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摘要
采用波长为515 nm的飞秒绿激光对AZ31镁合金进行实验研究，计算镁合金激光烧蚀阈值与烧蚀速率，研究镁合金的激光加工机理，对比分析AZ31镁合金有无表面微结构对其腐蚀速率的影响。结果表明：镁合金的激光烧蚀阈值为1.46 J/cm2，在能量密度为8.36 J/cm2时烧蚀速率为0.68 μm/pulse；随着能量密度的增大烧蚀速率增大，在能量密度为8.36 J/cm2，脉冲数为1000时可以加工出高质量的小孔。镁合金的腐蚀速率方面，微槽结构小于微柱结构，微柱结构小于光滑表面，拥有微结构表面的镁合金在24 h内的腐蚀速率约为光滑表面的1/3~1/2。
Abstract
In this paper, a femtosecond green laser with wavelength of 515 nm was used to process the AZ31 magnesium alloy. The laser ablation threshold and ablation rate of Mg alloy were calculated. The mechanism of femtosecond green laser process was determined. The effects of surface microstructures on corrosion rate of AZ31 magnesium alloy was compared and analyzed. The results show that the laser ablation threshold of AZ31 magnesium alloy is 1.46 J/cm2, the ablation rate is 0.68 μm/pulse in the laser fluence of 8.36 J/cm2, the ablation rate increases with the laser fluence increasing. The high-quality holes can be fabricated with the laser fluence of 8.36 J/cm2 and the pulse number of 1000. In terms of the corrosion rate of magnesium alloy, the groove structure is less than that of the columnar structure and less than that of the smooth surface, among which the corrosion rate on the microstructural surface is about 1/3~1/2 of that on the smooth surface in 24 hours.
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Key words:
- femtosecond laser /
- Mg alloy /
- structure /
- corrosion rate
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Overview
Overview:AZ31 magnesium alloy is a highly potential material in the field of implanted medical devices due to its biodegradable absorbability, mechanical compatibility and good biocompatibility. However, Mg alloy has relatively active chemical properties, low melting point, high thermal conductivity and big coefficient of thermal expansion, which result in poor processing performance. Therefore, the traditional mechanical processing method would not be able to meet the demand. Laser processing has the advantages of non-contact and high precision, among which the green laser is very suitable for the processing of magnesium alloys, so their application fields can be broadened. With the characteristics of short pulse width, low heat-affected zone, high peak power and processing accuracy, ultrafast laser is widely used in many fields, such as micro-nano structure processing and functional surface processing. Moreover, femtosecond green laser having shorter wavelength and better absorption for magnesium alloys contributes to the trend that it would be more suitable for the processing. In this paper, a femtosecond green laser with wavelength of 515 nm was applied to process the AZ31 magnesium alloy. The laser ablation threshold of Mg alloy and its ablation rate were calculated. By analyzing and comparing the SEM micrograph of different laser fluences, the mechanism of femtosecond green laser process has been illustrated. The effects of Mg alloy with or without microstructure on its corrosion rates in physiological saline were analyzed subsequently.
The results show the laser ablation threshold of AZ31 magnesium alloy is 1.46 J/cm2, the ablation rate is 0.68 μm/pulse in the laser fluence of 8.36 J/cm2, the ablation rate is 1.37 μm/pulse with the laser fluence of 15.79 J/cm2, the ablation rate is 2.29 μm/pulse with the laser fluence of 33.98 J/cm2. In conclusion, the ablation rate increases with the laser fluence increasing. The high-quality holes can be fabricated with the laser fluence of 8.36 J/cm2 and the pulse number of 1000. When the number of pulses is less than 100, the ablation mechanism of the Mg alloy was mainly controlled by phase explosion, while the number of pulse reach 500 the ablation mechanism of composites transfer from phase explosion to thermal evaporation. In terms of the corrosion rate of magnesium alloy, the groove structure is less than that of the columnar structure and less than that of the smooth surface, among which the corrosion rate on the microstructural surface is about 1/3~1/2 of that on the smooth surface in 24 hours, the reason is Mg(OH)2 precipitation film was formed in the microstructures, which could prevent the corrosion of microstructures.
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图 2 不同单脉冲能量作用下的镁合金样件表面烧蚀形貌电镜图。(a) 6.81 µJ；(b) 10.67 µJ；(c) 20.13 µJ；(d) 30.93 µJ；(e) 43.33 µJ；(f) 55.86 µJ
Figure 2. Surface ablation SEM images of Mg alloy sample under different laser pulse energy. (a) 6.81 µJ; (b) 10.67 µJ; (c) 20.13 µJ; (d) 30.93 µJ; (e) 43.33 µJ; (f) 55.86 µJ
图 3 入射脉冲能量E与烧蚀坑直径D2之间的关系
Figure 3. The relationship between the pulse energy E and the square of ablation diameter D2
图 4 能量密度为15.79 J/cm2时激光加工的镁合金表面图。(a)，(b)脉冲数为10、15的扫描电镜图；(c)，(d)相应的三维表面形貌；(e)，(f)相应的横截面
Figure 4. Mg alloy surface after femtosecond laser processing, the laser fluence was 15.79 J/cm2 in all cases. (a), (b) SEM images of Mg alloy surface with the pulse number of 10 and 15; (c), (d) Corresponding 3D surface topography; (e), (f) Corre-sponding cross-sectional surface profile
图 5 镁合金在不同脉冲数和能量密度下的烧蚀速率
Figure 5. The average etching rate of Mg alloy under various pulse number and laser fluence
图 6 不同脉冲数及能量密度下打点扫描电镜图
Figure 6. SEM image with different pulse numbers and laser fluence
图 7 能量密度8.36 J/cm2，脉冲数1000时镁合金表面扫描电镜图
Figure 7. SEM images of the Mg alloy surface for a laser fluence of 8.36 J/cm2 and pulse number of 1000
图 8 微槽深度、宽度与激光能量密度的关系
Figure 8. The relationship between the depth and width of the groove and the laser fluence
图 9 微槽深度、宽度与扫描速度的关系
Figure 9. The relationship between the depth and width of the groove and the scanning speed
图 10 (a) 微槽形貌图；(b)微槽截面轮廓图
Figure 10. (a) The morphology of groove; (b) The profile of groove section
图 11 (a) 柱状形貌图；(b)柱状截面轮廓图
Figure 11. (a) The morphology of columnar; (b) The profile of columnar section
图 13 腐蚀开始后前10 min不同时段三种表面腐蚀瞬态图
Figure 13. Three kinds of surface corrosion transient graphs at start 10 minutes
图 14 (a) 光滑表面；(b)微槽腐蚀形貌图；(c)柱状腐蚀形貌图
Figure 14. (a) The corrosion of smooth surface; (b) The corrosion morphology of groove; (c) The corrosion morphology of columnar
图 15 腐蚀前后镁合金表面对比图
Figure 15. Comparison of Magnesium alloys surface morphology before and after corrosion
表 1 镁合金化学成份及室温下的热物理性能参数
Table 1. The chemical composition and thermos-physical parameters of magnesium alloy at room temperature
Thermophysical
parametersDensity/
(g·cm-3)
1.74Specific heat capacity/
(kJ·kg-1·K-1)
0.871Thermal conductivity/
(W·m-1·K-1)
153.66Melting/Boiling
point/K
923/1380Thermal expansion
coefficient/K
25.0×10-6Composition Mg Al Zn Mn Ni Fe Cu Si Content/% 95 3.5 1.10 0.32 0.001 0.03 0.01 0.08 
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