Theoretical And Experimental Research On Surface Texturing By Laser Shock Processing

The applications of surface texturing can improve tribological, biomedical, and optical properties, as well as control surface wetting, and improve adhesion joint strength. From macro scale mechanical components to micro scale MEMS are all likely to require surface texturing. Surface texturing methods include: honing, LIGA technology and quasi-LIGA technology, micro electrochemical machining, reactive ion etching, ultrasonic machining, laser surface texturing. Among these, LST has been widely used due to the high degree of automation, good controllability, and high efficiency of laser machining technology. Besides, the development of laser devices and laser processing theories also promoted the development of LST. Thus a variety of surface texturing techniques based on laser were spawned. There are laser texturing based on ablation, laser texturing based on melting and laser shock surface texturing.In this thesis, on the basis of exploring surface quality of metal materials after laser shock process, the method to manufacture surface microstructure induced by laser shock is put forward. According to the method, the theoretical and experimental researches were conducted systematically. The main research contents and conclusions are as follows:(1)LSP was conducted on pure copper block using the aluminum foil as the absorbent material and water as the confining layer. The surface morphologies of copper block were characterized with 3D profiler, the optical microscopy(OM) or the scanning electron microscopy(SEM). It was found that there were a large number of micro defects on the target surface after LSP, including raised micro defects and crater-like raised micro defects. The temperature rise of metal surface due to LSP were evaluated theoretically. The mechanism of micro defects was analyzed. The conclusions are as follows: when aluminum foil was used as the absorbent material, the bubbles existed in the bonding material. The adiabatic compression induced by laser shock rose the air temperature within the bubbles rapidly, and then melted the metal materials at the locations of the bubbles, forming micro melting pool. At the same time, the secondary shock wave induced by the expanding of the bubbles launched on the micro melting pool and produced the crater-like micro defects on the metal surface. If the temperature rise generated by impact and high speed deformation is not enough to make metal melt,under unbalanced impact loading, micro defects without middle concavity will be formed on the metal surface. During LSP, the surface micro-defects caused by impactundermined the workpiece surface quality. However, as the above-depth study of the formation of micro embossments, grasping the form law, to achieve the formation process and the effect of forming controlled, then the active manufacturing of micro embossments on the metal surface can be realized.(2)We have actively designed and successfully produced an adhesive layer containing holes between the pure aluminum foil and workpiece surface, called hole mapping layer. In this experiment, the adhesive material is black paint with the thickness of several tens of micrometers, the cavities or recesses are etched on the black paint with laser marking machine. Laser shock was conducted with water as the constraining layer. Finally, the micro embossment array was formed successfully on the metal surface. The shapes and dimensions of micro embossments are consistent with the holes in the adhesive layer. ABAQUS software has been used to simulate the process of forming embossments. The results show that: when wide incident shock waves transmit through the hole mapping layer, it is spatially modulated to become the unbalanced shock wave. Under the unbalanced impact loading, the metal surface material flow from the high pressure area low pressure area. Finally, the micro embossments were formed on the metal surface.(3)The mechanical properties of target surface, and the influences of the laser energy, hole sizes and hole pattern on the micro embossments have been studied preliminarily. The microhardness on the laser shocked surfaces were measured. The results show that: the hardnesses on the entire laser-shocked zones improve remarkably.The micro embossments were characterized with 3D profiler. Below one certain value,the larger the laser energy, the higher the embossment. However, the embossment height can’t be increased indefinitely due to the pressure rise within the mapping holes.Furthermore, too large laser energy will cause some negative effect, such as softening the top of the produced embossment, and deteriorating its mechanical properties. The sizes of the mapping holes are another important factor to influence the height of the micro embossments. The bigger mapping holes are beneficial for increasing the heights of the micro embossments.