| The extended use of magnesium alloys is an efficient way for structural weight-reduction, but applications of magnesium alloys in many fields are restricted due to the poor surface performance. The modern bionics research shows that mechanical permormance can be improved by periodic microstructures on surface produced by pulsed laser ablation. In order to investigate the effect of the laser processing parameters on the morphology of craters, the laser ablation process of AZ91D alloy is simulated by modified finite element algorithm first, and then the micro/nano-indentation of Mg alloy surface with different fine roughnesses is conducted to discuss the strengthening effect of pulsed laser ablation.Owing to its high machining precision and simple usage, the short pulse width laser has been widely used. There are many researchers have investigated the relationship between laser processing parameters and the features of crater by FEM. Based on the previous work, this paper develops a new laser ablation algorithm for simulating the temperature field and crater morphology of the AZ91D alloy surface treated by nanosecond and sub-picosecond laser irradiation. The new method follows the actual physical process more closely, so it can describe the interaction relationship of Mg alloy and laser beam more precisely.The simulation results show that only when energy density is higher than the specific value, i.e.0.3X109W/cm2, the AZ91D alloy can be ablated off. When the pulse duration remains10ns, the ablation depth becomes larger and larger as the pulse energy density increases but the heat affecting zone decreases due to the plasma shielding. When the pulse energy density is kept at10J/cm2, and the pulse duration ranges from500ps to50ns, the ablation depth will firstly increase and then decrease, while the heat affecting zone will always increase. When the multi-pulse laser is used for AZ91D, the ablation depth becomes larger and larger as the number of pulse increases, the heat affecting zone hardly expands.Based on the laser ablation simulation results, micro-/nano-indentation simulation of a3D bulk Mg alloy with craters on its surface is carried out by using ABAQUS. The simulation results show that the bulk Mg alloy with fine rough surface have larger surface elastic modulus and hardness than the smooth ones in most of cases, the bulk Mg alloy with the surface roughness of500nm has the largest value. Generally, the surface hardness decreases as the indentation depth increases, and the Yong’s modulus increases as the depth increases and becomes unchanged in the end.Based on the above method, the extended study from short pulse laser to the femtosecond laser ablation micromachining is discussed. Under some hypothesis, the nanoindentation is also simulated for the rough surface with the nanometer and sub-micrometer scale craters. The nanoindentation simulation results show that the smaller the crater radius, the higher surface elastic modulus and hardness we can get. In nano-indentation scale, the variation tendency of elastic modulus and hardness has a similar character with the micro ones. The heat residual stress distribution also affects the surface mechanical properties deeply as well as the residual stress itself.Combining the laser ablation and micro-nanoindentation simulation result, it can be concluded that laser ablation technology can be used to make fine roughness surface, which has higher Youngs’modulus and hardness compared with the smooth one.The research of this paper is supported by the National Nature Science Foundation of China (No.11272072)... |
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