Femtosecond green laser processing of magnesium alloy
Zhu Yiliang1, Xie Xiaozhu1,2, Huang Qingpeng1, Hu Wei1, Ren Qinglei1     
1. Laser Micro/Nano Processing Lab, School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, China;
2. Department of Experiment Teaching, Guangdong University of Technology, Guangzhou, Guangdong 510006, China

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.

Supported by National Key Research and Development Program of China (2018YFB1107700), National Natural Sci-ence Foundation of China (51575114, 51805093), and Guangzhou Municipal Science and Technology Project (201607010156)