The Research Of Laser Induced Shock Wave Transmitting In Layered Materials And Its Applications

Nowadays,laser shock has been widely used in surface modification and sheet forming of various homogeneous materials,while its applications on laminated composites are relatively limited.That is because the shock wave will reflect and transmit at the interface,forming tensile waves and compressive waves,which make the propagation of laser-induced shock wave very complex.However,previous researches have indicated that,the shock wave can strengthen the material surface,even the interface of laminated materials.Besides,if we design the laminated layer reasonably,we can actively control the shock wave and get the modulated shock wave.As a result,we can not only realize the material modification,but also control the plastic flow mode and surface texturing.In this paper,based on the modulation theory of laser-induced shock wave transmitting in laminated materials,we investigated two applications of laser shock processing through experiment and simulation,”laser shock processing on laminated metal composites” and “fabricating micro embossments on metal surface through laser shock texturing”.The main works and conclusions are as follows:(1)Laser-induced shock wave transmitting modulation theory in laminated materials.We created the physical models of layered medium and simulate the shock wave transmitting process in laminated materials through Abaqus.The shock wave transmission rules were investigated.The stress wave transmission mechanism: if the stress waves transmit from higher-impedance material to lower-impedance material,the shock waves demonstrate abrupt attenuation at the interface,and the reflected waves form the tensile waves.For stress waves propagate from lower-impedance to higher-impedance,shock waves amplify at the interface,and the reflected waves form compression waves.For bimetallic composites,the residual stress distribution of the film is consistent with that of the pure material.When the shock wave transmit from the film to the substrate,the distribution trend of the residual stress is different due to the impedance matching.(2)The investigation of laser shock on aluminum/copper bimetallic composites.Through experiments and simulations,the effects of laser parameters and material impedance matching on the interface properties and composite performances have been investigated.The results are as follows: laser shock processing can be applied to strengthen bimetallic composites through choosing laser parameters reasonably.When LSP are applied on those composites with the higher-impedance film and lower-impedance matrix,the film can be strengthened,and the matrix could only be strengthened when the film is thin enough.For those with the lower-impedance film and higher-impedance matrix,not only film can be strengthened,but also the matrix has a strong probability to be reinforced because the impedance matching of the interface can amplify the transmitted compressive stress waves.(3)Fabricating micro embossments on the metal surface through laser shock texturing has been researched.The influences of key parameters,including laser energy,the mapping-layer material and the hole spacing,on the 3D morphologies of the embossments,have been investigated.Main conclusions are as follows: when the laser energy is lower than a certain value,the larger the laser energy,the higher the embossment height.But if the input energy exceeds some critical values,micro pits will appear on the tops of the embossments and thus reduce the overall heights of the micro embossments.Increasing the transverse sizes of the mapping holes is beneficial for obtaining higher embossments,because too small mapping-hole diameters(in the same order of magnitude of the grain sizes)will highlight the size effects and hinder the material flow into the mapping holes.Better acoustic-impedance matching between different layers,is more advantageous to enhance the amplitudes of the transmitted shock wave reaching the metal surface,and then increase the embossment height.Decreasing the hole spacing has great effect on the lateral shapes of the micro embossments due to changing the plastic flow of the materials between the embossments.Whether the micro hardnesses on the top of the micro embossments or those between the embossments are greatly increased comparing to that of the untreated material.So that,laser shock texturing has the composite effects of surface modification and micro-molding.