Study On Evolution Mechanism And Properties Of Surface Structure Of Femtosecond Laser Ablation AZ31 Magnesium Alloy

Femtosecond laser has the advantages of high power,high precision,small heat-affected zone,etc.It can directly remove surface materials and realize micro-nano processing.It has irreplaceable advantages in surface processing of materials such as aerospace,aerospace,semiconductor and biology.Magnesium alloy is not only a lightweight and high-strength structural material,but also a metal material with excellent performance and environmental friendliness,and is expected to become a new type of biodegradable biomaterial.Therefore,it has broad application prospects for the development of controllable functional magnesium alloy surface.However,the research on femtosecond laser processing of magnesium alloy is very limited.The surface morphology and mechanism of the formation of femtosecond laser and magnesium alloy are still unclear.There is little research on the microstructure and properties of magnesium alloy surface after processing..Therefore,this paper starts from the micro-nano structure,microstructure and properties of magnesium alloy to study the processing technology of femtosecond laser ablation of AZ31 magnesium alloy.Firstly,the metallographic sample is prepared from the magnesium alloy after femtosecond laser ablation.The metallographic structure was observed and the average grain size was calculated.Then,the surface and profile morphology were further observed by scanning electron microscopy,and the elemental changes and micro-nanostructures were analyzed.Next,the hardness of the magnesium alloy matrix and the ablated region was measured by a microhardness tester,and then the surface roughness of the magnesium alloy was tested by a white light interference three-dimensional profiler.Finally,the ablation threshold and ablation rate of the magnesium alloy were calculated.The experimental results show that the grain in the first layer(depth of 2-4 mm)is refined,and when AZ31 magnesium alloy is ablated using different laser parameters,there is almost no heat effect at different scanning speeds.At the repetition frequency and the number of scans,there was no significant change in the grain size except for the first layer.Femtosecond laser ablation of magnesium alloy at low energy density(<1.414 J/cm2)will form a micro-nano structure pore structure,which will ablate to form a trench structure at high energy density.We will change on the basis of trench structure.The ablation direction can be used to prepare the pore structure twice.After processing the magnesium alloy by femtosecond laser,the surface oxygen content is increased,and the aluminum and magnesium contents are reduced.The increase of laser scanning speed will reduce the hardness of magnesiumalloy.The increase of laser repetition frequency will increase the hardness of magnesium alloy surface.The scanning speed of laser will also affect the roughness of magnesium alloy surface.With the increase of scanning speed,roughness Increase;and as the repetition frequency increases,the roughness decreases.The ablation threshold of the single pulse of magnesium alloy was calculated to be 0.346±0.05J/cm2,and the ablation threshold at 500 pulses was 0.219 J/cm2.The ablation rate was calculated to obtain the highest ablation rate with a repetition rate of 640 k Hz and a scanning speed of 0.8 m/s,and the maximum value was 7.45×107 mm~3/s.