters on the micro-morphology of materials and the mechanism between lasers and materials. The experiments for slotting and grooving 316L stainless steel thin samples were used by the WGLM system developed by our research group in this work. The 2D micro-topography after experiments were tested by the Zeiss Vert.A1 metalloscope, and the 3D micro-topography of samples after experiments were tested by the Leica DVM6 optical microscope with the large depth of field & Bruke Contour Elite I white-light interferometer. Experimental results show that a certain width deposition layer can be occurred in the machining region, and the width of deposition layers does not change with the parameter of the machining time and the number of machining times. From the 2D micro-topography of samples, it can be found that the 'dr' of slotting samples and the 'wl' of grooving samples also do not change with the machining parameters. From the 3D micro-topography of grooving samples, it can be found that the cross-section shape is inverted trapezoid.
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Key words:
- water-jet guided laser machining /
- 316L stainless steel /
- grooving /
- slotting /
- micro-morphology
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Overview
Overview:With the increasing of the thrust & thrust weight ratio of aircraft engines, the operating temperature of aero-engine hot components can be reached above 1400 K. In order to ensure the normal working of blades at an extremely high temperature environment, the ceramic/metal gradient thermal barrier coatings and design of gas film cooling holes are selected in general. However, the process for gas film cooling holes of aero-engines has encountered a major challenge due to its complex material structures. Laser machining (LM) and electrical discharge machining (EDM) are usually used for machining gas film cooling holes. The LM technology utilizes the laser thermal effect, so this method has disadvantages of thick molten layer, micro-cracks, laser ablation, etc. Thus, the EDM is selected because it can reduce the thickness of the molten layer. However, EDM cannot guarantee the processing accuracy and the recrystallization would be occurred during the processing, which will affect the serve life of aircraft engines. In addition, the EDM is only applicable to metallic materials. In recent years, ceramic materials have been widely used in the aerospace field. The above two methods are unable to meet processing requirements gradually. Water-guided laser machining (WGLM) is a novel method by using water beam fibers to guide the laser to machine the work-piece surface, which can solve these problems. It has been widely applied in the precise machining field of aerospace, bio-medical, micro-electromechanical, and so on, due to advantages of almost no micro-cracks, small heat-affected zone, pollution-free, less recast layer, high processing accuracy, parallel cuffing, etc. This work aims to investigate the effect of different WGLM parameters on the micro-morphology of materials and the mechanism between lasers and materials. The experiments for slotting and grooving 316L stainless steel thin samples were used by the WGLM system developed by our research group. The 2D micro-topography after experiments were tested by the Zeiss Vert. A1 metalloscope, and the 3D micro-topography of samples after experiments were tested by the Leica DVM6 optical microscope with the large depth of field & Bruke Contour Elite I white-light interferometer. Experimental results show that a certain width deposition layer can be occurred in the machining region, and the width of deposition layer does not change with the parameter of the machining time and the number of machining times. From the 2D micro-topography of the machining region of samples, it can be found that the 'dr' of slotting samples and the 'wl' of grooving samples also do not change with the machining parameters. From the 3D micro-topography of the machining region of grooving samples, it can be found that the cross-section shape is inverted trapezoid.
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图 1 水导激光加工系统示意图
Figure 1. Schematic diagram of the water-guided laser processing system
图 3 显微镜测量二维形貌。(a) 切槽的二维形貌；(b)孔的二维形貌
Figure 3. Microscope measurement of 2D morphology. (a) 2D topography of the notch; (b) 2D topography of the hole
图 5 切槽三维形貌测量。(a)三维形貌；(b)切槽截面的二维轮廓
Figure 5. 3D shape measurement of the groove. (a) 3D topography; (b) 2D profile of the groove section
图 6 沉积层测量。(a) d1和d2随时间变化曲线；(b) dr随时间变化曲线；(c) wl随加工次数变化曲线
Figure 6. Recast layer measurement. (a) d1 and d2 curves with time; (b) dr curve with time; (c) wl curve with the number of processing times
表 1 316L不锈钢主要化学成分
Table 1. Main chemical composition of the 316L stainless steel
Chemical composition Content% C ≤0.03 Si ≤1.00 Mn ≤2.00 S ≤0.03 P ≤0.04 Cr 16~18 Ni 10~14 Mo 2~3 
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表 2 316L不锈钢的热物理性能参数
Table 2. Physical property parameters of the 316L stainless steel
Physical property parameters Value Yield strength/MPa ≥175 Density/(g.cm-3) 7.98 Thermal conductivity/(W·m-1·K-1) 15.1 Hardness/HRB ≤90 Specific heat capacity/(J·g-1·K-1) 0.502 Thermal expansion coefficient/(℃) 17.3×10-6
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表 3 水导激光加工系统的技术参数
Table 3. Technical parameters of the water-jet guided laser system
Technical parameters Value Laser energy/W 21 Laser wavelength/nm 532 Laser frequency/kHz 32.7 Water pressure/MPa 5 Nozzle diameter/µm 100 Cutting speed/(mm·s-1) 1 Water beam length at the cutting position/mm 15
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