| Higher requirements for micro-forming technology are proposed due to the wide application of micro-devices in some fields such as aerospace,micro-electromechanical systems and micro-medicine.However,traditional micro-forming technology hardly satisfies the forming requirements of metal foil in the micro-scale due to its difficulty in processing metal foil and aligning the punch and die.This paper proposes a novel laser cavitation shock micro forming technology.This method uses the plasma shock wave,bubble collapse shock wave and micro-jet impact generated by laser-induced cavitation so that the controllable forming of micro protrusions can be achieved.The mechanism of laser cavitation shock micro forming is studied based on experiments and numerical simulation.The main research contents and results are as follows:(1)The mechanism of laser-induced cavitation bubble was analyzed theoretically.Based on the theory of cavitation bubble pulsation,the influence of liquid parameters on bubble pulsation characteristics was investigated.A theoretical basis for studying the oscillation characteristics of cavitation bubble near the wall at different water temperatures was provided.The plastic deformation mechanism of material was explained from the shock wave and micro-jet generated by the pulsation process of cavitation bubble near a solid wall.(2)The experimental platform of laser cavitation shock micro forming was established.The pulsation law of cavitation bubble near the wall and shock wave pressure characteristics were studied by using high speed photography and acoustic measurement techniques.The results show that some discrete tiny bubbles generate around the cavitation bubble at low water temperature,but them disappear as the water temperature increases.Increasing water temperature and laser energy can enlarge the maximum equivalent radius of the cavitation bubble,and prolong the pulsation period.A maximum bubble energy is observed at the water temperature of 313 K.Increased laser energy and decreased defocusing amount increase the intensity of plasma shock wave and bubble collapse shock wave.The intensity of plasma shock wave is independent on water temperature.(3)The effects of laser energy,defocusing amount and water temperature on the surface morphology,forming depth,thinning ratio of cross-sectional thickness,micro hardness and surface roughness of the formed parts were analyzed experimentally.The results indicate that the forming depth is positively correlated with laser energy,and it is negatively correlated with defocusing amount.The forming depth first increases and then decreases with increased water temperature.The largest thickness reduction and hardness increase of the formed part occur at the fillet,and the thinning rate increases with the increase of laser energy.The aluminum foil protective layer can reduce the surface roughness.The above results indicate that controllable laser cavitation shock micro forming can be achieved by changing the laser energy,defocusing amount and water temperature.(4)ANSYS-Fluent software was used to simulate the distributions of pressure field,temperature field and velocity field during the bubble pulsation near the wall.The results show that the trend of bubble equivalent radius evolution is in good agreement with experimental results.The external pressure of the bubble is negative,which causes discrete micro-bubbles around the bubble.The temperature of flow field outside the cavitation bubble remains constant.Increased laser energy and decreased defocusing amount are both helpful to enhance the impact of micro-jet.The impact of micro-jet reaches the maximum at the water temperature of 313 K.The duration of micro-jet impacting the material surface increases with the increase of water temperature. |
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