| After aviation key components have been damaged or failured partly, the other parts of which are usually remain a high bearing capacity. It has been one of the core science issue in mechanical components remanufacturing engineering to repair damage key components, extend its service life and improve its reliability by using advanced surface technology. Laser shock peening(LSP) is a novel material surface strengthening technology, which has many unparalleled and dramatically technological advantages by contrast with conventional processing methods, such as high energy, high pressure, ultra-fast and ultra-high strain rates. Hence, laser shock peening can be used to eliminate the tensile residual stress and refine the original coarse grain in surface of the damage metal components which are repaired by thermal effect, so as to improve the mechanical properties and extend service life of damage components. However, there is a lack of the systematic research about the distributions of mechanical properties and microstructures in the cladding coating and welding zone. Meanwhile, the laser technology is also be listed as “frontier technology" and highest priority to developing in "Made in China 2025". Therefore, to carry out the further studies of the effects of laser shock peening on damage components in the field of remanufacturing industry has promising engineering application prospect and important academic value.In allusion to the problems mentioned above, in this dissertation, the laser cladding coating and the laser welding zone of 316 L stainless steel are regarded as the study subjects. Ultrasonic testing technique was used to detect the defects in the cladding coating, the influences of the chemical composition of the cladding coating on the causes of the defects were explored. The effects of massive laser shock peening on the distributions of micro-hardness, residual stress and microstructure in laser cladding coating and laser welding zone of 316 L stainless steel were investigated. The evolution process of microstructure of cladding coating and welding zone subjected to laser shock wave was characterized and analyzed. Some important conclusions and achievements of this work were listed as follows:Firstly, according to the test results, the relationships between three kinds of clad defects(bolwhole, micro-crack and shrinkage) and the typical characteristics of ultrasonic echo were found that for the cladding coating without defects, the ultrasonic wave exponentially attenuates, and the distance between the adjacent echoes is almost equal. Furthermore, all peaks of ultrasonic echoes exist a single peak, and their widths are similar. For the cladding coating with some blowholes, the peak of the first echo is sharp and its amplitude is higher than the standard amplitude. Meanwhile, the second echo and the third echo attenuate rapidly, and the roots of the third echo wave and the transverse echo wave are merged into a waveform. For the cladding coating with some microcracks, the amplitude of the first echo is lower than the standard amplitude. Furthermore, the amplitudes of the second ultrasonic echo and the third ultrasonic echo have a significant decrease. Meanwhile, the amplitudes of the second ultrasonic echo and the third ultrasonic echo are similar. For the cladding coating with shrinkage or loose structure, the whole echo waveform is similar to the echo waveform without defects, and the amplitude of ultrasonic echo attenuates exponentially. However, the amplitudes of all ultrasonic echoes are distinctly lower than the corresponding values. The relationships between the chemical composition of the cladding coating and the causes of the defects were found that the cause of bolwhole is a high content of C in the cladding coating, which will combine with O, then generate gas. A high content of Si will cause segregation during solidification, which is conducive to the initiation and propagation of micro-crack. There is a similar chemical composition between the shrinkage region and the defect-free region. The above-mentioned achievements have been published in the international well-known journal Lasers in Engineering.Secondly, the effects of massive laser shock peening treatment on the distributions of micro-hardness, residual stress and microstructure in laser cladding coating of 316 L stainless steel were investigated. Emphatically, the distributions of properties and microstructure in the top surface and the side bonding zone of the cladding coating were characterized and analyzed. Accordingly, the influence of massive laser shock peening on the microstructure of the cladding coating was obtained. Massive laser shock peening treatment brings an obvious improvement in micro-hardness, residual stress, and microstructure of the cladding coating, and it is an effective approach to eliminate tensile residual stress and to improve the micro-hardness in the near surface layer of the cladding coating. After massive laser shock peening treatment, the distribution of micro-hardness in the cross-section of the cladding coating is appear stepped, and the vlaue of micro-hardness in the inter-diffusion zone and transition zone are increased by 62.9 % and 31.3 %, respectively. Furthermore, the distribution of micro-hardness is strongly depends on the average size of local microstructure in different zones of the cladding coating(the finer the grains the higher the micro-hardness). The acicular structures in the side bonding zone are refined by the mechanical effect of the ultra-strong laser shock wave, which is helpful to increase the interfacial bonding strength between the cladding coating and the substrate, the columnar structures in the top surface of the cladding are refined into equiaxial structures after laser shock peening treatment. The porosity of the cladding coating is significantly decreased after massive laser shock peening treatment, and the microstructure is closer. Hence, massive laser shock peening treatment can eliminate the shrinkage structure defect in the caldding coating, which is caused by the uneven temperature and cladding powder distributions. After massive laser shock peening treatment, many obvious plastic deformations can be found in the near surface of 316 L stainless steel laser cladding coating and the substrate. The above-mentioned achievements have been published in the international well-known journal Journal of Alloys and Compounds.Finally, the effects of massive laser shock peening treatment on micro-hardness, residual stress and microstructure in laser welding zone of 316 L stainless steel were investigated, and the grain refinement mechanism of 316 L stainless steel laser welding zone and substrate which were treated by massive laser shock peening were analyzed. Massive laser shock peening treatment make a significant improvement in micro-hardness, residual stress, and microstructure of the laser welding zone. And it is an effective approach to eliminate tensile residual stress and improve the micro-hardness in the laser welding zone. However, the effect of massive laser shock peening treatment on micro-harness in the heat affect zone(HAZ) is quite small. It is also an effective approach for massive laser shock peening treatment to refine the grains in the laser welding zone and substrate, by which the columnar structures in the laser welding zone are refined into equiaxial structures. After massive laser shock peening treatment, a large number of dislocation lines, deformation twins and fault stackings have been found in the laser welding. Besides, many mechanical twins and lamellae twins also have been found in the substrate. The dislocation motion might be hindered by the boundaries of grains and twins. The mechanism governing deformation-induced grain refinement is believed to be dominated by high dense dislocations and deformation twins, which can result in grain boundary broken so as to form refine-grains. Meanwhile, the high density of dislocation tangles and dislocation walls in the coarse grains can form subgrain boundaries, and the continuous dynamic recrystallization caused by high pressure shock wave could make a progressive accumulation of boundary misorientations and finally lead to a gradual transition in boundary character with the formation of high angle grain boundaries. The above-mentioned achievements have been submitted in the international well-known journal Materials Science and Engineering: A.In summary, the research of this thesis provides a practical method to identify the typical defects in 316 L stainless steel laser cladding coating quickly and accurately. It also provides a reference for insight into the distributions of mechanical properties and microstructure in different regions of 316 L stainless steel laser cladding coating before and after massive laser shock peening treatment. Finally, it provides an important theoretical and experimental basis for roundly reveal the mechanisms of plastic deformation in 316 L stainless steel laser cladding coating and laser welding zone during massive laser shock peening treatment. |
PDF Full Text Request